Physiological Mechanism Of Three Kinds Of Potassium Fertilizer Regulating Cold Resistance Of Alfalfa In Horqin Sandy Land

In order to explore the effects of the type and dosage of potassium fertilizers on the cold resistance of alfalfa overwintering organs and the physiological mechanism of antioxidant enzyme activities,nitrogen content and carbohydrate protective substances changes.In this study,three kinds of potassium fertilizers,potassium chloride（KCl）,potassium sulfate（K₂SO₄）and dipotassium hydrogen phosphate（KH₂PO₄）,were applied to alfalfa in autumn at 100 kg·hm^-2,200 kg·hm^-2and 300 kg·hm^-2K₂O,represented by K₁,K₂and K₃respectively,with no potassium fertilizer as the control（CK）.The overwintering organs of alfalfa were excavated in the early freezing stage to simulate low temperature stress treatments of 4°C,-10°C,-20°C and-30°C,and the content of malondialdehyde and superoxide dismutase（SOD）,catalase（CAT）and peroxidase（POD）and other antioxidant enzymes activities,relative conductivity,content of nitrogen-containing protective substances,root activity and sugars such as soluble sugar,sucrose,fructose and starch substance content in the root crown of alfalfa.The results of the study are as follows:1.With the decrease of low temperature stress,the content of MDA in the root crown of alfalfa showed a continuous increase trend,and the activities of SOD,CAT and POD showed a trend of first increase and then decreasing,and all reached the maximum at-20℃;With the increase of potassium fertilizer application rate,the MDA content in the root crown of alfalfa treated with KCl,K₂SO₄and KH₂PO₄all showed a trend of first decreasing and then increasing,while the activities of SOD,CAT and POD showed a trend of increasing first and then decreasing;And under different low temperature stress treatments,K₂SO₄treatment reduced the MDA content of alfalfa root crown,and enhanced the activities of SOD,CAT and POD the most obvious.And when the application rate was200 kg·hm^-2K₂O,the content of MDA in the root crown of alfalfa was significantly different from that of CK（P<0.05）,which is 18.64%～44.44%lower than CK;Under this application amount,the activities of SOD,CAT and POD were the highest and were significantly different from CK（P<0.05）,the SOD activity increased by 30.02%～63.37%compared with CK under the four low temperature stress treatments,the activity of CAT increased by 37.37%～75.83%,and the activity of POD increased by 16.39%～59.10%.2.The relative conductivity of alfalfa root crown showed a continuous increase trend with the decrease of low temperature stress,while the soluble protein and proline contents showed a trend of first increase and then decreasing,and all reached the maximum at-20℃,while the free amino acid content showed a continuous decreasing trend.With the increase of potassium fertilizer application rate,the relative electrical conductivity of the root crown of alfalfa treated with KCl,K₂SO₄and KH₂PO₄decreased first and then increased under low temperature stress,the contents of soluble protein,proline and free amino acid showed a trend of increasing first and then decreasing;And under different low temperature stress treatments,K₂SO₄treatment had the most obvious effect on reducing the relative conductivity of alfalfa root crown and increasing the content of soluble protein,proline and free amino acids,and when the application amount was 200 kg·hm^-2K₂O,the root crown of alfalfa had the most obvious effect,alfalfa root crown relative conductivity is the lowest,and it was significantly different from CK（P<0.05）,which decreased by23.37%～38.02%compared with CK;The contents of soluble protein,proline and free amino acids were the highest and were significantly higher than those of CK（P<0.05）.Compared with CK,the soluble protein content increased by 32.48%～50.39%under the four low temperature stress treatments,the content of proline increased by43.95%～264.29%,the content of free amino acids increased by 56.50%～249.50%.3.With the decrease of low temperature stress,the root crown vitality and starch content of alfalfa treated with three potassium fertilizers of KCl,K₂SO₄and KH₂PO₄showed a continuous decreasing trend;The content of soluble sugar,sucrose and fructose showed a tendency to increase first and then The trend of decreasing changes,and all reach the maximum at-20°C.With the increase of the amount of potassium fertilizer,the root crown vitality,soluble sugar,sucrose,fructose and starch content of alfalfa treated with three potassium fertilizers of KCl,K₂SO₄and KH₂PO₄all showed a trend of first increasing and then decreasing;And under different low temperature stress treatments,the K₂SO₄treatment had the most obvious effect on improving the root crown vitality,soluble sugar,sucrose,fructose and starch content of alfalfa root crowns,and when the application rate was 200 kg·hm^-2K₂O,the alfalfa root crown activity,soluble sugar,sucrose,fructose and starch contents were significantly different from CK（P<0.05）.The alfalfa root crown activity increased by 56.03%～197.26%compared with CK under the four low temperature stress treatments,soluble sugar content increased by 16.38%～48.77%,sucrose content increased by 24.13%～46.22%,fructose content increased by 30.53%～59.95%,starch content increased by 35.73%～67.87%.In summary,among the three potassium fertilizers of KCl,K₂SO₄and KH₂PO₄,the alfalfa root crown antioxidant enzyme activity treated with 200 kg·hm^-2K₂O of K₂SO₄is the strongest,and the contents of nitrogen-containing protective substances and carbohydrate substances are also higher.That is more conducive to improving the antioxidant properties of alfalfa,regulating the cell osmotic potential of the alfalfa root crown,lowering the freezing point of the cytoplasm,alleviating the damage of plant cells,protecting the biofilm,and improving the cold resistance of alfalfa.