Asparagus Decline and Replant Problem: Autotoxicity, Autotoxic Substances, and Their Biological Functions

The cultivation of asparagus (Asparagus officinalis L.) is plagued by two serious issues: "asparagus decline" and "asparagus replant problem". The average lifespan of an asparagus plant is 15 to 20 years. However, its productivity decreases after a few years (asparagus decline). Even when these asparagus plants are replaced with new ones, the new plants remain unproductive (asparagus replant problem). The main causes of these problems are a Fusarium infection and asparagus autotoxicity. Several reviews have been conducted on Fusarium. Despite the accumulation of evidence on asparagus autotoxicity in the literature over the past four decades, no review has focused specifically on asparagus autotoxicity. It has been reported that asparagus growth is inhibited by asparagus root residues, leachates, root exudates, and rhizosphere soils. Several phenylpropanoids, including trans-cinnamic acid, p-coumaric acid, caffeic acid, and ferulic acid, have been identified as asparagus autotoxic substances in these root residues, root exudates, rhizosphere soils, growth media, and/or plant tissues. Tryptophan, 3,4-methylenedioxycinnamic acid, and iso-agatharesinol were also identified as asparagus autotoxic substances. These substances may cause autotoxicity by disrupting phytohormone levels, cellular metabolism, impairing membrane function, and by inducing oxidative stress. Although cinnamic, p-coumaric, caffeic, and ferulic acids have been reported to act as antibiotics, these compounds have also been shown to weaken the defense mechanisms of asparagus against pathogen infection, and enhance the Fusarium pathogenicity. The presence of these autotoxic substances, coupled with a Fusarium infection, may create a vicious cycle that worsens "asparagus decline" and "asparagus replant problem". This is the first review to focus on the asparagus autotoxicity.