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Crassulacean acid metabolism (CAM), one of three kinds of photosynthesis, is a water-use efficient adaptation to an arid environment. CAM is characterized by CO2 uptake via open stomata during the nighttime and refixation CO2 via the Calvin cycle during the daytime. Facultative CAM plants can shift the photosynthesis from C3 to CAM and exhibit greater plasticity in CAM expression under different environments. Though leaf thickness is an important anatomical feature of CAM plants, there may be no anatomical feature changes during the C3–CAM transition for all facultative CAM plants. The shift from C3 photosynthesis to CAM in facultative CAM plants is accompanied by significant changes in physiology including stomata opening, CO2 gas exchange and organic acid fluxes; the activities of many decarboxylating enzymes increase during the shift from C3 to CAM; the molecular changes occur during the photosynthesis C3–CAM shift involved DNA hypermethylation, transcriptional regulation, post-transcriptional regulation and protein level regulation. Recently, omics approaches were used to discover more proceedings underling the C3–CAM transition. However, there are few reviews on the mechanisms involved in this photosynthetic shift in facultative CAM plants. In this paper, we summarize the progress in the comparative analysis of anatomical, physiological, metabolic and molecular properties of facultative CAM plants between C3 and CAM photosynthesis. Facultative CAM plants also show the potential for sustainable food crop and biomass production. We also discuss the implications of the photosynthesis transition from C3 to CAM on horticultural crops and address future directions for research.

Portulaca grandiflora simultaneously utilizes both the C4 and Crassulacean acid metabolism (CAM) photosynthetic pathways. Our goal was to determine whether CAM developed and was functional simultaneously with the C4 pathway in cotyledons of P. grandiflora. We studied during development whether CAM would be induced with water stress by monitoring the enzyme activity, leaf structure, JO2 (rate of O2 evolution calculated by fluorescence analysis), and the changes in titratable acidity of 10 and 25 days old cotyledons. In the 10 days old cotyledons, C4 and CAM anatomy were evident within the leaf tissue. The cotyledons showed high titratable acid levels but a small CAM induction. In the 25 days old cotyledons, there was a significant acid fluctuation under 7 days of water stress. The overall enzyme activity was reduced in the 10 days old plants, while in the 25 days old plants CAM activity increased under water-stressed conditions. In addition to CAM, the research showed the presence of glycine decarboxylase in the CAM tissue. Thus, it appears both pathways develop simultaneously in the cotyledons but the CAM pathway, due to anatomical constraints, may be slower to develop than the C4 pathway. Cotyledons showed the ancestral Atriplicoid leaf anatomy, which leads to the question: Could a CAM cell be the precursor to the C4 pathway? Further study of this may lead to understanding into the evolution of C4 photosynthesis in the Portulaca.

The CO 2 concentration in Earth's atmosphere may double during this century. Plant responses to such an increase depend strongly on their nitrogen status, but the reasons have been uncertain. Here, we assessed shoot nitrate assimilation into amino acids via the shift in shoot CO 2 and O 2 fluxes when plants received nitrate instead of ammonium as a nitrogen source (ΔAQ). Shoot nitrate assimilation became negligible with increasing CO 2 in a taxonomically diverse group of eight C 3 plant species, was relatively insensitive to CO 2 in three C 4 species, and showed an intermediate sensitivity in two C 3 ‐C 4 intermediate species. We then examined the influence of CO 2 level and ammonium vs. nitrate nutrition on growth, assessed in terms of changes in fresh mass, of several C 3 species and a Crassulacean acid metabolism (CAM) species. Elevated CO 2 (720 μmol CO 2 /mol of all gases present) stimulated growth or had no effect in the five C 3 species tested when they received ammonium as a nitrogen source but inhibited growth or had no effect if they received nitrate. Under nitrate, two C 3 species grew faster at sub‐ambient (∼310 μmol/mol) than elevated CO 2 . A CAM species grew faster at ambient than elevated or sub‐ambient CO 2 under either ammonium or nitrate nutrition. This study establishes that CO 2 enrichment inhibits shoot nitrate assimilation in a wide variety of C 3 plants and that this phenomenon can have a profound effect on their growth. This indicates that shoot nitrate assimilation provides an important contribution to the nitrate assimilation of an entire C 3 plant. Thus, rising CO 2 and its effects on shoot nitrate assimilation may influence the distribution of C 3 plant species.

Are evolutionary outcomes predictable? Adaptations that show repeated evolutionary convergence across the Tree of Life provide a special opportunity to dissect the context surrounding their origins, and identify any commonalities that may predict why certain traits evolved many times in particular clades and yet never evolved in others. The remarkable convergence of C₄ and Crassulacean Acid Metabolism (CAM) photosynthesis in vascular plants makes them exceptional model systems for understanding the repeated evolution of complex phenotypes. This review highlights what we have learned about the recurring assembly of C₄ and CAM, focusing on the increasingly predictable stepwise evolutionary integration of anatomy and biochemistry. With the caveat that we currently understand C₄ evolution better than we do CAM, I propose a general model that explains and unites C₄ and CAM evolutionary trajectories. Available data suggest that anatomical modifications are the ‘rate-limiting step’ in each trajectory, which in large part determines the evolutionary accessibility of both syndromes. The idea that organismal structure exerts a primary influence on innovation is discussed in the context of other systems. Whether the rate-limiting step occurs early or late in the evolutionary assembly of a new phenotype may have profound implications for its distribution across the Tree of Life.

Stable carbon isotope ratios (δ13C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ13C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C3 plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO2 concentrations (c i/c a), coordination between stomatal conductance and photo-synthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C4 plants, variation in bundle-sheath leakiness to CO2 can either damp or amplify the effects of c i/c a on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night-time CO2 fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water-use efficiency (WUE), and enables CAM plants to inhabit water-limited environments such as semi-arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi-arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.

The key components of crassulacean acid metabolism (CAM) – nocturnal fixation of atmospheric CO2 and its processing via Rubisco in the subsequent light period – are now reasonably well understood in terms of the biochemical reactions defining this water-saving mode of carbon assimilation. Phenotypically, however, the degree to which plants engage in the CAM cycle relative to regular C3 photosynthesis is highly variable. Depending upon species, ontogeny and environment, the contribution of nocturnal CO2 fixation to 24-h carbon gain can range continuously from close to 0% to 100%. Nevertheless, not all possible combinations of light and dark CO2 fixation appear equally common. Large-scale surveys of carbon-isotope ratios typically show a strongly bimodal frequency distribution, with relatively few intermediate values. Recent research has revealed that many species capable of low-level CAM activity are nested within the peak of C3-type isotope signatures. While questions remain concerning the adaptive significance of dark CO2 fixation in such species, plants with low-level CAM should prove valuable models for investigating the discrete changes in genetic architecture and gene expression that have enabled the evolutionary transition from C3 to CAM.

• Despite growing evidence that niche shifts are more common in flowering plants than previously thought, little is known of whether such shifts are promoted by changes in photosynthetic pathways. • Here we combine the most complete phylogeny for epiphytic Malagasy Bulbophyllum orchids (c. 210 spp.) with climatic niche and carbon isotope ratios to infer the group’s spatialtemporal history, and the role of strongly expressed crassulacean acid metabolism (CAM) in facilitating niche shifts and diversification. • We find that most extant species still retain niche (Central Highland) and photosynthesis (C₃) states as present in the single mid-Miocene (c. 12.70 million yr ago (Ma)) ancestor colonizing Madagascar. However, we also infer a major transition to CAM, linked to a late Miocene (c. 7.36 Ma) invasion of species from the sub-humid highland first into the island’s humid eastern coastal, and then into the seasonally dry ‘Northwest Sambirano’ rainforests, yet without significant effect on diversification rates. • These findings indicate that CAM in tropical epiphytes may be selectively advantageous even in high rainfall habitats, rather than presenting a mere adaptation to dry environments or epiphytism per se. Overall, our study qualifies CAM as an evolutionary ‘gateway’ trait that considerably widened the spatial-ecological amplitude of Madagascar’s most species-rich orchid genus.

Opening of stomata in plants with crassulacean acid metabolism (CAM) is mainly shifted to the night period when atmospheric CO₂ is fixed by phosphoenolpyruvate carboxylase and stored as malic acid in the vacuole. As such, CAM plants ameliorate transpirational water losses and display substantially higher water-use efficiency compared with C₃ and C₄ plants. In the past decade significant technical advances have allowed an unprecedented exploration of genomes, transcriptomes, proteomes and metabolomes of CAM plants and efforts are ongoing to engineer the CAM pathway in C₃ plants. Whilst research efforts have traditionally focused on nocturnal carboxylation, less information is known regarding the drivers behind diurnal malate remobilisation from the vacuole that liberates CO₂ to be fixed by RuBisCo behind closed stomata. To shed more light on this process, we provide a stoichiometric analysis to identify potentially rate-limiting steps underpinning diurnal malate mobilisation and help direct future research efforts. Within this remit we address three key questions: Q1 Does light-dependent assimilation of CO₂ via RuBisCo dictate the rate of malate mobilisation? Q2: Do the enzymes responsible for malate decarboxylation limit daytime mobilisation from the vacuole? Q3: Does malate efflux from the vacuole set the pace of decarboxylation?

Surface energy partitioning governs canopy thermal conditions and water use by determining how available energy is dissipated as latent versus sensible heat. In tropical and subtropical croplands, compound heat–drought events increase atmospheric evaporative demand, yet crassulacean acid metabolism (CAM) crops may respond differently from C 3 /C 4 systems because daytime stomatal regulation constrains transpiration. Here we quantify subdaily energy partitioning in a subtropical CAM pineapple field in southern China using Bowen-ratio energy balance observations. Net radiation ( R n ), soil heat flux ( G ), and vertical gradients of temperature and vapor pressure were used to estimate sensible ( H ) and latent ( LE ) heat fluxes and to compute available energy ( A = R n − G ). To diagnose the coupling relationship between atmospheric demand and water supply, hourly data were classified into four weather types (WT4) using median thresholds of R n and vapor pressure deficit (VPD): LR n –LVPD (low R n , low VPD), LR n –HVPD (low R n , high VPD), HR n –LVPD (high R n , low VPD), and HR n –HVPD (high R n , high VPD). Soil water content (SWC) at a depth of 20 cm was further classified into three soil moisture states (SWC3) using percentiles (Dry, Normal, and Wet). Scenario medians and valid-hour counts were used to summarize LE/A, H/A, and G/Rn, complemented by Bowen ratio ( β = H/LE ) as an integrative indicator of sensible versus latent heat partitioning. The results showed that energy partitioning was dominated by compensation between LE / A and H / A , while G / R n remained small. LE / A was consistently lower under HR n –HVPD than under LR n –LVPD across soil-moisture states. Under LR n –LVPD, LE / A was 0.47 in Dry and Normal and increased to 0.56 in Wet; under HR n –HVPD it ranged from 0.26 (Dry) to 0.37 (Wet). Moisture effects were demand dependent: within HR n –HVPD, LE / A increased by 0.11 from Dry to Wet, whereas the LR n –LVPD Dry-to-Wet increase was 0.09. β responses reinforced this interaction. Under Dry conditions, β increased with VPD and remained high at high VPD, whereas under wet soils β was lower and tended to level off. Under HR n –HVPD, β declined steeply as SWC increased over 0.23–0.27 m 3 m -3 , then approached a plateau near 1.7; under non-HR n –HVPD conditions β showed weaker dependence and plateaued near 2.0. Composite daytime patterns further showed sustained H dominance under HR n –HVPD–Dry, while HR n –HVPD–Wet shifted partitioning toward higher LE . These results demonstrate that atmospheric demand sets a strong constraint on daytime evaporative cooling in this CAM pineapple system, and that soil water supply enhances latent heat dissipation most effectively under high-demand conditions, thereby informing model parameterization and targeted water management.