Morphological and physiological responses of the invasive weed Isatis tinctoria to contrasting light, soil-nitrogen and water

We evaluated the morphological and physiological responses to contrasting above- and below-ground resources for the invasive weed, Isatis tinctoria L. (dyerʹs woad). Plants were grown under low and high levels of light [shade (50% of ambient) and full sun], soil water (50 and 100 mL day^-1), and soil nitrogen (N) (0 and 20 mg N kg^-1 soil) in 8 L pots in 63-day glasshouse experiments conducted during winter and spring. Soil-N enrichment did not increase any of the growth variables (shoot and root dry mass, shoot:root ratio, leaf area, and specific leaf area) or physiological variables (stomatal conductance and 13C discrimination) in either experiment. The absence of plasticity in response to changes in soil-N supply suggests that I. tinctoria may survive and persist in nutrient-poor conditions by having low-N requirements, low-N productivity, or both. In contrast, plants compensated for shaded conditions by altering leaf area, specific leaf area, and shoot:root ratio to improve light capture. We discuss the potential mechanisms whereby response to these variable resources may be associated with a series of adaptations that favour the ability to tolerate and colonize harsh, nutrient-poor conditions, as well as invade shaded and undisturbed sites.