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Effect of Water Stress on ¹⁴CO₂ Fixation and Translocation in Wheat during Grain Filling¹

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 ¹⁴CO₂ 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 ¹⁴CO₂ 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 ¹⁴C had been transloeated from leaves and sheaths; in stressed plants 46% of the ¹⁴C (averaged over genotypes and dates of exposure) was found in the grain compared to 35% in control plants. Of the total ¹⁴C recovered from shoots at maturity, 83% was found in the grain of stressed plants and 69% in control plants. The lower percentage of ¹⁴C in grain of control plants at maturity was due to accumulation of ¹⁴C 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 ¹⁴CO₂ was fixed later in the grain filling period.

Key Words: Triticum aestivum L. • Photosynthesis

² Formerly graduate assistant (now assistant professor of agronomy, Univ. of Illinois, Urbana, IL 61801) and professor, respectively.

Received for publication January 24, 1976.

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