Can morpho-physiological responses to natural light conditions in Cybistax antisyphilitica (Mart.) Mart. explain its widespread occurrence in Brazilian Savannas?

Abstract

Cybistax antisyphilitica (Bignoniaceae) is extensively distributed in the Cerrado biome, from typical savannas to forest formations. In these areas, light varies widely in both time and space. The aim of this study was to investigate the initial growth and physiological responses of potted C. antisyphilitica seedlings to natural light conditions. To this end, we cultivated one group of seedlings under full sunlight (FS), a second group under a small gap with medium transmittance (MT) and a third group under canopy shade with low transmittance (LT), in an urban fragment of a tropical semideciduous forest. The daily course of chlorophyll a fluorescence was measured at the beginning, middle and end of the dry season. Also, at the end of the experiment, measurements of leaf gas exchange and growth were carried out. Dynamic photoinhibition was found in seedlings under FS. The photosynthetic rates, stomatal conductance and transpiration rates were higher under FS compared to seedlings under LT conditions. Overall, the stem diameter, the dry mass of leaves and stem, total dry mass and relative growth rate were higher in plants under FS and MT than plants under LT. Cybistax antisyphilitica allocated less biomass to stems and leaves and more to the roots, independent of growth conditions. The plasticity at leaf level suggests that C. antisyphilitica can exploit the heterogenous light environment of the semideciduous forest, but the massive development of the roots can contribute to its tolerance to dry periods typical of the Cerrado biome.