Table I.

Effect of O₂ and OAA on the transition state 2-to-state 1 in C. reinhardtii, wild type

Chlamydomonas cells containing 150 μg of chlorophyll mL⁻¹ were incubated at 25°C in the oxygen electrode cell in the minimal medium (see “Materials and Methods”) supplemented with 5 mm NaHCO₃ and catalase. The cells were allowed to consume O₂ in the dark until anaerobiosis was reached (several minutes were required, during which F₀ and F_m were monitored by the PAM fluorimeter). After 5 min in the dark anaerobic condition, state 2 was established, as indicated by the stable fluorescence parameters. The initial (state 1) F_v/F_m ratio was 0.72 ± 0.0 (30 measurements, with several cell cultures), and 0.41 in the dark anaerobic condition (state 2). No influence of OAA on these parameters was ever observed. Light was then turned on and O₂ was simultaneously generated (by catalase) as required upon the injection of H₂O₂. Light was of an intensity producing 20% to 25% of the maximal photosynthetic rate measured under light saturation. The lag time (see Fig. 1) in the control (no additions) is dependent upon light intensity at any particular chlorophyll concentration. Under the conditions indicated here (light intensity was 58 μE m⁻² s⁻¹), it was of 489 ± 86 s, and in each experiment the values observed in the treated samples were normalized to their control. Photosynthetic O₂ evolutions at steady state were, respectively, 35.1, 35.4, 35 μmol mg chlorophyll h⁻¹ in the control, with O₂, OAA, or O₂ + OAA added. The light-saturated rate was 146. All figures are averages ± se. The figures in parentheses indicate the number of experiments.

Additions
	None	O2 30 μm	OAA 2 mm	OAA + O2
Lag (s)	100	64.9 ± 3.2	79 ± 9.8	61 ± 10
		(4)	(9)	(6)