The Influence of Low Nutrient pH on Nitrate Assimilation and Nitrate Reductase Activity in Maize Seedlings

Abstract

Summary The effect of low pH on nitrate reduction in maize seedlings was investigated in the present study. Maize seedlings grown in a nutrient solution of pH 4.5 had a lower nitrite and a higher total nitrogen concentration in roots and shoots as compared to when the seedlings were grown at pH 6.5 (steady state condition), while total and metabolic nitrate concentrations were unaffected by the nutrient pH. An almost similar effect of low pH on nitrogenous components was recorded when excised roots were incubated in nutrient solution containing nitrate (inducing condition). The in vivo and in vitro NADH: NR and NADPH : NR activities in the roots and NADH: NR activity in the shoots were also lower at lower pH in both steady state as well as inducing conditions, except for in vivo NR activity in the roots of nitrate supplied seedlings, which was not affected by the change in pH. The in vitro enzyme activity in the roots and shoots incubated for 3.5 h in 0 or 5 mmol/L nitrate solution in water was also lower when the pH of the incubation medium was 4.5 as compared to that at 6.5. However, when the excised organs were incubated in 0.1 mol/L Tris buffer containing 5 mmol/L nitrate, the in vitro enzyme activities were substantially higher when the medium pH was 4.5 as compared to when it was 6.5. Usually the pH of the external medium did not change much when the original pH was 6.5, except for a substantial increase in pH when the medium was only water. However, when the original medium pH was 4.5, the pH increased substantially during a 24 h incubation of the excised roots. Supply of cycloheximide at both pH values and tungstate at pH 4.5 in unbuffered solution had little effect on the inducibility of NADH and NADPH : NR activities in the excised roots, but tungstate caused a substantial decline in the activities in other conditions. The supply of Mo in the aqueous medium not only abolished the pH 4.5 induced decline in NADH : NR activity in the roots but it further increased the activity over that of pH 6.5. However, supply of 1 to 100 μmol/L Mo in the Tris buffer did not affect the increase in enzyme activity at pH 4.5. The experiments demonstrate an inhibitory effect oflow (4.5) pH on nitrate reduction in an unbuffered aqueous nitrate solution or in complete nutrient solution, which might be due to restriction of the availability of Mo as cofactor for active NR formation.