Salinity Tolerance in the Mangroves Aegiceras corniculatum and Avicennia marina. I. Water Use in Relation to Growth, Carbon Partitioning, and Salt Balance

Abstract

The water use characteristics of two mangrove species, Aegiceras corniculatum and Avicennia marina, in salinities of 50, 250 and 500 mol m^-3 NaCI and leaf-to-air vapour pressure differences of 6, 12 and 24 mbar were studied in relation to growth, carbon partitioning and salt balance. The net water use efficiency in A. corniculatum declined with increasing salinity and decreasing humidity. In contrast, water use was more conservative in A. marina, which maintained the net water use efficiency almost constant with variation in salinity. Aegiceras corniculatum maintained higher rates of water uptake and higher leaf area/plant mass ratios than A. marina. Growth of both species declined with increasing salinity, with A. corniculatum being the more sensitive species. Differences in growth rates between species and between treatments were consistent with differences in the assimilation rate and leaf areal plant mass ratio. Salt exclusion by both species increased from 90 to 97% with increase in salinity from 50 to 500 mol m^-3 NaCl. The xylem Cl^- concentrations increased with increase in salinity, but decreased with increase in shoot evaporation rates such that the salt flux to the leaves did not increase with increase in evaporation rates at a given salinity. Despite similarities in the salt fluxes to leaves, the transport of Cl^- to the shoot per unit of shoot growth increased more with increasing salinity in A. corniculatum than in A. marina because the net water use efficiencies were lower in the former species. Thus, the amount of salt secreted per mole water transpired (and hence also per mole carbon gained) increased more with increasing salinity in A. corniculatum than in A. marina. These differences in salt balance may be associated with the greater sensitivity of A. corniculaturn to increasing salinity. The possible ecological significance of these findings is discussed.