LIGHT LIMITATION OF REPRODUCTION AND VARIATION IN THE ALLOMETRIC RELATIONSHIP BETWEEN REPRODUCTIVE AND VEGETATIVE BIOMASS IN AMARANTHUS-RETROFLEXUS (REDROOT PIGWEED)

1. Light limitation (induced by the canopy of Zea mays) of reproductio n in Amaranthus retroflexus and the allometric relationship between re productive and vegetative allocation in A. retroflexus was examined in the field. 2. Reproductive initiation in A. retroflexus was delayed a s increased densities of Z. mays caused increased light limitation. Su bsequent reproductive phenology, however, was unaffected. The sowing d ate of A. retroflexus, relative to the stage of development of Z. mays , did not affect reproductive phenology. 3. Although seed weight was u naffected, seed number per plant in A. retroflexus decreased as light limitation increased. 4. Reproductive effort (reproductive biomass/veg etative biomass) of A. retroflexus decreased with increasing density o f Z. mays in 1990 but was greatest at the high density of Z. mays in 1 991. This may indicate that density-dependent competition alters repro ductive effort; however, the relationship may be affected by genetic a nd/or environmental variation. 5. In 1990 there was an allometric rela tionship between these variables and reproductive allocation was relat ed inversely to vegetative allocation. In 1991 reproductive allocation was positively related to vegetative allocation but there was no allo metric relationship between vegetative and reproductive biomass. There may be genetic and/or environmental variation which affects allometri c relationships for A. retroflexus.