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Hoe lig bome water hoër as 10 meter op?

Hoe lig bome water hoër as 10 meter op?



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Die atmosfeerdruk is 10 meter water (ongeveer). Dit beteken dat dit onmoontlik is om water hoër as 10 meter te lig met vakuum of kapillêre aksie (op Aarde, onder normale toestande).

Daar is bome hoër as 10 meter.

Hoe lig hulle water na hul tops?

OPDATEER

Met ander woorde: hoe kan kohesie-spanningsteorie waar wees as dit klaarblyklik die wette van fisika weerspreek?

OPDATERING 2

Atmosferiese druk help om die water te laat styg, nie weerstaan ​​om te styg nie. Wat weerstand bied, is watergewig. Wanneer waterkolom 10 meter hoog is, kan atmosferiese druk nie meer help nie.

Enige adhesie-/kohesiemeganisme kan ook nie hier help nie, want dit werk net in dun molekulêre laag. Om aksiekrag verder oor te dra word die druk benodig, wat onvoldoende is op 10 meter.

OPDATERING 3

As ons kapillêre klein genoeg gehad het om water tot 10 meter te styg en dan sal ons kleiner kapillêre bou wat ons verwag water hoër sal styg, sal ons misluk. Waterkolom sal breek en klim nie hoër as 10 meter nie.

Menisci tree op soos 'n klein suier en kan nie help om water hoër as 10 meter op te styg nie.

OPDATERING 4

Algemene drukverspreiding in kapillêre is soos volg:

$P_0$ is atmosferiese druk. Soos jy sien, reg onder menisci, word die druk verlaag met $2 sigma / R$ waar $R$ die radius van menisci is en $sigma$ oppervlakspanning is. Die hele term word "Laplace-druk" genoem. Soos jy sien, kan dit nie atmosferiese druk vervang nie, want waterkontinuïteit sal in die kas verbreek word.

D.w.s. geen menisci kan water hoër as 10 meter styg nie.

Die bestaan ​​van hoër bome BEWYS dat daar 'n paar ander beduidende meganismes is, nie adhesie/kohesie nie, nie kapillêr nie.

OPDATERING 5

Huidige weergawe, soos ek dit verstaan ​​het, is gebaseer op 'n verklaring dat 'n water, as dit in dun kapillêre gesit word, soos 'n soliede liggaam kan optree. Dit kan veral spanning tot minus 15 atmosfeer weerstaan.

Hierdie is 'n treksterkte van beton, so ek glo dit nie sonder bykomende bewyse nie.

Ek dink dit is net nie moeilik om dun buis te maak, water daarin te gooi en te kyk hoe hoog dit kan klim nie.

Is dit ooit gedoen?


Disclaimer: Dit is nie my navorsingsveld nie.


Eerstens is dit nie 'n volledige antwoord op ons vraag nie. 'n Goeie verduideliking van die huidige hipotese van watervervoer in bome (Dixon-Joly kohesie-spanning teorie, oorspronklik voorgestel 1894) kan gevind word by Die wonderlike fisika van water in bome maar ook in Tyree (1997). Die sleutelpunte is dat die stoma (blaaroppervlakporieë) so klein is dat die menisci groot waterkolomme kan weerstaan, dat water sterk samehangende kragte het, en dat water vervoer word deur die negatiewe druk wat deur transpirasie geskep word. Die webblad wat hierbo gekoppel is, bevat 'n pragtige visualisering van hoe menigte huidmondjies en menisci sterk negatiewe druk veroorsaak:

Tweedens, baie van die huidige bespreking in die kommentaar ('n aanduiding dat die vraag dalk swak by Bio-SE pas?) wentel om die aanneemlikheid van die kohesie-spanningsteorie, en spesifiek oor die vraag of water sterk negatiewe druk kan onderhou. Caupin & Herbert (2006) hersien metastabiliteit en kavitasie in water (in 'n fisikajoernaal), en bevat eksperimentele resultate oor negatiewe druk in water. Die referaat verwys na 'n groot aantal eksperimente onder verskeie eksperimentele opstellings (ek kan dit nie regverdig beoordeel nie). In hul gevolgtrekking stel hulle dat:

Onder die ontelbare kavitasie-eksperimente kan slegs diegene met spesiale sorg vir watersuiwerheid groot negatiewe druk bereik; met 'n verskeidenheid tegnieke verkry hulle almal Pcav rondom -25 MPa by kamertemperatuur (sien Fig. 3 (b)), wat ver weg van die teoretiese waarde val (van -120 tot -140 MPa). Daar is 'n noemenswaardige uitsondering: eksperimente met mineraalinsluitings behaal -140 MPa. Die groot gaping tussen hierdie data verg spesiale aandag.

So basies lê die teoretiese skattings op -130MPa en empiriese resultate op -25MPa (-250 atmosfeer), en water kan duidelik groot negatiewe druk bereik. Dit sou ook beteken dat die huidige skattings baie groter is as wat nodig is vir die kohesie-spanningsteorie om te werk (atmosferiese druk= 0.1MPa, negatiewe druk in waterkolom by 50m ~ -0.5MPa).

Hulle het ook 'n afdeling wat spesifiek bome bespreek:

7.1. Water in die natuur
Die wet van hidrostatika leer ons dat die drukval in 'n waterkolom van 10,2 m 0,1 MPa is. Dit wys daarop dat negatiewe druk bereik kan word in die stygende sap van hoë bome. Trouens bykomende effekte (viskose vloei, droogte) maak die druk in die sap negatief selfs op kleiner hoogtes. Die kohesie-spanningsteorie, wat eers deur Dixon en Jolly voorgestel is [56], verduidelik dat die sapkolom aan die bokant deur die meniskus in die porie van die blare gehou word: volgens Laplace se wet laat die meniskuskromming 'n druksprong tussen die buitekant toe. lugdruk en die negatiewe druk in die sap. Die bome bevat dus groot hoeveelhede metastabiele vloeistowwe. Kavitasie kan soms voorkom, wat die vloeistofkolom ontwrig en die vloei stop (xileem-embolie). Die komplekse hidrouliese argitektuur van bome beperk die skade, en strategieë bestaan ​​om die geëmbolieerde xileemkanale te hervul. Baie werk is aan hierdie onderwerp gewy, en word in Verw. [110,111].

Daar is ook bewyse dat die risiko van xileemembolisme toeneem met boomhoogte, en dit skep 'n afweging tussen watervervoerdoeltreffendheid en strukturele aanpassings om embolisme te hanteer (Domec et al. 2008). Dit word byvoorbeeld vergemaklik deur die gatopening deursnee van trageïede, met openinge wat afneem met hoogte langs 'n boom, wat verhoogde weerstand teen embolisme veroorsaak, maar terselfdertyd laer watergeleiding. Dit sal duidelik die hoogte van bome beperk, en die papier dui aan dat die hoogste Douglas-sparre op die rand is van wat hulle kan bereik.

Nog 'n onlangs gepubliseerde artikel wat relevant behoort te wees, is 'Metodes om plant kwesbaarheid vir kavitasie te meet: 'n kritiese oorsig' deur Cochard et al. (2013), maar ek het nie tyd gehad om noukeurig hierna te kyk nie. Sien opsomming hieronder:

Opsomming:
Xileem-kavitasieweerstand het diepgaande implikasies vir plantfisiologie en -ekologie. Hierdie proses word gekenmerk deur 'n 'kwesbaarheidskurwe' (VC) wat die variasie van die persentasie kavitasie as 'n funksie van xileemdrukpotensiaal toon. Die vorm van hierdie VC wissel van 'sigmoïdaal' tot 'eksponensieel'. Hierdie resensie bied 'n panorama van die tegnieke wat gebruik is om so 'n kromme te genereer. Die tegnieke verskil deur (i) die manier waarop kavitasie geïnduseer word (bv. bankdehidrasie, sentrifugering of luginspuiting), en (ii) die manier waarop kavitasie gemeet word (bv. persentasie verlies van geleidingsvermoë (PLC) of akoestiese emissie), en 'n nomenklatuur word op grond van hierdie twee metodes voorgestel. 'n Opname van die literatuur van meer as 1200 VC's is gebruik om statistieke oor die gebruik van hierdie metodes en oor hul betroubaarheid en geldigheid te trek. Vier metodes was verantwoordelik vir meer as 96% van alle kurwes wat tot dusver geproduseer is: bankdehidrasie-PLC, sentrifugering-PLC, drukhuls-PLC, en Cavitron. Hoe die vorm van VCs verskil oor tegnieke en spesies xileem-anatomie is ook ontleed. Opvallend genoeg is gevind dat die oorgrote meerderheid kurwes verkry met die verwysingsbank dehidrasie-PLC metode 'sigmoïdaal' is. 'Eksponensiële' kurwes was meer tipies van die drie ander metodes en was merkwaardig gereeld vir spesies met groot xileembuise (ringporeus), wat gelei het tot 'n aansienlike oorskatting van die kwesbaarheid van kavitasie vir hierdie funksionele groep. Ons vermoed dat 'eksponensiële' kurwes 'n oop-vaartuig artefak kan weerspieël en vra vir meer voorsorgmaatreëls met die gebruik van die drukhuls en sentrifugering tegnieke.


Nog 'n vrywaring: dit is nie my vakgebied nie en ek is nie bevoeg om die inhoud van die referaat wat ek onder u aandag bring, te beoordeel nie.

Ek het hierbo gesê dat ek nie nog 'n bydrae sal maak nie, maar ek het iets anders gevind wat die moeite werd is om in hierdie konteks te deel en wat die antwoord van @fileunderwater aanvul

Wang Z et al. (2012) Kapillêre styging in 'n mikrokanaal van arbitrêre vorm en benatbaarheid: histerese lus. Langmuir 28: 16917-16926

Hierdie vraestel sluit modellering, vrye energieberekeninge en eksperimentering oor hierdie probleem in. Die wiskunde is ver verby my, maar hulle kom tot 'n baie interessante gevolgtrekking, naamlik dat alhoewel 'n boom nie kan begin vanaf 'n posisie van geen vloeistof in die xileem en dan tot bo vul nie, kan dit baie klein begin en bo die hoogte groei. wat deur eenvoudige kapillêre werking en tot 100 m in stand gehou kan word solank die waterkolom nooit gebreek word nie.

Die vraestel is agter 'n betaalmuur, maar ek reproduseer hieronder wat in wese die besprekingsafdeling is. Alhoewel dit 'n geur van die werk gee, moet ek beklemtoon dat dit 'n baie sterk teoretiese behandeling van die probleem bied en die moeite werd is om te kyk.

V. IMPLIKASIE VIR WATERVERVOER NA DIE BANTE VAN HOË BOME Die meeste plantfisioloë aanvaar die "kohesie-spanning teorie" as die verduideliking vir die styging van sap.26 In hierdie kwalitatiewe teorie hang die beweging van water af van drie belangrike fisies-chemiese eienskappe van water, wat eintlik ooreenstem met onderskeidelik kapillêre styging (kohesie), cavitasie (spanning) en die gehidreerde wand (lae kontakhoek). In hierdie afdeling fokus ons slegs op die implikasie van die algemene kragbalans en lushisterese in die kapillêre styging van 'n hoë boom. Die hoogte waartoe water in 'n boom styg, hang af van die grootte van die vervoerpype. As 'n mens 'n boom afkap en na binne kyk, is die kapillêre afmetings van die relatief groot pype (die xileembuis) in die orde van 100 μm.27 Gevolglik is die kapillêre styging ongeveer 0,1 m. As kapillêre druk alleen die wateropkoms na die boomtop van 'n 100 m hoë boom, soos die kusrooibosse van Kalifornië, sou verklaar, is 'n kapillêre radius van ongeveer 100 nm nodig. Daar is voorgestel dat die relevante kapillêre dimensie die lug-water raakvlakke in die selwande van die boonste blare is. Die matriks van sellulose mikrofibrille is hoogs benatbaar, en die spasiëring tussen hulle lewer effektiewe porieë deursnee van ongeveer 10 nm. Daar is daarop gewys dat dit nie nodig is vir die kapillêre om 'n klein boring oor sy lengte te hê nie. Slegs die boring by die meniskus (dws in die boonste blaar) is relevant.27 Hierdie gevolg is bewys in ons algemene uitdrukking van kragbalans, vgl 4. Let daarop dat 'n mikrokanaal wat hoeke of knots op sy dwarssnit bevat, nie in ag geneem word in die afleiding van vgl 4. Vloeibare filamente strek tot oneindig in die hoeke of knotse.28 Nietemin is die hoogte van die vloeistofkolom steeds omgekeerd eweredig aan die kenmerkende afmeting van die dwarssnit van die buis. Vir die oplossings wat die kragbalans bevredig, bestaan ​​daar nietemin die kwessie van fisiese stabiliteit. 'n Klein boring by die boonste blaar wat aan 'n groter xileemkanaal gekoppel is, toon die teenwoordigheid van 'n konvergente mikrokanaal. Gevolglik is veelvuldige stabiele hoogtes moontlik, soos beskryf in die voorgenoemde ontledings. Die finale toestand hang egter af van die aanvanklike toestand. Die vloeistof sal styg tot 'n stabiele hoogte wat ooreenstem met die groter xileemkanaal as die mikrokanaal aanvanklik leeg is. Met ander woorde, die vloeistof sal nie vanself styg tot die stabiele hoogte naby die bokant van die konvergente kanaal nie, want dit sal nie die groter kanaal van die kanaal kan deurkruis nie. Hierdie situasie is nietemin stabiel as die vloeistof na bo opgesuig word en dan die suiging verwyder word. Hoe kry 'n hoë boom sulke groot negatiewe (suig) druk van bo af? Soos gedemonstreer in ons eksperimente, is die geleidelike styging van 'n aanvanklik ondergedompelde keël in staat om die stabiliteit van die meniskus op die bokant van die afgeknotte keël te handhaaf solank die kragbalans bevredig is. Let daarop dat die kontakhoek in die omgewing van die klein porie mond aangepas kan word om die kragbalans te vervul wanneer die mikrokanaal nie hoog genoeg is nie. Die stadige groei van die boom kan beskou word as 'n geleidelike styging van die konvergente kanaal. Solank watervervoer na die porieë op die boonste blare nie deur die hele loop van die boom se groei tot 100 m onderbreek word nie, kan hierdie stabiele hoogte bereik word sonder om tot suiging oor te gaan.


Hier is Veritasium op YouTube het een verduideliking wat dieselfde is as @AlanBoyd se opmerking.

Meta-stabiele vloeistof kan negatiewe druk hê.


Die volgende artikel het werk oor die ondersoek van water by negatiewe druk ondersoek, vanaf die eerste poging honderd jaar gelede, toe die grootste spanning bereik is -3.4 MPa @ 24 degC, tot die mees onlangse meting van water by kamertemperatuur tot -26 MPa:

http://hal.archives-ouvertes.fr/docs/00/72/64/37/PDF/Caupin-JPCondensedMatter-2012.pdf

Daarom kan bome water hoër as 10 meter oplig omdat water deur negatiewe druk aan die bokant opgetrek word (Kohesie-spanningsteorie CTT). Die spanning wat nodig is om water na die hoogste bome op te lig is -1.2MPa, wat baie aanneemlik is, aangesien dit minder is as die waarde wat honderd jaar gelede gemeet is.


Hoe lig bome water hoër as 10 meter op? - Biologie

Plante is fenomenale hidrouliese ingenieurs. Deur slegs die basiese wette van fisika en die eenvoudige manipulasie van potensiële energie te gebruik, kan plante water na die top van 'n 116 meter hoë boom beweeg (Figuur 1a). Plante kan ook hidroulika gebruik om genoeg krag op te wek om klippe te kloof en sypaadjies vas te gespe (Figuur 1b). Plante bereik dit as gevolg van waterpotensiaal.

Figuur 1. Met hoogtes naby 116 meter, (a) kus rooibosse (Sequoia sempervirens) is die hoogste bome ter wêreld. Plantwortels kan maklik genoeg krag opwek om (b) betonsypaadjies vas te slaan en te breek, tot groot ontsteltenis van huiseienaars en stadsonderhoudsdepartemente. (krediet a: wysiging van werk deur Bernt Rostad krediet b: wysiging van werk deur Pedestrians Educating Drivers on Safety, Inc.)

Water potensiaal is 'n maatstaf van die potensiële energie in water. Plantfisioloë stel nie belang in die energie in enige spesifieke waterige sisteem nie, maar is baie geïnteresseerd in waterbeweging tussen twee sisteme. In praktiese terme is waterpotensiaal dus die verskil in potensiële energie tussen 'n gegewe watermonster en suiwer water (by atmosferiese druk en omgewingstemperatuur). Waterpotensiaal word aangedui deur die Griekse letter ψ (psi) en word uitgedruk in eenhede van druk (druk is 'n vorm van energie) genoem megapascals (MPa). Die potensiaal van suiwer water (Ψw suiwer H2O ) word, per definisie, 'n waarde van nul aangewys (al bevat suiwer water baie potensiële energie, word daardie energie geïgnoreer). Waterpotensiaalwaardes vir die water in 'n plantwortel, stam of blaar word dus relatief tot Ψ uitgedrukw suiwer H2O .

Die waterpotensiaal in plantoplossings word beïnvloed deur opgeloste stofkonsentrasie, druk, swaartekrag en faktore wat matrikseffekte genoem word. Waterpotensiaal kan in sy individuele komponente afgebreek word deur die volgende vergelyking te gebruik:

waar Ψs, Ψbl, Ψg, en Ψm verwys na onderskeidelik die opgeloste stof, druk, swaartekrag en matriekpotensiale. “Stelsel” kan verwys na die waterpotensiaal van die grondwater (Ψ grond ), wortelwater (Ψ wortel ), stamwater (Ψ stam ), blaarwater (Ψ blaar ) of die water in die atmosfeer (Ψ atmosfeer ): wat ook al waterige stelsel word oorweeg. Soos die individuele komponente verander, verhoog of verlaag hulle die totale waterpotensiaal van 'n stelsel. Wanneer dit gebeur, beweeg water om te ewewig, beweeg van die stelsel of kompartement met 'n hoër waterpotensiaal na die stelsel of kompartement met 'n laer waterpotensiaal. Dit bring die verskil in waterpotensiaal tussen die twee stelsels (ΔΨ) terug na nul (ΔΨ = 0). Daarom, vir water om deur die plant van die grond na die lug te beweeg ('n proses wat transpirasie genoem word), moet Ψ grond >Ψ wortel >Ψ stam >Ψ blaar >Ψ atmosfeer wees.

Water beweeg slegs in reaksie op ΔΨ, nie in reaksie op die individuele komponente nie. Omdat die individuele komponente egter die totale Ψ beïnvloedstelsel, deur die individuele komponente te manipuleer (veral Ψs), kan 'n plant waterbeweging beheer.


Druk, swaartekrag en matriekpotensiaal

Waterpotensiaal word beïnvloed deur faktore soos druk, swaartekrag en matriekpotensiale.

Leerdoelwitte

Onderskei tussen druk, swaartekrag en matriekpotensiale in plante

Sleutel wegneemetes

Kern punte

  • Hoe hoër die drukpotensiaal (Ψbl), hoe meer potensiële energie in 'n sisteem: 'n positiewe Ψbl verhoog Ψtotaal, terwyl 'n negatiewe Ψbl verminder Ψtotaal.
  • Positiewe druk binne selle word deur die selwand bevat, wat turgordruk veroorsaak, wat verantwoordelik is vir die handhawing van die struktuur van blare afwesigheid van turgordruk veroorsaak verwelking.
  • Plante verloor water (en turgordruk) deur transpirasie deur die huidmondjies in die blare en vul dit aan via positiewe druk in die wortels.
  • Drukpotensiaal word beheer deur opgeloste stofpotensiaal (wanneer opgelostestofpotensiaal afneem, verhoog drukpotensiaal) en die oop- en toemaak van huidmondjies.
  • Swaartekragpotensiaal (Ψg) verwyder potensiële energie uit die stelsel omdat swaartekrag water afwaarts na die grond trek, wat Ψ vermindertotaal.
  • Matriekpotensiaal (Ψm) verwyder energie uit die stelsel omdat watermolekules aan die sellulosematriks van die plant’ se selwande bind.

Sleutel terme

  • turgor druk: druk die plasmamembraan teen die selwand van plant wat veroorsaak word deur die osmotiese vloei van water van buite die sel in die sel se vakuool

Drukpotensiaal

Drukpotensiaal word ook turgorpotensiaal of turgordruk genoem en word deur Ψ voorgestelbl. Drukpotensiaal kan positief of negatief wees hoe hoër die druk, hoe groter potensiële energie in 'n sisteem, en omgekeerd. Daarom, 'n positiewe Ψbl (kompressie) verhoog Ψtotaal, terwyl 'n negatiewe Ψbl (spanning) verminder Ψtotaal. Positiewe druk binne selle word deur die selwand bevat, wat turgordruk in 'n plant veroorsaak. Turgordruk verseker dat 'n plant sy vorm kan behou. 'N Plant’s blare verwelk wanneer die turgor druk afneem en herleef wanneer die plant is natgemaak. Drukpotensiale is tipies rondom 0,6–0,8 MPa, maar kan so hoog as 1,5 MPa bereik in 'n goed natgemaakte plant. Ter vergelyking word die meeste motorbande teen 'n druk van 30–34 psi of ongeveer 0,207-0,234 MPa gehou. Water gaan uit die blare verlore deur transpirasie (nader Ψbl = 0 MPa by die verwelkpunt) en herstel deur opname via die wortels.

Turgor druk: Wanneer (a) totale waterpotensiaal (Ψtotaal) buite die selle laer is as binne, beweeg water uit die selle en die plant verwelk. Wanneer (b) die totale waterpotensiaal hoër is buite die plantselle as binne, beweeg water in die selle in, wat lei tot turgordruk (Ψp), wat die plant regop hou.

'n Plant kan Ψ manipuleerbl deur sy vermoë om Ψ te manipuleers (opgeloste potensiaal) en deur die proses van osmose. Plante moet die negatiewe kragte van swaartekragpotensiaal (Ψg) en matriekpotensiaal (Ψm) oorkom om 'n positiewe drukpotensiaal te handhaaf. As 'n plantsel die sitoplasmiese opgeloste stofkonsentrasie verhoog:

  1. Ψs sal afneem
  2. Ψtotaal sal afneem
  3. die Δ tussen die sel en die omliggende weefsel sal afneem
  4. water sal deur osmose in die sel inbeweeg
  5. Ψbl sal verhoog.

Plante kan ook Ψ reguleerbl deur die huidmondjies oop en toe te maak. Stomatale openinge laat water uit die blaar verdamp, wat Ψ verminderbl en Ψtotaal. Dit verhoog die waterpotensiaal tussen die water in die blaarblaar (basis van die blaar) en in die blaar, waardeur water aangemoedig word om vanaf die blaarblaar in die blaar te vloei.

Swaartekragpotensiaal

Swaartekragpotensiaal (Ψg) is altyd negatief of nul in 'n plant met geen hoogte nie. Sonder hoogte is daar geen potensiële energie in die sisteem nie. Die swaartekrag trek water afwaarts na die grond, wat die totale hoeveelheid potensiële energie in die water in die plant verminder (Ψtotaal). Hoe langer die plant, hoe hoër die waterkolom, en hoe meer invloedryke Ψg word. Op 'n sellulêre skaal en in kort plante is hierdie effek weglaatbaar en maklik geïgnoreer. Oor die hoogte van 'n hoë boom soos 'n reusagtige kusrooibos, moet die plant egter 'n ekstra 1MPa se weerstand oorkom as gevolg van die gravitasietrek van –0.1 MPa m -1.

Matriekpotensiaal

Matriekpotensiaal (Ψm) is die hoeveelheid water wat aan die matriks van 'n plant gebind is via waterstofbindings en is altyd negatief tot nul. In 'n droë stelsel kan dit so laag as –2 MPa in 'n droë saad wees of so hoog as nul in 'n waterversadigde stelsel. Elke plantsel het 'n sellulose selwand, wat hidrofiel is en 'n matriks vir wateradhesie verskaf, vandaar die naam matriekpotensiaal. Die binding van water aan 'n matriks verwyder of verbruik altyd potensiële energie uit die sisteem. Ψm is soortgelyk aan opgeloste stofpotensiaal omdat die waterstofbindings energie uit die totale sisteem verwyder. In opgeloste stofpotensiaal is die ander komponente egter oplosbare, hidrofiele opgeloste stofmolekules, terwyl in Ψm, die ander komponente is onoplosbare, hidrofiele molekules van die plantselwand. m kan nie deur die plant gemanipuleer word nie en word tipies geïgnoreer in goed natgemaakte wortels, stingels en blare.


Beweging van water teen swaartekrag

Hoe word water op 'n plant vervoer teen swaartekrag, wanneer daar geen “pomp” is om water deur 'n plant se vaskulêre weefsel te beweeg nie? Daar is drie hipoteses wat die beweging van water op 'n plant teen swaartekrag verduidelik. Hierdie hipoteses sluit mekaar nie uit nie, en dra elkeen by tot die beweging van water in 'n plant, maar slegs een kan die hoogte van hoë bome verduidelik:

  1. Worteldruk stoot water op
  2. Kapillêre werking trek water op binne die xileem
  3. Kohesie-spanning trek water die xileem op

Ons’ll oorweeg elk van hierdie op sy beurt.

Worteldruk staatmaak op positiewe druk wat in die wortels vorm soos water in die wortels van die grond af inbeweeg. Water beweeg in die wortels vanaf die grond deur osmose, as gevolg van die lae opgeloste stofpotensiaal in die wortels (laer Ψs in wortels as in grond). Hierdie inname van water in die wortels verhoog Ψp in die wortelxileem, wat water opdryf. In uiterste omstandighede lei worteldruk tot gevolg guttasie, of afskeiding van waterdruppels vanaf huidmondjies in die blare. Worteldruk kan water egter net 'n paar meter teen swaartekrag beweeg, so dit is nie sterk genoeg om water op die hoogte van 'n hoë boom te beweeg nie.

Kapillêre werking of kapillariteit is die neiging van 'n vloeistof om teen swaartekrag op te beweeg wanneer dit in 'n nou buis (kapillêre) beperk word. Kapillariteit vind plaas as gevolg van drie eienskappe van water:

  1. Oppervlakspanning, wat plaasvind omdat waterstofbinding tussen watermolekules sterker is by die lug-water-koppelvlak as tussen molekules binne die water.
  2. Adhesie, wat molekulêre aantrekkingskrag tussen “unlike” molekules is. In die geval van xileem vind adhesie plaas tussen watermolekules en die molekules van die xileemselwande.
  3. Kohesie, wat molekulêre aantrekkingskrag tussen “like” molekules is. In water vind kohesie plaas as gevolg van waterstofbinding tussen watermolekules.

Op sy eie kan kapillariteit goed werk binne 'n vertikale stam vir tot ongeveer 1 meter, so dit is nie sterk genoeg om water op 'n hoë boom te beweeg nie.

Hierdie video bied 'n oorsig van die belangrike eienskappe van water wat hierdie beweging fasiliteer:

Die cohesie-spanning hipotese is die mees algemeen aanvaarde model vir beweging van water in vaatplante. Kohesie-spanning kombineer in wese die proses van kapillêre werking met transpirasie, of die verdamping van water uit die huidmondjies van die plant. Transpirasie is uiteindelik die hoofdrywer van waterbeweging in xileem. Die kohesie-spanning model werk soos volg:

  1. Transpirasie (verdamping) vind plaas omdat huidmondjies oop is om gaswisseling vir fotosintese moontlik te maak. Soos transpirasie plaasvind, verdiep dit die meniskus van water in die blaar, wat negatiewe druk veroorsaak (ook genoem spanning of suiging).
  2. Die spanning geskep deur transpirasie “trek” water in die plant xileem, trek die water opwaarts in baie dieselfde manier wat jy trek water opwaarts wanneer jy suig aan 'n strooi.
  3. Kohesie (water wat aan mekaar kleef) veroorsaak dat meer watermolekules die gaping in die xileem vul aangesien die boonste water na die huidmondjies getrek word.

Hier is 'n bietjie meer detail oor hoe hierdie proses werk: Binne-in die blaar op sellulêre vlak versadig water op die oppervlak van mesofilselle die sellulose mikrofibrille van die primêre selwand. Die blaar bevat baie groot intersellulêre lugruimtes vir die uitruil van suurstof vir koolstofdioksied, wat nodig is vir fotosintese. Die nat selwand word aan hierdie blaar interne lugruimte blootgestel, en die water op die oppervlak van die selle verdamp in die lugruimtes, wat die dun film op die oppervlak van die mesofilselle verminder. Hierdie afname skep 'n groter spanning op die water in die mesofilselle, waardeur die trekkrag op die water in die xileemvate verhoog word. Die xileemvate en trageïede is struktureel aangepas om groot veranderinge in druk te hanteer. Ringe in die vate behou hul buisvorm, baie soos die ringe op 'n stofsuierslang die slang oop hou terwyl dit onder druk is. Klein perforasies tussen vatelemente verminder die aantal en grootte van gasborrels wat kan vorm via 'n proses wat kavitasie genoem word. Die vorming van gasborrels in xileem onderbreek die aaneenlopende stroom water vanaf die basis na die bokant van die plant, wat 'n breuk veroorsaak wat 'n embolisme in die vloei van xileemsap genoem word. Hoe hoër die boom, hoe groter is die spanningskragte wat nodig is om water te trek, en hoe meer kavitasie gebeure. In groter bome kan die gevolglike embolisme xileemvate toestop, wat hulle nie-funksioneel maak.

Die kohesie-spanning teorie van sap styging word getoon. Verdamping vanaf die mesofilselle produseer 'n negatiewe waterpotensiaalgradiënt wat veroorsaak dat water opwaarts beweeg vanaf die wortels deur die xileem. Beeldkrediet: OpenStax Biology

Hierdie video bied 'n oorsig van die verskillende prosesse wat veroorsaak dat water deur 'n plant beweeg (gebruik hierdie skakel na kyk hierdie video op YouTube, as dit nie vanaf die ingebedde video speel nie):


Antarktiese yslaag wat smelt om seevlak hoër te lig as wat gedink is, sê studie

Dit is in die Scotia See geneem tydens die kernveldtog in 2007. Krediet: Michael Weber

Globale styging in seevlak wat verband hou met die moontlike ineenstorting van die Wes-Antarktiese Yskap is aansienlik onderskat in vorige studies, wat beteken dat seevlak in 'n warm wêreld groter sal wees as wat verwag is, volgens 'n nuwe studie van Harvard-navorsers.

Die verslag, gepubliseer in Wetenskap vooruitgang, bevat nuwe berekeninge vir waarna navorsers verwys as 'n waterverdryfmeganisme. Dit vind plaas wanneer die soliede rots van die Wes-Antarktiese Yskap opwaarts terugspring soos die ys smelt en die totale gewig van die yslaag afneem. Die grondrots lê onder seevlak, so wanneer dit oplig, stoot dit water uit die omliggende gebied in die see, wat bydra tot globale seevlakstyging.

Die nuwe voorspellings toon dat in die geval van 'n totale ineenstorting van die yslaag, globale seevlakstygingsskattings binne 1 000 jaar met 'n bykomende meter versterk sal word.

"Die omvang van die effek het ons geskok," het Linda Pan, 'n Ph.D. in aard- en planetêre wetenskap in GSAS wat die studie saam met mede-nagraadse student Evelyn Powell gelei het. "Vorige studies wat die meganisme oorweeg het, het dit as onbelangrik afgemaak."

“As die Wes-Antarktiese Yskap ineenstort, is die mees algemene skatting van die gevolglike wêreldwye gemiddelde seevlakstyging wat sou lei, 3,2 meter,” het Powell gesê. "Wat ons gewys het, is dat die wateruitsettingsmeganisme 'n bykomende meter, of 30 persent, by die totaal sal voeg."

Maar dit is nie net 'n storie oor impak wat oor honderde jare gevoel sal word nie. Een van die simulasies wat Pan en Powell uitgevoer het, het aangedui dat teen die einde van hierdie eeu wêreldwye seevlakstyging wat veroorsaak word deur die smelting van die Wes-Antarktiese Yskap met 20 persent sal toeneem deur die waterverdryfmeganisme.

“Elke gepubliseerde projeksie van seevlakstyging as gevolg van die smelt van die Wes-Antarktiese yslaag wat op klimaatmodellering gebaseer is, of die projeksie tot die einde van hierdie eeu of langer in die toekoms strek, sal opwaarts hersien moet word omdat van hul werk," het Jerry X. Mitrovica, die Frank B. Baird Jr. Professor in Wetenskap in die Departement Aard- en Planetêre Wetenskappe en 'n senior skrywer op die koerant gesê. "Elke een."

Pan en Powell, albei navorsers in Mitrovica se laboratorium, het hierdie navorsing begin terwyl hulle aan 'n ander seevlakveranderingsprojek gewerk het, maar het na hierdie een oorgeskakel toe hulle meer waterverdryf uit die Wes-Antarktiese yslaag opgemerk het as wat hulle verwag het.

Die navorsers wou ondersoek instel na hoe die uitdryfmeganisme seevlakverandering beïnvloed wanneer die lae viskositeit, of die maklik vloeiende materiaal van die Aarde se mantel onder Wes-Antarktika, in ag geneem word. Toe hulle hierdie lae viskositeit in hul berekeninge ingesluit het, het hulle besef dat wateruitdrywing baie vinniger plaasgevind het as wat vorige modelle voorspel het.

“Maak nie saak watter scenario ons gebruik het vir die ineenstorting van die Wes-Antarktiese Yskap nie, ons het altyd gevind dat hierdie ekstra een meter se globale styging in die seevlak plaasgevind het,” het Pan gesê.

Die navorsers hoop dat hul berekeninge wys dat wetenskaplikes die wateruitdrywingseffek en die mantel se lae viskositeit onder Antarktika akkuraat moet skat om wêreldwye seevlakstyging wat met smeltende ysplate geassosieer word, akkuraat te skat.

"Stygging van seevlak hou nie op wanneer die ys ophou smelt nie," het Pan gesê. “Die skade wat ons aan ons kuslyne aanrig, sal vir eeue voortduur.”


Wetenskap Vrae

Wat veroorsaak 'n reënboog?


'n Reënboog in Indiana, Oktober 2015.
Bron: AP Photo/Michael Conroy

Alhoewel lig kleurloos lyk, bestaan ​​dit uit baie kleure - rooi, oranje, geel, groen, blou, indigo en violet. Hierdie kleure staan ​​bekend as die spektrum. Wanneer lig in water skyn, breek die ligstrale, of buig, teen verskillende hoeke. Verskillende kleure buig teen verskillende hoeke - rooi buig die minste en violet die meeste. Wanneer lig teen 'n sekere hoek deur 'n reëndruppel gaan, skei die strale in die kleure van die spektrum - en jy sien 'n pragtige reënboog.

Waarom word sommige voorwerpe, soos deure en vensters, groter en kleiner?

Het jy opgelet dat kasdeure nie so maklik in die somer toemaak soos in die winter nie? Dis omdat hulle in die hitte van die somer uitsit en in die koue winter saamtrek. Alles op aarde bestaan ​​uit klein deeltjies wat molekules genoem word, wat voortdurend in beweging is. Wanneer die molekules warm word, beweeg hulle vinniger en trek van mekaar af. Soos hulle uitmekaar beweeg, neem hulle meer spasie op, wat veroorsaak dat selfs soliede voorwerpe effens groter word. Molekules vertraag soos hulle afkoel, en hulle neem minder ruimte op. Dit veroorsaak dat dinge 'n bietjie krimp. (Water is 'n uitsondering. Wanneer dit vries, staan ​​die molekules so in lyn dat die ys meer spasie opneem.)

Hoekom land katte altyd op hul voete?

Katte het sommige van hul nege lewens te danke aan hul unieke skeletstruktuur. Katte het nie 'n sleutelbeen nie, en die bene in hul ruggraat is meer buigsaam as ander diere. Dit maak dit makliker van hulle af om hul liggame makliker te buig en te draai tydens 'n kort val. 'n Val van twee of meer vloere kan egter 'n kat ernstig beseer. 'n Kat se voete en bene kan gewoonlik nie die impak van 'n val van daardie afstand of hoër absorbeer nie.

Wat maak springmielies pop?

'n Springmieliespit is eintlik 'n saad. In die middel daarvan is 'n klein plantembrio, 'n lewensvorm in sy vroegste fase. Die embrio word omring deur sagte, styselagtige materiaal wat water bevat. Om die embrio is 'n harde dop. Wanneer die pit verhit word tot ongeveer 400 grade Fahrenheit, verander die water in stoom. Die druk van die stoom veroorsaak dat die pit se dop ontplof en die stysel uitstort. Jy moet die botter byvoeg!

Wat veroorsaak weerlig?

Wanneer lug binne 'n donderstorm opstyg en daal, vorm positiewe en negatiewe ladings in die wolk. Die onderkant van die donderwolk het 'n negatiewe lading, en die bokant het 'n positiewe lading. 'n Weerligflits gebeur wanneer 'n lading so sterk word dat die lug dit nie kan keer om van die wolk na die grond te spring nie, wat 'n positiewe lading het. Weerlig kan ook in die wolk vorm, wat tussen die positief en negatief gelaaide areas beweeg. Die gemiddelde weerligstraal kan 'n 100-watt gloeilamp vir meer as drie maande aanskakel. Die lug naby 'n weerlig is warmer as die oppervlak van die Son.

Hoekom voel ek duiselig wanneer ek draai?

Binne jou ore is buise gevul met vloeistof. Die vloeistof beweeg wanneer jy beweeg, en vertel jou brein in watter posisie jou liggaam is. Wanneer jy spin, draai die vloeistof ook. Die vloeistof bly draai nadat jy opgehou het. Jou brein dink jy draai nog steeds, so jy bly voel dat alles in sirkels loop – totdat die vloeistof ophou beweeg.

Hoekom lyk dit of 'n knokkelbal ?dans? na tuisbord toe?

Die bal val en sweef onvoorspelbaar omdat dit nie draai nie. Die gebrek aan vinnige draai verander die nate van die bofbal in klein vleuels?oppervlakke wat lig en sleep skep wanneer hulle deur die lug vlieg. Soos die lug oor die nate beweeg, word klein wervelings geskep, wat sakke van lae druk om die oppervlak van die bal veroorsaak. Soos lug instroom om die sakke te vul, word die bal in verskillende rigtings gedruk. As die bal te veel roteer, sal die nate 'n meer konsekwente oppervlak aan die wind bied, en die bal sal eweneens 'n gladder pad volg.

Hoekom verander blare van kleur in die herfs?


Herfsblare in Vermont, Oktober 2015.
Bron: AP Photo/Dave Gram

Een van die seker tekens van herfs (behalwe die begin van 'n nuwe skooljaar) is die verandering in kleur van blare van groen na heldergeel, oranje en rooi. Bome is soort van bere? hulle berg kos op gedurende die lente en somer en rus dan vir die winter. Oor die lente en somer gebruik bome 'n proses wat fotosintese genoem word om kos en energie te maak. ’n Groen pigment genaamd chlorofil laat fotosintese plaasvind. Gedurende die herfs en winter is daar nie genoeg lig of water vir fotosintese om plaas te vind nie, so die chlorofil begin vervaag. Soos die groen verdwyn, begin die ander kleure na vore kom. Hierdie kleure was heeltyd in die blare teenwoordig, maar hulle is oorheers deur die chlorofil

Hoekom rys my hare as ek my hoed afhaal op 'n koue, droë dag?

Alles wat jy sien bestaan ​​uit atome. Hulle bevat selfs kleiner deeltjies, genoem protone en elektrone. Protone het positiewe elektriese ladings en hulle beweeg nooit. Elektrone het 'n negatiewe lading en hulle beweeg rond. Atome het gewoonlik dieselfde aantal protone en elektrone, so hulle kanselleer mekaar uit en atoom is neutraal? dit het geen lading nie. Wanneer twee goed saam gevryf word, beweeg die elektrone soms van een ding na die ander. Die atoom wat elektrone verloor, word positief gelaai, en die atoom wat meer elektrone kry, word negatief gelaai. Twee dinge wat verskillende ladings het, trek na mekaar toe twee dinge met dieselfde ladings stoot van mekaar af. Wanneer jy jou hoed afhaal, beweeg elektrone van jou hoed aan na jou hare. Your individual hairs then have the same positive charge, so they move away from each other, and you look really funny.

How does a plane takeoff and fly?


An Alaska Airlines Boeing 737 in flight, Oct. 2015
Source: AP Photo/Ted S. Warren

It?s easy to understand how a bird can fly?it?s lightweight and has wings. But how does a huge airplane get off the ground? The plane?s engine pushes the plane forward. As it moves, air flowing around the wings creates lift. The lift increases as the plane gathers speed. The plane takes off once there?s enough lift to overtake gravity. When the plane?s in the air, thrust from the engines pushes the plane forward.

How do scientists know how to make a flu vaccine if viruses can be different every year?

The flu virus changes every year. However, scientists gather information about virus mutations, or changes, before the flu-virus season begins. This lets them predict what each year?s flu virus might look like. Based on that, a vaccine is made that we hope will be accurate enough to help people fight off major cases of the flu.

What are stem cells?

Stem cells, the basic building blocks of human development, are sometimes called ?magic seeds.? That?s because they can regenerate human tissue of various kinds. The use of stem cells is controversial because the best source for the cells is human embryos. Stem cells form four to five days after an egg is fertilized. These embryos must be destroyed to harvest the cells, and those opposing the research consider this the same as taking human life. Those who support stem cell research say that an embryo that is just a few days old is simply a miniscule cluster of cells and not the same as a human life. They maintain that stem cells have the potential to save human lives. Stem cells show promise in being able to one day be able to treat and cure many illnesses and diseases, such as Alzheimer's, diabetes, Parkinson's, spinal cord injuries and other medical conditions.

Why do stars twinkle?

We see the stars through the atmosphere. Their light passes through millions of miles of constantly moving pockets and streams of air, which distort the image of the stars. Even though many stars are much larger than planets, they're so far away from us that they seem smaller, like tiny dots. The distortions make it seem as if the shining lights are moving or blinking. In outer space, where there is no atmosphere, stars don't twinkle.

What causes thunder?

When a bolt of lightning shoots through the atmosphere, it heats the air to an amazing 50,000 degrees Fahrenheit in a fraction of a second. The superheated air rapidly expands, cools and then contracts, causing shock waves. These shock waves create sound waves, which we hear as thunder.

Why don't the oceans freeze?

In the Arctic and Antarctic, the oceans do freeze. The ice cap at the North Pole is entirely over ocean the ice, however, is only a few feet deep. Oceans don't freeze solid for because they contain a lot of water, which is constantly circulating around the world. In addition, water flowing from warmer oceans (and from areas near underground volcanoes) takes off some of the chill. Another important factor is that oceans contain salt water, which has a higher freezing point than fresh water.

Why do boats float?

A steel bar dropped into water sinks, but a huge boat made of steel floats. Hoekom? Most of the space in the boat is taken up by air. The air makes the boat less dense than water. Objects of lesser density float on liquids of greater density. This is also why holes in the bottom of a boat cause it to sink: as air floods out of the boat and water rushes in, the overall density of the boat increases to become more dense than the surrounding water.

What's the difference between bacteria and virus?

Bakterieë are tiny, one-celled living organisms that can only be seen with a microscope. They live and breed in warm, moist environments in the body and elsewhere, growing quickly and causing infection. Bacterial infections can usually be treated with an antibiotic. Virusse are smaller than bacteria and cannot be seen with a microscope. They grow inside the body and produce toxins (poisons) that can cause rashes, aches and fevers. Viruses cannot be killed with antibiotics.

Why do I have brown eyes?

The genes we inherit from our parents determine things like our height, looks, hair color and eye color. This passing of characteristics from parent to child is called heredity. If your mother has brown eyes, and your father has blue eyes, there?s a good chance you?ll have brown eyes. That?s because the brown-eye gene is dominant, and the blue-eye gene is recessive. The dominant gene usually prevails over the recessive one. It?s possible, however, for you to have blue eyes if both your parents have brown eyes. They probably inherited recessive blue-eye genes from their parents and passed them on to you.


Inhoud

Measurements can be made from living plants using specialised equipment. Among the most commonly used instruments are those that measure parameters related to photosynthesis (chlorophyll content, chlorophyll fluorescence, gas exchange) or water use (porometer, pressure bomb). In addition to these general purpose instruments, researchers often design or adapt other instruments tailored to the specific stress response they are studying.

Photosynthesis systems use infrared gas analyzers (IRGAS) for measuring photosynthesis. CO2 concentration changes in leaf chambers are measured to provide carbon assimilation values for leaves or whole plants. Research has shown that the rate of photosynthesis is directly related to the amount of carbon assimilated by the plant. Measuring CO2 in the air, before it enters the leaf chamber, and comparing it to air measured for CO2 after it leaves the leaf chamber, provides this value using proven equations. These systems also use IRGAs, or solid state humidity sensors, for measuring H2O changes in leaf chambers. This is done to measure leaf transpiration, and to correct CO2 measurements. The light absorption spectrum for CO2 en H2O overlap somewhat, therefore, a correction is necessary for reliable CO2 measuring results. [1] The critical measurement for most plant stress measurements is designated by "A" or carbon assimilation rate. When a plant is under stress, less carbon is assimilated. [2] CO2 IRGAs are capable of measuring to approximately +/- 1 μmol or 1ppm of CO2.

Because these systems are effective in measuring carbon assimilation and transpiration at low rates, as found in stressed plants, [3] they are often used as the standard to compare to other types of instruments. [4] Photosynthesis instruments come in field portable and laboratory versions. They are also designed to measure ambient environmental conditions, and some systems offer variable microclimate control of the measuring chamber. Microclimate control systems allow adjustment of the measuring chamber temperature, CO2 level, light level, and humidity level for more detailed investigation.

The combination of these systems with fluorometers, can be especially effective for some types of stress, and can be diagnostic, e.g. in the study of cold stress and drought stress. [5] [2] [6]

Chlorophyll fluorescence emitted from plant leaves gives an insight into the health of the photosynthetic systems within the leaf. Chlorophyll fluorometers are designed to measure variable fluorescence of photosystem II. This variable fluorescence can be used to measure the level of plant stress. The most commonly used protocols include those aimed at measuring the photosynthetic efficiency of photosystem II, both in the light (ΔF/Fm') and in a dark-adapted state (Fv/Fm). Chlorophyll fluorometers are, for the most part, less expensive tools than photosynthesis systems, they also have a faster measurement time and tend to be more portable. For these reasons they have become one of the most important tools for field measurements of plant stress.

Fv/Fm tests whether or not plant stress affects photosystem II in a dark adapted state. Fv/Fm is the most used chlorophyll fluorescence measuring parameter in the world. "The majority of fluorescence measurements are now made using modulated fluorometers with the leaf poised in a known state." (Neil Baker 2004) [5] [7]

Light that is absorbed by a leaf follows three competitive pathways. It may be used in photochemistry to produce ATP and NADPH used in photosynthesis, it can be re-emitted as fluorescence, or dissipated as heat. [2] The Fv/Fm test is designed to allow the maximum amount of the light energy to take the fluorescence pathway. It compares the dark-adapted leaf pre-photosynthetic fluorescent state, called minimum fluorescence, or Fo, to maximum fluorescence called Fm. In maximum fluorescence, the maximum number of reaction centers have been reduced or closed by a saturating light source. In general, the greater the plant stress, the fewer open reaction centers available, and the Fv/Fm ratio is lowered. Fv/Fm is a measuring protocol that works for many types of plant stress. [8] [9] [2]

In Fv/Fm measurements, after dark adaption, minimum fluorescence is measured, using a modulated light source. This is a measurement of antennae fluorescence using a modulated light intensity that is too low to drive photosynthesis. Next, an intense light flash, or saturation pulse, of a limited duration, is used, to expose the sample, and close all available reaction centers. With all available reaction centers closed, or chemically reduced, maximum fluorescence is measured. The difference between maximum fluorescence and minimum fluorescence is Fv, or variable fluorescence. Fv/Fm is a normalize ratio created by dividing variable fluorescence by maximum fluorescence. It is a measurement ratio that represents the maximum potential quantum efficiency of Photosystem II if all capable reaction centers were open. An Fv/Fm value in the range of 0.79 to 0.84 is the approximate optimal value for many plant species, with lowered values indicating plant stress (Maxwell K., Johnson G. N. 2000), (Kitajima and Butler, 1975). [10] Fv/Fm is a fast test that usually takes a few seconds. It was developed in and around 1975 by Kitajima and Butler. Dark adaptation times vary from about fifteen minutes to overnight. Some researchers will only use pre-dawn values. [8] [2]

Y(II) is a measuring protocol that was developed by Bernard Genty with the first publications in 1989 and 1990. [11] [12] It is a light adapted test that allows one to measure plant stress while the plant is undergoing the photosynthetic process at steady-state photosynthesis lighting conditions. Like FvFm, Y(II) represents a measurement ratio of plant efficiency, but in this case, it is an indication of the amount of energy used in photochemistry by photosystem II under steady-state photosynthetic lighting conditions. For most types of plant stress, Y(II) correlates to plant carbon assimilation in a linear fashion in C4 plante. In C3 plants, most types of plant stress correlate to carbon assimilation in a curve-linear fashion. According to Maxwell and Johnson, it takes between fifteen and twenty minutes for a plant to reach steady-state photosynthesis at a specific light level. In the field, plants in full sunlight, and not under canopy, or partly cloudy conditions, are considered to be at steady state. In this test, light irradiation levels and leaf temperature must be controlled or measured, because while the Y(II) parameter levels vary with most types of plant stress, it also varies with light level and temperature. [11] [12] Y(II) values will be higher at lower light levels than at higher light levels. Y(II) has the advantage that it is more sensitive to a larger number of plant stress types than Fv/Fm. [ aanhaling nodig ]

ETR, or electron transport rate, is also a light-adapted parameter that is directly related to Y(II) by the equation, ETR = Y(II) × PAR × 0.84 × 0.5. By multiplying Y(II) by the irradiation light level in the PAR range (400 nm to 700 nm) in μmols, multiplied by the average ratio of light absorbed by the leaf 0.84, and multiplied by the average ratio of PSII reaction centers to PSI reaction centers, 0.50, [4] [13] [14] relative ETR measurement is achieved. [15]

Relative ETR values are valuable for stress measurements when comparing one plant to another, as long as the plants to be compared have similar light absorption characteristics. [2] Leaf absorption characteristics can vary by water content, age, and other factors. [2] If absorption differences are a concern, absorption can be measured with the use of an integrating sphere. [9] For more accurate ETR values, the leaf absorption value and the ratio of PSII reaction centers to PSI reaction centers can be included in the equation. If different leaf absorption ratios are an issue, or they are an unwanted variable, then using Y(II) instead of ETR, may be the best choice. Four electrons must be transported for every CO2 molecule assimilated, or O2 molecule evolved, differences from gas exchange measurements, especially in C3 plants, can occur under conditions that promote photorespiration, cyclic electron transport, and nitrate reduction. [5] [2] [16] For more detailed information concerning the relationship between fluorescence and gas exchange measurements again refer to Opti-Sciences application note #0509 on Yield measurements.

Quenching measurements have been traditionally used for light stress, and heat stress measurements. [17] [ aanhaling nodig ] In addition, they have been used to study plant photoprotective mechanisms, state transitions, plant photoinhibition, and the distribution of light energy in plants. [18] [19] While they can be used for many types of plant stress measurement, the time required, and the additional expense required for this capability, limit their use. These tests commonly require overnight dark adaptation, and fifteen to twenty minutes in lighted conditions to reach steady state photosynthesis before measurement. [19]

Puddle model and lake model quenching parameters Edit

"Understanding of the organization of plant antennae, or plant light collection structures, and reaction centers, where the photosynthetic light reaction actually takes place, has changed over the years. It is now understood that a single antennae does not link only to a single reaction center, as was previously described in the puddle model. Current evidence indicates that reaction centers are connected with shared antennae in terrestrial plants." As a result, the parameters used to provide reliable measurements have changed to represent the newer understanding of this relationship. The model that represents the newer understanding of the antennae - reaction center relationship is called the lake model. [19]

Lake model parameters were provided by Dave Kramer in 2004. [20] Since then, Luke Hendrickson has provided simplified lake model parameters that allow the resurrection of the parameter NPQ, from the puddle model, back into the lake model. [21] [22] This is valuable because there have been so many scientific papers that have used NPQ for plant stress measurement, as compared to papers that have used lake model parameters. [19]

For an in-depth overview of quenching, refer to the OSI quenching application note. It discusses all of the parameters used in lake models by Kramer, Hendrickson, and Klughammer. [21] [22] It also reviews puddle model parameters, and quenching relaxation measurements. [19] Further, a deep review of all existing parameters is provided in Lazar (2015, J. Plant Physiol. 175, 131-147)

OJIP or OJIDP is a dark adapted chlorophyll fluorescence technique that is used for plant stress measurement. It has been found that by using a high time resolution scale, the rise to maximum fluorescence from minimum fluorescence has intermediate peaks and dips, designated by the OJID and P nomenclature. Over the years, there have been multiple theories of what the rise, time scale, peaks and dips mean. In addition, there is more than one school as to how this information should be used for plant stress testing (Strasser 2004), (Vredenburg 2004, 2009, 2011). [2] [23] [24] [25] [26] Like Fv/Fm, and the other protocols, the research shows that OJIP works better for some types of plant stress than it does for others. [ aanhaling nodig ]

When choosing the correct protocol, and measuring parameter, for a specific type of plant stress, it is important to understand the limitations of the instrument, and the protocol used. For example, it was found that when measuring Oak leaves, a photosynthesis system could detect heat stress at 30 °C and above, Y(II) could detect heat stress at 35 °C and above, NPQ could detect heat stress at 35 °C and above, and Fv/Fm could only detect heat stress at 45 °C and above. (Haldiman P, & Feller U. 2004) [27] OJIP was found to detect heat stress at 44 °C and above on samples tested. (Strasser 2004) [23]

The relationship between carbon assimilation measurements made by photosynthesis systems of the dark Calvin cycle, and measurements of variable fluorescence of photosystem II (PSII), made by chlorophyll fluorometers of the light reaction, are not always straightforward. [28] For this reason, choosing the correct chlorophyll fluorescence protocol can also be different for C3 en C4 plante. It has been found, for example, that Y(II) and ETR are good tests for drought stress in C4 plants, [29] [30] but a special assay is required to measure drought stress in most C3 plants at usable levels. [31] [32] In C3 plants, photorespiration, and the Mehler reaction, are thought to be a principal cause. (Flexas 2000) [16]

These are instruments that use light transmission through a leaf, at two wavelengths, to determine the greenness and thickness of leaves. Transmission in the infrared range provides a measurement related to leaf thickness, and a wavelength in the red light range is used to determine greenness. The ratio of the transmission of the two wavelengths provides a chlorophyll content index that is referred to as CCI or alternatively as a SPAD index. [33] [34] CCI is a linear scale, and SPAD is a logarithmic scale. [33] [34] These instruments and scales have been shown to correlate to chlorophyll chemical tests for chlorophyll content except at very high levels. [33] [34]

Chlorophyll content meters are commonly used for nutrient plant stress measurement, that includes nitrogen stress, and sulfur stress. Because research has shown, that if used correctly, chlorophyll content meters are reliable for nitrogen management work, these meters are often the instruments of choice for crop fertilizer management because they are relatively inexpensive. [35] [36] Research has demonstrated that by comparing well fertilized plants to test plants, the ratio of the chlorophyll content index of test plants, divided by the chlorophyll content index of well fertilized plants, will provide a ratio that is an indication of when fertilization should occur, and how much should be used. It is common to use a well fertilized stand of crops in a specific field and sometimes in different areas of the same field, as the fertilization reference, due to differences from field to field and within a field. The research done to date uses either [ opheldering nodig ] ten and thirty measurements on test and well fertilized crops, to provide average values. Studies have been done for corn and wheat. One paper suggests that when the ratio drops below 95%, it is time to fertigate. The amounts of fertilizer are also recommended. [35] [36]

Crop consultants also use these tools for fertilizer recommendations. However, because strict scientific protocols are more time consuming and more expensive, consultants sometimes use well-fertilized plants located in low-lying areas as the standard well-fertilized plants. They typically also use fewer measurements. The evidence for this approach involves anecdotal discussions with crop consultants. Chlorophyll content meters are sensitive to both nitrogen and sulfur stress at usable levels. Chlorophyll fluorometers require a special assay, involving a high actinic light level in combination with nitrogen stress, to measure nitrogen stress at usable levels. [37] In addition, chlorophyll fluorometers will only detect sulfur stress at starvation levels. [9] [2] For best results, chlorophyll content measurements should be made when water deficits are not present. [ aanhaling nodig ] Photosynthesis systems will detect both nitrogen and sulfur stress. [ aanhaling nodig ]


Transport in Plants

Transportation is a process in which a substance either synthesized or absorbed in one part of the body reaches another. In living things, many substances such as food, gases, minerals salts, hormones, and waste products have to be transported from one part of the body to another. Plants require inorganic substances like nitrogen, phosphorous, magnesium, manganese, sodium, etc. Soil provides such substances. In this article, we shall study transport in plants by osmosis and diffusion.

Plants and animals have a system of transporting substances throughout their body. In plants, water is the medium of transport. In higher plants (vascular plants) xylem conducts the water whereas the phloem conducts the food. All parts of the body are connected to these tissues.

To demonstrate Transportation in Plants:

Take a young herbaceous plant (e.g. sunflower) with roots and leaves. Wash the soil off the roots. Place it in a jar of water containing stains like eosin or red ink. After two days, cut sections of the stem and the root and observe it under a microscope. We see rings of red colour. This experiment shows transport of materials in plants.

In plants, water transport minerals salts through special tubes called xylem. Plants have root hairs on their primary and secondary roots. Plants absorb water and minerals salt from the soil with the help of root hairs. They absorb water by the process of osmosis. Osmosis is the movement of water molecules (solvent) from a lower concentration solution to a higher concentration solution through a semi permeable membrane. The concentration of the water molecules is lower in the root hair than in the soil. So the water moves into the root hairs through osmosis. Thus the cell of root hairs become turgid and exert pressure on the adjacent cells. This pressure is called root pressure.

The water and mineral diffuse from one layer to the next layer of cells and eventually reaches the xylem tubes in the centre of the root. Under the effect of root pressure, water and minerals reach xylem and continuously push forward. This root pressure is sufficient to lift water up in shrubs, small plants and small trees.

Demonstration of Osmosis ( Abbe Nollet Experiment):

A thistle funnel with a narrow long stem and wide mouth was taken. A semipermeable membrane is tied tightly around the wide mouth of the funnel. Now the stem of the funnel is filled with a sugar solution to a certain level. Then the thistle funnel is dipped in a beaker containing water with the help of an iron stand such that the broad mouth remains immersed in the water. The apparatus are left undisturbed for some time.

After some time it is observed that there is an increase in the level of sugar solution in the thistle funnel. This shows that there is at the flow of water (solvent molecules) into the solution through the semipermeable membrane. To stop this flow of solvent molecules into solution, we have to apply excess sufficient pressure from stem side of the thistle funnel on the solution. This excess pressure is the osmotic pressure.

Demonstration of Osmosis (Experiment – 2):

An egg has a shell made up of calcium carbonate. Below shell, there is a layer of semipermeable membrane. The calcium carbonate can be dissolved in acid like HCl. Place an egg in a beaker containing acid. Make sure that the egg does not float in acid. Allow it to remain in the acid until the shell completely dissolves. Leave the egg with the dissolved shell in water overnight. Remove the egg the next day and see its size. We will find that the egg will be swollen considerably.

If you carefully pierce the membrane of the egg with a needle, a jet of water will shoot into the air. It shows that the water from acid solution gets into the egg through a semipermeable membrane.

Mineral and salts are absorbed by a process called diffusion. Diffusion is the movement of solute molecules of a solid, a gas or liquid from a region of high concentration region to that of low concentration region.

Difference Between Osmosis and Diffusion:

In osmosis, solvent moves from lower concentration region to higher concentration region. In diffusion, solute particles move from higher concentration region to lower concentration region.

Ascent of Sap:

After reaching xylem tubes water is conducted from the root to the stem and then to the leaves. The continuous columns of water in the xylem tubes do not break due to strong cohesive force between the water molecules. The upward movement of cell sap containing water and minerals salts in a plant is called ascent of sap.

Transpiration:

Water evaporates from the leaves through the opening on them called stomata There are two guard cells at the opening of stomata. These cells control opening and closing of stomata.

The loss of water from the aerial parts of a plant is called transpiration. When transpiration takes place, the leaves lose water and become less turgid, or less swollen. They then absorb water from the xylem tubes in the veins. They pull water from the stem, which in turn pulls water from the roots. This process is called transpirational pull. Transpirational pull works due to the formation of a continuous column of water in the xylem.

The significance of Transpiration:

Minerals salts absorbed by the roots are taken along through this column of water during the ascent of sap and the essential elements needed by the plant reach the leaves and every part of the plant.

When the temperature is high or the atmosphere is dry or when there is an air current, the rate of transpiration becomes high. If there is no sufficient water in the soil, the leaves become less turgid. Transpiration pull is very important for big plants. By this method, water and minerals reach the different part of the body of the big plant. In the night the rate of transpiration is low, hence in night water and mineral transportation takes place by root pressure.

Translocation:

Glucose (sugar) is made in the leaves during the day in presence of sunlight by the process called photosynthesis. This food is required to be transported to other parts of the plant. This transportation of food material takes place through a special tube like tissues phloem in upward and downward direction.

During day glucose is formed and it is formed is converted into starch and stored in the leaves. As the day advances and manufacture of glucose (food) take place, the starch grains become more and more abundant. About the middle of the afternoon, the starch content reaches its maximum.

At night, due to the absence of sunlight plants do not make food. Then the stored starch in the cells of the leaves gets converted back into glucose, which is soluble in water. This sugar solution and other substances move from the leaves to other parts of the plant all through the night. This movement of food substance from leaves to the different parts of the body in the phloem is called translocation.

Before the daybreak, the food-making cells in the leaves are cleared of stored food and they become ready for food manufacture.


How donkeys digging wells help life thrive in the desert

A study has found wells dug by horses and donkeys increased water availability for many native desert species, and decreased the distances between important water sources during dry periods

For thousands of years, horses and donkeys have been some of our species' most important partners.

A new study published Thursday shows they're also friends to desert animals and plants, by digging deep wells that provide a vital source of water, especially at the height of summer.

Biologist Erick Lundgren, lead author of the paper in Wetenskap, told AFP he first began noticing the phenomenon while working in western Arizona as a field technician studying river systems.

"People just didn't think it was worthy of scientific attention," said the scientist, who is now at the University of Technology Sydney.

Lundgren had read about African elephants digging wells that were the only source of water for other animals during the dry season, and wanted to know if horses and donkeys might play a similar role in America.

The idea was intriguing, "especially since donkeys and horses are considered agents of biodiversity harm" as they are not native species in the region, he said.

Over the course of three summers, he and his team surveyed sites in the Sonoran Desert that stretches across Arizona and California.

Video sequence of well digging behavior by wild donkeys, and their utilization by other species. Credit: E. Lundgren

They documented the relative contribution of wells dug by horses and donkeys compared to the surface water that was available to animals from desert streams, some of which are intermittent while others are permanent.

The team also set up camera traps to learn how other animals were utilizing the wells.

They found that wells dug by the "equids" to depths of up to six feet (two meters) increased water availability for many native desert species, and decreased the distances between important water sources during dry periods.

The wells were especially important during the hottest and driest parts of summer, when they provided the only available water source at some sites.

Species that flocked to the equid-engineered wells and were caught on camera included mule deers, bobcats, Woodhouse's scrub jay and javelinas

Lundgren said the horses and donkeys acted as "buffers" against the extreme variability of desert streams from year to year.

"The donkey wells kept water in the system. And these features were used by pretty much every species you could picture, including some surprising ones like black bears, that we didn't expect to see in the desert," he said.

Other species that flocked to the wells and were caught on camera included mule deer, bobcats, Woodhouse's scrub jay and javelinas.

The team even spotted some river tree species sprouting from abandoned wells, indicating they also serve a role as plant nurseries.

Horses and donkeys were introduced to the Americas by Europeans to assist with the colonization of the continent, but their use declined with the advent of the internal combustion engine.

This undated image courtesy of Biologist Erick Lundgren shows a bobcat entering an equid well

Since then, they have been studied as "invasion biology," said Lundgren, which does not consider them to be a part of the local wildlife.

But this thinking is too tunnel-visioned and has prevented scientists from having a more nuanced understanding of their effects on their ecosystems, he argued.

Lundgren and his colleagues said in their paper that the wells will be increasingly important as human activity and climate change reduces the number of perennial streams in these regions.

Another element to the story is that the behavior of modern horses and donkeys might have an "ancient precedent," said Lundgren.

Horses, elephants and other large animals that roamed North America until a mysterious extinction event around 12,000 years ago could have once fulfilled a similar role.


Solute Transport in Plants [Terug na bo]

The phloem contains a very concentrated solution of dissolved solutes, mainly sucrose, but also other sugars, amino acids, and other metabolites. This solution is called the sap, and the transport of solutes in the phloem is called translokasie.

Unlike the water in the xylem, the contents of the phloem can move both up or down a plant stem, often simultaneously. It helps to identify where the sugar is being transported from (the source), and where to (the sink).

Surprisingly, the exact mechanism of sugar transport in the phloem is not known, but it is certainly far too fast to be simple diffusion. The main mechanism is thought to be the mass flow of fluid up the xylem and down the phloem, carrying dissolved solutes with it. Plants don t have hearts, so the mass flow is driven by a combination of active transport (energy from ATP) and evaporation (energy from the sun). This is called the mass flow theory, and it works like this:

This mass-flow certainly occurs, and it explains the fast speed of solute translocation. However there must be additional processes, since mass flow does not explain how different solutes can move at different speeds or even in different directions in the phloem. One significant process is cytoplasmic streaming: the active transport of molecules and small organelles around cells on the cytoskeleton.

Translocation Experiments [Terug na bo]

1. Puncture Experiments

If the phloem is punctured with a hollow tube then the sap oozes out, showing that there is hoë druk (compression) inside the phloem (this is how maple syrup is tapped). If the xylem is punctured then air is sucked in, showing that there is lae druk (tension) inside the xylem. This illustrates the main difference between transport in xylem and phloem: Water is pulled up in the xylem, sap is pushed down in the phloem.

2. Ringing Experiments

Since the phloem vessels are outside the xylem vessels, they can be selectively removed by cutting a ring in a stem just deep enough to cut the phloem but not the xylem. After a week there is a swelling above the ring, reduced growth below the ring and the leaves are unaffected. This was early evidence that sugars were transported downwards in the phloem.

3. Radioactive Tracer Experiments

Radioactive isotopes can be used trace precisely where different compounds are being transported from and to, as well as measuring the rate of transport. The radioactivity can be traced using photographic film (an autoradiograph) or a GM tube. This techniques can be used to trace sugars, ions or even water.

In a typical experiment a plant is grown in the lab and one leaf is exposed for a short time to carbon dioxide containing the radioactive isotope 14 C. This 14 CO2 will be taken up by photosynthesis and the 14 C incorporated into glucose and then sucrose. The plant is then frozen in liquid nitrogen to kill and fix it quickly, and placed onto photographic film in the dark. The resulting autoradiograph shows the location of compounds containing 14 C.

This experiment shows that organic compounds (presumably sugars) are transported downwards from the leaf to the roots. More sophisticated experiments using fluorescently labelled compounds can locate the compound specifically to the phloem cells.

4. Aphid Stylet Experiments

Aphids, such as greenfly, have specialised mouthparts called stylets, which they use to penetrate phloem tubes and sup of the sugary sap therein. If the aphids are anaesthetised with carbon dioxide and cut off, the stylet remains in the phloem so pure phloem sap can be collected through the stylet for analysis. This surprising technique is more accurate than a human with a syringe and the aphid s enzymes ensure that the stylet doesn t get blocked.