A growth-inhibitory substance exuded from freeze-dried mesquite (Prosopis juliflora (Sw.) DC.) leaves

When lettuce seeds were incubated in a concentric circle around freeze-dried mesquite leaves in a dish containing agar culture medium, the growth, especially radicle growth, of the seedlings was inhibited, the more strongly the closer they were to the leaves. This result indicates that allelopathic substance(s) inhibiting the lettuce growth are exuded from the mesquite leaves. A potent substance was isolated from the exudates of the freeze-dried mesquite leaves and identified as L-tryptophan by spectral analyses. L-Tryptophan inhibited the radicle growth of lettuce and barnyard grass at concentrations greater than 1.5 × 10^·3 mM. The content of L-tryptophan in the exudates of freeze-dried mesquite leaves (1 mg eq.) was 4.8 × 10^·3 mM. These results suggest that L-tryptophan may play an important role in the allelopathy of mesquite leaves.