The Amazonian Croton mollis (Euphorbiaceae): morphology and leaf anatomy help to understand its preference for the extreme igapó habitat

Abstract

Identifying traits contributing to the success of organisms living in harsh environments has always been of great interest in evolutionary ecology. Here we focused on morphological features that could be advantageous for the survival of Croton mollis, a slender shrub occurring in the extreme white-sand habitats of the Amazonian igapos (black/clear-water rivers). We examined macro-morphological characters, as well as leaf micro-morphology using light microscopy and scanning electron microscopy. We also estimated areas of habitat suitability for C. mollis based on climatic, soil, and elevation variables using species distribution modelling (SDM) and available herbarium collections. Our results show that C. mollis presents morphological attributes similar to those of rheophytes, which may allow the species to overcome drastic seasonal changes in water level. We report the presence of five secretory structures (extrafloral nectaries, colleters, idioblasts, glandular trichomes, and laticifers) in the leaves. The size and position of the secretory idioblasts support the hypothesis of the existence of a transitional storage for secondary metabolites from idioblasts to secretory trichomes. Besides glandular trichomes, we also found stellate trichomes and trichome-like emergences in leaves. We hypothesize that these emergences could have an important role in the species’ survival due to their capacity of absorbing atmospheric moisture. Finally, the SDM supported the known habitat preferences of C. mollis and estimated a relatively wider geographic range than the currently known distribution based on herbarium records, suggesting that collecting efforts in the Amazonian region need to be increased in the future.