Abstract
This study analyzed the yielding variation factor of sparsely planted rice cultivar “Akitakomachi” in Tohoku region, Japan. For four years (2010–2013), Akitakomachi was grown at densities of 15.1 hills m-2 and 11.2 hills m-2 with conventional cultivation. The grain yield in 2012 was lower than that in the other three years, in spite of the production of many panicles. In 2012, tillering was markedly stimulated, because of the high temperature and solar radiation after transplanting, and heavy precipitation during mid-summer drainage. On the other hand, the leaf color and plant height after the maximum tiller number stage in 2012 was comparably lower than in normal years. The number of spikelets per panicle was markedly decreased in 2012, resulting in 16–18% lower yield than in normal years under both sparse planting densities. There was a significant positive correlation between the spikelet number per panicle and the leaf color at meiotic phase, and the reduction of the spikelet number per panicle and the leaf color at meiotic phase, and the reduction in the number of spikelets per panicle was mainly due to the reduction in the number of spikelets at the secondary rachis-branch. These results suggest that maintaining moderate leaf color after maximum tiller number stage and prevention of decrease in the number of spikelets per panicle are important to achieve high yielding under sparse planting in a cool climate region.
