Identification Of Phosphorus Efficient Genotypes And Study On Efficient Utilization Of Phosphate Fertilizers In Sugarcane

Sugarcane（Saccharum officinarum L.）is the most important sugar crop in the tropical and subtropical regions,where the soils are acidic and deficient in phosphorus（P）bioavailibility due to the continuous high termperature and heavy rainfall.China is the third largest country for sugarcane cultivation area and sucrose production.Low P availablity in soils and low phosphate fertilizer utilization efficiency are major limiting factors for sugarcane production in China.Therefore,it is of importance to screen and breed P efficient genotypes,to investigate soil P budget and to recomand optimal P fertilization for improving yield and P efficiency in sugarcane.In this study,field investigation and soil culture experiments were conducted to verify the P fractions and P converting characteristics in the acidic soils from sugarcane plantation.Then,the genotypic variation in sugarcane P efficiency were characterized in the field,and the variation for insoluble P utilization efficiency were further evaluated in sand culture and hydroponics,which physiological mechanisms were investigated regarding to leaf photosynthetic characteristics,root morphology and secretions and so on.Based on above,mult-sites field experiments were carried out to elucidate soil P budget and optimal P fertilization level in sugarcane.The main results were listed as follows:1)Results from soil survey showed:most of the tested soils from sugarcane plantations were acidic and deficient in P availability with only 8.54%of available P to total P in soils.The three most abundant P forms in soils were iron bound P（Fe-P）,organic P（Org-P）and aluminium bound P（Al-P）,which account for 31.86%%,26.96%and 19.80%of the total P in soils,respectively.There are large varation is soil P forms,and the coefficient of variation（CV）for available P（Av-P）,Al-P,Fe-P,occluded P（Oc-P）,calcium bound P（Ca-P）and Org-P were 66.94%,64.90%,54.41%,78.06%,96.65%and43.88%,respectively.2)Soil culcture experiments indicated:the soils from sugarcane plantations coul d easily fix the applied P fertilizers,which were mainly converted into insoluble Al-P and Fe-P,and the Al-P were further converted to Fe-P with the prolonged culture time.Organic acids significantly affected the convertion of soil P forms.Without P fertilization,addition of organic acids could increase the contents of Av-P and Al-P,but decrease Fe-P.Both the type of organic acids and culture time affected the soil P conversion,and the activation capacity of organic acids was in order of citrate,ox alate and malate.With P fertilization,organic acids could facilitate P fertilizer converting to Av-P and Al-P,and therefore decrease P fixation and improve the bioavailability of P fertilizers.3)Field trials showed:there were significant genotypic variations in P effciency and P responsiveness in sugarcane,as reflected by shoot characteristics（such as seedling number,leaves number,plant height,leaves length,leaves width,leaf shape and CHL）,root parameters（root length,diameter,volume and weight）and P efficiency traits（plant biomass and P content）,suggesting that the tested 129 genotypes contain abundant genetic variation and selection potentials.In additon,the P efficiency of the 129sugarcane genotypes could be divided into four types:45 genotypes of P inefficiency with low responsiveness;25 genotypes of P inefficiency with high responsiveness;37genotypes of P efficiency with high responsiveness and 22 genotypes of P efficiency with low responsiveness,which might provide germplasm materials for breeding sugarcane varieties with high P efficiency.4)Sand culture experiments showed:there were significant genotypic differences in absorption and utilization of insoluble P,as indicated in the order of BC2-32,YN73-204and CP72-1210.Meanwhile,significant differences were also existed for P bioavailability among different insoluble P forms,which ranked in the order of Al-P,Org-P,Fe-P and Ca-P.The P efficient genotype BC2-32 showed higher root biomass,root length,root surface area and root volume than those of the other two genotypes,implying the importance of roots to efficient absorption and utilization of P in sugarcane.5)Results from hydroponics demonstrated that low P stress significantly promote the exudation of H⁺and organic acids from sugarcane roots with obivous genotypic varitions.Under low P stress,the P efficient genotype BC2-32 not only secreted more H⁺,but also exudated more organic acids,like tartrate,citrate and cxalate,and thereby had higher capability to activate inorganic P and increase P bioavailability.6)Multiple-site field experiments illustrated that P fertilization significantly increased sugarcane yield in all the tested sites regardless of the P fertilization level,demonstrating that P deficiency is the most limiting factor in sugarcane production regions.The rate increase of cane yield with P application decreased with the increasing content of soil Av-P.The agronomic critical values for Av-P in sugarcane could be monitored by three models,including linear-linear model,linear-plateau model and Mitscherlich.Among them,linear-linear model exhibited the best fit.Based on model fitting and yield responsive results,the soil Av-P budget was 22.9-49.9 mg/kg.In addition,P fertilizer application of P₂O₅ 120 kg/ha was recommended in this region considering the soil P budget and the current relative high level of soil Av-P.In conclusion,the current study evaluated soil P fractions and P converting characteristics,and identified the P efficient genotypes and analyzed the underlying physiological mechanisms at soil and crop levels in the main sugarcane production regions.Furthermore,soil P budget and recomand P fertilization level were obtained by multiple-site experiments.These results would play important roles in improving the utilization efficiency of P fertilizers,and promoting sustainable production of sugarcane in China.