High nitrogen input causes poor grain filling of spikelets at the panicle base of super hybrid rice

Abstract It is generally accepted that poor grain filling of rice plants under high nitrogen (N) input is due to increased competition for assimilates because high N rate usually results in large sink size with more spikelets per panicle. In this study, we examined grain filling parameters of spikelets in different panicle position under various N treatments to determine the effect of N rates on grain filling. A super rice cultivar with large panicle was grown under field conditions in Wuxue County, Hubei Province, central China in 2013 and 2014. Fully filled spikelet percentage measured at the crop level from 12 hills decreased by 10.9% when the total N rate increased from 0 to 250 kg ha−1, whereas spikelets per panicle remained almost constant across the N rates. At the individual panicle level, reduction in grain filling was evidenced by decreased fully filled spikelet percentage, fertilized spikelet weight, and grain plumpness, and increased partially filled spikelet percentage in the bottom part of the panicle as total N rates increased. Reduction in these grain filling parameters under high N input was relatively small in the middle part of the panicle and was not appeared in the top part of the panicle. Furthermore, high N rates at both mid-tillering and panicle initiation reduced fertilized spikelet weight in the bottom part of the panicle without an increase in spikelets per panicle. Our results suggest that the spikelets in the bottom part of the panicle were mainly responsible for poor grain filling under high N input and such high N-induced poor grain filling was not due to increased spikelets per panicle.