Effect of Gravity on Intercalary Growth in Rice Plant

The author found the fact in 1965 that a remarkable acceleration in elongation was brought about in the internodes of the paddy rice plants, in which the ninth foliage leaf was just emerging, if they were horizontally kept with their main axes for a six-days period. This fact was repeatedly observed in the plants treated similarly in 1967, in which the eighth foliage leaf was emerging about a half-length. The phenomenon of acceleration in intercalary growth of the horizontal stem was seen on the upper side as well as the lower side of the internode concerned, and was most conspicuous in the eighth internode, i. e. the one between the seventh node and the eighth, sofar as both experimentswere concerned, with the result that the length of the internode measured along its center line extended from 4.2 to 6.2 times as longas that of the control. Besides, the geotropic bending was occurred at the base of the eighth internode. From microscopical observation on the longitudinal sections of the eighth internode, it was clearly demonstrated that pith cells in a layer showed little difference in number on both upper and lower sides in the plants horizontally kept, and amounted to nearly three times as many as those in the control. The mean length of the pith cells was twice in the former as long as that in the latter. 3-Indoleacetic acid (IAA) given to the leaf sheath right bellow the apical one in the rice plants in a normal position, which were already in the stage of intercalary growth, showing the length of 19 mm or more in some of their internodes, influences so as to accelerate the intercalary growth. In the plants in a horizontal position, nevertheless, IAA exercised an inhibiting effect rather than a stimulating on the intercalary growth. Putting these evidences together with those obtained from the horizontal treatment, it may be safe to say that unilateral stimulation of gravitational pull to rice plants brings about not only an asymmetrical distribution of natural auxins existing, but also a renewed formation of them