Shifts in carbon isotope ratios of two C3 halophytes under natural and artificial conditions.

The total carbon δ13C values of two C3 halophytes,Salicornia europaea L. ssp.rubra (Nels.) Breitung andPuccinellia muttalliana (Schultes) Hitch., native to inland saline areas of Alberta, Canada, were determined for plants grown under controlled conditions of supplied NaCl in the nutrient solution, and for plants found growing in the field. Field specimens were collected along line transects which ran from areas of high salinity to areas of low salinity across the pattern of species zonation. The δ13C value of the two species seemed to reflect the water potential of the soil (ψ wsoil ) as measured arbitrarily at a depth of 10 cm, becoming less negative as the ψ wsoil decreased. Over a linear distance of 5.55 m,S. europaea spp.rubra showed a shift of +5.3‰ as the ψ wsoil went from-25x102 kPa to a minimum of-73x102 kPa. ForP. nuttalliana, the δ13C values differed by 3.4‰ over a distance of 7.45 m where the maximum difference in ψ wsoil was 12.7x102 kPa. However, δ13C values ofP. nuttalliana only roughly reflected the spatial trends in ψ wsoil at the time of collection. In the growth chamber, the δ13C value ofS. europaea ssp.rubra changed by a maximum of +8.0‰ when the solute potential of the nutrient solution (ψ wsoil ) was dropped from-0.25x102 kPa to-64.25x102 kPa; while the δ13C value ofP. nuttalliana changed by a maximum of +10.8‰ when the ψ wsoil was dropped from-0.25x102 kPa to-40.25x102 kPa. Linear regression analyses indicated that the δ13C values of both species were strongly correlated (P<0.2%) with ψ wsoil . The observed shifts in δ12C may represent changes in the mode of photosynthetic CO2 fixation. However, a number of other explanations, some of which are discussed in the text, are also possible. A proper ecophysiological interpretation of such shifts in δ13C values of C3 plants awaits a better understanding of the isotope fractionation mechanisms involved.