## 3 Results and Discussion
### Principal Component Analysis  
The data obtained from measurements of plant growth components and biochemical analyses were statistically analyzed using the principal component analysis (PCA) method for each cultivar. For the IMP cultivar (butterhead lettuce), the outcome of the analysis is given in Figure 7 (Figure 7a represents the scores of the samples grown on different growing media on the first two principal components; Figure 7b shows the vector of each variable on the first two principal components). The two first principal components represented 65.1% of the variance. The PC1 axis, which explained the 38.7% of the variability, was positively correlated with all growth parameters, which was correlated negatively with polyphenols, flavonoids, antioxidant activity, and only slightly with ascorbic acid. The PC2 axis, which explained the 26.4% of the variability, instead was strongly correlated with chlorophylls a and b, carotenoids, and in part with the other biochemical parameters except the antioxidant activity, while the correlation with morphological parameters was almost always low. A clear separation of individuals was observed between growing media with compost and those without compost. Samples grown on growing medium without compost, in fact, evidenced higher content of biochemicals, above all of the polyphenols, flavonoids, ascorbic acid, and in some cases of chlorophyll b, while samples grown on substrates with compost evidenced higher values of morphological parameters. It is to be noted, furthermore that C1 and C2 at dose 60 had the higher values of grown components.  
Figure 8 shows the PCA outcome of the cultivar VDI (romaine lettuce). The two first principal components, PC1 and PC2, represented 57.9% of the variance. The PC1 axis, which explained the 32.4% of the variability, was correlated positively with growth parameters, and slightly with chlorophyll a, and the chlorophyll a/chlorophyll b ratio, while it was correlated negatively with polyphenols, flavonoids, antioxidant activity, ascorbic acid, and chlorophyll b. The PC2 axis, which explained the 25.5% of the variability, instead was correlated negatively with chlorophylls a, carotenoids, and total chlorophyll. For the VDI, a clear separation of individuals was observed between samples grown on growing media with compost and samples grown on growing medium without compost. Samples grown on growing media with compost, except for C2-30, however, contrary to the cultivar IMP evidenced higher content of some bio-chemicals, above all polyphenols, flavonoids, ascorbic acid, antioxidant activity, while samples grown on substrates without compost stood out for the higher content of chlorophyll a and the higher chlorophyll a/chlorophyll b ratio. The C2 at dose 30 differed from the other growing media with compost because had higher values of grown components.  
These results indicate that cardoon-based compost allows for comparable or even better plant growth than that observed for peat-based growing media and supports the importance of the cultivar factor as a key tool for obtaining healthy food crops [73; 88; 89].  
To this end, according to [PERSON] [88], composting could become an important component of strategies to maximize the levels of bioactive molecules in lettuce.