Cotton belongs to the Malvaceae family within the genus Gossypium and is considered as an important commercial fibre crop of the world. It is an important cash crop often known as ‘white gold’. It plays a significant role in the agricultural, economic and social sector of the nation. Cotton is placed in the top fifteen important widely grown crops of the world even though; it is not used as a staple food (Wendel et al 2010). Cotton serves as a multipurpose plant, providing fundamental resources such as fibre, oil, oil cake, hulls and lint. The abundant fibre and energy in the form of fat and protein is present in whole cotton seeds, which are widely recognized as a popular feed for dairy cattle. The seeds are about 15% value of the crop which contain protein (20%), oil (20%) and starch (3.55%), after pressing to make oil and its cake used as animal feed 1. The oil obtained from the kernels of the seeds, once refined, becomes a valuable edible and nutritious resource. It is suitable for use as both cooking oil and in the preparation of salad dressings. Fibers emerge from the seed's outer covering, contributing in the development of a lint boll of cotton. This boll serves as a protective fruit; in commercial cultivation, it is removed from the seed through a process called ginning, and the lint is subsequently processed into cotton fibre.

India is a major producer of cotton in the world followed by China, USA, Brazil and Pakistan. India accounts about 21% of cotton production in the world 2, however, the lint productivity is 445 kg ha^-1, which is very less than the world average of 765 kg ha^-1. Cultivated cotton comprises of four species i.e G. herbaceum (Asian cotton), G. hirsutum (American Upland cotton), G. barbadense (Egyptian cotton) and Gossypium arboreum grown in India under diversified ecosystems. In India, Gossypium hirsutum constitutes 88% of the total cotton production, and all the existing Bt cotton hybrids are from G. hirsutum. Major cotton growing states in India are Maharashtra, Gujarat, Punjab, Madhya Pradesh, Haryana, Rajasthan, and Tamil Nadu.

Punjab is among one of the ten major cotton growing states in India. In kharif season, cotton is considered as second main crop in Punjab after paddy and a major cash crop of the south-western districts of the state. During 2020-21, American cotton occupied 2.48 lakh ha in Punjab with production of 10.14 lakh bales and average yield was 694 kg lint ha^-13. The Malwa belt of the state is also known as ‘cotton belt’ with more than 95 % area of cotton crop in state under Bt cotton hybrids 4. Although Punjab has highest lint productivity in India, it is still lesser than highly productive nations like China and Brazil with lint productivity around 1700-1800 kg ha^-1. Most developed nations with highest lint productivity plant cotton in HDPS (High Density Planting System) with planting density of 10 plants m^-2. However, in Punjab, farmers have been recommended to plant cotton with 2 plants m^-2 due to high cost of seed of Bt hybrids which is achieved by keeping plant to plant spacing at 75 cm. Recently, Punjab Agricultural University has developed and released three Bt cotton varieties, which will reduce the seed cost. With the availability of Bt cotton varieties, HDPS can be tested for these varieties for increasing the productivity. An optimum plant density may vary according to location, cultural practices, and environmental conditions (Silvertooth et al 1999). Cotton is a perennial plant that exhibits indeterminate growth, persisting in its vegetative growth even as fruiting has begun (Oosterhuis 2001) which may lead to competition for photosynthates. This suggests that vegetative growth of cotton is almost at the expense of cotton yield. Moreover, in dense planted cotton, excessive vegetative growth may occur, which may increase the shade in the plant canopy, reduces the supply of assimilates to reproductive structures, thereby causing fruit abscission and reduced yield (Mauney 1986 and York 1983).

In high density cotton production system, Plant Growth Regulators (PGR) are common tactics used to manipulate the growth of the plant canopy to promote early maturity and prevent yield reduction caused by auto shading and to decrease plant height (Hodges et al 1991 and Krieg and Kerby 1985). One such PGR, mepiquat chloride has been recommended by Punjab Agricultural University to check the rank growth in cotton commonly observed in highly fertile soils. Mepiquat chloride (MC) (1,1 dimethyl piperidinium) is an organic compound soluble in water, serving as a gibberellin suppressant. It hinders the synthesis of gibberellic acid, resulting in decrease in the rate of cell expansion and cell division (Srivastava 2002). The reduction in cell elongation after MC treatment leads to reduced plant height and lower cotton leaf area (Kerby 1985). Applying MC restricts the endogenous production of gibberellic acid, causing plants to exhibit a more compact and shortened structure. This is achieved by inhibiting cell elongation, ultimately resulting in decreased length of internodes (Rademacher 2000). MC is considered beneficial for managing plant architecture in conditions that encourage vegetative growth, which can negatively impact the quality and yield of fibres (Oosterhuis and Egilla 1996 and Constable 1991). Cotton plants treated with MC typically are compact, with lesser nodes (Reddy et al 1990), reduced internode length and lesser reproductive branches (Bogiani and Rosolem 2009). The application of MC leads to a decrease in leaf expansion, stem and petiole length, node count, as well as an advancement in the maturation of the cotton crop. This treatment yields varying outcomes in terms of yield (Bogiani and Rosolem 2009; Cook and Kennedy 2000; Mao et al 2015). The use of MC results in a reduction of leaf area per plant that surpasses the effect on boll load, consequently increasing the number of bolls per unit of leaf area. Mepiquat chloride application encourages boll formation on lower sympodia, thereby enhancing the synchronization of boll maturation and the demand for photosynthates (Gwathmey and Clement 2010). MC is commonly employed to maintain a balance between vegetative and reproductive growth, subsequently adjusting cotton yield (Zhao and Oosterhuis 2000; Yang et al 2014).

Boll density and boll weight are the two main factors used to determine cotton yield. These yield components can be managed by optimum Plant density and Mepiquat chloride (MC) application (Stewart 2005). Most researchers have studied the optimum plant density for high quality and yield in cotton. Some have depicted non-significant relation between plant density and yield (Jones and Wells 1998 and Bednarz and Robertz 2000), whereas others noted reduced yields at very low or high density (Bridge et al 1973 and Smith et al 1979). Optimum plant population in cotton depends on various factors such as rainfall, temperature, crop management and soil health. Also, variable yield responses have been reported with MC application, with some studies showed a negative response (Zhao and Oosterhuis 2000) while other showed no response (Nichols et al 2003 and Pettigrew and Johnson 2005). Favourable response to MC application has been reported, under situations that favour excessive vegetative growth viz., excessive nitrogen, warm and humid conditions (Gwathmey and Clement 2010) and with high plant density (York 1983).

Thus, the rational approach would be to curtail extravagant vegetative growth by applying plant growth regulators to enhance translocation of metabolites towards reproductive sinks. The plant spacing and the application of mepiquat chloride (MC) are thought to influence the dynamics of different factors related to growth and yield. Increasing plant density may increase the yield through higher plant population However, excess vegetative growth may affect the retention of fruiting bodies resulting in adverse effect of HDPS. Hence, MC application may help in limiting the vegetative growth thereby increasing the retention of bolls ultimately leading to higher yields. It is therefore hypothesized that higher plant density, when supplemented with Mepiquat chloride, will lead to improvement in reproductive bodies, resulting in higher seed cotton yield and enhanced overall productivity of the Bt cotton variety under consideration. This Research was undertaken with the two objectives i.e., to optimize the Mepiquat chloride application rate for dense planted Btcotton variety and to study the effect of increased plant density and Mepiquat chloride (MC) application on the cotton growth and productivity.

Monopods per plant, sympods per plant, boll retention, boll weight, opened bolls plant^-1, unopened bolls plant^-1 and total weight was influenced by spacings with wider spacing (67.5×60 cm) resulting in higher monopods, more boll retention, more opened bolls, lesser unopened bolls, more total bolls plant^-1 and higher boll weight. However, in densely planted crop, an increase in proportion of total seed cotton yield i.e., 2326 and 2190 kg ha^-1 was witnessed by Bt cotton sown at 67.5×30 cm and 67.5×15 cm at Abohar and Faridkot, respectively. Lesser days to square initiation, flower initiation, and boll formation were observed in widely sown crop, which brought earliness in the crop, thereby, Earliness index was significantly higher in cotton sown at 67.5 ×60 cm at both locations. Although highest gross returns and net returns were observed with 67.5×30 cm and 67.5×15 cm at Abohar and Faridkot, respectively, B:C ratio was highest in 67.5×15 cm at both the locations. This clearly established dense planting to be more remunerative over other spacings. Quality parameters like GOT, Micronaire value were not affected except halo length at Faridkot.

The study emphasized the substantial impact of plant spacing and Mepiquat chloride on cotton growth, yield, and economics. Wider plant spacing enhances dry matter partitioning towards leaves and fruiting bodies, while densely planted crops show increased proportion of total seed cotton yield. Mepiquat chloride, particularly at 750 ml ha^-1 during flowering, balances vegetative and reproductive growth, improving yield attributes. Economic analysis highlights the economic viability of dense planting, with higher benefit-cost ratios. Likewise, Mepiquat chloride at 750 ml ha^-1 during flowering yields improved net returns. This study provides valuable insights for cotton growers, emphasizing the importance of strategic plant spacing and growth regulator application for enhanced productivity and profitability.