Gas exchange characteristics of giant cacti species varying in stem morphology and life history strategy

Abstract

Premise of the StudyGiant cacti species possess long cylindrical stems that store massive amounts of water and other resources to draw on for photosynthesis, growth, and reproduction during hot and dry conditions. Across all giant cacti taxa, stem photosynthetic surface area to volume ratio (S:V) varies by several fold. This broad morphological diversity leads to the hypothesis that giant cacti function along a predictable resource use continuum from a “safe” strategy reflected in low S:V, low relative growth rates (RGR), and low net assimilation rates (Anet) to a high‐risk strategy that is reflected in high S:V, RGR, and Anet.MethodsTo test this hypothesis, whole‐plant gas exchange, chlorophyll fluorescence, and whole‐spine‐tissue carbon isotope ratios (δ13C) were measured in four giant cacti species varying in stem morphology and RGR. Measurements were conducted on five well‐watered, potted plants per species.Key ResultsUnder conditions of mild diel temperatures and low atmospheric vapor pressure deficit, Anet, transpiration (E), and stomatal conductance (Gs) were significantly higher, and water‐use efficiency (Anet : Gs) was lower in fast‐growing, multi‐stemmed species compared to the slower growing, single‐stemmed species. However, under warmer, less optimal conditions, gas exchange converged between stem types, and neither δ13C nor chlorophyll fluorescence varied among species.ConclusionsThe results add to a growing body of evidence that succulent‐stemmed plants function along a similar economic spectrum as leaf‐bearing plants such that functional traits including stem RGR, longevity, morphology, and gas exchange are correlated across species with varying life‐history strategies.

Relationships among climate, stem growth, and biomass δ13C in the giant saguaro cactus (Carnegiea gigantea)

“…These patterns suggest that this saguaro plant conducted some C3 photosynthesis, at least during the growing season, assuming that the spine tissue was constructed of recently fixed CO 2 with rapid carbon pool turnover rates. However, previous gas exchange experiments conducted throughout the year on mature saguaros at one of our field sites (Tumamoc Hill) and in potted saguaros and other giant cacti species in a controlled experiment yielded no evidence of daytime stomatal conductance and CO 2 uptake (Bronson et al 2011, Huber et al 2018). On the other hand, both studies did detect potential CO 2 leakage during Phase III (Bronson et al 2011, Huber et al 2018).…”

“…However, previous gas exchange experiments conducted throughout the year on mature saguaros at one of our field sites (Tumamoc Hill) and in potted saguaros and other giant cacti species in a controlled experiment yielded no evidence of daytime stomatal conductance and CO 2 uptake (Bronson et al 2011, Huber et al 2018). On the other hand, both studies did detect potential CO 2 leakage during Phase III (Bronson et al 2011, Huber et al 2018). Because of the large 13 C discrimination associated with Rubisco, leaked CO 2 (if present) results in photosynthetic products that can be considerably more negative than the originally fixed organic acids during Phase I (O'Leary 1988, Winter andHoltum 2002).…”

“…, Huber et al. ). On the other hand, both studies did detect potential CO 2 leakage during Phase III (Bronson et al.…”

“…, Huber et al. ). Because of the large 13 C discrimination associated with Rubisco, leaked CO 2 (if present) results in photosynthetic products that can be considerably more negative than the originally fixed organic acids during Phase I (O'Leary , Winter and Holtum ).…”

“…) and on potted saguaros under controlled watering experiments (Huber et al. ) found that stomatal conductance and photosynthetic carbon gain were highest during the growing season in mid‐summer, even though nocturnal vapor pressure deficit was much higher than other periods of the year when gas exchange was substantially lower. These gas exchange studies combined with the daily correlation between the δ 13 C ratio of spine tissues and nocturnal vapor pressure deficit mentioned earlier (English et al.…”