Hydrophobic cell-wall barriers and vacuolar sequestration of Na+ ions are among the key mechanisms conferring high salinity tolerance in a biofuel tree species, Pongamia pinnata L. pierre

Abstract

Gradual soil-salinization is enhancing the proportion of non-arable salinized land areas. Developing strategies to utilize salinized lands for balanced economical productivity are highly desirable. Salt-tolerating Pongamia pinnata has gained significant attraction as a potential biofuel tree species and hence, could act as an efficient energy- crop alternative for cultivation in salinized lands. However, mechanisms conferring salt-tolerance to Pongamia are not yet demonstrated. It is highly crucial to understand the tolerance mechanisms for future breeding purposes for enhanced productivity under saline conditions. Hydroponically grown 30 days old seedlings of Pongamia are treated with two different salt concentrations (300 and 500 mM NaCl) for 8 days and analysed at regular intervals of 1, 4 and 8 days after salt exposure. Physiological parameters were recorded using infrared gas analyser and portable mini-PAM. Ion (Na+, K+, Cl−, and Ca2+) accumulation in leaves and roots were analysed through atomic absorption spectroscopy and Na+ localization was tracked through confocal laser scanning microscopy. Histochemical detection of lignin and suberin depositions in leaves and roots were carried out. Pongamia roots act as ultra-filters/strong barriers to avoid accumulation of excess Na+ levels in the leaves. The Na+ probe fluorescence analysis demonstrated effective vacuolar sequestration of Na+ in the roots. Formation of suberized multiseriate exodermis in the roots, along with extensive lignification maximized water permeability in both leaves and the roots. The present study clearly demonstrates the key cellular mechanisms conferring salinity tolerance in P. pinnata, which can be sustainably grown in salinized marginal lands as a potential biofuel tree species.