Effect of calcium on improving zinc tolerance in hydroponically grown Mentha longifolia L. through modulation of metabolic pathways and ion homeostasis

The purpose of this research was to examine the impacts of varying levels of Ca concentrations (0, 5, and 10 mM) on mint plants (Mentha longifolia L.) cultivated hydroponically, specifically under conditions of zinc (Zn) toxicity (300, and 600 μM). Various aspects, including chlorophyll and proline metabolisms, defense mechanisms (antioxidant and glyoxalase systems), ion balance, and sulfur assimilation process, were examined to assess the effects. Zn treatment resulted in the initiation of oxidative stress, hindered nutrient absorption, and led to an increase in Zn leaf concentration. These effects together contributed to the impairment of the photosynthesis apparatus and a subsequent decrease in the growth of mint seedlings. However, adding Ca by modulating proline and chlorophyll metabolisms raised proline and chlorophyll levels. Ca treatment (especially 10 mM) reduced methylglyoxal (31.1 and 38%), malondialdehyde (44.9 and 42.1%), and hydrogen peroxide (31.2 and 48.5%) levels and electrolyte leakage (22 and 30.6%) in Zn-stressed plants by strengthening the antioxidant machinery. The application of Ca by attenuating Zn amount and facilitating the uptake of mineral nutrients maintained the ionic balance under Zn toxicity. Ca addition resulted in the upregulation of enzymes associated with the sulfur metabolism, leading to elevated amounts of cysteine, glutathione, and phytochelatins. Therefore, our findings point out that Ca increased the adaptation of mint seedlings during Zn phytotoxicity by modulating proline and chlorophyll metabolism, reinforcing the antioxidant capacity, maintaining ionic homeostasis, and inducing the sulfur assimilation mechanism.