Nitrate reductase activation state in leaves of molybdenum-deficient winter wheat

Nitrate reductase (NR, EC 1.6.6.1) activity, NR activation state, and in vivo nitrate reduction were measured in leaves of winter wheat (Triticum aestivum L. cv. Sadovo 1) in order to study the effect of molybdenum (Mo) deficiency on light-induced changes in NR activation. It was established that dark to light transition of the plants resulted in a fast increase in actual and maximum NR activities (NRAact and NRAmax) and vice versa; the deactivation of NR occurred soon after light to dark transition. The diurnal rhythm of NRAact and NRAmax was similar in phase in Mo-deficient and Mo-supplied plants. However, the absolute values of NRAact and NRAmax were about two to three times higher in Mosupplied compared to Mo-deficient plants, i.e. only the amplitude of the rhythm was affected by Mo deficiency. The activation state of NR, given as percentage ratio of NRAact to NRAmax, was higher in Mo-deficient plants, reaching a maximum of 60-66% in the light, independent of the light intensity over 210 μmol m-2 s-1. In both Mo-deficient and Mo-supplied plants, the level of NRAact — thought to reflect the in situ NR activity — exceeded by several times the rate of in vivo nitrate reduction. The results indicate that Mo deficiency itself does not alter the timing of light/dark changes in NR activity, but it reduces significantly the levels of NRAmax and NRAact and results in a higher NR activation state in Mo-deficient plants. A possible mechanism of this effect is discussed.