"Evaluation of microalgae and Ascophyllum nodosum extracts as biostimulants for abiotic stress mitigation"

Drought and salinity are two of the major factors affecting crop productivity. In the coming years, these abiotic stresses will significantly increase and by 2050 the conventional agricultural practices are expected to be abandoned. A challenge in the field of food security is developing strategies to grow crops productively in increasingly extreme environments. The use of organic inputs to enhance abiotic tolerance in plants has been subject to increasing investigation, and seaweed extracts, from micro- and macro-algae, are the most promising, introduced so far in agriculture. They belong to a specific product category named biostimulants, which promote plant growth and development throughout the full crop cycle. These beneficial effects include increasing crop yield, general crop and fruit quality, improving tolerance, recovery from abiotic stress and enhancing nutrient uptake. The research for new potential biostimulants components, among biological organisms, and the investigation about their mechanisms of action represent an interesting topic of research. The object of this work was to evaluate the effects of some micro-algae extracts, and one of the most used macro-algae extracts, Ascophyllum nodosum, for improving drought and salt stress tolerance in plants. Thirty-six prototypes, obtained from nine microalgae strains and four prototypes, obtained from Ascophyllum nodosum, were tested. This study has been conducted in collaboration with Valagro S.p.A. (Atessa, CH, Italy), who developed and provide the prototypes. The tolerance tests to drought and salt stress were performed using Arabidopsis thaliana plants, grown in hydroponic system. Interestingly the results showed that, among the investigated microalgae strains, one prototype from H.pluvialis and two prototypes from Spirulina sp. induced respectively drought and salt stress tolerance in Arabidopsis plants. Four Ascophyllum nodosum prototypes (SEA7-11-13-14) were screened for preliminary drought stress tolerance induction on treated plants. According to the results, obtained from these preliminary tests, the research was focused on just one Ascophyllum nodosum prototype, named SEA11. A secondary screening was developed, in order to evaluate a possible field application of Ascophyllum nodosum SEA11 prototype, and two different methods of application were evaluated, root drench and foliar spray. The results showed a significant tolerance increased in treated plants, compared to the untreated control. Finally, the mode of action of Ascophyllum nodosum SEA11 was investigated. In order to study the physiological responses in plants treated with this product, leaf gas exchange parameters were measured. Moreover, the expression of genes correlated with stomatal movements and mesophyll conductance, such as genes involved in the protection of photosynthesis machinery, in ABA-responsive and ROS signaling were analysed. We found that pre-treatment with Ascophyllum nodosum SEA11 induced a partial stomatal closure, associated with changes in the expression levels of genes involved in ABA-responsive and antioxidant system pathways. The pre-activation of these pathways results in a greater ability of Ascophyllum nodosum SEA11-treated plants to maintain better photosynthetic performance compared to control plants throughout the dehydration period, combined with a higher capacity to dissipate the excess of energy as heat in their photosystem II reaction centers.