Effects of ionic toxicity of sodium chloride on some physiological and biochemical characteristics of spearmint (Mentha spicata L.)

Abstract
Background and objectives: Soil salinity is one of the most important limited factors of plant productions. Physiological drought resulting from salinity stress is one of the main reasons of limiting water absorption from soil. Moreover, increasing the salt absorption by plants cause to disorder in physiological and cellular processes. Spearmint (Mentha spicata L.) is from Lamiaceae family. This plant grows well in acidic sandy soils and prefers middle light and high soil humidity conditions. Spearmint is rich source of polyphenolic compounds, so it has antioxidant property. This experiment is conducted to study the effect of some physiological and biochemical processes.
Materials and methods: This study was performed in Agricultural Faculty of Ferdowsi University of Mashhad at 2013 as factorial based on randomized complete design with four replications. In this experiment the effects of ionic toxicity of Na+ and Cl- on some physiological and biochemical characteristics of spearmint was investigated. The treatments of experiment were included five levels of salinity (0, 30, 60, 90 and 120 mM) sodium chloride and three sampling times (90, 100 and 110 days after planting). The measured characteristics were included chlorophyll concentration, leaf relative water contents, electrolyte leakage, proline, soluble sugars, leaf SPAD, antioxidant activity, total phenol and stomatal conductance.
Results: The results showed that salinity stress and sampling time each one individually had significant effect on relative water content, chlorophyll a and b, total chlorophyll, leaf SPAD, antioxidant activity and electrolyte leakage. Salinity also had significant effect on stomatal conductance, whereas sampling time had no influence on these characteristics. Interaction effects of salinity stress and sampling time on SPAD, proline and leaf relative water contents were significant.
The highest (106.72%) and lowest (65.38%) relative water contents were observed at control and first sampling time, and 90 mM sodium chloride and third sampling time, respectively. The highest (50.49%) and lowest (19.42%) electrolyte leakage were obtained in 120 mM sodium chloride and control, respectively. Sampling time had significant effect on soluble sugars, total phenol and proline. Relative water content, membrane stability, chlorophyll a, b and total chlorophyll decreased over the time.
Conclusion: According to the obtained results, rising the salinity levels cause to decrease relative water contents, chlorophyll and stomatal conductance, increase electrolyte leakage and antioxidant activity, and had no effect on proline, soluble sugars and total phenol. It appears that spearmint is sensitive to salinity stress and it can't tolerate salinity levels more than 30 mM sodium chloride.