Salinity and cation foliar application: Implications on essential oil yield and composition of hydroponically grown spearmint plants

Abstract

Intensified cropping systems and irrational use of fertilizers and agrochemicals has reduced quality and availability of water for agricultural uses due to high salinity. In the present study, the effect of salinity (0 mM, 25 mM, 50 mM and 100 mM NaCl) and foliar application of distilled water (dH2O), K, Zn, and Si (1250 mg/L, 144 mg/L, and 725 mg/L of K, Zn, and Si, respectively) on essential oil yield and composition of hydroponically grown spearmint plants was investigated. The results showed that saline conditions and foliar application of cations did not affect essential oil yield, except for the case of Zn application and the highest salinity level (100 mM NaCl) where a significant increase of oil yield by 44% and 47% was observed comparing to control treatments (sprayed plants +0 mM NaCl, and non-sprayed plants +100 mM NaCl, respectively). The major volatile constituents were carvone and limonene, which ranged between 59.74%–72.87% and 9.76%–18.76%, respectively. Other compounds detected in significant amounts 1,8-cineole, germacrene D, β-caroyphyllene, β-pinene, bicyclogermacrene, and α-pinene. Moreover, carvone's content decreased with increasing salinity by up to 15.8% for the highest salinity level (100 mM NaCl), whereas contrasting effects were observed for limonene which increased by up to 75.6% for the highest salinity level. Foliar application of K, Zn and Si alleviated negative salinity effects on carvone's content at the highest salinity level (reduction by 7.4%, 7.7%, and 8.0% for K, Zn, and Si respectively), while limonene content showed similar increasing trends to the unsprayed plants, especially at salinity levels higher than 50 mM NaCl. In conclusion, irrigation with saline water is possible when foliar application of K and Zn is also considered, allowing us to stabilize major compounds content and increase essential oil yield, respectively.