Light Limitation of Reproduction and Variation in the Allometric Relationship Between Reproductive and Vegetative Biomass in Amaranthus retroflexus (Redroot Pigweed)

Abstract

1. Light limitation (induced by the canopy of Zea mays) of reproduction in Amaranthus retroflexus and the allometric relationship between reproductive and vegetative allocation in A. retroflexus was examined in the field. 2. Reproductive initiation in A. retroflexus was delayed as increased densities of Z. mays caused increased light limitation. Subsequent reproductive phenology, however, was unaffected. The sowing date of A. retroflexus, relative to the stage of development of Z. mays, did not affect reproductive phenology. 3. Although seed weight was unaffected, seed number per plant in A. retroflexus decreased as light limitation increased. 4. Reproductive effort (reproductive biomass/vegetative biomass) of A. retroflexus decreased with increasing density of Z. mays in 1990 but was greatest at the high density of Z. mays in 1991. This may indicate that density-dependent competition alters reproductive effort; however, the relationship may be affected by genetic and/or environmental variation. 5. In 1990 there was an allometric relationship between these variables and reproductive allocation was related inversely to vegetative allocation. In 1991 reproductive allocation was positively related to vegetative allocation but there was no allometric relationship between vegetative and reproductive biomass. There may be genetic and/or environmental variation which affects allometric relationships for A. retroflexus