Potassium is a potential toxicant for Arabidopsis thaliana under saline conditions

Background and aims : Most physiological and biochemical studies on salt stress are NaCl‐based. However, other ions (e.g., K⁺, Ca²⁺, Mg²⁺, and ${\mathrm{SO}}_4^{2-}$) also contribute to salt stress in special circumstances. In this study, salt stress induced by various salts was investigated for a better understanding of salinity. Methods : Arabidopsis thaliana plants were stepwise acclimated to five iso‐osmotic salts as follows: NaCl, KCl, Na₂SO₄, K₂SO₄, and CaCl₂. Results and Conclusions : Exposure to KCl and K₂SO₄ led to more severe toxicity symptoms, smaller biomass, and lower level of chlorophyll than exposure to NaCl and Na₂SO₄, indicating that Arabidopsis plants are more sensitive to potassium salts. The strongly reduced growth was negatively correlated with the accumulation of soluble sugars observed in KCl‐ and K₂SO₄‐treated plants, suggesting a blockage in the utilization of sugars for growth. We found that exposure to KCl and K₂SO₄ suppressed or even blocked sucrose degradation, thus leading to strong accumulation of sucrose in shoots, which then probably inhibited photosynthesis via feedback inhibition. Moreover, K⁺ was more accumulated in shoots than Na⁺ after corresponding potassium or sodium salt treatments, thus resulting in decreased Ca²⁺ and Mg²⁺ concentrations in response to KCl and K₂SO₄. However, K₂SO₄ caused more severe toxicity symptoms than iso‐osmotic KCl, even when the K⁺ level was lower in K₂SO₄‐treated plants. We found that Na₂SO₄ and K₂SO₄ induced strong accumulation of tricarboxylic acid intermediates, especially fumarate and succinate which might induce oxidative stress. Thus, the severe toxicity symptoms found in K₂SO₄‐treated plants were also attributed to ${\mathrm{SO}}_4^{2-}$ in addition to the massive accumulation of K⁺.