Combined cadmium and ozone treatments affect photosynthesis and ascorbate-dependent defences in sunflower

•  The combined effects of the two pollutants, cadmium and ozone, on sunflower (Helianthus annuus) metabolism are analysed here. •  Photosysnthetic processes and ascorbate metabolism were studied in sunflower plants grown for 15 d in the presence of cadmium and exposed to acute O₃ treatments. •  CO₂ assimilation rate was reduced in plants subjected to Cd(II) and/or O₃ treatments, but no alterations in stomatal conductance and F_v  : F_m ratio were observed. Rubisco activity was significantly reduced only in plants grown in the presence of cadmium indicating that the photosynthetic process is mainly altered by this factor. Photochemical quenching and the quantum efficiency of PSII in steady-state conditions were significantly depressed and nonphotochemical quenching increased in stressed plants. Cd(II) and O₃ also strongly affected ascorbate metabolism. •  The changes in ascorbate redox state and the increase in ascorbate-redox enzymes strongly supported an ascorbate over-utilization in Cd(II) and/or O₃ -treated plants. However, the increase in ascorbate-based detoxification mechanisms did not provide complete protection against the oxidative stress imposed by the two pollutants, since an increase in lipid peroxidation and protein oxidation accompanied a decrease in photosynthesis under pollutant exposure.