A pulse-labelling method to generate 13C- enriched plant materials

Plant materials labelled with C-13 can be used to trace litter decomposition and root carbon flow, but only if the isotope is uniformly distributed in the plant. We postulated that if (CO2)-C-13 were applied at regular intervals, in direct proportion to the rate of photosynthesis, then the abundance of C-13 would be uniform among plant parts. To test this hypothesis, wheat plants were grown in the greenhouse, and exposed weekly to (CO2)-C-13 for six hours in a closed chamber. A constant dose of (CO2)-C-13 (about 33 atom%) was injected whenever CO2 concentration fell below a prescribed limit, so that (CO2)-C-13 was added in proportion to photosynthetic rate. Wheat exposed for 13 weeks (starting 11 days after seeding) had reasonably consistent C-13 abundance among plant parts: grain = 3.41, chaff = 3.41, stem = 3.65, and root = 3.50 atom%. The `leaf' fraction had slightly higher abundance (3.99 atom%), perhaps because recently-fixed C-13 was not translocated from senescing tissue. Exposing plants only during early stages of the growing season increased differences among plant parts. The approach offers a practical way to label plants with C-13.