Abstract
On the basis of the results of the experiment given in the Report Nos. I-VIII, the author made an analytical study of the resistabilities of paddy rice varieties to unseasonable low temperatures, at the same time showing the relations between them and the phenomenon of low temperature injury, and also making observations of the possibilities of breeding resistent varieties, and methods of determining the resistabilities to low temperature injury. A summary of our findings is given below. (1) The mechanism of the resistability of a paddy rice variety to unseasonable low temperature is very complex, a number of factors taking part in it, but it can be roughly divided into insensibility to cold and insensibility to the blast disease, or analyzed into the three factors of environmental adaptability, resistability to cold and resistability to the blast desease. It is thought that resistability to coid and environmental adaptbility take part in insensibility to cold, while resistability to the blast disease and environmental adaptability go into insensibility to disease. (2) The mechanism of the resistability of paddy rice to cold varies according as its organs and tissues differ during the process of its growth. The function of forming primordia of stamens and pistils, function of forming generative cells and matured pollens etc. are remarkably sensitive to injury by cold, and there exists an inter-varietal differenee in this resistability to cold. (3) The insensibility of a paddy rice variety to cold is determined by a combination of the difference in the degree of decline of the physiological function of the plant body due to low temperature and the difference in the resistability of the organs and tissues to cold, and the insensibility of a paddy rice variety to cold in its broad sense can be represented, for pnctical purposes, by the insensibility to cold at the stage of formation of generative cells. (4) The mechanism of injury by cold and that of injury by cold water in a paddy rice variety is not always identical, but generally speaking, insensibility to cold and insensibility to cold water among varieties are practically shown in a parallel relation (Ref. Table 1). The inseiasibillty of a paddy rice variety to cold water which is expressed in injury to fertilization or per-centage of decrease in the weight of refined grains due to long-term irrigation with cold watet can, from a practical point of view, be considered as showing its insensibility to cold. (5) There exists an inter-varietal difference in the degree of decline of the physiological function of the plant body due to adverse weather, i.e. in the environmental adaptability, which is thought to play an important role in the insensibilities of a paddy rice variety to cold and to disease. (6) The injury of a paddy rice variety due to unseasonable low temperatures manifests itself as the composite results in which its resistability to low temperature and cultural environments take part. We made an inquiry into this relationship, about the main varieties used in this study by comparing the resistabilities to low temperature as shown by an experimental inquiry with the injury that actually took place in unseasonable low temperature years. (7) Different genetic factors take part in the iasensibilities of a paddy rice variety to cold and to blast (Ref. Table 2), and therefore it is thought possible to breed a variety possessed of these two characters, i.e. a variety both highly adaptable to environments and highly resistant to cold and to blast. (8) Different genetic factors take part in the resistability to unseasoual low temperatures, time of maturation and yieldability of a paddy rice variety (Ref. Table 2), and it is inferred therefrom that the breeding of a variety highiy yielding and highly resistant to unseasonal low temperatures is possible. (9) As a method of determining the cold resistant variety in plant-breeding experiment, a comparative study of the per-cent
