Down-Regulation of Photosynthetic Electron Transport and Decline in CO₂ Assimilation under Low Frequencies of Pulsed Lights

The decline in CO₂ assimilation in leaves exposed to decreasing frequencies of pulsed light is well characterized, in contrast to the regulation of photosynthetic electron transport under these conditions. Thus, we exposed sunflower leaves to pulsed lights of different frequencies but with the same duty ratio (25%) and averaged light intensity (575 μmoles photons m⁻² s⁻¹). The rates of net photosynthesis P<i>n</i> were constant from 125 to 10 Hz, and declined by 70% from 10 to 0.1 Hz. This decline coincided with (1) a marked increase in nonphotochemical quenching (NPQ), and (2) the completion after 25 ms of illumination of the first phase of P₇₀₀ photooxidation, the primary electron donor of PSI. Under longer light pulses (<5 Hz), there was a slower and larger P₇₀₀ photooxidation phase that could be attributed to the larger NPQ and to a resistance of electron flow on the PSI donor side indicated by 44% slower kinetics of a P₇₀₀⁺ dark reduction. In addition, at low frequencies, the decrease in quantum yield of photochemistry was 2.3-times larger for PSII than for PSI. Globally, our results indicate that the decline in CO₂ assimilation at 10 Hz and lower frequencies coincide with the formation of NPQ and a restriction of electron flows toward PSI, favoring the accumulation of harmless P₇₀₀⁺.