# Identification and Isolation Techniques for Plant Growth Inhibitors in Rice
## 1 Introduction  
According to the Food and Agriculture Organization (FAO), world rice production in 2022 was 519.5 million tonnes, which supplies starchy cereal grain to more than half of the world's population [1, 2]. Basically, rice grain consists of endosperms, husks, brass, and germs at around 70%, 20%, 8%, and 2% of the rough rice weight, respectively, which are a plentiful source of nutrients such as minerals, fiber, carbohydrates, vitamins, and proteins [3, 4]. Along with nutritional and pharmaceutical values, rice products and byproducts from rice production have attracted more consideration in terms of application for weed and pest control. Among the proposed approaches, the exploitation of naturalmolecules in rice for suppressing the growth of weeds and pests is a promising direction thanks to its availability and environmentally friendly advantage. Laboratory, greenhouse, and field experiments indicate that rice produces secondary metabolites comprising phenolics, flavonoids, terpenoids, steroids, and alkaloids which exhibit inhibitory activities against weeds and ambient species [5; 6; 7; 8; 9; 10; 11]. Such phytocompounds are considered plant growth inhibitors (PGIs) or allelochemicals. Rice can either accelerate or retard the growth of neighboring plants and natural enemies by exudating allelochemicals into the surrounding environment (soil, water, and air) [12]. The yield and quality of rice are influenced by their allelochemicals, which can cause positive or negative effects on the plant-plant, plant-microorganism, and microorganism-microorganism relationships. Additionally, the allelopathic mechanisms that impact receiver plants and soil microorganisms have been studied through physiological, biochemical, and molecular pathways [13; 14]. In the 1970s, scientists reported the positive side of PGIs released from rice straw and rice hulls, which acted as natural herbicides against weeds via plant models including barnyard grass (_Echinochla crus-gall_), orchardgrass (_Dactyllis glomerata_), lettuce (_Lactuca sativa_ L.), and duck salad (_Heteranthera limosa_). [PERSON] et al. [15] performed experiments on five thousand rice accessions to evaluate their suppression ability on the growth of duck salad and other aquatic weeds, which resulted in around 3.82% of cultivars exhibiting allelopathic activity. In 2008, [PERSON] et al. [16] indicated that the water extracted from 18 Vietnamese rice varieties could inhibit 81.5% of the root growth of both creas (_Lepidium sativum_ L.) and barnyard grass (_Echinochla crus-gall_). Previous reports also revealed that allelopathic rice varieties suppressed 75% of the growth of indicator plants in comparison with non-allelopathic cultivars [17]. Parallelly, popular rice diseases such as bacterial blight (_Xanthomonas oryzae_ pv. _oryzicola_), blast (_Magnaporthe oryzae_), and fungi were efficiently treated by phytochemicals derived from rice [18; 19; 20; 21; 22]. In recent years, allelochemicals with medicinal properties including anti-cancer, anti-diabetic, anti-inflammatory, antioxidant, anti-HIV, and anti-SARS-CoV-2 potential have been increasingly elaborated upon [23; 24; 25; 26; 27; 28]. To date, there have been numerous reports regarding the characterization and identification of PGIs from rice. The major groups of PGIs can be listed as phenolic acids, polyphenols, terpenoids, phenylamides, fatty acids, indoles, and steroids. Among these, phenolic acids are known as a major class of allelochemicals [29]. In addition, sesquiterpenoids detected in aqueous extracts of some rice varieties were indicated as important rice PGIs [30; 31]. Noticeably, momilactones belonging to the diterpene lactone group were detected in minor amounts in rice husks but expressed potent allelopathic activity against weeds [32]. In addition, tricin (5,7\(A^{t}\)-trihydroxy-3\({}^{t}\),5\({}^{t}\)-dimethoxyflavone) found in rice bran revealed potent allelopathic activities [19].