Long-term steady-state labelling of wheat plants by use of natural ¹³CO₂/¹²CO₂ mixtures in an open, rapidly turned-over system

Abstract

A photosynthate labelling method is presented which takes advantage of the natural difference in carbon-isotope composition (δ¹) 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 54m³·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 cm³ · m⁻³) and vapor-pressure deficits.