Figure 5.

Constitutive nitrate-uptake activity in WT and chl1 plants. Seedlings were grown in liquid culture with 12 mM ammonium succinate without nitrate for 7 days at pH 6.5 (see Materials and Methods) and then preincubated with fresh medium (5 mM ammonium succinate and no nitrate) at pH 5.5 for 3 h. Seedlings were then washed with 5 ml of wash/uptake medium (as described in Fig. 4) and then resuspended in 2 ml of uptake medium with different concentrations of either KNO₃ (A and C) or NH₄NO₃ (B) at pH 5.5. Nitrate-uptake activity was monitored for 15–45 min as described in Materials and Methods. The graphs show uptake kinetics in the high-affinity range (A and B) and low-affinity range (C). Open circles, chl1-5 plants; closed circles, WT plants.