The study investigated the potentials of alkaloidal fraction of leaf and stem of Crotalaria retusa with a view to understanding its mechanism of allelopathy in bean (Phaseolus vulgaris) seedlings. The study involved collection of C. retusa (leaves and stems), identification, dried, pulverized and extracted with methanol to yield methanolic extract (MECR). Crude alkaloid fraction was prepared from MECR by a procedure that consisted of acidification, basification and extraction with chloroform. Brown beans were grown in varying concentrations (0, 50 and 100 µg/ml) of crude alkaloid fraction with strychnine (10 µg/ml) as reference alkaloid. Leaves and stems of the seedlings were collected for biochemical analyses which included determination of percentage germination and evaluation of biochemical parameters. The results showed that percentage germination of bean seeds reduced with increasing concentrations of alkaloid fraction. The activities of antioxidant enzymes (superoxide dismutase and catalase) increased with alkaloid concentration in the leaves and stems of bean seedlings. The levels of metabolites (proline, reduced glutathione and ascorbic acid) increased significantly (p ≤ 0.05) in the stems and leaves of treated bean seedlings. However, there was reduction in the total protein and sugar contents of the leaves and stems of bean seedlings which implied stress. Alkaloidal fractions of C. retusa elicit significant allelopathic effects on germination of bean seedlings by reducing its germination rate. The alkaloid also affected some metabolites in bean seedlings which are markers of environmental stress. The study revealed that the mechanisms of action of the alkaloid fraction of C. retusa involved the induction of oxidative stress that resulted in the generation of reactive oxygen species (ROS) that caused metabolic derangement in the bean seedlings.
Key words: Alkaloidal fraction, allelopathy, strychnine, basification, extract, antioxidant enzymes.
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