Biostimulants Enhance the Growth and Nutritional Quality of Lettuce (Lactuca sativa L.)

Biostimulants have emerged as effective tools for enhancing both the productivity and quality of crops. In this study, we assessed the impact of the two commercial biostimulant products (Kiana Earth® and Kiana Climate®) on the growth, yield, and quality of lettuce (Lactuca sativa L.). Eight treatments were established, comprising six different biostimulant formulations, a normal control (no fertilizer applied), and a positive control (chemical fertilizer application). Biostimulant treatments significantly improved plant and stem diameters, fresh and dry biomass, and yield (p < 0.01). The best yields and morphological performance were obtained with samples receiving T6 (Kiana Climate® + 75:50:75 kg ha−1 N:P:K) and T7 (Kiana Earth® + 150:100:150 kg ha−1 N:P:K) applications, which comprised biostimulant–fertilizer combinations. Chlorophyll a, chlorophyll b, and total chlorophyll levels were significantly higher with than without biostimulant treatment, indicating that the biostimulants enhanced photosynthetic efficiency. Biochemical analyses further identified significant increases in vitamin C levels, total antioxidant capacity, total phenolic compounds, and flavonoid contents, especially with treatments T5 (Kiana Earth® + 75:50:75 kg ha−1 N:P:K)–T8 (Kiana Climate® + 150:100:150 kg ha−1 N:P:K). Nitrogen assimilation analysis showed that leaf NO3− levels were lower with the combined treatment than with chemical fertilizer alone, suggesting that the biostimulants improved nitrogen-use efficiency. Micronutrient (Fe, Zn, Cu, Mn, Na) and macronutrient (N, P, K, Ca, Mg, S) levels were significantly increased with biostimulant-enriched treatments, alongside a rise in soil organic matter. Biostimulants, especially when combined with mineral fertilization, significantly enhanced lettuce growth, yield, and nutritional quality, while also promoting soil fertility. These findings highlight the potential of biostimulants as valuable tools in conventional, regenerative, and organic agricultural practices, offering a sustainable approach to enhancing agricultural productivity while ensuring long-term soil fertility.