GRAIN-YIELD PERFORMANCE OF RICE GENOTYPES AT SUBOPTIMAL LEVELS OF SOIL N AS AFFECTED BY N UPTAKE AND UTILIZATION EFFICIENCY

Field screening trials were conducted in two dry seasons to assess var iability in grain yield, N uptake and N utilization for 180 rice genot ypes, mostly lowland indica improved varieties or elite breeding lines of different growth durations [60 each of early (100-110 days), mediu m (111-119 days) and late (120-135 days)], without the addition of N f ertilizer and to identify genotypes with the potential to produce high yields at suboptimal N levels through efficient uptake and/or utiliza tion of N. Available soil N was lower in year 2 (20 kg ha(-1)) than in year 1 (64 kg ha(-1)). In both years significant differences in grain yield, N uptake and N-utilization efficiency (NUE) were observed amon g genotypes within each growth-duration group. Genotypes varied in the ir response to change in available soil N. The average increase in gra in yield for each kilogram increase in N uptake was 61.9 kg in year 1 and 82.7 kg in year 2. However, some genotypes absorbed similar amount s of N but produced different grain yields and/or total dry matter. So me genotypes with similar harvest index exhibited significantly differ ent NUEs - e.g., IR50363-61-1-2-2 (NUE=64.4 in year 1 and 71.9 in year 2) and IR51009-155-2-3-3 (55.3 and 60.8) in the early-duration group; BG380-2 (70 and 78.8) and IR50391-100-2-3-3-2 (56.3 and 58.9) in the medium; and IR27325-63-2-2 (69.7 and 69.8) and BR51-46-1-Cl (57.9 and 60.5) in the late. Those with higher NUE had lower percentage straw N at maturity. Grain N concentration also affected NUE, but the coeffici ent of variation in percentage grain N among genotypes was less than t hat of percentage straw N. The relative performance of genotypes in te rms of NUE was more consistent than plant N uptake, based on rank corr elations between the two trials. High N uptake and NUE were observed i n IR13429-150-3-2-1-2 (NUE 65.4, N uptake 9.1 g m(-1)) in the early-du ration group, IR44 (NUE 67.2, N uptake 8.3 g m(-2)) in the medium-dura tion group and IR39323-182-2-3-3-2 (NUE 64.8, N uptake 9.3 g m(-2)) in the late-duration group. The study identified genotypes which may pos sess promising traits for improved N uptake and utilization efficiency . Further studies are needed to elucidate the mechanisms involved in h igh N uptake and N-use efficiency.