Studies on Utilization of Solar Radiation by Crop Stands

II. Utilization of short wave and photosynthetically active radiation by rice and soybean plant populations

Abstract

If we want to know the strictly meaning utilization efficiency of solar energy by crop plants, it is necessary to make clear absorption rates of photosynthetically active radiation (PhAR), because they can never convert any radiation except PhAR to the energy for their growth. Also for researching the development of solar energy utilization by crop stands, it would be useful to compare the various types of crop stands in conversional efficiencies of solar radiation. For this purpose, rice (var. Tyotama) and soybean (var. Tamanishiki) plant populations were used to study. In order to obtain the absorption rates of short wave radiation and PhAR in the stands, following items of radiant energy were measured; incident energy to the top of canopy, albdo of canopy and transmissive energy into the ground surface. On the other hand, to estimate photosynthetically fixed energy by the stands, total dry matter produced and its combustion energy were measured on successive growing period.
Results are summarized as follows:
1. Rate of energy absorption increased with leaf area index (LAI). The absorp tion rate of soybean stand was higher than rice at low LAI, but the maximum absorption rate at high LAI was not different in both stands. These values were 73% for short wave radiation and 88% for PhAR (fig. 4).
2. Combustion energy per unit dry weight of whole plant was little changed throughout growing season. Combustion energy of soybean was higher than that of rice. These averaged values were 3676 and 4165 cal/g for rice and soybean plants respectively (table 4).
3. Utilizational efficiencies of radiation (Eueff), which were obtained from dividing the fixed energy into stand by incident solar energy onto canopy, were low at early growing stage. There was no difference in Eueff between rice and soybean stands. But later, Eueff increased and reached maximum, then decreased gradually. The maximum value of Eueff of the PhAR were estimated to be 4.5-5% for rice stand and 3-3.5% for soybean (fig. 6).
4. Conversional efficiencies (Eøeff), which were obtained from dividing the fixed energy in stand by solar energy absorbed into the canopy, were almost constant at early stage but decreased progressively at later for both crop stands. Eøeff of rice stand were always higher than those of soybean. The mean value of Eøeff of PhAR in early sage is 5-6% for rice stand and 3-4% for soybean (fig. 7).