Studies on Utilization of Solar Radiation by Crop Stands : IV. Estimated absorptivities of solar energy in each leaf layer of rice and soybean canopies

Abstract

We have shown in the previous paper that no difference in absorptivity of solar energy could be found between populations of soybean and rice at later stage of growth with high LAI, while the dry matter increment of soybean was considerably lower than that of rice indicating that soybean population might have lower efficiency for utilization and conversion of solar energy than that of rice population. The present investigation was carried out to elucidate the cause of the lower efficiencies of soybean in terms of absorptivity of solar energy in each layer of both crop canopies. 1. Incident radiation being absorbed and reflected by leaf layers, its flux density attenuates downward in the canopy. On the other hand, the upward radiations reflected by soil surface and leaf layers penetrate in the canopy, and the former attenuates upward for being absorbed by leaf layers and the latter increases upward for being accumulated in the canopy (Fig. 1). To evaluate the absorptivity in each leaf layer of canopy, these two components of upward flux need to be measured separately. But the separation of these components is impossible technically. In this study, therefore, the upward flux was divided into two components numerically. The absorptivities of solar energy in each leaf layer were obtained from calculation using the simulating formula. 2. At the earlier stage of growth with lower LAI, the absorptivities of solar energy in each leaf layer were higher throughout all layers in soybean canopy than in those of rice. At the later stage of growth with higher LAI, the relations tended to differ with height in canpoy; in upper layer of the canopy absorptivities of soybean were higher than those of rice, but in lower layer they were higher in rice. These tendencies were more significant in the case of photosynthetically active radiation (Fig. 7). These relations were considered as one of the determinant factors which made the utilization and conversion rates of solar energy in soybean canopy lower than those of rice at the later stage of growth with higher LAI.