Increased nitrate reductase activity in leaf tissue after application of the fungicide Kresoxim-methyl

Spinach (Spinacia oleracea L.) leaf discs floating on buffer solution were treated with Kresoxim-methyl (KROM), an inhibitor of respiratory electron transport. In the leaf tissue, actual and maximal nitrate reductase (NR) activities, nitrite content and ATP levels were determined. In darkened leaf discs incubated without KROM (control) actual NR activity decreased to 20% after 6 h in the dark. Treatment with 10 mu g ml(-1) (corresponding to 32 mu M) KROM totally prevented inactivation of NR in the dark and also diminished NR-protein degradation during prolonged darkness. Due to restricted nitrite reduction in darkened leaf tissues, nitrite accumulated in KROM-treated discs. Inhibition of respiration decreased ATP and increased AMP levels in KROM-treated discs. In illuminated leaf discs, NR was highly activated to 65%. Nevertheless, KROM-treatment caused an additional activation of NR (activation state 76%) in the light. Possible side-effects of KROM on nitrite reduction and photosynthesis were also checked in the leaf-disc system. Neither nitrite reduction nor photosynthesis were altered in KROM-treated discs. The extent of KROM-induced activation of NR was dependent on the applied concentration and on the pH of the external medium. The highest activation of NR was achieved at an external pH of 4.8, confirming previous results (Kaiser and Brendle-Behnisch, 1995, Planta 196. 1-6) that cytosolic acidification might play an important role in the modulation of NR activity.