Effects Of Plant Growth Regulators On Cotton Floral Bud Differentiation,Yield And Quality

Floral bud differentiation is the beginning of flower development and is closely associated with growth rate,flower quantity and quality,and cotton yield.During floral bud development,some floral buds fail to form square due to various reasons such as environmental stress,nutritional status,fertilizer and water conditions.Square stage is sensitive to cotton yield and quality formation.Plant growth regulators can consolidate the differentiated floral buds into square and expand the material basis for the regulation of cotton yield and quality.In order to explore the effect of plant growth regulators on physiological mechanism of yield formation and floral bud differentiation,a hydroponic experiment was conduct in Nanjing,Jiangsu in 2016.Seeds of barely/direct-seeded cotton cultivar CCRI 50 were presoaked with 5 mg L-1 gibberellin(GA3),25 mg L-1 benzyl aminopurine(6-BA),and 150 mg L-1 dimethyl piperidinium chloride(DPC),with water set as control.At the same time,field experiments were conducted in Dafeng,Jiangsu Province(33°20’ N,120°46’ E)in 2016 and 2017.GA3,6-BA,and DPC applied with three methods[seed soaking(method 1),foliar spraying(method 2),seed soaking and foliar spraying(method 3)].The objectives of this study were(1)to explore the physiological mechanism of plant growth regulator(PGRs)on cotton floral bud differentiation;(2)to investigate the development of cotton square and the effects of PGRs with different application methods on squaring,flowering and boll setting,and consequently analyze the physiological mechanism effect of PGRs on cotton yield;(3)to evaluate the effect of PGRs with different application methods on seed quality based on sucrose metabolism.The main findings were as follows:1.Effects of plant growth regulators on cotton floral bud differentiationPlants from seed pre-treated with GA3 and 6-BA differentiated more floral buds relative to control,while DPC application initiated fewer floral buds than control.GA3 and 6-BA treatments significantly increased the content of zeatin riboside(ZR)and gibberellic acid(GA3),but decreased the indole-3-acetic acid(IAA)content.Consequently,the ZR/IAA and GA3/IAA ratios were markedly increased,contributing to higher floral bud numbers.Meanwhile,GA3 and 6-BA treatments up-regulated GhSOC1,GhMADS13 and GhAGL24 expression,which was related to higher sucrose contents,inducing floral initiation.Whereas the GhMADS13 was down-regulated to suppress floral bud differentiation under DPC application.Surprisingly,the floral-associated genes were more sensitive to GA3 than 6-BA,which induced the differences in bud numbers between GA3 and 6-BA at the beginning of flower bud differentiation.The differentiation rate of floral was higher in 6-BA treatment than in GA3 treatment.Sucrose,as a signal molecule,provides energy for initiation of flower bud differentiation,since sucrose content was not affected during the middle and late stages of floral bud differentiation.In conclusion,seed pre-treated with GA3 and 6-BA affected hormone content,induced sucrose accumulation in apical buds and regulated genes involved in floral induction,which impacted floral bud differentiation.2.Effects of plant growth regulators on change of floral organs during boll formationGA3 and 6-BA increased squaring rate,and the increase in 6-BA is larger than in GA3 with method 1 and method 3 whereas DPC decreased squaring rate with method 1.GA3 and 6-BA significantly increased photosynthesis rate(Pn)accordingly affecting the subtending leaf carbohydrate content and translocation,resulting in higher sucrose and starch contents in floral buds,which substantially enhanced dry matter of floral buds and reduced abscission rate,inducing higher boll retention and boll opening rate.GA3 advanced anthesis and boll opening date,shortened the boll period for 1-2 d while 6-B A shortened boll period for 2-3 d with all application methods relative to control.PGRs also improved the proportion of high quality boll(summer boll and early autumn boll).3.Effects of plant growth regulators on cotton yieldSeed cotton yield was increased by GA3 and 6-BA with all application methods compared to control.DPC treatment increased cotton yield only with application method 3.The number of bolls is most sensitive to the response of plant growth regulators,which is the main reason for the difference in seed cotton yield.Additionally,6-BA showed equal effects to GA3 on yield and yield components with application method 2,whereas induced a greater promotion than GA3 with application method 1 and 3.Moreover,PGRs’ application achieved the greatest yield with method 3 due to increased boll number and boll weight.Cottonseed yield and embryo oil and protein yield were increased by GA3 and 6-BA relative to control.6-BA induced greater promotion in seed yield and embryo oil yield than GA3 with application method 1 and 3.No differences were observed in embryo protein yield between GA3 and 6-BA with application method 1 and 2.4.Effects of plant growth regulators on fiber and seed qualityGA3 and 6-BA improved fiber quality by increasing fiber length with application method 1 and 3.Cottonseed is the "intermediate carrier" of fiber development,GA3 and 6-BA significantly increased the seed index,kernel index,oil and carbohydrate content with all application methods.DPC improved seed index,kernel index,oil and carbohydrate content only with application method 3.Additionally,boll number was the dominant driver of yield increment for 6-BA and the increase in boll number was larger for 6-BA than that of GA3,contributing to the difference in cotton seed yield between GA3 and 6-BA with application method 1 and 3.DPC increased the cotton seed yield only with application method 3.Specially,the activities of sucrose synthase(SuSy),acid invertase(C-INV)and sucrose synthase(SPS)in 6-BA were significantly higher than those in GA3 treatment with application method 1 and 3,the oil content in embryo is significantly positively correlated with the carbohydrate content and the increased carbohydrate is beneficial to oil synthesis.Therefore,6-BA had higher oil content than GA3 with application method 1 and 3.The activity of SPS showed sensitivity to PGRs with different application methods.The response of SPS activity to GA3 and 6-BA with application method 1 and 3 occurred earlier(31 DPA)than application method 2(45 DPA)In summary,GA3 and 6-BA promoted cotton floral bud differentiation,increased bud number,consolidated the number of floral bud developing into square,improved squaring rate and boll number.Meanwhile,GA3 and 6-BA improved proportion of high quality boll and boll opening rate,laid the theoretical basis for high quality and yield,and mechanized harvest of barely/direct-seeded cotton.