Comparison of five agro-industrial waste-based composts as growing media for lettuce: Effect on yield, phenolic compounds and vitamin C

• Compost is a good substitute for non-renewable resource in organic lettuce production.
• A good compromise for lettuce yield and antioxidants can be attained with composts.
• Chestnut wastes showed a greater reuse potential in improving yield and antioxidants.
• The white grape compost revealed a possible trade-off between yield and antioxidants.
• All five composts tested led to decreased vitamin C content in lettuce.

Overall phenolic content in plants is on average higher in organic farming, including when renewable resources such as composts are used as soil amendments. In most cases, however, the composting process needs to be optimized to reach the desired outcome. Using composts obtained from chestnut, red and white grapes, olive and broccoli wastes, the relative antioxidative abilities of lettuces cultivated in greenhouse were examined. Results clearly coupled high phenolic levels with high yield in lettuce grown on the chestnut-based compost. A huge accumulation of phenolics was observed with the white grape-based compost, but this coincided with low yield. Three compounds were identified as discriminating factors between treated samples, namely quercetin 3-O-glucoside, luteolin 7-O-glucoside, and cyanidin 3-O-(6″-malonyl)-β-d-glucoside; these are also some of the compounds receiving health claims on lettuce consumption. On a negative note, all composts led to decreased vitamin C levels. Collectively, the data suggest that compost amendments can help add value to lettuce by increasing its antioxidant activity as compared to other organic resources.