Long-term steady-state labelling of wheat plants by use of natural (13)CO 2/ (12)CO 2 mixtures in an open, rapidly turned-over system.

A photosynthate labelling method is presented which takes advantage of the natural difference in carbon-isotope composition (δ Carbon-isotope composition of a sample is conventionally presented as δ (‰) = [(RP/RS) -1] · 1000, where RP and RS are the molar abundance ratios, (13)C/(12)C, of the sample and of the standard (PDB carbonate), respectively ) which exists between atmospheric CO2 (δ≈-8‰) and commercially available compressed CO2. Carbon dioxide with δ-4.0 and -27.9%., respectively, has been used for labelling. A plant growth cabinet served as the labelling compartment. CO2-free air was continuously injected at a rate of up to 54m(3)·h(-1). Dilution of cabinet CO2 by CO2-free air was counterbalanced by addition of CO2 with known constant δ. Since the labelling-cabinet atmosphere was continuously exchanged at a high rate, photosynthetic carbon-isotope discrimination was fully expressed. In order to study the distribution of carbon acquired by the plant during a defined growth period, the δ of CO2 was modified by replacing, for example, atmospheric CO2 by CO2 with δ -27.9%. and the weight and 5 of plant carbon pools was monitored over time. In such an experiment the δ change of CO2 was followed by a rapid change of the δ of sucrose in mature flag-leaf blades of wheat (Triticum aestivum L.). The 5 of sucrose stabilized near -51%., indicating complete exchange by current photosynthate. In contrast 83% of the total carbon in mature flag-leaf blades was not exchanged after 14 d continuous labelling. Differential labelling of pre- and post-anthesis photosynthate indicated that 13% of grain carbon originated from pre-anthesis photosynthesis. Carbon-isotope discrimination and its consideration in experimentation and labelling data evaluation are discussed in detail. Since the air supplied to the labelling cabinet is dry and free of CO2, carbon-isotope discrimination and carbon turnover and partitioning can be studied over a wide range of CO2 concentrations (0-2600 cm(3) · m(-3)) and vapor-pressure deficits.