Above- and below-ground responses of Calamagrostis purpurea to UV-B radiation and elevated CO₂ under phosphorus limitation

UV-B radiation and elevated CO₂ may impact rhizosphere processes through altered below-ground plant resource allocation and root exudation, changes that may have implications for nutrient acquisition. As nutrients limit plant growth in many habitats, their supply may dictate plant response under elevated CO₂. This study investigated UV-B exposure and elevated CO₂ effects, including interactions, on plant growth, tissue chemistry and rooting responses relating to P acquisition. The sub-arctic grass Calamagrostis purpurea was subjected to UV-B (0 or 3.04 kJ m⁻² day⁻¹) and CO₂ (ambient 380 or 650 ppmv) treatments in a factorial glasshouse experiment, with sparingly soluble P (0 or 0.152 mg P per plant as FePO₄) a further factor. It was hypothesized that UV-B exposure and elevated CO₂would change plant resource allocation, with CO₂ mitigating adverse responses to UV-B exposure and aiding P uptake. Plant biomass and morphology, tissue composition and rhizosphere leachate properties were measured. UV-B directly affected chemical composition of shoots and interacted with CO₂ to give a greater root biomass. Elevated CO₂ altered the composition of both shoots and roots and increased shoot biomass and secondary root length, while leachate pH decreased. Below-ground responses to CO₂ did not affect P acquisition although P limitation progressively reduced leachate pH and increased secondary root length. Although direct plant growth, foliar composition and below-ground nutrient acquisition responses were dominated by CO₂ treatments, UV-B modified these CO₂ responses significantly. These interactions have implications for plant responses to future atmospheric conditions.