作物群落の計量植物学的研究 : 第2報 品種・生育段階による水稲の群落構造の変化

Abstract

Canopy structure (geometrical structure) that plays an important role in light interception and air mixing was measured with two rice cultivars: Manryo and IR-8: cultivated at rectangular spacing of 30 and 17.4 cm. The silhouette method was used for obtaining phytomeirical data of rice canopies. The phytometrical measurements were repeated at intervals of about two weeks during the rice growing season from JuIy to Sept.28 in 1971. Results can besummarized as follows: 1. Fig.2 shows that there is no pronounced difference in the profile of leaf area density between the two cultivars, despite the difference in absolute value of leaf area density. The leaf area density profties for these cultivars were found to be a grass with its maximum in middle layer of the canopy. The height at which the maximum in leaf area density is observed moved gradually upward with the development of rice canopy. The such movement was somewhat marked in Manryo canopy comparing with IR-8 canopy, this was due mainly to the differenc in light penetration between these two canopies. 2. The spatial distributions of leaf area density around the hill are shown in Fig. 3. The leaf area density decreased drastically with increasing the distance from the center of hill, implying that the almost all of leaf area was within the confines of cylinder with the diameter.of 20cm. In order to characterize the dispersion of spatial distribution of leaf area density, the values of kurtosis were calculated as a measure of disprsion. During the whole growing season of rice crops, the values of kurtosis obtained were larger than 3.0 for normal distribution curve. The values of kurtosis reached the maximum at the stage with largest leaf area index and were 3.48 and 4.31 for Manryo and IR-8 canopies, respectively. 3. Although the leaf area distribution of the two cultivars was non preferential as to azimuth angle throughout the growing season (see Fig. 5), the leaf area distribution with respect to inclination angle was manifest to be far from the uniform distribution and to be a distribution in which vertical leaves are most frequent. The leaf area inclination index, XL, calculated from Eq. 9 is shown in Table 1. The values of XL were less than 0 throughout the growing period, indicating that the leaf distribution is preferential in the large inclination angle. Except the early stage of rice growth, IR-8 canopy consisted of strongly vertical leaves comparing with Manryo canopy. Manryo - canopy shifted clearly from the Vertical distribution to the near spherical distribution at the heading stage, whereas IR-8 canopy was consistently most frequent in vertical leaves. Fig. 7 shows that such a difference in canopy structure is due mainly to varying leaf angle with depth in the Manryo canopy. Before the heading time, Manryo canopy had upper strate composed of vertical leaves and becoming more horizontal in display with increasing depth and might be more efficient in light utilization. Manryo canopy, after the heading time, however, showed the quite inverse leaf arrangement. Such clear change in leaf arrangement should give very strong influences on light utilization in photosynthesis. 4. Comparison of the leaf area inclination index with the kurtosis of spatial distribution of leaf area density revealed that there exists negative correlation between them (Fig. 8).The gathering type arrangement with kurtosis larger than 3.0 composes more strongly vertical leaves and operates to make light distribution in the canopy uniform. This result agrees well with that obtained by Tsunoda. It is concluded from the results described here that the studied two rice cultivars, particulary IR-8 have advantage of rice cultivars with dispersing type arrangement in canopy structure.