Figure 2.

Inhibition of PSII photochemistry and of photosynthetic CO₂ fixation during strong light stress. (A) F_v/F_m during a strong light stress (1,500 μmol of photons·m⁻²·s⁻¹) of wild-type and npq1 Arabidopsis leaves. The hatched areas indicate the night periods. Each experimental point is the mean value of 4–15 measurements. Chl fluorescence was measured only on green leaves or the green parts of partially bleached npq1 leaves (see Fig. 3). (A + Z)/(V + A + Z) values: wild type, 0.07 and 0.53 before and after light stress (60 hr); npq1, 0 and 0.025 before and after light stress (60 hr). (B) Net photosynthetic CO₂ fixation by one Arabidopsis plant (▵, wild type; ▴, npq1 mutant) exposed to strong, white light. At time 0, plants were shifted from 300 μmol of photons·m⁻²·s⁻¹ to 1,500 μmol of photons·m⁻²·s⁻¹, and the rate of CO₂ fixation was measured every day during 1 hr from 11:30 a.m. to 12:30 p.m. Transpiration of npq1 (●) and wild-type (○) plants was comparable, indicating that stomatal closure was not responsible for the lower CO₂ fixation activity of npq1.