Establishment And Application Of Critical Phosphorus Concentration Dilution Curve For Cotton

Phosphorus（P）is a component of important life substances such as nucleic acid,protein and ATP and participates in the regulation,of most physiological and biochemical processes in plants.Cotton is an important economic crop in China.As an essential nutrient element for the growth and development of cotton,P plays an important roles in stress resistance,cotton yield and quality formation.P deficiency will significantly hinder cotton growth and reduce cotton yield.Only the appropriate amount of phosphorus application could ensure cotton yield and reduce input.The critical phosphorus concentration dilution model could quantitative the critical P concentration changes,while the phosphorus nutrient index（PNI）based on the dilution model could accurately guide the determination of suitable phosphorus application amount for cotton.Therefore,field experiments were conducted using Lu 54（low-P-sensitive cultivar）and Yuzaomian 9110（low-P-tolerant cultivar）as materials under five phosphorus levels(0,50,100,150 and 200 kg P₂O₅ hm^-2)in 2017 and 2018 in Dafeng,Jiangsu province.By analyzing the effect of phosphorus application on biomass,yield and phosphorus accumulation of cotton,the critical phosphorus concentration dilution curve of cotton cultivars with contrasting phosphorus sensitivity was established,and the stability of the model was verified.At the same time,PNI was calculated based on the dilution curve to determine the appropriate phosphorus application rate for cotton.The main results are as follows:1.Effects of phosphorus application on cotton yield and yield compositionPhosphorus application had no significant effect on cotton boll weight,but significantly increased boll number,and consequently increased cotton yield.Under P application of 50,100,150,and 200 kg P₂O₅ hm^-2,boll number of Lu 54 increased by 16.0%,28.0%,36.0%,and 36.9%in 2017,and by 19.1%,31.6%,39.4%,and 37.9%in 2018,respectively,comepared with 0 kg P₂O₅ hm^-2;while that of Yuzaomian 9110 increased by 7.4%,27.0%,32.3%,and 32.6%in 2017,and by 6.3%,18.4%,23.0%and 25.6%in 2018,respectively.Cotton yield of Lu 54 increased by 16.6%,29.0%,39.2%,and 39.8%in 2017,and by 20.3%,34.6%,44.3%,44.9%in 2018;yield of Yuzaomian 9110 increased by 8.7%,28.9%,34.2%and 35.6%,in 2017,and by 6.6%,21.0%,26.6%and 29.0%in 2018,respectively,under P application of 50,100,150,and 200 kg P₂O₅ hm^-2.Thus,the yield increase of Lu 54 was larger than that of Yuzaomian 9110.More importantly,cotton yield significantly increased with P application increase at the application range of 0-150 kg P₂O₅ hm^-2,the yield increase was not significant when P application was greater than 150 kg P₂O₅ hm^-2.2.Effects of phosphorus application on cotton biomass and phosphorus accumulation and distributionP application significantly increased the biomass and P accumulation of cotton at each growth stage except seedling stage.The root shoot ratio of cotton decreased with the increase of P application.Similar with the result of yield,the aboveground biomass and P accumulation,organs’（including the main stem,main stem leaf,fruit branch,fruit branch leaf,reproductive organs）biomass and P accumulation were all increased as P application increase within the range of 0-150 kg P₂O₅ hm^-2.As P application increased,biomass and P distribution in fruit branch,fruit branch leaf and reproductive organs were significantly increased,but biomass and P distribution in main stem were decreased.Effects of P on main stem leaf biomass and P distribution varied in different growth stage.Besides,P utilization rate was significantly decreased with P application increasing from 0 to 150 kg P₂O₅ hm^-2.P utilization rate was relatively larger,biomass and P accumulation and distribution of reproductive organs were more appropriate under 150 kg P₂O₅ hm^-2 than that under 200 kg P₂O₅ hm^-2,indicating that appropriate amount of P application could not only improve cotton growth,but also sustain relatively higher phosphorus utilization rate in cotton.3.Cotton critical phosphorus concentration dilution model and phosphorus nutrient index（PNI）According to the relationship between aboveground biomass and P concentration in 2017,the critical phosphorus dilution curve models of the two varieties were established(Lu 54:Pc=0.784W^-0.221,Yuzaomian 9110:Pc=0.774W^-0.198).RMSE of the two dilution curve models were 0.1296 and 0.1383,respectively.The n-RMSE was 17.8504%and 18.5447%,respectively,indicating that the model has good stability,and the stability of the model of Lu 54 is slightly higher than that of Yuzaomian 9110.Compared with the model parameters of Yuzaomian 9110,the parameter a and b of Lu 54 increased by 1.29%and 11.62%,respectively,indicating that phosphorus-sensitive cotton cultivar accumulated more P in the early growth period.PNI which calculated based on the dilution model was first increased and then decreased with cotton growth progressed.On the same sampling period,PNI increased with phosphorus application increase.Under P application of 150 kg P₂O₅ hm^-2,the overall PNI value was around 1;under P application of 200 kg P₂O₅ hm^-2,PNI value was bigger than 1,indication that P in cotton plant was surplus;while P application rate was lower than 150 kg P₂O₅ hm^-2,PNI value was smaller than 1,indicating that the supply of P was insufficient.To sum up,considering comprehensively the effect of P application rate on cotton yield,biomass and PNI,150 kg P₂O₅ hm^-2 was considered the recommended optimal P application rate in this region.