Figure 6

Effects of the different inorganic N sources on GS activity in maize root. Maize plants were grown without N sources for 10 days and then exposed for the last 30 h to absence of N (c), to 10 mM NO₃ ⁻ (n), to 10 mM NH₄ ⁺ (a) or to 5 mM NO₃ ⁻ + 5 mM NH₄ ⁺ (na). Both the GS synthetase (A, B) and the transferase activities (C) were evaluated and expressed as specific activity. The synthetase activity was measured in the enriched fraction of plastidial proteins (A) and in total protein fraction (B) representing the GS2 activity and the total activity, respectively. The transferase activity was measured in total protein fraction prior to and after the exposure of the samples to 45°C for 10 min. The % thermal inactivation (C) represents the portion of transferase activity lost after this thermic treatment. Graphs show average values ± SE (n = 3). The upper letters indicate differences among the four treatments according to Student’s t-test (p < 0.05).