Bannister Peter, Strong Graham L
Department of Botany, University of Otago, PO Box 56, Dunedin, New Zealand.
Oecologia. 2001 Jan;126(1):10-20. doi: 10.1007/s004420000495. Epub 2001 Jan 1.
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.
新西兰槲寄生（-29.51±0.10‰）及其寄主（-28.89±0.12‰）的碳同位素比率（δC）总体上更负，且槲寄生与其寄主之间的差异比以往研究中发现的要小。在37%的被检测配对中，槲寄生的δC比其寄主的δC负性更小。这些逆转与寄主材料相对于槲寄生的相对位置（近端或远端）无关。寄主和槲寄生之间的差异在δC负性较小的寄主上往往更大。槲寄生叶片的氮含量和同位素比率（δN）都与它们寄主的氮含量和同位素比率密切相关。在低氮含量时，槲寄生叶片的氮含量与它们寄主的氮含量相似，但在高氮含量的寄主上，槲寄生叶片的氮含量按比例减少，而槲寄生的δN则始终与其寄主的δN相似。槲寄生的δC与寄主的氮含量和δN都有关，但寄主组织中的δC与两者均无关，这表明槲寄生从其寄主中获取氮和碳。寄主和槲寄生的δC都与叶片导度和二氧化碳浓度显著相关，但与蒸腾作用和水分利用效率的关系不显著。在所有情况下，寄主和槲寄生的响应之间没有明显的区分。这可能与所研究的槲寄生及其寄主的气孔导度、蒸腾作用和光合作用的相似性有关，并且与本研究中发现的槲寄生和寄主之间δC的微小差异一致。因此，从碳同位素分馏来估计槲寄生的异养作用变得复杂，因为寄主和槲寄生碳同位素分馏之间的微小差异同样可能是由于光合作用和水分关系的相似性，或者是由于对寄主衍生碳的异养同化作用。我们的研究与以往研究（大多来自季节性干燥或半干旱至干旱环境）之间的差异可能与这些槲寄生生长的温带环境有关。水分可自由获取，因此槲寄生能够获得足够的水分和溶解养分，而无需维持从水分胁迫的寄主中提取水分所需的高蒸腾速率和低水势。同样，槲寄生的光合作用受水分胁迫的抑制较小。温带环境中槲寄生和寄主之间的生理相似性反映在它们相似的δC值上。
