Photosynthesis and water relations of the mistletoe, Phoradendron villosum, and its host, the California valley oak, Quercus lobata.

Water vapor and CO2 exchange characteristics were studied in Phoradendron villosum, a hemiparasitic mistletoe and its host, Quercus lobata. The hemiparasite had stomatal conductances equal to or higher than the host but a much lower capacity to fix carbon. Respiration was high in the mistletoe relative to maximum photosynthesis because of the high leaf specific weight (weight per unit area) in this species. The temperature optima for photosynthesis were similar in both species although photosynthesis in Phoradendron declined more steeply below 20 degrees C. Consequences of the high conductance and low rate of photosynthesis in the mistletoe include a low water-use efficiency and a relatively high internal concentration of CO2.