Studies on CO2 Uptake and CO2 Evolution in Each Part of Crop Plants : (III) Variation of photosynthetic rate in different part of leaf on rice, corn and sorghum plant

It is important to make clear the differences of the photosythetic rate on each part of the leaf for analyzing the structure of canopy photosynthesis of crop plant population. The rate of photosynthesis of each part of the leaf was measured with a small chamber as shown in figure 1 for rice, corn and sorghum leaves. The plants were grown in pots. The rice plants were grown as a group of many potted plants, and the others in a thin density. 1) The photosynthetic rates of top-, middle- and base-part of leaf in the three crops were measured respectively with the small assimilation chamber (Fig.1). There are lage difference in maximum rate of photosynthesis among three parts of leaf, and also in light saturation point of that. It considered that the difference of the phosynthetic rate among the leaf parts due to nitrogen content per unit leaf area related to its chlorophyll content. 2) The maximum rate of net photosynthesis of the rice leaf in mg CO2 absorbed per dm2 per hour was 30 and that of corn and sorghum was 50-60 under the conditions of light saturations, 300 ppm CO2 concentration and about 30°C temperature. The relationship between the light intensity (I) in K lux and the rate of gross photosynthesis (P) in each part of leaf can be expressed by the following equation: P=bI/I+aI, where a and b is constant. The rice leaves exisiting at a lower layer within the population and having a lower rate of photosynthesis were considered to be the shade leaves according to the values of constant in the above equation and the light saturating point decreased. 3) The rate of photosynthesis in each part of leaves of three crops had a high positive correlation to the nitrogen content and the respiratory rate of its part. Corn leaves, especially, had a negative correlation to specific leaf area (leaf area in cm2 per gram dry leaf weight). There were large differences in the regression line between the rate of net photosynthesis and the nitrogen content in leaves, moreover, photosynthesis per unit of leaf nitrogen could be ranged in following order: sorghum, corn and rice