Effects Of Amino Acids And Calcium Magnesium Phosphate Fertilizer On Characteristic Of Cadmium Accumulation In Rice And Their Mechanism

Rice is the main food crop in China.In southern acidic soils,cadmium（Cd）,from soil and agricultural inputs,is easy to accumulate in rice vegetative organs and transfer to rice grains.It can enter human body through food chain and threaten human health.Therefore,exploring effective methods to reduce Cd content in rice has great significance to ensure the food security of China.This paper studied the effects and mechanism of foliar spraying of amino acids on Cd reduction,as well as the influence of calcium magnesium phosphate fertilizer（CMP）and amino acids on Cd accumulation in rice,so as to provide a theoretical basis for the safety production technology of rice in Cd-polluted farmland and the research of Cd-reducing foliar fertilizer.The main results were as follows:1.The content of amino acids and nutrient elements are key factors affecting the Cd content in rice grains.The content of amino acids and nutrient elements in rice grains was significantly different between early rice and late rice varieties.The content of five kinds of amino acids including glutamate（Glu）,alanine（Ala）,phenylalanine（Phe）,glycine（Gly）and threonine（Thr）were closely related to the content of Cd in rice grains.When rice was planted in light Cd-polluted farmland,the Glu content of high cadmium accumulation（HCA）cultivars was significantly higher than that of low cadmium accumulation（LCA）cultivars.When rice was planted in heavy Cd-polluted farmland,the Glu content declined dramatically,and there was no longer an obvious difference in Glu content between HCA and LCA cultivars.Cd in rice had a significant inhibitory effect on the synthesis of Glu and other amino acids.When Cd content in rice was less than 0.40 mg·kg^-1,Glu played the main detoxification effect while when the Cd content in rice was in the range of 0.40-1.16 mg·kg^-1,Mn played the main detoxification effect.2.The Cd content in rice is reduced significantly after spraying amino acid foliar fertilizer.In Cd-contaminated farmland,spraying 1.0 g·L^-1·m^-2 methionine,0.4 g·L^-1·m^-2 isoleucine,1.0 g·L^-1·m^-2 glycine,0.2 mg·L^-1·m^-2 tryptophan,0.1 g·L^-1·m^-2 phenylalanine,0.1 g·L^-1·m^-2 serine and 1.0 g·L^-1·m^-2 threonine at flowering stage could significantly reduce Cd content in grains,rachises and nodes of both early and late rice.The content of Mn and Zn in grains,rachises,nodes and stem was decreased at the same time.3.Application of CMP in the rhizosphere can significantly promote the development of roots and reduce the Cd content in grains.Applying 0.1-0.5 kg·m^-2 of CMP in the field could increase the root quantity of early rice and late rice.The root quantity of early rice and late rice was increased by the basal application with 0.3 kg·m^-2 CMP fertilizer by 185.0%and 61.6%.The proportion of fine roots increased by 28.7%and 9.7%,respectively.The surface area and iron plaque content of fine roots of late rice increased by 92.7%and 63.2%.It also reduced Cd content in fine,medium and thick roots by 21.7%,27.9%and 23.2%.The Cd content of grains,rachises,necks,nodes,flag leavies and stem bases decreased by 75.9%,76.4%,79.8%,83.5%,45.2%and 61.3%.4.Increasing application of CMP can reduce the Cd content in rice protein components significantly.Compared with control,the addition of 0.3 kg·m^-2 of CMP reduced the Cd and Zn content in rice protein components by 85.7%and 21.6%,the K content increased by 67.7%;the rice N and P content increased by 389.9%and 182.3%,the total amino acid content increased by 24.8%.Increasing application of CMP had a little effect on the content of Cd and other elements in rice starch.5.Increasing the concentration of phosphate in the rhizosphere can inhibit the absorption of Cd by the root system and Cd transport from roots to the shoots remarkably.Increasing the concentration of KH₂PO₄ in the nutrient solution containing Cd can significantly increase the content of N and P in the leaves of Zhongzao 35 and T215;adding 10 mmol·L^-1 KH₂PO₄ increased the N content of Zhongzao 35and T215 by 61.2%and 61.4%,respectively.The P content increased by 38.4%and 71.4%.The Cd content decreased by 45.3%and 22.9%in the shoots,as well as the Cd content decreased by 32.9%and74.6%in the roots.At the same time,Ca content in the shoots of Zhongzao 35 and T215 increased by24.4%and 28.5%,respectively.Ca content in roots increased by 11.3%and 60.6%;Mn content in roots was reduced by 50.2%and 74.9%,Zn content was reduced by 30.8%and 36.4%.The total amount of free amino acids increased by 34.3%and 130.3%in shoots,while decreased by 42.8%and 56.1%in roots of Zhongzao 35 and T215,respectively.In summary,the amino acid metabolism and the ability of the accumulation of essential elements in rice vegetative organs and grains are closely related to the cadmium accumulation characteristics and cadmium tolerance of rice cultivars.Spraying specific concentrations of amino acids on the leaves can remarkably reduce the Cd content in rachises and grains of rice.Increasing the phosphate concentration in the rhizosphere environment can effectively increase the quantity of roots,root surface area and the content of iron plaque,which inhibits the absorption and transport of Cd by rice plants.These results are of great significance for the improvement of agronomic measures to reduce Cd in rice and the research of foliar fertilizer products.