Abstract
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.
Highlights
It is important for this region to develop and implement new disease control measures for both crops, that rely less on chemical-based strategies. Considering this problem, we investigated the use of a commercial Stimplex, A. nodosum seaweed extract as an alternative to chemical inputs
We have studied the mode of action of seaweed extract by assessing the induction of host defense mechanisms including defense enzyme activities and upregulation of defense-gene transcripts
Foliar application of A. nodosum extract (AN) significantly improved all the plant growth parameters including plant height, leaf number, root length and dry biomass compared to the fungicide (P < 0.05) (Tables 2 and 3)
Summary
Indiscriminate use of synthetic chemicals in agriculture has led to many challenges facing the sector globally, including development of pathogen and pest resistance, chemical residue. Biostimulatory activities of Ascophyllum nodosum extract in tropics carry-over in food produce, increased production cost and causation of irreparable negative impact on the environment [1]. Researchers have been exploring the use of organic-based biostimulants for enhancing plant growth and defense mechanisms [2]. One promising source of biostimulants is seaweed extracts which have been shown to possess both phytostimulatory and phytoelicitor properties [3,4]. The potential of extracts of several marine algal species has been demonstrated as plant biostimulants, the most important species used globally for commercial extraction is the brown seaweed Ascophyllum nodosum [5,6]
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