Figure 1.

Changes in the respiration rate (μmol m⁻² s⁻¹; A) and δ¹³C (‰; B) of CO₂ respired in the dark after isotopic labeling in French bean leaves. Leaves were labeled with commercial CO₂ with δ¹³C of −51.2‰ for 20 min (10 mmol C fixed per m²; squares), 75 min (30 mmol C fixed per m²; circles), and 180 min (80 mmol C fixed per m²; triangles), respectively. The arrow indicates the beginning of the dark period. The δ¹³C value of the dark-respired CO₂ for a 20-h dark period for unlabeled leaves is also shown (diamonds). Data are the means of three replicates (se values are shown when larger than the symbols). At the beginning of the dark measurements, the percent of new carbon in respired CO₂ was 18%, 29%, and 34% for the 10, 30, and 80 mmol C m⁻² labeling curves, respectively. Inset, Schematic drawing of the surfaces delimited by (1) the control δ¹³C and the δ¹³C of the 20-min labeling (10 mmol C fixed per m²) curves; and (2) the δ¹³C of the 75-min labeling (30 mmol C fixed per m²) curve and the steady-rate line (δ¹³C = −27‰).