## 3 Subcellular Influence of Light Intensity and Spectrum on the Redox System
### Light-Intensity-Dependent Subcellular Changes in ROS and Antioxidants  
The redox system in plant cell compartments reacts fast to light fluctuations, and changes in the level of various ROS may have a special regulatory function. For instance, 1 h of excess PPFD (1000 umol m\({}^{-2}\) s\({}^{-1}\)) increased H\({}_{2}\)O\({}_{2}\) in genetically modified tobacco HyPer2 (fluorescent H\({}_{2}\)O\({}_{2}\) biosensor) chloroplast stroma, cytosol, and nuclei [98]. The chloroplast-derived H\({}_{2}\)O\({}_{2}\) accumulation in nuclei may indicate a signalling function for fast transcriptional response to HL intensity [98]. Especially under sudden light intensity changes (exposure of understory plants/leaves to full sunlight), the efficiency of electron transport between the photosystems may not be adjusted sufficiently. We may assume that the subcellular accumulation and/or distribution of H\({}_{2}\)O\({}_{2}\) and other ROS differ in plants or leaves adapted to an environment with sudden light intensity changes according to specific coping mechanisms. However, at the cell compartment level, data addressing changes in H\({}_{2}\)O\({}_{2}\) as a consequence of high PPFD are, to date, mainly available from model plants such as tobacco [98] and _Arabidopsis_[99]. Depending on PPFD intensity and length of exposure, [PERSON] et al. [99] showed that in _Arabidopsis_, excess H\({}_{2}\)O\({}_{2}\) from overstrained electron transport chains in chloroplasts and from photorespiration in peroxisomes subsequently accumulates in the cytosol and later in the vacuole. Similarly, a transient increase in H\({}_{2}\)O\({}_{2}\) (detected via roGFP2-Orp1) and glutathione redox potential (\(E_{\text{GSH}}\)) (detected via Grx1-roGFP2) upon photo-oxidative stress was detected in chloroplasts, followed by the cytosol and mitochondria in genetically modified _Arabidopsis_[100, 101]. We may expect a subsequent increase in H\({}_{2}\)O\({}_{2}\) in vacuoles, which, however, requires the development of a vacuole-targeted biosensor line. Further, \({}^{1}\)O\({}_{2}\) accumulates in chloroplasts when electron transport is inefficient under high PPFD stress, which leads to photo-inhibition by irreversible damage to photosystem II. The subcellular redox system is particularly strained under a combination of stresses, such as high PPFD and osmotic stress. Closure of the stomata (to prevent water loss) limits the availability of the electron acceptors ADP and NADP and the regeneration of 3-phosphoglycerate in chloroplasts. To prevent electron transfer overload and ROS formation close to thylakoid membranes, ADP and NADP (and 3-phosphoglycerate for primary metabolism) are provided by photorespiration. A side reaction of photorespiration, however, increases the H\({}_{2}\)O\({}_{2}\) concentration in peroxisomes. Limited ADP availability likewise contributes to an above-normal ROS (initially O\({}^{\bullet-}\), followed by H\({}_{2}\)O\({}_{2}\)) accumulation in mitochondria (Figure 3) [101, 102, 103, 104]. Under high PPFD stress, H\({}_{2}\)O\({}_{2}\) accumulation in chloroplasts can be scavenged by AsA, GSH, and APX activity, in the cytosol and peroxisomes by AsA, GSH, APX, and CAT activities, and in vacuoles by AsA and POD (EC 1.11.1.7) activities (Figure 3) [99, 105, 106, 107]. AsA concentration rises with increasing light intensity, and AsA recycling has been localized in peroxisomes, mitochondria, and the cytosol [108]. Overexpression or silencing of MDHR3 (monodehydroacerbate reductase 3, one of the iso-enzymes which catalyse the regeneration of AsA via oxidation of GSH) in tomatoes resulted in lower or higher AsA concentrations, respectively. Fluorescent targeting with GFP and mCherry suggests a dual localisation of MDHR3 in peroxisomes and the cytosol [109]. Darkness and light strongly influence the subcellular glutathione level, as shown by its diurnal rhythm in _Arabidopsis_ mesophyll cells, reaching a maximum after 3 h of light period and a minimum at the end of the night [110].