PHOTOSYNTHATE RESERVES DURING GRAIN FILLING IN WINTER-WHEAT

How much does stored or reserve photosynthate contribute to grain growth in winter wheat (Triticum aestivum L.)? Three models were developed to assess this contribution. In Model I, all current photosynthate was used for vegetative or reproductive growth, and there was no reserve. In Model II, photosynthate accumulated in a reserve was used preferentially to support maintenance respiration, while current photosynthate accumulated in the grain. In Model III, the maintenance component extracted from the reserve was continuously replenished from current photosynthate, thus requiring a greater contribution from the reserve for dry matter in the grain. Each model related dry matter accumulation at the whole-plant level to photosynthesis, respiration, and the distribution and retention of C-14 from (CO2)-C-14, assimilated photosynthetically. To test the models, these characteristics were measured in four cultivars of New York soft white winter wheat grown in the field in Connecticut in three years. Observations showed that canopy respiration 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 reserve) terminated grain filling early and predicted a lower harvest index than was observed. The observed time course of the distribution and retention of radioactivity from a pulse of (CO2)-C-14, assimilated photosynthetically was most consistent with predictions of Model III (continuous replenishment of the reserve). In this model, preanthesis reserves contributed from 0.2 to 0.5 of grain dry weight of winter wheat in Connecticut, depending on harvest index. This contribution is expected to exceed 0.5 in more stressful climates.