## 2 The Effect of Light Intensity and Spectrum on ROS and Antioxidants in Plant Tissues
### Light Intensity-Dependent Changes in ROS and Antioxidant Levels
Interestingly, high air humidity (85%) and heat stress mitigate systemic and local H\({}_{2}\)O\({}_{2}\) accumulation under HL [65; 66], which indicates their activating effects on the antioxidant system. However, the activation of antioxidants by light alone was also reported in several plant species, including the model plant _Arabidopsis_ and the crop species wheat (Figure 1b) [55; 67]. This induction was due to the transcriptional regulation of GSH metabolism-related enzymes at HL in both species [62; 67]. LL increased, in turn, the activities of SOD, POD, APX, and CAT in soybean leaves (_Glycine max_ L.) [68]. Based on these works, we may conclude that different ROS, in an interaction with other regulatory compounds, participate in the light intensity-dependent adjustment of the antioxidant levels. The specific role of the various ROS and antioxidants in this light (and stress) adaptation process should be clarified in further studies. The light intensity-dependent adjustment of ROS and antioxidant levels is important in the reduction in stress-induced damages. Light promoted H\({}_{2}\)O\({}_{2}\) accumulation and disease resistance in tobacco leaves during _Pseudomonas_ infection, compared to dark conditions [69] and elicitor-induced systemic O\({}_{2}\)\({}^{\bullet-}\) accumulation, occurred also only in  
Figure 1: Effect of light intensity on reactive oxygen species and antioxidants. (**a**) The presence or absence of light in shoots and roots, respectively, greatly affects reactive oxygen species (ROS) accumulation and antioxidant levels under various environmental conditions. (**b**) Rapid alterations in light intensity (3–6 h) trigger a systemic ROS response (ROS wave) mediated by H\({}_{2}\)O\({}_{2}\) and respiratory burst oxidase homolog protein D (RBOHD). Long-term response (1–7 days) of ROS homeostasis to low light or high light is also light intensity-dependent, but it is also influenced by the other environmental conditions (temperature, water availability). SOD: superoxide dismutase; CAT: catalase; GR: glutathione reductase; GSH: reduced glutathione; APX: ascorbate peroxidase; PhyB: phytochrome B; PPFD: photosynthetic photon flux density; SA: salicylic acid; ABA: abscisic acid; PQ pool: plastoquinone pool; RH: relative humidity; POD: guaiacol peroxidase; \(\gamma\)ECS: \(\gamma\)-glutamylcysteine synthase; GST: glutathione-S-transferase; AsA: ascorbic acid. light (Figure 1a) [70]. These observations indicate that the various ROS have specific roles during the light-dependent protective processes against different biotic stresses. Although, under biotic stresses, the light-induced accumulation of ROS can be useful for killing the pathogens, the reduction in their amount would be more favourable during abiotic stresses to avoid tissue damage, as observed in _Arabidopsis_ plants under heat stress [71]. The excess ROS can be removed by the efficient function of antioxidants. Pre-treatment of wheat seedlings by HL before drought resulted in an increased GSH content, which was due to the induction of _\(\gamma\)-glutamyl-cysteine synthase (\(\gamma\)ECS)_ and _GSH synthase 2_ gene expressions [61; 72]. Such an effect was also shown for genes encoding various antioxidant enzymes (APX, GR, and GST). Similarly, there was a positive correlation between the activation of APX, SOD, GSH, and ascorbic acid (AsA) synthesis by HL and drought response in cashew (_Anocardium occidentale_ L.) [73]. However, under extreme abiotic stress conditions, the antioxidant level can be higher due to the impairment of the antioxidant system, as shown in Cd-treated _Arabidopsis_ cell culture [74] and cold-treated _Arabidopsis_ leaves [75]. These results demonstrate that under controlled conditions (cultivation of horticultural plants in glasshouses), the proper adjustment of light intensity may improve stress tolerance.