Figure 4.

Nitrate-uptake rates as a function of pH. Seedlings were grown in liquid culture with 12 mM ammonium succinate without nitrate for 7 days at pH 6.5 as described in Materials and Methods. Seedlings were then preincubated with fresh medium (at the indicated pH) containing 5 mM ammonium succinate for 3 h. The preincubated seedlings were then washed with 5 ml of growth medium containing 50 μM KNO₃ and no ammonium succinate at indicated pH values. Uptake assays were performed by resuspending seedlings in 2 ml of uptake medium at the indicated pH (same constituents as wash medium in previous step) and monitoring loss of nitrate from the medium after 30 min as described in Materials and Methods. The histogram shows nitrate-uptake activity by WT (black) and chl1-5 (grey) plants at the indicated pH.