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In the present study, 40 mg L−1 N (NH treatment) and 20 mg L−1 6‐benzylaminopurine (6‐BA; BA treatment) were used to stimulate tiller bud growth in rice (Oryza sativa L.). The hormone changes in the tiller nodes and tiller buds and the N levels in the leaf blades, leaf sheaths, and tiller nodes were measured. The results indicate that N and 6‐BA promoted tiller bud germination and increased the indole‐3‐acetic acid (IAA) and zeatin (Z) plus zeatin riboside (ZR) levels in tiller buds and tiller nodes but decreased the abscisic acid (ABA) levels in tiller buds. Before tiller bud germination, the IAA and Z plus ZR levels increased but the ABA did not change, suggesting that IAA and Z plus ZR but not ABA may be the key regulators of tiller bud growth. From 4 d posttreatment, the tiller bud growth of the BA‐treated plants markedly slowed but the buds of the NH‐treated plants continued to grow quickly. We also found that the N levels in the leaf blades, leaf sheaths, and tiller nodes of the NH‐treated plants increased significantly as compared to those of the control plants (the plants treated with 10 mg L−1 N). However, no significant differences in N levels were observed between the plants of BA and control treatments. This study suggests that N may regulate tiller bud growth in two ways: by regulating N metabolism and regulating endogenous hormones. External 6‐BA promoted tiller bud germination by regulating endogenous hormones, but tiller bud growth was not maintained because it could not increase the N levels in the plants.

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This study aims to quantify nitrogen (N) effect on occurrence of perfect rice kernel (PRK) and imperfect grains which includes white-belly rice kernel (WBRK), white-core rice kernel (WCRK), green rice kernel (GRK), opaque rice kernel (ORK), and other imperfect grains (OTHERS). Two-year field experiments involving six japonica rice cultivars and seven N treatments were performed. The structural differences between white-belly and white-core tissues were compared using scanning electron microscope. Averaged over cultivars, grain yield increased progressively with N rate. PRK increased with N rate in 2008, but decreased with increased N rate in 2009. WBRK and WCRK decreased as N rate increased for both years. High N input resulted in higher occurrence of GRK and OTHERS for both years. Most starch granules in white-belly tissues are intact and surrounded by globular protein bodies, with many air spaces between them; while in white-core tissues, starch granules are easily broken into many single granules and no protein bodies are visible. Our results suggest that N has suppressing influence on chalky grains but favorable effect on other imperfect grains, and indicate different mechanism between WBRK and WCRK.