Role of leaf photosynthesis and stomatal conductance in determining the genotypic variation in whole-plant Transpiration Efficiency (TE) in Sorghum

Abstract

Traits such as transpiration efficiency (TE) that are influenced by plant water use can be used to characterise the adaptability of crops to specific growth environments. TE is defined as the amount of biomass produced per unit of water used, and can ensure continued crop production in droughtprone regions. Where TE is associated with reduced use of soil water during the vegetative growth phase, water availability during grain filling may be greater, which can delay the onset of drought stress and increase grain yield under water-limited conditions. This may become even more pertinent with predicted increases in severity and frequency of droughts with climate change. The aims of this study were to firstly dissect TE into its leaf-level physiological components to better understand the effects of genetic variation in these components on TE in sorghum. Secondly, to examine whether TE responses observed under well-watered conditions were preserved under drought, and whether transpiration response was an adaptive response to drought. Twenty-seven genotypes were screened for TE under well-watered conditions using a fully automated lysimetry platform to obtain accurate plant water use data. To determine whether variation in TE among these genotypes was associated with differences in maximum photosynthesis (Amax) or leaf conductance (g) we measured the net carbon assimilation rate of the second last fully expanded leaf at high light intensity, using an infrared gas analyser, and leaf water flux was measured using a porometer, as a proxy for conductance. Genotypic variation in TE among the sorghum germplasm used was mainly associated with differences in the response of transpiration rates to vapour pressure deficit (VPD).Genotypes with low transpiration rates per unit green leaf area (T/GLA) tended to have high TE.Variation in Amax explained some of the differences in TE that could not be explained by T/GLA and may have been a result of mechanisms associated with differences in biochemical pathways that affect the efficiency of conversion of CO2 into photosynthate. While drought tended to increase TE, genotypic variation in TE was largely conserved. However, the response of transpiration rates to drought stress differed across genotypes, with some genotypes showing reduced T/GLA under drought when VPD was high, whereas others did not. These contrasting responses were associated with differences in stomatal responses to drought stress, such that some genotypes were better able to conserve water under drought stress than others. This adaptive response was not related to TE per se and may have important implications for adaptation to drought stress. Hence, the phenotyping of sorghum lines using the associated physiological traits underpinning TE differences is beneficial in identifying traits that may support growth in certain environments and can optimise grain yield production under water-limited conditions. III Declaration by AuthorThis thesis is composed of my original work, and contains no material previously ...