This review discusses the current status of research on phenotypic diversity and plasticity of CO2 assimilation in plants with crassulacean acid metabolism photosynthesis.
Abstract
In plants ...exhibiting crassulacean acid metabolism (CAM), CAM photosynthesis almost always occurs together with C3 photosynthesis, and occasionally with C4 photosynthesis. Depending on species, ontogeny, and environment, CAM input to total carbon gain can vary from values of <1% to 100%. The wide range of CAM phenotypes between and within species is a fascinating example of functional diversity and plasticity, but poses a significant challenge when attempting to define CAM. CO2 gas exchange experiments designed for this review illustrate key patterns of CAM expression and highlight distinguishing features of constitutive and facultative CAM. Furthermore, they help to address frequently recurring questions on CAM terminology. The functional and evolutionary significance of contrasting CAM phenotypes and of intermediate states between extremes is discussed. Results from a study on nocturnal malate accumulation in 50 species of Aizoaceae exposed to drought and salinity stress suggest that facultative CAM is more widespread amongst vascular plants than previously thought.
Tropical forests contribute significantly to the global carbon cycle, but little is known about the temperature response of photosynthetic carbon uptake in tropical species, and how this varies ...within and across forests.
We determined in situ photosynthetic temperature–response curves for upper canopy leaves of 42 tree and liana species from two tropical forests in Panama with contrasting rainfall regimes. On the basis of seedling studies, we hypothesized that species with high photosynthetic capacity – light-demanding, fast-growing species – would have a higher temperature optimum of photosynthesis (T
Opt) than species with low photosynthetic capacity – shadetolerant slow-growing species – and that, therefore, T
Opt would scale with the position of a species on the slow–fast continuum of plant functional traits.
T
Opt was remarkably similar across species, regardless of their photosynthetic capacity and other plant functional traits. Community-average T
Opt was almost identical to mean maximum daytime temperature, which was higher in the dry forest. Photosynthesis above T
Opt appeared to be more strongly limited by stomatal conductance in the dry forest than in the wet forest.
The observation that all species in a community shared similar T
Opt values suggests that photosynthetic performance is optimized under current temperature regimes. These results should facilitate the scaling up of photosynthesis in relation to temperature from leaf to stand level in species-rich tropical forests.
Tropical forests have a mitigating effect on man-made climate change by acting as a carbon sink. For that effect to continue, tropical trees will have to acclimate to rising temperatures, but it is ...currently unknown whether they have this capacity. We grew seedlings of three tropical tree species over a range of temperature regimes (T
Growth = 25, 30, 35 °C) and measured the temperature response of photosynthetic CO₂ uptake. All species showed signs of acclimation: the temperature-response curves shifted, such that the temperature at which photosynthesis peaked (T
Opt) increased with increasing T
Growth. However, although T
Opt shifted, it did not reach T
Growth at high temperature, and this difference between T
Opt and T
Growth increased with increasing T
Growth, indicating that plants were operating at supraoptimal temperatures for photosynthesis when grown at high temperatures. The high-temperature CO₂ compensation point did not increase with T
Growth. Hence, temperature-response curves narrowed with increasing T
Growth. T
Opt correlated with the ratio of the RuBP regeneration capacity over the RuBP carboxylation capacity, suggesting that at high T
Growth photosynthetic electron transport rate associated with RuBP regeneration had greater control over net photosynthesis. The results show that although photosynthesis of tropical trees can acclimate to moderate warming, carbon gain decreases with more severe warming.
Net photosynthetic carbon uptake of Panamanian lowland tropical forest species is typically optimal at 30–32 °C. The processes responsible for the decrease in photosynthesis at higher temperatures ...are not fully understood for tropical trees. We determined temperature responses of maximum rates of RuBP‐carboxylation (VCMax) and RuBP‐regeneration (JMax), stomatal conductance (Gs), and respiration in the light (RLight) in situ for 4 lowland tropical tree species in Panama. Gs had the lowest temperature optimum (TOpt), similar to that of net photosynthesis, and photosynthesis became increasingly limited by stomatal conductance as temperature increased. JMax peaked at 34–37 °C and VCMax ~2 °C above that, except in the late‐successional species Calophyllum longifolium, in which both peaked at ~33 °C. RLight significantly increased with increasing temperature, but simulations with a photosynthesis model indicated that this had only a small effect on net photosynthesis. We found no evidence for Rubisco‐activase limitation of photosynthesis. TOpt of VCMax and JMax fell within the observed in situ leaf temperature range, but our study nonetheless suggests that net photosynthesis of tropical trees is more strongly influenced by the indirect effects of high temperature—for example, through elevated vapour pressure deficit and resulting decreases in stomatal conductance—than by direct temperature effects on photosynthetic biochemistry and respiration.
Photosynthetic carbon uptake in tropical forests decreases at high temperature. To investigate the mechanisms underlying this decrease we analysed the temperature sensitivities of biochemical and stomatal controls over net photosynthesis for four lowland tropical tree species in Panama. While net photosynthesis and stomatal conductance peaked near current ambient temperatures, biochemical control factors VCMax and JMax peaked at much higher temperatures. This, combined with model simulations, suggests that the decreased carbon uptake at high temperatures caused stomatal closure, for example, in response to increased vapour pressure deficit, and not by a direct temperature effect on the biochemical machinery of photosynthesis.
Abstract Objective This study evaluated the frequency, severity and outcome of complications in the clinical course of tako-tsubo cardiomyopathy (TTC). Background TTC is regarded as a benign disease ...since left ventricular (LV) function returns to normal within a short time. However, severe complications have been reported in selected patients. Methods From 37 hospitals, 209 patients (189 female, age 69 ± 12 years) were prospectively included in a TTC registry. Results Complications developed in 108/209 patients (52%); 23 (11%) had > 2 complications. Complications occurred median 1 day after symptom onset, and 77% were seen within 3 days. Arrhythmias were documented in 45/209 patients (22%) including atrial fibrillation in 32 (15%) and ventricular tachycardia in 17 (8%). Of 8 patients resuscitated (4%), 6 survived. Additional complications were right ventricular involvement (24%), pulmonary edema (13%), cardiogenic shock (7%), transient intraventricular pressure gradients (5%), LV thrombi (3%) and stroke (1%). During hospitalization, 5/209 patients (2.5%) died. Patients with complications were older (70 ± 13 vs 67 ± 10 years, p = 0.012), had a higher heart rate (91 ± 26 vs 83 ± 19/min, p = 0.025), more frequently Q\ waves on the admission ECG (36% vs 21%, p = 0.019) and a lower LV ejection fraction (47 ± 15 vs 54 ± 14%, p = 0.002). Multivariate regression analysis identified Q-waves on admission (OR 2.49, 95% CI 1.23–5.05, p = 0.021) and ejection fraction ≤ 30% (OR 4.03, 95% CI 1.04–15.67, p = 0.022) as independent predictors for complications. Conclusions TTC may be associated with severe complications in half of the patients. Since the majority of complications occur up to day 3, monitoring is advisable for this time period.
The effects of global warming on tropical forest growth and carbon storage are uncertain. While observations on canopy trees indicate negative correlations between temperature and growth, some ...seedling studies suggest the opposite. These contrasting results may reflect ontogenetic differences in temperature responses, or differences between the performance of potted plants under controlled conditions and plants under more variable conditions in the field.
To try to bridge the gap between highly controlled experiments on small seedlings and field observations on canopy trees we conducted two sets of outdoor experiments on saplings up to 2.5 m tall; one set to study the effects of night warming, and another set on the effects of day warming. To test the hypothesis that night warming would reduce growth in tropical saplings through stimulation of respiration, we grew the early‐successional species Ochroma pyramidale in large 380‐L soil containers under ambient night‐time temperature and ambient +4.5°C. To test the hypothesis that day warming would reduce growth by reducing photosynthesis we compared plants in multi‐species and single‐species mesocosms rooted in the ground under ambient and passively warmed daytime conditions. In all experiments we monitored growth and measured foliar physiology and plant biomass allocation.
Neither night warming nor day warming significantly affected biomass accumulation and allocation. Height growth increased with night warming in O. pyramidale, but decreased with day warming in late‐successional species. Night warming resulted in acclimation of dark respiration. Day warming resulted in acclimation of photosynthesis in early‐successional species, but warming decreased photosynthesis in late‐successional tree species.
The seedling‐to‐sapling transition is a critical stage in the life of trees. We found no evidence that in this juvenile growth phase moderate increases in mean temperature reduce the performance of tropical trees, although increases in peak daytime temperature may negatively impact photosynthesis, especially in late‐successional species.
A plain language summary is available for this article.
Plain Language Summary
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.
Predictions of how tropical forests will respond to future climate change are constrained by the paucity of data on the performance of tropical species under elevated growth temperatures. In ...particular, little is known about the potential of tropical species to acclimate physiologically to future increases in temperature. Seedlings of 10 neo-tropical tree species from different functional groups were cultivated in controlled-environment chambers under four day/night temperature regimes between 30/22 °C and 39/31 °C. Under well-watered conditions, all species showed optimal growth at temperatures above those currently found in their native range. While non-pioneer species experienced catastrophic failure or a substantially reduced growth rate under the highest temperature regime employed (i.e. daily average of 35 °C), growth in three lowland pioneers showed only a marginal reduction. In a subsequent experiment, three species (Ficus insipida, Ormosia macrocalyx, and Ochroma pyramidale) were cultivated at two temperatures determined as sub- and superoptimal for growth, but which resulted in similar biomass accumulation despite a 6°C difference in growth temperature. Through reciprocal transfer and temperature adjustment, the role of thermal acclimation in photosynthesis and respiration was investigated. Acclimation potential varied among species, with two distinct patterns of respiration acclimation identified. The study highlights the role of both inherent temperature tolerance and thermal acclimation in determining the ability of tropical tree species to cope with enhanced temperatures.
Tropical forests play a critical role in the global carbon cycle, but our limited understanding of the physiological sensitivity of tropical forest trees to environmental factors complicates ...predictions of tropical carbon fluxes in a changing climate. We determined the short-term temperature response of leaf photosynthesis and respiration of seedlings of three tropical tree species from Panama. For one of the species net CO2 exchange was also measured in situ. Dark respiration of all species increased linearly â not exponentially â over a
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