1. Introduction
Cotton (
Gossypium hirustum L.) is a leading cash crop considered as “white gold” and cultivated in more than 80 countries throughout the world [
1]. It contributes to the economy of cotton-producing countries. According to statistics, India is the largest producer of cotton with 6.2 million metric tons in the year 2017–2018, while China, USA, Pakistan and Brazil is included in top five cotton-producing countries [
2]. Approximately 80% of world total cotton production comes from these five countries. Cotton production is very important not only for its economic benefits but also for the socio-economic value in the country. The USA is the largest exporter with 3.4 million metric tons of cotton and Bangladesh is the largest importer of cotton with 1.65 million metric ton [
2]. China contributes about 30% of the world cotton production [
3]. Cotton is a unique crop which provides oil and clothes to humans, chaff for livestock feed, organic matter to soil and many other products to industries [
4]. Currently, more than half of clothes worn by all people in the world is made of cotton fiber [
5].
Proper spacing between plants is an important agronomic factor which affect optimal use of resources and increase crop productivity [
6]. Plant density is a key factor for optimizing structure and increasing the photosynthetic capacity of the cotton canopy. Crop geometry and plant density are agronomic factors which enhance yield and profitability [
7]. Plant density affects light interception, moisture availability and wind movement which further affect plant height, architecture, boll behavior, crop maturity and yield. An optimal plant density not only enhances the yield and fiber quality of cotton but also reduces fertilizer application and labor cost as compared to high plant densities without compromising yield [
8]. Fertilizer and irrigation can also be efficiently utilized in optimal plant density regimes. Globally, high planting density has become common in the cotton production systems. High plant density has more leaf shedding in late season along with lower weight boll production. High plant density (>10 plants m
−2) and the associated shading may lead to disease infestation, fruit shedding, reduced boll size, delayed maturity and decreased individual plant development and light interception [
9,
10]. Current recommended and practiced plant densities in China is 5.3 × 10
4–7.5 × 10
4 plants ha
−1 in the Yellow River Valley [
11], 3.0 × 10
4 plants ha
−1 in the Yangtze River Valley [
9] and 22.7 × 10
4 plants ha
−1 in the Northwest region.
Cotton yield can be separated into different components such as boll number, boll weight and lint percentage [
12]. Boll density is a major contributor to lint yield [
13]. Cotton fiber is an extension of the epidermal cell of the seed, and the most basic element of lint yield can be further dissected into smaller units such as seeds number per boll [
13] and number of fibers per seed [
14]. Seed size affects fiber numbers per unit seed surface area and lint mass [
10,
15]. High plant density produces more bolls per unit area and contributes to final yield, however, it also leads to a decrease in individual plant yield [
9,
16]. Cotton growth, yield and quality perfection through optimal management practices is the continuous goal of cotton agronomists.
Plant density and boll retention have a direct and complex relationship which is influenced by many factors like temperature, nutrition, physiology, genotype, water stress, competition for photosynthates, insects and/or a combination of these [
17,
18,
19,
20,
21]. Biomass production is also the prerequisite of cotton yield and biomass partitioned to reproductive organs contributes to the final yield [
22]. Due to its indeterminate growth, cotton accumulates high vegetative biomass. The biomass accumulation in cotton increases as cotton crops change from one growth stage to another; however, in the last growth stages, biomass decreases due to fruit and leaves shedding [
8,
23]. In early growth phases, more light intercepts to lower parts of the plant due to a less dense canopy which helps in the establishment of a good stand and increases the biomass.
The purpose of this research was to investigate the effect of plant density on cotton growth parameters, seed cotton yield, lint yield, and yield components, and to find the optimal cotton plant density for Henan province in China.
4. Discussion
The present two-year study was conducted to investigate the effects of different PD on cotton growth, yield, boll retention and biomass accumulation. High PD is practiced for higher production in terms of number of bolls per unit area, however, it has been observed that yield increases up to a certain moderate plant density while too high a PD negatively affects the yield resulting in lower production.
An optimal cotton PD is influenced by various conditions, including soil, microclimate, planting pattern, irrigation type, fertilizer application method, cultivar and farmer’s field management. Three major regions of China’s cotton belt have huge differences in their PD. An optimal higher PD is advantageous for producing high yield. Xinjiang region has the highest PD of 21.0 × 10
4–24.0 × 10
4 plants ha
−1 [
24], followed by Yellow River Valley, with a PPD of 3.0 × 10
4, 4.5 × 10
4 and 6.0 × 10
4 plants ha
−1 for hybrid Bt cotton, indigenous Bt cotton, and Bt cotton, respectively [
25,
26], whereas, for late sowing PD it is 7.5 × 10
4 ha
−1 [
27]. Similarly, Yangtze River Valley in which hybrid seeds are commonly used, has the PD of 3.0 × 10
4 plants ha
−1 [
28]. Similar results are also presented in this paper that cotton yield increases with increasing PD up to certain limit (87,000 plants ha
−1) in Yellow River Valley while yield reduction occurred with very high or very low PD [
29].
Growth and development of cotton is highly affected by PD. As PD increases, squaring, flowering, boll setting, and maturity is delayed. Late maturity is related to low temperature and light interception. An increase in PD decreases the amount of light interception to the lower parts of the plant and increases resource competition among plants which affect cotton phenological development [
30].
Increasing plant density reduced plant height, main-stem nodes per plant, number of bolls per plant, and individual boll weight [
31,
32]. Decreasing PD increased bolls at the 1st, 2nd and 3rd positions of the plant with less yield and unnecessary vegetative growth that led to undesirable fruit shedding and boll rotting [
33]. In the present study, it was observed that plant height, number of bolls and fruiting nodes per plant decreased, while shedding percentage increased with increasing PD. Similar results were also reported in previous investigations [
16,
34].
In this study, the plant density D5 produced the highest yield and yield contributors except boll weight and lint percentage in the year 2018. Cotton yield and yield contributors of the different plant densities decreased in the following trend, D5 > D4 > D6 > D3 > D2 > D1, with the highest and lowest yield in D5 and D1, respectively. During both years, the increase in yield might be due to higher number of bolls and fruiting nodes m
−2. Individual plant yield and bolls m
−2 of D5 was lower as compared to D1, D2, D3, D4 but was higher per unit area. Our results are in agreement with Mao et al. (2015), who reported an increased number of bolls m
−2 and a lower weight of single boll in higher PD [
35]. Exceptionally high plant densities reduced boll weight and light penetration to lower parts of the plant which increased ethylene/sugar ratio and resulted in greater shedding with low yield [
16]. Our results are also in agreement with Dong et al. (2010) that high PD produce more bolls but decrease boll weight [
36]. High PD resulted in fruit shedding, poor boll filling, delay maturity and disease infestation which resulted in reduced cotton yield [
9,
10,
23]. Our results showed that too high (D6) and too low (D1) plant population led to a decline in cotton yield. An optimal plant density (D5) not only increases yield but also utilize less inputs without yield reduction as compared to high PD [
8].
Boll retention was affected by plant density and an inverse relationship was found between fruiting position and plant density. Bednarz et al. (2000) reported that plant population affected boll numbers per plant in cotton [
17]. In this study, the rate of boll retention at both fruiting position and node decreased as plant population increased. This might be due to high resource competition as density increases [
8]. Our result are in agreement with the findings of Siebert and Stewart (2006) that decrease of plant population increased bolls at the 1st, 2nd and 3rd positions of the plant [
33]. Higher boll numbers were retained above node 8 while less retention rate was observed at node 4–10. Increasing plant density decreased the bolls at the 1st position on nodes 6–10 [
31]. Different studies conducted on boll retention rate reported that more bolls were retained at nodes 7–13 [
37] or at nodes 13–22 [
38]. Shade during the first bloom, peak bloom and boll development increased fruit shedding which consequently reduced boll retention rate [
39]. Pettigrew et al. (2004) reported that higher plant nodes produced more bolls in irrigated cotton [
40], while Guo-zheng et al. (2010) linked high boll retention with increased boll rot, low open boll rate and concluded that dense population create shade, increase canopy humidity, making the canopy environment suitable for pest injury, spread of disease and remaining safe from insecticides or pesticide sprays due to dense leaves [
34]. The lower boll retention rate might also be due to less light interception in the canopy. Xue et al. (2015) concluded that light interception decreases with an increase in plant height because of a sealed canopy [
41]. Light interception increases speedily after planting and starts decreasing after canopy sealing whereas less of a difference in light interception was observed in different plant densities at early growth stages [
41]. Therefore, it might be possible that due to more light interception in the early growth stages, the first nodes had more boll retention and canopy sealed slowly affecting light interception and boll retention. After tip removal, the plants stopped branching and the upper branches received more light up to the end of season which probably resulted an increase in boll retention in the nodes above 8.
Biomass accumulation in cotton changes with different treatments and follows a parabolic curve. High plant density increased plant total biomass in terms of kg ha
−1 but the individual biomass of a cotton plant decreased [
42]. Plant density D5 produced maximum biomass and had maximum biomass accumulation. As the canopy becomes dense, the light interception in the canopy decreases and resource competition increases which affect the growth of the crop [
41,
43]. Vegetative organ biomass accumulation increases as density increases but reproductive organ biomass accumulation decreases at too high a density. Biomass above ground is directly related with PD and increasing plant density increases light-use efficiency at the reproductive stage [
44]. The increase in total and vegetative biomass may also be due to higher number of plants per unit area with high vegetative growth. Reproductive organ biomass accumulation decreases with too high and too low plant densities[
9]. Our results recommended that high ROB can be obtained by adjusting plant density accordingly.