photorespiratory flux. Collectively, our study provides strong evi-dence that rates of leaf RLare indeed lower than those of RD(withlight inhibiting leaf R by 17–47%; Table 2, Fig. 3) and that variationsin RLcan be predicted with knowledge of the underlying rates ofRDand 0.5voor perhaps vc(Fig. 5). Moreover, in contrast to theinhibitory effect of low [CO2] on A1000(Fig. 2, Table 2), we found thatlow atmospheric [CO2] had no significant effect on rates of root Ror leaf RD. As a result, a greater proportion of daily photosyntheticCO2uptake is likely released by root and leaf R in plants grownunder low [CO2] concentrations characteristic of the pre-industrialperiod