## 3 Results and Discussion
### Effect of Growing Medium on the Quality of Lettuce
#### 3.3.2 Effect of Growing Medium on the Leaf Antioxidant Compounds and Antioxidant Activity
IMP \(\times\) TC showed the highest polyphenol content while VDI \(\times\) TC was the lowest (551.1 vs. 367.1 mg 100 g\({}^{-1}\) f.w.). Leaf polyphenols of IMP \(\times\) C1-60 and IMP \(\times\) C2-60 were lower than those of IMP \(\times\) TC; instead, the content of VDI \(\times\) C1-60 and VDI \(\times\) C2-60 were higher than VDI \(\times\) TC. VDI \(\times\) C2-30 e VDI \(\times\) C3-60 did not differ from VDI \(\times\) TC (Figure 5a). Polyphenols, a family of naturally occurring organic substances comprising a large number of highly bioactive polyfunctional compounds, are involved in the response to abiotic and biotic stresses and are induced by genetic, environmental, and eliciting factors. The increased polyphenol content could be linked to the activation of phenylalanine ammonia-lyase (PAL), the regulatory enzyme of phenylpropanol metabolism [86]. Also with regard to flavonoid content, IMP \(\times\) TC showed the highest average value (331.9 mg 100 g\({}^{-1}\) f.w.) while _VDI \(\times\) TC_ was the lowest (209.5 mg 100 g\({}^{-1}\) f.w.). The flavonoid content of IMP \(\times\) C1-60 (261.4 mg 100 g\({}^{-1}\) f.w.) was lower than that of IMP \(\times\) TC; VDI \(\times\) C2-30 (255.1 mg 100 g\({}^{-1}\) f.w. ), which did not differ from VDI \(\times\) TC (Figure 5b). Figure 5: Effect of the cultivar (C) and growing medium (GM) interaction on the leaf polyphenols (significant at \(p<0.001\)) (**a**) and flavonoids (significant at \(p<0.001\)) (**b**) at harvesting. Vertical bars represent the standard deviation of mean values. Different letters mean that the values are significantly different according to [PERSON]’s test at \(p\leq 0.05\). GAE: Gallic acid Equivalent; f.w. : fresh weight; CE: Catechine Equivalent. IMP: _Imperiale_ lettuce cv; VDI: _Verde d’Inverno_ lettuce cv; TC: Peat-based substrate; C1-60: 60% Composst 1 + 40% Peat; C1-30: 30% Composst 1 + 70% Peat; C2-60: 60% Composst 2 + 40% Peat; C2-30: 30% Composst 2 + 70% Peat; C3-30: 30% Composst 3 + 70% Peat; C3-60: 60% Composst 3 + 40% Peat. The average values of total flavonoid content found in this study were in the range of 272.2.6 to 331.9 mg 100 g\({}^{-1}\) f.w. These data are in agreement with the results of [PERSON] et al. [73] who reported baby leaf mean values of about 2300 to 2400 mg kg\({}^{-1}\) f.w. Flavonoids constitute a large and diverse group of phenolic compounds ubiquitous in higher plants in which they fulfill many functions. Flavonoids are involved in mechanisms of response to biotic and abiotic stresses, color and chemical signaling, and other auxiliary roles. They constitute the first line of defense against photodamage by UV [87]. As well as polyphenols, we observed a rise of flavonoid content in VDI lettuce when grown in medium containing compost that presumably led to cultivar-specific activation of a relative biosynthetic pathway. With regards to the antioxidant activity, the highest value was found in IMP \(\times\) C3-60 (2769.3 \(\upmu\)mol TE 100 g\({}^{-1}\) f.w.) and the lowest in VDI \(\times\) TC (1548.7 \(\upmu\)mol TE 100 g\({}^{-1}\) f.w.). The antioxidant activity of IMP \(\times\) C1-60 and IMP \(\times\) C1-30 was lower than those of IMP \(\times\) TC; VDI \(\times\) C2-60 and VDI \(\times\) C2-30 did not differ from VDI \(\times\) TC (Figure 6). Leaf antioxidant activity found in this study was in the range from 1548.7 to 2769.3 \(\upmu\)mol TE 100 g\({}^{-1}\) f.w. (from 32 to 69 expressed as % of inhibition). These data agree with the results of [PERSON] et al. [47], who reported in 11 baby leaf lettuce varieties mean values from 32.61 to 87.02% of inhibition. Similar results were found by [PERSON] et al. [78], who found in baby leaf lettuce (cv Zarina) an antioxidant activity of 26.2 mmol TE 100 g\({}^{-1}\) d.w. Indeed, our results were in the range from 10.3 to 18.5 expressed as mmol TE 100 g\({}^{-1}\) d.w., taking into account that the average dry matter (d.w.) value found in our samples was about 15%. Antioxidant capacity is related to the phenolic compound content.