Morphological response of weed and crop species to nitrogen stress in interaction with shading

Reducing herbicide use may result in residual weed flora in arable fields, increasing crop-weed competition for resources. In temperate cropping systems, light and nitrogen are the main resources for which plants compete. As weed competition increases, soil mineral nitrogen might become less abundant as the use of mineral fertiliser is reduced for environmental reasons (pollutions and biodiversity loss). Better understanding crops and weeds competitive abilities for nitrogen and light might help manage weeds and promote weed biological regulation by sowing more competitive crops. Our objectives were to (1) quantify the diversity in species morphological response to nitrogen stress, in interaction with shading and (2) investigate relationships between species responses and species features (clade, crop vs. weed, Ellenberg indexes). Response of three aboveground variables, important for light competition, (specific leaf area [SLA], leaf to aboveground biomass ratio [LBR] and height per unit aboveground biomass [HBR]) to nitrogen stress was measured on four crops and four weeds grown in a greenhouse under various nitrogen treatments and two light levels. Increasing SLA, LBR and HBR in response to nitrogen stress improve plant light competitiveness in a situation of nitrogen deficiency. SLA decreased in response to nitrogen stress for all species, with a higher decrease for nitrophilic species. LBR response to nitrogen stress varied between crops and weeds, weeds decreasing LBR while crops tended to increase it. HBR was independent of nitrogen stress for most species. Shading had a small to no effect on the morphological response to nitrogen stress. These results suggest that the investigated parameters characterise species intrinsic response of morphological variables to nitrogen stress. In the next future, integrating these species properties into a mechanistic model and performing simulations will allow drawing generic conclusions on species competitiveness in various situations.