Studies on the Canopy Photosynthesis of the Horticultural Crops in Controlled Environment

(2) Distribution of percent sunlit leaf area in hedgerow cucumber canopies

Abstract

Light is the primary ecological factor limiting growth and yield of crops cultivated in glass or vinyl houses. A simple relation proposed in a previous paper (Iwakiri and Inayama, 1974) was applied to determine the penetration of direct sunlight into cucumber canopy rows with four different azimuth angles such as φ_r=0°(N-S), φ_r=90°(E-W), φ_r=45°(E45°N-W45°S), and °_r=135°(E45°S-W45°N). It is assumed in the calculations that the length of each cucumber row is infinite so that the end effect can be neglected. Phytometrical data obtained on May 16 (at five weeks after transplanting) were used to calculate the extinction coefficients (k_d^↓ and k_d^→) as influenced by both sun and row directions. As in the previous paper, the cross section of each cucumber row was first divided into twenty five sub-blocks of 20cm wide and 30cm deep. The twenty five blocks were divided into the two parts in which leaves are irradiated by direct sunlight incident on the top surface and on the lateral face, respectively. The following relations were used to calculate the attenuation of percent sunlit leaf area with the optical depth along light path:
d_n=exp{-Σⁿ_n=1k_d^↓, _nΔF_n^↓},
d_m=exp{-Σ^m_m=1k_{d, m}^←ΔF_m^←},
where d_n and d_m denote percent sunlit leaf areas at the boundary of blocks n and m (integers n and m being the orders of block counted from top and lateral surfaces of cucumber row, respectively); k_{d, n}^↓ and ΔF^↓_n extinction coefficient and partial leaf area index in the block irradiated by sunlight incident on top surface, respectively, and k_{d, m}^← and ΔF^←_m extinction coefficient and partial leaf area index of the block irradiated by sunlight incident on lateral surface of row. The mean sunlit leaf area in blocks n and m were determined by
d_n={d_n+d_n-1}/2,
d_m={d_m+d_m-1}/2,
where d_n and d_m are respective percent sunlit leaf areas in blocks n and m.
The results so obtained can be described as follows:
1. The light environment in an isolated cucumber row was firstly studied in relation to the direction of sun and row. The percent sunlit leaf area of a cucumber row as defined by F_l/F_t, where F_t and F_l are leaf area index and sunlit leaf area index, respectively, showed a characteristic diurnal change (Figs. 8 and 9).
The percent sunlit leaf area of an N-S oriented row decreased with sun altitude in all seasons. This is mainly due to that the direct sunlight incident on a side surface that plays a very significant role in the light environment when sunrays are perpendicular to the side surface of an N-S oriented row. The role of direct sunlight incident on a side surface becomes less with an incident angle.