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

Leaf water potentials in the mistletoe, Ileostylus micranthus growing outdoors decreased rapidly during the early part of the day but remained relatively steady in the early afternoon despite increases in atmospheric vapour pressure deficit (vpd). Minimum water potentials of the mistletoe were relatively constant. They were held at values lower than those of hosts when the latter maintained high water potentials but approached or even exceeded those of hosts when they developed low water potentials. In contrast, cut shoots of Ileostylus usually maintained higher water contents and leaf water potentials than those of its hosts when both were desiccated separately in the laboratory. Pressure-volume analyses indicated that Ileostylus had lower water potential at full turgor, a lower water potential but higher relative water content at turgor loss, and a higher bulk modulus of elasticity than the following four hosts: the native Kunzea ericoides and Coprosma propinqua, and the introduced Ribes sanguineum and Teline monspessulana. Water potential at turgor loss (psi(tlp)) was strongly correlated with the minimum field water potential of both mistletoes and hosts. When psi(tlp) of mistletoe 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 hosts that maintain high field water potentials than on hosts that develop low water potentials. (C) 1999 Annals of Botany Company.