Photosynthetic characteristics of dwarf and fringe Rhizophora mangle L. in a Belizean mangrove

Twin Cays (Belize) is a highly oligotrophic mangrove archipelago dominated by Rhizophora mangle L. Ocean-fringing trees are 3–7 m tall with a leaf area index (LAI) of 2.3, whereas in the interior, dwarf zone, trees are 1.5 m or less, and the LAI is 0.7. P-fertilization of dwarf trees dramatically increases growth. As a partial explanation of these characteristics, it was hypothesized that differences in stature and growth rates would reflect differences in leaf photosynthetic capacity, as determined by the photochemical and biochemical characteristics at the chloroplast level. Gas exchange and chlorophyll fluorescence were used to compare photosynthesis of dwarf, fringe and fertilized trees. Regardless of zonation or treatment, net CO 2 exchange ( A ) and photosynthetic electron transport were light saturated at less than 500 m mol photons m 2 s 1 , and low-light quantum efficiencies were typical for healthy C 3 plants. On the other hand, light-saturated A was linearly related to stomatal conductance ( g s ), with seasonal, zonal and treatment differences in photosynthesis corresponding linearly to differences in the mean g s . Overall, photosynthetic capacity appeared to be co-regulated with stomatal conductance, minimizing the variability of C i at ambient CO 2 (and hence, C i / C a ). Based on the results of in vitro assays, regulation of photosynthesis in R. mangle appeared to be accomplished, at least in part, by regulation of Rubisco activity. Key-words : oligotrophy; phosphorus fertilization; Rubisco; stomatal conductance.