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 t ransport. In the leaf tissue, actual and maximal nitrate reductase (NR) act ivities, nitrite content and ATP levels were determined. In darkened leaf d iscs incubated without KROM (control) actual NR activity decreased to 20% a fter 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 diminish ed NR-protein degradation during prolonged darkness. Due to restricted nitr ite 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 (ac tivation state 76%) in the light. Possible side-effects of KROM on nitrite reduction and photosynthesis were also checked in the leaf-disc system. Nei ther nitrite reduction nor photosynthesis were altered in KROM-treated disc s. 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 acidific ation might play an important role in the modulation of NR activity.