Iso-osmotic calcium nitrate and sodium chloride stresses have differential effects on growth and photosynthetic capacity in tomato

Calcium nitrate [Ca(NO₃)₂] and sodium chloride (NaCl) are among the main salts used in cultivation and present in salinized soil, respectively. These salts at high levels would reduce photosynthetic capacity and eventually cause a reduction in tomato growth. To better understand the growth disruption caused by Ca(NO₃)₂ salt (from the fertilizer) and the other predominant salt NaCl in salinized soils, tomato plants were grown in half-strength Hoagland's nutrient solution with three treatments. Through 7 days of salt stress and later 7 days of recovery, we investigated tomato seedling growth, nonstructural carbohydrates accumulation, antioxidant capacity, photosynthetic gas exchange, and chlorophyll fluorescence parameters. We found that Ca(NO₃)₂ and NaCl stresses increased the levels of nonstructural carbohydrates, reactive oxygen species (ROS), and various antioxidant enzyme activities in tomatoes while lowering foliar photosynthetic capacity. Under iso-osmotic salt stress conditions, the disruption of photosynthesis by NaCl was greater than that brought about by the Ca(NO₃)₂ treatment, which was attributed to Na⁺ accumulation. Interestingly, Na⁺ increased the levels of nonstructural carbohydrates and disrupted the feedback inhibition of photosynthesis. In addition, Na⁺ inhibited the activities of antioxidant enzymes, leading to more ROS production that would disrupt the processes within various photosynthetic apparatus. Our study further revealed that foliar nonstructural carbohydrates played an important role in delaying the onset of photoinhibition and rapid recovery of photosystem damage after Na⁺stress.