Salt acclimation mechanisms in the mangrove Bruguiera Cylindrica (L.) Blume

Abstract

Plant growth and productivity is severely affected by high salinity. Saline soils have high amount of soluble salts and exhibit electrical conductivity (ECe) of more than 4 dS m-1. High salinity leads to hyperionic and hyperosmotic stress at cellular level as well as whole plant level. Plants have developed an array of morphological, physiological, biochemical and molecular mechanisms to withstand salt stress. Presence of salt glands on leaves is a morphological adaptation shown by halophytes to salt stress. Other halophytes show physiological adaptations in which salt is removed from the cytoplasm to avoid metabolic toxicity. These include (a) Pumping of Na+ ions into vacuoles by a vacuolar Na+/H+ antiporter, (b) Synthesis of compatible solutes such as amino acids, quarternary ammonium compounds, polyols etc. to counterbalance the osmotic potential in cytoplasm, (c) Increase in respiration rate, which provides ATP for vacuolar sequestration of salt and osmolyte synthesis, (d) Detoxification of reactive oxygen species (ROS) that may be generated as a consequence of metabolic impairment caused by salt stress. Mangroves are woody trees and shrubs that grow in estuarine or brackish environment of tropical and sub-tropical coastal systems. Flourishing in hostile environmental conditions such as high salinity (with irregular periods of fresh water conditions), hypoxia, excess light and nutrient deficiencies, make mangroves ideal models for salt stress research. In this thesis, we have compared some physiological responses of a mangrove Bruguiera cylindrical (L.) Blume to dehydration and high salt stress imposed on saplings irrigated with fresh water or salt water. The objective was to understand the salt adaptation mechanisms in this mangrove. The aspects dealt with in this thesis are: 1. The role of salt acclimation in the ability of Bruguiera cylindrica plants to withstand dehydration and osmotic stresses 2. Salt-induced respiration and its role in salt adaptation 3. Identification and characterization of a plasma membrane Ca2+/H+ antiporter 4. Transcriptional regulation of ion-transporters.