Change in Nitrate-Reducing Activity in Squash Seedlings

Nitrogen dioxide (NOi) 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 (Cucwbita maxima Duch.) seedlings. The inhibition was recovered when the seedlings were transferred to NOj-free conditions, indicating that the effect of NOj was reversible. The NADH content in the cotyledons, photosynthetic Oj evolution and respiratory Oi uptake did not change notably under NOj fumigation. Nitrate contents in the cotyledons and first leaves did not change with NOi fumigation, but nitrite, ammonium and rapidly-metabolized amino acids contents increased. The inhibitory effect of NOi 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 NOj 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 NOi fumigation. Key words: Cucurbita maxima — Nicotinamide dinucleotide — Nitrate reductase — Nitrite — Nitrogen dioxide (NOs) — Squash cotyledon