Table II.

Midday mean chlorophyll fluorescence parameters in September 1996

J_PSII, F_v′/F_m′ and q_P were measured on sun-exposed branches in two FACE experiments during September 1996: (a) The FACE prototype, which was a single elevated pCO₂ ring and control that had been operated over three consecutive growing seasons prior to these measurements. (b) The adjacent full experiment of three replicate elevated and three replicate current pCO₂ rings that had been operated for just 1 week prior to these measurements. Values are the means (se) for two fascicles measured on each of three trees in the prototype ring and a control ring; and means (se) for the three replicate elevated and control pCO₂ rings in the full experiment. Two-way ANOVA tested the effect of pCO₂ (F_1,14), experiment (F_1,14), and their interaction (F_1,14) on each parameter. F values for the effect of pCO₂ are shown, *indicates P < 0.05; n.s. indicates P > 0.05. No interaction was found for any of the parameters (F_1,14 < 3.2; P > 0.05). E/C indicates the ratio of J_PSII and q_P measured at elevated pCO₂ to that measured at the current ambient pCO₂. J_PSII is expressed in μmol m⁻² s⁻¹, F_v′/F_m′ and q_P are dimensionless.

Parameter	FACE Prototype		Full Experiment		F Statistic
	Current	Elevated	Current	Elevated
JPSII	227.0  (12.0)	297.0  (17.0)	170.3   (3.5)	241.9  (15.8)	19.9*
E/C of JPSII	1.31		1.42
Fv′/Fm′	0.73  (0.03)	0.67  (0.02)	0.65  (0.05)	0.72  (0.01)	0.03, n.s.
qP	0.47  (0.02)	0.67  (0.04)	0.43  (0.04)	0.56  (0.02)	22.8*
E/C of qP	1.43		1.30