## 3 Results and Discussion
### Biological Activity of Vanillic Acid and Trans-Ferulic Acid  
Vanillic acid and _trans_-ferulic acid showed phytotoxicity against the four model plants' seedling length (Figures 3 and 4). At the concentration 1 mM of vanillic acid, the shoot length of _L. sativum_, _M. sativa_, _P. pratense_, and _E. crus-galli_ was inhibited to 47.42, 66.32, 74.34, and 66.87% of control length, respectively, whereas that of root length was inhibited to 47.79, 66.01, 63.22, and 54.75% of control length, respectively (Figure 3). On the other hand, 1 mM of _trans_-ferulic acid inhibited the shoot length of _L. sativum_, _M. sativa_, _P. pratense_, and _E. crus-galli_ to 45.87, 65.81, 57.00, and 53.68% of control length, respectively, whereas root length was inhibited to 41.91, 56.25, 57.00, and 29.28% of control length, respectively (Figure 4). The correlation coefficient between vanmillic acid and _trans_-ferulic acid and the seedling length of the model plants was very high, with values ranging from \(-\)0.962 to \(-\)0.996 (_p_ < 0.01). The IC\({}_{50}\) values of vanillic acid and _trans_-ferulic acid for the seedling length of the model plants were 0.73 to 3.17 mM and 0.42 to 2.43 mM, respectively (Table 1). Figure 3: Effect of vanillic acid isolated from the _Afzelia xylocarpa_ leaf extracts on the model plants (**A**) _Lepidium sativum_, (**B**) _Medicago sativa_, (**C**) _Phleum pratense_, (**D**) _Echinochla crus-galli_, and (**E**) effect of vanillic acid on the seedling length of the model plants after treatment for 48 h. Means \(\pm\) SE with three replicates (10 seeds/replicates). The different letters along a curve indicate significant differences ([PERSON]’s HSD, \(p\) < 0.05). Statistical significance is represented by asterisks (two-tailed Pearson’s correlation, \"_p_ < 0.01 and \"**_p_ < 0.001). correlation coefficient (r). These results indicate that vanillic acid and _trans_-ferulic acid have phytotoxic activity against the seedling length of the model plants in a concentration-dependent manner. According to the results of the bioassay-directed fractionation, both compounds may be the main compounds responsible for the phytotoxic activity of _A. xylocarpa_ leaf. These results are supported by previous studies, which reported that vanillic acid and _trans_-ferulic acid isolated from _Triticum estirum_ and _Saccharum officinarum_ act as potential phytotoxic chemicals [64; 65]. In addition, the IC\({}_{50}\) values showed the different responses of both compounds against the four model plants, indicating inhibition was species-specific (Table 1). Differences in phytotoxic effects of compounds against dicotyledonous and monocotyledonous plants have been found in many previous studies [66; 67]. The different sensitivities of the model plant species may be related to the genetic, biochemical, and physiological characteristics of each specific species [68; 69]. In the present study, the _P. pratense_ seedlings (monocotyledonous weed) were found to be the most sensitive to the phytotoxic effects of vanillic acid and _trans_-ferulic acid. Of  
Figure 4: Effect of _trans_-ferulic acid isolated from the _Afzelia xylocarpa_ leaf extracts on the model plants (**A**) _Lepidium sativum_, (**B**) _Medicago sativa_, (**C**) _Pileum pratense_, (**D**) _Echinachloa crus-galli_, and (**E**) effect of _trans_-ferulic acid on the seedling length of the model plants after treatment for 48 h. Means \(\pm\) SE with three replicates (10 seeds/replicates). The different letters along a curve indicate significant differences ([PERSON]’s HSD, \(p\) < 0.05). Statistical significance is represented by asterisks (two-tailed Pearson’s correlation, **_p_ < 0.01 and ***_p_ < 0.001). correlation coefficient (r). the model plants used in this study, the seeds of _P. pratense_ were the smallest; hence, seed size may be one factor that contributed to the sensitivity to the phytotoxic substances. This supposition is consistent with the research of [PERSON] et al.