Beneficial effects of an Ascophyllum nodosum extract on tomato (Solanum lycopersicum L.) during water stress

Abiotic stress, especially drought, is one of the principal constraints of crop productivity and is expected to worsen with climate change in the near future. This warrants the development of new technologies to decrease the impact of drought on crop productivity. Biostimulants made from seaweed extracts are one of the relatively new biological based inputs that can help mitigate the negative effects of abiotic stress on plants by enhancing growth, development, and stress tolerance. The current study evaluated the impact of Ascophyllum nodosum extract (SWE) on tomato plants grown under different levels of water stress. Greenhouse studies were conducted to assess the effect of foliar spray (0.5%) of the extract on stomatal conductance, photosynthesis, chlorophyll content, and biomass, for plants grown at moisture levels of 100%, 50%, and 25% of field capacity. A field study was also conducted with irrigation maintained at 100% and 25% (irrigation capacity). Additionally, the expression levels of marker genes associated with drought tolerance were evaluated using qPCR studies for Metacaspase 1 (LeMCA1), Non-specific lipid-transfer protein 2 (Itpg2), Late embryogenesis abundant protein (LEA) and Delta 1-pyrroline-5- carboxylate synthetase (P5CS) genes. Assays were also conducted on plants for osmolyte content including proline and glycine betaine. Furthermore, the activities of antioxidant defense enzymes were evaluated for superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX) and guaiacol peroxidase (GPX). The results of the greenhouse study indicated higher stomatal conductance, chlorophyll content, and Fv/Fm in stressed plants that were foliar sprayed with the A. nodosum extract. Seaweed extract-treated plants under water-stress conditions also had significantly greater shoot and root dry weights compared to control plants. The relative expression levels of the marker genes were also significantly increased in SWE sprayed plants compared to control plants along with increased antioxidant enzymes activities and osmolyte contents. The field experiments revealed that seaweed extract-treated plants had significantly higher plant height and total yield under reduced irrigation compared to control plants, although both parameters remained lower than those seen under full irrigation. This study has produced encouraging findings on the application of SWE in crop systems, indicating that it may have a positive role in mitigating the effects of drought stress.