Journal of Experimental Botany, Vol. 54, No. 389, pp. 1985-1993,
August 1, 2003
© 2003 Oxford University Press
Water flows in the parasitic association Rhinanthus minor/Hordeum vulgare
Received 20 January 2003; Accepted 12 May 2003
Julius von Sachs Institut für Biowissenschaften der Universität, Lehrstuhl Botanik I, Julius von Sachs Platz 2, D-97082 Würzburg, Germany
* To whom correspondence should be addressed. Fax +49 931 888 6158. E-mail: hartung@botanik.uni-wuerzburg.de
Using the facultative root hemiparasite Rhinanthus minor and its host Hordeum vulgare several aspects of water relations have been measured in this parasitic association. Extraction of xylem sap by the parasite from the host’s roots is facilitated by con siderably higher transpiration per leaf area in the parasite than in the host and by the fact that stomata of attached Rhinanthus were open all day and night despite extremely high ABA concentrations in the leaves. By comparison, another root hemiparasite, Melampyrum arvense, parasitizing various grasses in the field, showed normal diurnal stomatal behaviour. The abnormal behaviour of Rhinanthus stomata was not due to anatomical reasons as closure could be induced by applying high external ABA concentrations. Remarkable differences have been detected between the hydraulic conductance of barley seminal roots showing relatively low values and that of Rhinanthus seminal roots showing very high values. The latter could be related to the observed high ABA concentrations in these roots. Whole plant water uptake, transpirational losses, growth-dependent deposition, and the flows of water within the plants have been measured in singly growing Rhinanthus and Hordeum plants and in the parasitic association between the two. Water uptake, deposition and transpiration in Rhinanthus were dramatically increased after attachment to the barley host; most of the water used by the parasite was extracted as xylem sap from the host, thereby scavenging 20% of the total water taken up by the host’s roots. This water uptake by the parasitized host, however, due to a parasite-induced reduction in the host’s growth, was decreased by 22% as compared to non-parasitized barley. The overall changes in growth-related water deposition in the host and parasite pointed to decreased shoot growth and relatively favoured root growth in the host and to strongly favoured shoot growth in the parasite. These changes in the host became more severe, when more than one Rhinanthus was parasitizing one barley plant.
Key words: Hordeum, parasitic association, Rhinanthus, water relations.
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