Early growth reduction in Vicia faba L. under alkali salt stress is mainly caused by excess bicarbonate and related to citrate and malate over accumulation

Abstract

Rather than high pH or Na+, excess bicarbonate, by inducing over-accumulation of organic acids (through dark fixation) in plant roots, was hypothesized to be the main growth-limiting factor under alkali salt stress. Thus, the effect of four treatments viz. control (pH 6.0), high pH (9.0), high pH (9.0) plus sodium sulfate (25 mM) and sodium bicarbonate (50 mM, pH 9.0) on growth, gas exchange and leaf water content, organic acid accumulation, ion relations, oxidative damage and changes in the expression of key genes was investigated in Vicia faba. The bicarbonate treatment showed the highest growth reduction, PEPCase gene expression and accumulation of citrate and malate in roots. Bicarbonate treated plants preferentially accumulated Fe2+ and Ca2+ in roots and reduced root exudation of citrate and malate. Moreover, these plants experienced K+ and Ca2+ deficiencies, which caused a massive increase in H2O2 concentration in leaves and roots, leading to severe oxidative damage. Bicarbonate treated plants showed reduced levels of PM H+-ATPase, SOS1, CS, and MDH gene expression in roots. The cascade of events confirms the hypothesized central role of organic acid accumulation in bicarbonate induced stress, making bicarbonate more detrimental to plant growth than high pH or its combination with sodium salt.