PSII and PSI activities under chilling stress in N. commune
Responses of PSII and PSI activities to chilling stress are shown in Fig.1A. During the exposure to chilling temperature (4 ºC), the PSII maximun quantum yield Fv/Fm showed a slight rise within 4 h, then gradually decreased, and significantly reduced by 35% at 24 h compared with the initial values (Turkey HSD, P < 0.05), but
the maximum photo-oxidizable P700 Pm, as an indicator of the quantity of efficient PSI complex, did not decrease and maintained stable (Fig. 1). During the following recovery at 25 ºC, Fv/Fm exhibited a rapid increase, up to the levels of the initial values, Pm also remained stable (Fig. 1A&B).
To explore the underlying mechanism, we further carried out western blot assay for photosystem core proteins of N. commune. In agreement with the physiological results, after exposed to chilling stress (4 ºC) for 24 h, the abundance of PSII core protein D1 in N. commune obviously decreased but the abundance of PSI core protein PsaA remained stable, and during the following recovery experiments (25 ºC) for 24 h, the D1 abundance recovered approximately to the initial levels whereas the PsaA/B abundance also remained stable. These results showed that PSII was sensitive to chilling stress whereas PSI was chilling-tolerant in N. commune.
The PSII excitation pressure maintained stable under chilling stress in N. commune
Effects of chilling stress on the chlorophyll fluorescence parameters Fv'/Fm',1-qP and rETR(II) in N. commune were indicated in Fig. 2. During the 24 h exposure to chilling stress, both the light-adapted maximum quantum yield of PSII Fv'/Fm' and the relative electron transport rate around PSII rETR(II) continuously decreased, up to 65% of the original values, and returned to the initial values during the following recovery at 25 ºC. However, the parameter 1-qP, as an indicator of the excitation pressure in PSII or the proportion of closed PSII reaction centers, maintained stable during the chilling treatment and the subsequent recovery at 25 ºC.
non-photochemical quenching increased under chilling stress in N. commune
Effects of chilling stress on light energy distribution in PSII were illustrated in Fig. 3. Since the effective quantum yield of PSII Y(II) is the product of the light-adapted maximum quantum yield of PSII Fv'/Fm' multiplied by the photochemical quenching coefficient qP, when exposed to chilling stress for 24 h, qP maintained stable, so the decrease in Y(II) by 35% was only due to the loss of Fv'/Fm' which was also reduced by 35% (Fig. 2&3). During chilling stress at 4 ºC, the quantum yield of non-regulated energy dissipation in PSII (Y(NO), reflecting the inability to protect itself against damage by excess light energy, gradually increased by 16% at 24 h, the quantum yield of regulated energy dissipation in PSII Y(NPQ) was stimulated, significantly rose by 41% at 24 h compared to the initial values. During the recovery, Y(II) rapidly increased, Y(NO) and Y(NPQ) fell down quickly, reaching the original levels (Fig. 3).
Cyclic electron flow was stimulated upon the transfer to chilling stress and subsequent recovery in N. commune
Responses of cyclic electron flow (CEF) in N. commune to chilling stress at 4 ºC and subsequent recovery at 25 ºC were assayed in Fig. 4. When the samples was transferred to chilling stress at 4 ºC, the cyclic electron flow was significantly stimulated, its cyclic electron transfer rate of 2.03±0.39 s−1 at 0.5 h rose by 46% compared to the initial values (1.39±0.10 s−1), then cyclic electron flow gradually decreased, reaching 50% of the original values at 24 h. Once the chilling-treated samples were transferred to the optimum temperature condition at 25 ºC, its cyclic electron flow was sharply stimulated again, its cyclic electron transfer rate of 1.40±0.39 s−1 at 0.5 h reached nearly two-fold that of chilling-treated cells (0.7±0.10 s−1), then gradually increased within 4 h, and then maintain relative stable levels, specifically its cyclic electron transfer rate reached 126-137% of the initial values (Fig. 3 A-B).
A similar reduction of PSII activity was induced by methyl viologen and chloramphenicol under chilling stress in N. commune
We further examined the decrease of PSII activity in the presence of methyl viologen (MV) or chloramphenicol (CMP) during chilling stress as shown in Fig. 5. CMP is the protein translation inhibitor and can interrupt PSII repair cycle, and MV accepts electrons from the PSI acceptor side and can block the cyclic electron flow around PSI (Yu et al. 1993). After 24 h exposure to chilling stress at 4 ºC, the Fv/Fm value decreased to 69% of the initial values, whereas Fv/Fm further decreased to 48% of the initial values in the presence of chloramphenicol, implying the PSII repair contributed to 21% of the PSII activity during chilling stress. When the samples were exposed to MV during chilling stress, its PSII activity showed a similar reduction in Fv/Fm values (21%), suggesting that cyclic electron flow may contribute to the PSII repair under chilling stress in N. commune.