C4 and CAM photosynthesis are evolutionarily derived from C3 photosynthesis. The morphological and biochemical modifications necessary to achieve either C4 or CAM photosynthesis are thought to have independently arisen numerous times within different higher plant taxa. It is thought that C4 photosynthesis evolved in response to the low atmospheric CO2 concentrations that arose sometime after the end of the Cretaceous. Low CO2 concentrations result in significant increases in photorespiration of C3 plants, reducing productivity; both C3-C4 intermediate and C4 plants exhibit reduced photorespiration rates. In contrast, it may be argued that CAM arose either in response to selection of increased water-use efficiency or for increased carbon gain. Globally, all three pathways are widely distributed today, with a tendency toward ecological adaptation of C4 plants into warm, monsoonal climates and CAM plants into water-limited habitats. In an anthropogenically altered CQ2 environment, C4 plants may lose their competitive advantage over C3 plants. 411

Evolutionary and Ecological Aspects of Photosynthetic Pathway Variation

Summary Stable carbon isotope ratios (d 13 C) of terrestrial plants are employed across a diverse range of applicationsinenvironmentalandplantsciences;however,thekindofinformationthatisdesired fromthed 13 Csignaloftendiffers.Attheextremes,itrangesbetweenpurely environmentaland purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (D) in terrestrial plants, and the biological processes that can either damp or amplifytheresponse.ForC3plants,whereDisprimarilycontrolledbytheratioofintercellularto ambient CO2 concentrations (ci/ca), coordination between stomatal conductance and photo- synthesisandleafareaadjustmenttendstoconstrainthepotentialenvironmentallydrivenrange of D. For C4 plants, variation in bundle-sheath leakiness to CO2 can either damp or amplify the effects of ci/ca on D. For plants with crassulacean acid metabolism (CAM), D varies over a relatively largerange asafunctionoftheproportion ofdaytimetonight-time CO2fixation. This range can be substantially broadened by environmental effects onD when carbon uptake takes place primarily during the day. The effective use of D across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.12423

Environmental and physiological determinants of carbon isotope discrimination in terrestrial plants

Ecophysiology of Crassulacean Acid Metabolism (CAM)

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.

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

For diverse communities of omnivorous insects such as ants, the extent of direct consumption of plant-derived resources vs. predation is largely unknown. However, determination of the extent of "herbivory" among ants may be crucial to understand the hyper-dominance of ants in tropical tree crowns, where prey organisms tend to occur scarcely and unpredictably. We therefore examined N and C stable isotope ratios (delta(15)N and delta(13)C) in 50 ant species and associated insects and plants from a tropical rainforest in North Queensland, Australia. Variation between ant species was pronounced (range of species means: 7.1 per thousand in delta(15)N and 6.8 per thousand in delta(13)C). Isotope signatures of the entire ant community overlapped with those of several herbivorous as well as predacious arthropods. Variability in delta(15)N between ants was not correlated with plant delta(15)N from which they were collected. Ant species spread out in a continuum between largely herbivorous and purely predacious taxa, with a high degree of omnivory. Ant species' delta(15)N were consistent with the trophic level predicted by natural feeding observations, but not their delta(13)C. Low delta(15)N levels were recorded for ant species that commonly forage for nectar on understorey or canopy plants, intermediate levels for species with large colonies that were highly abundant on nectar and honeydew sources and were predacious, and the highest levels for predominantly predatory ground-foraging species. Colonies of the dominant weaver-ants (Oecophylla smaragdina) had significantly lower delta(15)N in mature forests (where preferred honeydew and nectar sources are abundant) than in open secondary vegetation. N concentration of ant dry mass showed only very limited variability across species and no correlation with trophic levels. This study demonstrates that stable isotopes provide a powerful tool for quantitative analyses of trophic niche partitioning and plasticity in complex and diverse tropical omnivore communities.

Disentangling a rainforest food web using stable isotopes: dietary diversity in a species-rich ant community

The occurrence of crassulacean acid metabolism (CAM) among epiphytes and related plant species from tropical and subtropical rainforests in Eastern Australia was investigated. As judged from δ13C value and the absence of Kranz anatomy, indications of CAM were found in 66 species belonging to the families, Polypodiaceae (3), Orchidaceae (55), Asclepiadaceae (6) and Rubiaceae (2).Two thirds of orchidaceous plants examined appeared to use CAM. Those species with thicker leaves generally had less negative δ13C values, as did those species growing on more exposed sites; leaves thicker than about 1 mm in most species yielded δ13C values indicative of pronounced CAM. Two leafless species, Chiloschista phyllorhiza and Taeniophyllum malianum, which depend on chloroplast-containing, stomata-less roots for photosynthesis also showed δ13C values typical of CAM. Pseudobulbs and swollen stems, a characteristic of many orchids, were usually somewhat enriched in 13C compared to corresponding leaves.In Polypodiaceae CAM was found in the genus Pyrrosia. While δ13C values were generally less negative with increasing frond thickness, the leaf morphology was extremely variable within species. Pyrrosia confluens plants from shaded habitats had long, relatively thin and darkgreen fronds whereas specimens from sun-exposed sites were characterized by short, thick, bleached fronds. Both types showed the capacity for nocturnal accumulation of titratable acidity and exhibited continuous net CO2 fixation during 12 h light/12 h dark cycles under laboratory conditions. Shade-fronds showed this capacity even when irradiance was lower than 2% of full sunlight during the 12 h light period.In Asclepiadaceae CAM was found in species of two genera which usually have fleshy leaves, Hoya and Dischidia. In Rubiaceae CAM was recorded in two genera of epiphytic ant plants, Hydnophytum and Myrmecodia.It is concluded that CAM is widespread in Australian epiphytes. It is most prevalent in species found in exposed microhabitats where the growing conditions are characterised by relatively high light intensities and short but frequent periods of water stress.

Ben J. Wallace

Papers

Crassulacean acid metabolism in australian vascular epiphytes and some related species