Influence of ambient acidity on the absorption of NO3−and NH4+by tomato punts

The effects of ambient acidity on NO3 − and Nh4 + absorption by 26‐day‐old tomato plants (Lycopersicon esculentum Mill.) were examined in solution culture. The absorption rate per unit root mass was measured for 6 hr. The NO3 − absorption rate from 0.4 mM NaNO, was 36% greater at pH 4.5 than at pH 6.5. In contrast, the NhY absorption rate was approximately 42% greater at pH 5.5 or 6.5 than at pH 4.5. The presence of equimolar NHr from 0.4 mM NH^NO, decreased the NO, absorption rate at pH 5.5 or 6.5 but did not reduce the rate at pH 4.5. The NO, absorption rate was inhibited less at pH 5.5 when equimolar NHr was supplied from 0.2 mM (NH4)2S04 as opposed to NH4NO3. At pH 5.5, the N03 − absorption rate increased with increased #OPNH4#CP2SO4 concentration. The presence of equimolar NO3 − supplied as either NaNO3 or NH4NO3 had no effect on the NH4 + absorption rate at pH 5.5 or 6.5. However, at pH 4.5, the NH4 + absorption rate was slightly reduced from NH4NO3 solutions relative to that from a (NH4)2S04 solution.