The aim of the present study was to investigate the effects of
elevated CO2 on the antioxidative systems and the
contents of pigments, soluble protein and lipid peroxidation in leaves
of adult oaks, Quercus pubescens and Quercus
ilex, grown at naturally enriched CO2 concentrations. For
this purpose, a field study was conducted at two CO2
springs in Central Italy. Measurements of the pre-dawn water potentials
indicated less drought stress in trees close
to CO2 springs than in those grown at ambient CO2
concentrations. Most leaf constituents investigated showed
significant variability between sampling dates, species and sites. The
foliar contents of protein and chlorophylls
were not affected in trees grown close to the CO2 vents compared
with those in ambient conditions. Increases in
glutathione and other soluble thiols were observed, but these responses
might have been caused by a low pollution
of the vents with sulphurous gases. At CO2 vents, glutathione
reductase was unaffected, and superoxide dismutase
activity was significantly diminished, in both species. Generally, the
activities of catalase, guaiacol peroxidase and
ascorbate peroxidase as well as the sum of dehydroascorbate and ascorbate
were decreased in leaves from trees
grown in naturally CO2-enriched environments compared with those
grown at ambient CO2 concentrations. The
reduction in protective enzymes did not result in increased lipid peroxidation,
but increased monodehydroascorbate
radical reductase and dehydroascorbate reductase activities found in leaves
of Q. pubescens
suggest that the smaller pool of ascorbate was subjected to higher turnover
rates. These data show that changes
in leaf physiology persist, even after lifetime exposure to enhanced atmospheric
CO2. The results suggest that the
down-regulation of protective systems, which has also previously been found
in young trees or seedlings under
controlled exposure to elevated CO2 concentrations, might reflect
a realistic response of antioxidative defences in
mature trees in a future high-CO2 world.
