Abstract
This study was undertaken to elucidate the growth process of young panicle of rice. The growth of each spikelet was investigated in relation to the positional differences and was compared with that treated with thinning and top-dressing at the end of spikelet differentiation stage (16 days before heading). Three spikelet positions, the upper (T-1), the middle (M-2) and the lower (B-2'), were selected for this experiment (Fig. 1a, b). The results obtained are summarized as follows: 1. In controls, the length of lemma, empty glumes and the width of spikelet at maturation decreased on lower position. But there was little differences in the length of anther and pistil, and the diameter of pollen among the spikelets at different positions on panicle (Fig. 2, Table 3 and 4). 2. The growth of each spikelet proceeded succesively from the upper to the lower position on panicle (Fig. 3). 3. Interrelationships of the growth of each spikelet organ were almost the same in all spikelets on panicle. Lemma, anther and pistil in a spikelet began to grow simultaneously. Marked growth occurred in lemma from 16 to 8 days before flowering, in anther and pistil from 16 to 3 or 2 days before flowering, and in the width of spikelet from 16 to 6 days before flowering (Fig. 4). 4. Regardless of spikelet position on panicle, meiosis of pollen was found to occur in the anther of the same length. However, the lemma length at meiotic stage in the upper spikelet was different from that in the lower one. Meiosis of the upper spikelet occurred when it was smaller than the lower spikelet (Fig. 2 a, c). 5. When the nutritive conditions were improved by thinning and top-dressing, the length of panicle, the number of spikelet, the length of primary branches and the diameter of rachis increased. The increase of the spikelet number was due to the decrease of degenerative spikelets (Table 1 and 2). 6. The lemma length of middle and lower spikelets, the length of empty glumes and pistil of all spikelcts, were increased by the treatment. The anther length and the pollen diameter in all spikelets were not increased and the growth period of anther in the lower spikelet was shortened by the treatment. The growth of the upper spikelet was not promoted by the treatment (Fig. 5, Table 3 and 4). Thus, the spikelet organs, which seemed to be differentiated in various manner from the apex, showed the different responses to the treatment. From these results, it is assumed that the positional differences in the spikelet growth were not only dependent upon the distribution of translocated substances but also upon the physiological specificity of each organ of the spikelet.
