Hyperstomaty in Hawaiian Chamaesyce, a remarkable case of adaptive radiation

Background/Question/Methods

The Hawaiian Chamaesyce (Euphorbiaceae) are a group of C₄ dicots that radiated from one colonizing species into nearly 30 taxa.  This group is present on all of the main islands and includes a variety of life forms, from creeping woody sub-shrubs to trees over 6 m tall, with taxa adapted to diverse habitats, from rain forest to dry forest to coastal vegetation.  The leaves of the Hawaiian Chamaesyce taxa vary strongly, but no study so far has examined stomatal traits.  Typically plant species have stomata distributed only on the abaxial (lower) surface of the leaf (hypostomaty), or on both the abaxial and adaxial (upper) surfaces (amphistomaty).  The distribution of stomata only on the adaxial surface (hyperstomaty) is rare, previously documented in various aquatic plants, and in herbs that fold their leaves to prevent water loss, but not in typical terrestrial dicots.  For 29 Chamaesyce taxa we determined stomatal characteristics using scanning electron microscopy of leaf surfaces, light microscopy of epidermal peels, and porometry.  For each taxon we quantified stomatal numbers, dimensions and distribution; we tested for correlation of these traits with the source population elevation, mean annual rainfall, temperature, and relative humidity, and with leaf size and thickness.

Results/Conclusions
We found an unexpectedly large diversity in stomatal density and in stomatal distribution, including hypostomaty, amphistomaty and, for several taxa, hyperstomaty.  Apparently, Hawaii’s isolated location and strong climatic gradients have driven stomatal trait diversification for Chamaesyce beyond any previously characterized in a given lineage.  We explore possible hypotheses for the evolutionary factors involved, and the functional significance of this diversity.