植物群落における光要因と光合成の理論的解析 (1)

葉層構造と直射光, 散光および自然光との関係

抄録

Under natural light conditions, the radiation impinging upon foliage from sky is composed of the direct and the diffuse radiation. The intensity of direct radiation within foliage is determined with the extinction coefficient for one beam, and the intensity of diffuse radiation with Monsi and Saeki's formula concerning the ratio of light intensity inside and outside of foliage in isotropic light beam.
Our theoretical extinction coefficient for one beam (K_D) corresponds to the ratio of the horizontal projection to the actual area in a leaf. In the foliage that consists of leaves orientating at right angles to the sun, K_D=cosecH (H, sun elevation). In the foliage with random arrangement of leaves inclined at a fixed angle (α) to the horizontal, K_D=cosα in H≥α, and in H<α K_D={cosα⋅sin^-1 (cotα⋅tanH)+(sinα⋅cotH)√1-cot²α⋅tan²H}2/π. Using these equations and Monsi and Saeki's formula, the relative light intensities under sunny conditions were calculated as a function of leaf area index and sun elevation. The calculated light intensity had a diurnal minimum at H=30°, in α=80° as well as in leaves with a phototropic movement. This result agrees with the observation of Brougham in perennial ryegrass community and in white clover community under sunny conditions. Disagreement among some observers as to the relative light intensity measurements was elucidated on the basis of our theoretical analysis of extinction coefficient for one beam. Light transmission of each leaf leads to the increase of light intensity within foliage, and light reflection from foliage surface results in the decrease of light intensity. Consequently, the theoretical equations whether which include or not the terms concerned with transmission and reflection bring about the same light intensities.