Water Relations of the Mistletoe, Amyema miquelii, and Its Host Eucalyptus fasciculosa

Abstract

Mistletoes have frequently been found to transpire more rapidly than their hosts, and usually maintain a more negative water potential. Leaf water potential and diffusive conductance were monitored from the end of a dry summer (March) through a wet winter (to August) on the mistletoe Amyema miquelii (Lehm. ex Miq.) Tieghem and its host Eucalyptus fasciculosa F. Muell.

The calculated transpiration rate of the mistletoe was greater than that of its host, but water potential data showed two unusual features. Firstly, the parasite pre-dawn water potential always remained lower than that of the host, regardless of the host's water status. Secondly, the parasite water potential during the day was on one occasion less negative than that of the host, i.e. a reverse gradient.

Tissue water relations studies showed that A. miquelii leaves had more negative solute potential and a larger water capacitance than E. fasciculosa leaves. The unusual features of the seasonal data were explained in terms of a large haustorial resistance to water flow and hysteresis in the relationship between transpiration and water potential in the mistletoe leaf. This hysteresis was thought to be due to the contribution of stored leaf water to transpiration.