Changes in gas exchange and chlorophyll fluorescence characteristics of two mangroves and a mangrove associate in response to salinity in the natural environment

Photosynthetic responses of two mangroves and a mangrove associate were investigated in a low salinity site at Beachwood (<12‰) and a high salinity site at Durban Bay (35‰). Measurements were made of CO2 exchange, chlorophyll fluorescence, ion and water relations of Avicennia marina, Bruguiera gymnorrhiza and Hibiscus tiliaceus at the two sites. At Beachwood, CO2 exchange was highest in H. tiliaceus (9.12 µmol m–2s–1) and least in A. marina (3.27 µmol m–2s–1). At the Durban Bay site, CO2 exchange in A. marina was greater than that at Beachwood by 79%, that in H. tiliaceus decreased by 59% while that in B. gymnorrhiza remained unchanged. In all species, photosystem II (PSII) quantum yield decreased with increase in photon flux density (PFD) up to 2,000 µmol m–2s–1. Electron transport rate (ETR) through PSII increased with increase in PFD, reaching saturation at 1000 µmol m–2s–1. At saturating PFD, PSII quantum yield and ETR were higher in the mangroves but lower in H. tiliaceus at the high salinity site. Photochemical and non-photochemical quenching were, however, lower in the mangroves but higher in H. tiliaceus at the high salinity site. Predawn and midday leaf water potential were higher at Beachwood and significantly reduced at Durban Bay, values being highest in H. tiliaceus (–0.4 and –1.8 MPa) and least in A. marina (–2.5 and –3.8 MPa). Leaf concentrations of Na+, Cl–, Ca2+ and Mg2+ in mangroves were significantly higher at Durban Bay than at Beachwood. In H. tiliaceus there were no differences in ion concentrations between sites except for K+, which was significantly higher at Durban Bay.