The effects of elevated concentrations of atmospheric carbon dioxide and ozone on diurnal patterns of
photosynthesis have been investigated in field-grown spring wheat (Triticum aestivum). Plants cultivated under
realistic agronomic conditions, in open-top chambers, were exposed from emergence to harvest to reciprocal
combinations of two carbon dioxide and two ozone treatments: [CO2] at ambient (380 μmol mol−1, seasonal mean)
or elevated (692 μmol mol−1) levels, [O3] at ambient (27 nmol mol−1, 7 hr seasonal mean) or elevated (61 nmol
mol−1) levels. After anthesis, diurnal measurements were made of flag-leaf gas-exchange and in vitro Rubisco
activity and content. Elevated [CO2] resulted in an increase in photoassimilation rate and a loss of excess
Rubisco activity. Elevated [O3] caused a loss of Rubisco and a decline in photoassimilation rate late in flag-leaf
development. Elevated [CO2] ameliorated O3 damage. The mechanisms of amelioration included a protective
stomatal restriction of O3 flux to the mesophyll, and a compensatory effect of increased substrate on
photoassimilation and photosynthetic control. However, the degree of protection and compensation appeared to
be affected by the natural seasonal and diurnal variations in light, temperature and water status.