Light use in relation to carbon gain in the mangrove, Avicennia marina, under hypersaline conditions

Photosynthesis was studied in relation to light use in the mangrove, Avicennia marina (Forsk.) Vierh. var. australasica (Walp.) Moldenke, growing under soil salinities equivalent to one and two times seawater (i.e. 35 and 60‰). Midday CO₂ assimilation rates averaged 7.6 ± 0.7 and 4.3 ± 0.3 μmol m^-2 s^-1 at the seawater and hypersaline sites, respectively. Despite this difference, xanthophyll pool sizes per Chl and epoxidation states were similar at both sites. Non-photochemical quenching also indicated comparable energy dissipation from pigment beds. Electron transport rates calculated from fluorescence characteristics were also similar and exceeded the requirements to sustain measured assimilation rates. However, cell wall conductance was low in seawater plants (75 mmol m^-2 s^-1) and declined to 40 mmol m^-2 s^-1 in hypersaline plants. This would cause CO₂ concentrations in chloroplasts (C(c)) to be lower than expected from measurements of intercellular CO₂ concentrations (C(i)). In seawater plants, C(c) was estimated to be 144 μmol mol^-1 when C(i) was 245 μmol mol^-1, while values for C(c) and C(i) in hypersaline plants were 78 and 212 μmol mol^-1, respectively. Reductions in C(c) would enhance rates of photorespiration relative to assimilation, with the higher photorespiratory rates being sufficient to account for apparent excess electron transport rates.