Chlorophylls are degraded and flavonoids synthesized during autumn senescence of deciduous trees. In the present study, chlorophyll and flavonol contents of individual leaves were monitored non-destructively throughout the autumn. Loss of chlorophyll and synthesis of flavonols were not gradual. Instead, each leaf maintained steady chlorophyll content until rapid chlorophyll degradation, accompanied by flavonol synthesis, was triggered. In ~1 week, the leaf turns yellow and falls. The pattern was similar in birch (Betula pendula), maple (Acer platanoides) and bird cherry (Prunus padus); in rowan (Sorbus aucuparia), very slow gradual chlorophyll degradation occurred on top of the main pattern.

The volumetric productivity of the highlight tolerant strain hit2 of Chlamydomonas reinhardtii was found to be higher than that of the parental strain CC124 during continuous growth at PPFD from 200 to 1,500 µmol m-2 s-1. At PPFD of 1,250 µmol m-2 s-1 , hit2 produced 2.53 ± 0.18 and CC124 produced 2.05 ± 0.12 g(biomass) dm-3 d-1. The rate constant of photoinhibition of hit2 was less than half of that of CC124, suggesting that hit2 produces more biomass than CC124 because hit2 does not need to allocate as much resources for PSII repair as CC124. Growth in high light triggered similar loss of chlorophyll, increase in the carotenoid-to-chlorophyll ratio, and decrease in PSI fluorescence in both strains. Thermoluminescence B band was shifted toward the Q band in hit2, suggesting that low redox potential of the QB/QBpair contributes to the photoinhibition tolerance of hit2.

Most photosynthetic organisms are sensitive to very high light, although acclimation mechanisms enable them to deal with exposure to strong light up to a point. Here we show that cultures of wild-type Chlamydomonas reinhardtii strain cc124, when exposed to photosynthetic photon flux density 3000 μmol m−2 s−1 for a couple of days, are able to suddenly attain the ability to grow and thrive. We compared the phenotypes of control cells and cells acclimated to this extreme light (EL). The results suggest that genetic or epigenetic variation, developing during maintenance of the population in moderate light, contributes to the acclimation capability. EL acclimation was associated with a high carotenoid-to-chlorophyll ratio and slowed down PSII charge recombination reactions, probably by affecting the pre-exponential Arrhenius factor of the rate constant. In agreement with these findings, EL acclimated cells showed only one tenth of the 1O2 level of control cells. In spite of low 1O2 levels, the rate of the damaging reaction of PSII photoinhibition was similar in EL acclimated and control cells. Furthermore, EL acclimation was associated with slow PSII electron transfer to artificial quinone acceptors. The data show that ability to grow and thrive in extremely strong light is not restricted to photoinhibition-resistant organisms such as Chlorella ohadii or to high-light tolerant mutants, but a wild-type strain of a common model microalga has this ability as well.

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Abbreviations: ETR -electron transfer rate; ETR(II) -electron transport rate calculated by taking into account the relative absorption cross-section of PSII; ETRMAX -maximum electron transport rate; Fv/Fm -ratio of variable to maximum fluorescence; Ik -minimum saturating light intensity; OD -optical density; RLC -rapid light curve; α -light-use efficiency; σII(λ) -wavelength-dependent functional cross-section of PSII.