Plant topping effects on growth, yield, and earliness of field-grown cotton as mediated by plant density and ecological conditions
Introduction
Cotton (Gossypium hirsutum L.) owns an indeterminate growth habit with strong apical dominance. Manual removal of the main-stem growth tip (manual topping) has been widely adopted in China and other cotton-growing countries with abundant agricultural labor (Dai and Dong, 2014) because it effectively breaks apical dominance and increases the partitioning of assimilates to reproductive organs, leading to more squares, flowers, bolls, and lint yield (Hosny et al., 1995, Li et al., 2006). However, manual topping is labor intensive, and meeting labor needs has become a great challenge because rural laborers are increasingly migrating to cities and towns to work in secondary and tertiary industries in the rapid urbanization of China, greatly reducing both the quantity and quality of rural labor (Dai et al., 2017a). Therefore, the development of a new plant topping method that is not labor-based to replace manual topping has attracted great attention.
Chemical topping may be a promising alternative that uses plant growth regulators to inhibit apical dominance in crop plants (Ren et al., 2013, Rosolem et al., 2013, Siebert and Stewart, 2006). Research on chemical topping has been conducted since the 1960s (Cathey et al., 1966, Sill and Nelson, 1970, Su et al., 2012, Uhring, 1971, Zhu et al., 2020). Chemical suckercides are used to control axillary shoot growth and to maximize yield in tomato (Gianfagna et al., 1998, Logendra et al., 2004), red pepper (Kim and Ho, 1980), and tobacco (Mahmood et al., 2007, Taylor et al., 2008). They also effectively inhibit sucker formation and repress the expression of genes in auxin and cytokinin signaling pathways that are involved in axillary shoot formation in tobacco (Singh et al., 2015). Chemical topping with plant growth regulators has also been used in cotton. Kang et al. (2015) found that chemical topping reduces plant height and number of fruiting branches, shortens the length of fruiting branches and petiole from internode 1 to internode 6, and increases the yield compared with non-topping. Similar increases in lint yield and yield components with chemical topping compared with non-topping are reported in full-season cotton under single cropping (Li et al., 2018, Ren et al., 2013) and in short-season cotton under double cropping (Liu et al., 2019, Yu et al., 2021). There are other benefits of chemical topping in cotton, including reduced labor inputs (Dong et al., 2017a), improved seed quality (Ge et al., 2015), and compacted plant type (Zhao et al., 2019, Zhu et al., 2020). However, according to the study of Ye et al. (2017), although chemical topping increased the number of lower and inner within-plant bolls, it did not increase boll weight, yield, or lint percentage compared with those with manual topping or non-topping. In our previous research, yield decreased with chemical topping compared with manual topping under low plant density (Dong, 2019). The differences in chemical topping efficacy suggest that it may be affected by other factors (Liu et al., 2019, Ye et al., 2017). Therefore, the factors that influence the effects of chemical topping in cotton need to be identified and studied.
Application of plant growth regulators, such as mepiquat chloride, to regulate plant growth and development has long been widely adopted in cotton (Li et al., 1991). The effect of mepiquat chloride can vary with plant density (Dong, 2019, Zhao et al., 2019) and ecological conditions (Rosolem et al., 2013, Ye et al., 2017). Therefore, we hypothesized that the efficacy of chemical topping is also mediated by planting density and ecological condition. These factors may be the most important reasons for the differences in chemical topping effects reported previously. Thus, we conducted a three-year field experiment with a split-plot design with plant density as the main plot treatment and plant-topping mode as the subplot treatment in different ecological regions of China to study the effects of plant topping. The objectives of the study were to determine a) the effects of plant topping, planting density, and ecological condition on seed cotton yield and net return; b) whether plant density or ecological condition mediated the efficacy of plant topping. This study will further expand the application of plant growth regulators in cotton production, which has important significance or reference for China and other cotton-producing countries in the world.
Section snippets
Experimental site and cultivar
Field experiments were conducted in three sites with different ecological conditions (Table S1) in China from 2016 to 2018. Site 1 was in Hutubi County (44°68′N, 87°12′E) in Xinjiang in the Northwest inland cotton-growing region. The experimental area is in a warm-temperate arid zone and has a continental climate. The average annual precipitation is 184.2 mm, the frost-free crop growth season is 165 d, and evaporation is 1850 mm. Annual effective accumulated temperature is 3443 °C. Xinluzao 64,
Yield, yield components, and earliness
Seed cotton yield was affected by the interaction between topping mode and plant density in each experimental site, but the interaction was not affected by experimental year (Table 1, Table 2, Table 3). Averaged across the three years, at low plant density, chemical topping reduced seed cotton yield by 4% at site 1, 4.7% at site 2, and 6% at site 3 compared with no topping and by 10.8% at site 1, 5.5% at site 2, and 8.6% at site 3 compared with manual topping. By contrast, at moderate and high
Discussion
In this study, manual topping increased seed cotton yield and earliness compared with no topping regardless of plant density or ecological region. More importantly, chemical topping also increased seed cotton yield and earliness at moderate and high plant densities, although not at low plant density, indicating that plant density mediated the effects of chemical topping. This study also provides new academic insights into how plant topping improves seed cotton yield through regulating
Conclusions
Manual and chemical topping increased seed cotton yield compared with no topping at moderate and high plant densities regardless of ecological conditions, attributing to increased partitioning of assimilates into reproductive tissues. Although manual topping increased seed cotton yield at low plant density, chemical topping decreased yield because of greater reductions in biological yield than those with no topping. Thus, plant density mediated the effect of chemical topping. Although seed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Key Research and Development Program of China (2020YFD1001002), National Natural Science Foundation of China (31771718, 31801307), China Agricultural Research System (CARS-15-15), Seed-Industrialized Development Program in Shandong Province (2020LZGC002), Agricultural Scientific and Technological Innovation Project of Shandong Academy of Agricultural Sciences (CXGC2021A06).
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2023, Industrial Crops and ProductsCitation Excerpt :However, no seed cotton yield loss or fiber damage was observed in this study, even though defoliant was applied at approximately 10 % of open bolls in Alaer and 30 % of open bolls in Luntai (Table 2; Fig. 3). Mature bolls are promoted by manual or chemical topping in China (Dai et al., 2022), and suitable air temperature after defoliant treatments at both sites in 2019 may explain the reasons for no yield loss (Fig. 2). Learning the spatial distribution and number of unopen bolls is essential for improving seed cotton yield and harvest aids efficacy.