干旱区生态环境知识资源中心

Gas exchange patterns and water relations of two mistletoes, Tapinanthus oleifolius and Viscum rotundifolium on the same deciduous host tree, Acacia nebrownii, have been studied intensively in the dry and wet seasons in semi-arid south-eastern Namibia, where rain falls only in the summer. Comparative measurements of Tapinanthus oleifolius on the stem-succulent CAM plant, Euphorbia virosa, and on Ziziphus mucronata in the same area were also made. All measurements of CO2 exchange and transpiration were performed in fully acclimatized cuvettes of a gas exchange 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 water 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 CO2 concentration and water-use efficiency (WUE) contradicted commonly accepted generalizations. Transpiration and leaf conductance were found to be higher for the host than for any of its common mistletoes as long 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 oleifolius, but not Viscum rotundifolium, transpired at higher rates than the host. CO2 balance over 24 hours and photosynthetic CO2 uptake were significantly higher for the host than for any of its parasites. Although the WUE over 24 h was slightly better for Acacia nebrownii compared 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 Tapinanthus oleifolius and Viscum rotundifolium behaved in opposite ways in the dry and wet seasons, indicating that different phenological cycles of both species might be involved in determining mistletoe performance on the same host. The results presented contribute to a better understanding of the host/parasite relationship and indicate that commonly accepted generalizations may not always be applicable.