Ecophysiological comportment of the tropical CAM-tree Clusia in the field: I. Growth of Clusia rosea Jacq. on St John, US Virgin Islands, Lesser Antilles

Facultative crassulacean acid metabolism (CAM) plants: powerful tools for unravelling the functional elements of CAM photosynthesis

Facultative crassulacean acid metabolism (CAM) describes the optional use of CAM photosynthesis, typically under conditions of drought stress, in plants that otherwise employ C3 or C4 photosynthesis. In its cleanest form, the upregulation of CAM is fully reversible upon removal of stress. Reversibility distinguishes facultative CAM from ontogenetically programmed unidirectional C3-to-CAM shifts inherent in constitutive CAM plants. Using mainly measurements of 24h CO2 exchange, defining features of facultative CAM are highlighted in five terrestrial species, Clusia pratensis, Calandrinia polyandra, Mesembryanthemum crystallinum, Portulaca oleracea and Talinum triangulare. For these, we provide detailed chronologies of the shifts between photosynthetic modes and comment on their usefulness as experimental systems. Photosynthetic flexibility is also reviewed in an aquatic CAM plant, Isoetes howellii. Through comparisons of C3 and CAM states in facultative CAM species, many fundamental biochemical principles of the CAM pathway have been uncovered. Facultative CAM species will be of even greater relevance now that new sequencing technologies facilitate the mapping of genomes and tracking of the expression patterns of multiple genes. These technologies and facultative CAM systems, when joined, are expected to contribute in a major way towards our goal of understanding the essence of CAM.

Adaptations of strangler figs to life in the rainforest canopy

Figs are rainforest keystone species. Non-strangler figs establish on the forest floor; strangler figs establish epiphytically, followed by a dramatic transition from epiphyte to free-standing tree that kills its hosts. Free-standing figs display vigorous growth and resource demand suggesting that epiphytic strangler figs require special adaptations to deal with resource limitations imposed by the epiphytic environment. We studied epiphytic and free-standing strangler figs, and non-strangler figs in tropical rainforest and in cultivation, as well as strangler figs in controlled conditions. We investigated whether the transition from epiphyte to free-standing tree is characterised by morphological and physiological plasticity. Epiphyte substrate had higher levels of plant-available ammonium and phosphate, and similar levels of nitrate compared with rainforest soil, suggesting that N and P are initially not limiting resources. A relationship was found between taxonomic groups and plant N physiology; strangler figs, all members of subgenus Urostigma, had mostly low foliar nitrate assimilation rates whereas non-strangler figs, in subgenera Pharmacocycea, Sycidium, Sycomorus or Synoecia, had moderate to high rates. Nitrate is an energetically expensive N source, and low nitrate use may be an adaptation of strangler figs for conserving energy during epiphytic growth. Interestingly, significant amounts of nitrate were stored in fleshy taproot tubers of epiphytic stranglers. Supporting the concept of plasticity, leaves of epiphytic Ficus benjamina L. had lower N and C content per unit leaf area, lower stomatal density and 80% greater specific leaf area than leaves of conspecific free-standing trees. Similarly, glasshouse-grown stranglers strongly increased biomass allocation to roots under water limitation. Epiphytic and free-standing F. benjamina had similar average foliar δ¹³C, but epiphytes had more extreme values; this indicates that both groups of plants use the C₃ pathway of CO₂ fixation but that water availability is highly variable for epiphytes. We hypothesise that epiphytic figs use fleshy stem tubers to avoid water stress, and that nitrate acts as an osmotic compound in tubers. We conclude that strangler figs are a unique experimental system for studying the transition from rainforest epiphyte to tree, and the genetic and environmental triggers involved.

The physiological ecology of vascular epiphytes: current knowledge, open questions

The current knowledge of the physiological ecology of vascular epiphytes is reviewed here with an emphasis on the most recent literature. It is argued that by far the most relevant abiotic constraint for growth and vegetative function of vascular epiphytes is water shortage, while other factors such as nutrient availability or irradiation, are generally of inferior importance. However, it is shown that the present understanding of epiphyte biology is still highly biased, both taxonomically and ecologically, and it is concluded that any generalizations are still preliminary. Future studies should include a much wider range of taxa and growing sites within the canopy to reach a better understanding how abiotic factors are limiting epiphyte growth and survival which, in turn, should affect epiphyte community composition. Finally, a more integrative approach to epiphyte biology is encouraged: physiological investigations should be balanced by studies of other possible constraints, for example, substrate instability, dispersal limitation, competition or herbivory.

A one-year study on carbon, water and nutrient relationships in a tropical C3 -CAM hemi-epiphyte, Clusia uvitana Pittier

Diel (24 h) courses of CO₂ and water-vapour exchange of leaves of hemi-epiphytic plants of Clusia uvitana Pittier (Clusiaceae) were measured under natural tropical conditions in the semi-evergreen moist forest of Barro Colorado Island, Panama, from January 1991 until January 1992. Plants were studied at two sites, in the crown of a 47-m tall tree (Ceiba pentandra) and on the shore of Lake Gatun, at a height of about 2-4 m. The following results were obtained: (1) Diel carbon gain was mainly a function of photosynthetic photon fluence rate (PPFR) on individual days. PPFR also strongly affected CAM activity. A leafless period of the host tree Ceiba pentandra resulted in higher incident PPFRs and slightly lower nighttime temperatures in the canopy of C uvitana; this led to increases in both daytime and nighttime CO₂ fixation. (2) In fully mature sun leaves from the two sites, nocturnal net uptake of atmospheric CO₂ occurred on almost all of the 71 days measured and nocturnal carbon gain was enhanced during the dry season. (3) In C. uvitana at the Lake site, 24-h carbon gain during the wet and dry season was similar to C. uvitana at the Ceiba site during the leafless period of the host tree. Overall CAM activity was lower at the Lake site. (4) Recycling of respiratory CO₂ was a major route for nocturnal acid synthesis. Nocturnal net uptake of atmospheric CO₂ was closely correlated with changes in titratable acidity, but accounted for only about 30% of the nocturnal increase in organic acids. (5) Mature shade leaves performed CAM only during the dry season, whereas in the wet season they showed atmospheric CO₂ uptake exclusively in the light. (6) Independent of exposure or season, leaves less than about 12 wk old showed a C₃ pattern of diel gas exchange and the level of titratable acidity was high day and night. (7) The annual carbon budget of outer canopy leaves of C uvitana in the Ceiba site was 1780 g CO₂ m^-2 a^-1 and the average long-term water-use efficiency was 23 × 10^-3 g CO₂ g^-1 H₂ O. (8) Vegetative growth was strongly seasonal. Branch length increment and leaf area development was much higher in the wet season. Mineral element contents in these evergreen plants showed no age-related changes, but a significant proportion of elements was retrieved before abscission.

On the ecophysiology of the Clusiaceae in Trinidad: expression of CAM in Clusia minor L. during the transition from wet to dry season and characterization of three endemic species

A study was made of photosynthesis and expression of crassulacean acid metabolism (CAM) in naturally exposed and shaded populations of Clusia minor L. during the transition from wet to dry season in Trinidad (mid-February to mid-April, 1990). At the start of the dry season, plants from exposed and shaded habitats showed a capacity for CAM either through the fixation of external or internal (respiratory) CO₂ . Exposed plants showed continuous uptake of CO₂ over 24 h although dark fixation accounted for only a small proportion of CO₂ fixed over the day. The expression of CAM was considerably enhanced as the dry season progressed with substantial increases in the overnight accumulation of titratable acidity, particularly in leaves of exposed plants. This was accompanied by a reduction in day-time photosynthesis and an increase in dark fixation, with shaded plants showing only night-time fixation of CO₂ . The magnitude of CAM in C. minor was substantial with a maximum ΔH⁺ of 1410 mol m^-3 measured in leaves from exposed branches. Both malic and citric acids were accumulated overnight. The highest citric:malic acid ratios were found in young leaves from exposed plants with 250 mol m^-3 malic and 125 mol m^-3 citric acid accumulated near the time of maximum CAM activity. Photosynthetic efficiency, measured as light responses of O₂ evolution, also varied on a daily basis dependent on the incident photosynthetic photon flux density (PPFD). Apparent quantum yield and photosynthetic capacity showed marked reductions depending on the degree of exposure, suggesting that photoinhibitory responses are important under natural conditions. An analysis of three members of the Clusiaceae endemic to Trinidad showed that each had the capacity to induce CAM activity, despite being found in a narrow range of habitats which have higher rainfall than those of C. minor. However, despite the variable expression of CAM activity, carbon isotope composition suggested that when integrated throughout the year, carbon accumulation is predominantly mediated via the C₃ pathway in all the species studied.

Ecophysiological comportment of the tropical CAM-tree Clusia in the field: II. Modes of photosynthesis in trees and seedlings

Measurements were performed on leaves of Clusia rosea Jacq. trees in the moist central mountains (330 to 365 m above sea level) and at the dry south coast of St John Island (US Virgin Islands, Lesser Antilles). Seedlings of C. rosea were also studied in the central hills. During the study period (March 1989) all trees showed crassulacean acid metabolism (CAM), in which net CO₂ uptake extended for a remarkably long time in the morning (phase II of CAM: until about 11 to 12 h) and contributed about 1/3 of total net CO₂ -uptake. During the night (phase I of CAM) malic acid and citric acid were accumulated concurrently at a molar ratio of malic: citric acid of about 1.6. Internal recycling of respiratory CO₂ was 20% of total CO₂ fixed during the night. Water-use-efficiency (mol CO₂ taken up: mol H₂ O transpired) was 0.014 to 0.022. The pH of leaf-cell sap at the end of the dark period was 2.85. This would still allow an H⁺ -ATPase at the tonoplast to transport 2H⁺ into the vacuole per ATP hydrolysed when operating near thermodynamic equilibrium. Free sugars, glucose and fructose, and starch were used as precursors for the CO₂ -acceptor phosphoenolpyruvate during the dark period; contributions of the two hexoses were about equal and together four-times that of starch. Xylem tensions showed increases of up to 8 bar during day-time. Leaf-sap osmotic pressures did not change significantly; the trend was a small decline during day-time. Among the seedlings, three different modes of photosynthesis were encountered, namely C₃ -photosynthesis in terrestrial and in epiphytic seedlings, continuous stomatal opening and CO₂ -uptake day and night in epiphytic seedlings, and CAM in seedlings growing in the tanks of Aechmea lingulata (L.) Baker.