description
- Improving transpiration efficiency (TE) offers a mechanistic pathway to enhance yield under drought by modulating the balance between carbon assimilation and water loss. This study quantified genotypic variation in TE and dissected the physiological processes underlying this variation across a diverse panel of wheat genotypes. Following an initial experiment with six cultivars, 105 genetically diverse lines were evaluated under well-watered conditions across naturally fluctuating vapour pressure deficit (VPD). Transpiration was measured at 10-minute intervals using a high-throughput lysimeter platform and normalised daily at low VPD to minimise confounding effects arising from genotypic differences in canopy size. Variation in TE was strongly associated with reduced normalised transpiration rate at high VPD (TR). No relationship was detected with maximum photosynthetic capacity, indicating that TE differences were driven primarily by regulation of water loss rather than carbon gain. High-TE genotypes achieved either greater biomass for a given water use or equivalent biomass with reduced water use, consistent with conservative stomatal regulation under high evaporative demand. A complementary experiment conducted under low VPD revealed limited genotypic variation in intrinsic TE, suggesting that genetic control of TE is predominantly expressed under high atmospheric demand. The consistency of TE–TR relationships across experiments highlights TR as a robust and physiologically meaningful phenotyping target. Several high-TE genotypes outperformed modern cultivars, offering novel sources of allelic variation for transpiration regulation. Collectively, these results define a mechanistically grounded, scalable phenotyping framework to target VPD-responsive water-use traits and support breeding strategies aimed at improving drought resilience and water productivity.