Effect of Transplanting Plant Numbers per Hill on Heterosis in Hybrid Rice

Rice (Oryza sativa L.)is a staple food crop, with China contributing over one-fourth of global rice production. It provides essential energy and nutrition to approximately 65% of the Chinese population[1]. Hybrid rice has played a critical role in ensuring food security, yielding 15–20% more than inbred cultivars[2]. The best cultivation measures, including transplanting density, number of transplanted plants, and fertilization measures, can achieve the highest yield of hybrid rice.This yield advantage is primarily attributed to increased grain weight, biomass accumulation, and extended growth duration, which enables better utilization of environmental resources such as temperature, light, and heat[3-6]. With the widespread adoption of hybrid rice, optimizing cultivation practices is essential to fully exploit its yield potential. Among agronomic factors, planting density—including inter-plant spacing and the number of transplanted plants per hill—is a fundamental determinant of rice productivity[7]. Recent advancements in agricultural mechanization, alongside labor shortages, have made machine transplanting the preferred method due to its efficiency and control over spacing and transplanting density [8]. The number of seedlings per hill at transplanting is particularly influential, directly affecting rice growth and yield formation [9]. Thus, systematic evaluation of transplanting density is necessary to refine high-yield cultivation strategies for hybrid rice and inbred varieties.

Hybrid rice yield is strongly influenced by planting density, which modulates critical yield traits such as effective panicles per hill (EPN), spikelets per panicle (SPP), seed setting rate (SSR), and 1000-grain weight (KGW). Proper density management balances inter-plant competition and compensation, enhancing yield. Historically, inbred rice yield in China was increased in the 1960s through high-density planting to maximize panicle number per unit area. In contrast, the 1980s saw the adoption of sparse planting for large-panicle hybrid rice, which optimized plant architecture by reducing excessive tillering and increasing panicle size rather than panicle number [10, 11]. The number of seedlings per hill is a key factor in yield optimization. Under constant planting density, an increase in seedlings per hill initially enhances effective panicle formation but eventually reduces yield due to a decline in spikelets per panicle and increased competition[12]. Excessive seedling numbers per hill promote ineffective tillering, reducing panicle productivity and limiting yield gains [13]. Conversely, an appropriate seedling number per hill, coupled with optimal spacing, can improve population structure, minimize ineffective tillers, enhance tiller-to-panicle conversion rates, and optimize yield component relationships [14].

The yield potential of super high-yielding hybrid rice in China has risen from 10.5 t/ha to 15 t/ha [15]. Given the importance of transplanting practices, it is hypothesized that the number of plants per hill significantly affects the expression of heterosis in hybrid rice. Since farmers commonly adopt sparse planting densities, it is crucial to evaluate the impact of transplanting plant numbers per hill under these conditions. This study systematically compares key physiological and yield traits indifferent hybrid rice genotypes, their parental lines, and inbred varieties, clarifying the role of transplanting density in the expression of heterosis. The findings provide a theoretical basis for breeding, high-yield cultivation, and the mechanization of hybrid rice production, contributing to food security.

TABLE 1 MAIN RICE VARIETIES USED IN THE EXPERIMENT Male steril Restore Super Variety Certification Breeding unit line line rice National approved Hunan Hybrid Rice Research LYP9 P64s R9311 1st 2001001 Center National approved Hunan Hybrid Rice Research YLY1 Y58s R9311 2nd 2008001 Center National approved Hunan Hybrid Rice Research YLY2 Y58s YH2 3rd 2013027 Center National approved Hunan Hybrid Rice Research YLY900 Y58s R900 4th 2015034 Center National approved Rice Research Institute, HHZ 2007018 Guangdong National approved XWX17 Hunan Rice Research Institute 2008035 TABLE 2 MAIN SOIL PROPERTIES OF EXPERIMENTAL PADDY FIELD Soil sample Total N(g/kg) Total N(g/kg) Total N(g/kg) pH Organic matter (%) 1 1.75 0.93 9.93 5.7 2.92 2 0.79 8.14 1.53 6.0 2.89 3 0.63 10.69 1.17 5.8 1.99 4 0.78 10.13 1.91 6.5 3.75 5 0.73 10.98 1.91 6.4 3.94 6 1.74 0.91 10.39 6.4 3.52 Mean 1.67 0.80 10.04 6.13 3.17 TABLE 3 MAIN RICE VARIETIES ARRANGEMENTS AND GROWTH PERIOD IN THE EXPERIMENT Sowing Transplanting Heading Milk ripening Mature Whole growth Year Variety date date date date date period 2016 LYP9 2016/5/28 2016/6/22 2016/9/13 2016/9/15 2016/10/4 129 2016 YLY1 2016/5/28 2016/6/22 2016/9/13 2016/9/15 2016/10/4 129 2016 R9311 2016/5/28 2016/6/22 2016/9/13 2016/9/15 2016/10/4 129 2016 YLY2 2016/5/28 2016/6/22 2016/9/13 2016/9/15 2016/10/4 129 2016 YH2 2016/5/28 2016/6/22 2016/10/4 2016/10/5 2016/10/28 153 2016 YLY900 2016/5/28 2016/6/22 2016/9/13 2016/9/15 2016/10/4 129 2016 R900 2016/5/28 2016/6/22 2016/10/4 2016/10/5 2016/10/28 153 2016 HHZ 2016/5/28 2016/6/22 2016/8/29 2016/9/1 2016/9/25 120 2016 XWX17 2016/5/28 2016/6/22 2016/8/29 2016/9/1 2016/9/25 120