Studies on the Form of Plant in Rice Varieties with Particular Reference to the Efficiency in Utilizing Sunlight. : I. The significance of extinction coefficient in rice plant communities.

Abstract

It has been hitherto assured by many researchers that the efficiency in utilizing sunlight in photosynthesis of plant community plays an important role in its dry matter production. The sunlight transmitting into plant community is generally reduced from its initial intensity, but there is considerable variation in its reduction rate among communities owing to their structural difference, which might cause consequently great influence on the efficiency in its utilization. In this respect, the form of plant in each stand which make up the structure of community could be of most importance as the fundamental factor affecting on the efficiency in utilizing sunlight. From this point of view, the form of plant in rice varieties has been studied since 1959. In the present report, as the first step, the significance of extinction coefficient in rice plant communities was examined in relation to the efficiency in utilizing sunlight and also to the form of plant. In rice plant community, it is well known that the form of plant shows considerable changes accompanied by variety, growth stage, cultivating condition, etc.. The experiment, therefore, was conducted at the period of heading and of about three weeks later, and the changes in growth attributes as well as light factors were measured with the communities of fourteen rice varieties. The results obtained would be described as follows: Light transmission rate (k) showed, in general, an exponential decrease with increase in leaf area index (F), but the considerable variation was found even among the communities of same F. This is considered to prove positively the influence of the form of plant on k. A very close negative correlation obtained between extinction coefficient (K) and F was clear enough to explain that the community with bigger F could be secured only in small K. More, indicating the remarkable influence of K on F when much amout of leaves are produced, the gradient of regression line which was dependent on the earliness of varieties turned steeper for late varieties. Suppose the dead leaves in lower part of community is caused by low illumination near or below the compensation point, the community of a smaller K is expected to have a smaller percent of dead leaves (D) because of its advantage in light transmissibility. This was clearly proved by a very close positive correlation between D and K. Evidence that K might account for the changes in the efficiency in uitlizing sunlight was given when daily increase in dry matter production of each community (ΔW) was plotted against F discriminated on K. Although the data obtained were not accurate enough for further discussion, there showed a tendency to increase both maximum ΔW (ΔW_max) and F (F_opt) correspondent to ΔW_max with decrease in K. This also proved that F_opt is decided dependently not only by intensity of incident sunlight but also by K. From these results mentioned above, K could be grasped as an important index deciding at one time the conditions in both side of light transmission and of light receiving in community, and through which, in due course, affecting its dry matter production. This also denotes that the form of plant must be studied in relation to K. Among the characteristics of rice plant so far investigated, the mean inclination of leaf adjusted by its thickness resulted in a fairly close negative correlation with K. This shows the advantage of community consisted with more erect and thinner leaves. A significant positive correlation of culm length to K might not be originated from its direct influence on K but be derived from its very close negative correlation (-0.739^**) to inclination of leaf.