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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 he affected by the natural seasonal and diurnal variations in light, temperature and water status. [ABSTRACT FROM AUTHOR]

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m-2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bisphosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO2-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No majoreffects of UV-B were observed on end-product inhibition and stomatallimitation of photosynthesis or the rate of photorespiration relative to CO2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO2 assimilation. [ABSTRACT FROM AUTHOR]