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Keywords:

  • Acidity;
  • carboxylates;
  • ion balance;
  • Triticum aestivum;
  • xylem exudate

Summary

The effect of Ca2+, K+ and Na+ on uptake, transport and reduction of NO3 was studied in 10-day-old plants of Triticum aestivum L. cv. Sicco grown in water culture in relation to the ion balance in plant, medium and xylem exudate.

When Ca2+ (0.25 mM) was the only nutrient cation, NO3 uptake and accumulation of NO3 in the leaves were low as compared to treatments with K+ or Na+ (0.5 mM). With Na+, the uptake of NO3 was similar to that with K+, but virtually no Na+ and 75 % less NO3 accumulated in the leaves.

Medium pH dropped initially with 0.5 mM KNO3 but rose immediately when Ca2+ was the only cation. The carboxylate content rose in roots and leaves of K+ plants and, to a lesser extent, in Na+ plants. Roots of Ca(NO3)2 plants showed negative ash alkalinity indicating the presence of acidity.

The pH of the xylem exudate increased with the cation/NO3 ratio in the exudate and was about one unit higher with Ca(NO3)2 than with NaNO3 or KNO3.

We conclude that roots of plants grown in the absence of monovalent cations have serious pH problems because of a high rate of alkalinity release to the medium in exchange for NO3 uptake, and the secretion of alkalinity into the xylem.