## 4 Discussion  
The results revealed that \(G\). _xantholchymus_ extract markedly inhibited both the dicot species (lettuce and rapeseed) and the monocot species (Italian ryegrass and timothy). The phytotoxicity of the extract against all the test plants increased with the increase in concentration. Other researchers have also documented such concentration-dependent inhibitory activity of different plant extracts [20; 21; 22; 23; 24; 25; 26]. Our previous experiment with _Garcinia pedunculata_ also showed strong inhibition against several test species [27]. In addition, the phytotoxic activity of the extracts varied against different test plants. [PERSON] et al. [28] also reported species specificity of _Castanea crenata_ extracts against radish, lettuce, and barnyard grass. These concentration-dependent and species-specific phytotoxicity of the \(G\). _xantholchymus_ leaf extract led us to assume that the extract contains potential phytotoxic substances. Bio-guided isolation resulted in obtaining two phytotoxic substances from the \(G\). _xantholchymus_ leaf extract, which were identified as vanilla acid, and methyl phloreate through \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Tested Species**} & \multicolumn{3}{c}{**Vanillic Acid**} & \multicolumn{3}{c}{**Methyl Phloreate**} \\ \cline{2-6}  & & **Shoot*** & **Root*** & **Shoot*** & **Root*** \\ \hline dicot & Cress & 331.7 \(\pm\) 7.1 & 314.7 \(\pm\) 8.3 & 113.7 \(\pm\) 3.8 & 104.7 \(\pm\) 2.9 \\ \hline Monocot & Timothy & 118.8 \(\pm\) 4.4 & 107.3 \(\pm\) 4.1 & 53.4 \(\pm\) 2.8 & 40.6 \(\pm\) 1.9 \\ \hline \hline \end{tabular}  
Significant differences between vanillic acid and methyl phloreate represented by ***\(p\) \(<\) 0.001 (paired \(t\)-test).  
\end{table} 
Table 2: The \(I_{50}\) value (concentration causing 50% of growth inhibition) of vanillic acid and methyl phloreate (\(\mu\)M) on cress and timothy standard deviation. Figure 4: Effects of the vanillic acid (**a,b**) and methyl phloreate (**c,d**) on cress and timothy. Values represent means \(\pm\) SE from three replicates (n = 30). The significant variations between control and treatment are denoted by *\(p\) \(<\) 0.05 and ***\(p\) \(<\) 0.001.  
spectral analysis. Vanillic acid is one of the most common phenolic compounds found in different plant parts. It was previously identified in extracts from different plants such as _Almus japonica_, _Gossyrium mexicanum_, _Rosa canina_, _Panax ginseng_[29], and _Chenopodium murale_[30]. Vanillic acid can act as an antimicrobial, antioxidant [31], anti-inflammatory, and antidiabetic agent [32]. However, phytotoxic effects of the vanillic acid have also been narrated by many researchers [33]. Methyl chlorate is a methyl ester of phenylpropanol phenol, phloretic acid [34], which is a naturally occurring phenolic compound that can be obtained from \(p\)-coumaric acid hydrogenation or synthesized from phloretin, a secondary metabolite of apple leaves [35]. In phloretic acid, there is a propionic acid side chain that is suitable for esterification, leading to production of methyl phloreate [36]. Methyl phloretate is a potential synthetic intermediate and can be used to prepare antidiabetic agents [37]. In our study, the inhibitory activity of vanillic acid and methyl phloretate depended on the concentration and species. In previous reports such inhibitory activities were noted from different phytotoxic substances [20,38]. [PERSON] et al. reported that vanillic acid has strong growth-inhibitory activity against cress and timothy, and the inhibition is more pronounced against timothy than cress [39]. This species specificity of allelochemicals might be because of different physical and physiological features of receptor seeds including thickness of seed coat, cell membrane permeability, and sensitive enzymes in seeds [40]. In our experiment, timothy was much more affected by both the allelochemicals compared with cress. Our finding is in line with that by [PERSON] et al.