Abstract
Algal biomass, extracts, or derivatives have long been considered a valuable material to bring benefits to humans and cultivated plants. In the last decades, it became evident that algal formulations can induce multiple effects on crops (including an increase in biomass, yield, and quality), and that algal extracts contain a series of bioactive compounds and signaling molecules, in addition to mineral and organic nutrients. The need to reduce the non-renewable chemical input in agriculture has recently prompted an increase in the use of algal extracts as a plant biostimulant, also because of their ability to promote plant growth in suboptimal conditions such as saline environments is beneficial. In this article, we discuss some research areas that are critical for the implementation in agriculture of macro- and microalgae extracts as plant biostimulants. Specifically, we provide an overview of current knowledge and achievements about extraction methods, compositions, and action mechanisms of algal extracts, focusing on salt-stress tolerance. We also outline current limitations and possible research avenues. We conclude that the comparison and the integration of knowledge on the molecular and physiological response of plants to salt and to algal extracts should also guide the extraction procedures and application methods. The effects of algal biostimulants have been mainly investigated from an applied perspective, and the exploitation of different scientific disciplines is still much needed for the development of new sustainable strategies to increase crop tolerance to salt stress.
Highlights
Algae are autotrophic photosynthetic organisms that are able to colonize even complex habitats.Algae are exposed to continuous and sometimes harsh changes in light intensity, salinity, temperature, and nutrient availability; they evolved the ability to synthesize a large array of secondary metabolites
The aim for this review is to offer a critical discussion of the extraction techniques and the chemical composition of seaweed- and microalgae-based extracts and biostimulants
The plant response to algal biostimulation is comparatively much less detailed than salt-stress response, yet it is notable that this commonality is broad, referring for instance to mechanisms related to ion homeostasis, ion exclusion, biosynthesis of osmoprotectants, and increases of antioxidant compounds, enzymatic activities, and phytohormones playing important roles in plant response to environmental stress
Summary
Algae are autotrophic photosynthetic organisms that are able to colonize even complex habitats. AEs have an unexplored potential as PB for salinity, which is still one of the most harmful abiotic stresses in agriculture, for different reasons They include the AEs’ chemical and biochemical diversity, the range of biological effects on crops, and the shared molecular pathways between algae and higher plants. Recent studies highlight that AEs can elicit different biochemical, physiological, and molecular mechanisms to enhance plant tolerance to salinity [60,107] They are able to mitigate the effects of salt stress inducing protective metabolites and/or the activation of metabolic pathways, which contribute to enhance plant growth and yield and increase quality [11,108,109] (Tables 2 and 3).
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