Plant-Induced Changes to Rhizosphere pH Impact Leaf Accumulation of Lamotrigine but Not Carbamazepine

Many ionizable organic contaminants (IOCs) are present in treated wastewater used to irrigate edible crops and accumulate in plants under field conditions. Phytoavailability of IOCs with pKa values between 4 and 9 may be affected by the pH of the rhizosphere (the water and soil within 2-3 mm of the root surface). Plants can alter rhizosphere pH by 2-3 units in either direction in response to nutrient availability. The effects of plant modulation of rhizosphere pH on IOC accumulation have not been previously reported. Here we provide direct evidence that plant-driven changes in rhizosphere pH impact accumulation of an IOC in plant leaves. Using a modified hydroponic system, we found that rhizosphere pH was higher by 1.5-2.4 units when plants received only nitrate rather than a combination of nitrate and ammonium. Plant-driven changes to rhizosphere pH altered accumulation of lamotrigine (pKa = 5.7) but not carbamazepine, a non-ionizable contaminant. Lamotrigine accumulation in leaves correlated strongly with the concentration of the neutral species available in porewater. We expect plant-driven changes in rhizosphere pH to be important across a wide range of plant species and IOCs. Consideration of plant modulation of rhizosphere pH may be necessary to accurately predict IOC bioaccumulation.