Proportional relationship between spikelet number and dry weight at heading, rather than with nitrogen content, more accurately reflects cultivar differences in spikelet production efficiency in rice

Spikelet number per unit area is the most important yield component of rice (Oryza sativa L.). While spikelet number in rice is often related to total N content in whole plant, several studies have reported that it is more strongly correlated with dry weight (DW). The aims of this study are to identify the most critical factor determining spikelet number and to evaluate cultivar differences in spikelet production efficiency. In a simple yet unique experiment, we investigated the relationships between spikelet number, DW, and N content of the same plants at heading stage in 27 cultivars. In all cultivars except Tsukisuzuka, with an extremely short panicle, DW at heading stage had a clear proportional relationship with spikelet number. N content had no consistent relationship with spikelet number. The constant of the proportionality between DW and spikelet number estimated from our 2-year dataset at a single site agreed with that estimated from multisite data in previous studies. We concluded that DW at heading was the most critical factor determining spikelet number. Its proportional relationship can be used as a simple model for predicting spikelet number. The proportionality constant reflected cultivar differences in spikelet production efficiency. This study not only settles the question of the most critical factor determining spikelet number, but also efficiently evaluated cultivar differences in spikelet production efficiency, which we expect to contribute significantly to regulating spikelet number in rice.