A New Measurement Technique Reveals Temporal Variation in δ18O of Leaf-Respired CO2

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The oxygen isotope composition of CO2 respired by Ricinus communis leaves (δ18OR) 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, δ18OR 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 δ18OR. 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 δ18OR when stomatal conductance was rapidly changing. Under non-steady-state conditions, δ18ORvaried 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 δ18OR 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.


Plant, Cell & Environment





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