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.

Interactions of Silicon With Essential and Beneficial Elements in Plants

“…Decreased Ca accumulation in response to Si application can be attributed to lowered transpiration caused by deposition of Si in the leaves (Ma and Takahashi, 1993), reduced Ca 2+ uptake due to biosilicification of root structures (Fleck et al, 2015) and Si-Ca interaction in the growing media or apoplast (Dishon et al, 2011). Recently, Bosnic et al (2019c) reported that high Si supply may also decrease activity of Ca 2+ in nutrient solution high in Na + under alkaline conditions. Increased Ca uptake can result as a consequence of Si-mediated alleviation of primary stress, due to restored plasma membrane integrity and increased activity of H + -ATPase (Liang, 1999;Kaya et al, 2006).…”

“…Bosnic et al (2019c) demonstrated that Si supply increases binding capacity of cell walls of maize roots for Na + , thereby decreasing free Na + for transport through the plasma membrane. Considering that influx of Na + is mediated by various transporters some of which are sensitive to Ca 2+ (Davenport and Tester, 2000;Demidchik and Tester, 2002), interaction of Si and Ca in growing media can lead to increased Na accumulation in roots (Dishon et al, 2011;Bosnic et al, 2019c). Bosnic et al (2018) demonstrated that Si supply to moderately NaCl-stressed maize plants reduced Na concentration in the root symplast by up-regulating the expression of SOS1 (responsible for Na + efflux) and down-regulating the expression of HKT1 (responsible for Na + influx).…”

“…Although evidence of tissue Na + complexation by Si is lacking, Si co-precipitation with Na + in the extracellular matrix was reported for wheat ( Saqib et al, 2008 ). Bosnic et al (2019c) demonstrated that Si supply increases binding capacity of cell walls of maize roots for Na + , thereby decreasing free Na + for transport through the plasma membrane. Considering that influx of Na + is mediated by various transporters some of which are sensitive to Ca 2+ ( Davenport and Tester, 2000 ; Demidchik and Tester, 2002 ), interaction of Si and Ca in growing media can lead to increased Na accumulation in roots ( Dishon et al, 2011 ; Bosnic et al, 2019c ).…”

Mechanistic Basis of Silicon Mediated Cold Stress Tolerance in Alfalfa (Medicago sativa L.)

“…Silicon (Si) showed potential interaction with other essential and beneficial elements in plants [13]. Our data show that exogenous supplementation of Si has a potential impact in uptake under stress response [12]. However, our data suggested that the potential increase of endogenous Si, Mg in root and shoot helps to enhance CS tolerance in alfalfa.…”

“…Si shows interaction with other essential and beneficial elements. For instance, exogenous Si enhances endogenous Si [6], declines the activity of Ca 2+ [12], enhances the regulation of magnesium (Mg) and sulfur (S) [13]. These interactions lead to enhance plant growth, fitness, and stress adaptation in plants.…”