Carbon assimilation and export from leaves of diverse cultivars of sorghum

Abstract

Preflowering and postflowering classes of stress response have served as selection criteria in traditional plant breeding programs. These phenotypic classes provide a source of variation for identifying and quantifying physiological processes that contribute to stress tolerance. However, relationships between physiological stress responses and preflowering and postflowering stress tolerance in sorghum are inconsistent among prior research projects. This research identifies and compares physiological processes between preflowering and postflowering tolerant sorghum lines (Tx430 and B35, respectively) and a hybrid thereof. The objective of this project was to compare leaf carbon exchange rate (CER), carbohydrate concentrations, and sucrose synthesis rate, and [ 14 C]assimilate partitioning within labeled leaves and the whole plant. All genotypes were grown under irrigated and water limited regimes. Plants were exposed to 14CO2 for 3 h under steady state labeling conditions and harvested immediately. Subsamples of plant organs were combusted and radioactivity was counted using liquid scintillation spectroscopy. Rates of sucrose synthesis and leaf export of [ 14 C]assimilate were quantified. Nonstructural carbohydrates in labeled leaves were quantified using colorimetry. B35 exhibited lower rates of [ 14 C]assimilate export than Tx430 or the hybrid under well-watered conditions. Grain weight during grain filling was significantly lower in B35 than Tx430 or the hybrid, although differences in aerial biomass at earlier growth stages did not differ among the genotypes. Under well-watered conditions, starch concentrations in labeled blades were higher in B35 than Tx430 and the hybrid, and allocation of radiolabel to starch was greater in B35 than the other genotypes at grain filling. The carbon partitioning to leaf starch indicated B35 metabolized newly-formed carbohydrates differently than Tx430 or the hybrid. The similar biomass among genotypes at preboot and anthesis indicated that the slower rate of export and greater partitioning of [14 C] assimilate to leaf starch were not due to a slower plant growth rate in B35 than Tx430 and the hybrid. It is hypothesized that photoassimilate retention in leaves under well-watered conditions predisposes B35 to postflowering tolerance to water stress.