Upon a dark–light transient, it would be expected that maximal fluorescence signals would decrease as a result of elevated non-photochemical fluorescence quenching (Krause and

Weis 1991; Campbell et al. 1998). In this study, however, F m ′ values increased compared to F m in the block light treatment (Fig. 2). The F m ′ increase (and therefore NPQ down-regulation) was induced after approximately 1 min of actinic light onset, continued for ca 2.5 min, and was followed by a somewhat slower, but steady, decline until the signal was perturbed by addition of 160 μM DIC. Alvelestat manufacturer F m ′ correlated strongly with F′ (m = 1.39; r 2 = 0.91–0.96). A strong correlation between F′ and F m ′ in FRRF measurements suggests a change in the absorption cross section of PSII during the transient, although the functional absorption cross section was found to be stable throughout the actinic light phase (Fig. 2b). The initial rise in F m ′ might be an indication of the dissipation of chlororespiration, but the following decrease in both F′ and F m ′ might be due to both induction of qE or a change in the absorption cross section of PSII due to a state-transition. We applied low-temperature chlorophyll fluorescence emission spectra to investigate the occurrence

of state-transitions. 77 K ICG-001 cell line emission spectra Figure 4 shows a typical chlorophyll fluorescence emission spectrum in D. tertiolecta. Fluorescence emission peaks were not very distinct, with a small contribution at 695 nm

(F 695) (PSII reaction centre). Emission at 715 nm (F 715) is regarded as a contribution from PSI, F 730 is considered as a vibration, while the origin of F 702 remains unclear. Emission spectra were normalised to the fluorescence yield at F 685 (light harvesting complexes of PSII). Murakami (1997) showed that the PSI/(PSII + PSI) ratio determined with biochemical techniques many could be estimated accurately from the F PSI/(F PSII + F PSI) ratio for different algal species. We used the F 685/F 715 ratio as a proxy for changes in the ratio of PSII to PSI. Fig. 4 Representative fluorescence emission spectrum measured at 77 K (a) and residuals remaining after de-convolution (b). A minimum of three measurements per sample were averaged and baseline corrected. The fit was forced through peaks at 685 nm (light harvesting compounds of PSII), 695 nm (PSII reaction core), 702 nm (origin not clear), 715 nm (PSI) and 730 nm (PSI, or vibration). Top curve: dots data points, line resulting fit from de-convolution. Although the origin of the F 702 is obscure, leaving it out resulted in poor fits. Spectra were normalised to F 658 nm. Residuals (b) show the quality of the fit and remained below 0.05 for all samples analysed. Emission peak height data were used for PSII/PSI ratio (F 685/F 715 nm). Excitation wavelength was 435 nm F 685/F 715 ratios F 685/F 715 ratio remained relatively constant at approximately 3.4 during the dark to light transient (Fig. 5).