Nitrogen metabolism disorder in watermelon leaf caused by fusaric acid

The effect of fusaric acid (FA) on the activity of leaf nitrogen (N) metabolism enzymes in watermelon seedlings supplied with different N forms was studied. The results showed that FA inhibited nitrogen uptake and caused decreased leaf amide and protein but increased the content of ammonium and amino acids. When treated with FA the activities of enzymes in the pathway for the synthesis of amino acid in leaves (GS, GOGAT, and GDH) were decreased by 15–23%, 13–40%, and 71–86%, respectively. The activity of asparagine synthetase was decreased by 34–57%. The proteinase activity was initially increased by 37–125% at 12 h after treatment of FA but then subsequently decreased. The activity of glutamate-pyruvate aminotransferase was increased by 280–400%, though the activity of glutamate-oxaloacetate aminotransferase was decreased by 30–63%. It was suggested that FA inhibited the uptake of ammonium in seedlings and suppressed the activities of amino acid and amide synthases, while stimulating proteinase activity.A new pathogenic mechanism of watermelon infection by Fusarium spp. was found as FA caused the complete disorder and collapse of the host plant's nitrogen metabolism. This work provides a new insight into the progression of watermelon wilting caused by Fusarium oxysporum f.sp. niveum.