## 2 The Effect of Light Intensity and Spectrum on ROS and Antioxidants in Plant Tissues
### Light Quality-Dependent Alterations in ROS and Antioxidant Levels  
Similar to light intensity, light quality may also affect ROS production through the specific effect of BL, RL, and FRL on ROS and antioxidants, as shown in several plant species (Figure 2). In leaves of _Campotheca acuminata_ seedlings, BL and yellow light, for instance, increased H\({}_{2}\)O\({}_{2}\), O\({}_{2}^{\bullet-}\) and malondialdehydedehyde (MDA) accumulation compared to white light, whereas RL decreased their amount [76], similarly to ramie (_Boelmeria nivex_ L.) leaves [77]. Conversely, in Chinese cabbage (_Brassica campestris_ L. ssp. _Pekinensis Rupr_. cv. [PERSON]), BL exposure lowered H\({}_{2}\)O\({}_{2}\) accumulation drastically compared to white light (WL) [78]. In addition, RL enhanced H\({}_{2}\)O\({}_{2}\) production and cell death in _Arabidopsis_, and the effect was dependent on HY5-mediated light signalling [79]. These results indicate that the spectral components affect various ROS species in individual plant species differently. Therefore, general models of the regulatory mechanisms should be created very carefully. The light-responsive HY5 transcription factor (see introduction) may have a major role in this process through its many target genes. BL is more efficient than RL in the induction of its accumulation [80]. HY5 can mediate the effect of RL and BL on ROS (H\({}_{2}\)O\({}_{2}\) or O\({}_{2}^{\bullet-}\)) through Phys and Crys (Figure 2) [81; 82]. Furthermore, Crys indirectly participate in several ROS signalling pathways, and there are overlaps between ROS- and Cry-regulated genes [83]. During \({}^{1}\)O\({}_{2}\) mediated cell death in the _Arabidopsis flu_ mutant (which accumulates protokhorophyllide under dark, then produces \({}^{1}\)O\({}_{2}\) under dark to light transition), Cry signalling and BL exposure were needed to trigger programmed cell death (PCD) after 1 h WL illumination [83]. Light quality-dependent changes in the amount of ROS also modify antioxidant levels (Figure 2). Accordingly, under low red:blue light ratio (1:3) lettuce (_Lactuca sativa_ L.), plants accumulated AsA [84]. In addition, in buckwheat (_Fagopyrum esculent_ L.), BL increased the amount of total phenolics and flavonoids and the activity of antioxidant enzymes compared to RL [85]. Similar differences were found between BL and RL regarding their effects on the activities of SOD and POD and the amount of proline in ramie [77]. These observations indicate that BL seems to be more effective than RL in the induction of the antioxidant system, but this trend should be confirmed in further plant species. The ratio of blue, red, and FR regions is very important in the adjustment of ROS and antioxidant levels to changes in the environmental conditions (Figure 2). For example, a low R:FR ratio increased H\({}_{2}\)O\({}_{2}\) production and MDA accumulation in tomatoes under control conditions, while under salt stress, it acted oppositely on ROS formation [86]. The regulation of ROS levels by the alteration in the R:FR ratio was also confirmed during low-temperature stress in _Arabidopsis_[87], wheat, and barley [88]. Light spectrum-dependent alterations in ROS content under changing environmental conditions result in an appropriate adjustment of the antioxidant levels, which improves stress tolerance (Figure 2). In this respect, the low R:FR ratio reduced the salinity-caused damage by the induction of SOD, POD, CAT, and APX at gene expression and enzyme activity levels in tomatoes [89; 90]. The spectral control of the antioxidant-dependent adaptive responses was also demonstrated in wheat [91]. Many-times lower cystine (CySS) and glutathione disulphide (GSSG) contents, as well as CySS/Cys and GSSG/GSH ratios, were detected during drought in the seedlings of the cold-tolerant wheat genotypes grown in FRL compared to those cultivated in WL, BL, or pink light. These parameters indicate that the redox environment was shifted into more reducing directions, which can ensure the appropriate function of many proteins.