Carbon 13/12 isotope ratios have been determined from leaves of a hundred C plant species (or ecotypes) from all major mountain ranges of the globe, avoiding drought stressed areas. A general increase in C content was found with increasing altitude, i.e. overall discrimination against the heavy isotope is reduced at high elevation. The steepest decline of discrimination is observed in taxa typically ranging to highest elevations (e.g. the genus Ranunculus). Mean δC for all samples collected between 2500 and 5600 m altitude is-26.15‰ compared to the lowland average of-28.80‰ (P<0.001). Forbs from highest elevations reach-24‰. According to theory of C discrimination this indicates decreasing relative limitation of carbon uptake by carboxylation. In other words, we estimate that the ratio of internal to external partial pressure of CO (p /p )in leaves of high elevation plants is lower than in leaves of low altitude. These results confirm recent gas exchange analyses in high and low elevation plants.

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Seedlings of two mangrove species, Avicennia marina and Aegiceras corniculatum, were grown in a range of salinities and humidities in controlled environment chambers, and Phaseolus vulgaris plants were grown in the glasshouse. The fractionation of carbon isotopes in the three species was correlated with the ratio of intercellular and ambient partial pressures of CO. The results are consistent with fractionation being due both to diffusion in air and to carboxylation in the leaf. It was concluded that the latter process discriminates against CO relative to CO by about 27‰.

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 δC 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 δC values, as did those species growing on more exposed sites; leaves thicker than about 1 mm in most species yielded δC 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 δC values typical of CAM. Pseudobulbs and swollen stems, a characteristic of many orchids, were usually somewhat enriched in C compared to corresponding leaves.In Polypodiaceae CAM was found in the genus Pyrrosia. While δC 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 CO 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.

The δC values of submerged aquatic plants from contrasting but relatively defined habitats, and the δC values of emergent, floating and submerged leaves of dimorphic aquatic plants, were measured. In many instances the δC values of dissolved inorganic carbon in the water were also measured. Plant δC values in the vicinity of-40 to-50‰ were found in rapidly flowing spring waters with carbonate δC values of-16 to-21‰, consistent with the notion that species such as Fontinalis antipyretica almost exclusively assimilate free CO via RuP carboxylase. Plant δC values in the vicinity of-10 to-15‰ in sluggish water with carbonate δC values of about-5‰ were observed, consistent with the notion that boundary layer diffusion and/or HCO uptake may determine the δC value of submerged aquatic plants in these circumstances. Comparisons of δC values of the same or related species growing in waters of similar carbonate δC value but different flow rates confirmed this view; more negative δC values were frequently associated with plants in fast moving water. In Britain, but not in Finland, the δC values of submerged leaves of dimorphic plants were almost invariably more negative than in aerial leaves. The δC value of carbonate from chalk streams and in acid springs indicate substantial inputs of respiratory CO, as opposed to atmospheric carbon. The contributions of these variations in δC of the carbon source, and of isotope fractionation in diffusion, to the δC value of submerged parts of dimorphic plants is discussed.

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Ultrastructural and physiological characteristics of the C3-C4 intertnediate Neuraehnc minor S. T. Blake (Poaceae) ate cotnpated with those of Cj atid C4 relatives, and CJ-C4 Panicum milioides Nees ex Trin , . A', minor consistently exhibits very low CO2 cotnpensation points (F: < 1.0, usually 0.3-0.6 Pa) yet has Cj-like (5'^C values. CO2 assitnilation rates {A) tespotid like those of C3 plants to a decrease in O2 partial pressure (2 x 10*-l.9 x 10"* Pa) at atnbient CO2 levels, but this response is progressively attenuated until negligible at very low CO2. By contrast, other species of the Neut"achneae are clearly either C4 (two spp.) or C3 (seven spp.). For plants grown and tneasured at different photon flux densities (PFDs), F for N. minor and P. milioides increases ftotn 0.5 to 1.0, and frotn LO to 2.1 Pa, respeetively, as PFD is decreased frotn I860 to 460 /(tnol tn~^ s~ '. In A', minor, the O2 response of F is either biphasic as in P. milioides, but tnuch ditninished and with a higher transition point, or is very C4-Iike. As in C4 relatives, inner sheath cells contain nutnetous chlotoplasts. Their walls possess a suberized latnella, which tnay tnake thetn more CO2-tight than bundle sheath cells of P. milioides, contributing to the altnost C4-like F characteristics of A', minor. The biochetnical basis of C3 C4 intennediacy is considered.