Abstract
Summary The in vivo and in vitro nitrate reductase activities during the ontogeny of the first leaf showed maximum values at the time of maximum rate of leaf expansion. During this phase the changes of the enzyme activity depend on the level of endogenous nitrate in the tissues, which accumulated preferentialy in young leaves. It was shown that the in vitro nitrate reductase instability increases as leaf age progresses. Inclusion of 3 % Casein in the enzyme extraction medium significantly improved enzyme activity suggesting that caScin could effect the balance between some activating and inactivating component associated with the protein molecule. When nitrate was supplied via the petioles of excised leaves it could be demonstrated that old leaves accumulated less nitrate than young ones. In addition it was shown that the nitrate concentration in the leaf alter the relation between in vivo and in vitro nitrate reductase activities.
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