## 3 Results
### The Biological Effects of the Isolated Compounds  
The biological effects of dehydrovomifoliol and loliolide on cress were evaluated. The results from the bioassay showed that cress seedling growth was significantly suppressed by both substances (Figures 5 and 6). The inhibition level of the substances was raised by raising the concentration, indicating that suppression was dose-dependent. Dehydrovomifoliol and loliolide significantly inhibited the cress seedling growth in the concentrations of 0.1 and 0.01 mM, respectively (Figures 5 and 6). Dehydrovomifoliol showed the highest inhibition of 36.7% and 34.9% for the shoot and root growth of cress, respectively, at a concentration of 4.0 mM, and loliolide showed the highest inhibition of 5.9% and 5.8% for the shoot and root growth of cress, respectively, at a concentration of 1.5 mM, compared with the control seedling growth. The I50 values for dehydrovomifoliol against the cress were 3.16 mM for the shoots and 3.01 mM for the roots, and for loliolide the I50 values were 0.03 and 0.02 mM, respectively (Table 2). Accordingly, loliolide had higher growth suppressing potential against cress compared with dehydrovomifoliol. Moreover, the roots showed higher sensitivity to the compounds than the shoots. Figure 4: The chemical structures of the isolated allebotathic compounds dehydrovomifoliol and loliolide from the extracts of _Albizia richardiana_. Figure 3: The allebotathic effect of _Albizia richardiana_ extracts on shoot and root growth of cress. The cress seeds were treated in the concentration of 1.2 g DW equivalent extract/mL using the following fractions: F1 (20% aqueous methanol), F2 (40% aqueous methanol), F3 (60% aqueous methanol), F4 (80% aqueous methanol), and F5 (methanol). The means \(\pm\) standard errors for every treatment from the two separate experiments with 10 seedlings are displayed. Various letters denote the significant differences according to [PERSON]’s HSD test at a 0.05 probability level. ing suppression of the growth of seedlings of the tested species was related to the increased extract concentration. The I50 values of the tested species were different, indicating that the inhibitory effects were test-plant specific (Table 1). The concentration-dependent and test-species-dependent inhibitory effects of the leaf extract of _Albizia richardiana_ indicate that this leaf extract might possess phytotoxic compounds, and this inhibitory activity has been reported in various studies [40; 41; 42; 43]. Alleopathic substances can influence physiological activities and factors such as seed germination, photosynthesis, respiration, transpiration, ion uptake, water status, stomatal opening, enzyme activity, and the hormone levels of plants [44]. These phytotoxic substances also influence cell division and differentiation, gene expression, signal transduction, cell wall structure, cell membranes, and cell permeability [45; 46]. The results of this study show that the shoots of the test species were less susceptable to the _Albizia richardiana_ extracts than the roots. Plant roots play a vital role in the adaptation of the plant to edaphic restrictions, as well as biotic and abiotic factors [47]. The elongation of shoots and roots is usually used to measure allebotathic potential [48]. However, researchers also report that growth suppression activity of plant extracts are more active against roots than shoots [49; 50]. Roots show greater sensitivity to plant extracts because of direct interaction with allebotathic substances [51] and because root tissues are more highly permeable to phytochemicals than shoot tissues [52; 53]. In addition, root growth depends on the proliferation of cells, which is greatly influenced by allelochemicals, leading to the inhibition of root growth [54]. The plant extracts for this experiment were subjected to different purification steps, and the two allebotathic compounds were identified and characterized through spectral analysis as dehydrocomifoliol and loliolide.