Excessive nitrogen fertilizer is often applied to achieve higher rice yield in many regions of the world. However, the effect of excessive nitrogen on grain filling of inferior spikelets and its underlying mechanisms are still poorly understood. A three-year field experiment was conducted to reveal the mechanisms by which excessive nitrogen application regulated grain filling. We found that excessive application of nitrogen reduced the starch biosynthesis and grain weight of inferior spikelets but not superior spikelets. Plant hormone detection revealed that both cytokinin (CTK) and 3-Indoleacetic acid (IAA) were reduced by excessive nitrogen at the early stage of grain filling in inferior spiekelts. Quantitative real-time PCR results showed that inhibition of biosynthesis genes led to a decrease in IAA content, while elevation of CTK oxidase genes resulted in a lower level of CTK under excessive nitrogen treatment. Application of exogenous IAA or CTK significantly impaired the inhibitory effect of excessive nitrogen on starch accumulation and grain weight of inferior spikelets. Further enzyme assays revealed that excessive nitrogen application significantly suppressed the enzyme activities of sucrose synthase (SuS), adenosine diphosphate glucose pyrophosphorylase (AGP) and starch synthase (StS) in inferior spikelets. Application of exogenous IAA or CTK to the panicle under excessive nitrogen treatment significantly increased the enzyme activities of SuS, AGP and StS. Taken together, our results revealed that excessive nitrogen application in field-grown rice suppressed grain filling of inferior spikelets by reducing the accumulation of cytokinin and auxin.

在世界许多地区，经常施用过量的氮肥以提高水稻产量。然而，过量的氮对劣质小穗灌浆的影响及其潜在机制仍然知之甚少。进行了为期三年的田间试验，以揭示过量施氮调节籽粒灌浆的机制。我们发现过量施氮会降低劣质小穗的淀粉生物合成和粒重，但不会降低优等穗的粒重。植物激素检测表明，在下质穗灌浆早期，过量的氮降低了细胞分裂素（CTK）和3-吲哚乙酸（IAA）的含量。定量实时 PCR 结果表明，抑制生物合成基因导致 IAA 含量降低，而在过量氮处理下，CTK 氧化酶基因的升高导致 CTK 水平降低。外源IAA或CTK的施用显着削弱了过量氮对劣质小穗淀粉积累和粒重的抑制作用。进一步的酶测定表明，过量施氮显着抑制了劣质小穗中蔗糖合酶 (SuS)、二磷酸腺苷葡萄糖焦磷酸化酶 (AGP) 和淀粉合酶 (StS) 的酶活性。外源IAA或CTK施用于过量氮处理下的穗部显着提高了SuS、AGP和StS的酶活性。综合起来，