Studies of Energy and Gas Exchange within Crop Canopies (5) : Geometrical structure of barley canopies and the penetration of direct solar radiation into the Canopy

Abstract

Biometrical measurements of leaf area density (including the surface area of ears and (stems) and spatial distribution of leaves were carried out in a barley field in order to get the data necessary to calculate the penetration of direct solar radiation into the canopy. Similar measurements were also done in potted barley canopies. Sunlit leaf area index and the intensity of direct solar radiation on the foliage surface (leaf, ear and stem) were determined on the basis of the biometrical data and the penetrating rate of direct solar radiation. The following relations proposed by Roos and Nilson (1965) were used to calculate the penetrating rate of direct solar radiation at each depth of the canopy: [numerical formula] and α(w)=α(w)αs(w)αE(w) where αL(w), αs(w), αE(w) and α(w) are the extinction factor due to leaves, the extinction factor due to stems, the extinction factor due to ears and the extinction factor due to whole biomass, respectively, fL(w'), fS(w') and fE(w') are the respective surface area densities of leaves, stems and ears at a depth w', GL(w') is the effective leaf area function. The vertical distribution of the leaf area density was found te be approximately symmetrical in relation to the half of canopy height at each growing stage. As can be seen in fig. 1, the density of surface area of the ear was twice as large as that of stem. The distribution function of the leaf area in relation to the inclination angle was characterized by the establishment of a sharp peak between 60°and 75°. The distribution function of the leaf area in relation to the azimuth angle was found to be slightly lengthened to the direction perpendicular to the planting row (fig. 1). In the potted barley canopy with sparse density, a peak between 60°and 75°in the leaf area distribution function in relation to the inclination angle became distinctive with growth of crops (see fig. 3). Fig. 4 indicates that the magnitude of the effective leaf area function decreases with increasing sun altitude. This is quite opposite to results of corn fields reported in a previous paper. Values of the effective leaf area function in both the upper and bottom layers of the canopy showed relatively small hourly change, though the values of the middle layer change largely with time (namely sun altitude). This implies that the penetration of direct solar radiation into the canopy is considerably affected by the leaves in middle layer when the sun is high. On the other hand, quite different relatioships between effective leaf area function and sun altitude were observed in the potted barley canopy. The attenuation of direct solar radiation at the low sun altitude was considerable in the upper layer, while the attenuation at the higher sun altitude was considerable in the middle layer (fig. 5). The sun altitude dependence of the mean extinction coefficient for the barley canopy is in good agreement with that for a canopy of leaves with uniform inclination angle of 80°(fig. 6). The percentage of the sunlit area was highest for the ear and that of the stem lowest. The sunlit leaf area index of the barley canopy (Ft=2.8) was about 1.1 when the sun was higher. This was smaller than that obtained in corn field. The sunlit leaf area that is exposed abaxially to direct solar radiation was found to decrease almost linearly with increasing sun altitude except the initial and final parts (fig. 8). Frequency distribution of intensities of direct solar radiation falling on the whole surface was studied to make clear light conditions of each photosynthesizing organ (leaves ears and sheaths) under sunlight. The frequency curves were found to have a weak tendency such that its mode moves toward a weak radiation intensity with increasing sun altitude (fig. 9).