Net photosynthesis, dark respiration and the CO2 compensation point were determined for several wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) cultivars at temperatures ranging from 5 to 35 C. Temperature optima for net photosynthesis were as follows: vegetative plants—wheat 25 C; barley 20 C; reproductive plants (leaves plus developing spikes)—wheat and barley 15 C. Temperature optimum for the flag leaf of wheat was 20 C while that for the spike was 10 C. Respiration rates increased with increasing temperatures giving a Q10 of between 1.4 and 2.3 for the range of 15 to 35 C; for 5 to 15 C the Q10 was between 1.9 and 3.7. The CO2 compensation point averaged from 39 ppm at 7 C to 73 ppm at 30 C. Thus part of the reduction in temperature optimum for net photosynthesis can be attributed in the reproductive stage to the increasing volume of nonphotosynthetic tissue in the developing spike although there appears to be a lower optimum for the flag leaf as well. These declines in temperature optima very likely contribute to the lower wheat yields observed in many Great Plains wheat-growing areas where maximum daily temperatures during heading are often in the range of 25 to 35 C, temperatures that caused from 3 to 53% reduction in net photosynthesis in wheat plants in the reproductive stage and from 13 to 58% reductions in barley plants at a similar stage.
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