Evaluation of by-products of plant food (potato and apple) as potential biostimulants for green leafy vegetables

A large amount of organic waste is produced from agro-food processing industries. In particular, the processing waste of fruit and vegetables, could be an excellent source of bioactive compounds perceived by consumers as low health risk substances. In this work we are interested in potato peels and apple residual pulp (e.g. apple pomace: the residue after the extraction of apple juice) as natural biostimulants to improve growth and nutrient uptake of “Romaine” lettuce seedlings (Lactuca sativa L. ‘Longifolia’). Skin of organic red potatoes “Blue Salad” and residual of organic “Fuji” apple after squeezing were minced, macerated in deionized water (500 g in 1 L) for 14 and 21 days, in the dark, at room temperature (RT). The aqueous extracts were filtered and properly diluted in water (to 1 or 10 mL L−1). Lettuce seedlings were grown in a floating system under greenhouse conditions. Treatment solutions were sprayed in the morning onto leaves until run-off, every three days for 5 times. Positive effects on the biomass of lettuce seedlings were observed with application of by-products extracts. Moreover, results showed that by-product extracts of apple, potato and a mixture of them affected positively the root growth and development. Overall the data reported in this study represent an interesting and promising possibility of using waste products of apple and potato processing industry to obtain biostimulants able to support the growth of agricultural productions in greenhouse using hydroponic system.