Phytometrical Studies of Crop Canopies (1)

Abstract

Biometrical measurements of the leaf inclination, the leaf azimuth and the leaf area density were made within a soybean field at certain intervals during the growing season of 1969 (from June 9 to August 28). The penetration of direct solar radiation into the canopy was also determined with sunlit area measuring rods (see Fig. 9). These data were analysed to make clear the characteristics of geometrical structure of the canopy. The results obtained can be summarized as follows:1. Fig. 2a indicates that the leaf of soybean crops is non preferential as to the azimuth angle. The leaf distribution with respect to the inclination angle is characterized by curves with a pronounced peak between 30 to 45°, implying that the leaf orientation of soybean canopy is horizontal (see Fig. 2b). The effective leaf area function GL(w) characterizing the extinction of sun light within the canopy changed markedly with the sun altitude, as shown in Fig. 3. GL(w)-profiles of the soybean canopy are similar to those of a broad bean canopy reported by Ross and NILSON (1965) in shape and in absolute value. Mean effective leaf area function GL calculated from Eq. (3) increased from 0.3 to 0.9 (see Fig. 4) with the sun altitude. The sun altitude dependence of GL was well approximated by Eq. (4).2. Extinction coefficients (kd) for direct sun light obtained from GL(w) and leaf area density fL(w) are shown as a function of the sun altitude in Fig. 8. The extinction coefficient in the range of sun altitude over 40° was independent of th sun altitude, varing from 0.75 to 0.85 with the growth stages, while in the range lower 40° it increased drastically with decreasing the sun altitude. From the results presented in Fig. 8. it is concluded that the soybean canopy behaves as a leaf canopy consisting of leaves with the inclination angle of 30 to 40°.3. Profiles of sunlit area within the canopy were determined by the sunlit area measuring rods and compared with those calculated from Eq. (1) with good accordance in Figs. 5 and 10. On August 28, somewhat large discrepancy was observed between measured and calculated ones. This seems to be caused by the disturbance (increase in leaf cluster) of the canopy structure due to the typhoon hit before 5 days. With the aid of measured profiles of the sunlit area and the leaf area density profiles, the extinction coefficient was determined and compared with those determined from Eq. (8) in Fig. 11. With the exception of lower sun altitude, there is a good accordance between them. This implies that use of the sunlit area measuring red accompanied with measurements of leaf area density function is one of the most practical methods in researches of canopy structure of crop plants.