Adaptation of tropical mangroves to high solar radiation

Lovelock, Catherine Ellen (1991) Adaptation of tropical mangroves to high solar radiation. PhD thesis, James Cook University.

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Abstract

Mangroves grow in solar radiation environments that are high in visible and UV radiation. Both UV and visible radiation have been shown to be responsible for damage to the photosynthetic apparatus of plants (Caldwell, 1981; Powles, 1984). The general aim of this study was to investigate mechanisms that allow mangroves to tolerate their high solar radiation environment. Investigation into tolerance of visible light centered on the xanthophylls, while for UV radiation, emphasis was placed upon the accumulation of UV-absorbing phenolic compounds.

Mangroves have similar xanthophyll/chlorophyll ratios to other plant species. Xanthophyll/chlorophyll ratios were sensitive to the light environment in which leaves grow, decreasing as light levels decreased over a vertical transect through a forest canopy. The Xanthophyll/chlorophyll ratio also varied between species. In sun leaves the xanthophyll/chlorophyll ratios over all species correlated with the proportion of leaf area displayed on a horizontal plane, which is determined by leaf angle. Thus leaf angle and xanthophyll/chlorophyll ratios may be equally important in providing protection from high light levels in mangrove species.

A canopy survey assessed whether xanthophyll/chlorophyll ratios could be correlated to species dominance of exposed positions in forest canopies. Rhizophora mangroves, with near-vertical leaf angles, and B.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 two species. The first strategy is avoidance, through near vertical leaf angles, and the second is a large capacity to dissipate energy through zeaxanthin. The xanthophyll/chlorophyll ratio was also negatively correlated with the epoxidation state of the xanthophylls (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 xanthophyll/chlorophyll ratio. Thus, an investigation into the factors that influence epoxidation state was undertaken.

Within a data set where solar radiation was greater than 750μmol.m⁻².s⁻¹ photosynthetic rates accounted for a greater proportion of the variation in epoxidation state than solar radiation. With a data set collected over a wide range of solar radiation levels, leaf temperatures, and photosynthetic rates, photosynthesis and solar radiation were found to account for similar proportions of the variation in epoxidation state. Leaf temperature was not significant in directly explaining the variation in epoxidation state, but exerted its influence indirectly through its effect on photosynthesis. As solar radiation influences photosynthesis, leaf temperature and epoxidation state, solar radiation is probably the most important factor in determining the epoxidation state, and thus the total xanthophyll concentration in mangrove leaves.

UV-absorbing phenolic compounds, that have been shown to be protective against the damaging effects of UV-B radiation (Tevini et al., 1991) were found in the epidermis of mangrove species. An extensive field survey showed that the concentration of these compounds varied between species, sites and sun and shade leaves. Sun leaves have greater concentrations than shade leaves, and more saline sites have plants with greater concentrations in their leaves than less saline sites. It was concluded from this study that although these compounds form a UV screen in the epidermis of mangrove leaves, UV radiation may not be the only factor influencing their accumulation.

In an experiment to assess the adaptation of three species of mangroves to UV radiation, UV radiation resulted in a greater concentration of UV-absorbing compounds in B.parviflora, but not the other two species, and lower chlorophyll concentrations occurred in R.apiculata, but not the other two species. There was no difference in leaf morphology, carotenoid/chlorophyll ratios, or chlorophyll a/b ratios between UV treatments, although these varied between species; B.parviflora having the highest carotenoid/chlorophyll ratio and R.apiculata having the lowest. Chlorophyll a/b ratios were correlated with the concentration of UV-absorbing compounds over all species. Thus it was proposed that the effects of UV radiation measured in these experiments may be associated with the total energy absorbed by leaves, and that differences between species response to UV radiation may be associated with their abilities to dissipate the increased energy associated with UV radiation.

Item ID: 27499
Item Type: Thesis (PhD)
Keywords: mangroves; solar radiation tolerance; xanthophyll/chlorophyll ratios; canopy dominance attributes; epoxidation state; mangrove adaptation
Date Deposited: 25 Jun 2013 04:37
FoR Codes: 06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060799 Plant Biology not elsewhere classified @ 70%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 30%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 51%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 49%
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