## 2 The Effect of Light Intensity and Spectrum on ROS and Antioxidants in Plant Tissues  
### Light Intensity-Dependent Changes in ROS and Antioxidant Levels  
Changes in light intensity affect the redox environment through their effect on ROS formation and antioxidant activity (Figure 1a) [22]. While under natural conditions, the light intensity gradually increases and decreases during sunrise and sunset, respectively, in most experiments, researchers apply sudden strong changes in this parameter. Thus, the physiological and biochemical changes observed in the latter case should be interpreted carefully with respect to their role under field conditions. Interestingly, even the lack of light (2 days) can lead to ROS accumulation, as was observed in leaves of _Arabidopsis_[51]. This change was the result of a decrease in the amount of glutathione (GSH) and in the activity of glutathione reductase (GR), dehydroacorbate reductase (DHAR), and CAT. However, prolonged darkness for 4 days increased the activity of SOD, CAT, and APX enzymes in the leaves of _Pelargonium zonale_ L. [52]. These results indicate that the length of the dark period strongly affects dark-induced changes in ROS and antioxidant levels. While, in leaves, the lack of light alters the redox environment in the tissues, in roots, even the very low light intensity reaching them through fractured soil layers may result in enhanced ROS accumulation (Figure 1a). This promotes root growth in the short term as a light-escape mechanism. Upon prolonged illumination, however, ROS accumulation may cause oxidative damage and reduce root length. Light-grown roots accumulated more H\({}_{2}\)O\({}_{2}\), but not O\({}_{2}\)*-, a process that was regulated by PhyB-induced ABA signals of the shoot in _Arabidopsis_, indicating a coordinated control of ROS content in the whole plant [53]. This observation indicates that the exposure of individual plant organs to altered light intensity may also influence the ROS and antioxidant levels in other organs; therefore, the redox environment should be monitored in all organs. Exposure to light in the case of roots and darkness in the case of shoots affects the redox environment in plants. Besides that, it is likewise strongly affected by changes in light intensity. A few hours (3-6) of exposure to high light intensity (HL) increased O\({}_{2}\)*- accumulation and stress symptoms in _Arabidopsis_[54] and pea [55] leaves, while prolonged HL stress had a similar effect in wheat (Figure 1b) [56]. The changes in ROS levels occur immediately after the exposure to HL and provide a signalling function in order to initiate various adaptive mechanisms, as demonstrated in _Arabidopsis_[57]. Partial very short (30 min) HL illumination of _Arabidopsis_ leaves exposed to low light intensity (LL) can trigger H\({}_{2}\)O\({}_{2}\) accumulation and systemic acquired acclimation (SAA) in non-illuminated leaves as well (Figure 1b) [57; 58; 59]. H\({}_{2}\)O\({}_{2}\) proved to be a systemic signal in tomato leaves, too [60]. The detected fast systemic response was termed an 'ROS wave'. ROS waves were observed during several types of abiotic and biotic stresses, and this signal may thus have an important role in the stress adaptation process. Although H\({}_{2}\)O\({}_{2}\) has a major role in signalling [57; 61; 62], \({}^{1}\)O\({}_{2-}\) and O\({}_{2}\)*- responsive transcripts were also upregulated in systemic tissues of _Arabidopsis_ after HL treatment [61; 63]. This finding indicates that the various ROS participate simultaneously in the control of HL-inducible genes. In addition, RBHOD (O\({}_{2}\)*- production) and the redox state of the plastoquinone pool in chloroplasts were reported to mediate SAA during HL stress [57]. ROS signals may control the response to HL together with other regulatory systems [59]. According to this assumption, ROS waves depend on phyB [64] and ABA [65], and they are mediated by vascular bundles in _Arabidopsis_[66].