Seasonal photosynthetic gas exchange and water-use efficiency in a constitutive CAM plant, the giant saguaro cactus (Carnegiea gigantea)

Abstract

Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use efficiency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to climate change. We monitored net CO(2) assimilation (A(net)), stomatal conductance (g(s)), and transpiration (E) rates periodically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tucson, Arizona USA to investigate environmental and physiological controls over carbon gain and water loss in this ecologically important plant. We hypothesized that seasonal changes in daily integrated water use efficiency (WUE(day)) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO(2) uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE(day) occurred during time periods with extreme high and low air vapor pressure deficit (D(a)). The diurnal with the highest D(a) had low WUE(day) due to minimal net carbon gain across the 24 h period. Low WUE(day) was also observed under conditions of low D(a); however, it was due to significant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D(a) confirmed the relationship between D(a) and g(s). Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succulent CAM plants of warm desert environments.

From facilitative to competitive interaction between Larrea tridentata and Cylindropuntia leptocaulis in the southern Chihuahuan Desert

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Long‐term changes in abundances of Sonoran Desert lizards reveal complex responses to climatic variation

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Dominant Trees in a Subtropical Forest Respond to Drought Mainly via Adjusting Tissue Soluble Sugar and Proline Content

“…Changes in precipitation regimes have considerable ecological and evolutionary impacts on terrestrial ecosystems (Pratt and Mooney, 2013; Moran et al, 2014; Zeppel et al, 2014; Knapp et al, 2015). Physiological properties of plant species (Coe and Sparks, 2014); encroachment of woody plants (Kulmatiski and Beard, 2013); tree mortality (Anderegg et al, 2015); forest structure (Hoeppner and Dukes, 2012); evapotranspiration dynamics (Raz-Yaseef et al, 2012); plant community composition (Cleland et al, 2013); biomass and net primary productivity (Hovenden et al, 2014; Wilcox et al, 2015); litterfall (Travers and Eldridge, 2013); the availability, uptake, transport, and accumulation of plant nutrients (including N, P, S, K, Ca, and Mg) (Rouphael et al, 2012); soil respiration (Thomey et al, 2011); and soil microbial communities (Zeglin et al, 2013), have been well documented to be affected by the rainfall patterns.…”