Effects of planting density and topdressing at the panicle initiation stage on spikelet number per unit area

Abstract

Dense planting is an important cultural technique to increase the number of spikelets per unit area, and is thought to be favorable for grain filing because it increases the percentage of spikelets on the primary rachis-branches (PBs) to the total spikelets. The objective of this experiment was to analyze the relationship between planting density and spikelet number with reference to the percentage of spikelets on the PBs. An active tillering cultivar, Nipponbare was used. Four planting densities (11.1 to 44.4 hills m-2) were combined with four rates of nitrogen topdressing (0 to 7.5 g m-2). Denser planting increased the number of differentiated and final spikelets. The effect of the dense planting on spikelet number was more prominent, the higher the rate of nitrogen application at panicle initiation. The dense planting increased panicle number per unit area, but decreased the number of differentiated PBs per panicle. The number of differentiated secondary rachis-branches per PBs and the percentage of spikelet degeneration were scarcely affected by the density. Contrary to the expectation, the denser planting did not increase the percentage of spikelets on the PBs. This is because the number of differentiated secondary rachis-branches per differentiated PB was not changed by planting density and the degeneration of spikelets on the PBs increased with increasing planting density. This experiment suggests that dense planting does not necessarily increase the percentage of spikelets on PBs.