Gilbert et al. (2002) studied the possible role of salt excretion by mangroves as a defense against pathogenic fungi in a mangrove forest in Panama. Although presumably evolved for other reasons, salt excretion by leaves of some mangrove species may serve as an important defense against fungal attack, reducing the vulnerability of typically high-density, monospecific forest stands to severe disease pressure. In their study, Gilbert et al. found that Black Mangrove (Avicennia germinans) suffered much less fungal leaf damage from than did White Mangrove (Laguncularia racemosa) or Red Mangrove (Rhizophora mangle). Black Mangrove leaves also supported the least fungal growth on the leaf surface, the least endophytic colonization, and the lowest fungal diversity, followed by White Mangrove and Red Mangrove.
Host specificity of leaf-colonizing fungi was greater than expected at random. The fungal assemblage found on Black Mangrove appears to be a subset of the fungi that can grow on the leaves of Red and White Mangrove. The authors suggested that the different salt tolerance mechanisms in the three mangrove species may differentially regulate fungal colonization. The mangroves differ in their salt tolerance mechanisms such that Black Mangrove (which excretes salt through leaf glands) has the highest salinity of residual rain water on leaves, White Mangrove (which accumulates salt in the leaves) has the greatest bulk salt concentration, and Red Mangrove (which excludes salt at the roots) has little salt associated with leaves. The high salt concentrations associated with leaves of Black and White Mangrove, but not the low salinity of Red Mangrove, were sufficient to inhibit the germination of many fungi associated with mangrove forests. The authors suggest that efficient defenses against pathogens may be especially important in natural communities, such as mangrove forests, where host diversity is low and the density of individual hosts is high – ideal conditions for diseases to have strong impacts on plant populations.
Mangrove forests are unusual among tropical forests for their low tree species diversity and associated high population density of individual species. Mangrove species are unusual in their ability to grow in flooded, saline soils and for the array of mechanisms they have evolved to tolerate high salt concentrations. The work by Gilbert et al. suggests that some mangrove species may also be unusual in their escape from strong disease pressures, even when growing at high densities, through the inhibitory effects of
high foliar (leaf) salt concentration on fungal infection. (Gilbert et al. 2002)
The leaves of mangroves in general have abundant and diverse glands and their function and physiological and ecological importance appear to be poorly known. The glands on the petioles (leaf stalks) of White Mangrove and Buttonwood, for example are extrafloral nectaries (i.e., structures outside of flowers that secrete nectar), but the ecological significance of these structures seems not to have been investigated (these glands are not salt-secreting structures, as is sometimes claimed). The glands on the leaf blade of White Mangrove accumulate salt, but it is not secreted to the leaf surface, so in contrast to a Black Mangrove leaf, a White Mangrove leaf does not taste salty when licked. It is sometimes stated that the glands on the leaf blade of White Mangrove serve as domatia (special shelters) for beneficial mites. While this may be true, this appears to be another intriguing topic that remains unstudied.
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