# Identification and Isolation Techniques for Plant Growth Inhibitors in Rice
## 4 Major PGIs in Rice  
### Phenolic Acids  
Phenolic acids are known as derivatives of benzoic and cinnamic acids, which are popularly present in cereals. These natural compounds are detected and quantified with different contents among rice cultivars and among plant parts. Structurally, they are grouped into insoluble-bound, soluble-free, and soluble-conjugated forms [39; 69; 70]. In 1976, allebotathic compounds, including \(p\)-hydroxybenzoic, \(p\)-coumaric, vanillic, ferulic, and \(o\)-hydroxyphenyl acetic acids, were detected in extracts of rice straw by thin-layer chromatography (TLC) [71]. Subsequently, by using HPLC-EI/MS, phenolic phytotoxic compounds were intermittently discovered in the leaf and stem of four rice cultivars, including Philippine 2 (0.05 mg/g of ferulic acid), Gin Shun and Juma 10 (6.87 mg/g and 6.34 mg/g of \(p\)-hydroxybenzoic acid, respectively), and Kasawala mundara (0.34 mg/g of \(p\)-coumaric acid) [40]. The structures of phenolic acids, which are confirmed as PGIs in rice, are shown in Figure 2. In addition, the previous studies also indicated that these compounds significantly inhibit the germination and seedling length of barnyard grass. For instance, the reduction in seed germination and in root and shoot growth caused by \(10^{-3}\) M \(p\)-hydroxybenzoic acid (PHDB) was 27%, 28%, and 43%, respectively [40]. [PERSON] et al. [72] also indicated that rice straw of wild rice (_Oryzaa longstaminita_, S37) contains fumaric acid, \(p\)-coumaric acid, and \(p\)-hydroxybenzoic acid. At 100 ppm, fumaric acid reduced plant height by approximately 38.12% and significantly inhibited the root length and fresh weight of _Echinolha crus-galli_ as well. In addition, foliar application of the two phenolic acids PHDB and vanillic acid (VB) enhanced the drought tolerance of two rice cultivars, Q2 and Q8 [73]. Furthermore, sinagic acid and ferulic acid were found only in high-yield rice cultivars at the vegetative stage at amounts of 3.7 mg/g and 1.2 mg/g, respectively [74]. In reality, Asian farmers traditionally leave a huge amount of rice straw in the field after harvesting, which causes price yield loss in the next cultivation. [PERSON] et al. [75] found that the productivity of rice decreased by 25% compared to the first crop in Taiwan due to the autotoxicity produced during the decomposition of rice residues left in the soil. Another hypothesis noted that allelochemicals in rice and emitted from rice roots are different. With self-defense, the living rice can detect the receiver plants and release allelochemicals at higher concentrations. However, the allelochemical content is over the threshold (demand on the agronomic traits of rice cultivars) that inhibits rice growth [75]. This problem is discussed in this section, owing to phenolic acids accounting for the main allelochemicals in rice. Figure 2: _Cont._  
Recently, epidemiological studies demonstrated that the existence of antioxidant compounds such as phenolic acids in rice may contribute to reducing the risks of certain chronic diseases in some rice-consuming regions of the world. Chemically, the antioxidant properties of such compounds depend on the number and the position of hydroxyl groups on the phenolic ring [76]. In addition, previous reports indicated that soluble free, soluble, conjugated, and insoluble bound forms of phenolic acids are highly associated with hydroxylation and methylation in the human body [42]. Among the nineteen prevalently identified phenolic acids in rice, ferulic acid is the most abundant substance, constituting 56% of the total phenolic acids in rice bran and 77% in polished rice, while caffeic and cinnamic acids occupy under 1% [69; 70; 77]. These phenolic acids have also been researched for their role in some human diseases [78]. In detail, research in 2000 showed that 50 uM of caffeic acid extracted from brown rice bran reduced the number of MDA-MB-468 breast cells and colon-derived SW-480 and human colonic epithelial cells [78].