The physiological and genetic underpinnings that influence the xylem properties of Eucalyptus grandis, when subjected to different levels of water availability

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dc.contributor.advisorHills, Paul Normanen_ZA
dc.contributor.advisorDrew, David Michaelen_ZA
dc.contributor.authorKeret, Rafaelen_ZA
dc.contributor.otherStellenbosch University. Faculty of AgriSciences. Department of Genetics & Institute of Plant Biotechnology.en_ZA
dc.date.accessioned2024-02-29T13:07:24Z
dc.date.accessioned2024-04-26T20:33:54Z
dc.date.available2024-02-29T13:07:24Z
dc.date.available2024-04-26T20:33:54Z
dc.date.issued2024-03
dc.descriptionThesis (PhDAgric)--Stellenbosch University, 2024.en_ZA
dc.description.abstractENGLISH ABSTRACT: Prospects for climate change forecast increases in temperature, with unpredictable rainfall. These future environmental scenarios are not promising for commercial forestry, especially since the main culprit of tree dieback is runaway cavitation, caused by drought. Consequently, foresters have opted to propagate more adaptable, fast-growing tree species, of which the genus Eucalyptus offers some promising candidates and, as such, has become an extensively propagated hardwood, carrying global significance. Given the wide native range of Eucalyptus, particularly E. grandis, these trees often display diverse physiological responses to abiotic stress that typically manifest in phenotypic plasticity in the xylem to prevent hydraulic failure. However, the broad range of physiological responses in Eucalyptus, coupled with the complexities associated with xylogenesis, have caused great difficulty in identifying the core mechanisms that govern the commercially valuable wood properties. To uncover how the environmental inputs are translated into anatomical outputs in the xylem of E. grandis, highly controlled drought experiments were conducted, with careful consideration of the physiology, anatomy, and transcriptome. As an early adaptive strategy, isohydric stomatal responses were elicited in the leaves of water-stressed plants to mitigate water loss through transpiration and maintain a stable water potential. Gaseous exchange was preserved at lower levels under drought, which likely provided the photosynthates necessary for the development of hydraulically-safer xylem. A novel method for procuring quantitative wood anatomy data from transverse stem microsections was developed in QuPath (v0.4.4) to assess the adaptive xylem anatomy stimulated under drought. Predominantly, smaller vessels and fibers were produced at a higher density, which enhanced the hydraulic- safety through improved implosion resistance and functional redundancy. Although these features reduced the theoretical hydraulic conductance, the associated decrease in water flow to the leaves may have reduced the transpiration rate to further mitigate water loss. Likewise, smaller ray parenchyma cells were elicited, which manifested in an increased number of rays and ray-vessel contacts. This arrangement increased the ray network reach within the xylem and may have facilitated a more efficient distribution of solutes for embolism prevention and repair. To assess the underlying mechanisms that govern cellular plasticity in E. grandis during water deficit, a comparative transcriptomics approach was conducted on the mRNA transcripts of the xylem. Genes associated with responses to reactive oxygen species and hypoxia were shown to exist at the core of the stress response, triggering a molecular cascade of events that ultimately led to the droughted xylem phenotype. For instance, cell cycle processes including DNA replication, cellular expansion, and division, were severely downregulated in water-stressed plants, which reduced vegetative growth. Similarly, genes associated with cell wall remodelling and component biosynthesis were expressed to a considerably lower degree compared to the controls. These gene expression patterns strongly suggest putative candidates for controlling cell size in E. grandis. Lastly, the expression of genes associated with the metabolism and transport of lipid-surfactants and osmotica suggest the capacity of E. grandis for embolism prevention or repair. This study has thus identified numerous regulatory mechanisms that influence the wood structure and physiology, to improve survivability under drought.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Vooruitsigte vir klimaatsverandering voorspel stygings in temperatuur, met onvoorspelbare reënval. Hierdie toekomstige omgewingsituasies is nie belowend vir kommersiële bosbou nie, veral aangesien die hoof oorsaak van boomafsterwing weghol-holte formasie is, wat deur droogte veroorsaak word. Gevolglik het bosbouers gekies om meer aanpasbare, vinnig-groeiende boomspesies te vermeerder, waarvan die genus Eucalyptus 'n paar belowende kandidate bied en, as sodanig, 'n omvattende vermeerderde hardehout geword het, wat wêreldwye betekenis dra. Gegewe die wye inheemse verskeidenheid van Eucalyptus, veral E. grandis, vertoon hierdie bome dikwels diverse fisiologiese reaksies op abiotiese stres wat tipies manifesteer in fenotipiese plastisiteit in die xileem om hidrouliese mislukking te voorkom. Die wye reeks fisiologiese reaksies in Eucalyptus, tesame met die kompleksiteite wat met xilogenese geassosieer word, het egter groot probleme veroorsaak om die kernmeganismes te identifiseer wat die kommersiële waardevolle houteienskappe beheer. Om te ontbloot hoe die omgewingsinsette in anatomiese uitsette in die xileem van E. grandis vertaal word, is hoogs beheerde droogte-eksperimente uitgevoer, met noukeurige oorweging van die fisiologie, anatomie en transkriptoom. As 'n vroeë aanpasbare strategie, is isohidriese huidmondjie- reaksies in die blare van water-gestresde plante ontlok om waterverlies deur transpirasie te versag en 'n stabiele waterpotensiaal te handhaaf. Gaswisseling is op laer vlakke onder droogte waargeneem, wat waarskynlik die fotosintate verskaf het wat nodig was vir die ontwikkeling van hidroulies-veiliger xileem. 'n Nuwe metode vir die verkryging van kwantitatiewe houtanatomiedata vanaf dwarsstam-mikroseksies is in QuPath (v0.4.4) ontwikkel om die adaptiewe xileem-anatomie wat onder droogte gestimuleer is, te assesseer. Oorwegend is kleiner vate en vesels teen 'n hoër digtheid vervaardig, wat die hidrouliese veiligheid verbeter het deur verbeterde inploffingsweerstand en funksionele oortolligheid. Alhoewel hierdie kenmerke die teoretiese hidrouliese geleiding verminder het, kan die gepaardgaande afname in watervloei na die blare die transpirasie-tempo verminder om waterverlies verder te versag. Net so is kleiner straal-parenchiemselle ontlok, wat gemanifesteer het in 'n verhoogde aantal strale en straalvatkontakte. Hierdie rangskikking het die straalnetwerkbereik binne die xileem vergroot en het moontlik 'n meer doeltreffende verspreiding van opgeloste stowwe vir embolisme-voorkoming en -herstel moontlik gemaak. Om die onderliggende meganismes wat sellulêre plastisiteit in E. grandis beheer tydens watertekort te bepaal, is 'n vergelykende transkriptomiese benadering op die mRNA-transkripsies van die xileem uitgevoer. Gene wat geassosieer word met reaksies op reaktiewe suurstofspesies en hipoksie is getoon as die kern van die stresreaksie, wat 'n molekulêre kaskade van gebeure veroorsaak het wat uiteindelik gelei het tot die droogte xileem-fenotipe. Selsiklusprosesse, insluitend DNS-replikasie, sellulêre uitbreiding en -deling, is byvoorbeeld ernstig afgereguleer in water-gestresde plante, wat vegetatiewe groei verminder het. Net so is gene geassosieer met selwandhermodellering en komponentbiosintese, tot 'n aansienlik laer mate uitgedruk in vergelyking met die kontroles. Hierdie geenuitdrukkingspatrone stel sterk vermoedelike kandidate voor vir die beheer van selgrootte in E. grandis. Laastens dui die uitdrukking van gene wat verband hou met die metabolisme en vervoer van lipied-oppervlakaktiewe middels en osmotika die kapasiteit van E. grandis vir embolisme- voorkoming of -herstel. Hierdie studie het dus talle reguleringsmeganismes geïdentifiseer wat die houtstruktuur en fisiologie beïnvloed, om oorlewingsvermoë onder droogte te verbeter.af_ZA
dc.description.versionDoctorateen_ZA
dc.format.extent185 pages : illustrationsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/130519
dc.language.isoen_ZAen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshEucalyptus grandis -- Breedingen_ZA
dc.subject.lcshEucalyptus grandis -- Effect of drought onen_ZA
dc.subject.lcshXylemen_ZA
dc.subject.lcshPhenotypic plasticityen_ZA
dc.subject.lcshGenotype-environment interactionen_ZA
dc.subject.lcshWater-deficit irrigationen_ZA
dc.subject.lcshCavitation resistanceen_ZA
dc.subject.lcshDroughtsen_ZA
dc.titleThe physiological and genetic underpinnings that influence the xylem properties of Eucalyptus grandis, when subjected to different levels of water availabilityen_ZA
dc.typeThesisen_ZA
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