In strong light, after several minutes darkness, fluorescence induction curves for green algae exhibit the following features: a low initial level O, a first peak P1 at about 15 msec, a minimum M1 at about 50 msec, a high second peak P2 lasting from about 0.2 to about 1 sec, a minimum M2 at about 30 sec, a third peak P3 at 60 to 120 sec, and a steady-state level S reached after several minutes. Simultaneously measured induction curves of rate of O2 evolution indicate a maximum reached in well under 100 msec, a zero rate while fluorescence is at the high P2 level, a first phase a1 of accelerating rate complementary to the P2-M2 decline of fluorescence, and a slower second phase a2 during which oxygen evolution rises in parallel with the M2-P3 fluorescence climb. When strong light is given after a preillumination, the P1, M1, and P2 fluorescence levels are substantially heightened. However, the steady-state level is unchanged, and the course of fluorescence from P2 to S no longer passes through a minimum nor exhibits the M2-P3 rise. In addition, the amount of oxygen evolved in the spike is increased, and the acceleration of O2 evolution during the a2 phase is faster and kinetically similar to that of the a1 phase. The characteristics of fluorescence and O2 induction, both after darkness and after preillumination, can be explained by assuming a slow activation of System-II units. After darkness, perhaps half of the units would be initially active and capable of fluorescence. As a result, the P1, M1, and P2 levels, and the amount of O2 in the spike, are relatively low. During the about 30th to about 120th sec of induction, the initially inactive, nonfluorescent units are slowly converted to the active form. The M2-P3 rise and the parallel acceleration of O2 evolution of the a2 phase are manifestations of this activation. After preillumination, all System-II units would be active. In consequence, the P1, M1 and P2 levels are higher, the M2-P3 rise is abolished, and the rate of O2 evolution rises faster and more nearly continuously with that of the a1 phase. The M2-P3 fluorescence rise occurs in wild type and System II, but not System-I mutants, and in the presence, as well as the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Furthermore, with monochromatic preilluminations, 708 nm is about three times more effective than 650 nm in maintaining the active form of System-II units. These facts imply that System I sensitizes the slow activation of System-II units. In darkness, inactivation of the units occurs with a half time of about 4 min. Altogether, three different activations appear to underly induction. One is the fast activation of System II discovered by Joliot and accounting for the parallel rises of fluorescence and O2-evolving activity during the first 10-20 msec. The second is the activation of a dark step which permits regeneration of System II oxidant, and which is manifested in the complementary courses of fluorescence during P2-M2 and O2 evolution during the a1 phase. The third is the slow activation of System-II units underlying the parallel rises of fluorescence (M2-P2) and O2 evolution (a2 phase). These three activations appear to provide the basis for a general understanding of the first 2 min of photosynthetic induction. © 1968.
