Effect of Mepiquat Chloride on Phenology, Yield and Quality of Cotton as a Function of Application Time Using Different Sowing Techniques

: Mepiquat chloride (MC) is a plant growth regulator used to manage the rampant vegetative growth of cotton. A two-year ﬁeld experiment was conducted at the Postgraduate Agricultural Research Station, University of Agriculture, Faisalabad, Pakistan, during 2017 and 2018 to investigate the inﬂuence of MC applied at different times on phenology, morphology, lint yield and quality of cotton cultivated using different sowing techniques. MC was applied 50 days after sowing (DAS), 60 DAS and 70 DAS to cotton planted in ﬂat ﬁelds (ﬂat sowing), ridges (ridge sowing) and beds (bed sowing). The interactive effect of MC application time and sowing technique did not inﬂuence crop phenology, morphology, and lint yield and quality. It was revealed that the crop planted on beds took fewer days to ﬂower (10%) as compared to that on the ﬂat ﬁeld, and the bed-sown crop produced a higher number of opened bolls (60%) and was characterized by a higher boll weight (32%) and seed cotton yield (50%) in comparison to the ﬂat-sown crop. A late application of MC (at 70 DAS) caused a signiﬁcant reduction in the time to ﬂowering (8%), with a simultaneous increase in the number of opened bolls (60%), boll weight (32%), ginning out turn (8%) and lint yield (27%) as compared to MC application at 50 DAS. In terms of lint quality, cotton planted on beds had better ﬁber uniformity (8%) compared to that on the ﬂat ﬁeld, while MC applied at 70 DAS produced better ﬁber ﬁneness by 27% in comparison to MC applied earlier. Overall, cotton planting on beds and MC application at 70 DAS may help improve cotton yield and ﬁber quality and may help in the mechanical picking of cotton.


Introduction
Cotton (Gossypium hirsutum L.), a natural fiber, is cultivated across the globe as an annual crop specifically for lint, oil, animal feed, and as an industrial commodity [1,2].It is also the most important cash crop in Asian countries including India and Pakistan and many Latin American countries [3].Similar to other countries, cotton plays an imperative role in Pakistan economy as it is a source of earning for millions of people involved in the farming, textile, ginning, oil, and garments industries of Pakistan [4][5][6][7].
The cotton plant has the unique characteristic of indeterminate growth; hence, the growing environment, e.g., planting density or planting technique, has a strong influence on its morphological adaptation [8][9][10][11].In a sub-tropical climate, excessive rainfall during the crop's growth, despite good management practices in warm and sunny days enhancing the crop's growth, may result in self-shading, increased insect pest attack (rotting of bolls and cotton leaf curl virus), and reduced yield [12][13][14].Besides having a complex growth behavior, cotton plants are very sensitive toward field management and adverse climatic conditions [15].Among agronomic practices, planting technique and application of plant growth regulator have prime importance because they not only influence the growth, yield, and quality of cotton but also help maintain a good crop stand, improving radiation use efficiency, and modulating plant canopy [16][17][18][19].
To ensure a balanced crop growth and development as well as a high crop yield, an effective sowing technique is required that can provide favorable soil conditions with suitable temperature, moisture content, and least resistance to root penetration [20].Nevertheless, each sowing technique has its pros and cons in different environmental conditions; therefore, it is imperative to determine the suitability and performance of different sowing techniques on a site-specific basis.The conventional sowing technique (flat sowing) of cotton is inefficient due to poor germination and reduced water use efficiency and crop yields [21].Conversely, improved sowing techniques (bed sowing and ridge sowing) not only ensure better germination and optimum crop stand but also increase the resource (land, light, fertilizer, and water) use efficiency [22,23].Similarly, bed sowing allows good drainage during the rainy season, better weed control, no crust formation, and a higher lint yield [24][25][26][27].Similarly, the ridge sowing technique of cotton offers better emergence and root penetration, soil moisture contents, higher yields, and improvement in soil properties compared to flat sowing [20,[28][29][30].
Extensive vegetative growth can be managed by topping and thinning; however, these techniques are labor-intensive [31].In case of labor scarcity, the use plant growth regulators is an important agronomic tool to manage cotton plants foliage by tuning the plant's hormonal balance, transforming canopy stature, and improving the source-sink relationship [32][33][34][35].Mepiquat chloride (MC) is one of the most extensively used plant growth regulator for cotton across the globe to manage plant height and to attain better production [36][37][38].The foliar application of MC inhibits the biosynthesis of gibberellic acid as it can be easily absorbed from the leaves and diffuse into the whole plant body [35,39].The application of MC induces morphological alterations in terms of reduced intermodal distance [40], plant height [41], number of nodes [33], and height-to-node ratio [42], while increasing light interception, source-sink ratio, and photoassimilate partitioning [39,[43][44][45].Moreover, MC application also increases flower and fruit retention on the lower branches due to better light interception by the lower leaves [40,46,47].
Yield attributes, lint quality, and phenology were previously studied using different sowing techniques; however, information about the influence of MC on crop phenology, yield attributes, and lint quality of cotton cultivated using different sowing techniques is not available.Moreover, previous research studies in advanced countries have warranted an investigation of the influence of MC applied at different times on cotton grown in Punjab, Pakistan.The objective of this study was to explore the influence of MC application at different times on phenology, plant stature, yield attributes, and lint quality of cotton grown using different sowing techniques.

Crop Phenology
The analysis of the data revealed that floral bud initiation and boll maturation periods (in 2017) were not influenced by any of the examined factor (sowing technique or MC application) or their interaction.However, days to flowering were significantly influenced by sowing technique and MC application, but their interactive effect was non-significant (Table S1).
The bed sowing technique and MC application at 70 DAS reduced the time taken from floral bud initiation to flowering and to boll maturation (data not shown).However, a significant reduction (by 10%) in days to flowering was observed in bed sowing as compared to flat sowing, across the years (Table 1).Similarly, on average, MC application at 70 DAS reduced the time to flowering by 8% as compared to MC application at 50 DAS (Table 1).

Morphological Traits
Different sowing techniques substantially influenced plant height (in 2018), sympodial branches, total number of bolls plant −1 , and opened bolls plant −1 , but monopodial branches and un-opened bolls plant −1 were not influenced by the sowing technique across the years.However, MC application time influenced all these morphological attributes significantly, except for un-opened bolls plant −1 across the years (Tables 1 and 2).The highest values of plant height, sympodial branches, total bolls plant −1 , and opened bolls plant −1 were recorded for the bed sowing technique and were either statistically similar to those recorded for ridge sowing or significantly higher than those obtained for flat sowing (Table 1).On average, the bed sowing technique increased the plant height, sympodial branches, total bolls plant −1 , and opened bolls plant −1 by 7%, 24%, 43%, and 60%, respectively, in comparison to the flat sowing technique.Regarding the MC application, application of MC at 50 DAS reduced plant height, sympodial branches as well as total bolls plant −1 and opened bolls plant −1 as compared to a late application of MC.On average, MC application at 70 DAS increased opened bolls plant −1 by 60% as compared to MC application at 50 DAS (Table 1).

Yield Attributes
All yield attributes were substantially influenced by the sowing technique except for GOT in both years of experimentation.Similarly, the MC application timing influenced the yield attributes significantly, except for boll weight in 2017 and GOT in 2018 (Tables 2 and 3).It was recorded that the highest seed cotton yield plant −1 , boll weight, lint yield, 100-cotton seed weight, and final cotton yield were recorded from bed sowing, and the values were either statistically comparable to those found for ridge sowing or higher than those obtained for ridge sowing.Bed sowing of cotton increased the seed cotton yield plant −1 , boll weight, lint yield, 100-cotton seed weight, and final cotton yield by 50%, 32%, 39%, 29%, and 50%, respectively, in comparison to flat sowing (Tables 2 and 3).As regards MC, application of MC at 70 DAS increased seed cotton yield plant −1 (23%), boll weight (32%), GOT (8%), lint yield (27%), 100-cotton seed weight (16%), and final cotton yield (23%) as compared to MC application at 50 DAS (Tables 2 and 3).

Lint Quality
The data on lint quality showed that different sowing techniques only influenced fiber length substantially in 2017 and fiber uniformity in both years of experimentation.However, the MC application timing only influenced fiber fineness significantly only in 2017 (Table 3).It was observed that flat planting of cotton was associated with the lowest values of fiber length, fiber strength, fiber uniformity, and fiber fineness.However, on average, bed sowing increased fiber uniformity by 8%, while ridge sowing increased fiber length by 9% in comparison to flat sowing.MC application at 70 DAS increased fiber fineness by 27% as compared to the earlier application of MC (Table 3).

Discussion
The present study revealed that the sowing technique significantly influenced cotton's days to flower (Table 1), fiber length, and most morphological and yield attributes significantly (Tables 1-3).Similarly, an early MC application (50 DAS) influenced cotton's morphological attributes (Table 1), while a late application (70 DAS) influenced fiber fineness and yield attributes significantly (Tables 2 and 3).The yield attributes were worse in 2017 as compared to 2018, possibly due to unexpected weather conditions (Figure 1).Reduced yield attributes might be ascribed to more rainfall and higher relative humidity at the start of the reproductive stage, which promoted insect pest attacks.

Sowing Techniques
It was observed that the in-bed planting technique produced early flowering (Table 1) and boll maturation, which might be attributed to better aeration and increased uptake of nutrients related to better root growth and development [48].

Sowing Techniques
It was observed that the in-bed planting technique produced early flowering (Table 1) and boll maturation, which might be attributed to better aeration and increased uptake of nutrients related to better root growth and development [48].

Mepiquat Chloride Application
This study revealed that MC application at 70 DAS reduced the days to flowering (Table 1), which could be due to the fact that MC inhibited the plant vegetative growth and altered photoassimilate translocation toward the reproductive organs [58,59].Similarly, Biles and Cothren [60] reported that the application of MC improved the growth of the reproductive parts by regulating plant development, with the benefit of increased flowering.Tung et al. [61] suggested that MC application also helped in retaining a balance between reproductive and vegetative growth, as any variation in the metabolic or physiological pathway could influence the photoassimilate translocation and partitioning.
An early application of MC (50 DAS) reduced the morphological attributes of cotton (Table 1), which might be attributed to a decrease in cell elongation due to inhibition of gibberellin biosynthesis [35,[62][63][64].However, late application of MC (at 70 DAS) increased the total number of bolls plant −1 (Table 1) [65], opened bolls plant −1 , boll weight, 100-cotton seed weight, lint yield (Tables 1-3) [66] and seed cotton yields (Table 2) [9,67] which might be attributed to an alteration in canopy structure and source-sink ratio and to a fine-tuning of plant's hormonal balance [35].Pettigrew and Johnson [47] highlighted that MC induced flower production in the early season, while Nuti et al. [40] and Prince et al. [68] stated that MC helped to retain bolls on lower branches.The increased retention of bolls on lower branches could be due to increased light penetration to the lower branches or to an alteration of photoassimilate translocation towards boll development [46].Similarly, Nawalagatti et al. [69] reported that MC application at the flowering and square development stages has the potential to enhance the final yields by accelerating the flowering and improving the fruit yield percentage [59].A late application of MC increased the cotton yield, possibly due to a reduction in regrowth and an enhancement of leaf maturity [70].
The application of MC at different times did not have any influence on GOT, fiber length, fiber strength, and uniformity, as also concluded by Khanzada and Khanzada [71] who reported a non-significant influence of plant growth regulators (MC) on GOT and fiber quality.4. The weather data of the experimental site during the course of studies are presented in Figure 1.

Design and Treatments
The experiment was conducted in a split-plot arrangement with three replications.Three sowing techniques, i.e., flat sowing = S 1 , ridge sowing = S 2 , and bed sowing = S 3 , were allocated and randomized in main plots, while in sub-plots, mepiquat chloride (150 mL ha −1 ) was applied at three different times: 50 days after sowing (DAS) = T 1 , 60 DAS = T 2 , and 70 DAS =T 3 .Each sub-plot contained 8 rows of cotton having an interrow spacing of 75 cm and an inter-plant spacing of 30 cm.The gross plot size of each experimental unit was 10 m × 6 m.

Experimental Material
Delinted seeds of the high-yielding cotton line PB-896 were obtained from the Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan.Fertilizers (Urea, diammonium phosphate = DAP, and Muriate of Potash = MOP), herbicides, and all pesticides used during the study were acquired from the Department of Agronomy, University of Agriculture, Faisalabad.Mepiquat chloride was obtained from Bayer Crop Science, Pakistan.

Field Management
Each year, the field was irrigated with soaking irrigation (4-acre inch depth) before the preparation of the seedbed.Stubbles of the previous crop (Egyptian clover) and weeds were incorporated with a moldboard plow.The field was plowed and planked when soil mellowness was considered physically suitable.Ridger and bed shaper were used to build ridges (75 cm apart ridges) and beds (bed width 75 cm, and bed-to-bed distance of 75 cm).Before sowing of the crop, a uniform dose of fertilizer was applied in all experimental units.Nitrogen, phosphorous, and potassium were applied at the rate of 200, 115, and 95 kg ha −1 , respectively.Sowing of the crop was accomplished in all treatments on 26th and 27th May of 2017 and 2018 using 20 kg ha −1 seed rate.The delinted cotton seeds were sown using the manual hill-drop sowing method for the ridge and bed sowing techniques, while a single-row hand drill was used for flat sowing.For flat sowing, the inter-row spacing was maintained at 75 cm.Four to five seeds were dropped on each hill when using the ridge and bed sowing techniques and covered with soil.At the four-leaf stage, (BBCF identification code = 14) [72], the seedlings were manually thinned to maintain similar plant populations for all sowing techniques.Similarly, hoeing was performed at the vegetative side shoot development stage (BBCF identification code = 25) by using a spade to keep the research area free from weeds, for earthing up, and to break hard surfaces among the rows of crop.Foliar application of mepiquat chloride 5SL was carried out at pre-defined crop times (50, 60 and 70 days after sowing) using a manual Knapsack sprayer having a hollow conetype nozzle.The crop was irrigated using canal water as per crop requirement, while other management practices (insecticides, weedicides application) are not detailed because all were accomplished according to local agronomic practices.
Cotton picking was carried out at 50% boll opening, while the last picking was carried out on 10th and 18th November of 2017 and 2018, respectively.

Data Recorded
Data on crop phenology, agronomic, yield, yield component, and lint quality were recorded using standard procedures.

Crop Phenology
Five plants were tagged before mepiquat chloride foliar application to record the phenological data.The dates of floral bud initiation, flowering, and boll maturation period were recorded by estimating the number of days taken to accomplish these phenological stages.

Morphological Features, Yield and Yield Component
Five plants were tagged in each experimental unit (before mepiquat chloride foliar application) to measure plant height (cm), monopodial and sympodial branches, total bolls plant −1 , opened bolls plant −1 , and un-opened bolls plant −1 , which were averaged.Before the 2 nd harvesting, 50 fully matured and opened bolls were handpicked from each experimental unit, sun-dried, and manually ginned to determine single boll weight (g) as indicated in Equation ( 1), ginning out turn (GOT) percentage in Equation ( 2), lint yield (g) as indicated in Equation ( 3), and 100-seed cotton weight (g).The formulas to calculate each trait are shown below.Data regarding plant height and monopodial and sympodial branches were recorded at the 2nd harvesting (10th November in 2017 and 18th November in 2018), while data regarding the bolls were obtained at 50% boll opening and final harvesting, and then the average was calculated.

Lint Quality
Fiber length and fiber strength were measured by following the method of Moore [73] and expressed in cm and g tex −1 , respectively.Fiber length is the upper half mean length of the fiber, while fiber strength (g/tex) represents the force needed to breakdown the fibers of unit linear density.Fiber uniformity (%) and fiber fineness (µg inch −1 ) were measured by using a spin lab high volume HVI-900 instrument, as detailed by Ehsanullah et al. [17].

Statistical Analysis
The data obtained were analyzed using statistics 10, a computer program, Fisher's ANOVA technique, and Tukey's (HSD) test at 5% probability level, comparing the treatments' means [74].The interactive effect of the sowing technique and mapiquat chloride application time was non-significant, so only the main factors are presented in the tables.

Conclusions
The major objective of this study was to investigate the influence of MC application timing on phenology, morphology, yield attributes, and lint quality of cotton cultivated using different sowing techniques.The results revealed that the bed sowing technique not only reduced the days to flowering and the boll maturation period but was also associated with the highest values of morphological attributes, yield and yield-related traits, and fiber uniformity.For ridge sowing, most of cotton morphological traits and yield-related traits were either lower or statistically similar to those obtained with the bed planting technique.However, cotton performance in terms of phenology, yield attributes, and quality was poor when it was sown using flat sowing technique.As regards MC, the early application of MC at 50 DAS decreased the rampant vegetative growth of cotton but did not increase the yield and yield -related attributes.Contrarily, the late application of MC reduced the days to flowering significantly, while increasing the yield and yield-related traits.Moreover, the late application of MC increased the fiber fineness but did not influence the other quality traits of lint, crop phenology, and GOT.In short, it can be concluded that the bed sowing technique and MC application at 70 DAS could be useful to promote cotton mechanization in developing countries like Pakistan.

Figure 1 .
Figure 1.Weather data for the experimental site during the cotton season of 2017-2018.

Figure 1 .
Figure 1.Weather data for the experimental site during the cotton season of 2017-2018.
This field-based study was conducted at the Postgraduate Agricultural Research Station (Latitude = 32.26• N, Longitude = 73.04• and Altitude = 184 m), University of Agriculture, Faisalabad, Punjab, Pakistan, during the summer seasons of 2017 and 2018.The soil of the experimental site belongs to the Lyallpur soil series, Haplic Yermosols (FAO classification) mixed, aridisol-fine-silty, and Haplarged, hyperthermic Ustalfic (USDA classification) classes.The physico-chemical and biological indices of the soil are detailed in Table

:
Variance components and Tukey's HSD values (p ≤ 0.05) for phenological attributes of cotton under the influence of different mepiquat chloride application times and different sowing techniques.Author Contributions: N.A., S.H. and S.A.A. conceived and designed the experiments; K.M. and M.I. carried out experiments; K.M. performed the quality traits; M.I., N.A. and S.A.A. analyzed the data and wrote the manuscript.N.A.B., S.H., A.M.A. and A.A.H. revised the manuscript.All authors have read and agreed to the published version of the manuscript.

Table 1 .
Influence of different sowing techniques and timing of mepiquat chloride application on phenological and morphological traits of cotton.

Table 2 .
Effect of different sowing techniques and timing of mepiquat chloride application on morphological and yield-related attributes of cotton.
g = gram; df= Degree of freedom; ns = not significant at p > 0.05; * = Significant at p < 0.05; ** = Significant at p < 0.01; HSD = Honestly significant difference; Values sharing same lettering (in superscript lowercase letters) for a parameter are statistically similar (p ≤ 0.05) by the Tukey's HSD test.

Table 3 .
Effect of different sowing techniques and timing of mepiquat chloride application on yield and lint quality-related attributes of cotton.

Table 4 .
Physico-chemical indices of the experimental soil for the years 2017 and 2018.Adequate Each value indicates the average soil analysis values for both years 2017 and 2018.