Maintenence of the Grain Growth in Rice (Oryza sativa L.) Subject to Water Stress during the Early Grain Filling

Abstract

The glain growth of rice was found to be highly tolerant to water stress during the poriod of rapid starch seposition, despie of a large reduction in the total dry-matter production. The objective of this study was to examine whether the water stress in the earlier stage of grain growth when the process of cell division and expansion is predominant affects the growth of endosporm as well as final agrain yield per plant. Rice plants (Oryza sativa L. cv. Nipponbare) were grows in 4-liter pots under outdoors conditions. Twenty plants were grown in each pot in which the soil was submerged till ears emerged. All tellers removed as they appeared, only the main culm being kept growing. After the ear emergence, the amount of water applied to 15 pots was reduced for 10 day period inpose the development of defictis in plants (dry treatment 1) Water application was also reduced for another 15 pots between 5 days and 14 days after ear emergence (dry treatment 2). The remaining pots continued to be waterd as before (control). Pots of bith dry treatments were well watered as control after 10 days drying poriod. At intervals during the stress and recovery cycle, the relative water content (RWC) was monitored on the flag leaves between 1300 and 1500 h. During the period between 9 days and 13 days after ear emergence, the measurements of diffusion resistance 9 were taken late in the morning on the flag leaves. The number of spikelets at flowering was noted every day for eavh ear. Plants were harvested at both the being and the end of treatments and at maturity to determani the dry weights of grain, leaf and others. Leaf areas were also determined on these occasions. At the end of dry treatment, the grains which had formed the aleuron, and their dry weight and volume were measured individually. Water deficits, as indicatcd in thc daily trend of RWC, devcloped between 2 days and 10 days after ear emrgence in the plants of dry treatment 1 and between 7 days and 14 days after ear emergence in the plants of dry treatment 2. The renge of RWC in the control was small, between 98 and 100%. Leaf conducatance in plants subjected to desiccation was also reduced, being approximetely 75% of well watered plants. Both RWC and the leaf condutance recovered to same levels as the control plants a days or two after rewatering. Leaf senecence after ear emergence was accelerated in stressed plants, and plants in dry trsament 1 lost green leaves more rapodly than in dry treatment 2. As the flowering of flowring most 1 were subject to wter deficits while they were flowring whereas the watwer stress developed after flowring in the plants of dry treatment 2. Rcduction in both the leaf area and the leaf conductance in stressed plants resulted in less dry matter production than in well watered plants over the period of soil desiccation, but grain growth of the plants, in terms of dry weight, in two dry treatments over this period was not affected at all. Since the dry weight of plant parts other than grain decreased more in stressed plants than in control, the maintenance of grain growth may have resulted from the increased supply of assimilates that had been sttred before anthesis. The result the upper parts of grasins formed the aleuron laver up layer up to 10 days after ear emergence and those of flower of lower parts of ear completed to form it between 10 and 14 days after ear emergence indicates that water deficits developed primarily during the period of repid cell division and expansion in the endosperm of the upper grains of the plants in 1 and in the endosperm of lower grains of plants in dry treatment 2. Thurs, the divison and exapansion of the endosperm cell appeared not to be affected water deficits during its cell development was essentially the same that of control. Since turgor is generally regarded as a primary driving force to control the cell division and expasion, the maintenance of cell division and exapansion may be mediated through the turgor maint