Can Ascophyllum nodosum extract application before or at drought stress trigger different metabolic adaptation responses in soybean plants?

Soybean is essential for food security worldwide and water restriction is a rising challenge for its production. In this scenario the use of natural sources such as Ascophyllum nodosum extracts (ANE) which contribute to the better adaptation of plants to environmental stresses, gains relevance. Nonetheless, understanding the metabolic effect of ANE according to the time of application could be better explored, such as, if the application before or during the stress occurrence can trigger different metabolic responses. Thus, this work aimed to identify the physiological and biochemical effects of foliar application of ANE at different times, in soybean plants subjected to water restriction. The experiment was carried out in pots with plants at the vegetative stage, subjected to foliar sprays of ANE in the following treatments: (i) application of ANE at adequate humidity with 80% of the substrate water holding capacity (WHC); (ii) application of ANE three days before the humidity reached 50% of the substrate WHC; (iii) application of ANE on the day that WHC reached 50% (established as drought stress condition); (iv) positive control without application of ANE and WHC 50%; (v) absolute control, without application of ANE and without water restriction. The results allow to point out that ANE application three days before drought stress improved nitrate reductase enzyme activity, chlorophyll and proline content, decrease lipid peroxidation and presented more effective improvement of peroxidase enzyme activity; while application at stress condition resulting in more effective reduction of stomata resistance, long term expressive proline content increment, more effective improvement of superoxide dismutase and catalase enzymes activity and reduce chlorophyll degradation. This works results also could help to answer some field practice questions, such as; it is worth applying ANE when plants are already under drought stress.