Mechanism Of Phosphate Fertilizer Inhibiting Cadmium Accumulation In Rice

Cadmium(Cd)is a kind of heavy metal element with high toxicity,high biological activity and wide distribution.Cadmium pollution not only affects crop growth and development,but also enters human body through food chain and increases human health risk.Rice is the main food crop in South China,and shows strong tolerance to cadmium stress.Therefore,it is of great significance to study agronomic measures with low cost,good effect of cadmium reduction for improving the safe utilization efficiency of cadmium contaminated farmland and ensuring the food safety production in China.Phosphorus is one of the essential elements for plant growth.Application of phosphate fertilizer can reduce the bioavailability of cadmium in soil and regulate the transport process of cadmium in rice.Through hydroponic,pot and field experiments,the mechanism of phosphate inhibiting the transport of Cd from rhizosphere environment to rice roots and from vegetative organs to grains was explored.The main results are as follows:1.Phosphate significantly inhibited the uptake of cadmium by rice roots and the transport of Cd from roots to shoots.In the cadmium stress environment,the cadmium absorbed by rice seedlings was mainly distributed in the cell wall and soluble fractions of root cells.Adding 1.0?10.0 mmol/L KH₂PO₄ to the nutrient solution containing 0.3 mg/L Cd significantly reduced the cadmium content in roots and shoots of high-Cd-accumulating cultivar T705 and low-Cd-accumulating cultivar X24.After adding 5.0mmol/LKH₂PO₄,Cd content of T705 and X24 was decreased by 60.6%and 70.1%,respectively.2.Under Cd stress,phosphate effectively promoted the synthesis of cysteine(Cys)and phytochelatins(PCs)in rice roots.The contents of Cys and PCs in T705 and X24 roots were significantly increased under Cd stress.The addition of 5.0 mmol/L KH₂PO₄ further increased the contents of Cys and PCs in rice roots.The contents of PCs in roots and shoots were negatively correlated with Cd content.Under the same cadmium stress environment,the content of PCs in rice roots was higher than that in shoots.The content of PCs in T705 was higher than that in X24.Compared with Cd exposure treatment,0.3mg/L Cd²⁺and 5mmol/L KH₂PO₄ combined treatment increased the contents of PCs in roots and shoots of T705 by 13.40%and 32.03%,and increased the contents of PCs in roots and shoots of X24 by 30.24%and 33.87%,respectively.3.Application of calcium magnesium phosphate(CMP)significantly inhibited the uptake of cadmium by rice and its transport to rice grains but increased the content of amino acids in rice grains in Cd-contaminated soil.Applying 0.5?2.5 g/kg CMP fertilizer to Cd-contaminated soil significantly reduced Cd concentration in rice roots,flag leaves,rachises and grains.The application of CMP increased the soil p H value and decreased the soil Eh during the rice growing period,decreased the content of sulfur in rice roots and increased the contents of phosphorus and iron in rice roots,increased the contents of free amino acids in rachises and grains.The application of 2.5 g/kg CMP fertilizer increased the contents of essential amino acids(EAAs)and non-essential amino acids(NEAAs)in grain by 17.8%and 71.7%respectively.4.Application of CMP fertilizer significantly reduced the Cd content in rachises and grains when rice planted in Cd-polluted farmland.After applying 0.1,0.3 and 0.5 g/m² CMP fertilizer to three paddy fields with different levels of cadmium pollution,the cadmium content in rachises and grains of six rice varieties decreased with the increase of CMP fertilizer dosage.The higher the level of cadmium pollution in paddy fields,the more obvious the effect of CMP fertilizer on reducing Cd in grains.After applying 0.3 g/m²CMP fertilizer in high,medium and low Cd-contaminated farmland,the Cd content in grains of late rice variety Jiyou decreased by 75.8%,54.2%and 32.2%,respectively.Applying 0.5 g/m² CMP fertilizer in low Cd-contaminated farmland,the Cd content in grains of six rice varieties decreased to below 0.2 mg/kg.5.Effect of CMP fertilizer on reducing Cd was affected by the application rate and rice growing seasons.When the application rate of CMP fertilizer was less than 0.1 kg/m²,its Cd reducing effect was only obvious in a few years or in a few varieties.However,when the application rate increased to 0.3 kg/m²,the Cd content in rice grains decreased by more than 30%.The response of early rice to CMP fertilizer was relatively slow,and the amount of Cd reduction in grains was small.There was a significant negative correlation between Cd content and phosphorus content in grains of late rice.It was found that the Cd accumulation in rice grains could be effectively inhibited either by increasing the application of phosphate fertilizer or by improving the transport of phosphorus to grains.In conclusion,increasing the phosphate concentration in rhizosphere environment can promote the development of iron-manganese plaque on rice root surface and the synthesis of phytochelatins in roots,inhibit the uptake of Cd by root and Cd transportation to shoots.Increasing the phosphorus content in rice can not only directly inhibit the accumulation of Cd in rice grains,but also increase the amino acid content,which indirectly inhibit the accumulation of cadmium in rice.Application of CMP Fertilizer is an important measure to reduce Cd pollution risk of rice in Cd-contaminated farmland in South China.