Weed photosynthetic and morphological responses to heterogeneous canopy light environments

Light is an important and often limiting resource for plants, but it serves also as a signal that triggers plant responses to heterogeneous light microenvironments. Weed plants growing within dominant crop canopies must respond to heterogeneous light microenvironments to maximize competitive ability. Previous studies have shown that leaf nitrogen (N) declines with canopy light and that leaf N content and photosynthetic capacity are highly correlated in monotypic plant communities. Similarly, previous research has demonstrated that plant perception of reduced red:far-red (R:FR) spectral content of light (caused by plant neighbors in the absence of shading) elicits morphological adjustments that may increase competitive ability in light-limited environments. However, little information regarding weed photosynthetic and morphological responses to heterogeneous light quantity and light quality microenvironments exists. Therefore, research was conducted to examine canopy patterns of leaf N and photosynthetic capacity of two weed species, common lambsquarters ( Chenopodium album L.) and redroot pigweed (Amaranthus retroflexus L.) when grown in monotypic stands and in corn-weed communities. Additionally, studies were conducted to determine whether vegetative-stage exposure to reduced R:FR, in the absence of mutual shading, was associated with changes in morphology or productivity of common lambsquarters and giant foxtail (Setaria faberi Herrm.). The results of the photosynthetic studies indicated that leaf N content and photosynthetic capacity were greater for leaves of subordinate weed plants grown in mixed communities than for leaves of dominant weed plants grown in monotypic communities at the same photon flux density (PFD) level. Results of light quality studies indicated that greater specific leaf area, greater leaf area allocated to mainstems than branches or tillers, greater specific stem length, and greater allocation of stem biomass to mainstems compared to branches or tillers occurred for common lambsquarters and giant foxtail in response to reduced vegetative-stage R:FR. However, vegetative-stage exposure to altered R:FR was not associated with substantial differences in reproductive-stage morphology, fecundity, or biomass of either species grown in shade or sun during the reproductive stage. These results provide a basis for improved descriptions of photosynthetic biomass production and biomass allocation in process-based models of crop and weed growth.