Insecticide-free reduction of Myzus persicae aphids on sweet pepper (Capsicum annuum L.) through MgSO4-faciliated amino acid and sugar reallocation

Fruits of sweet pepper (Capsicum annuum L.) are among the most consumed fresh fruits, however, their production can be affected by aphids, which feed on the amino acids in the leaves. Mg2+ in leaves plays a crucial role in the reallocation of amino acids from source organs (leaves) to sink organs (fruits). We hypothesize that foliar application of Mg2+ can reduce free amino acid concentration in leaves by promoting their transfer to sink fruits, thereby compromising aphid colonization by reducing their nutritional basis. To test this, a two-factorial experiment was conducted with sweet pepper subjected to aphid infestation with and without leaf MgSO4 application. The concentration of amino acids, sugars, organic acids and mineral nutrients was analysed in the leaves, leaf phloem sap and fruits. Mg2+application decreased the concentrations of amino acids and sugars that are key for aphid nutrition in both the leaf and phloem sap and increased the size and number fruits, indicating an increased sink strength in response to Mg2+. This response was accompanied by a reduced number of aphids on the entire plant. In results, the sugar and amino acid concentration in the phloem got less attractive for the aphids. We conclude that Mg2+ worsens the nutritional situation for aphids by facilitating the reallocation of metabolites to form fruits.