Studies on Matter Production in Wheat Plant : VI. Quantitative estimate of photosynthetic depression in the field

Abstract

The authors attempted to estimate the amount of depression in apparent photo-synthesis due to some different factors under field conditions. Here, we deal with three kinds of depression; the one due to a direct effect of current low temperature in the daytime, the one due to an after-effect of low temperature at the preceding night, and the one occurring in the afternoon on fine days in spring. 1. In winter air temperature in the field was lower than optimum for photosynthesis throughout the daytime. Therefore, it was considered that in winter photo-synthetic depression by current low temperature in the daytime always occurred throughout the daytime. This depression was larger in the morning than in the afternoon. 2. In order to confirm the existence of photosynthetic depression due to an after-effect of low temperature at the preceding night, we devised to separate the depression due to current low temperature in the daytime from the total depression. As a result, it was confirmed that on fine days subsequent to a cool night in winter, photosynthetic value in the morning was smaller than in the afternoon due to an after-effect of low temperature at the preceding night. 3. On fine days in winter slight photosynthetic depression occurred in the afternoon. This was in nature the same as the one observed on fine days in spring. 4. The photosynthetic depression due to current low temperature occurred throughout the daytime until the beginning of May. 5. On fine days in spring great photosynthetic depression due to unknown cause occurred in the afternoon. When the depression due to a direct effect of current low temperature was eliminated by calculation, this depression became greater. 6. We tried to estimate above-mentioned two kinds of photosynthetic depression on three fine days in winter. Correcting apparent photosynthesis data for the two kinds of photosynthetic depression, we calculated total amount of apparent photosynthesis during daytime. As compared with the total amount of apparent photosynthesis, the photosynthetic depression due to a direct effect of current low temperature in the daytime was 5∼20% and that due to an after-effect of low temperature at the preceding night was about 10%. 7. On fine days in spring, it was inferred that the depression due to a direct effect of low temperature decreased gradually with the progress of season, and that the depression in the afternoon was about 10%.