Drought and salinity synergistically modulate the physiology and growth pattern of the facultative halophyte red mangrove (Rhizophora mangle)

Abstract

Under natural conditions, most plants are exposed to a plethora of stress factors like salinity and drought which can act in synergy to undermine their growth responses. The red mangrove, Rhizophora mangle, is endemic to estuarine ecosystems and is prone to salinity and drought stresses under changing climatic conditions. This work aimed at elucidating the combined impact of drought and salinity on the growth pattern and physiology of red mangroves. The use of factorial experimental layout in a completely randomised design was employed in the study to impose either a single or a combination of different drought regimes and levels of salt stress on red mangrove plants, which culminated in nine treatments to uncover both the combined and individualistic impact of salinity and drought stresses on the red mangrove. Morphological, physiological and biochemical responses of the facultative halophyte were evaluated following the imposition of salinity and drought stress. The results revealed that application of both salinity and drought stresses simultaneously on red mangrove seedlings led to a decline in plant growth indices, chlorophyll content, transpiration rate [E], stomatal conductance [gs] and net photosynthesis rate [PN], as compared to other plants exposed to single stress treatment. Besides, combined salinity and drought treatment increased oxidative stress rapidly, thereby increasing malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) accumulation. However, the red mangrove exhibited a certain level of stress resistance to the simulated salinity and drought stresses which was attributable to the mechanisms such as hyperactivation of catalase (CAT) and superoxide dismutase (SOD) activities and accumulation of osmoprotectants (soluble sugar, Na⁺ and Cl⁻). The results recorded indicate that gas exchange attributes, photosynthetic content, CAT and APX activities and MDA are reliable screening parameters for salinity and drought stress in the plants because they have roles in the level of combined stress tolerance exhibited by the red mangrove.