CHANGES IN NITRATE REDUCTASE-ACTIVITY AND THE PROTECTIVE EFFECT OF MOLYBDENUM DURING COLD STRESS IN WINTER-WHEAT GROWN ON ACID SOIL

Two cultivars (frost resistant Sadovo 1 and frost susceptible San Past ore) of winter wheat (Triticum aestivum L.) were used ro study the pro tective effect of molybdenum during freezing of plants grown on acid s oil. We followed the effect of cold acclimation (2 degrees C for 7 day s) and freezing (-5 degrees C for 16 h) on the activities of nitrate r eductase (NR), nitrite reductase (NiR) and glutamine synthetase (GS) i n the leaves of both wheat cultivars grown on soil with pH 6.0 and pH 4.5. No difference was observed between the cultivars in the temperatu re induced changes of enzyme activities. During cold acclimation the a ctivities of the three enzymes increased irrespective of the soil's pH ; this increase, which was most pronounced for NADH:NR activity was pa ralleled by an increase in NR protein. Freezing affected only NR activ ity in plants grown on acid soil; the sharpest decline was found for N ADH:NR activity and FADH:NR partial activity. Molybdenum supplied to w inter wheat cv. Sadovo I grown on soil with pH 4.5 prevented the frost -induced decline of NADH:NR activity. The possible mechanism of the pr otective effect is discussed.