GAS-EXCHANGE AND WATER RELATIONS OF 2 MISTLETOES, TAPINANTHUS-OLEIFOLIUS AND VISCUM-ROTUNDIFOLIUM, ON THE SAME HOST, ACACIA-NEBROWNII, IN SOUTH-EASTERN NAMIBIA

Gas exchange patterns and water relations of two mistletoes, Tapinanth us oleifolius and Viscum rotundifolium on the same deciduous host tree , Acacia nebrownii, have been studied intensively in the dry and wet s easons in semi-arid south-eastern Namibia, where rain falls only in th e summer. Comparative measurements of Tapinanthus oleifolius on the st em-succulent CAM plant, Euphorbia virosa, and on Ziziphus mucronata in the same area were also made. All measurements of CO2 exchange and tr anspiration were performed in fully acclimatized cuvettes of a gas exc hange system which allowed continuous measurements of the host and the parasite simultaneously. The water potential results confirm previous observations that mistletoes have a lower water potential throughout the day than their hosts. In the dry season, pre-dawn and minimum wate r potential was about 1 MPa lower than in the wet season, for both the leafless and fully foliated host and the evergreen mistletoes. There was an effective stomatal control in all mistletoe plants investigated . The results obtained for transpiration, leaf conductance, internal C O2 concentration and water-use efficiency (WUE) contradicted commonly accepted generalizations. Transpiration and leaf conductance were foun d to be higher for the host than for any of its common mistletoes as l ong as the leaf-to-air water vapour pressure difference (Delta w) did not exceed 30 mPa Pa-1. At higher values of Delta w, Tapinanthus oleif olius, but not Viscum rotundifolium, transpired at higher rates than t he host. CO2 balance over 24 hours and photosynthetic CO2 uptake were significantly higher for the host than for any of its parasites. Altho ugh the WUE over 24 h was slightly better for Acacia nebrownii compare d with Tapinanthus oleifolius, apparent WUE during photosynthetic CO2 uptake was always better for both mistletoes. Measurements of CO2 and water balance over 24 h, photosynthetic CO2 uptake, night respiration, internal CO2 concentration, leaf conductance and WUE indicate that Ta pinanthus oleifolius and Viscum rotundifolium behaved in opposite ways in the dry and wet seasons, indicating that different phenological cy cles of both species might be involved in determining mistletoe perfor mance on the same host. The results presented contribute to a better u nderstanding of the host/parasite relationship and indicate that commo nly accepted generalizations may not always be applicable.