Leaf Area Index, Tillering Capacity, and Grain Yield of Tropical Rice as Affected by Plant Density and Nitrogen Level¹

To study the effects of plant density and N level on tillering capacity, leaf area index, and other growth characteristics and grain yield of flooded rice ( Oryza sativa L.), we planted an improved tropical rice cultivar, ‘IR8,’ two experimental lines, ‘IR305‐4‐12‐1‐3’ and IR127‐ 80‐1‐10’, and a traditional tall tropical cultivar, ‘Peta,’ at three nitrogen levels in puddled soil. All cultivars were transplanted at five spacings and also broadcast. Of the three improved semidwarf rices, ‘IR305‐4‐12‐1‐3’ had the highest tillering capacity. Increasing plant density and nitrogen level increased the leaf area index (LAI) of all varieties at flowering. Broadcast‐planted plots had higher LAI values than transplanted plots. In the sunny dry season, broadcast‐planted ‘Peta’ had the highest LAI value, 14.6. In comparison, ‘IR127‐80‐I‐I0’, a low‐tillering line, had a LAI value of only 7.0. Grain yield increased with increased LAI values in the dry season, except for ‘Peta’ which lodged severely just after flowering. In the cloudy wet season, broadcast‐planted ‘IR8’ had the highest LAI value, 10.2, but it lodged. ‘IR8’ that was transplanted at close spacings did not lodge and it had higher yields. Thus even with an improved cultivar like ‘IR8’, grain yield declines at a high value if the crop lodges.