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Six wheat cultivars with more than a twofold range in stem height and weight were grown at 21/16°C in 16-hr days of high light intensity. Changes with time in photosynthesis, respiration, dry weight, and the distribution of 14C fed to the flag leaf or ear of the main stem were followed in all cultivars. The effects on grain yields, stem weights, and 14C distribution of prevention of photosynthesis by the ears or by the rest of the plant during grain filling were also examined. Substantial losses of dry weight from the stem occurred during the most rapid period of grain filling, but stem weights then rose again in most cultivars. About one-third of the early loss in stem weight of control plants was due to stem respiration, the remainder to mobilization, but cultivars differed in these proportions, the respiration rate of stems per unit dry weight being higher in the shorter cultivars. The assimilates mobilized from the stems were equivalent to 2.7–12.2% of final grain weight in control ears. Mobilization increased when photosynthesis was reduced, particularly that from the lower internodes, which indicated that grain yields in the control plants were not limited by lack of assimilate. Balance sheets derived from gas analysis measurements supported this conclusion. The amount of material lost from the stems of plants in all the conditions employed, measured as dry weight or as 14C, was unrelated to stem height. Tall cultivars were no more dependent than short ones on stem reserves, and no more able to draw on reserves for grain filling in conditions limiting photosynthesis. Nor was there evidence that stem growth in tall cultivars was more likely to compete with grain growth. The cultivars differed more than threefold in the rate of ear filling. High growth rate per ear was associated with high yield and grain number per ear and with slow initial growth of the grains.
Rawson et al. (Wed,) studied this question.