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Optimized nitrogen (N) management (OPT), with reduced total N input and more N applied during panicle development, has been proved to increase grain yield of rice through panicle enlargement. However, the changes in panicle architecture and source of variation are not well understood. A hybrid rice variety named Tianyou 3618 was subjected to OPT and farmer's fertilizer practice (FFP) in early cropping seasons of 2016 and 2017. With 16.7 % less N input, OPT increased panicle size by 8.6 % and 27.4 %, and grain yield by 13.8 % and 12.3 % for 2016 and 2017, respectively. OPT had greater dry matter accumulation and N uptake from panicle initiation to heading, which bolstered panicle enlargement. The number of surviving florets per branch was quite constant under different N treatments for all primary, secondary, and tertiary branches, implying that panicle size was mainly determined by the number of branches rather than the number of florets per branch. Little change was observed between OPT and FFP in differentiation, degeneration and survival of primary branches and their florets. Surviving secondary and tertiary branches and their florets were significantly more under OPT than those under FFP. The increase in surviving secondary branches under OPT resulted from both enhanced differentiation and reduced degeneration. While the increase in surviving tertiary branches under OPT was merely from enhanced differentiation though their degeneration was also dramatically increased. Among the increased differentiated florets under OPT, 32.4%–36.3 % and 61.6%–67.7 % came from secondary and tertiary branches, respectively. Among the increased surviving florets under OPT, 62.2%–65.2 % and 32.5%–37.8 % came from secondary and tertiary branches, respectively. Both secondary branches and tertiary branches were principal contributors to the increase in panicle size of OPT. To our knowledge, this is the first report on the detailed changes in panicle architecture and their involvement in panicle enlargement and yield gain under OPT.

Short basal internodes are important for lodging resistance of rice (Oryza sativa L.). Several canopy indices affect the elongation of basal internodes, but uncertainty as to the key factors determining elongation of basal internodes persists. The objectives of this study were (1) to identify key factors affecting the elongation of basal internodes and (2) to establish a quantitative relationship between basal internode length and canopy indices. An inbred rice cultivar, Yinjingruanzhan, was grown in two split-plot field experiments with three N rates (0, 75, and 150 kg N ha−1 in early season and 0, 90, and 180 kg N ha−1 in late season) as main plots, three seedling densities (16.7, 75.0, and 187.5 seedlings m−2) as subplots, and three replications in the 2015 early and late seasons in Guangzhou, China. Light intensity at base of canopy (Lb), light quality as determined from red/far-red light ratio (R/FR), light transmission ratio (LTR), leaf area index (LAI), leaf N concentration (NLV) and final length of second internode (counted from soil surface upward) (FIL) were recorded. Higher N rate and seedling density resulted in significantly longer FIL. FIL was negatively correlated with Lb, LTR, and R/FR (P 0.05). Stepwise linear regression analysis showed that FIL was strongly associated with Lb and LAI (R2 = 0.82). Heavy N application to pot-grown rice at the beginning of first internode elongation did not change FIL. We conclude that FIL is determined mainly by Lb and LAI at jointing stage. NLV has no direct effect on the elongation of basal internodes. N application indirectly affects FIL by changing LAI and light conditions in the rice canopy. Reducing LAI and improving canopy light transmission at jointing stage can shorten the basal internodes and increase the lodging resistance of rice. Keywords: Internode elongation, Leaf area index, Light intensity, Light quality, R/FR, Light transmission ratio, Leaf N concentration