Yoshida - 1981 - Fundamentals of Rice Crop Science

200 FUNDAMENTALS OF RICE CROP SCIENCE
a. Incident solar radiation. Incident solar radiation in rice-growing regions has
been discussed in Chapter 2. High solar radiation is a prerequisite for high rates of
photosynthesis.
b. Photosynthetic rate per unit of leaf area. The net photosynthetic rate of rice
leaves varies with leaf position, nutritional status, water status, and growth stage.
The net photosynthetic rate of active, healthy single leaves is about 40–50 mg
CO 2 /dm 2 per hour under light saturation. The net photosynthesis of single leaves
reaches maximum at about 40–60 klx, about one-half of full sunlight. Photosynthesis
of a well-developed canopy, however, increases with increasing light
intensity up to full sunlight and there is no indication of light saturation (Murata
1961).
The light-photosynthesis curve of single leaves indicates that net photosynthesis
is zero at a light intensity of about 400 1x (light compensation point), and it
increases with increasing light intensity up to about 50 klx (Fig. 5.1). Photosynthetic
efficiency, when it is defined as photosynthesis per unit of light energy, is
higher at lower light intensities. In Figure 5.1, photosynthetic efficiency is indicated
by b for low light intensity and by b’ for high light intensity. Obviously, b is
much greater than b’. This implies that when the total amount of light energy
available for photosynthesis is held constant, the total photosynthesis would be
greater if the light is provided to leaves at lower intensities and, hence, for longer
durations.
5.1. Light photosynthesis curve of single leaves (schematic).