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

The total carbon δC values of two C 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 δC value of the two species seemed to reflect the water potential of the soil (ψ ) as measured arbitrarily at a depth of 10 cm, becoming less negative as the ψ decreased. Over a linear distance of 5.55 m,S. europaea spp.rubra showed a shift of +5.3‰ as the ψ went from-25x10 kPa to a minimum of-73x10 kPa. ForP. nuttalliana, the δC values differed by 3.4‰ over a distance of 7.45 m where the maximum difference in ψ was 12.7x10 kPa. However, δC values ofP. nuttalliana only roughly reflected the spatial trends in ψ at the time of collection. In the growth chamber, the δC value ofS. europaea ssp.rubra changed by a maximum of +8.0‰ when the solute potential of the nutrient solution (ψ ) was dropped from-0.25x10 kPa to-64.25x10 kPa; while the δC value ofP. nuttalliana changed by a maximum of +10.8‰ when the ψ was dropped from-0.25x10 kPa to-40.25x10 kPa. Linear regression analyses indicated that the δC values of both species were strongly correlated (P<0.2%) with ψ . The observed shifts in δC may represent changes in the mode of photosynthetic CO 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 δC values of C plants awaits a better understanding of the isotope fractionation mechanisms involved.