*2.2.3 Growth manipulation*

The very purpose of evolving genotypes suitable for HDPS would be achieved only by manipulation of row spacing, plant density and the spatial arrangements of cotton plants for obtaining higher yields. The most commonly tested plant densities range from 5 to 15 plants per sq. m [17] resulting in a population of 50,000 to 150,000 plants per ha. The UNR cotton plants produce less number of bolls per plant than conventional cotton but retain a higher percentage of the total number of good opened bolls per unit area in the first sympodial position and a lower percentage in the second position [18]. The other advantages include better light interception, efficient leaf area development and early canopy closure which shades out the weeds and reduce their competitiveness [19]. The early maturity in soils that do not support excessive vegetative growth [20] can make this system ideal for shallow to medium soils under rainfed condition where conventional late maturity hybrids experience terminal drought.

Cotton growth must be regulated and eventually terminated by chemical means, due to the plants' intrinsic indeterminate growth habit. Plant growth retardants are natural or synthetic organic compounds that control or modify one or more physiological events in plants. These synthetic compounds are widely used in cotton for reducing plant height. The plant growth retardants affect many physiological functions in plants. The crop growth regulator Mepiquat Chloride (MC) is commonly used in cotton production in China and elsewhere to maximize cotton yield and fiber quality [21, 22]. The application of MC increases leaf thickness, reduces leaf area [23], shortens internodes [24] and decreases plant height [25], and thus results in a more compact plant architecture [26] which had been witnessed in the CO 17 culture as well (**Figure 2**).

**Figure 2.** *Application of Mepiquat chloride in cotton variety CO 17.*

Modification of cotton structure increases the light interception in the middle part of the canopy [27]. In addition, Light Use Efficiency (LUE) of cotton is increased by MC application [28]. Furthermore, cotton canopy structure is affected by population density [29] and practices such as wheat–cotton intercropping [30] which influence the crop light interception and fruit formation, thereby biomass growth and yield.

High population densities increase leaf area index (LAI) but reduces the individual leaf area [29]. Like most species, cotton plant height increases with population density [31]. Field experiments raised with CO 17 wherein cotton plant structure was obviously affected by MC and plant density. As MC could bring in changes in the architecture of the applied plant resulting structural changes, it leads to a challenge in maintaining the cotton plant's architecture suiting to the mechanized cultivation. Application of MC is essentially responsible for controlling cell elongation and shoot and stem growth [32]. When plant growth retardants are applied to plants, internodes become shorter and leaves become thicker and greener which leads to altered plant morphology and altered assimilate partitioning resulting in reduced plant growth. The response of plants to PGR applications can differ with plant growth stage, rates of application, and environmental conditions during the applications [33]. Cotton producers and researchers have, therefore, used plant growth retardants as a means to manage the balance between vegetative and reproductive growth for efficient cotton production. But research on application of growth retardants in conjunction high density planting will pave way for synchronized maturity of the crop with uniform plant height that may help in harvesting of seed cotton mechanically at large scale level. This research is at its nascent level in India.

Mepiquat Chloride (1,1-dimethyl-piperidinium Chloride), a plant growth regulator is widely used to manage cotton structure, regulate plant development and hasten maturity under high plating densities [34]. Although plant growth regulators have been thoroughly widely tested in cotton in India, specific recommendations regarding their dose and timing for modifying the architecture at high planting densities are yet to be arrived for adoption on a holistic perspective. Reduction in plant height, decrease in height/node ratio, an increase in boll weight and a delay in maturity with the application of growth regulators were observed with non-significant effect on yield. Application of Mepiquat Chloride reduced the leaf area and increased the number of bolls per unit area at high plant density. It also helped in retention of bolls on lower sympodia and increased the synchrony of boll maturation [29].

However, the effect of Mepiquat Chloride on cotton is affected by environmental conditions, particularly temperature [35]. Studies taken up at the Department of Cotton indicated the differential response of cultures and its performance across centres. At Coimbatore, there was a variety dependent response to Mepiquat Chloride application @ 75 g ai/ha in three splits on 45, 60 and 75 DAS in winter irrigated cotton planted at 45 x 15 cm spacing and the effect was more pronounced in CO 17 (**Figure 3**).

There was a reduction in plant height, sympodial length and LAI and an enhanced the number of burst boll/sq. m leading to an increase in yield at Coimbatore. Across the cultivars, application of Mepiquat Chloride increased seed cotton yield from 1330 kg/ha to 1530 kg/ha. Interaction effect of cultivars and application of Mepiquat Chloride was significant. Taller cultivars namely TCH 1608 and TCH 1705 (CO 15) benefitted more from the application of Mepiquat Chloride compared to the other cultivars having a compact growth habit. Cultivars with a more indeterminate growth habit responded more positively to Mepiquat Chloride application [36]. There is a need for detailed investigations on this aspect before any recommendations are given.

*High Density Planting System of Cotton in India: Status and Breeding Strategies DOI: http://dx.doi.org/10.5772/intechopen.94905*

#### **Figure 3.** *Effect of Mepiquat chloride on cotton varieties.*

*2.2.4 Effect of Mepiquat chloride application on seed cotton yield in different genotypes*
