Water relations of temperate mistletoes on various hosts

The daily field water relations and gas exchange of the temperate mistletoes Ileostylus micranthus (Hook.f.) Tiegh. and Tupeia antarctica Cham. et Schlecht. on various hosts were examined seasonally in Dunedin, New Zealand during 1996-1998. Mistletoes commonly have higher transpiration rates (E) than their hosts, and this is generally cited as the reason why mistletoes develop lower water potentials (ψ) than their hosts. The mistletoe-host pairs that we examined showed no significant overall differences in E and stomatal conductance (g), and we used them to test the hypothesis that lowered ψ in mistletoes result from higher E. Despite the lack of differences in E and g, osmotic potentials, predawn and daily minimum ψ (ψmin) were significantly more negative in mistletoes, although differences between host and mistletoe ψ were less on hosts with low osmotic potentials and ψ. Mistletoes maintained lower ψ than their hosts both when unshaded and under artificial shading, had lower ψ than hosts at equal E, but had shoot hydraulic resistances similar to that of their hosts. E and ψ of hosts and mistletoes tended to be coordinated only in summer, when hosts were most water-stressed. Mistletoes maintained higher relative water contents at turgor loss, symplastic water contents, and bulk moduli of elasticity (ε) than their hosts. We conclude that the lower ψ in these temperate mistletoes are a consequence of greater mistletoe E only when host ψ are low, but are otherwise maintained by greater succulence and higher ε than in their hosts.