Fig. 5.

Direct effects of NO on the nitrate reductase activity of tomato: (A) effects of NO gas and cPTIO on the nitrate reductase activity of enzyme extracts; (B) the reversible or irreversible effect of NO gas on the nitrate reductase activity of enzyme extracts. The enzyme was extracted from the roots fed with 0·5 (Low N) or 5 (High N) mm nitrate. The mixtures of enzyme extract and assay solution were directly incubated without (control) or with 1, 2 or 3 cm³ NO gas (0·04 µmol L⁻¹) or 2 mm or 5 µm cPTIO. For the investigation of reversible or irreversible effect, the enzyme extract was pre-treated with 3 cm³ NO gas for 30 min, and then flushed with N₂ and added assay solution (NO + N₂) to initiate the enzyme reaction. The concentrations of NO₂⁻ in the mixtures were measured 30 min after enzyme reaction, and the enzyme activities were then calculated. Error bars show the s.d. (n = 5), asterisks denote a significant difference from the control at P < 0·05.