Baccharis L. is an important genus in the high-altitude environments of South America. In the State of Rio de Janeiro, southeastern Brazil, the greatest richness is found at montane and high-montane ecological refuges, known as high-altitude grasslands. The high altitude provides several environmental factors that turn the high-altitude grasslands into singular tropical habitats. In this article we describe the leaf anatomy of six shrubby species of Baccharis, pointing out ecological aspects of the structures, and testing the use of statistical analyses to compare the species. Using light and scanning electron microscopy, we compared the anatomical features of the leaves and performed a cluster and principal component analyses. The results show novel features in the leaf anatomy for the genus, such as an undulated cuticle, three secretory ducts in the midrib, secretory ducts associated with the xylem, and biseriate glandular trichomes with an irregular organization of the cells. Our statistical analyses indicate that the anatomical features of the leaves, especially trichomes, are useful for distinguishing the different species, whereas others, such as isobilateral mesophyll, amphistomatic leaves, and an epidermis with thick cell walls correlate with their high-elevation habitat.

This study aimed to investigate leaf anatomy, as well as photosynthetic gas exchange, that underlie the improvement in light foraging capacity, which appears to occur in aroid vines seeking light exposure. Three levels of plant height (soil level, 3 m and 6 m) were categorised for the aroid vine Rhodospatha oblongata Poepp. to represent the transition from ground to canopy. Compared with shaded leaves, leaves exposed to high light conditions were thicker, presenting a larger, spongy parenchyma characterised by a larger transversal area of intercellular spaces. In addition to the increase in maximum CO2 assimilation (Amax) and thicker and larger leaf lamina, we found an increased light saturation point, light compensation point and water use efficiency at 500 µmol PPFD. Nitrogen content per leaf dry mass remained constant across habitats, but Amax/N was 1.5-times greater in the canopy position than in the leaves at soil level, suggesting that CO2 gain did not rely on an N-related biochemical apparatus. The lower δ13C discrimination observed at high canopy leaves corroborated the higher photosynthesis. Altogether, these results suggest that the large and exposed aroid leaves maintained carbon gain coupled with light gain through investing in a more efficient proportion of intercellular spaces and photosynthetic cell surface, which likely allowed a less pronounced CO2 gradient in substomatal-intercellular space.

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