Change in Nitrate-Reducing Activity in Squash Seedlings with NO2 Fumigation

Nitrogen dioxide (NO2) fumigation inhibited nitrate reductase (NR, EC 1.6.6.1) activity assayed by an in vivo system in the cotyledons, but not in the first leaves, of squash (Cucurbita maxima Duch.) seedlings. The inhibition was recovered when the seedlings were transferred to NO2-free conditions, indicating that the effect of NO2 was reversible. The NADH content in the cotyledons, photosynthetic O2 evolution and respiratory O2 uptake did not change notably under NO2 fumigation. Nitrate contents in the cotyledons and first leaves did not change with NO2 fumigation, but nitrite, ammonium and rapidly-metabolized amino acids contents increased. The inhibitory effect of NO2 was also observed in the in vitro assay, though the inhibition rate was smaller than that in the in vivo assay. These results indicate that the inhibitory effect of NO2 on NR activity in squash cotyledons was derived in part from the decrease in the amount of active NR due to ammonium and/or amino acids accumulated in the tissue under NO2 fumigation.