JXB Advance Access published online on April 24, 2008
Journal of Experimental Botany, doi:10.1093/jxb/ern080
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RESEARCH PAPER |
On the nature of facultative and constitutive CAM: environmental and developmental control of CAM expression during early growth of Clusia, Kalanchoë, and Opuntia
1Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancon, Republic of Panama
2School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia
* To whom correspondence should be addressed. E-mail: winterk{at}si.edu
The capacity to induce crassulacean acid metabolism developmentally (constitutive CAM) and to up-regulate CAM expression in response to drought stress (facultative CAM) was studied in whole shoots of seven species by measuring net CO2 gas exchange for up to 120 day–night cycles during early growth. In Clusia rosea, CAM was largely induced developmentally. Well-watered seedlings began their life cycle as C3 plants and developed net dark CO2 fixation indicative of CAM after the initiation of the fourth leaf pair following the cotyledons. Thereafter, CAM activity increased progressively and drought stress led to only small additional, reversible increases in dark CO2 fixation. In contrast, CAM expression was overwhelmingly under environmental control in seedlings and mature plants of Clusia pratensis. C3-type CO2 exchange was maintained under well-watered conditions, but upon drought stress, CO2 exchange shifted, in a fully reversible manner, to a CAM-type pattern. Clusia minor showed CO2 exchange reponses intermediate to those of C. rosea and C. pratensis. Clusia cretosa operated in the C3 mode at all times. Notably, reversible stress-induced increases of dark CO2 fixation were also observed during the developmental progression to pronounced CAM in young Kalanchoë daigremontiana and Kalanchoë pinnata, two species considered constitutive CAM species. Drought-induced up-regulation of CAM was even detected in young cladodes of a cactus, Opuntia ficus-indica, an archetypal constitutive CAM species. Evidently, the defining characteristics of constitutive and facultative CAM are shared, to variable degrees, by all species.
Key words: Carbon dioxide uptake, Clusia, constitutive CAM, crassulacean acid metabolism, development, drought stress, environment, facultative CAM, Kalanchoë, Opuntia
Received 29 January 2008; Revised 21 February 2008 Accepted 26 February 2008