A new measurement technique reveals temporal variation in Δ¹⁸O of leaf-respired CO₂

The oxygen isotope composition of CO₂ respired by Ricinus communis leaves (Δ¹⁸O_R) was measured under non-steady-state conditions with a temporal resolution of 3 min using a tunable diode laser (TDL) absorption spectrometer coupled to a portable gas exchange system. The SD of Δ¹⁸O measurement by the TDL was ± 0.2‰ and close to that of traditional mass spectrometers. Further, Δ¹⁸O_R values at isotopic steady state were comparable to those obtained using traditional flask sampling and mass spectrometric techniques for R. communis grown and measured in similar environmental conditions. As well as higher temporal resolution, the online TDL method described here has a number of advantages over mass spectrometric techniques. At isotopic steady state among plants grown at high light, the 'one-way flux' model was required to accurately predict Δ ¹⁸O_R. A comparison of measurements and the model suggests that plants grown under low-light conditions have either a lower proportion of chloroplast CO₂ that isotopically equilibrates with chloroplast water, or more enriched Δ¹⁸O of CO₂ in the chloroplast that has not equilibrated with local water. The high temporal resolution of isotopic measurements allowed the first measurements of Δ¹⁸O_R when stomatal conductance was rapidly changing. Under non-steady-state conditions, Δ¹⁸O_R varied between 50 and 220‰ for leaves of plants grown under different light and water environments, and varied by as much as 100‰ within 10 min for a single leaf. Stomatal conductance ranged from 0.001 to 1.586 mol m ^-2 s^-1, and had an important influence on Δ¹⁸O_R under non-steady-state conditions not only via effects on leaf water H₂¹⁸O enrichment, but also via effects on the rate of the one-way fluxes of CO₂ into and out of the leaf.