Find in Library

Rate-limiting steps in the dark-to-light transition of Photosystem II (PSII) were discovered by measuring the variable chlorophyll-<i>a</i> fluorescence transients elicited by single-turnover saturating flashes (STSFs). It was shown that in diuron-treated samples: (i) the first STSF, despite fully reducing the Q_A quinone acceptor molecule, generated only an <i>F</i>₁(<<i>F</i>_m) fluorescence level; (ii) to produce the maximum (<i>F</i>_m) level, additional excitations were required, which, however, (iii) were effective only with sufficiently long Δ<i>τ</i> waiting times between consecutive STSFs. Detailed studies revealed the gradual formation of the light-adapted charge-separated state, PSII_L. The data presented here substantiate this assignment: (i) the Δ<i>τ</i>_1/2 half-increment rise (or half-waiting) times of the diuron-treated isolated PSII core complexes (CCs) of <i>Thermostichus vulcanus</i> and spinach thylakoid membranes displayed similar temperature dependences between 5 and –80 °C, with substantially increased values at low temperatures; (ii) the Δ<i>τ</i>_1/2 values in PSII CC were essentially invariant on the <i>F</i>_k−to-<i>F</i>_k+1 (<i>k</i> = 1–4) increments both at 5 and at −80 °C, indicating the involvement of the same physical mechanism during the light-adaptation process of PSII_L. These data are in harmony with the earlier proposed role of dielectric relaxation processes in the formation of the light-adapted charge-separated state and in the variable chlorophyll-<i>a</i> fluorescence of PSII.