Studies on Branching Habits in Crop Plants : 4. On the growth of tillering buds after enlargement of space by thinning in thick-sown rice seedlings.

Abstract

This experiment was made for the purpose of studying the possibility of growth in tillering buds which have ceased to grow. Rice seeds were sown relatively thickly, i. e., one seed every 3 cm × 1.5 cm in 1959, and one seed every 3 cm × 1.5 cm or 6 cm × 3 cm in 1961. These rice seedlings were thinned alternatively so that the spaces among the seedlings were enlarged to 6 cm × 3 cm or to 12 cm × 6 cm. Thinning treatments were made at the seedling age 5, 6, 7 or 8 by means of clipping the seedlings with scissors. at the ground surface After that, the growth and the leaf formation of each tillering bud were observed at every seedling age. Moreover, the measurements of respiratory rates were made 2 days after thinning in 1961. During the two years of experiments, the tillering buds which grew surely after each age thinning were as follows: In 1959 (Fig. 2 and Table 3), the 2nd, 3rd, 4th, 5th and 6th tillering buds after the 5th age thinning, the 2nd, 4th, 5th and 6th ones after the 6th age thinning, the 6th, 7th and 8th ones after the 7th age thinning, and the 7th and 8th ones after the 8th age thinning; in 1961 (Fig. 3, Fig. 4 and Table 3), the 5th, 6th and 7th tillering buds after the 6th age thinning, and the 6th, 7th and 8th ones after the 7th and 8th age thinnings. The 2nd and 3rd buds grew after thinning of seedling age 5 or 6 in 1959, but did not grow after thinning of seedling age 6 in 1961. The difference between the results of two years' experiments is thought to be due to the facts that these buds were in the course of growth at seedling age 5 and 6 in 1959, although have ceased to grow at the same age in 1961. The differentiation of leaves on each tillering bud were observed at every seedling age with cross section (Table 1 and Fig. 5). In these observations the author could recognize the following facts; first, even in a bud which has ceased to grow, new leaves are being formed on the growing point though very slowly, second, leaf formation is accelerated by thinning not only in the tillering buds which grow after thinning (ex. the 6th bud in Fig. 5), but also in those which do not grow after thinning (ex. the 2nd and 4th ones in Fig. 5). As regards the respiratory rate, similarly, it was shown to be activated by thinning even in the buds which do not grow after thinning (Table 2). In a previous work, the author found that at the seedling age n in thick-sown rice seedlings, the nth and n-1th tillering buds are in the course of growth, the n-3th and lower ones have ceased to grow. In this experiment, the following facts were observed : If the spaces among seedlings are enlarged at seedling age n in the similar seedlings, the nth and n-1th buds grow surely and normally, the n-3th and lower ones do not grow, and the n-2th bud, which is in a critical stage, grow temporarily or very slightly with only a few exceptions. In those buds that do not grow, however, the formation of leaves are accelerated and the respiration activated. The main factors which inhibit the growth of these buds, in spite of activation of leaf formation and respiration, are going to be studied.