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Ecological Section

Edwards, Erika [1], Diaz, Miriam [2], Donoghue, Michael J. [1].

Water relations of Pereskia guamacho, a cactus with leaves.

The unusual water relations of typical leafless, stem-succulent cacti are well documented. This is not true, however, for Pereskia (Cactaceae), the 17 species of leafy trees and shrubs that have long been thought to represent the “ancestral cactus.” Recent molecular phylogenetic studies strongly suggest that Pereskia is not monophyletic, with one clade of Pereskia species being more closely related to the rest of the cacti. Understanding how members of these different Pereskia lineages regulate their water use should be of considerable value in reconstructing the events that led to stem succulence and an evolutionary loss of leaves in Cactaceae. Here we report on the water relations of Pereskia guamacho, a small tree of the semi-arid scrubland of Venezuela’s Caribbean coast. Sapwood-specific xylem conductivity is moderately low when compared to other vessel-bearing trees, but leaf-specific xylem conductivity is one of the highest reported in the literature. This is due to an exceptionally high Huber Value, which is afforded by P. guamacho’s short shoot system, allowing for continued use of older wood. Our constructed vulnerability curves indicate that P. guamacho xylem is not particularly vulnerable to drought-induced cavitation, especially considering the high leaf water potentials maintained year round. Our curves were confirmed with observations of little to no variation in stem xylem conductivity between wet and dry seasons, or before and during leaf shedding. P. guamacho appears to be a facultative CAM plant, and maximum leaf stomatal conductance is low whether the leaf is performing either C3 or CAM photosynthesis. δ13C values similarly indicate high photosynthetic water use efficiency in this species. Our findings contradict a hypothesized relationship between leaf performance and stem hydraulic capacities previously discussed in the literature. We discuss this result in terms of what it may suggest for early cactus evolution.

1 - Yale University, Department of Ecology and Evolutionary Biology, P.O. Box 208105, New Haven, Connecticut, 06520-8105, USA
2 - Universidad Experimental Francisco de Miranda, CIEZA, Coro, Falcon, , Venezuela

CAM photosynthesis
hydraulic conductivity
vulnerability curve
water relations

Presentation Type: Paper
Session: 17-13
Location: Wasatch (Cliff Lodge)
Date: Monday, August 2nd, 2004
Time: 4:30 PM
Abstract ID:663

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