Do various levels of salinity change chlorophyll fluorescence, nutrient uptake, and physiological characteristics of Mentha ecotypes?

Mint (Lamiaceae) is an economically important crop with many uses in the herbal, ornamental, culinary, pharmaceutical and food industries. To expand global cultivation, a better understanding of the growth and physiological responses of mint to soil salinity conditions is increasingly a strategic necessity. This study was aimed to investigate the NaCl (0, 2.5, 5, and 7.5 dS m−1) salinity stress effects on chlorophyll fluorescence, nutrient uptake, and physiological characteristics of different mint species [(18 ecotypes(E)] under the greenhouse environment. Based on the results obtained, the highest and the lowest soluble sugar contents (93.41 and 39.61 mg g−1 dry matter, respectively) were found in E15 at 7.5 dS m−1 salinity and in E6 under non-saline conditions that was raised by 57.5% upon 7.5 dS m−1 NaCl treatment over the control. The leaf relative water content was decreased in response to increasing salinity levels with the lowest values occurring at the highest salinity treatment by 42.2%, where ecotype 16 had the highest relative water content (53.2%) in the same salinity levels. Although, stomatal conductivity decreased as salinity intensity increased, the amount of this increase varied among ecotypes. At the highest level of salinity, E16 and E6 showed maximum and minimum stomatal conductivity, 7.23 and 4.13 mmol (H2O) m−2 s−1, respectively, which was a 42.8% difference between them. By increasing salinity levels, the nitrogen content of the plants was significantly decreased, which varied from 0.74 mg g−1 dry matter for ecotype 1–0.98 mg g−1 DW for ecotype 16. In term of 7.5 dS m−1 NaCl, the highest level of phosphorus (0.25 mg g−1 dry matter) was observed in ecotype 18. As salinity levels increased, the amount of potassium in shoot was decreased in a dose dependent manner in the mint ecotypes tested. At the highest salinity level, E8 and E15 had the highest, and E6 and E16 had the lowest level of potassium in shoot (1.34 mg g−1 dry matter and 1.24 mg g−1 dry matter, respectively). Overall, this research indicates that the mint E14, E10, and E18 are the most tolerant ecotypes displaying a high Fv/Fm index at all employed NaCl salinity levels. Thus, saline conditions may change the physiological processes and defense pathway resulting in dramatic changes in plant nutrients, photosynthesis efficiency and essential oil content of Mentha species.