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Photosynthesis, photorespiration and related aspects of CO2 exchange by wheat, corn and Amaranthus edulis Jolliffe, Peter Alfred


Certain aspects of CO₂ exchange by wheat (Triticum aestivum L.), corn (Zea mays L.) and the grain Amaranth (Amaranthus edulis Speg.) were investigated. The effects of O₂ concentration on apparent photosynthesis of juvenile wheat and corn shoots were measured at different temperatures, CO₂ concentrations and light intensities using infra-red CO₂ analysis and both open and closed gas-flow systems. The only condition in which apparent photosynthesis of wheat was not inhibited by O₂ was in 20.8% O₂ when the CO₂ concentration was saturating and the temperature was 30° C or less. The degree of inhibition increased with increasing O₂ concentration, increasing temperature, and decreasing CO₂ concentration and was independent of the light intensity. During some of the growing season in temperate regions, the effect of atmospheric O₂ on the photosynthetic productivity of wheat may be negligible. The effect of O₂ on wheat was ascribed to both a stimulation of photorespiration and an inhibition of photosynthesis by O₂. In corn, which apparently lacks photorespiration, photosynthesis was also inhibited by 99% O₂- No inhibition occurred at 20.8% O₂, however, and the stability and reversibility of the inhibition observed at 99% O₂ differed from that found with wheat. These differences between wheat and corn are correlated with differences in leaf anatomy and photosynthetic carbon metabolism and with differences in the response of apparent photosynthesis and the CO₂ compensation point to environmental conditions. Many of the gas exchange characteristics of corn and similar plants seem to be advantageous for the warm dry areas they often inhabit. Initially high rates of CO₂ production are exhibited by wheat immediately following illumination, and it has been suggested that this post-illumination CO₂ burst is an extension of photorespiration into the dark period. In accord with this, CO₂ concentration was found to influence both the post-illumination CO₂ burst and the inhibition of apparent photosynthesis by O₂ in a similar way. Except for Amaranthus edulis, plants which lack photorespiration also lack significant post-illumination CO₂ bursts. On the basis of its response to O₂ concentration, however, the burst of Amaranthus edulis is concluded to be unrelated to photorespiration. Measurements of ¹²CO₂ and ¹⁴CO₂ exchange were used to estimate the quantity of carbon in wheat and corn shoots which was in free exchange with atmospheric CO₂. The free exchange pool size was found to be very small and it cannot be an important factor in the CO₂ concentration response of photosynthesis or in the post-illumination CO₂ burst.

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