Leaf Photoassimilation and Partitioning in Stress‐Tolerant Sorghum

Abstract

Previous attempts to relate variation in physiological measurements of carbon and water exchange to prettowering and postflowering tolerance to water stress in Sorghum bicolor (L.) Moench have yielded inconsistent results. The objective of this research was to compare physiological indicators of sorghum photoassimilation and partitioning among a prefiowering‐tolerant line (Tx430), postflowering tolerant line (B35), and an F1 hybrid thereof. Leaf CO2 exchange and sucrose synthesis rates, carbohydrate concentrations, and 14C‐assimilate partitioning were quantified in field‐grown plants that were exposed to 14CO2 for 3 h under steady‐state labeling conditions. Leaf CO2 exchange and sucrose synthesis rates were slower, and concentrations and radio‐activity in blade starch were greater, in B35 than in Tx430 under well‐watered conditions. In addition, the postflowering‐tolerant cultivar B35 retained 70% more 14C‐assimilate in the labeled blade than did Tx430. The differences in leaf traits were associated with 33% smaller grain weights in B35 than Tx430 or the hybrid, but aerial biomass and upper stem carbohydrate concentrations at preboot and anthesis did not differ among the cultivars under well‐watered conditions. The consistency of differences between B35 and Tx430 across sampling stages suggested that metabolic and unloading processes in leaf blades, rather than limitations to assimilate unloading in growing and storage tissues, contributed to slower rates of CO2 exchange, sucrose synthesis, and 14C‐assimilate export in B35. Leaf traits and dry weights did not differ among cultivars when water stress was imposed on potted plants during preboot, anthesis, and grain filling. The lack of a relationship between leaf traits under well‐watered conditions and postflowering tolerance to water stress indicates tolerant sorghum phenotypes could be selected to avoid the relatively low rates of CO2 exchange, sucrose synthesis, and export observed for B35.