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_w^soil) as measured arbitrarily at a depth of 10 cm, becoming less negative as the ψ_w^soil decreased. Over a linear distance of 5.55 m,S. europaea spp.rubra showed a shift of +5.3‰ as the ψ_w^soil 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_w^soil was 12.7x10² kPa. However, δ¹³C values ofP. nuttalliana only roughly reflected the spatial trends in ψ_w^soil 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_w^soil) 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_w^soil 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_w^soil. 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.