Biofertilizers, such as arbuscular mycorrhiza fungi (AMF) and seaweed extract (SWE), have been effective in environmental and agricultural ecosystems. In this study, the effects of AMF, SWE, and their co-application were assayed on the growth and antioxidant potential of lettuce plants. The experiment was conducted as a factorial based on a completely randomized design with two factors and four replications under greenhouse conditions. The first factor was AMF (Glomus mosseae) at two levels consisting of AMF application (20 g pot⁻¹), and without using AMF; and the second factor was SWE foliar spraying (Ascophyllum nodosum) at 0.5, 1.5 and 3 g L⁻¹ concentration. The results revealed that the highest root colonization (85%) belonged to AMF and SWE (3 g L⁻¹) × AMF; the lowest colonization rate (65%) was observed for AMF × SWE (0.5 g L⁻¹) treatment. The highest growth parameters (leaf number, shoot and root fresh weight, head diameter), biochemical traits (total soluble proteins, carbohydrates content) and TAA, total antioxidant activity by FRAP method and ascorbic acid, total phenolics, and flavonoids content were obtained with the co-applications. Therefore, the best results of the evaluated traits were achieved with AMF × SWE (3 g L⁻¹). The TAA value was increased three-fold compared to the control. Total phenolics and flavonoids content were 2.24 and 6.59 times higher than the control, respectively. On the other hand, leaf dry weight was decreased with the further growth of the plants. Overall, the co-application of AMF with SWE can be recommended to producers as an alternative and environment-friendly strategy to improve the qualitative and quantitative traits of the lettuce crop.

Beneficial plant–microbe interaction for enhancing crop yield and quality is a sustainable way to achieve eco-friendly, desirable agricultural productions. The main objective of this experiment was to evaluate the individual and combined effects of an arbuscular mycorrhizal fungus (AMF) strain (Funneliformis mosseae) and a seaweed extract (SWE) derived from Ascophyllum nodosum, on the growth and physiological responses of lettuce (Lactuca sativa L.). Lettuce plants were inoculated with commercial AMF inoculum (5 g kg⁻¹ soil), and SWE foliar application was done at three levels (0.5, 1.5, and 3 g L⁻¹). The findings revealed that AMF along with SWE generated the greatest impact. In fact, co-application of AMF inoculation and 3 g L⁻¹ SWE considerably enhanced root colonization, chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, and mineral content in the shoots and roots (N, P, K, Ca, Fe, Zn, and Mn content) of lettuce plants. This combination improved initial fluorescence (F₀), photochemical efficiency of PSII (F_V/F_m) and Y(NO) and total antioxidant activity (TAA), whereas the maximum fluorescence, (F_m) and Y(II), showed the highest increase in lettuce plants treated with AMF and 1.5 g L⁻¹ SWE. Furthermore, AMF inoculation along with SWE, at concentrations 1.5 and 3 g L⁻¹, considerably enhanced variable fluorescence (F_V) and the activity of water decomposition in electron donor photosystem II (F_V/F₀). As a result of these findings, it can be stated that the co-application of AMF and SWE positively improves the growth and development of lettuce plants.