COS 33-10
Water use of mistletoe and its host tree
Loranthus europaeus and Viscum album belong to the group of hemiparasitic plants. Their modified roots (haustoria) tap the xylem of a host plant and, developing more negative water potential, acquire water directly from the hosts' vessels. The effect of mistletoe on its host palnt sap flow is so severe that branches infested by Loranthus europaeus often dies behind the point of infestation. On the other hand, phloem connection between mistletoe and host plant is not established. Therefore no products of photosynthesis are given back to the host tree by the mistletoe and all acquired water is lost through tranpiration. The research aimed to answer three questions related to water use of this complex of two organisms: (1) Is the amount of water transpiraed per unit of leaf area different between the host plant and mistletoe? (2) Are the diurnal patterns of stomatal conductance different? (3) Does mistletoe affect whole tree level and posibly stand level transpiration? Heat balance method with external heating (EMS62, EMS Brno, Czech Republic) was used to measure sap flow of three oak branches and four Loranthus plants. The same experimental design was used for the second experiment with willow Salix alba and another species of mistletoe - Viscum album. Measured sap flow was standardized per unit of leaf area in these two experiments. In a third experiment, sap flow was measured at the tree level in stems of three infested (10 - 15 individuals of Viscum album per tree) and three healthy mature (30 years old) poplars (Populus nigra). We used trunk heat balance method with internal heating (EMS51, EMS Brno, Czech Republic). The regression betwen three breast height diameter and sap flow was established for both groups of healthy and infested trees. Stomatal conductance was calculated from sap flow and inverted Penman-Monteith equation for all three experiments.
Results/Conclusions
Both species of mistletoe maintained by 5 – 10 bars lower midday water potential than their host plants. Therefore, tranpiraton of mistletoe was usually 1.5 – 2 times higher than of a host tree. Stomata of host and hemiparasite differed in their sensitivity to air vapor pressure deficit (D): stomatal conductances of mistletoes were less limited upon high D. Infested trees transpired more water than healthy ones at the stem level. The sap flow of the trees infested by mistletoe was by 10 – 15 % higher than in heathy ones, which could increase a stand transpiration by 50 mm year-1.