Stable isotope physiology of stem succulents across a broad range of volume-to-surface area ratio
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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 δ13C, δ2H and δ18O 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—δ13C and δ18O increased 2.2 and 11.5 ‰, respectively, with a 22-fold increase in V:S. Both δ13C 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.
KeywordsCactaceae Climate niche Deuterium in CAM Euphorbiaceae Stable isotopes
The authors thank the Desert Botanical Garden for providing access to plant material in their Living Collections. The authors thank D. Dehn and D. Koepke for their technical assistance in the lab and in the field.
Author contribution statement
KRH and DGW originally formulated the project’s idea, KRH and RPM conducted the field work, DGW and DLD generated the isotope data, BJB performed the climate envelope modeling, KRH performed the statistical analysis and wrote the manuscript, and all authors provided editorial advice.
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