Fig. 1. Photos and illustrations of spikelet degeneration. The ellipses in the figure highlight the degeneration of spikelets.

Fig. 2. Number of differentiated spikelets and spikelet degeneration at different panicle initiation periods under high temperatures compared with normal temperatures. Data are from the authors’ pot experiment conducted in 2014 with two rice varieties Huanghuazhan and Fengliangyou 6. The experiment included a high-temperature treatment with a peak temperature of 40 ºC (HT) and a normal-temperature treatment with a peak temperature of 32 ºC as the control (NT). The experiment was conducted for 7 d at 35, 28, 21, and 14 d before panicle heading, respectively. Data are Mean ± SD (n = 3).

Fig. 3. Frequency of spikelet degeneration and its variability within panicle positions. The data are from an experiment conducted in 2023. The panicle positions were determined by counting the primary branches from the top to the base and were designated as positions 1 through 30. The panicle was divided into three parts, the upper, middle, and basal parts. The upper part was defined as including the top 1/3 of the branches, the middle part as including the middle 1/3 of the branches, and the basal part as including the bottom 1/3 of branches. A spikelet that appeared white and withered on a panicle was defined as degenerated. Data are Mean ± SD (n = 3).

Fig. 4. Characteristics of spikelet differentiation and degeneration in various rice varieties. The data are from a seeding experiment with the application of 180 kg/hm2 nitrogen in 2020. Seeding dates were as follows: S1, 1 April; S2, 21 April; S3, 11 May; S4, 31 May; and S5, 20 June. Rice panicles of uniform size were sampled at the heading stage, and all existing and degenerated primary and secondary spikelets were counted. The number of differentiated spikelets was determined as the total number of developed and degenerated spikelets. The ratio of degenerated spikelets was calculated by dividing the number of all degenerated spikelets (including both primary and secondary spikelets) at each panicle position by the total number of differentiated spikelets. Data are Mean ± SD (n = 3).

Fig. 5. Physiological pathways underlying positional differences in spikelet degeneration within panicles. Photosynthetic products from the top three leaves of rice plants are transported from the base of the apex of the panicle via the panicle stem. Sucrose transport, facilitated by vascular tissues and plasmodesmata, is regulated by the sucrose transporter SUT. Apical dominance in sucrose utilization may contribute to the growth of the upper part of the spikelet, although the exact relationship is not well understood and may be influenced by BR. The content of BR is higher in the upper part of the spikelet compared with the lower part, and this higher BR content in the upper part of the spikelet may enhance the activity of enzymes involved in sucrose degradation. Similarly, the levels of ATPase and ROS are higher in the upper part of the spikelet, which can promote spikelet growth and development, whereas they are lower in the basal part of the spikelet. Alternatively, a lower frequency of spikelet degeneration may be attributed to programmed cell death, a process regulated by both genes and ROS accumulation. Red font indicates high or increased levels, blue font indicates low or decreased levels, and ‘?’ denotes mechanisms that require further investigation. AOS, Antioxidant system; ROS, Reactive oxygen species; CTK, Cytokinin; BR, Brassinolide; ETH, Ethylene; ABA, Abscisic acid; SA, Salicylic acid; HSPs, Heat shock proteins; PCD, Programmed cell death; PDH, Pyruvate dehydrogenase; ICDHm, Isocitrate dehydrogenase; α-KGDH, Alpha-ketoglutarate dehydrogenase; MDHm, Malate dehydrogenase.

Fig. 6. Effects of different fertilization methods on dry matter accumulation in single spikelets and spikelet degeneration. Data are from authors’ experiment conducted in 2023. The experiment involved three fertilization methods: T1, Application of 100% flowering- promoting fertilizer at 30‒35 d before heading; T2, Application of 50% flowering-promoting fertilizer at 30‒35 d before heading and 50% flowering-preserving fertilizer at 15‒20 d before heading; T3, Application of 100% flowering-preserving fertilizer at 15‒20 d before heading. The total nitrogen application rate was 270 kg/hm2. Data are Mean ± SD (n = 3).