It has been reported that PSI photoinhibition is induced even in wild-type plants of Arabidopsis thaliana, rice and other species by exposure of leaves to fluctuating light (FL) for a few hours. Because plants are exposed to FL in nature, they must possess protective mechanisms against the FL-induced photodamage. Here, using A. thaliana grown at various irradiances, we examined PSI photoprotection by far-red (FR) light at intensities comparable with those observed in nature. Dark-treated leaves were illuminated by red FL alternating high/low light at 1,200/30 µmol m⁻² s⁻¹ for 800 ms/10 s. By this FL treatment without FR light for 120 min, the level of photo-oxidizable P700 was decreased by 30% even in the plants grown at high irradiances. The addition of continuous FR light during the FL suppressed this damage almost completely. With FR light, P700 was kept in a more oxidized state in both low- and high-light phases. The protective effect of FR light was diminished more in mutants of the NADH dehydrogenase-like complex (NDH)-mediated cyclic electron flow around PSI (CEF-PSI) than in the PGR5 (proton gradient regulation 5)-mediated CEF-PSI, indicating that the NDH-mediated CEF-PSI would be a major contributor to PSI photoprotection in the presence of FR light. We also confirmed that PSI photoinhibition decreased with the increase in growth irradiance in A. thaliana and field-grown plants, and that this PSI photodamage was largely suppressed by addition of FR light. These results clearly indicate that the most effective PSI protection is realized in the presence of FR light.

It has been claimed that the cyclic electron flow around PSI (CEF-PSI) plays an important role in protection of PSI against the fluctuating light photoinhibition. However, the photoprotective mechanism of PSI is not fully elucidated. Here, we approached the mechanism, using the two CEF-PSI mutants of Arabidopsis thaliana, and antimycin A, an inhibitor of the PGR5-mediated CEF-PSI. Dark-adapted leaves in these plants were illuminated in the fluctuating light alternating high-light at 1200 µmol m⁻² s⁻¹ and low-light at 30 µmol m⁻² s⁻¹ every 2 min, and PSI and PSII parameters were simultaneously measured for 160 min with 830 nm absorption and Chl fluorescence, respectively. When CEF-PSI, especially the PGR5-mediated CEF-PSI, didn’t operate, the acceptor-side limitation of PSI, Y(NA), increased stepwise, leading to marked PSI photoinhibition. The deficiency of CFE-PSI decreased not only the electron transport rate through PSI but also the donor-side limitation of PSI, Y(ND), in high light phases. These results showed that the large Y(ND), observed only when CEF-PSI operated, contributed to suppression of PSI photoinhibition. In combination with our previous report that high Y(NA) was alleviated by the enhancement of CEF-PSI, a model for the protective mechanisms of PSI are proposed. In this model, both alleviation of Y(NA) and acceleration of Y(ND) are indispensable, and for realization of such the situation, regulation of the electron flows, especially the PGR5-mediated CEF-PSI, plays a key role. It is important for effective protection to regulate the balance of Y(ND) and Y(NA) through CEF-PSI.

Heterogeneous distribution of PSI and PSII in thick grana in shade chloroplasts is argued to hinder spillover of chlorophyll excitations from PSII to PSI. To examine this dogma, we measured fluorescence induction at 77K at 690 nm (PSII) and 760 nm (mostly PSI) in the leaf discs of Spinacia oleracea, Cucumis sativus and shade tolerant Alocasia odora, grown at high and low light, and quantified their spillover capacities. PSI fluorescence (FI) consists of the intrinsic PSI fluorescence (FI_α) and fluorescence caused by excitations spilt over from PSII (FI_β). When FI and FII parameters between State 1 and State 2, induced by weak far-red and blue light, were compared, PSII maximum fluorescence (FII_m) and FI_β were greater, and FI_α was smaller in State 1 and thereby the spillover ratio, FI_β/(FI_α+FI_β), was greater in State 1. When non-photochemical quenching (NPQ) was induced, the spillover ratio decreased. Since analyses of F_v/F_m spectra tentatively suggested that about 15% of F_m at 760 nm was from PSII, all data were corrected accordingly. Even after the correction, the spillover ratio in FI_m in State 1 ranged from 16 to 28%. The spillover ratios did not greatly differ between the species or growth light levels. Although extensive grana in low light grown plants would suggest that PSII and PSI are too separated for spillover, the ratios of non-appressed thylakoid membranes/total thylakoid membranes in A. odora chloroplasts were little affected by growth light and more than 40%. Abundant non-appressed thylakoids and margins of appressed-thylakoids would enable efficient spillover. (250 words)