CO2 starvation experiments provide support for the carbon-limited hypothesis on the evolution of CAM-like behaviour in Isoëtes

Background and Aims Crassulacean acid metabolism (CAM) is an adaptation to increase water use efficiency in dry environments. Similar biochemical patterns occur in the aquatic lycophyte genus Isoëtes. It has long been assumed and accepted that CAM-like behaviour in these aquatic plants is an adaptation to low daytime carbon levels in aquatic ecosystems, but this has never been directly tested. Methods To test this hypothesis, populations of Isoëtes engelmannii and I. tuckermanii were grown in climate-controlled chambers and starved of atmospheric CO2 during the day while pH was measured for 24 h. Key Results We demonstrate that terrestrial plants exposed to low atmospheric CO2 display diel acidity cycles similar to those in both xerophytic CAM plants and submerged Isoëtes. Conclusions Daytime CO2 starvation induces CAM-like nocturnal acid accumulation in terrestrial Isoëtes, substantiating the hypothesis that carbon starvation is a selective pressure for this physiological behaviour.

Are thick leaves, large mesophyll cells and small intercellular air spaces requisites for CAM?

Background and Aims It is commonly accepted that the leaf of a crassulacean acid metabolism (CAM) plant is thick, with large mesophyll cells and vacuoles that can accommodate the malic acid produced during the night. The link between mesophyll characteristics and CAM mode, whether obligate or C3/CAM, was evaluated. Methods Published values of the carbon isotopic ratio (δ 13C) as an indicator of CAM, leaf thickness, leaf micrographs and other evidence of CAM operation were used to correlate cell density, cell area, the proportion of intercellular space in the mesophyll (IAS) and the length of cell wall facing the intercellular air spaces (Lmes/A) with CAM mode. Key Results Based on 81 species and relatively unrelated families (15) belonging to nine orders, neither leaf thickness nor mesophyll traits helped explain the degree of CAM expression. A strong correlation was found between leaf thickness and δ 13C in some species of Crassulaceae and between leaf thickness and nocturnal acid accumulation in a few obligate CAM species of Bromeliaceae but, when all 81 species were pooled together, no significant changes with δ 13C were observed in cell density, cell area, IAS or Lmes/A. Conclusions An influence of phylogeny on leaf anatomy was evidenced in a few cases but this precluded generalization for widely separate taxa containing CAM species. The possible relationships between leaf anatomy and CAM mode should be interpreted cautiously.

Synergistic Effects of Elevated CO2 and Fertilization on Net CO2 Uptake and Growth of the CAM Plant Hylocereus undatus

This study examined the response of the crassulacean acid metabolism (CAM) vine-cactus fruit crop species Hylocereus undatus to two CO2 regimes [enrichment (1000 μmol·mol−1) vs. ambient control (380 μmol·mol−1)] and to two fertilization regimes [0.5- vs. 0.1-strength Hoagland's solution (designated high and low, respectively)]. CO2 enrichment increased total daily net CO2 uptake, nocturnal acid accumulation, shoot elongation, and total dry mass by 39%, 24%, 14%, and 6% (averaging the two fertilization regimes) versus ambient CO2 treatment, respectively. Plants exposed to high fertilization demonstrated 36%, 21%, 198%, and 79% (averaging the two CO2 regimes) increases versus those receiving the low fertilization regime in total daily net CO2 uptake, nocturnal acid accumulation, stem elongation, and total dry mass, respectively. Plants exposed to high fertilization and elevated CO2 demonstrated 108%, 77%, 264%, and 111% increases versus those receiving the low fertilization regime at the ambient CO2 concentration in total daily net CO2 uptake, nocturnal acid accumulation, stem elongation, and total dry mass, respectively. This response was 25% to 71% higher than the summed effects of the separate responses to each factor, indicating a synergistic effect of elevated CO2 and high fertilization. Thus, it is apparent that H. undatus crops grown under a high-fertilization agromanagement regime may benefit from elevated CO2 to a greater extent than those grown with low fertilization.

Distribution of crassulacean acid metabolism in orchids of Panama: evidence of selection for weak and strong modes

Crassulacean acid metabolism (CAM) is one of three metabolic pathways found in vascular plants for the assimilation of carbon dioxide. In this study, we investigate the occurrence of CAM photosynthesis in 200 native orchid species from Panama and 14 non-native species by carbon isotopic composition (δ13C) and compare these values with nocturnal acid accumulation measured by titration in 173 species. Foliar δ13C showed a bimodal distribution with the majority of species exhibiting values of approximately –28‰ (typically associated with the C3 pathway), or –15‰ (strong CAM). Although thick leaves were related to δ13C values in the CAM range, some thin-leaved orchids were capable of CAM photosynthesis, as demonstrated by acid titration. We also found species with C3 isotopic values and significant acid accumulation at night. Of 128 species with δ13C more negative than –22‰, 42 species showed nocturnal acid accumulation per unit fresh mass characteristic of weakly expressed CAM. These data suggest that among CAM orchids, there may be preferential selection for species to exhibit strong CAM or weak CAM, rather than intermediate metabolism.

Mode of photosynthesis during different life stages of hemiepiphytic Clusia species

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. Carbon isotope fractionation and nocturnal acid accumulation in Clusia osaensis Hammel-ined., C. �peninsulae Hammel-ined. and C. valerii Standl. were investigated during the seedling, epiphytic and hemiepiphytic phases in a lowland tropical forest in Costa Ricato study photosynthetic adaptations of different plant growth stages to their habitat. Foliar δ 13C values around -24 to -32‰ indicate predominant C3 fixation of CO2 and low crassulacean acid metabolism (CAM) activity in all three Clusia species. Only terrestrially rooted plants of C. osaensis showed increased CAM expression. However, all developmental stages exhibited significant CAM cycling as shown by significant day-night fluctuations of titratable protons and of malic and citric acid. In C. valerii and C. peninsulae, an increase in CAM expression with plant development was not observed, and CAM cycling in hemiepiphytic-stage plants was completely repressed during the high rainfall season. The expression of CAM in the three Clusia species is therefore not developmentally controlled but triggered by environmental factors such as water availability and light intensity. These factors remain relatively stable in this ecosystem and CAM is therefore not fully expressed. However, CAM cycling may be of ecophysiological significance in all life stages as it serves as a mechanism to improve carbon economy by reducing respiratory CO2 losses.

Effects of photoperiod and drought on the induction of crassulacean acid metabolism and the reproduction of plants of Talinum triangulare

To examine the effects of day length on the induction of crassulacean acid metabolism (CAM) by drought in the tropical species, Talinum triangulare (Jacq.) Willd. (Portulacaceae), plants were subjected to drought under different photoperiods. Nocturnal acid accumulation was 52 µmol H+··g-1 fresh mass (FM) in plants grown under a 10 h light : 14 h dark photoperiod and 76 µmol H+·g-1 FM in plants grown under 13 h light : 11 h dark, whereas it was only 10 µmol H+·g-1 FM in plants grown under 18 h light : 6 h dark. Plants were subjected to drought under short days and under short days with a night interruption of 1.5 h white light, aiming to simulate a long day, while minimally affecting daily carbon balance. Only droughted plants under normal short days accumulated acids during the night. Absence of CAM could not be attributed to differences due to photoperiod in either biomass allocation, chlorophyll content, or leaf water content. Photoperiod did not significantly affect fecundity in watered plants, whereas drought markedly reduced fecundity in plants with night interruption relative to plants under normal short days. Reproductive effort, calculated as seeds per gram leaf, was significantly higher in droughted plants under normal short days and watered plants with and without night interruption than in droughted plants with night interruption.Key words: CAM, crassulacean acid metabolism, drought, fecundity, induction, photoperiod, reproductive effort, reproduction, Talinum triangulare

A relationship between fecundity, survival, and the operation of crassulacean acid metabolism in Talinum triangulare

In plants of the perennial, deciduous herb Talinum triangulare, crassulacean acid metabolism (CAM) is induced by drought; therefore, CAM may be an adaptation to water deficit in this species. The present study was undertaken to determine the effects of water deficit on fecundity and CAM in plants of T. triangulare. Plants were grown from seed in the greenhouse and the life table was constructed with dynamic cohorts. CAM was induced by drought in plants as young as 45 days old, and its induction was associated with a significant rise in fecundity; values of survival beta mean fecundity by age-class were 30-50% higher in plants subjected to drought than in control plants due to a rise in fecundity. Plants subjected to drought produced more and lighter seeds, which germinated faster than their watered controls. These characteristics could be advantageous for a colonizing species such as T. triangulare. Plants obtained from the germination of seeds of plants subjected to drought did not show higher values of nocturnal acid accumulation when subjected to drought than the droughted offspring of watered plants but they showed higher survival and an earlier and higher reproductive effort than plants obtained from the germination of seeds of watered plants. The fact that values of survival beta mean fecundity were higher in plants subjected to drought than in watered plants suggests, within the context of the life history, that characters associated with the CAM syndrome may be adaptive.Key words: fitness, inducible CAM, life table.