Aspects of the water relations of Ileostylus micranthus (Hook. f.) Tieghem, a New Zealand mistletoe

Leaf water potentials in the mistletoe, Ileostylus micranthus growing outdo ors decreased rapidly during the early part of the day but remained relativ ely steady in the early afternoon despite increases in atmospheric vapour p ressure deficit (vpd). Minimum water potentials of the mistletoe were relat ively constant. They were held at values lower than those of hosts when the latter maintained high water potentials but approached or even exceeded th ose of hosts when they developed low water potentials. In contrast, cut sho ots of Ileostylus usually maintained higher water contents and leaf water p otentials than those of its hosts when both were desiccated separately in t he laboratory. Pressure-volume analyses indicated that Ileostylus had lower water potential at full turgor, a lower water potential but higher relativ e water content at turgor loss, and a higher bulk modulus of elasticity tha n the following four hosts: the native Kunzea ericoides and Coprosma propin qua, and the introduced Ribes sanguineum and Teline monspessulana. Water po tential at turgor loss (psi(tlp)) was strongly correlated with the minimum field water potential of both mistletoes and hosts. When psi(tlp) of mistle toe and host is similar (as on Kunzea and Ribes) field water potentials are also similar, but when psi(tlp) is lower in the mistletoe (as on Coprosma and Teline), the field water potential of the mistletoe is lower than that of its host. Consequently, I. micranthus is likely to be more frequent on h osts that maintain high field water potentials than on hosts that develop l ow water potentials. (C) 1999 Annals of Botany Company.