NONSTRUCTURAL CARBOHYDRATE UTILIZATION BY SORGHUM AND MAIZE SHADED DURING GRAIN-GROWTH

Grain sorghum [Sorghum bicolor (L.) Moench] and maize (Zea mays L.) in dryland conditions can experience environmental stress during grain filling, causing increased dependency on stored assimilate. Estimates of maintenance costs and efficiency of production of grain from stored assimilate are important for modeling yield production in such conditions. Our primary objectives in this study were to estimate the maintenance coefficient, the maximum decrease in leaf and stem dry matter, and the efficiency of production of grain from stored assimilate, all with severe stress during grain filling. We measured the loss of sorghum and maize stem dry weight and nonstructural carbohydrate (NCO) content and the increase in panicle and ear dry weight when severe shading stress occurred after anthesis. Extreme shading treatments were imposed from 7 to 8 d after anthesis until physiological maturity on three sorghum cultivars and a maize hybrid in the field at Temple, TX. Shading reduced the combined culm and leaf sheath weights of sorghum by 20 to 33% relative to the weights at anthesis. Measurements of NCO and dry matter of the maize hybrid indicated that 190 to 240 g kg-1 of the stem dry weight at anthesis represented assimilate available for respiration and growth. Mean maintenance coefficient values were 13 mg g-1 dry wt. d-1 for sorghum and 8 mg g-1 dry wt. d-1 for maize. Efficiencies of grain produced from stored assimilate were less than theoretical values. The importance of NCO stored prior to grain filling for maize and sorghum yield modeling appeared to be minimal.