Mid-season nitrogen application strategies for rice varieties differing in panicle size

Application of nitrogen (N) during the mid-season is an important practice in rice production. However, little is known if its timing should vary with the varieties differing in panicle size. We investigated the effectiveness of various mid-season N applications on the grain yield and nitrogen use efficiency (NUE) for rice varieties differing in panicle size. Three high-yielding japonica rice varieties, Yangfujing-8, Lianjing-7 and Huaidao-9 with a small, medium and large panicle size respectively, were grown in the field. Four N treatments during the mid-season, applied at the stages of panicle initiation (PI), spikelet differentiation (SD), heading time (HT) or all the three stages, were adopted with no N application as control. Results show that their effectiveness on grain yield and NUE varied substantially with the variety panicle size. For Yangfujing-8, N applied at PI was the most effective in increasing yield and NUE, while for Huaidao-9, N applied at SD or HT was the most effective. The effectiveness was similar for Lianjing-7 when N applied at PI, SD or N split application at PI, SD, and HT. Nitrogen applied at SD markedly increased crop growth rate during the 15-day period proceeding heading and pre-anthesis non-structural carbohydrate accumulation in the stem at heading. While N applied at HT significantly increased the photosynthesis of leaves. Both treatments of N applied at SD and HT could increase activities of sucrose synthase and adenosine diphosphoglucose pyrophosphorylase in the inferior spikelets of HD-9 at the mid and late grain filling stages. According the results, a mid-season N application strategy is suggested: applying N at PI to increase sink capacity for a variety with a small panicle size, at SD or HT to increase filling efficiency for a variety with a large panicle size, and at either PI or SD for a variety with a medium panicle size.