Allelopathic potential of Verbesina encelioides root leachate in soil

Verbesina encelioides (Cav.) Benth. & Hook. F. ex. A. Gray (Asteraceae) is a perennial weed that interferes with the growth and establishment of crop species in semiarid regions of India. The present research was designed to understand the probable involvement of allelopathy in its interference mech anism. We studied the effect of soils amended with different dilutions of V . encelioides root leachate (full strength and 1:2 and 1:4 (v/v) ratios of root leachate to water) on the growth of radish seedlings (Raphanus sativus L.). Soils that were not amended were used as controls. We also investigat ed the influence of different levels of N fertilization (1, 5, and 10 mM) o n the modification of the allelopathic potential of amended soils, in terms of their effect on soil total phenolics and radish seedling growth. The ad dition of both full strength and 1:2 dilution of V. encelioides root leacha te resulted in significant (P < 0.05) suppression of root (-25.7 and -17.2% , respectively) and shoot (-21.3 and -13.8%, respectively) growth of radish seedlings. The total level of phenolics in soil amended with full-strength (8.53 +/- 0.55 mu g/g), 1:2 dilution (5.43 +/- 0.4 mu g/g), and 1:4 diluti on (4.17 +/- 0.36 mu g/g) was significantly (P < 0.05) higher compared with that of control (2.98 +/- 0.47 mu g/g) soil. Although the different amount s of N fertilization in soil amended with V. encelioides root leachate coul d not counteract the probable allelopathic interference, we observed an inc rease in root growth of radish seedlings in soil amended with 10 mM N ferti lization plus either a 1:2 dilution (+21.2%) or a 1:4 dilution (+36.5%) of root leachate. No significant differences in NO3- and NH4+ concentration we re observed between control soil and soil amended with different amounts of root leachate and N fertlization. Since allelopathic activities include bo th inhibitory and stimulatory growth responses, the radish seedling growth responses to V. encelioides root leachate can be explained by allelopathy. High performance liquid chromatography data indicate qualitative and quanti tative differences in phenolic peaks of both control and amended soil. Our research demonstrates the allelopathic potential of V. encelioides roots an d the probable involvement of allelopathy in its interference success.