The hemiparasitic shrub Osyris lanceolata (Santalaceae) does not disturb the ecophysiology of its hosts

Osyris lanceolata is a hemiparasitic dioecious shrub species growing on stabilised dunes. This study aimed to determine the spatial structure of O. lanceolata and its hosts, to understand the ecophysiological mechanisms involved in water and nutrient transfer, and quantify the physiological stress on its hosts. In the Asperillo stabilised dunes (Doñana, Spain) we selected one mixed plot (50 m × 50 m) with Pinus pinea and Juniperus phoenicea in which the coordinates of every individual, the hosts and the hemiparasitic species, were recorded. Additionally, we selected two study areas in which O. lanceolata was well represented, one dominated by P. pinea and the other by J. phoenicea. We marked 60 plants: 20 O. lanceolata with their respective hosts (10 Pinus and 10 Juniperus) and 20 free host plants. In all the marked plants we measured: leaf water potential, gas exchange, photochemical efficiency, morpho-anatomical leaf traits, pigments, and proline content. Our results evidenced that O. lanceolata is spatially associated with its hosts. Midday leaf water potential values were always between 1.5 to 3 times more negative and transpiration rates were 6-fold higher for the hemiparasite than for the hosts. Additionally, O. lanceolata plants showed a high accumulation of proline in leaves and haustorium. Although the hemiparasitic species maintains an active photosynthetic canopy, its rates of CO2 assimilation were 35–48% lower than in the hosts, which caused a 10-fold lower instantaneous water use efficiency. Through these mechanisms, the hemiparasitic plant can absorb water and nutrients from the host species. Despite this parasitic relationship, there were no significant differences in the physiological performance of the hosts in comparison to the parasite-free plants. We conclude that O. lanceolata presents a specific strategy to absorb water in this dry ecosystem, without exerting negative effects on the plant community, which might indicate that there is an equilibrium in the tradeoffs between parasitism and mutualism in the interaction between species.