Comparisons of physiological characteristics related a high drought resistance among rice cultivars

水稻产量主要是水分=限制在高地和灌溉的低地条件下。水稻品种“Suweon”是一个高产的粳稻印度杂交低地品种，即使在干旱生长期的一年中，在高地条件下也显示出高产。在相同土壤干燥条件下，该品种叶片光合速率和干物质生产速率均高于日本高地品种。供试品种间叶水势下降趋势相似。因此，“Suweon”具有较高的耐旱能力。该品种在轻度水分胁迫条件下仍能持续光合作用，气孔导度下降幅度较小，且在白天具有较高的糖浓度。这种高糖浓度降低了叶片的渗透势，并在土壤干燥过程中在较低的叶水势下维持较高的蒸腾速率。为了研究水分胁迫条件下根系活力与叶片生理的关系，比较了反映根系活力的伤流率。叶片伤流率随土壤含水量的降低而降低，供试品种间的下降趋势基本一致。此外，伤流液中细胞分裂素、玉米素核苷和二氢玉米素核苷含量在品种间差异不显著。结合型细胞分裂素是水稻中一种重要的细胞分裂素，有待于进一步研究。高地条件下，愈伤后干物质生产量高的品种，其伤流率高，叶片衰老慢。在这些品种中，愈合后种子根的数量增加。小麦新种子根的形成对于维持整个根系的高活性具有重要意义，是今后高地抗旱高产研究的挑战之一。

Rice yields are primarily water=limited under upland and rainfed lowland conditions. A rice cultivar "Suweon", which is a high yielding Japonicax lndicahybrid lowland cultivar, showed high yield under upland conditions even in a year which had a dry growing period. This cultivar had a higher leaf photosynthetic rate and a higher dry matter production rate than upland cultivars in Japan under the same soil drying conditions. The leaf water potential decreased similarly among cultivars tested. Thus "Suweon" had a high drought tolerant ability. This cultivar could continue photosynthesis under the mild water stress conditions with relatively small decrease in the stomatal conductance, and had a tendency to possess higher sugar concentration during the day time. This high sugar concentration decreased leaf osmotic potential and maintain high turgar under the low leaf water potential during soil drying. In order to examine the relation of a root activity and leaf physiology under the water stress conditions, the bleeding rates which reflect a root activity were compared. The bleeding rates decreased with decreasing soil water content and the decreasing tendency was almost the same among the cultivars tested. In addition, the cytokinins in the bleeding sap, zeatine riboside and dihydrozeatineriboside concentration were not different significantly among cultivars. Conjugated cytokinins, a large cytokinin fruction of rice plant should be determined further. Under the upland field conditions, some cultivars which had a high dry matter production after healing, had a tendency of higher bleeding rate and slower leaf senescence. In these cultivars, the numbers of seminal roots increased after healing. Wheatear the formation of new seminal roots is important for maintaining a high activity of the whole root system will be one of the future research challenges to improve drought resistance and high yielding of upland rice production.