Investigating the photosynthetic and hydraulic trade-off during drought recovery in eucalypts

Saunders, Alta (2022-04)

Thesis (PhD)--Stellenbosch University, 2022.

Thesis

ENGLISH ABSTRACT: Stomatal regulation plays a vital role in maintaining the water status of a plant by minimising water loss, however, decreases in stomatal conductance can lead to reductions in carbon uptake. The stomata balance a trade-off between water loss and carbon gain. The hydraulic system and stomatal conductance are closely linked but play opposing roles within a plant. The hydraulic system ensures that there is a sufficient water supply to leaves, while stomatal conductance regulates the loss of water from the leaves. During periods of drought, reductions in hydraulic conductance due to embolism formation can be seen, however stomatal regulation can help reduce embolism formation or prevent runaway cavitation during drought. Understanding how plant hydraulics and stomatal regulation influence production rates is becoming more important to model stomatal responses in a changing climate, especially for Eucalyptus species which is often grown in drought prone environments. Plants use a wide range of strategies to reduce or mitigate the negative impact of embolism formation, with this study focusing on the strategies utilised by commercially significant Eucalyptus hybrids. Two Eucalyptus hybrids, E. grandis X camaldulensis (GC) and E. urophylla X grandis (UG), where subjected to a drought-recovery treatment where they were periodically droughted. During the experimental period the stomatal responses, together with transpiration rates, photosynthetic capacity and biomass allocation was measured. Hydraulic measurements together with CT-scan imaging was also utilised to determine whether these plants can recover lost hydraulic pathways post-drought, and what the underlying mechanism for this might be. During the study two distinct hydraulic strategies were observed. GC was more resistant to embolism formation compared to UG, however GC showed lower levels of hydraulic recovery after rewatering. The drought responses could also be split into a resilient v. a resistant response, with the more resilient hybrid, UG, maintaining stomatal conductance throughout drought periods running the risk of hydraulic failure, however with the ability to recover lost hydraulic pathways through refilling post- drought. This is in comparison with the resistant strategy seen in GC, where water loss during drought was minimised, however this also reduced carbon uptake and production. From this study the stomatal responses could also be accurately modelled using a gain-risk model that assumes stomata optimise the trade-off between water loss and carbon gain. It was however clear, that the strategy used by plants need to be considered when using a gain-risk model, with the gain-risk model performing better for the Eucalyptus hybrid that utilised a resistant strategy, since plants that uses a resilient strategy will maintain stomatal conductance during drought, regardless of the hydraulic risk. The use of alternative models was also investigated during this study, with Machine Learning models being able to accurately predict stomatal responses on a global scale. Major increases in tree mortality are predicted due to changes in climate. To properly predict these changes, accurate models of plant responses to water limitations and other environmental changes are therefore becoming increasingly important for effective forest management. Understanding how hydraulic traits and stomatal regulation covary, can help model drought-induced tree mortality in a changing climate.

AFRIKAANSE OPSOMMING: Die regulasie van gaswisseling deur die huidmondjies is belangrik aangesien dit nie net die verlies van water deur transpirasie reguleer nie, maar ook die opname van CO2 wat nodig is vir fotosintese en produksie. Die hidroulise sisteem van ‘n plant en huidmondjies is gekoppel, maar speel teenoorgestelde rolle. Die hidrouliese sisteem beheer hoeveel water beskikbaar is vir die plant, terwyl die huidmondjies water verlies deur transpirasie beheer. Tydens droogtes, word die hidrouliese geleiding van water deur die plant bellemer deur die vorming van embolisme/lugborrels. Die huidmondjies kan wel die verspreiding van hierdie lugborrels verminder of verhoed deur water verlies te beheer. Dit is dus belangrik om te verstaan wat die interaksie tussen die huidmondjies en die hidrouliese sisteem van ‘n plant is om akurate modelle van gaswisseling te bou, veral vir Eucalyptus spesies, aangesien hulle gereeld in droogte geteisterde areas geplant word. Plante geskik van ‘n verskeidenheid van strategieë om die negatiewe impak van embolisme te vermider. Die studie het dus gefokus op die strategieë wat Eucalyptus kruise gebruik. Die gaswisseling en groei was gemeet van twee Eucalyptus kruise, E. grands X camaldulensis (GC) en E. urophulla X grandis (UG), tydens ‘n droogte eksperiment waar die plante vir sekere periodes nie water ontvang het nie. Meetings van die hidroulise geleiding en hidroulise einskappe was ook gemeet deur gebruik te maak van beide traditionele metodes, sowel as rekenaartomografie-skanderings, om te bepaal of die plante van die embolisme/lugborrels in die hidrouliese sisteem kan vul na ‘n droogte. Tydens die studie was dit duidelik dat die twee Eucalyptus kruise twee verskillend hidrouliese strategieë benut. GC was meer bestand teen droogte en die vorming van embolisme in vergelyking met UG, maar GC kon nie die embolisme in die hidroulises sisteem weer vul met water na die droogte nie. Die strategieë wat benut was kon ook beskryf word as ‘n weerstandbiedige strategie teenoor ‘n veerkragtige strategie. UG het ‘n veerkragtige strategie benut, waar gaswisseling gehandhaaf was tydens droogte, maar het ‘n hoër risiko gehad dat die hidrouliese sisteem misluk, terwyl GC ‘n weerstandbiedige strategie gehandhaaf het, waar gaswissinling and water verlies deur transpirasie verminder was, maar so ook CO2 opname. Tydens die study kon ons ook die gaswisseling voorspel deur ‘n wins-risiko model te gebruik wat aaneem dat die huidmondjies ‘n balans vind tusen water verlies en CO2 opname. Dit was egter duidelik dat die hidrouliese strategieeë in ag geneem moet word, aangesien die model meer akuraat was vir plante wat ‘n weerstandbiedige strategie benut. Dit is aangesien ‘n plant wat ‘n veerkragtige strategie benut gaswisseling sal handhaaf tydens droogte, ongeag die hidrouliese risiko. Ondersoek was ook ingestel om alternatiewe modelle te gebruik om gaswisseling te modelleer en dit was gevind dat masjienleermodelle ook gebruik kan word. Grootskaalse sterftes van bome word verwag asgevolg van klimaats verandering. Om die impak van dit te voorspel, is dit dus belangrik om akkurate modelle van gaswisseling in reaksie tot klimaats verandering te ontwikkel. As ons verstaan hoe die hidrouliese en gaswisseling sisteme van plante reageer, kan ons beter modelle bou om boom streftes te voorspel.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/124576
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