Variable salinity is one of the defining environmental features of mangrove habitats. Studies of mangroves have identified physiological attributes associated with increasing salt tolerance that have far reaching effects on growth. These attributes that promote growth at either low or high salinities may be mutually exclusive. Indeed, in the Indo-Pacific region, there are numerous genera in which closely related species differ in salt tolerance, the implication being that differences in salt tolerance have contributed to the evolution of species. Pairs of species from the genera Avicennia, Bruguiera, Ceriops, and Rhizophora were grown in dilutions of seawater to determine what factors reduce growth at sub-optimal and supra-optimal salinity. Growth analyses used the identity: Relative growth rate (RGR) = Net Assimilation Rate (NAR) x Leaf Area Ratio (LAR). The relative contributions of change in NAR and LAR to decrease in RGR in response to sub or supra-optimal salinity were evaluated through ratios of the parameters measured under either fresh or sea water (F, S) relative to those obtained at a salinity where RGR was maximal: (RGRF,S)/(RGRmax) = (NARF,S)/(NARmax) x (LARF,S)/(LARmax).
Results/Conclusions
Each species exhibited a unique growth response to salinity. However, salinity tolerance differed within each congeneric pair, with the species showing greater growth in fresh water having lower growth rates in sea water. When grown in fresh water, RGR declined by 54% and 71% from maximal values, respectively, in the more and less tolerant species of suboptimal salinity in each congeneric pair. In both cases, NAR declined by 46%. Maintenance of higher growth rates in the species more tolerant of suboptimal salinity was due to maintenance of greater LAR. When grown in seawater, RGR declined by 52% and 72% from maximal values, respectively, in the more and less tolerant species of supra-optimal salinity in each congeneric pair. In both cases, LAR declined by 47%. Maintenance of higher growth rates in species more tolerant of supra-optimal salinity was due to maintenance of greater NAR. Thus, inhibition of growth under salt deficiency differs from that due to salt excess. Differentiation in salt tolerance appears related to evolution of species within genera. However, consistency in the nature of growth inhibitions across genera and families implies that they are an inevitable consequence of common physiological mechanisms of salt tolerance.
