Scientific basis for the use of minimally processed homogenates of Kappaphycus alvarezii (red) and Sargassum wightii (brown) seaweeds as crop biostimulants

Abstract

Seaweed extracts act as biostimulants that boost crop −productivity, −quality and ameliorate stress. The precise constituents and mechanism behind the positive responses have still remained elusive and speculative. In-depth investigation was contemplated to reveal the metabolites that become constituent of extracts after seaweed processing and appraise whether they are present in physiologically relevant concentration at a given foliar application level. The minimally processed homogenate(s) (MPHs) of Kappaphycus alvarezii (KA) and Sargassum wightii (SW) were biochemically characterized. In two different experiments, six different formulations prepared using one or both of MPHs in proportions ranging from 0 to 100 % were assayed on tomato at three dilution levels of 0 %, 0.8 %, and 1.6 %, under well-watered and soil-moisture deficit conditions. The two MPHs differed between them with respect to composition and content. The tomato yield increased over control at 0.8 % application level under both water regimes, but also increased at higher concentrations under duress. The formulations made using only one of the two seaweed MPHs (KA:SW 0:100 and 100:0) recorded 20–31 % higher yields over control, although combined formulations using at least 60 % of either of the MPHs also produced higher yield. Both the MPHs contained gallic acid, catechin hydrate, myo-inositol, mannitol, glycine, leucine, putrescine, tyramine, IAA, canavanine, and many other reported bioactive compounds having defined roles and mode of action. At the application level of 0.8 %, many of these compounds were deduced to be present in physiologically relevant concentrations. This was inferred based on the literature cited concentrations of same compounds that elicited beneficial plant response upon exogenous application in pure form. It is implied that multiple bioactive compounds in seaweed extracts act in tandem and improve plant performance in totality, despite possible synergistic or antagonistic interactions among them. The findings prescribe a scientific basis for formulating other seaweed biostimulants and determining their optimal application doses.