The Daily Course of Water Potential and Leaf Conductance in some New Zealand Plants

Abstract

The field water potentials of some 23 species, mostly woody plants and ferns native to New Zealand (but including one introduced fern), were measured on various occasions, mostly during the 1982/83 summer. The minimum water potentials recorded in the field were highly correlated with the ability of cut shoots and leaves to avoid low water deficits and low water potentials when artificially desiccated in the laboratory. Species that maintained high water potentials in the field also showed lesser fluctuations in water potential than those that developed low water potentials. Leaf conductances and transpiration rates were measured in five species. Low conductances and transpiration rates were generally associated with those species that maintained high water potentials in the field (e.g. Phymatosorus diversifolius (W illd .) P ic . S er .), whereas high conductances and transpiration rates were generally associated with species that developed low water potentials (e.g Brachyglottis repanda F orst .). One species, Metrosideros robusta A. C unn . showed high conductances and transpiration rates but maintained a higher water potential in the field. Those species that showed the largest reductions of conductance in the early afternoon also showed the best recovery of shoot water potential. For the species examined, the artifical desiccation of cut shoots and leaves in the laboratory provides a guide to minimum water potentials in the field. These appear to be determined by rates of water loss from the shoot rather than the hydraulic conductivity of the soil-plant-atmosphere system.