Biostimulant potential of Brazilian macroalgae: seasonal variations and effects on early growth and germination of lettuce

A wide variety of commercial seaweed-based biostimulants is available worldwide for improving plant growth and development for agriculture and gardening purposes. Biostimulant properties are influenced by species, seasonality, preparation methods, and harvest site. This study evaluated the biostimulant properties of the methanolic, hot aqueous and aqueous extracts of three Brazilian macroalgae— Sargassum vulgare C. Agardh (Ochrophyta, Phaeophyceae), Palisada flagellifera (J. Agardh) K. W. Nam (Rhodophyta), and Ulva fasciata Delile (Chlorophyta)—in two different periods (dry and wet seasons) on germination and early development of Lactuca sativa Linnaeus. All three algae showed biostimulant activities, with the root length being the primary factor that exhibited growth enhancement. The length of the roots increased by an average of 40% when in contact with the extracts, ranging from 28 to 55% for P. flagellifera , 37–48% for S. vulgare , and 28–79% for U. fasciata . The extracts promoted resource allocation for elongation of roots, which can aid the plant survival by improving competition for water and nutrients. In general, the biostimulant properties of seaweed extracts varied between harvesting period, since the activity of methanolic and aqueous extracts were influenced by the seasons. Methanolic extracts from the dry season and aqueous extracts from the wet season showed the most biostimulant effect, although overall, the dry season extracts were the most effective. In conclusion, the studied extracts of three Brazilian macroalgae have significant biostimulant properties, which can vary depending on the harvesting period and extract type.