Plant growth, leaf protein and chlorophyll content, and chloroplast ultrastructure as affected by nitrogen (N) were examined in four rice (Oryza sativa L.) cultivars grown in culture solution under controlled conditions. Increasing N concentration generally increased height and shoot dry weight of all cultivars. Cultivar differences were significant at normal N level (40 ppm). Among cultivars, IR8 was most responsive to increasing N, having the significantly highest shoot dry weight and protein content. Total chlorophyll and protein contents varied with cultivar and N, but chlorophyll a/b ratio remained constant. At the ultrastructural level, chloroplasts had generally well-developed grana and stroma lamellae at 40 ppm N. Chloroplasts at high N had from one to four times as many grana as the N-deficient chloroplasts. Nitrogen deficiency reduced the size of the chloroplast, grana-stroma lamellae and resulted in fewer poorly stacked grana. Increasing the N level (120 ppm) above the normal level did not significantly affect chloroplast size of any cultivar, except for IR8 which had the largest chloroplast. A reduction in the number of starch grains was observed in IR8, but more were present in IR36 under N-deficient conditions. The size of starch grains was not affected by N and did not differ among cultivars. Plastoglobuli appeared to be larger under N-deficient conditions. Nitrogen had no effect on the number of plastoglobuli but cultivar differences existed. The highly N-responsive IR8 (based on dry weight) had the largest chloroplast which increased with N level. The increase in chloroplast size accounted for the increase in both chlorophyll and protein contents and, consequently, dry weight.
