The Physiological Role of Ascorbate as Photosystem II Electron Donor: Protection against Photoinactivation in Heat-Stressed Leaves    

Abstract

Previously, we showed that ascorbate (Asc), by donating electrons to photosystem II (PSII), supports a sustained electron transport activity in leaves in which the oxygen-evolving complexes were inactivated with a heat pulse (49°C, 40 s). Here, by using wild-type, Asc-overproducing, and -deficient Arabidopsis (Arabidopsis thaliana) mutants (miox4 and vtc2-3, respectively), we investigated the physiological role of Asc as PSII electron donor in heat-stressed leaves (40°C, 15 min), lacking active oxygen-evolving complexes. Chlorophyll-a fluorescence transients show that in leaves excited with trains of saturating singleturnover flashes spaced 200 ms apart, allowing continual electron donation from Asc to PSII, the reaction centers remained functional even after thousands of turnovers. Higher flash frequencies or continuous illumination (300 mmol photons m 22 s 21 ) gradually inactivated them, a process that appeared to be initiated by a dramatic deceleration of the electron transfer from Tyr Z to P680 + , followed by the complete loss of charge separation activity. These processes occurred with half-times of 1.2 and 10 min, 2.8 and 23 min, and 4.1 and 51 min in vtc2-3, the wild type, and miox4, respectively, indicating that the rate of inactivation strongly depended on the Asc content of the leaves. The recovery of PSII activity, following the degradation of PSII proteins (D1, CP43, and PsbO), in moderate light (100 mmol photons m 22 s 21 , comparable to growth light), was also retarded in the Ascdeficient mutant. These data show that high Asc content of leaves contributes significantly to the ability of plants to withstand heat-stress conditions.

Alterations in photochemical efficiency of photosystem II in wheat plant on hot summer day

“…It was probably due to the fact that this work was done under natural conditions where the plant was getting sunlight continuously. This explanation finds support from earlier observations in which a K step is observed in leaves exposed to continuous illumination (Toth et al 2011;Brestic et al 2012). An additional H-G step was observed in the heat treated plants (Fig.…”

Interaction of ascorbate with photosystem I

“…Ascorbate oxidation by photoreduced H 2 O 2 stimulates electron transport in chloroplasts. Under stress conditions (e.g., in heat-stressed leaves with inactive oxygen evolving complexes), ascorbate may play the role of alternative electron donor to PSII, protecting photosynthetic apparatus against photoinactivation (Mano et al 1997(Mano et al , 2004Tóth et al 2009Tóth et al , 2011. Besides the donor capacity of ascorbate, its oxidized forms, MDA and DHA, may act as electron acceptors.…”

Dynamic properties of photosystem II membranes at physiological temperatures characterized by elastic incoherent neutron scattering. Increased flexibility associated with the inactivation of the oxygen evolving complex

“…Specific reorganizations in the membrane ultrastructure are induced at elevated temperatures both in the dark and in combination with strong illumination (Anderson and Andersson 1988;Dobrikova et al 2003;Páli et al 2003). Upon heat stress, the oxygen evolving complex (OEC), the most heat sensitive component of the photosynthetic electron transport chain, is damaged due to the dissociation of the 33 kDa protein from the lumenal side of PSII around 313 K (Enami et al 1994), while the activity of the reaction centers and the membrane energization can be retained (Tóth et al 2011(Tóth et al , 2009. In this study our attention is focused on reorganizations induced by elevated temperatures.…”