Effects And Physiological Mechanisms Of Heterogeneous Phosphorus Application On Seedling Growth Of Citrus Rootstocks

Citrus is the second most important fruit and occupied the largest fruit cultivated areas in South China.It plays an important role in national economy.Soils of the main producing areas for citrus in China are mainly red soil,which has low phosphorus availability.The low phosphorus availability can seriously affect the production and quality of citrus.Currently,in order to supply the phosphorus,phosphorus fertilization was applicated in the forms of broadcasting,hole application or fertigation with water soluble fertilizer.These phosphorus application methods are the main cause of soil phosphorus heterogeneity.However,few systematic researches have been conducted on the growth and development of the citrus with the soil phosphorus heterogeneity.The present study was performed to investigate the effects of phosphorus on ten citurs rootstock seedlings through conducting hydroponic cultures and soils cultures with heterogeneous phosphorus application.An optimum phosphorus concentration for citrus seedling growth was developed and used for further comparison of different phosphorus fertilizer application techniques.Three citrus rootstocks were selected for further research on the effects of phophorus heterogeneity on citrus seedling.The response on root and shoot of citrus seedling to heterogeneous phosphorus was analyzed to evaluate the mechanisms of heterogeneous phosphorus on seedling growth in the citrus rootstocks.The main conclusions were as follows:(1)Phosphorus availability significantly affected the growth and nutrient accumulation of citrus rootstocks.Medium and high levels of phosphorus could increase the biomass of citrus seedlings,but decrease the root-shoot ratio of citrus seedlings.The highest root dry weight of Cuningmeng(Citrus jambhiri Lush)and Suanju(Citrus reticulata Blanco)were identified under 1.00 mmol/L phosphorus.Additionally,Xiangyuan(Citrus medica L.),Suancheng(Citrus aurantium L.),Honglimeng(Citrus limonia Osb.),Zhike(Poncirus trifoliate(L.)Raf.),Goutoucheng(Citrus aurantium L.)and Zhicheng(hybird of Citrus sinensis Osbeck.× Poncirus trifoliate Raf),had the highest root dry weight under the condition of 0.50mmol/L phosphorus,while Hongju(Citrus reticulata Blanco cv.Red tangerine)and Xiangcheng(Citrus junos Sieb.ex Tanaka)had the highest root dry weight at 0.01mmol/L phosphorus.The highest shoot dry weight of Cuningmeng,Suancheng and Zhicheng was identified with 0.50 mmol/L phosphorus.In addition,the other seven rootstock seedlings obtained the highest shoot dry weight with 1.00mmol/L phosphorus.Low phosphorus treatment increased the root-shoot ratio of nine citrus seedlings except Zhike.With regard to nutrients,the content and accumulation of nitrogen,phosphorus,potassium and magnesium in the plants were increased at the middle and high level of phosphorus supplyment.In conclusion,there was a significant difference in the seedling varieties of citrus rootstock seedlings in response to low phosphorus stress with comprehensive index evaluation.(2)The citrus rootstocks of Hongju,Suanju and Honglimeng were selected for the further study.The results showed that when phosphorus concentration was equal to/higher than 2.0mmol/L,the growth of both root and shoot would be severely inhibited,so did the nutrients uptake.Moreover,1.00 to 1.50 mmol/L of phosphorus was sufficient for the growth of Hongju and Honglimeng while 0.50-1.00mmol/L of phosphorus was suitable for Suanju.The correlation analysis showed that the seedling root length,total surface area,plant height,leaf area,dry weight of each part and total biomass,nutrient accumulation of three rootstocks were significantly related with parabolic relationships.(3)Compared with the no phosphorus treatment,five phosphorus application measures increased the total biomass and accumulation of nitrogen,phosphorus and potassium in both Suanju and Honglimeng.It could be concluded that the localized phosphorus supply was beneficial to the growth of Honglimeng rootstock seedlings.The treatment of localized phosphorus supply with humic acid was most suitable for Suanju seedlings growth.(4)The treatments of water and phosphorus heterogeneous supply increased the shoot elongation,the leaf area and the shoot biomass of Suanju.The treatments of one-sided phosphorus supply were beneficial to the nitrogen,phosphorus and potassium absorption and accumulation in the shoot of Suanju.The leaf and the total biomass of Hongju were inhibited with water and phosphorus heterogeneous supply.(5)Nitrogen and phosphorus heterogeneous supply have negative effect on the shoot growth of Suanju.It reduced the plant height,the leaf area and the biomass of leaf and shoot.However,the treatments could promote the shoot growth of the Hongju,and increased the plant height,the stem diameters,the dry weight of the stem and leaf.The absorption of nitrogen and phosphorus nutrients in shoot of Suanju were reduced by the treatments of nitrogen and phosphorus heterogeneous supply.On the contrary,the accumulation of nitrogen,phosphorus and potassium in shoot of Hongju were improved by those treatments.(6)The treatments of potassium and phosphorus heterogeneous supply decreased the leaf and the total biomass of Hongju.One-sided potassium and phosphorus supply was beneficial to the absorption of nitrogen and phosphorus in shoot of Hongju.The treatment of potassium and phosphorus separately supplied on both sides of the root significant inhibited the shoot growth of Suanju.One-sided phosphorus supply was optimum for the growth and absorption of nitrogen and potassium in shoots of Suanju.(7)In the experiments of phosphorus heterogeneity in root-split box,the same trend was found that the treatments of homogeneous supply of water,nitrogen and potassium with heterogeneous phosphorus supply were most beneficial to root morphogenesis and biomass of two rootstocks.Furthermore,the root growth had better performance in the treatments of one-sided phosphorus supply than that in one-sided nitrogen or potassium supply.