Effect of CO2 on growth, seed yield and nitrogen uptake in sunflower

Sunflower Hybrid (KBSH-1) was cultivated in Open Top Chambers (OTCs) at ambient (380 ppm) and elevated CO2 (550 and 700ppm) conditions. Biomass accumulation in root, stem, head and seed was significantly greater at elevated CO2 (550 ppm and 700ppm) compared with ambient (380 ppm).The improvement in total biomass with 550 and 700 ppm CO2was 18.7 per cent and 40.5 percent and seed yield was 21.0 per cent and 45.9 per cent respectively compared to ambient. The positive growth response to increased atmospheric CO2 resulted in N and C concentrations. Although elevated CO2 levels tended to lower N concentration in some plant parts compared to ambient, elevated CO2 enhanced total N accumulation in each plant component via an increase in biomass.Elevated CO2 decreased the N per cent compared to ambient in seed meal, leaf and stem. At the recommended dose of nutrients, the total N uptake was high in plants grown at 700 ppm (1.25 g per plant), followed by 550 ppm (1.11g per plant) and ambient (0.96 11 g per plant). At the recommended dose of nutrients, physiological N use efficiency (PE) and grain N use efficiency (NUEg) was also high in plants grown at elevated CO2 in OTCs and nitrogen harvest index (NHI) was similar for ambient (70.3%) and elevated CO2 conditions (70.4 and 70.2 per cent at 550 ppm and 700 ppm).The total carbon accumulation was high in plants grown at 700 ppm, followed by 550 ppm and ambient.The C/N ratio of plants was high at elevated CO2 conditions being 21.2, 20.0 and 19.0 at 700, 550ppm and ambient control respectively.