Contrasting gas exchange effects on the interactions of two mistletoe species and their host Acacia schaffneri

Abstract

Mistletoes nutrient and water uptake can alter the gas exchange activity of their host plants, leading them to a diminished photosynthetic capacity and poor water use efficiency. This is particularly important in semi-arid areas, where water availability is a limiting factor. The aim of this study was to evaluate gas exchange (photosynthetic net rate (A), stomatal conductance (G) and intrinsic water use efficiency (iWUE)) of uninfected Acacia schaffneri trees, compared to trees infected by two mistletoe species, Psittacanthus calyculatus and Phoradendron brachystachyum, in a semi-arid zone in central Mexico. We also registered the physiological performance of the two-infecting species. All variables were measured in a monthly basis for one year. Physiological parameters for Acacia trees and for the two-mistletoe species, varied along the year, suggesting an important effect of the environmental conditions. Infected versus uninfected trees were not different for A and iWUE, however G in trees infected with P. brachystachyum was significantly lower, particularly on severely infected trees. Mistletoe parameters also varied with time and severity of infection, the latter suggesting a density dependent pattern: P. brachystachyum seems to be competing with its conspecific since A and iWUE were lower when infection was high, whereas P. calyculatus was favored with a greater iWUE when coexisting with its conspecifics. Results suggest that seasonality effect is more important than mistletoe infection in the physiological performance of the host.