Table S1.

Gas exchange and isotopomer data obtained from experiments with sunflower leaves (group A)

Variable	CO2 treatment, ppm
	180	280	450	700	1,500
Ci, µM	5.2 ± 0.1	7.8 ± 0.2	11.65 ± 0.3	19.7 ± 0.6	49.1 ± 0.7
ϕ	0.51	0.34	0.23	0.14	0.05
D6S/D6R	1.187	1.107	0.944	0.962	0.883
	1.108	1.097	1.014	0.877	0.893
			1.007	0.925	0.904
				0.931	0.863
				0.923	0.921

The leaf temperature was 22 °C, giving an intercellular (dissolved) O₂ concentration of 277 μM and K_sp of 104 (59). C_i was derived from gas exchange measurements on sunflower leaves, and D6^S/D6^R ratios were measured on leaf starch. Values of C_i are averages ± SE. Samples represent individual plants, except for 180 ppm and 280 ppm, where material from two to four plants had to be pooled to obtain sufficient sample amount.