Carbon dioxide fertilization provides a way of increasing crop photosynthesis and can be used to indicate when and to what extent crop photosynthesis limits grain yield and its components. A CO 2 fertilization experiment was conducted using a dwarf spring wheat ( Triticum aestivum L. ‘Yecora 70’) in each of three winter cropping cycles (1970‐73) in northwest Mexico. The wheat was grown in the field under irrigation and optimal agronomy. Small areas of crop (1 to 2 m 2 ) were enclosed in transparent chambers ventilated with air enriched to approximately 750 ppm CO 2 ; chambers ventilated with normal air and unenclosed areas of crop acted as controls. Fertilization was carried out in separate plots during each of four 1‐ month periods covering the crop cycle. Fertilization with CO 2 increased crop dry matter production. Grain yield was either unaffected or increased by up to 23% by extra CO 2 . Biggest yield increases came generally from CO 2 , in the 2nd and 3rd months and were associated with more spikes/m 2 and grains/spikelet, hence more grains/m 2 . During the 4th month or grain filling period, kernel weight and grain yield were significantly increased in 1972, but clearly not in 1973. Results indicate greatest yield limitation by crop photosynthesis in the 2 months preceding grain filling, when grains/m 2 is determined. They are discussed in relation to adjacent shading experiments with the same cultivar, to other CO 2 fertilization studies with wheat and other crops, and to possible avenues for increasing yield potential.
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Fischer et al. (1976) studied this question.