Studies on Dry Matter Production of Soybean Plant : VI. Changes in spectral composition of solar radiation penetrating through leaf canopy and photosynthetic rate of single leaf as affected by light quality

[摘要] Two soybean varieties-Tachisuzunari and Yamabedaizu-were grown under usual field conditions and penetration of radiation through the leaf canopies was investigated with its change in intensity and spectral composition. Photosynthetic response of leaf to light quality was further studied by measuring photosynthetic rate of single leaf under light of blue, green, red and white provided with colored fluorescent lamps. The results obtained were as follows: (1) The intensity of each spectral component of incident radiation decreases at different rates depending on the spectral region as it penetrates through leaf canopy. Infra red radiation penetrates through leaf canopy more than visible part. Of three components in visible part (blue, green, red) the green region penetrates the canopy most. (2) Relation between radiation intensity in leaf canopy and thickness of leaf layer is expressed for each spectral region by the following equation: Ii=Ioie-KiF where, Ii: intensity of radiation in spectral region i under leaf layer F. Ioi: intensity of incident radiation in spectral region i upon leaf canopy. e: base of natural logarithm. Ki: extinction coefficient of leaf canopy for spectral region i. F: thickness of leaf layer (LAI) cumulated from top surface. Extinction coefficient of Tachisuzunari canopy for blue (390∼490 mμ), green (490∼590 mμ), red (590∼690 mμ), visible region (390∼690 mμ), infra red (690m<) and total radiation (250mμ<) were 0.838, 0.721, 0.894, 0.804, 0.337 and 0.474 respectively. Those of Yamabedaizu canopy are 0.839, 0.668, 0.887, 0.787, 0.388 and 0:525. Extinction coefficient for the light assessed by illuminometer are 0.775 for Tachisuzunari and 0.751 for Yamabedaizu. (3) Spectral composition of radiation changes as the radiation penetrates through leaf canopy because of the uneven extinction of the spectral components. That is, proportion of infra red to total radiation increases whereas that of visible part decreases. Moreover, proportion of green region in visible part increases while that of blue and of red decreases. (4) Photosynthetic rate of single leaf on the basis of equal incident energy varies with the spectral region. Further, the photosynthetic response to light quality differs with kind of leaves. (i) Upper leaves of main stem located at the top of leaf canopy photosynthesize more under red light than under blue light, and under green light in-between. (ii) Lower leaves of main stem located at the bottom of leaf canopy photosynthesize more under red light than under other spectral lights as is observed in the upper leaves of main stem. But their photosynthetic rate under green light is either equal to that under blue light or rather less than the latter, showing a remarkable difference in photosynthetic response from the upper leaves of main stem. (iii) Branch leaves located at the bottom of leaf canopy show a similar photosynthetic response to the lower leaves of main stem, though the relative rates of photosynthesis under green and blue light are higher than those of the latter.

[学科分类] 农业科学（综合）