Plant allelochemicals: agronomic, nutritional and ecological relevance in the soil system

Abstract

Background Allelopathy is an ecological phenomenon consisting of both positive and negative effects between organisms determined by the release of secondary metabolites into the environment. Root exudation represents the most important pathway of releasing allelochemicals. Once released into the soil, allelochemicals interact with the organic and inorganic soil phases, as well as with soil microorganisms. The set of these interactions fix allelochemicals bioavailability and phytotoxic level. Scope Here we critically review the interactions between plant allelochemicals and physical, chemical and biological soil characteristics by reporting the literature available and pointing out both positive and negative relationships affecting allelochemicals phytotoxicity and nutrient availability. In addition, we have reported a qualitative balance of allelochemicals in the soil. Thirdly, we reviewed the exudation process of allelochemicals and the transport mechanisms across plasma membranes. Conclusions A two-way relationship exists between soil characteristics and allelochemicals. The level of phytotoxicity is not affected only by a single soil characteristic, but they are closely linked to each other and exert a multiple-effect on retention, transport and transformation processes of allelochemicals in soil. Further efforts are needed to better understand the interactions involved in soil allelopathy and to create new opportunities for a sustainable control of agroecosystems.

Long-Term Effect of Cover Crops on Species Abundance and Diversity of Weed Flora

“…These secondary metabolites can be directly exuded into the soil or released by decomposition of plant residues. Once present into the rhizosphere, allelochemicals interact with the complex of physical, chemical and biological soil characteristics, which altogether fix their availability [23]. Unfortunately, competitive and allelopathic effects are very difficult to distinguish in field experiments.…”

Integrated Weed Management in Herbaceous Field Crops

“…Such weeds, thanks to root branching and deepening, may help to increase water infiltration into the soil and improve the water holding capacity and soil structure. Regarding the latter aspect, it can be explained not only in physical terms, but also through the root exudation process which promotes the formation of aggregates thanks to the adsorption of rhizodeposits (e.g., ions such as Ca 2+ , Fe 2+ , Al 3+ , K + , mucillages and several organic acids) with colloids, and the stimulation of microorganisms [23]. In addition, the joint action of root exudates and weed living and dead mulch contribute to enhance the soil organic matter content.…”

“…In all cases, they prevent weed emergence both physically and chemically [55]: the former by increasing the competition with weeds for space, water, light and nutrients, while the latter through the release of phytotoxic compounds able to inhibit seed germination, weed emergence, establishment and early growth. The herbicidal potential of cover crops is closely dependent on cover crop genotype and management (e.g., sowing date, date of incorporation, agricultural practices), weed community composition, environmental and pedological conditions, amount of the plant residues and rate of decomposition [23,56]. Several practical applications of cover cropping for field herbaceous field crops have been suggested: rye, wheat, sorghum, oat, hairy vetch, subterranean clover and alfalfa cover crops are indicated by numerous authors, in different agricultural systems, to exert significant effects on weed control in cotton, maize, soybean and tomato [57][58][59].…”

“…The above-mentioned effects of crop rotation can be further exacerbated by including an allelopathic crop within a crop rotation in order to overcome the autotoxicity and decrease the pressure of plant pests [127]. In particular, allelochemicals exuded into the rhizosphere exert, directly and/or indirectly (by microbial interactions), inhibitory effects on seed germination and weed density [23]. For this reason, several crop sequences such as soybean-wheat-maize [128], sugar beet-cotton [129], sunflower-wheat [130], etc., are suggested.…”

Research Advances of Beneficial Microbiota Associated with Crop Plants

“…A plant can release various chemical compounds into the environment that impose either direct or indirect allelochemical effect on another plant [149] (Figure 1b). In addition, studies have revealed that the production of allelochemicals by plants significantly alters the structure of associated microbiomes [150][151][152]. However, the influence of microbiota on the allelochemical production in plants remains unclear.…”