Analysis of carbon fluxes in plant and crop-environment interactions. Use of stable and radioactive isotopes

Carbon fluxes within plants and between plants and their environment may be analyzed using radiotracers (14C or 11C) and stable isotopes (13C/12C ratio). Tracers are used to test models of the systems, to estimate the fluxes between specific components (pools) or to assess the pool sizes and their rates of turnover. Radiotracers are most suited to controlled environment analyses of carbon fluxes in photosynthesis and metabolism and between organs, which may be related to the photosynthetic rate, to the assimilate pools and to the environmental conditions; the data are used to analyze the models. The advantages of 14C are its ready availability at low cost, the long half-life, ease of use to produce 14CO2 and the readily available sample preparation methods (e.g. combustion to 14CO2). The disadvantages of 14C are its ionizing radiation of low energy, which reduces detection within a plant, making transport studies more difficult, and the long half-life for some applications. In contrast, the short half-life, more easily detectable 11C has advantages for short term analysis of carbon fluxes, but it is not readily available and requires plant containment. Stable carbon isotopes (13C and 12C) are naturally occurring and without health hazards. The differences in their relative abundance (13C/12C ratio) in plants of different photosynthetic metabolism (C3, C4 and crassulacean acid metabolism) provide a natural tracer for carbon in plants and ecosystems. The ratios relate events at a macroscale (e.g. identification of sources and fluxes of carbon in ecosystems) to plant processes, providing a method for assessing plant-environment interactions (e.g. the water use efficiency of crops) integrated over long periods and areas. The choice of stable and radioactive isotope methods for specific purposes related to the assessment of agricultural sustainability and plant processes in the field and under controlled conditions is discussed. 18 refs, 4 figs