title: Stable isotope physiology of stem succulents across a broad range of volume-to-surface area ratio.
abstract: Volume-to-surface area ratio (V:S) across stem succulent taxa varies by almost two orders of magnitude. The broad range in V:S of cacti and other succulent species likely has considerable importance for adaptation since stem volume determines the storage capacity of water, carbon and nutrients and stem surface area is directly related to whole-stem photosynthetic capacity. We examined the intrinsic physiological tradeoffs across diverse stem morphologies in three divergent evolutionary groups where stem succulence is common: Cactoideae, Opuntioideae (Cactaceae) and Euphorbiaceae. We predicted that variation in physiological response to environmental conditions would be (1) constrained by stem V:S, and (2) detectable in the stable isotope ratios of plant tissues. Stable isotope ratios were measured in the spines/prickles of 62 stem-succulent species occurring in a common garden setting in Phoenix, AZ, USA. Biomass δ(13)C, δ(2)H and δ(18)O increased with V:S in Cactoideae only, possibly reflecting various levels of Crassulacean acid metabolism (CAM) strength in the other lineages. Within Cactoideae-group with the highest CAM strength and largest range in V:S-δ(13)C and δ(18)O increased 2.2 and 11.5 ‰, respectively, with a 22-fold increase in V:S. Both δ(13)C and V:S decreased with species climate-niche estimates of precipitation, indicating that stem morphology and physiology in Cactoideae may be constrained by available moisture. Taken together, these data suggest that physiological tradeoffs associated with stem V:S are detectable across broad evolutionary groups despite differences in CAM strength.
authors.affiliation: Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ, USA. khultine@dbg.org.
authors.forename: Kevin R
authors.initials: KR
authors.lastname: Hultine
authors.affiliation: Department of Botany, University of Wyoming, Laramie, WY, USA.
authors.forename: David G
authors.initials: DG
authors.lastname: Williams
authors.affiliation: Department of Geosciences, University of Arizona, Tucson, AZ, USA.
authors.forename: David L
authors.initials: DL
authors.lastname: Dettman
authors.affiliation: Merriam Powell Center for Environmental Research and Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA.
authors.forename: Bradley J
authors.initials: BJ
authors.lastname: Butterfield
authors.affiliation: Department of Research, Conservation and Collections, Desert Botanical Garden, Phoenix, AZ, USA.
authors.forename: Raul
authors.initials: R
authors.lastname: Puente-Martinez
mesh_terms: D029421:Cactaceae; D002247:Carbon Isotopes; D002980:Climate; D007554:Isotopes; D010788:Photosynthesis; D018547:Plant Stems / Q000737:chemistry*
chemical_list: D002247:Carbon Isotopes; D007554:Isotopes
keywords: Cactaceae; Climate niche; Deuterium in CAM; Euphorbiaceae; Stable isotopes