Differential expression of physiological and biochemical characters of some Indian mangroves towards salt tolerance.

Mangroves are physiologically interesting as potential models for stress tolerance and as sources of alternative ideas about physiological strategies relevant at the ecosystem level. Variation in habitat has great impact on the physiological behavior and biochemical expression level of a particular plant species. Five species of mangroves, growing in saline and fresh water conditions were assessed for their ecological fitness in two different habitats. Assessments were based on some physiological and biochemical parameters measured from the fully exposed mature leaves under saline (15-27 PPT) and non-saline (1.2-2 PPT) conditions. Among the five species considered for investigation Bruguiera gymnorrhiza, Excoecaria agallocha and Phoenix paludosa grow luxuriously in the Sundarbans forest, while the rest two (Heritiera fomes, Xylocarpus granatum) are scanty. A comparative account of photosynthetic efficiency, chlorophyll content, mesophyll and stomatal conductance, specific leaf area, photosynthetic nitrogen use efficiency, total foliar free amino acids and differential expression of some antioxidant isoenzymes in leaf were estimated between the saline and non-saline plants. Elevated assimilation rate coupled with increased chlorophyll content, increased conductance and higher specific leaf area in non-saline condition indicates ability of these mangroves to grow even under minimal substrate salinity. The optimum PAR acquisition for photosynthesis in B. gymnorrhiza, E. agallocha and P. paludosa was higher under salt stress, while the maximum assimilation rate was lower in control plants. The opposite trend occurred in H. fomes and X. granatum, where the peak photosynthesis was lower under non-saline conditions even at a higher irradiance than in the saline forest. The isoform patterns of peroxidase, acid phosphatase and esterase indicated considerable difference in regulation of these enzymes due to salt stress and /or reverse adaptation.