Yoshida - 1981 - Fundamentals of Rice Crop Science

198 FUNDAMENTALS OF RICE CROP SCIENCE
change is expressed as (McCree 1974):
dW/dt = D – N, (5.7)
where D = daytime net total of CO 2 taken up by the plant and
N = night total of CO 2 evolved by the plant.
When dW/dt is expressed per unit of ground area (g dry matter/m 2 per day or
week), it is called crop growth rate (CGR). The crop growth rate is used as a
measure of primary productivity of crops in the field.
5.1.2. Characteristics of rice photosynthesis
Rice is generally believed to have a C-3 photosynthetic pathway (Ishii et al 1977b).
There is, however, a report that suggests the operation of both C-3 and C-4
pathways in a salt-tolerant indica variety (Hegde and Joshi 1974).
As a C-3 plant, rice has a high CO 2 compensation point, exhibits photorespiration,
and lacks bundle sheath chloroplasts (Table 5.2). Compared with other C-3
species, however, rice has a relatively higher net photosynthetic rate per unit of
leaf area. Within Oryza sativa, indica rices have a higher optimum temperature
than japonica rices. Early papers on rice photosynthesis reported a leaf photosynthetic
rate of 10–20 mg CO 2 /dm 2 per hour, while recent papers indicate a rate
of 40–50 mg CO 2 /dm 2 per hour. This difference could be attributed primarily to
improvements in the measuring technique (Yoshida and Shioya 1976). In early
studies on rice photosynthesis, the CO 2 exchange rate was measured by enclosing
several cut leaves in aplastic chamber. Later studies, however, use an intact leaf.
The photorespiration rate increases with increasing light intensity, but the rate
of photorespiration relative to the rate of CO 2 fixation is higher at lower light
intensities. At light intensities lower than 10 klx, photorespiration accounts for
70–90% of CO 2 fixation. At 40 klx, it accounts for 40% (Ishii et al 1977c).
At the moment, lowering the oxygen concentration in air is the most convenient
means of reducing photorespiration. A group of plant growth regulators can also
reduce photorespiration and increase photosynthetic activity (Zelitch 1979).
When rice was grown in a low-oxygen atmosphere, both photosynthesis and dry
matter production increased; grain yield decreased due to an increased percentage
of unfertile grains. The percentage spikelet fertility of Tangin Bozu and Hoyoku
varieties decreased from 86 and 74% in the 21% O 2 atmosphere to 22 and 46% in
the 3% O 2 atmosphere, respectively (Akita and Tanaka 1973). Thus, inhibiting
photorespiration by lowering the oxygen concentration did not increase the grain
yield.
5.1.3. Crop photosynthesis
Crop photosynthesis in the field is primarily determined by incident solar radiation,
photosynthetic rate per unit leaf area, leaf area index, and leaf orientation.