**1. Introduction**

Crops are susceptible to attacks from pests and diseases and competition from weeds, which lead to considerable yield losses. Unlike pests and pathogens, which generally invade the crop from external sources, weeds are active within the same cultivation area as the crop and compete for light, nutrients, and water. In the most severe situations, weed competition can lead to complete loss of the crop [1]. Cultivation practices often exacerbate weed germination and development, and weed control is thus essential in most cropping situations. Substantial economic resources are used by farmers for weed-control practices, such as herbicide application [2,3]. Use of allelopathy may allow weeds to be managed in a more cost-effective manner.

Allelopathy is the ability of plants to inhibit or stimulate growth of plants in the neighboring environment through the activity of exuded bioactive secondary metabolites referred to as allelochemicals [4]. Allelopathic potential in rice was found to be proportional to the amount and type of phytotoxic compounds produced, including phenolic acids and momilactones [5]. Allelochemicals can elicit a wide range of effects, including changes to plant membrane permeability that impact nutrient absorption; suppression of metabolic activities, such as photosynthesis, respiration, and diverse

enzyme functions; and disruption of growth and development through inhibition of cell division and elongation and alterations to submicroscopic structures [6].

Plants with allelopathic potential are termed donor plants, whereas plants a ffected by the allelopathic compounds from the donor plant are referred to as receiver plants. Donor and receiver plants can a ffect one another through both allelopathy and competition. The combined e ffect of these two interactions is termed interference [7]. Allelopathic interactions are complex, and it is di fficult to exclude the e ffects of competition and the environment when assessing allelopathic potential. Although field-based screening is an important component of plant-breeding programs, it is almost impossible to distinguish allelopathic potentials from competition under natural field conditions. A range of plant species can be used as receivers in bioassays, to assess allelopathic activity [8] and several considerations, such as susceptibility and genetic uniformity, are important when selecting a receiver species for testing allelopathic potential. Some standard species, such as lettuce (Lactuca sativa), radish (Raphanus sativa), and duckweed (Lemna minor), are recommended for preliminary testing because of their availability and high sensitivity to allelopathic actions [9].

Although many agronomic traits in rice have been studied at the genetic level, relatively few studies have been conducted on the genetic basis of allelopathy. Most published studies to date have involved screening and evaluation of existing plant materials. One quantitative trait loci (QTL) study examined the allelopathic potential of 150 recombinant inbred lines (RILs) generated from a cross between 'AC1423, a highly allelopathic rice cultivar, and a minimally allelopathic line, 'Aus196 . Allelopathic potential was evaluated against the vigorous weed species Echinochloa crus-galli (L.) in laboratory and greenhouse conditions. QTLs linked to allelopathic traits were found on chromosomes 3–10 and 12 [10]. A separate QTL study examined the allelopathic potential of an F2 population derived from 'PI312777, a highly allelopathic indica cultivar, and 'Rexmont', a minimally allelopathic japonica cultivar. Water-soluble extracts from the F2 seedlings were supplied to lettuce seedlings as receiver plants, and candidate allelopathic QTLs were identified on chromosomes 1, 3, 5, 6, 7, 11, and 12 [11].

Recent advances in molecular breeding technologies, such as the development of high-density DNA markers, DNA chips, and next-generation sequencing (NGS), have facilitated the identification and characterization of many genes associated with quantitative traits. However, genetic studies to uncover the basis of allelopathic traits in rice remain in their infancy. This is due to the complications arising from distinguishing allelopathic potential from the e ffects of competition and the environment, which necessitate performing large numbers of replicated studies in field and greenhouse conditions, as well as in the laboratory. In this study, QTL analysis to identify candidate regions associated with allelopathic traits in rice was conducted in controlled laboratory conditions.
