NR (a) and NiR (b) specific activities. Seedlings were grown for 14 days in a Hoagland modified complete nutrient solution (0) and treated for the last 2 days with HS concentrations of 0, 1 and 5 mg C L−1. The specific activities were expressed in unit per milligramme protein. One unit corresponds to the production (NR) or consumption (NiR) of 1 μmol of NO2
− per minute at the assay conditions. Values are the means of three independent determinations (±SE). Letters above bars indicates significant differences (P < 0.05).