A Study on the Phytotoxicity of Bark and Root Extracts of the Invasive Senna spectabilis (DC.) H.S. Irwin and Barneby

The study was carried out to evaluate the impact of Senna spectabilis bark and root extracts upon the germination and growth of other plant species, including Vigna radiata (L.) R. Wilczek, Cicer arietinum L., and Amaranthus cruentus L., and to identify the major allelochemicals present. After the bioassay analysis, it was found that the quarter-strength (25%) bark extract was enough to reduce the shoot length of V. radiata seedlings by 85%, C. arietinum seedlings by 40%, and A. cruentus seedlings by 65%. Similarly, the quarter- strength root extract reduced the shoot length of the V. radiata seedlings by 50%, C. arietinum by 12%, and A. cruentus by about 45%. LC-HRMS-QTOF with Agilent Mass Hunter data acquisition software and Agilent Mass Hunter Qualitative Analysis B.06 data processing software was used to analyze the butanol fraction in the bark extract and the aqueous fraction in the root extract. From the bark extract, ten allelochemicals were identified, including phenols, cardiac glycosides, flavonoids, phenolic glycosides, and hydroxycoumarin, whereas three allelochemicals were identified from the root extract, including flavonoids, polyphenols, and hydroxycoumarin. Soil sterilization experiments with the test species Bambusa bambos L. did not show any significant role of microbes in the phytotoxic effect of the bark and root leachates. Further studies on the impact of the identified allelochemicals on the soil environment will provide a clear understanding of their role in the invasion of S. spectabilis.