| Summary: | Lettuce seedlings often require the use of fertilizers for their cultivation management to achieve appropriate yield. However, for eco-sustainable chemical-fertilizers-free agronomy, the implementation of totally organic farming often cannot support lettuce productivity, therefore new natural biostimulants able to increase lettuce yield could be considered of great interest. In this preliminary work, the foliar spray application of a <i>Chlorella vulgaris</i> extract in lettuce seedlings was investigated in order to achieve better yield performance. Its biostimulant effect was evaluated by monitoring the morphobiometric parameters, chlorophylls, carotenoids, total protein contents, and several enzymatic activities involved in primary and secondary metabolisms of the plant. The experimental trials were carried out by growing lettuce seedlings on inert substrate (pumice) with a 16 h photoperiod for 21 days. The treatment consisted of three consecutive applications by foliar spraying using a concentration of the <i>C. vulgaris</i> extract, corresponding to 1 mg C<sub>org</sub> L<sup>−1</sup>, which were performed one week apart. The results showed that the <i>C. vulgaris</i> extract positively influenced the growth of lettuce seedlings, by increasing the fresh and dry weights, chlorophylls, carotenoids, protein content, and ashes at shoot level. From a biochemical point of view, primary and secondary metabolisms of shoots, in particular nitrogen metabolism, were positively influenced. At the root level, the extract increased dry matter, proteins, and ash content.
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