Field canopies of two semi‐dwarf wheat ( Triticum aestivum L.) genotypes were subjected to water stress that caused visible wilting during the grain filling stage to determine the distribution of photosynthesis within the canopies and the pattern of translocation of labeled assimilates following 14 CO 2 uptake. The stress treatment caused a 14% reduction in kernel weight and a 20% reduction in grain yield compared to control plants. Plants in the control and stress plots were exposed to 14 CO 2 at 12, 16, and 23 days after heading. During this time, the severity of stress increased on the stress plots. Under water stress, the site of photosynthesis tended to shift away from leaf lamina to the upper leaf sheaths, upper portions of the stem, and the spike. Within 24 hours after labeling, the majority of the 14 C had been transloeated from leaves and sheaths; in stressed plants 46% of the 14 C (averaged over genotypes and dates of exposure) was found in the grain compared to 35% in control plants. Of the total 14 C recovered from shoots at maturity, 83% was found in the grain of stressed plants and 69% in control plants. The lower percentage of 14 C in grain of control plants at maturity was due to accumulation of 14 C in stem segments, primarily in the form of structural carbohydrates. As the lower leaves became senescent, the relative contribution of spikes, upper sheaths, and stems to total photosynthesis increased. The percentage of the present in the grain at 24 hours and at maturity was greatest when 14 CO 2 was fixed later in the grain filling period.
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Johnson et al. (1976) studied this question.