Gleason, S. M.; Stewart, J. J.; Polutchko, S. K.; Allen, B. S.

Limited transpiration (LT) traits aim to conserve early-season water to benefit late-season grain development. While theoretical and modeling efforts support LT efficacy, empirical tests directly measuring water loss from leaves and canopies are scarce. This study evaluates the performance of LT genotypes in achieving reduced early-season water use and improved late-season growth and yield in semi-arid Colorado. The research involved near-isogenic lines (NILs) derived from sorghum inbred lines, subjected to different irrigation treatments. Measurements included stomatal conductance, net CO2 assimilation, and photosystem II (PSII) efficiency. Results indicate that LT genotypes did not consistently exhibit lower early-season water use or higher late-season growth compared to non-LT genotypes. Early-season water use was positively correlated with above-ground biomass, challenging the assumption that early-season water conservation can be leveraged for late-season benefits. We question the efficacy of LT traits, highlighting the physiological link between water use and carbon gain, and the potential opportunity costs of reduced early-season growth. We suggests that breeding strategies should focus on enhancing deep soil water access and maximizing carbon gain rather than merely reducing transpiration or shifting water use in arid and semi-arid environments.