Plant extract improves quality traits of green and red lettuce cultivars

The use of biostimulants in agriculture has been rapidly increasing in recent years, because of their positive effects on crop yield, product quality, and tolerance to abiotic stresses. In the present study the efficacy of multiple applications of a plant-derived biostimulant, obtained from the aqueous maceration of borage (Borago officinalis, L.) flowers on two lettuce cultivars, namely a green (Lactuca sativa L. cv. Expertise RZ) and red (Lactuca sativa L. cv. Codex RZ) Salanova® was evaluated. The treatment was applied at 10 mL L^-1 as foliar spray three times, once a week starting from two weeks after transplant. Control plants were treated with water. Non-destructive measurements (pigments, leaf nitrogen index, chlorophyll a fluorescence) were taken during plant growth after at each treatment application. At the end of the experiment, destructive analyses were performed to assess qualitative traits. The research work was focused on the evaluation of physiological parameter changes during plant growth, and on primary and secondary metabolism. Foliar applications did not affect the accumulation of total sugars (4.56 mg g^-1 in Expertise, 3.5 mg g^-1 in Codex) in either cultivar. However, the lettuce head weight was negatively affected by the extract application in red cultivar (-10 g/plant), while no changes were observed in the green lettuce. The nitrogen-flavonol index (NFI) increased after the third application of borage extract in green cultivar (+67 %), suggesting an improvement of nitrogen nutrition status or a reduced stress condition. A different response resulted in term of maximum quantum efficiency of PSII (F_V/F_M), performance index (PI), nitrate, and anthocyanin accumulation in leaves. The F_V/F_M ratio significantly increased in green cultivar after the first application (from 0.80 to 0.84) and at harvest (from 0.79 to 0.84). The PI showed a slight but not significant increase at the same time points. On the contrary, the PI was significantly higher in red cultivar after the third application (+9.4 %). Interestingly, the borage extract induced a significant decrease of nitrate accumulation in lettuce leaves of the red cultivar (from 4149.7 to 2711.6 mg/kg, -34 %). At the same time a positive variation of anthocyanin content was observed in red lettuce (+24.7 %). The application of biostimulant products might improve the quality of some lettuce varieties as regards the accumulation of metabolites useful for the plant to overcome stress conditions and fundamental in human healthy diet, increasing the leaf concentration of Ca, Na, and Mg.