Hydraulic conductance differences among sorghum genotypes to explain variation in restricted transpiration rates

Sorghum (Sorghum bicolor L.) is an important crop for production in dryland regions of the globe. Traits identified in many sorghum lines that apparently make them adapted for dryland conditions are restricted transpiration rate both early in the soil drying cycle and under high atmospheric vapour pressure deficit. It was hypothesised that these responses could be a result of differences in hydraulic conductance of the plants: those with low hydraulic conductance would be more likely to express restricted transpiration rates. The location of the lower hydraulic conductance in the plant could also be important with a low conductance in the leaf xylem to stomata pathway possibly being more advantageous than in the root. In this study, the amount and location of the hydraulic conductance was measured in 20 sorghum genotypes. Those genotypes that expressed an early decrease in transpiration rate with soil drying had greater plant and leaf hydraulic conductance than those genotypes that had the later decreases in transpiration rate, which was in contrast with what was hypothesised. However, sorghum genotypes that segregated between two groups based on expression of a maximum transpiration trait also segregated based on their hydraulic conductance. Those genotypes that expressed the maximum transpiration trait had lower hydraulic conductance for the intact plant and in the leaves.