On the Solar Energy Balance of Rice Population in Relation to the Growth Stage

Abstract

The utilizing efficiency of solar energy (Eu) in dry matter production of plant population consists of two components: one is the efficiency for absorbing the solar energy by the stand (α) and the other is the efficiency for fixing the absorbing energy for dry matter production (Eφ). With the aim to find out the actual values of these components as well as the utilizing efficiency in a rice population, measurement of the following items was carried out using rice plants, Norin No. 29 which were cultivated in the paddy field at the density of 16 hills/m2 (25×25cm): total incident energy (S), the rate of energy reflection by the canopy (albedo, a), the rate of energy transmittance at the surface of flooding water (t), the rate of energy reflection by the surface of water, the amount of dry matter produced in a certain period (ΔW), the leaf area index (LAI), and the heat of combustion per gram dry matter. Measurements were made continuously from August to October. The results are as follows: 1. The albedo (a) of the rice population before heading stage was almost constant (22%). However, it decreased gradually after heading to reach 18 per cent at maturity. This decrease may be related to the fact that the albedo of ears is lower than that of leaves (table 1). 2. The decrease in the rate of energy transmittance by the canopy according to the advancing growth stage was attributed both to the increase in LAI before heading, and to such changes in the structure of the crop stand as the emergence of ears and decrease in the inclination of leaf-blade after heading (fig. 6, table 1). 3. The efficiency for absorbing the solar energy (α) increased with the growth of rice plants to reach the maximum value (about 70 %) on 30 days after heading. The change of α was closely related to that of t (fig. 4). 4. The heat of combustion per gram of each organ in each growth period was measured by use of a calorimeter. The value, K, was in the following order: Leaf-blade>Leaf-sheath>Root but its variation during the whole growth season was as small as four per cent (table 2). The mean value of K of the whole plant was 3.76 Kcal per gram. 5. The efficiency for fixing the absorbed solar energy (Eφ) was calculated by the following formula: Eφ=(K·ΔW)/(α·ΣS)×100 (%) The value of Eφ for 40 days in total before and after heading varied between 4.1 and 6.9, maximum value being seen at heading stage (table 3). 6. The utilizing-efficiency of incident energy (Eu) is given by the following formula: Eu=(K·ΔW)/(ΣS)×100 (%) The value of Eu fluctuated from 0.8 to 3.7 by growth stage, maximun value being found at heading stage (table 3).