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Articles

Organic Acids and Ionic Balance in Xylem Exudate of Wheat during Nitrate or Sulfate Absorption ¹

Botany Department, University of Maryland, College Park, Maryland 20742

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 SO₄^2– was negligible in both treatments. In a second experiment, exudate was collected every 4 hours during the daytime throughout a 72-hour treatment with 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⁺.

³ To whom reprint requests should be addressed.

¹ Scientific Article No. A2624, Contribution No. 5662 of the Maryland Agricultural Experiment Station, Department of Botany.