R. Di Cagno et al., Combined cadmium and ozone treatments affect photosynthesis and ascorbate-dependent defences in sunflower, NEW PHYTOL, 151(3), 2001, pp. 627-636
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 sunflowe
r plants grown for 15 d in the presence of cadmium and exposed to acute O-3
treatments.
CO2 assimilation rate was reduced in plants subjected to Cd(II) and/or O-3
treatments, but no alterations in stomatal conductance and F-V : F-m ratio
were observed. Rubisco activity was significantly reduced only in plants gr
own in the presence of cadmium indicating that the photosynthetic process i
s mainly altered by this factor. Photochemical quenching and the quantum ef
ficiency of PSII in steady-state conditions were significantly depressed an
d nonphotochemical quenching increased in stressed plants. Cd(II) and O-3 a
lso strongly affected ascorbate metabolism.
The changes in ascorbate redox state and the increase in ascorbate-redox en
zymes strongly supported an ascorbate over-utilization in Cd(II) and/or O-3
-treated plants. However, the increase in ascorbate-based detoxification me
chanisms did not provide complete protection against the oxidative stress i
mposed by the two pollutants, since an increase in lipid peroxidation and p
rotein oxidation accompanied a decrease in photosynthesis under pollutant e
xposure. (C) New Phytologist (2001).