Influence of solar radiation and leaf angle on leaf xanthophyll concentrations in mangroves

Mangroves have similar xanthophyll cycle components/chlorophyll ratios [i.e. (V + A + Z)/chl] to other plant species. (V + A + Z)/chl ratios were sensitive to the light environment in which leaves grew, decreasing as light levels decreased over a vertical transect through a forest canopy. The (V + A + Z)/chl ratio also varied among species. However, in sun leaves over all species, the (V + A + Z)/chl ratios correlate with the proportion of leaf area displayed on a horizontal plane, which is determined by leaf angle. Thus, leaf angle and the xanthophyll cycle may both be important in providing protection from high light levels in mangrove species. A canopy survey assessed whether (V + A + Z)/chl ratios could be correlated with species dominance of exposed positions in forest canopies. Rhizophora mangroves, with near-vertical leaf angles, and Bruguiera parviflora, with small, horizontal, xanthophyll-rich leaves, dominated the canopy, while B. gymnorrhiza, a species with large, horizontally arranged leaves, was less abundant at the top of the canopy. Thus, two different strategies for adapting to high solar radiation levels may exist in these species. The first strategy is avoidance through near vertical leaf angles, and the second is a large capacity to dissipate energy through zeaxanthin. The (V + A + Z)/chl ratio was also negatively correlated with the epoxidation state of the xanthophyll cycle pool (the proportion present as violaxanthin and half that present as antheraxanthin) at midday. This suggested that the requirement for dissipation of excess light (represented by the midday epoxidation state) may influence the (V + A + Z)/chl ratio.