High monosilicic acid supply rapidly increases Na accumulation in maize roots by decreasing external Ca2+ activity

Abstract

Both calcium (Ca2+) and silicon (Si) improve plant performance under salt (NaCl) stress. Although these two mineral elements share numerous similarities, the information on how their extracellular interactions in the root apoplast affect uptake of sodium (Na+) is still lacking. Here, we investigated the effect of high Si supply in the bioavailable form of monosilicic acid (H4SiO4) on the activity of Ca2+ in the external root solution, and subsequent root uptake and compartmentation of Na in maize (Zea mays L.). In the short-term experiments (6 h), 14-d-old maize plants were exposed to various concentrations of Ca2+ at three different pH-values (6.5, 7.5, and 8.5) and two Si concentrations, i.e., low (1 mM) and high (4 mM) supply of H4SiO4. The activity of Ca2+ and Na+ in the external solution as well as the root concentrations of total and cell sap and BaCl2-exchangeble apoplastic fractions of both elements were analyzed. The pH of the nutrient solution affected neither the ion activities nor the root accumulation of both Ca2+ and Na+. At higher pH values (7.5 and 8.5) the interactions of Ca2+ and Si at high Si supply led to a decrease of Ca2+ activity and, hence, an increase of Na+ : Ca2+ activity ratio in the external root solution. Concomitantly, despite the elevated exchangeable apoplastic fraction of both Ca2+ and Na+, the total and cell sap concentrations were remarkably decreased for Ca2+ and increased for Na+ by the addition of 4 mM H4SiO4. This work demonstrates that at high Si supply extracellular Ca-Si interactions leading to lowered activity of Ca2+ might rapidly compromise the ameliorative effect of Ca2+ on Na+ accumulation in roots. Practically, Si over-fertilization of saline and, in particular, sodic soils may further promote the accumulation of Na+ in root tissues hours after Si application and, hence, increase a potential risk of Na+ toxicity.