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

Abstract

Summary 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 (NRA act and NRA max ) and vice versa; the deactivation of NR occurred soon after light to dark transition. The diurnal rhythm of NRA act and NRA max was similar in phase in Mo-deficient and Mo-supplied plants. However, the absolute values of NRA act and NRA max 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 NRA act to NRA max , 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 NRA act — 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 NRA max and NRA act and results in a higher NR activation state in Mo-deficient plants. A possible mechanism of this effect is discussed.