Foliar Applied Mepiquat Chloride Induced Morpho-physiological Changes In Late-planted Cotton (Gossypium Hirsutum L.) At High Density

Cotton is the most important fiber crop globally.China is one of the major cotton producers with its first production rank in the world for more than 30 years.Double cropping systems are adopted in China,such as cotton-rapeseed and cotton-wheat cropping systems have been widely practiced in the Yangtze River Valley of China,contributing12.3%of the total national production.This type of cotton cropping system is a laborious and increasingly costly,as more Chinese laborers are migrating to cities.Therefore,in recent years cotton production faces significant reduction as per high cost due to multiple inputs of management and materials.In this case,a new profitable planting model has been practiced with lower cost without compromising yield which is characterized with late and direct sowing,high density and low fertilization,which is a promising alternative for cotton production in this region.But we hypothesize that cotton production cost could be further reduced by ignoring mepiquat chloride（MC）application which is widely used under the conventional planting model.A two-year（2015-2016）field experiment was conducted at Huazhong Agricultural University,Wuhan,China with a randomized complete block design for five MC treatments（MC0-MC120）and four replications.MC was applied three times at 5 main-stem leaf intervals initiated from the 6^thleaf stage in respective dosages(MC0,MC30,MC60,MC90,and MC120 represent 0,30,60,90,and 120 g ha^-1).The main findings are summarized below:1.MC application significantly enlarged the boll setting period in the first growing season while no differences were found in seedling and squaring periods for both years.Cotton plant height,stem diameter,number of nodes,internode length,green leaves,number of fruiting branches,leaf area index and leaf area duration were markedly reduced by 9-40%,2-13%,4-27%,3-19%,6-24%,2-11%,8-38%and 12-36%,respectively,under MC application as compared to control（MC0）at different growth intervals.2.Lint yield was reduced by 6-29%with increment of MC dosage due to reduced cotton plants biomass(from 10365 kg ha^-1of MC0 to 8439 kg ha^-1of MC120),particularly the reproductive organs biomass(from 4923 kg ha^-1of MC0 to 3764 kg ha^-1of MC120)averaged across two years.Biomass accumulation of MC0 had a higher rate for plant biomass and a longer period for reproductive organs biomass during the fast accumulation period.3.Nutrients（nitrogen（N）,phosphorus（P）,potassium（K））uptake,distribution and accumulation rate remained unaffected in MC treated plants,specifically the uptake in the reproductive organs ranged from MC0 to MC120 of 57 to 53 kg N ha^-1,20 to 19 kg P ha^-1,and 45 to 44 kg K ha^-1averaged across two years.4.Net photosynthesis was evidently decreased by 1-28%in MC treated plants at various time intervals followed by the similar trend of stomatal conductance and transpiration rate.Conversely,water use efficiency was considerably increased while intercellular CO₂ concentration did not significantly influence by MC application.The activity of Ribulose-1,5-bisphosphate carboxylase-oxygenase was decreased with the increment in MC dosage and positively associated with CO₂ exchange rate.5.MC application substantially increased SPAD value,chlorophyll a,chlorophyll b and total chlorophyll contents by 2-20%,3-56%,2-45%and 3-51%,respectively,with MC dosage at different intervals.Further,carbohydrate contents in the 4^thleaf from the top were increased by 4-48%,3-48%,2-63%and 2-43%for sucrose,glucose,fructose and starch,respectively,at various sampling intervals.6.The activities of enzymes involved in carbohydrate metabolism decreased in MC treated plants by 2-25%,2-16%,3-19%and 1-17%for sucrose phosphate synthase,alpha amylase,beta amylase and total amylase,but increased by 2-40%,2-30%and 3-52%for sucrose synthase,soluble acid invertase and neutral invertase,respectively.Further,the relationship of sucrose with sucrose phosphate synthase and that of starch with alpha,beta and total amylase showed negative association.Consequently,MC induced reduction in photosynthesis and sucrose phosphate synthase activity led to yield reduction possibly due to diminished sucrose translocation from source to sink tissues.These findings suggest that MC application could be omitted under the new cotton planting model to harvest more profit.