Physiological and Biochemical Responses of Lettuce to Arbuscular Mycorrhizal Inoculation and Landoltia punctata Extract Applications

The use of biostimulants offers a sustainable strategy to improve crop quality. This study assessed the effects of an arbuscular mycorrhizal fungi inoculum (consisting of species Claroideoglomus claroideum, Claroideoglomus etunicatum, Funneliformis geosporum, Funneliformis mosseae and Rhizophagus irregularis) and an 0.5% aqueous extract of Landoltia punctata on the growth and biochemical composition of lettuce (Lactuca sativa L. cv. ‘Dubáček’) under indoor conditions. Four variants were tested: control (C), mycorrhiza (M), L. punctata extract (L), and their combination (M + L), with biometric, physiological, and biochemical parameters evaluated. Simultaneously, the amino acid profile of Landoltia extract was determined, and the degree of plant colonization by mycorrhizal fungi was evaluated. While biostimulant treatments did not affect above-ground biomass, L. punctata extract (L and M + L) significantly raised chlorophyll a (by 15.9% and 16.0%) and chlorophyll b (by 55.5% and 42.8%) compared to the control. The combined treatment (M + L) achieved the highest total phenolic content (254.28 mg/kg). All treated variants significantly reduced leaf nitrate content, with M and M + L being most effective (−35.1% and −33.6%). Amino acid metabolomic analysis showed that the extract is rich in γ-aminobutyric acid, valine, phenylalanine, tryptophan, and other proteinogenic amino acids that may drive its biostimulant effects. Microscopy confirmed successful root colonisation in mycorrhizal variants (58% in M, 42% in M + L). Although the biostimulants did not significantly affect growth, their application is recommended to improve lettuce quality by enhancing photosynthetic pigments and phenolic compounds while reducing nitrate content, indicating their potential for producing safe, higher-quality crops.