The physiology of shade avoidance

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Competition between crop and weed has been regarded traditionally as a struggle for resources such as light, water and nutrients. The physiological mechanism by which weeds cause yield losses in crops is understood poorly. Early detection of neighbouring plants through the red-to-far red (R:FR ) ratio may be an important mechanism affecting plant to plant competition. Crop plants can detect changes in the R:FR ratio caused by the presence of neighbouring weeds and as a result will grow initially taller, with less branching and less root development. This shade avoidance response may delay phenological development and come at a cost to reproductive fitness. No study has been conducted previously to associate the early detection of the R:FR ratio as a mechanism to explain the outcome of crop and weed competition. In this study, I propose to explore the physiological and molecular mechanisms involved in the shade avoidance response of corn and soybean caused by the presence of weeds. Preliminary research has identified a wide range in corn and soybean germplasm phenotypic response to R:FR ratio reflected from the leaf surface of weeds. I plan to use this within crop variation to explore the following hypotheses: Objective 1: to determine if corn hybrids and soybean cultivars differing in response to R:FR are constrained phenotypically during their development and perform suboptimally in terms of biomass accumulation. Objective 2: to determine if corn hybrids and soybean cultivars differing in response to R:FR experience a similar cost of shade avoidance, as measured by a loss of phenotypic plasticity to subsequent abiotic stresses or expressed as yield loss. Objective 3: to discover if alterations exist in key metabolic and signalling pathways associated with weed-induced shade avoidance in corn hybrids and soybean cultivars that differ in their response to R:FR ratio. New knowledge generated from this study will be used by plant breeders to develop more stress tolerant corn hybrids and soybean cultivars.