The use of seaweed-based bioproducts has been gaining momentum in crop production systems owing to their unique bioactive components and effects. They have phytostimulatory properties that result in increased plant growth and yield parameters in several important crop plants. They have phytoelicitor activity as their components evoke defense responses in plants that contribute to resistance to several pests, diseases, and abiotic stresses including drought, salinity, and cold. This is often linked to the upregulation of important defense-related genes and pathways in the plant system, priming the plant defenses against future attacks. They also evoke phytohormonal responses due to their specific components and interaction with plant growth regulation. Treatment by seaweed extracts and products also causes significant changes in the microbiome components of soil and plant in support of sustainable plant growth. Seaweed extracts contain a plethora of substances which are mostly organic, but trace levels of inorganic nutrient elements are also present. Fractionation of seaweed extracts into their components and their respective bioassays, however, has not yielded favorable growth effects. Only the whole seaweed extracts have been consistently proven to be very effective, which highlights the role of multiple components and their complex interactive effects on plant growth processes. Since seaweed extracts are highly organic, they are ideally suited for organic farming and environmentally sensitive crop production. They are also very compatible with other crop inputs, paving the way for an integrated management approach geared towards sustainability. The current review discusses the growth and functional effects evoked by seaweed extracts and their modes and mechanisms of action in crop plants which are responsible for elicitor and phytostimulatory activities. The review further analyses the potential value of seaweed extracts in integrated crop management systems towards sustainable crop production.

This study evaluated the effectiveness of a commercially available Ascophyllum nodosum alkaline extract as a plant growth stimulant and defense elicitor against foliar diseases of tomato and sweet pepper caused by Xanthomonas campestris pv. vesicatoria and Alternaria solani in a tropical environment. Foliar applications of 0.5% A. nodosum extract (AN) at 10-day intervals resulted in significant (P < 0.05) increase in plant growth parameters, including plant height (40%), leaf number (50%), plant dry biomass (52%), root length (59%) and chlorophyll content (20%) compared to control. Treated plants also had a significantly higher number of flower clusters, flower numbers, fruits per cluster and total harvested fruit yield. The Ascophyllum extract significantly (P < 0.05) reduced disease incidence by the pathogens in both crops under greenhouse and field conditions. The combinatory treatment of seaweed extract and a minimum dose of contact fungicide in field trials, recorded the overall lowest disease levels (60% reduction) and highest yield (57% increase). Investigations into the mechanisms of disease suppression revealed the effects of the extract in inducing the activities of defense-related enzymes including phenylalanine ammonia lyase, peroxidase, polyphenol oxidase, chitinase and β-1,3-glucanase, as well as the levels of total phenolic compounds. The effect on SA, JA and ET-mediated signalling defense pathways was examined by quantifying expression levels of marker genes including PR1-a, PinII and ETR-1, for the above pathways respectively. Both crop plants treated with A. nodosum extract had significantly higher expression levels of the PinII and ETR-1 marker genes than controls. This was coupled with a marked increase in gene transcripts involved in auxin (IAA), gibberellin (Ga2Ox) and cytokinin (IPT) biosynthesis, which provides possible evidence for induced growth in plants treated with AN extract. Cross-talks between growth and defense responses as a result of seaweed extract application could evidently implicate the benefits of seaweed extract usage in sustainable crop production.

Tomato plants (Lycopersicum esculentum Mill) grown under tropical field conditions were treated with an alkaline seaweed extract made from Ascophyllum nodosum (ASWE). Two field experiments and one greenhouse experiment were conducted to evaluate methods of application, dosage of application, and the impact of each on plant growth parameters and on the quality and yield of fruit. Field experiment 1 included 0.2 % ASWE spray, 0.2 % ASWE root drench, fungicide spray and combinations of the above. Plants foliar-sprayed with 0.2 % ASWE had significantly increased plant height (10 %) and plant fruit yield (51 %) when compared to control plants. Similar results were observed for ASWE spray alternated with fungicide or with ASWE root drench. Field experiment 2 included 0.5 % ASWE spray, fungicide spray and ASWE spray alternated with fungicide. The higher concentration of ASWE resulted in a significant increase in plant height (37 %) and plant fruit yield (63 %) compared to control plants. The third experiment under greenhouse conditions also showed that 0.5 % ASWE spray caused a significant increase in plant height (20 %) and plant fruit yield (54 %) compared to control plants. In the greenhouse, ASWE-treated plants had larger root systems and increased concentrations of minerals in the shoots. Fruit from plants treated with ASWE showed significant increases in quality attributes including, size, colour, firmness, total soluble solids, ascorbic acid levels and mineral levels. Overall, the use of ASWE resulted in clear improvements in tomato fruit yield and quality under tropical growing conditions.