High nitrogen reduces seedling survival after submergence in rice

Early flooding that persists for several days usually reduces the survival of rice seedlings in farmers' fields. Ensuring proper nutrient management early in the season could help improve seedling survival. The authors attempt to evaluate the effect of early nitrogen (N) management of seedling survival after a few days of flooding. Three experiments were conducted in the greenhouse using one tolerant (FR13A) and one intolerant (IR42) cultivars with 3 levels of N treatments: no N, N at planting (early), and N at 14 d after seeding (late). Seedlings ( 21 d old) were completely submerged in concrete tank for 11 d. Leaf, root, and stem carbohydrates contents were measured before and after submergence and plant survival was recorded 21 d after submergence. Nitrogen content of leaves measured just before submergence significantly increased with both early and late N treatments. Survival of both cultivars decreased significantly with N applications, yet, FRI3A showed higher survival under all treatments. There was a negative correlation (r=-0.54) between leaf N content and plant survival. Plant survival correlated positively with soluble sugar contents of leaves, roots and stems (r=0.91, 0.68, and 0.87, respectively), and with starch content of stem (r=0.89). Chlorophyll content, measured 3 d after desubmergence decreased significantly in IR42 regardless of N treatment. With early or late N treatment, photosynthetic gas exchange and root/shoot ratio decreased substantially in both cultivars. The negative effect of high N on root growth may have enhanced susceptibility of rice seedlings to flooding and decreased seedling survival. This study showed that seedling survival could be enhanced in areas where untimely flooding is anticipated by avoiding undue N application early in season