Organic Acids and Ionic Balance in Xylem Exudate of Wheat during Nitrate or Sulfate Absorption

Experiments were designed to study the importance of organic acids as counterions for K(+) translocation in the xylem during excess cation uptake. A comparison was made of xylem exudate from wheat seedlings treated 72 hours with either 1.0 millimolar KNO(3) or 0.5 millimolar K(2)SO(4), both in the presence of 0.2 millimolar CaSO(4). Exudation from KNO(3) plants had twice the volume and twice the K(+) and Ca(2+) fluxes or rate of delivery to shoots, as K(2)SO(4) plants. Malate flux was 25% higher in K(2)SO(4) than in KNO(3) exudate. Malate was the principal anion accompanying K(+) or Ca(2+) in K(2)SO(4) treatment, while in the KNO(3) treatment, NO(3) (-) was the principal anion. The contribution of SO(4) (2-) was negligible in both treatments. In a second experiment, exudate was collected every 4 hours during the daytime throughout a 72-hour treatment with KNO(3). Malate was the only anion present in exudate at first, just after the CaSO(4) pretreatment had ended. Malate concentration decreased and NO(3) (-) concentration increased with time and these concentrations were negatively correlated. By 62 hours, NO(3) (-) represented 80% of exudate anions. K(+) and NO(3) (-) concentrations in exudate were strongly correlated with K(+) and NO(3) (-) uptake, respectively. The first 36 hours of absorption from KNO(3) solution resembled the continuous absorption of K(2)SO(4), in that malate was the principal counterion for translocation of K(+).