Transpiration and stomatal conductance of mistletoe (Loranthus europaeus) and its host plant, downy oak (Quercus pubescens)

Sap flow rate was measured in the crown of a solitary specimen of downy oak (Quercus pubescens) infested by mistletoe (Loranthus europaeus). Five oak branches and two mistletoe plants were selected for analysis. The seasonal sum of transpired water expressed per leaf area unit was five times higher in the mistletoe than in the oak. In addition, the diurnal curves of sap flow were different between the plants. In the morning, the sap flow measured in the mistletoe lagged one hour behind the sap flow measured in an oak branch unencumbered by mistletoe. In contrast, no time lag was observed in the evening. The proportion of water transpired at night relative to the total transpiration was 7% in both species. The stomatal conductances derived from the inverted Penman-Monteith equation and their dependence on global radiation and the vapour pressure deficit (D) revealed that D exerts a different behaviour in stomatal control of transpiration in the mistletoe. We also determined that the concentration of calcium in the leaf mass could serve as a proxy for transpiration rate, however the relationship was not proportional.