Abstract
The mechanism of tuber formation in potato plant may be of great scientific interest and practical value. In the previous paper, the author has reported that small tubers were formed spontaneously on the stem segments under the sterile condition, and that this tuber formation was supposed to be concerned with the concentration of sucrose in the nutrient medium. In the present investigation, studies on the influences of environmental conditions on the tuberization of stem segments of potatoes were undertaken in vitro from the view point of the auxin physiology. The experimental results obtained may be summarized as follows : (1) The variation of the cultural temperature and, of pH value of the nutrient medium have certain influences on the tuber formation of stem segments. Namely, when the stem segments of etiolated sprouts were cultured at 23°C and pH 6.0, new small tubers were formed most easily. It could hardly be deduced, however, that these tuber formations may take place only within the limits of the environmental conditions described above. (2) The apical pieces of the sprouts which were taken just after the germination of the potato tubers did not formed any new tubers in vitro, regardless the sugar concentration in the nutrient medium. With respect to the tuber formation on the stem segments of the etiolated sprouts, on the contrary, the tuber formations were inversely related to the distance from the apex at which the stem segments were obtained. (3) In the apical tips of the sprouts at the early stage of growth, high contents of auxin, especially in a free form, and low activities of IAA oxidase were found, which in turn let us assume that the highly enzymatical conversion of tryptophan to IAA might take place there. In the stem segments which were obtained from the etiolated sprout, less amounts of free auxin and high activities of IA-A oxidase were found and in this case the reverse seemed to hold for the auxin balance as compared to the former case. (4) It seems quite reasonable to assume from the facts stated above that the sprouts and tubers may be the controling factor for the tuber formation on the stem segments of potatoes due to the auxin balance in them.
