Effects of elevated carbon dioxide on plant growth and leaf photosynthesis of annual ryegrass along a phosphorus deficiency gradient.

Introduction: Soil phosphorus (P) deficiency limits plant growth and productivity in grassland ecosystems and may moderate the growth-promoting effects of "carbon dioxide (CO2) fertilization effect". Methods: To evaluate the interactive effects of these two factors on the growth and physiology for annual ryegrass (Lolium multiflorum Lam.), plants were grown in controlled growth chambers with a range of P supply (0.004, 0.012, 0.02, 0.06, 0.1 and 0.5 mM) under two levels of CO2 (400 and 800 µmol mol-1, respectively). Results: Elevated [CO2] dramatically increased the aboveground biomass and net photosynthetic rates of annual ryegrass by 14.5% and 25.3% under sufficient P supply (0.5 mM), respectively, whereas decreased the belowground biomass and net photosynthetic rates under lower P supply of P0.004, P0.02, and P0.06. Two-way ANOVA results showed that CO2 × P (p < 0.001) significantly affected stomatal traits, leaf photosynthesis and biomass. The stimulation of growth and photosynthesis by elevated CO2 concentration (e[CO2]) was reduced or highly suppressed, indicating that the sensitivity of annual ryegrass to P deficiency was enhanced under e[CO2]. Discussion: These results indicated that P limitation may offset the positive effects of e[CO2] on plant growth by altering stomatal traits, leaf photochemical processes and biochemical composition in annual ryegrass.