Abstract
This study was undertaken to clarify the effects of nitrogen application rate in basal to top dressing at panicle formation stage and planting density on the relative development of sink (number of spikelets per m^2) and source (Leaf Area Index [LAI]at heading time), that is, sink/source ratio. We also discussed the relationship between the sink/source ratio and the dry matter production or yielding ability for the establishment of a stable high-yielding cultural method, using young seedlings, in the southern district of Japan. The results are summarized as follows : 1) Sink/source ratio changed mainly depending o the amount of source, not on that of sink. As LAI was increased in proportion to planting density, the higher the planing density was, the lower was the sink/source ratio was less, and it was little changed among plots applied rate in basal dressing on the sink/source ratio was less, and it was little changed among plots applied 20-100% nitrogen in basal dressing, except the non-basal nitrogen application plot, in which sink/source ratio was increased because of less development of leaf area. Though sink/source ratio of the plots bearing more than 3.5 million spikelets per m^2 was about 50-80 (spikelets/dm^2 leaf area), it was noteworthy that the sink/source ratio did not decrease according to increase of the sink. 2) The effect of planting density on the yield was markedly greater than that of nitrogen application rate in basal dressing. With increasing the planting density, the higher brown rice yield obtained was due to both superior percentage of ripened grain and 1,000 grain weight. 3) The sink/source ratio showed significant negative correlation with the top dry weight at heading time, but not any significant correlations with the amount of absorbed nitrogen until then or dry matter production during the grain-filling period. Brown rice yield level in the plots with high sink/source ratio, viz., non-application of basal nitrogen or sparse density, was considered to be lower than that in the plots with lower sink/source ratio at the same sink level, because of the inferior percentage of ripened grain through the less amount of reserved carbohydrate in leaf sheath and culm per unit sink (=per spikelet) at heading time.
