009*) <0.001 0.594 0.562 0.067 0.743 0.234 0.228 Treatments (0.208*) <0.001 <0.001 0.258 <0.001 <0.0011 <0.0011 0.538 Interaction (accessions  ×  treatments)

<0.001 0.694 0.103 0.185 0.378 0.400 0.437 0.915 Effects of accessions (Col-0. C24 and Eri) and treatments (C 50 and SSF 1250/6) on different parameters were tested. Shown are P values for each set of test. Significant effects are marked italics * Due to significant interactions between accessions and treatments, the main effect of each GSK458 chemical structure factor cannot be properly determined Discussion Acclimation to fluctuating light environment: effects of light intensity, duration, and frequency Figure 11 gives an outline of the responses of Col-0 during acclimation to different light regimes. The 7-day treatments were long enough to study these acclimatory

changes in Arabidopsis plants. The NPQ capacity increased in mature leaves of the SSF plants in which QA was more strongly reduced upon HL exposure (Figs. 1 and 2); as 1-qp decreased on day 7 to reach a level as low as in C 85 and LSF 650 (SSF 650/6) or to restore the initial level on day 0 (SSF 1250/12, SSF 1250/6), deceleration of NPQ upregulation was observed. Likewise, the NPQ capacity in C 85, C 120, and LSF 650 did not change, or even declined slightly (Fig. 1), as the capacity for QA oxidation and electron transport increased in these plants (Figs. 2 and 3). These results underline opposite and complementary responses of NPQ and electron transport under the different LY294002 cell line light conditions used in this study (Fig. 11, upper

boxes). Fig. 11 A diagram summarizing the responses of Arabidopsis (Col-0) Thiamine-diphosphate kinase during 7-day acclimation to constant (C 85, C 120) or fluctuating light environment with long (LSF 650) or short sunflecks (SSF 650/6, SSF 1250/12, SSF 1250/6). All plants were acclimated to the C 50 condition before starting the experiments on day 0 Our data in SSF 650/6 clearly show that NPQ enhancement precedes upregulation of electron transport during acclimation to SSF (Figs. 1d, 2d, and 3d) presumably to cope with an acute threat of photo-oxidation. Since both SSF 1250/12 and SSF 1250/6 increased the maximal NPQ and suppressed the upregulation of QA oxidation and electron transport almost equally and more strongly than SSF 650/6 (Figs. 1–3), it seems that the intensity of SSF has a great impact on these acclimatory responses in Arabidopsis plants. How about the duration and the frequency of sunflecks? The two treatments SSF 650/6 and LSF 650 revealed distinct initial effects of the sunflecks with AZD1152 contrasting duration and frequency (but the same intensity): upregulation of NPQ and photoprotection in SSF 650/6 and upregulation of QA oxidation and electron transport in LSF 650 (Fig. 11).