Field experiments were conducted in Yongan (28°09′N, 113°37′E, 43 m asl), Hunan Province, China in the late rice-growing season in 2017 and 2018. The site has a moist subtropical monsoon climate. Average daily solar radiation and mean temperature during the rice-growing season were 12.9 MJ m−2 d−1 and 25.7 °C in 2017 and 15.1 MJ m−2 d−1 and 25.0 °C in 2018, respectively (Fig. 3a,b). The soil of the experimental field was a clay with pH 6.20, organic matter 38.6 g kg−1, available N 168 mg kg−1, available P 18.5 mg kg−1, and available K 183 mg kg−1. Soil tests were based on samples taken from the upper 20 cm layer before the experiment started in 2017.
In each year, two hybrid rice cultivars, Longjingyou 1212 (L1212) and Taiyou 390 (T390), were grown under two hill densities, 25 cm × 11 cm (D11) and 25 cm × 21 cm (D21). The cultivars were selected because they are commonly grown in the study region. The experiment was laid out in a split-plot design using hill densities as main plots and cultivars as subplots, with three replications and a subplot size of 80 m2.
Rice seedlings were raised according to the method described by Huang and Zou17, and 27-day-old seedlings were transplanted with one seedling per hill, using a high-speed rice transplanter (PZ80-25, Dongfeng Iseki Agricultural Machinery Co., Ltd., Xiangyang, China). Missing plants were manually replanted within 7 days after transplanting to obtain a uniform plant population. Fertilizers used were urea for N, single superphosphate for P, and potassium chloride for K at rates of 150 kg N ha−1, 75 kg P2O5 ha−1, and 150 kg K2O ha−1, respectively. The N fertilizer was applied in three splits: 50% as basal fertilizer (1 d before transplanting), 20% at early-tillering (7 days after transplanting), and 30% at panicle initiation. The P fertilizer was applied as basal fertilizer. The K fertilizer was split equally at basal fertilization and panicle initiation. The experimental field was kept flooded with a water depth of 3–5 cm from transplanting until 7 days before maturity. Weeds, pests, and diseases were intensively controlled by chemicals to avoid yield loss.
Ten hills of rice plants were sampled from each subplot and separated into three subsamples: main stems, primary tillers, and secondary tillers. The main stem is the first plant stem. Primary and secondary tillers are those that emerged from the main stem and primary tillers, respectively. Panicle number was counted to calculate panicles per hill and per m2 for each subsample. The panicles were hand threshed, and filled and unfilled spikelets were counted to calculate spikelets per panicle and spikelet filling percentage. Dry weight of filled grains was determined after oven-drying at 70 °C to a constant weight, and grain weight was calculated. Total grain yield was the summation of grain yields (filled grain weights) of main stems, primary tillers, and secondary tillers.
Data were analysed using analysis of variance (ANOVA) and linear regression (Statistix 8.0, Analytical Software, Tallahassee, FL, USA). The statistical model for the ANOVA included replication, cultivar (C), density (D), year (Y), the two-factor interactions of C × D, C × Y, and D × Y, and the three-factor interaction of C × D × Y. Statistical significance was set at the 0.05 probability level.
Source: Ecology - nature.com
