Amino acids in watermelon root exudates and their effect on growth of Fusarium oxysporum f.sp. nevium

Watermelon cultivars resistant or susceptible to fusarium wilt were grown hydroponically as seedlings or grafted onto calabash or pumpkin rootstocks. To investigate disease resistance, the composition and content of amino acids in root exudates were analysed using an automatic amino acid analyser. Fifteen amino acids were detected in the exudates. Amino acid content was lowest in resistant cultivars and highest in susceptible cultivars, but there were no significant differences between the two rootstocks. The amino acid content was significantly higher in root exudates from resistant cultivars grafted on calabash rootstock than those from watermelon seedlings, whereas, susceptible cultivars grafted on pumpkin rootstock had lower amino acid content than watermelon seedlings. The effects of different amino acids concentrations (0, 0.6, 1.2, and 1.8 gL⁻¹) on growth and development of F. oxysporum f.sp. nevium were measured. Asparagine, methionine, leucine, glutamic acid and phenylalanine promoted the growth and development of F. oxysporum f.sp. nevium at concentrations of 1.2 gL⁻¹ or more; but methionine and phenylalanine inhibited the growth and development of F. oxysporum f.sp. nevium at conc of 0.6 gL⁻¹ or less.