Abstract
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 KNO3 or 0.5 millimolar K2SO4, both in the presence of 0.2 millimolar CaSO4. Exudation from KNO3 plants had twice the volume and twice the K+ and Ca2+ fluxes or rate of delivery to shoots, as K2SO4 plants. Malate flux was 25% higher in K2SO4 than in KNO3 exudate. Malate was the principal anion accompanying K+ or Ca2+ in K2SO4 treatment, while in the KNO3 treatment, NO3− was the principal anion. The contribution of SO42− was negligible in both treatments. In a second experiment, exudate was collected every 4 hours during the daytime throughout a 72-hour treatment with KNO3. Malate was the only anion present in exudate at first, just after the CaSO4 pretreatment had ended. Malate concentration decreased and NO3− concentration increased with time and these concentrations were negatively correlated. By 62 hours, NO3− represented 80% of exudate anions. K+ and NO3− concentrations in exudate were strongly correlated with K+ and NO3− uptake, respectively. The first 36 hours of absorption from KNO3 solution resembled the continuous absorption of K2SO4, in that malate was the principal counterion for translocation of K+.
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