Physiological mechanism of potassium application in decreasing density of aphid (Sitobion avenae F.) in wheat

Objectives Based on the view of plant basic metabolism, the experiment revealed the mechanism of reducing aphid (Sitobion avenae F.) population density of wheat by applying potassium, and provided scientific basis for ecological regulation of wheat aphids in the field.Methods A solution culture method and fully balancing design with five potassium levels (0.005, 0.05, 0.5, 2 and 10 mmol/L KCl, respectively) and two aphid infection levels (aphid infection and not infection) were used in this experiment, and three repetitions were arranged for each treatment. When the wheat seedlings had 5 expanded leaves, 5 adult wingless aphids in the same size and age were set on the latest fully expanded leaves per wheat plant. Then the density of aphids were surveyed and the wheat samples were collected respectively at the fourth and eighth days after the aphid infection to study effects of potassium level and aphid feeding on free amino acids, soluble protein and the content of soluble sugar of wheat leaves.Results The results showed that at the fourth day after the infection of aphids and in the range of K 0.005-0.5 mmol/L, the aphid density of unit dry weight of wheat decreased significantly as potassium level increased, and if improving the potassium level further, the aphids population density did not increase significantly. At the eighth day after the aphid infection, the aphid population density decreased significantly with the increase of potassium levels in the arranged range. As potassium level increased, the free amino acid contents in the wheat plants uninfected by aphids showed a decreasing trend, while both soluble protein and soluble sugar contents showed increasing trends at first and then showed decreasing trends. In the wheat plants infected by aphids, the free amino acids and soluble protein contents showed the same changing trend as in the former, however, the soluble sugar content showed a continuously increasing trend. Comparing the treatment infected by aphids with the treatment uninfected by aphids at the same time nodes, as potassium level increased, the free amino acids and soluble protein contents of wheat plant were decreased by aphid infection, while the soluble sugar contents were increased. By comparison of the involved indexes in the two time nodes, as the time prolongation, there were average increments of 2.6%, 18.1%, 2.0% for free amino acid, soluble protein and soluble sugar contents respectively in the uninfected wheat, and of 67.3%, 20.9% and 22.9% for infected wheat respectively, which showed greater increase for free amino acids and soluble sugar content in infected wheat plants than in uninfected wheat plant. In addition, the contents of induced free amino acid, soluble protein and soluble sugar, had significant or extremely significant correlation with the aphid population density in relevant each time node, and the correlation coefficients were 0.948, 0.920 and -0.908 in the fourth day, and 0.944, 0.985 and -0.991 in the eighth day respectively. The contents of constitutive soluble protein and soluble sugar had no significant correlation with aphid density, but the content of constitutive free amino acid had extremely significant correlation with aphid density, and the correlation coefficients were 0.995 and 0.944 respectively at the fourth and eighth days after the aphid infection.Conclusions Our results showed that increasing potassium supply could significantly reduce aphid population density by significantly reducing constitutive free amino acid content, inhibiting the accumulation of free amino acids and soluble protein and promoting the induced soluble sugar content.