Carbon and nitrogen assimilation during development of sorghum as related to genotype and water stress

Water stress is a major deterrent of grain yield in much of the Great Plains and the world. Grain sorghum is more drought tolerant than other cereals and is grown on vast areas in West Texas. For sorghum to become a more important feed grain crop, then genotypes that are more water efficient and water stress tolerant must be developed. The purpose of this study was to determine the relative contribution of both carbon and nitrogen assimilated prior to flowering to the grain filling process as affected by water stress and genotypic differences. Sorghum genotypes differing in degree of leaf senescence during grain filling were grown in field studies with and without irrigation. At flowering the average sorghum plant had accumulated 63% and 70% of the total dry matter (DM) and nitrogen (N), respectively, of that present at physiological maturity. For the average sorghum plant, 15% and 40% of final grain DM and N, respectively, were due to reallocation from assimilates present at flowering.

Significant differences in the assimilation and reallocation of DM and N in the sorghum plants occurred due to water stress and genotype. All water stressed plants relied more heavily on DM and N accumulated by flowering in leaves and stalks to fill the grain than nonstressed plants indicating the severe effect of stress on both photosynthesis and protein synthesis. Both genotypes used existing DM and N present at flowering in addition to absorbing more during grain filling. The senescent plants relied more heavily on reallocation of assimilates from stalks and leaves acquired by flowering to fill the grain than did the nonsenescent plants.

Findings from this study should aid in understanding the effects of water stress and genotype selection upon the use of net assimilation and remobilization of DM and N in the grain filling process. The use of established reserves by the grain sorghum plant could be an important mechanism to allow plants to mature if no detrimental effects due to stalk or leaf remobilization occur.