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The adverse effect of elongation growth and the beneficial effect of carbohydrate supply on tolerance to complete submergence for 10 days were evaluated in five kinds of transplanted rice seedlings, with plant ages of 2.0, 2.3, 2.5, 2.7, and 3.4 in leaf number in 1993 and 1994. The percent survival of seedlings with a plant age of 3.4 was lowest, and seedlings with a plant age of 2.5 showed the highest level of submergence tolerance among the seedlings in both years. The percent survival of seedlings positively correlated (r=0.951*) with the increase in shoot dry weight during submergence. In contrast, a negative correlation berween elongation growth (% of nonsubmerged seedlings) and percent survival was observed in each year. When the length of leaf sheath was longer than that of the leaf blade under submergence, the percent survival decreased and the percent survival in both years was reflected by a regression line (r=0.757*). Furthermore, a strong positive correlation (r=0.998***) was found between the ratio of increase in shoot dry weight to length of leaf sheath at the end of submergence and the percent survival. These findings suggest that the difference in submedrgence tolerance among seedlings is due to the difference of elongation growth, depending on the developmental stage of a leaf and the supplied carbohydrates to the growth zone of elongating leaves.

Rice seedlings (Oryza sativa L.cv.Sasanishiki) with plant ages of 2.0, 2.3, 2.5, 2.7, and 3.4 in leaf number were transplanted and completely submerged for 0, 5, and 10 days. Plant length and total root length were measured 10 days after transplanting (DAT), and the percent survival and total dry weight of the survived plants were recorded at 25 DAT to evaluate the difference in submergence tolerance among the seedlings. When the seedlings were submerged for 5 days, all survived. When they were submerged for 10 days, however, the elongation of plant length was promoted and growth of the roots was conversely suppressed. The percent survival of the seedlings with a plant age of 2.0-2.7 ranged from 53% to 81%, and it was higher than the 22% of seedlings with a plant age of 3.4. In particular, the percent survival of seedlings with a plant age of 2.5 was highest among seedlings. When endosperm reserves were removed from the seedlings just prior to transplantation, the reduction of elongation growth and the percent survival was hardly observed in the seedlings, except for those with a plant age of 2.0. The results indicate that the difference in percent survival among the seedlings was not caused by the amount of the residual endosperm reserves at the start of submergence.

To examine the factors affecting the emergence and establishment of rice seedlings, we tested FAO-designated germ plasm 207 cultivars (indica rice 137 cultivars, japonica rice 56 cultivars, and unexplained ecospecies 14 cultivars) collected from throughout the world. In the first experiment, the seeds were sown in nursery soil at a seeding depth of 3.0cm. Seeded pots were kept in a greenhouse at 30/25°C for 21 days. The ratio of the established plants (REP) to total plants was measured at 21 days after seeding. As a result, 13 cultivars, 6.3% of the total tested showed the best REP, i.e., 100-80%. These include 7 indica types and 6 japonica types of rice. In the second experiment, 75 cultivars were selected as the materials. The plants were grown under the same conditions as the first experiment, but the seeding depths were 1.0 and 3.0 cm. The chlorophyll contents of the second leaf blades and REP were measured at 2 and 3 weeks after seeding, respectively. The REP showed no correlation to seed weight, whereas correlation existed between REP and the values of chlorophyll contents of the second leaf blades at 1 cm and 3 cm seeding depths 2 weeks after seeding. In conclusion, seedling establishment seemed to be affected by the rate of increase in the chlorophyll contents of the leaves.