Elevated concentrations of atmospheric CO2 protect against and compensate for O-3 damage to photosynthetic tissues of field-grown wheat

The effects of elevated concentrations of atmospheric carbon dioxide and oz one on diurnal patterns of photosynthesis have been investigated in field-g rown spring wheat (Triticum aestivum). Plants cultivated under realistic ag ronomic conditions, in open-top chambers, were exposed from emergence to ha rvest to reciprocal combinations of two carbon dioxide and two ozone treatm ents: [CO2] at ambient (380 mu mol mol(-1), seasonal mean) or elevated (692 mu mol mol(-1)) levels, [O-3] at ambient (27 nmol mol(-1), 7 hr seasonal m ean) or elevated (61 nmol mol(-1)) levels. After anthesis, diurnal measurem ents were made of flag-leaf gas-exchange and in vitro Rubisco activity and content. Elevated [CO2] resulted in an increase in photoassimilation rate a nd a loss of excess Rubisco activity. Elevated [O-3] caused a loss of Rubis co and a decline in photoassimilation rate late in flag-leaf development. E levated [CO2] ameliorated O-3 damage. The mechanisms of amelioration includ ed a protective stomatal restriction of O-3 flux to the mesophyll, and a co mpensatory effect of increased substrate on photoassimilation and photosynt hetic control. However, the degree of protection and compensation appeared to be affected by the natural seasonal and diurnal variations in light, tem perature and water status.