作物群落の計量植物学的研究 : 第3報 水稲群落内の光環境

抄録

The structure of radiation environment within the canopies of two rice cultivars of Manryo and IR-8 was studied using the phytometrical data reported in a previous paper. The data of Aug. 25 and Sept. 14 which were before and after heading were used in the calculations. In the calculations, special interest was mainly directed toward clarifying the diffuse radiation fluxes due to scattering of solar radiation by plant elements, and the radiation energy partition between leaves. the distribution functions of leaf area with respect to the angle between leaf normal γ_L and the direction of solar γ₀ presented in Table 1 were used as the original data for studying the structure of radiation environment within rice canopies. Adopting reasonable assumptions concerning the scattering of radiation fluxes by plant elements, the upward flux U(j) and downward flux D(i) of complementary diffuse radiation due to light scattering by plant elements through reflection and transmission can be approximated by U(j)=B(j)+U(j-1)exp.[-τ(_{s, j}f_{L, i}], D(i)=C(i)+D(i-1)exp[-τ(_{s, j}f_{L, i}], where B(j) is the diffuse radiation flux emanated upward from the upper boundary of the j-th layer due to first scattering of radiation flux in this layer, C(i) the diffuse radiation flux emanated downward from the bottom boundary of the i-th layer due to first scattering of radiation in this layer, τ=1-T the term for considering the effect of the second scattering of radiation flux on light distribution approximately, and T leaf ransmissibility assumed to be 0.3. When the scattering of direct solar radiation by plant elements is considered, B(j) and C(i) are given by B(j)=Q₀(h_o)a(j, h_o)f_{L, i}·RΣ^^(10)__(j=1)P(j, l)N(l), C(i)=Q₀(h_o)a(j, h_o)f_{L, i}·TΣ^^(10)__(i=1)P(i, l)N(l), where Q₀(h_o) is the direct solar radiation intensity on surface normal to sun's ray at sun altitude of h_o, a(j, h_o) and a(i, h_o) are the sunlit area in j- and i-th layers at sun altitude of h_o, P(j, l) and P(i, l) the leaf area distribution functions with respect to |cos <γ_Lγ_o>^^^^^| of j- and i-th layers, N(l) is the mean value of |cos <γ_Lγ_o>^^^^^| for class l, and R is the albedo of leaf surfaces to total short-wave radiation and assumed to be 0.30. For the sake of simplicity, R is assumed to be equal to T and independent of the incident angle of light. The rice canopies were divided into twenty layers with the 5cm depth (i=j=20). The intiger i is counted downward from the canopy top and the integer j upward from the canopy bottom. Important results obtained can be summarized as follows: 1. With increasing sun altitude, the extinction coefficient of direct solar radiation decreased drastically from 2-3 at sun altitude of 10° to 0.4-0.6 at sun altitude of 60°. Although the sun altitude dependence of k_d changed somewhat during the growing stage of rice plants, two rice canopies were found to behave to direct solar radiation like a plant canopy forming from leaves with inclination angle between 60 and 75°. IR-8 canopy with more erected leaves indicates somewhat larger extinction coefficient at low sun altitude and smaller extinction coefficient at high sun altitude than those of manryo canopy (see Fig. 2). 2. A simple tool for measuring the sunlit area (measurement rod of sunlit area) was used to determine the penetration of direct solar radiation into the rice canopy. The data obtained from measurement of sunlit area using the rod were analyzed by E_q. (8) to yield the effective leaf area projection G_L(h_o), and the extinction coefficient of direct solar radiation k_d. [the rest omitted]