Physiological And Biochemical Mechanism Towards Improved Lint Yield Of Cotton Cultivars Under Different Planting Densities

Manipulation in the planting density and choice of cultivar are effective management components in any cropping system that targets to increase the fit between the environmental resource availability and crop requirements.An expanding population necessitates global efforts to increase crop production,especially those fulfilling the food and clothing need.On the other hand,migration of the labors to cities,lack of information regarding cultivar and density selection,high input costs,with the use of multiple management and material inputs have threatened cotton productivity.Therefore,in recent years cotton production faces significant yield reduction.This situation require strategies for efficient cotton production i.e.the wise use of crop management inputs may help to produce more crop per unit area with reduced production costs.Therefore,we hypothesize that this problem can be counteracted by a beta growth planting model which is characterized by once only low nitrogen fertilization at flowering stage with a moderately populated stand by late and direct sowing was proposed.A two-year（2017-2018）field experiment was conducted at Guangxi University,Nanning,China with a split plot arrangement for three planting densities(low,2.00×10⁴;moderate,2.67×10⁴ and high 3.33×10⁴ ha^-1)and two cultivars（Zhongmian-16 and J-4B）.This study sought to elucidate how changing planting density affect leaf structural（stomatal density,stomata length,stomata width,pore perimeter and leaf thickness）,leaf functional（leaf gas exchange（net photosynthetic rate,stomatal conductance,intercellular CO₂ concentration and transpiration rate）chlorophyll fluorescence（photosystem II,actual quantum yield of PSII,maximal photochemical efficiency of PSII,electron transport rate,non-photochemical quenching,photochemical quenching）leaf nitrogen metabolisms,osmolyte contents）,lint yield,nutrient uptake,growth,biomass accumulation and fiber quality of field grown cotton cultivars at various developmental stages.This also explores the quantitative relationship between cotton cultivar and planting density.Results revealed that cotton lint yield,fiber quality,growth,nutrient uptake,biomass yield,chlorophyll fluorescence,leaf gas exchange and leaf structural attributes were considerably influenced by planting density and cultivar in both years.Moderate planting density substantially improved leaf net photosynthetic rate,stomatal conductance,intercellular CO₂concentration,leaf structural characteristics i.e.stomatal density,number of stomata,pore perimeter,length and width as well increased activity of photochemical quenching,actual quantum yield of photosystem II and maximum quantum yield of PSII.High planting density significantly repressed cotton functional leaf sugar,protein,amino acid,chlorophyll contents,nitrogen metabolism,and lint yield.Moderate density crops produced 15.9%-16.5%and 12.3%-10.5%more bolls m^-2 with 20.6%-13.4%and 28.9%-24.1%greater lint yield compared with low and high density,respectively.This increment in yield under moderate density was the result of improved leaf structural,leaf gas exchange,chlorophyll fluorescence traits,protein,Chl a,Chl b,amino acid and nitrogen metabolism during late reproductive growth contributed carbon assimilation and subsequently boll development.Favorable leaf microclimate could also have sustained the activities of leaf nitrate reductase and nitrite reductase;increased amino acid and protein contents with better distribution of nitrogen metabolic assimilate.In contrast,substantial reductions in lint yield under higher density for Zhongmian-16 can be associated with decreased in the leaf functional attributes（photosynthetic and chlorophyll fluorescence）led to depression in leaf photosynthetic capacity.Across the cultivars,J-4B performed better in terms of lint yield and physiological traits in both years.In addition,high planting density resulted in 6.2 and 12.6%larger stems,and fruiting nodes m^-2,while low density produced 37.5 and 59.4%maximum height to node ratio compared with medium and high density,respectively.Increasing planting density caused a remarkable reduction in total biomass production,however moderate density produced 26.4-15.5%,24.7-12.6%and 10.5-13.6%higher biomass accumulation rate at peak bloom,boll set and plant removal stage over low and high density crops in both years,respectively.It is cleared that at early growth stages,planting density has no effects on the reproductive organs biomass accumulation.This gain in biomass formation was associated to greater phosphorus（P）accumulation at the high density and moderate density crops in J-4B,which had higher both average(1.9 V_T kg.ha^-1.d^-1),and maximum(2.2 V_M kg.ha^-1.d^-1)total plant P accumulation rate as well as both higher average(0.8 V_T kg.ha^-1.d^-1)and maximum(1.0V_M kg.ha^-1.d^-1)reproductive organs P acquisition,respectively.These data highlighted that late reproductive structure development is closely allied with leaf physiological attributes.In addition,plants under low density produced 5.3-18.5%,9.5-15%,7.8-12.8%better fiber length,fiber strength and fiber micronaire values,over moderate and high planting density,respectively.Increasing planting density had a severe negative impact on cotton fiber quality and reproductive organ biomass yield due to the poor light penetration and distribution into the lower crop canopy and thus,during boll filling poor rate of P acquisition.Conclusively,the data underscore the importance of proper agronomic methods for cotton production that cv.J-4B and Zhongmian-16 grown under moderate and lower density could be a promising option based on improved lint yield under a beta growth model in subtropical regions.