PHOTOSYNTHATE RESERVES DURING GRAIN FILLING IN WINTER-WHEAT

How much does stored or reserve photosynthate contribute to grain grow th in winter wheat (Triticum aestivum L.)? Three models were developed to assess this contribution. In Model I, all current photosynthate wa s used for vegetative or reproductive growth, and there was no reserve . In Model II, photosynthate accumulated in a reserve was used prefere ntially to support maintenance respiration, while current photosynthat e accumulated in the grain. In Model III, the maintenance component ex tracted from the reserve was continuously replenished from current pho tosynthate, thus requiring a greater contribution from the reserve for dry matter in the grain. Each model related dry matter accumulation a t the whole-plant level to photosynthesis, respiration, and the distri bution and retention of C-14 from (CO2)-C-14, assimilated photosynthet ically. To test the models, these characteristics were measured in fou r cultivars of New York soft white winter wheat grown in the field in Connecticut in three years. Observations showed that canopy respiratio n and grain dry matter accumulation were approximately equal sinks for photosynthate and, together, were greater than canopy photosynthesis late in grain filling. Consequently, in two years, Model I (with no re serve) terminated grain filling early and predicted a lower harvest in dex than was observed. The observed time course of the distribution an d retention of radioactivity from a pulse of (CO2)-C-14, assimilated p hotosynthetically was most consistent with predictions of Model III (c ontinuous replenishment of the reserve). In this model, preanthesis re serves contributed from 0.2 to 0.5 of grain dry weight of winter wheat in Connecticut, depending on harvest index. This contribution is expe cted to exceed 0.5 in more stressful climates.