Oxidative stress as an indicator of niche-width preference of mangrove Rhizophora stylosa

Inundation, elevation gradient and salinity fluctuations are considered the major abiotic drivers that influence the growth and distribution of mangroves. In certain conditions, these environmental stressors trigger excessive generation of reactive oxygen species (ROS), affecting mangrove physiology and homeostasis, leading to oxidative stress and mortality if the condition remains exacerbated. As a natural defense to quench the deleterious effects of ROS, mangroves have developed an antioxidant system to scavenge the toxic effects of excessive ROS. This study investigated environmental stress due to salinity, inundation, and elevation gradient using the biochemical responses in the leaves of Rhizophora stylosa as an oxidative stress indicator in greenhouse experiments and field conditions. From the variations of biochemical responses and levels of oxidative stress, the niche-width preference was extrapolated. The results of the study showed that the observed environmental factors significantly induced the generation of high H2O2 concentrations in the leaves of R. stylosa, which in return activated the antioxidant defense system. Inundation of the whole plant imposed a higher-order oxidative stress compared with the effect of salinity in the greenhouse condition, as shown by the significant increase in H2O2, and even caused sublethal damage, as manifested by the chlorotic leaves when prolonged. In the field, rare inundation and high elevation are also considered stressful to R. stylosa, as shown by the significantly higher H2O2 levels compared with those in the frequently inundated (but plants not submerged) areas. The long-term negative effects of high H2O2 at the plant and community levels were manifested in the reduction of growth rate in plants cultured in the greenhouse and the reduction of height in the 30-year-old R. stylosa plantation. Integrating the levels of oxidative stress induced by salinity, inundation and elevation from both greenhouse experiments and field conditions, it appears that relatively low oxidative stress results in a preference of the niche-width of R. stylosa in inundated areas as long as the leaves remain emerged, even during the spring tide. Thus, this species typically dominates middle intertidal areas. These findings could have valuable implications for the selection of areas appropriate for mangrove rehabilitation.