PRESENT AND POTENTIAL USES OF THE NATURAL ABUNDANCE OF STABLE ISOTOPES IN PLANT-SCIENCE, WITH ILLUSTRATIONS FROM THE MARINE-ENVIRONMENT

The use of stable isotope natural abundance measurements in plant ecophysiological research is discussed in the context of studies of C-13/C-12 ratios in marine plants, with emphasis on the uniqueness of the information given by natural abundance measurements and of the importance of complementary data obtained by other techniques in making full use of the natural abundance data. (1) Inorganic C acquisition and assimilation in marine plants can involve diffusive entry of CO2, or the occurrence of a CO2-concentrating mechanism frequently involving active HCO3- influx. For diffusive CO2 entry, the deltaC-13 measurements can give unique information on the fractional limitation of photosynthesis by CO2 transport which, with photosynthetic rate measurements, can be used to compute transport conductances. For active HCO3-, influx, the deltaC-13 values uniquely permit computation of the ratio of the bidirection fluxes (influx/efflux) which, with photon yield data, can be used to given information on the mechanism of the efflux. The analyses are absolutely dependent on external (non-deltaC-13) data distinguishing between diffusive CO2 entry and the occurrence of a CO2 concentrating mechanism. (2) deltaC-13 measurements on marine photolithotrophs and on members of other trophic levels collected from the sea can give unique data on food webs, with measurements of delta values for other isotopes and compositional data adding precision to the interpretations. (3) Measurements of in situ deltaC-13 values for extant marine photolithotrophs, compared with deltaC-13 values for ancient atmospheric CO2, can give unique information on the mechanism of atmospheric CO2 draw-down at the start of glacials; other information permits more concrete conclusions to be drawn.