Nitrate reductase (NR) activity in the presence of Mg2+ (NR act) representing the non-phosphorylated NR state and the activity in the presence of
EDTA (NR max) representing maximum NR activity was measured in roots and shoots of 15 d grown
aluminium and water stressed rice seedlings to examine changes in NR activation state due to these stresses. Seedlings subjected to a moderate
water stress level of -0.5 MPa for 24 h or grown in presence of 80 microM Al3+showed decreased level of NR max but resulted in higher NR act and NR activation state. However, seedlings grown in presence of a higher level of 160 microM Al3+ showed a decline in NR act as well as NR max. With a higher
water stress Level of -2.0MPa a marked decline in the levels of both NR act and NR max was observed, whereas NR activation state remained almost unaltered with severe
water stress. NR activity appeared to be sensitive to H2O2, PEG-6000, NaCl and various
metal salts. Incorporation of these components in the
enzyme assay medium led to decreased affinity of
enzyme towards its substrate with increase in Km and decrease in Vmax values. Addition of each of the osmolytes i.e. 1 mol/
L proline, 1 mol/L
glycine betaine or 1 mol/L
sucrose in the
enzyme assay medium caused a considerable protection to the
enzyme against the damaging effects of stressful components. An enhanced level of
proline and
glycine betaine was observed in Al-stressed seedlings and
sucrose in Al as well as water stressed seedlings. Results suggest that Al toxicity and
water stress decrease total amount of functional NR in rice seedlings and the osmolytes
proline,
glycine betaine and
sucrose appear to have a direct protective action on
enzyme NR under stressful conditions