The carbon isotope ratio (δC) of New Zealand mistletoes (-29.51±0.10‰) and their hosts (-28.89±0.12‰) is generally more negative, and shows less difference between mistletoes and their hosts, than found in previous studies. In 37% of the examined pairs, the δC of mistletoes was less negative than that of their hosts. These reversals were not associated with the relative position (proximal or distal) of the host material with regard to the mistletoe. Differences between host and mistletoe tended to be greater on hosts with less negative δC. Both nitrogen content and isotope ratio (δN) of the mistletoe leaves were strongly correlated with those of their hosts. Nitrogen contents of mistletoe leaves were similar to those of their hosts at low nitrogen contents but proportionately less on hosts with a high nitrogen content, whereas δN of mistletoes was consistently similar to that of their hosts. The δC of mistletoes was related to both host nitrogen content and δN, but δC in host tissue was related to neither, suggesting that the mistletoes derived both nitrogen and carbon from their hosts. The δC of both hosts and mistletoes were significantly related to leaf conductance and carbon dioxide concentration but relationships with transpiration and water use efficiency were not significant. In all cases there was no clear separation between the responses of hosts and mistletoes. This may be related to the similarity of stomatal conductance, transpiration and photosynthesis in the studied mistletoes and their hosts and is consistent with the small differences in δC between mistletoes and hosts found in this study. Consequently, the estimation of mistletoe heterotrophy from carbon discrimination is confounded, as the small difference between host and mistletoe carbon discrimination could equally well result from either similarities in photosynthesis and water relations or heterotrophic assimilation of host-derived carbon. The differences between our study and previous studies (which are mostly from seasonally dry or semi-arid to arid environments) may be related to the temperate environment in which these mistletoes grow. Water is freely available so that the mistletoe is able to obtain sufficient water and dissolved nutrients without having to maintain the high transpiration rate and low water potentials that are needed to extract water from a water-stressed host. Similarly, mistletoe photosynthesis is less inhibited by water stress. The physiological similarities between mistletoe and hosts from a temperate environment are reflected in their similar δC values.
It has been generally assumed that differential accumulation of mineral nutrients, leading to greater accumulation of elements in mistletoe tissues, is associated with greater transpiration in the mistletoe than in the host. Only a few investigations have measured both tissue element concentrations and transpiration, or transpiration-related parameters such as carbon isotope ratios (δ13C). Seasonal means for foliar concentrations of Ca, Mg, K, Na, P, N, transpiration and δ13C were obtained from ten mistletoe–host pairs, nine involving the mistletoe Ileostylus micranthus and one with Tupeia antarctica. Annual means of transpiration and δ13C were similar in mistletoes and hosts, but hosts showed greater variation in both transpiration and δ13C than mistletoes. Foliar concentrations of Ca and Mg in mistletoes were similar to those of their hosts, N concentrations were less, and foliar concentrations of K, Na and P were greater in mistletoes than in their hosts. Ratios of foliar concentrations of Ca, Mg and P in mistletoes to those in their hosts (M/H) were greatest when host transpiration was low, when host δ13C was least negative, and when the difference between mistletoe and host δ13C was most negative. The lack of a phloem connection between host and mistletoe, combined with circulation of elements in the host phloem and their transfer into the host xylem, provides a mechanism that explains the accumulation of phloem-mobile elements in the mistletoe.
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.
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