## 3 Results and Discussion  
### Biological Activity of the Aqueous Methanol Extracts of A. xylocarpa Leaves  
The _A. xylocarpa_ leaf extracts significantly reduced the seedling length of the four model plants (_p_ < 0.05) (Figure 1). At the concentration of 30 mg DW equivalent extract/mL, the shoots of _L. sativum_ were completely inhibited, whereas the shoots of _M. sativa_, _P. pratense_, and _E. crus-galli_ were inhibited to 1.68, 30.08, and 52.44% of control length, respectively (Figure 1A). In contrast to the shoot length, the leaf extracts inhibited the root length of _L. sativum_, _M. sativa_, _P. pratense_, and _E. crus-galli_ to 10.85, 10.60, 2.65, and 29.04% of control length, respectively (Figure 1B). The phytotoxic activity of the leaf extracts against the model plants significantly correlated with the extract concentrations (r = \(-\)0.92 to \(-\)0.98, \(p\) < 0.01) (Figure 1A,B). The IC\({}_{50}\) values of the leaf extracts for the seedling length of the model plants ranged from 6.42 to 24.21 mg DW equivalent extract/mL (Table 1). In addition, the leaf extracts inhibited the root length of _L. sativum_, _P. pratense_, and _E. crus-galli_ more than the shoot length (_P. pratense_ was most sensitive to the extracts; inhibition of its roots was 2.3-times greater than for its shoots).  
Figure 1: Effect of the _Afzelia xylocarpa_ leaf extracts on the (**A**) shoot and (**B**) root length of the model plants exposed to six concentrations. Means \(\pm\) SE with six replicates (10 seeds/replicates). The different letters on the columns of each model plant indicate significant differences (Tukey’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).  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{Test plant species} & \multicolumn{3}{c}{IC\({}_{50}\) values of _Afzelia xylocarpa_ leaf extracts} \\  & \multicolumn{3}{c}{(mg DW equivalent extract/mL)} \\ \cline{2-3}  & \multicolumn{2}{c}{Shoot length} & \multicolumn{2}{c}{Root length} \\ \hline _Lepidium sativum_ & 7.78  \\ _Medicago sativa_ & 8.50  \\ _Pileum pratense_ & 15.28  \\ _Echinochla crus-galli_ & 24.21  \\ \hline \multirow{2}{*}{Test plant species} & \multicolumn{3}{c}{IC\({}_{50}\) values of vanillic acid (mM)} \\ \cline{2-3}  & \multicolumn{2}{c}{Shoot length} & \multicolumn{2}{c}{Root length} \\ \hline _Lepidium sativum_ & 0.81  \\ _Medicago sativa_ & 2.72  \\ _Pileum pratense_ & 0.80  \\ _Echinochla crus-galli_ & 3.17  \\ \hline \multirow{2}{*}{Test plant species} & \multicolumn{3}{c}{IC\({}_{50}\) values of _trans_-ferulic acid (mM)} \\ \cline{2-3}  & \multicolumn{2}{c}{Shoot length} & \multicolumn{2}{c}{Root length} \\ \hline _Lepidium sativum_ & 0.65  \\ _Medicago sativa_ & 2.43  \\ _Pileum pratense_ & 0.61  \\ _Echinochla crus-galli_ & 1.10  \\ \hline \hline \end{tabular}  
Data with different letters in a column indicate a significant difference ([PERSON]’s HSD, \(p\) < 0.05).  
\end{table}
Table 1: The concentration of the aqueous methanol leaf extracts and vanillic acid and _trans_-ferulic acid from _Afzelia xylocarpa_ required for 50% inhibition (IC\({}_{50}\) value) of the shoot and root length of the model plants.  
These results suggest that the leaves of _A. xylocarppa_ may contain photovoltaic substances with phytotoxic activity. In addition, the results indicate that seedling length varied among the model plants, and the level of inhibition depended on the extract concentration (Figure 1). Similarly, several previous studies have also reported that the phytotoxic effect is concentration-dependent [48; 49; 50; 51; 52; 53]. Moreover, many previous studies noted that test plant species' response varied depending on the types of phytotoxic substances [54]. Hence, it is important to understand which compounds play a major role in plant allelopathy to be used for weed control.