LEAF PHOTOASSIMILATION AND PARTITIONING IN STRESS-TOLERANT SORGHUM

Previous attempts to relate variation in physiological measurements of carbon and water exchange to preflowering and postflowering tolerance to water stress in Sorghum bicolor (L.) Moench have yielded inconsist ent results. The objective of this research was to compare physiologic al indicators of sorghum photoassimilation and partitioning among a pr eflowering-tolerant line (Tx430), a postflowering tolerant line (B35), and an F-1 hybrid thereof. Leaf CO2 exchange and sucrose synthesis ra tes, carbohydrate concentrations, and C-14-assimilate partitioning wer e quantified in held-grown plants that were exposed to (CO2)-C-14 for 3 h under steady-state labeling conditions. Leaf CO2 exchange and sucr ose synthesis rates were slower, and concentrations and radioactivity in blade starch were greater, in B35 than in Tx430 under well-watered conditions. In addition, the postflowering-tolerant cultivar B35 retai ned 70% more C-14-assimilate in the labeled blade than did Tx430. The differences in leaf traits were associated with 33% smaller grain weig hts in B35 than Tx430 or the hybrid, but aerial biomass and upper stem carbohydrate concentrations at preboot and anthesis did not differ am ong the cultivars under well-watered conditions. The consistency of di fferences between B35 and Tx430 across sampling stages suggested that metabolic and unloading processes in leaf blades, rather than limitati ons to assimilate unloading in growing and storage tissues, contribute d to slower rates of CO2 exchange, sucrose synthesis, and C-14-assimil ate export in B35. Leaf traits and dry weights did not differ among cu ltivars 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 wa ter stress indicates tolerant sorghum phenotypes could be selected to avoid the relatively low rates of CO2 exchange, sucrose synthesis, and export observed for B35.