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RESEARCH PRODUCT

The response of weed and crop species to shading: measurement and prediction from traits

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description

International audience; Crops compete with weeds for light, and choosing competitive crop species is a major lever for managing weeds. The present study aimed to (1) measure the range of species parameters that drive light competition in contrasting crop and weed species of temperate European arable crops, (2) relate the parameter values which are difficult to measure to species traits that are easier to access, by establishing trait-parameter relationships, (3) integrate the measured parameter values into the FlorSys model which simulates weed dynamics and crop canopy growth in virtual fields over the years with a daily time step, and (4) run simulations to investigate which crop and weed parameters are linked to weed harmfulness for crop production. 25 weed and 30 crop species were investigated. Parameters driving initial growth were measured in optimal light and nutrient conditions in a greenhouse with automatic non-destructive measurements. Parameters describing potential morphology in unshaded conditions were measured on individual plants grown in optimal light and nutrient conditions in garden plots and harvested at 4-5 stages during plant cycle. Shading response was measured by comparing potential morphology to that of plants grown under shading nets in these same gardens. Crops vs weeds presented a larger leaf area at emergence, a smaller specific leaf area (SLA); they were narrower for a given plant biomass, and their leaf area was less evenly distributed along plant height. Weeds vs crops responded more to shading, increasing their SLA, leaf biomass ratio, plant height and width per unit biomass. All parameters were predicted with functional relationships from seed (weight, lipid content...), plant (epigeal vs hypogeal growth, form...) and general traits or characteristics (clade, base temperature, plant lifespan, legume vs non-legume...). For instance, relative growth rate (RGR) was larger for monocotyledonous vs dicotyledonous species; it increased with base temperature, seed weight and lipid content. When shaded, the species that the most increased their plant height per unit biomass were rosette-shaped, summer annuals, legumes, shade-loving (low Ellenberg L indicator) etc. These parameters were introduced into the FlorSys weed dynamics model. 272 cropping systems from 7 regions in France and Spain were simulated over 30 years and repeated with 10 weather series. Crop species with the lowest weed-caused yield loss grew fast after emergence (high RGR), had thinner larger leaves (high SLA), were wide rather than tall (larger plant width and smaller plant height per unit plant biomass), and allocated biomass preferentially to stems vs leaves (lower leaf biomass ratio). Harmful weed species presented a large leaf area at emergence and strongly responded to shade, by increasing their height, leaf biomass and area per plant biomass unit.