We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Carbon Isotope Discrimination and Photosynthesis

Variation in carbon-isotope composition among and between wheat genotypes was correlated with variation in water-use efficiency in separate pot experiments conducted in spring-summer and in winter. In the main, winter experiment, the water-use efficiencies ranged from 2.0 to 3.7 mmolC/mol H2O (means of four replicates) while the corresponding isotope effects for leaf material ranged from 1.0225 to 1.0194. 13C was more abundant in grain than in leaves and stems. It is suggested that carbon-isotope analysis may be a useful tool in selection for improved water-use efficiency in breeding programmes for C3 species.

Isotopic Composition of Plant Carbon Correlates With Water-Use Efficiency of Wheat Genotypes

he efficiency of photosynthesis continues to interest biochemists, biologists, and plant physiologists. Scientists interested in CO2 uptake are concerned about the extent to which the uptake rate is limited by such factors as stomatal diffusion and the chemistry of the CO2 absorption process. The fractionation of carbon isotopes that occurs during photosynthesis is one of the most useful techniques for investigating the efficiency of CO2 uptake. Atmospheric carbon dioxide contains approximately 1.1% of the nonradioactive isotope carbon-13 and 98.9% of carbon-12. During photosynthesis, plants discriminate against C because of small differences in chemical and physical properties imparted by the difference in mass. This discrimination can be used to assign plants to various photosynthetic groups. The isotope fractionation also reflects limitations on photosynthetic efficiency imposed by the various diffusional and chemical components of CO2 uptake. When analyzed in detail, this fractionation provides information .about water use efficiency and indicates that different strategies are needed for improving wateruse efficiency in different kinds of plants. Isotope fractionation in simple physical and chemical processes is well understood and is commonly Current studies include

Carbon Isotopes in PhotosynthesisFractionation techniques may reveal new aspects of carbon dynamics in plants

Stable isotope ratios provide clues about the origins and transformations of organic matter. A few key reactions control the isotopic composition of most organic matter. Isotopic variations introduced by these reactions are often passed on with little change so that isotopic measurements can indicate natural pathways and flows “downstream” from these key reactions. When chemical and metabolic processes scramble the information content of molecules, isotopic compositions are often preserved. This realization has prompted increasing use of stable isotope analyses as a tool for understanding complex ecological processes.

δ13C Measurements as Indicators of Carbon Flow in Marine and Freshwater Ecosystems

The use of stable carbon isotopes as a means of studying energy flow is increasing in ecology and paleo- ecology. However, secondary fractionation and turnover of stable isotopes in animals are poorly understood pro- cesses. This study shows that tissues of the gerbil (Meriones unguienlatus) have different 613C values when equilibrated on corn (C4) or wheat (C~) diets with constant 13c/1ac contents. Lipids were depleted 3.0%o and hair was enriched 1.0%o relative to the C4 diet. Tissue 6t3C values were ranked hair > brain > muscle > liver > fat. After changing the gerbils to a wheat (C3) diet, isotope ratios of the tissues shifted in the direction of the 6a3C value of the new diet. The rate at which carbon derived from the corn diet was replaced by carbon derived from the wheat diet was ade- quately described by a negative exponential decay model for all tissues examined. More metabolically active tissues such as liver and fat had more rapid turnover rates than less metabolically active tissues such as hair. The half-life for carbon ranged from 6.4 days in liver to 47.5 days in hair. The results of this study have important implications for the use of 6~3C values as indicators of animal diet. Both fractionation and turnover of stable carbon isotopes in animal tissues may obscure the relative contributions of isotopically distinct dietary components (such as C3 vs. C,, or marine vs. terrestrial) if an animal's diet varies through time. These complications deserve attention in any study using stable isotope ratios of animal tissue as dietary indicators and might be minimized by analysis of several tissues or products covering a range of turnover times.

Fractionation and turnover of stable carbon isotopes in animal tissues: Implications for δ13C analysis of diet

The δ13C values of submerged aquatic plants from contrasting but relatively defined habitats, and the δ13C values of emergent, floating and submerged leaves of dimorphic aquatic plants, were measured. In many instances the δ13C values of dissolved inorganic carbon in the water were also measured. Plant δ13C values in the vicinity of-40 to-50‰ were found in rapidly flowing spring waters with carbonate δ13C values of-16 to-21‰, consistent with the notion that species such as Fontinalis antipyretica almost exclusively assimilate free CO2 via RuP2 carboxylase. Plant δ13C values in the vicinity of-10 to-15‰ in sluggish water with carbonate δ13C values of about-5‰ were observed, consistent with the notion that boundary layer diffusion and/or HCO3- uptake may determine the δ13C value of submerged aquatic plants in these circumstances. Comparisons of δ13C values of the same or related species growing in waters of similar carbonate δ13C value but different flow rates confirmed this view; more negative δ13C values were frequently associated with plants in fast moving water. In Britain, but not in Finland, the δ13C values of submerged leaves of dimorphic plants were almost invariably more negative than in aerial leaves. The δ13C value of carbonate from chalk streams and in acid springs indicate substantial inputs of respiratory CO2, as opposed to atmospheric carbon. The contributions of these variations in δ13C of the carbon source, and of isotope fractionation in diffusion, to the δ13C value of submerged parts of dimorphic plants is discussed.

Comparisons of δ13C values in leaves of aquatic macrophytes from different habitats in Britain and Finland; some implications for photosynthetic processes in aquatic plants.

S. M. Haslam

Papers

Comparisons of δ13C values in leaves of aquatic macrophytes from different habitats in Britain and Finland; some implications for photosynthetic processes in aquatic plants.