title: Trends in (13)C/ (12)C ratios and C isotope discrimination of wheat since 1845.
abstract: Archived wheat grain and straw samples from the unfertilised plot of the Broadbalk Continuous Wheat Experiment at Rothamsted, England, were used to investigate the trends in stable C isotope ratios since 1845. Grain δ(13)C was higher than straw δ(13)C. Both grain and straw δ(13)C have decreased by approximately 2.5-2.8‰ over the last 153 years, and since the 1960s the decrease has been more rapid than the decrease of the δ(13)C of atmospheric CO2. The C isotope discrimination (Δ), and hence the ratio of CO2 concentration in leaf intercellular space to that in the ambient air (c i /c a), remained relatively stable or decreased slightly between 1845 and the mid-1960s, then increased considerably. The period with increasing Δ and c i /c a corresponds to the introduction of the modern short-straw cultivars of wheat. When grown on the unfertilised plot in the same seasons, a modern wheat cultivar had slightly higher Δ values than an old cultivar. The c i derived from both grain and straw Δ have increased by 33-37% over the last 153 years, with the increase being more rapid after than before the mid-1960s. However, there was no clear trend in the yields of straw and grain, primarily because of the limitations of N and other nutrients, and also because of large year-to-year variations.
authors.affiliation: Agriculture and Environment Division, IACR-Rothamsted, AL5 2JQ, Harpenden, Hertfordshire, UK.
authors.forename: Fang-Jie
authors.initials: FJ
authors.lastname: Zhao
authors.affiliation: Kingsley Dunham Centre, NERC Isotope Geosciences Laboratory, NG12 5GG, Keyworth, Nottingham, UK.
authors.forename: Baruch
authors.initials: B
authors.lastname: Spiro
authors.affiliation: Agriculture and Environment Division, IACR-Rothamsted, AL5 2JQ, Harpenden, Hertfordshire, UK.
authors.forename: Steve P
authors.initials: SP
authors.lastname: McGrath