Morpho-biometric and biochemical responses in lettuce seedlings treated by different application methods of Chlorella vulgaris extract: foliar spray or root drench?

Microalgae-based products applied as biostimulants have recently attracted the attention of researchers. The effect of two different methods of application of a Chlorella vulgaris extract, foliar spray and root drenching, was evaluated in lettuce seedlings by monitoring their morpho-biometric parameters and chlorophyll, carotenoid, and total protein contents. The biochemical response, through the study of enzymatic activities involved in primary and secondary metabolism, was also evaluated. Two consecutive applications, 1 week apart, of the C. vulgaris extract were carried out on the lettuce seedlings grown on an inert substrate (pumice) irrigated with Hoagland solution. Lettuce seedlings were then collected at 1, 4, and 7 days after the first treatment and at 7 days after the second treatment. Both application methods positively affected the growth of lettuce seedlings, increasing the dry matter, chlorophyll, carotenoid, and protein contents in the edible portion of the plant. From a biochemical point of view, the extract application methods influenced the primary and secondary metabolism by coordinated regulation of C and N metabolic pathways, which may represent the key point in the mechanism of action. The foliar application mostly influenced the activities of enzymes involved in nitrogen primary metabolism, whereas the root drenching application mainly affected the enzymatic activities involved in carbon primary metabolism. These results are very promising since both application methods of C. vulgaris extract acted as a biostimulant on lettuce seedlings, although their mechanism of action seems to be quite different. Graphical abstract: [Figure not available: see fulltext.].