Journal of Chemical Ecology, Vol. 9, No. 8, 1983 STRESS MODIFICATION OF ALLELOPATHY OF Helianthus annuus L. DEBRIS ON SEEDLING BIOMASS PRODUCTION OF Amaranthus retroflexus L.1 ANTHONY B. HALL, 2 UDO BLUM, 3 and ROGER C. FITES 3 2Dow Chemical USA, 8050 Hosbrook Rd. Cincinnatti, Ohio 45236 3Department of Botany, North Carolina State University Raleigh, North Carolina 27650 (Received September 12, 1982; revised October 18, 1982) Abstract--When added to soil, Helianthus annuus debris from plants grown under various nutrient stresses in the greenhouse and field had a significant negative effect on Amaranthus retroflexus seedling dry weight production. The regression models for dry weight versus total phenolic compounds and the variation in N, P, and K contents of the Helianthus debris were significant. It was possible to partially simulate debris inhibition of A. retroflexus seedling dry weight production when chlorogenic acid alone was added to the soil instead of Helianthus debris. The inhibition of Amaranthus growth by Helianthus debris and chlorogenic acid was not evident when nutrient solution was applied to the soil. Key Words--Helianthus annuus, Amaranthus retroJ7exus, allelopathy, chlorogenic acid, seedling growth, plant debris, stress. INTRODUCTION It has been shown that the addition ofHelianthus annuus L. debris, obtained from plants grown under different nutrient regimes in the greenhouse and field, significantly inhibited seed germination of Amaranthus retroflexus L. (Hall et al., 1982). Seed germination was found to be correlated more closely with total phenolics added to the soil than with any other variable measured. This observation confirmed a long-held postulate that environmentally ~Paper No. 8329 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina. 1213 0098-033Ii83:0~00-I2t3503.00,'0 9 1983 Plenum Publishing Corporation