Four distinct leaf types in the Brazilian Cerrado, based on bundle sheath extension morphology

Abstract

Plant species have been classified as being either heterobaric or homobaric based on the presence or absence (respectively) of bundle sheath extensions (BSE) in leaf veins. These leaf types differ in their physiological performance, and their occurrences tend to be associated with environment. This study investigated the presence/absence of BSE and their structural peculiarities in 49 woody species (28 angiosperm families) in the Brazilian Cerrado using anatomical methods. We measured the gas exchange indices and stomatal density. In addition to typical homobaric (61.2%) and heterobaric (22.4%) species, we also found species with intermediate BSE features. These we call “larger heterobaric” (12.2%) and “semi-heterobaric” (4.1%) species. In the former, BSE occurs around only the secondary veins, whereas in the latter the BSE projects only towards the adaxial leaf surface. The net CO2 assimilation rates, the stomatal conductance, the transpiration rates, and the carboxylation efficiencies were statistically higher in heterobaric than in homobaric species. In the larger heterobaric species, the photosynthetic indices were similar to those in the heterobaric species. Meanwhile, semi-heterobaric species showed photosynthetic indices similar to the homobaric species. We demonstrated that structural variation in the BSE is linked to physiological leaf performance, suggesting some correlation between leaf morphology and plant adaptations.

Drought-adapted leaves are produced even when more water is available in dry tropical forest

“…Consequently, BSEs consist of colorless tissue and surrounding bundles and extend from one epidermis to the other, completely or not, creating compartments in the mesophyll (McClendon 1992; Rodrigues et al 2017). B. acuruana, P. moniliformis, and T. molle are classi ed as heterobaric based on the presence of BSEs tissue (Rodrigues et al 2017) as the CO 2 pressure varies between different parts of the leaf (Zsögön et al 2015). This reduces air diffusion and allows the leaf to open or close groups of stomata in different regions, and consequently, has more control over water loss.…”

Competition for epidermal space in the evolution of leaves with high physiological rates

“…This function, again, can apply to some, but certainly not all, leaves with BSEs since it requires BSEs that span the full leaf thickness to both upper and lower epidermis while some BSEs are observed to reach only one epidermis (e.g. Rodrigues et al ., ). Heterobaric leaves also require closed areoles, a trait that is similarly not universal (Rodrigues et al ., ).…”

“…Rodrigues et al ., ). Heterobaric leaves also require closed areoles, a trait that is similarly not universal (Rodrigues et al ., ). Furthermore, heterobaric leaves require isolation of relatively small areoles while parallel or open dichotomous venations can only involve the enclosure of large and highly elongate leaf areas (e.g.…”