Antioxidant capacity along the leaf blade of the C-CAM facultative bromeliad Guzmania monostachia under water deficit conditions.

Guzmania monostachia (L.) Rusby ex Mez is an epiphytic, rosette-shaped bromeliad that displays variable degrees of crassulacean acid metabolism (CAM) along the leaf under water deficit. The aim of our study was to evaluate whether the production-scavenging system of reactive oxygen species (ROS) along the leaf length of G. monostachia plants is related to the foliar gradient of CAM when irrigation is withheld. Among the leaf portions, the apex was exposed to the highest photosynthetic flux density and presented the highest relative water content, CAM activity, hydrogen peroxide and lipid peroxidation after treatment. Hence, the most intense CAM at the leaf apex may not have prevented higher oxidative burden in that region during water deficiency. However, the photosynthetic efficiency in the apex seemed unaffected by irrigation withholding or light intensity. The leaf apex also had the highest carotenoid content and increased superoxide dismutase and ascorbate peroxidase activities under treatment. Hence, G. monostachia was able to maintain ROS under tolerable levels by increasing antioxidant capacity. Our results suggest the metabolic differences within the same leaf under water deficit may derive from the gradient of light incidence, which emphasises the physiological plasticity this bromeliad applies to adapt to the adverse conditions of the canopy.