Factors affecting ¹³C/¹²C ratios of inland halophytes. I. Controlled studies on growth and isotopic composition of Puccinellia nuttalliana

Studies on various factors affecting the growth and stable carbon isotope composition of the graminaceous C 3 halophyte Puccinellia nuttalliana (Schultes) Hitch. were initiated as a step towards interpreting δ 13 C variations in nature. For isotope analysis, combustion at 900 °C resulted in higher CO 2 yield than at 550 °C but did not affect δ 13 C values. Differences in δ 13 C between leaves of different insertion level were unimportant, but roots were about 1‰ more positive than shoots. Trends in δ 13 C with salinity were the same in all plant parts. Depressions of growth by NaCl or Na 2 SO 4 were similar, but plants grown in Na 2 SO 4 displayed a greater shift in δ 13 C relative to controls. Growth rates were affected more by salinity than were previously reported photosynthetic rates. At typical salinities, δ 13 C changed linearly with salinity. The supply of nitrate to stressed and unstressed plants had no important influence on δ 13 C. Growth in polyethylene glycol produced δ 13 C values consistent with a high level of stress. After a salinity step-up, changes in δ 13 C were complete within 10 days. During winter, data were found to be heavily influenced by unintentional, human-respired CO 2 enrichment. This represents a potentially serious research problem in laboratories of temperate climes.