# The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae
## 6 The Role of Ascorbate in Non-Photochemical Quenching  
Excess light may lead to photooxidative stress involving the formation of ROS and light damage (recently reviewed by, e.g., [128; 129; 130]). Non-photochemical quenching (NPQ) of excitation energy is a complex photoproduction mechanism, including energy-dependent (qE), zeaxanthin-related (qZ), state-transition-related (qT), and photoinhibitory (qI) components (e.g., [131; 132]). The pH-regulated qE component is formed in response to light-intensity changes within a few minutes. In vascular plants, activation of qE is mediated by PsbS that acts as a lumen pH sensor and induces LHCII antenna rearrangement in a zeaxanthin-dependent manner [133; 134]. The qZ component of NPQ is activated in the time range of 10 to 30 min, which correlates with the formation of zeaxanthin by violaxanthin de-epoxidase (VDE) in the lipid phase of the thylakoid membrane of vascular plants [135; 136; 137]. VDE belongs to the lipocalin protein family, and it utilizes Asc as a co-substrate (Figure 1), providing the reducing power for de-epoxidation [138]. At the pH-optimum of VDE of pH 5.0, the Km value for Asc is approximately 1.0 mM [138; 139], which is probably in the range of luminal Asc concentration [102]. During the operation of the violaxanthin cycle, VDE attaches to the luminal side of the thylakoid membrane following its pH-dependent activation [140; 141; 142]. The active VDE is probably a dimer, capable of binding violaxanthin and Asc [143; 144]. The maximum VDE activity is several times faster than zeaxanthin epoxidase activity [144]. Consequently, zeaxanthin accumulates in strong light when a low lumen pH is established. In contrast, zeaxanthin is epoxidized only when VDE activity is low, i.e., under low light or in darkness. It was recently demonstrated that up-regulation of VDE, Psb5, and zeaxanthin epoxidase in soybean significantly accelerated the violaxanthin cycle, leading to faster induction and relaxation of NPQ. This increased the efficiency of CO\({}_{2}\) assimilation and PSII electron transport in fluctuating light conditions and significantly improved the biomass yield in field studies, demonstrating that the violaxanthin cycle plays a crucial role in the regulation of photosynthesis, thereby relating to plant productivity [145]. Besides playing a role in NPQ, zeaxanthin may also contribute to photoproduction by acting as an antioxidant and possibly by modulating thylakoid membrane properties [146; 147]. In addition, zeaxanthin may be required for the PSII repair cycle [148]. The physiological importance of Asc for zeaxanthin formation was demonstrated in vivo using _vtc_ mutants of Arabidopsis: it was shown that they accumulate less zeaxanthin in high light, and consequently, they have diminished and/or delayed NPQ induction [6; 45; 78]. In the _miox4_ Asc-overproducing mutant [107], a slightly higher NPQ level was obtained than in the wild type [78], demonstrating that Asc may be limiting NPQ formation [44]. In contrast to the _vtc2_ mutant of Arabidopsis, the _C. reinhardtii vtc2_ mutant (_Crvtc2-I_) with strongly decreased Asc content performs normal violaxanthin de-epoxidation [75]. _C. reinhardtii_ lacks a plant-type VDE and instead uses an unrelated enzyme belonging to lycopene cyclases, called _Chlorophycan_ VDE (CVDE). It is located on the stromal side of the thylakoid membrane [149], and it does not require Asc for violaxanthin de-epoxidation [75]. On the other hand, a slow, H\({}_{2}\)O\({}_{2}\)-dependent NPQ component (probably qI) is enhanced upon Asc-deficiency in _C. reinhardtii_[75]. Many green alga species, including _Chlorella vulgaris_, contain plant-type VDEs [150], which are crucial for photoprotective NPQ. Thus, there is an evolutionary divergence of photoprotective mechanisms among _Chlorophyta_[151]. _Chromalveolate_ algae, such as _Phueodactylum tricontum_, use the diadinoxanthin cycle instead of the violaxanthin cycle in a fashion similar to vascular plants to support qE [152].