NONSTRUCTURAL CARBOHYDRATE UTILIZATION BY WHEAT SHADED DURING GRAIN-GROWTH

Wheat (Triticum aestivum L.) is commonly grown in dryland conditions, where environmental stress during grain filling can increase the depen dency on stored assimilate. The objective of this study was to quantif y the loss of total nonstructural carbohydrates (TNC) from wheat leave s and stems and the increase in seed dry mass when severe shading stre ss occurred after anthesis. Beginning 10 to 11 d after anthesis, sever e shading was imposed on 'Mit' wheat in the field at Temple, TX, in 19 91 and 1992. During shading, plants with spikes had reductions in stem dry mass of 290 g kg-1 in 1991 and 140 g kg-1 in 1992. Leaf dry mass decreased 240 g kg-1 in 1991 and increased 10 g kg-1 in 1992. Fructan and sucrose comprised the largest proportion of the storage carbohydra te reduction in stems and leaves. Starch in stems and leaves was not r emobilized during shading. Estimates of maintenance coefficient were 4 .3 mg carbohydrate g-1 dry mass d-1 in 1991 and 1.8 in 1992. Maintenan ce respiration consumed 68% of the TNC present at the start of shading in 1991 and 18% in 1992. There was 0.68 g of grain produced per gram of assimilate lost from leaves, stems, and the nongrain portions of sp ikes in 1991 and 0.78 in 1992. When estimated costs of maintenance res piration were removed, 0.82 and 0.93 g of grain was produced per gram of assimilate that disappeared. Stored carbohydrate represented an imp ortant buffer for yield production when stress occurred during grain f illing.