Studies on the Physiology of Rice. VI. Effect of Photoperiod on Development of the Shoot Apex

Abstract

1. Three-week-old seedlings of a winter variety (Rupsail) of rice (Oryza sativa) were subjected to 8-hour photoperiods in 24-hour cycles for 1, 2, 3, or 6 weeks. In another set of experiments a week of natural days was interposed in the third, fourth, or fifth week of the 6 weeks of short-day treatment. Plants were otherwise maintained on natural daylengths longer than the critical one. 2. Short days induced earliness accompanied by increased tiller production. With 1 week of short-day exposure none of the plants flowered, but with 2 weeks 3%, with 3 weeks 67%, and with 4 weeks 100% of them did so. 3. The interposition of natural daylengths during exposure to short days had a delaying effect on the flowering of the primary axis; the earlier the interposition, the greater was the delay. Flowering of the tillers was also adversely affected. 4. Though 4 weeks of short-day exposure were sufficient for 100% flowering of the primary axis, 2 weeks more were needed to induce flowering of the tillers. The tiller buds formed during the period of short-day exposure received the stimulus and flowered earlier than did the late tillers. 5. The vegetative apex of rice is very small and extends only a few millimeters above the soil. With short-day treatment the apex elongates and is pushed up from the soil level. Prior to floral initiation there is a depression of the tip associated with the development near the epidermis of groups of specialized cells with large nuclei and rich in protoplasm. These groups of cells bulge out and later develop into branches of the inflorescence bearing more than one spikelet. Stages of transition from the vegetative apex to inflorescence development are illustrated. Changes in the shoot apex take place in the third week of photoinductive treatment. All plants at a given stage of growth are not equally sensitive. 6. The flowering behavior of rice as influenced by photoperiods is discussed.