AspAT (a) and AS (b) specific activities. The 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 AspAT and AS specific activities were expressed in as unit per milligramme protein. For AspAT activity, 1 U corresponds to the oxidation of 1 μmol of β-NADH per minute at the assay conditions. For AS, 1 U corresponds to 1 μmol of Asn produced per minute at the assay conditions. Values are means of three independent determinations (±SE). Letters above bars indicate significant differences (P < 0.05).