Allelopathic effects of three invasive plant species on shoot growth of a test species: Strong in vitro bioactivity

Aqueous leaf extracts of three widespread pantropical invasive plants, Alternanthera sessilis, Synedrella nodiflora, and Sphagneticola trilobata - were tested for allelopathic effects on shoot elongation of Abelmoschus esculentus (L.) Moench L. under controlled in vitro and field (in vivo) conditions. In vitro assays revealed strong and concentration-independent inhibition by A. sessilis (>87% reduction even at 10%, Welch’s t-test p0.2), although S. trilobata retained a non-significant positive trend (+31% at 50%). The results exemplify the classic lab-field paradox, whereby potent phytochemical activity observed in sterile systems is completely or largely neutralised in soil through microbial degradation, adsorption, dilution, and environmental interactions. These findings demonstrate that in vitro bioassays have limited predictive value for real-world allelopathic interactions and suggest that allelopathy is unlikely to be a major driver of invasive success for these three species in natural ecosystems.