Enhancement of plant photosynthesis by seaweed extracts under optimal and abiotic stress conditions: physiological and molecular mechanisms

Global climate change is intensifying abiotic stress in crops, posing a serious threat to agricultural productivity and global food security. In this context, seaweed extracts (SWEs) have emerged as key sustainable biostimulants to optimize plant performance. This review critically examines recent scientific evidence on the physiological, biochemical, and molecular mechanisms by which SWEs enhance photosynthesis under both optimal growth conditions and environmental stress scenarios (drought, salinity, and extreme temperatures). The literature shows that, under favorable conditions, SWEs increase pigment biosynthesis and improve gas exchange parameters. Under abiotic stress, these extracts mitigate damage to the photosynthetic system by regulating stomatal conductance, protecting RuBisCO activity, and enhancing cellular antioxidant systems. At the molecular level, evidence increasingly supports these responses are explained by physiological priming, whereby polysaccharide-derived oligosaccharides act as elicitors that trigger early reactive oxygen species (ROS) bursts, Ca² + fluxes, and protein-kinase signaling. These early events initiate transcriptional responses integrating hormonal and other stress-signaling pathways that ultimately protect pigments, thylakoid membranes, and carbon assimilation. Finally, current knowledge gaps are discussed, highlighting that the integration of ROS/redox, Ca² + , protein-kinase, hormonal, and chloroplast retrograde signals into SWE-induced transcriptional responses remains poorly understood. Time-resolved studies combining pathway-specific perturbations with transcriptional and physiological measurements will be essential to establish how these signals converge to protect photosynthesis under abiotic stress.