Studies on Utilization of Solar Radiation by Crop Stands

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Abstract
If a precise determination of utilization efficiency of solar energy by crop plants is to be made, it is necessary to determine absorption rates of photosynthetically active radiation (PhAR), because plants can never convert any radiation except PhAR to energy for their growth. It would also be useful to compare the various types of crop stands in conversional efficiencies of solar radiation. Rice (var. Tyotama) and soybean (var. Tamanishiki) plant populations were used to study. To obtain the absorption rates of short wave radiation and PhAR in the stands, the following items of radiant energy were measured; incident energy to the top of canopy, albedo of canopy and transmissive energy into the ground surface. To estimate photosynthetically fixed energy by the stands, total dry matter produced and its combustion energy were measured during the growing periood. Rate of energy absorption increased with leaf area index (LAI). The absorption 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. Combustion energy per unit dry weight of whole plant was little changed throughout the 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. 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 growth stages. 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. Conversional efficiencies (E.vphi.eff), 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 later for both crop stands. E.vphi.eff of rice stand were always higher than those of soybean. The mean value of E.vphi.eff of PhAR in early stages is 5-6% for rice stand and 3-4% for soybean.

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