Stndies on the Changes in the Photosynthetic Activity of a Crop Leaf during its Development and Senescence : I. Changes in the developmental stage of a rice leaf

Photosynthetic activity measured by O2 evolution and other physiological characteristics such as content of total nitrogen, soluble protein, fraction I protein and chlorophyll (a+b) were investigated in the developing 9th leaf of rice grown under conditions different in air-temperature, light intensity or nitrogen supply. The results obtained are as follows: 1. When the leaf blade emerges out from the preceding leaf sheath, the quantity of fraction I protein increases most rapidly as compared with other leaf constituents. 2. High temperature given during leaf expansion increases photosynthetic activity as well as the content of soluble protein, of fraction I protein and of chlorophyll (a+b). However, high temperature gives no effect on these physiological characteristics after the leaf has attained its final size. 3. Both photosynthetic activity and the content of various leaf constituents increase with increasing nitrogen supplied. Under the same level of nitrogen supply, fraction I protein synthesis is subject to the greatest influence of light intensity, and the transformation from absorbed nitrogen being inhibited by low light intensity. 4. Although considerable amount of total nitrogen and soluble protein are accumulated in the non-emerged part of leaf blade, little amount of fraction I protein is contained with a very low photosynthetic activity. 5. Variation in the photosynthetic activity of a rice leaf during its development (0-12 days after leaf emergence), inducing the effect of environmental conditions, shows the closest curvilinear correlationship with the content of fraction I protein in the leaf, among various leaf constituents. Thus, the content of fraction I protein which is most rapidly synthesized during the leaf development, seems to be the most important limiting factor for the photosynthetic activity of a leaf.