Factors that contribute to dwarfing in the mangrove Avicennia marina

What is it about?

Background and Aims In Richards Bay, South Africa, Avicennia marina frequently exhibits a distinct productivity gradient, with tree height decreasing markedly from 6–10m in the fringe zone to <15m in the dwarf zone which is 120m inland ataslightly higher elevation. In this investigation, soil physico-chemical conditions between fringe and dwarf A. marina were compared and the constraints imposed by any differences on mangrove ecophysiology and productivity determined.
Methods Soil and plant samples were analysed for inorganic ions using spectrophotometry. Gas exchange measurements were taken with an infrared gas analyser and chlorophyll fluorescence with a fluorometer. Xylem y was determined with a pressure chamber and chlorophyll content with a chlorophyll absorbance meter.
Results In the dwarf site, soil salinity, total cations, electrical conductivity and soil concentrations of Na+,K+, Ca 2+, Mg2+, Zn 2+, Mn 2+ and Cu2+ were significantly higher than those in the fringe zone. Soil water potential and the concentration of soil P, however, were significantly lower in the dwarf site. In the leaves, Na+ was the predominant ion and its concentration was 24% higher in dwarf than fringe mangroves. Leaf concentrations of K+, Ca 2+, Mg 2+, Mn2+ and P, however, were significantly lower in dwarf mangroves. Photosynthetic performance, measured by gas exchange and chlorophyll fluorescence, was significantly reduced in the dwarf plants.
Conclusions The results suggest that hydro-edaphic factors contribute to high soil salinities, low water potentials, water stress and ion imbalance within tissues including P deficiency, which in interaction, contribute to dwarfing in Avicennia marina.

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