# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 4. Ascorbate Is an Alternative, 'Emergency' Donor to Photosystem II  
Asc is a weak reductant that has the potential to reduce amino acid radicals, such as tyrosine and tryptophan [94]. This feature makes it capable of donating electrons to Tyrz\({}^{+}\) in PSII with inactive oxygen-evolving complexes (OEC). This was initially demonstrated in vitro on TRIS-washed, UV-B-irradiated, and heat-treated isolated thylakoids [95; 96; 97] and later on heat-treated intact leaves [98; 99].  
Heat stress results in the removal of the extrinsic proteins and the release of Ca- and Mn-ions from their binding sites, resulting in the inactivation of OEC [100; 101]. It has been shown that electron donation from Asc to Tyrz\({}^{+}\) occurs in heat-stressed leaves; thus, Asc is a naturally occurring electron donor that can replace water, the terminal electron donor of PSII [99] (Figure 1). With the aid of chlorophyll \(a\) fluorescence induced by short (5-ms) light pulses it was shown that the halftime of electron donation from Asc to PSII is in the range of 25 ms in wild-type Arabidopsis leaves and about 55 ms in Asc-deficient _vtc2_ mutants [78; 99]. This alternative electron transport occurs in Arabidopsis, pea, barley, _Marchantia polymorpha_, _Nephrolepis exaltata_, _C. reinhardtii_, etc.; thus, it appears to be ubiquitous in the plant kingdom [99]. The electron transfer rate from Asc to PSII depends on the species and their physiological state, which is most probably related to the availability of Asc in the lumen (probably in the range of a few mM, [102]).  
Isolated photosynthetic samples with inactivated OECs are extremely susceptible to illumination. The impaired electron donation from the OEC results in the accumulation of highly oxidizing radicals, including P680\({}^{+}\), Tyrz\({}^{+}\), and superoxide and hydroxyl radicals [103; 104], leading to a rapid inactivation and degradation of PSII reaction centers [105; 106]. This type of photodamage is called weak light or donor-side-induced photoinhibition. By using intact leaves of wild-type, Asc-overproducing (_miox4_) [107], and Asc-deficient Arabidopsis mutants (_vtc2-3_) [6] subjected to heat stress (40 \({}^{\circ}\)C, 15 min), it was demonstrated that the continuous electron flow from Asc to PSII alleviates PSII photoinactivation [78]. Gradual inactivation of PSII charge separation activity occurred on a time scale of tens of minutes, along with extensive protein degradation, including probably the complete disassembly of PSII [78]. Besides the rate of photoinactivation, the recovery rate from the photoinactivated state also depended on leaf Asc content. Thereby, Asc contributes significantly to the ability of plants to withstand heat stress conditions and aids recovery [78; 108].  
Asc may also act as an alternative electron donor in bundle sheath chloroplasts. These are found in the so-called NADP\({}^{+}\) malic enzyme type species carrying out C4-photosynthesis, such as maize and sorghum. The amount of PSII in bundle sheath chloroplasts is small, and their OECs have low activity. It was shown that Asc is an effective electron donor for PSII in bundle sheath chloroplasts in vivo [109; 110]. On the other hand, since the number of PSII reaction centers is low, photosynthetic electron transport is moderate, but it is sufficient to maintain PSI cyclic electron flow, ensuring thylakoid membrane energization and ATP synthesis for the Calvin-Benson cycle [109; 110]. Moreover, the replacement of water by Asc as a PSII electron donor also ensures low O\({}_{2}\) concentration within the bundle sheath chloroplasts, diminishing the risk of competition of O\({}_{2}\) with CO\({}_{2}\) molecules for the catalytic sites of RuBisCO (reviewed by [111]).  
Asc also donates electrons to PSI in isolated thylakoid membranes (see, e.g., [112]) and in DCMU-treated bundle sheath cells isolated from maize leaves [113]. However, in vivo, Asc was a far more effective electron donor for PSII than for PSI [99; 109; 110].