Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

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Abstract
Wheat plants were grown in a controlled environment with day temperatures of 18 °C and with 500 μ Einsteins m−2 8−1 of photosynthetically active radiation for 16 h. Before anthesis and 2 to 3 weeks after, rates of net photosynthesis were measured for leaves in 2 or 21% O2 containing 350 vpm CO2 at 13, 18, 23, and 28 °C and with 500 μEinsteins m−2 s−1 of photosynthetically active radiation. Also, under the same conditions of light intensity and temperature, the rates of efflux of CO2 into CO2-free air were measured and, for mature flag leaves 3 to 4 weeks after anthesis, gross and net photosynthesis from air containing 320 vpm 14CO2 of specific activity 39·7 nCi μmol−1. When the O2 concentration was decreased from 21 to 2% (v/v) the rate of net photosynthesis increased by 32 per cent at the lowest temperature and 54 per cent at the highest temperature. Efflux of CO2 into CO2-free air ranged from 38 per cent of net photosynthesis at 13 °C to 86 per cent at 28 °C. Gross photosynthesis, measured by the 14C assimilated during 40 s, was greater than net photosynthesis by some 10 per cent at 13 °C and 17 per cent at 28 °C. These data indicate that photorespiration was relatively greater at higher temperatures.