Calcium salt anion toxicity: Cl- and NO3- differently inhibit Ceratostigma willmottianum photosynthesis

Abstract

Background and aims

Calcium salts are prevalent in soils, and excessive amounts of these salts can subject crops to abiotic stress, leading to yield reduction or death. While the effects of Ca²⁺ in calcium salt stress have been widely reported, the role of the anions remains unclear.

Methods

The response of the calcium-secreting plant Ceratostigma willmottianum to five (0, 25, 50, 100, and 200 mM) equimolar concentrations (also iso-osmotic) of Ca(NO₃)₂ and CaCl₂ in terms of growth, morpho-anatomy, photosynthesis, physiology and biochemistry, and ion content was evaluated.

Results

Plants were more sensitive to CaCl₂ than to equal concentrations of Ca(NO₃)₂, which caused more severe water deficit, oxidative damage, and inhibition of photosynthesis and growth. The CaCl₂ sensitivity may be related to the toxicity of Cl^-, which accumulates in large amounts in leaves (661–2149 mM); however, under the Ca(NO₃)₂ treatments, the leaf NO₃^- concentrations were 42–210 mM. Cl^- inhibited chlorophyll synthesis and accelerated chlorophyll degradation, leading to photosystem disruption, and its inhibition of photosynthesis may involve both stomatal and nonstomatal limitation. In contrast, NO₃^- was not ionotoxic but rather promoted nitrogen assimilation and chlorophyll synthesis. The inhibition of photosynthesis by 100–200 mM Ca(NO₃)₂ originated mainly from stomatal limitation triggered by osmotic water loss. In addition, the Ca²⁺ secretion rate increased under calcium salt stress, which may represent a strategy for adaptation to high-calcium environments.

Conclusion

The present study provides valuable information for a comprehensive understanding of calcium salt injury mechanisms and plant adaptation to high-calcium environments.