Physiological Mechanism Of Phosphorus Deficiency Impacting Cotton Yield And Fiber Development

China is a big country of cotton production and cotton textile industry.The cotton industry provides economic income for cotton farmers and raw materials for the textile industry,and plays an important role in the national economy.Agricultural science and technology input in our contury has continuously improved the per unit area yield and total yield of cotton.In addition,consumers put forward higher requirements for cotton fiber quality.In order to ensure cotton lint yield and fiber quality,the cotton plant needs to absorb a large amount of phosphorus（P）nutrition during the whole growth period.In China,cotton production is concentrated in Xinjiang cotton area,saline-alkali land,beaches,and sandy dry land.The soil types of above cotton fields are mostly calcareous soil,which has the strong fixation effect on P.Coupled with drought and little rain,the problem of lack of available phosphorus（AP）content in the soil is prominent.Therefore,it is necessary to apply a large amount of P fertilizer to ensure the high yield and quality of cotton,which further leads to the reduction of P fertilizer utilization efficiency in cotton fields.However,there is a shortage of high-grade P rock resources in China,and the mining recovery rate is low,which may cause a shortage of P fertilizer in the future.Therefore,the cotton cultivars with low-P tolerance were screened to study the physiological mechanism of cotton yield formation and fiber development under low-P stress tolerance,so as to provide technical support for cotton production under soil P deficiency.This study was conducted in Anyang（36°06′N and 114°21′E）in 2018-2020.Tests on25 cotton cultivars（strains）used as experimental materials have been done for studying the effects of low-P on cotton agronomic traits,yield and yield components and fiber quality.Two cotton cultivars with different low-P tolerant ability were screened out to further study the physiological basis of the differences in low-P tolerant ability.This study included three experiments:To screen the low-P tolerance cotton cultivars by hydroponics test;（2）To screen and verify the low-P tolerance cotton cultivars by field trail;（3）To study the physiological basis of yield formation and fiber development in different low-P tolerance cotton cultivars by pool-culture experiment.1.Screening of low-P tolerant cotton cultivars under hydroponics culture and field verificationsIn this study,25 cotton cultivars（strains）were compared in hydroponic culture at two P levels(0.01 and 0.5 mmol L^-1KH₂PO₄)to study P efficiency at seedling stage and conducted two P rates(0 and 90 kg P₂O₅ha^-1)to investigate yield characters and fiber quality traits in field condition.Under low-P stress in hydroponic culture(0.01 mmol L^-1KH₂PO₄),there were significant differences in agronomic traits and physiological indicators among different cultivars（strains）of cotton,and the coefficient of variation of each character ranged from10.0%to 162.1%.In field conditions,P deficiency(0 kg P₂O₅ha^-1)reduced the single cotton plant biomass by 19.7%,mainly due to the reduction of stem and branch biomass by 34.5%.The boll number per plant and boll weight decreased by 8.0%and 8.3%respectively,resulting in a 10.1%decrease in lint yield.Meanwhile,the fiber length and strength decreased by 1.5mm and 3.0 c N tex^-1respectively.Phosphorus uptake efficiency（PUp E）decreased by 26.3%,but phosphorus utilization efficiency（PUt E）increased by 9.1%in low-P deficiency.The results of principal component analysis（PCA）based on 21 indexes measured at seedling stage showed that the cumulative variance contribution rate of the first five principal components reached 82.6%,and the five indicators including the relative values of the single plant PUp E,shoot PUt E,plant height,SPAD,net photosynthesis rate（Pn）could effectively evaluate the low-P tolerant ability of cotton cultivars at seedling stage.According to cluster analyses（CAs）of these 25 cotton cultivars（strains）based on these ten indicators at seedling stage,yield characters,and fiber quality traits,a comprehensive analysis of these three different CA results showed that P efficiency at seedling stage could affect yield and fiber quality to a certain extent.Under the condition of low-P stress,the cultivars with low-P tolerant at seedling stage,such as ZMS-79,ZMS-12,JM-958,and ZMS-112,had a higher yield and better fiber quality,while the low-P sensitive cultivars including LMY-28,LNM-18,JM-169,and XM-33B,had a lower yield and poor fiber quality.2.Soil available phosphorus deficiency reduces boll biomass and lint yield by affecting sucrose metabolism in cotton-boll subtending leavesThe pool-culture experiments were conducted using two cotton cultivars（ZMS-79,low-P tolerant;LMY-28,low-P sensitive）under three soil available phosphorus（AP）levels:(P₀（severe soil AP deficiency）:3±0.5,P₁（mild soil AP deficiency）:6±0.5,and P₂（control）:15±0.5 mg kg^-1)to ascertain the effect of soil AP on boll biomass and lint yield.P₀and P₁reduced the P concentration and net photosynthetic rate（Pn）of subtending leaves by12.1%～31.8%and 3.3%～17.3%,thus,reducing boll biomass and lint yield by 3.4%～26.9%and 13.2%～62.1%.Additionally,soil AP deficiency decreased boll wall:boll,lint:boll,and lint:seed,and increased seed:boll ratio.The decline of sucrose transformation rate（Tr）and activities of initial ribulose-1,5-bisphosphate carboxylase-oxygenase（Rubisco）and sucrose synthase（Su Sy）was less on fruiting branch 10-11(FB_10–11)than FB_2–3and FB_6–7under low soil AP.These results suggest that higher lint:seed and the proportion of total lint yield on upper fruiting branch positions(FB_9–12)are positive responses to soil AP deficiency.Moreover,soil AP deficiency also reduced the Tr and activities of sucrose-metabolizing enzymes,such as Rubisco,Su Sy,and sucrose phosphate synthase（SPS）,while increased carbohydrate levels（soluble sugar,sucrose,and starch）and the activity of cytosolic fructose-1,6-bisphosphatase（cy-FBPase）in the subtending leaves.The sucrose transport from"source（subtending leaves）"was blocked,resulting in excessive accumulation of carbohydrate,which further reduced the activity of sucrose metabolizing enzymes.It might be the main physiological reason for the reduction of cotton boll biomass and lint yield.Higher Tr and activities of initial Rubisco and Su Sy in the subtending leaves improved boll biomass and lint yield.The sucrose and starch contents,cy-FBPase,and SPS activities of LMY-28were more sensitive to low soil AP than ZMS-79,possibly explaining the different sensitivities of both cultivars to low soil AP.3.Soil available phosphorus deficiency reduced fiber length by decreasing the maximum velocity of fiber elongation(V_Lmax)Soil AP deficiency（P₀and P₁）reduced the contents of cotton fiber osmotically active solutes（potassium（K⁺）,malate,soluble sugar,and sucrose）by 10.6%～18.2%、18.1%～28.5%、11.0%～18.0%,and 13.7%～22.1%,respectively,affects vacuoles to provide power for fiber elongation through osmoregulation.Moreover,it reduced the activities of enzymes engaged in fiber elongation（plasma membrane H⁺-ATPase（PM-H⁺-ATPase）,vacuole membrane H⁺-ATPase（V-H⁺-ATPase）,vacuole membrane H⁺-PPase（V-H⁺-PPase）,and phosphoenolpyruvate carboxylase（PEPC））by 12.4%～16.5%、10.6%～17.0%、18.5%～26.2%,and 21.8%～27.5%,respectively,which is more unfavorable to provide transmembrane transport power for osmotically active solutes and acidified condition for fiber cell wall,thus limiting the fiber elongation.Finally,soil AP deficiency reduced fiber length by 0.7～1.7 mm,mainly due to the reduction of the maximum velocity of fiber elongation(V_Lmax)by 5.9%～11.8%.Higher malate content and V-H⁺-ATPase and V-H⁺-PPase activities in the cotton fiber improved fiber length.The contents K⁺and sucrose,and activities of V-H⁺-ATPase and PEPC in the fiber of LMY-28 were more sensitive to low soil AP than ZMS-79,possibly explaining the sensitivity of fiber length in LMY-28 to low soil AP.4.Soil available phosphorus deficiency reduced fiber strength by decreasing the maximum velocity of fiber strength increment(V_Smax)Soil AP deficiency（P₀and P₁）decreased the activities of related enzymes engaged in fiber thickening（soluble acid invertase（SAI）,Su Sy,SPS,andβ-1,3-glucanase）by11.8%～27.8%、19.3%～32.4%、8.8%～30.1%,and 24.2%～35.7%,respectively.Further,the cellulose synthesis,one of the main material for the formation of fiber strength,was blocked.Eventually,soil AP deficiency reduced fiber strength by 1.4～3.3 c N tex^-1,mainly due to the reduction of the maximum velocity of fiber strength increment(V_Smax)by 9.5%～11.7%.Higher cellulose content and SAI andβ-1,3-glucanase activities in the cotton fiber improved fiber strength.Compared with LMY-28（low-P sensitive）,ZMS-79（low-P tolerant）had more stable performance of the fiber strength in the face of low-P stress was mainly due to the more stableβ-1,3-glucan and cellulose content and SPS activity of cotton fiber.