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Apex and Bristol cultivars of oilseed rape {Brassica napus) were irradiated with 0-63 W m~^ 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 major effects of UV-B were observed on end-product inhibition and stomatal limitation 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.Key-words: Brassica napus; electron transport; fluorescence; leaf gas exchange; photosynthesis; photosystem II; Rubisco; ultraviolet-B radiation.Abbreviations: A, net CO2 assimilation rate; A^.^^, light-saturated net CO2 assimilation rate; Cj, intercellular CO2 concentration; Fo, F^, E^, minimal, maximal and variable fluorescence yields; F,^', EJ, E^, maximal, variable and steady-state fluorescence yields in a light-adapted state; ^max.RuBP' maximum potential rate of electron transport contributing to RuBP regeneration; imax.psii' rate of PSII electron transport at saturating light and CO2; PPFD, photosynthetically active photon flux density; RuBP, ribulose 1,5-bisphosphate; Rubisco, ribulose 1,5-bisphosphate carboxylase/oxygenase; V^..^^^, maximum carboxylation velocity of Rubisco; ^psn, relative quantum efficiency of PSII photochemistry.

Long-term exposure of plants to elevated partial pressures of CO 2 (pCO 2 ) often depresses photosynthetic capacity. The mechanistic basis for this photosynthetic acclimation may involve accumulation of carbohydrate and may be promoted by nutrient limitation. However, our current knowledge is inadequate for making reliable predictions concerning the onset and extent of acclimation. Many studies have sought to investigate the effects of N supply but the methodologies used generally do not allow separation of the direct effects of limited N availability from those caused by a N dilution effect due to accelerated growth at elevated pCO 2 . To dissociate these interactions, wheat (Triticum aestivum L.) was grown hydroponically and N was added in direct proportion to plant growth. Photosynthesis did not acclimate to elevated pCO 2 even when growth was restricted by a low-N relative addition rate. Ribulose-1, 5-bisphosphate carboxylase/oxygenase activity and quantity were maintained, there was no evidence for triose phosphate limitation of photosynthesis, and tissue N content remained within the range recorded for healthy wheat plants. In contrast, wheat grown in sand culture with N supplied at a fixed concentration suffered photosynthetic acclimation at elevated pCO 2 in a low-N treatment. This was accompanied by a significant reduction in the quantity of active ribulose-1, 5-bisphosphate carboxylase/oxygenase and leaf N content.Growth at elevated pCO 2 frequently brings about change in plant physiology that is commonly interpreted as acclimation (Drake et al., 1997). Photosynthesis is inextricably involved because CO 2 is the substrate in C 3 species that is limiting at the current atmospheric pCO 2 . However, results from investigations on the effects of elevated pCO 2 on photosynthesis have been inconsistent. The stimulatory response brought about when pCO 2 is suddenly increased (Long, 1991) has often been found to decline with increasing duration of exposure (for review, see Gunderson and Wullschleger, 1994;Sage, 1994; Drake et al., 1997), but some experiments have failed to find any long-term effect, either in controlled environments (Radoglou and Jarvis, 1990;Wong, 1990) or in the field (Arp and Drake, 1991;Jones et al., 1995;Pinter et al., 1996). Why, then, is the acclimatory response so varied? Species differences can no doubt account for some of the variability, but often the same species in apparently similar conditions can yield different results with different investigators (Sage, 1994). This fact in itself suggests that there may be some uncontrolled factor(s) in the experimental design that may be crucial to the acclimatory response of photosynthesis.Evidence that additional factors may be interacting with the CO 2 response was brought to prominence by Arp (1991), who, after reviewing the data from several investigations using a variety of experimental designs, suggested that root restriction by pot size had a significant effect on the acclimatory response. Limited rooting volume was suggested to creat...

SUMMARYElevated [CO^] has been shown to protect photosynthesis and growth of wheat against moderately eIe-\-ated [O^]. To investigate the role of ozone exclusion and detoxification in this protection, spring wheat {Triticum aestivitm L. cv. Wembley) was grown from seed, in controlled-environment chambers, under reciprocal combinations of [CO^] at 350 or 700 //mol mol"' and [Og] peaking at < 5 or 60 nmol mol"', respectively. Cumulative ozone dose to the mesophyll and antioxidant status were determined throughout flag leaf development. Catalase activity correlated with rates of photorespiration and declined in response to elevated [COJ and/or [O,J. Superoxide dismutase activity was not significantly affected by either condition. Neither ascorbate nor glutathione content was enhanced hy elevated [COJ. In wheat, at moderately elevated [Oj], our results show that stomatal exclusion plays a major role in the protective effect of elevated [CO^] against O3 damage.