Studies on Dry Matter Production in Soybean Plant : V. Photosynthetic system of soybean plant population

Abstract

Photosynthetic system of plant population were investigated with two early soybean varieties, Shirobana-Sai No. 1 and Tachisuzunari, growing in the field with particular reference to its structure, formation process, light regime and photosynthetic properties of leaves composing the system. The results obtained were as follows: (1) Primary branches are classified into two groups, that is, "branches on lower nodes of mainstem" and "branches on upper nodes of mainstem". The former are formed on lower nodes of mainstem in early stage of growth, having relatively many leaves and can form secondary branches. On the contrary, the latter are formed on upper nodes of main stem in later stage of growth, having only a few leaves at the most and cannot form secondary branch. The development of the latter branches begins when the uppermost leaf attains to full size, after a rest period following the termination of forming the former branches. (2) Leaves formed either on mainstem or on those branches develop successively in accordance with order of the positions on stems. Time interval of unfolding of succesive leaves is longer for leaves on lower nodes of mainstem compared with those on middle or higher nodes. The interval is longer for leaves of branches than for those of mainstem. (3) Duration from unfolding to defoliation varies from leaf to leaf. The duration of the leaves of mainstem is moderate for lower leaves, short for middle leaves and long for upper leaves. That of braches, however, is shortest for lowermost leaves and the higher the position of leaves the longer the duration. (4) At early stage of growth, only the leaves of mainstem constitute photosynthetic system of the population. Leaf area density in space is sparse, particularly at the space between rows, resulting in a good light penetration into the canopy. At the beginnig of flowering, the leaves of main stem and those of "branches on lower nodes of main stem" constitute the photosynthetic system. As the result of vigorous development of these leaves, population is luxuriated: leaf canopy is thick and space between rows is also filled up by leaves, though leaf area density is somewhat lower at this space. The top part of photosynthetic system is constituted with upper leaves of main stem, while the botom part is constituted with lower leaves of "branches on lower nodes of main stem" and with aged lower leaves of main stem. Owing to the luxuriance of the population light penetration into leaf canopy decrease considerably. At the middle of ripening period photosynthetic system is constituted with those leaves of main stem, of "branches of lower nodes of main stem", of "branches of upper nodes of main stem" and of some secondary branches. Height and LAI of the population continue to increase after the beginning of flowering, accompaning noticable cnange in spatial distribution of leaf area. At lower half of population leaf area density decreases as the result of defoliation of lower leaves and of upward shifting of some leaves from lower space to upper space. The gap built is partially filled by newly developed leaves of some "branches on upper nodes of main stem" and those of secondary brances. But this replacement is insufficient, and extensive hollow is formed at the bottom of leaf canopy. On the other hand, in upper hallf of population, leaf area density increase remarkably due to formation of new leaves, expansion of existing leaves and upward shifting of some leaves from lower space. Consequently, the photosynthetic system of this stage is characterized by relatively thin but danse leaf canopy under which thick empty layer extends. Light flux, therefore, decreases steepy as it penetrates the leaf canopy. (5) Light condition of leaf during the development varies with the position on main stem on branches. [the rest omitted]