The Sequence of Change within the Photosynthetic Apparatus of Wheat following Short-Term Exposure to Ozone

Abstract

The basis of inhibition of photosynthesis by single acute 03 exposures was investigated in vivo using analyses based on leaf gas exchange measurements. The fully expanded second leaves of wheat plants (Triticum aestivum L. cv Avalon) were fumigated with either 200 or 400 nanomoles per mole 03 for between 4 and 16 hours. This reduced significantly the light-saturated rate of CO2 uptake and was accompanied by a parallel decrease in stomatal conductance. However, the stomatal limitation, estimated from the relationship between CO2 uptake and the intemal CO2 concentration, only increased significantly during the first 8 hours of exposure to 400 nanomoles per mole 03; no significant increase occurred for any of the other treatments. Analysis of the response of CO2 uptake to the internal CO2 concentration implied that the predominant factor responsible for the reduction in lightsaturated CO2 uptake was a decrease in the efficiency of carboxylation. This was 58 and 21% of the control value after 16 hours at 200 and 400 nanomoles per mole 03, respectively. At saturating concentrations of C02, photosynthesis was inhibited by no more than 22% after 16 hours, indicating that the capacity for regeneration of ribulose bisphosphate was less susceptible to 03. Ozone fumigations also had a less pronounced effect on lightlimited photosynthesis. The maximum quantum yield of CO2 uptake and the quantum yield of oxygen evolution showed no significant decline after 16 hours with 200 nanomoles per mole 03, requiring 8 hours at 400 nanomoles per mole 03 before a significant reduction occurred. The photochemical efficiency of photosystem 11 estimated from the ratio of variable to maximum chlorophyll fluorescence and the atrazine-binding capacity of isolated thylakoids demonstrated that photochemical reactions were not responsible for the initial inhibition of CO2 uptake. The results suggest that the apparent carboxylation efficiency appears to be the initial cause of decline in photosynthesis in vivo following acute 03 fumigation.Ozone is widely recognized as a major air pollutant affecting crop yields ( 12

Effects of exposure to ozone and water stress on the following season's growth of beech (Fagus sylvatica L.)

“…Tbe closure of the stomata observed during ozone exposure may have reduced assimilate acquisition. The negative impact of ozone on photosynthesis has been well documented (Reich & Amundson, 1985;Moldau, Sober & Sober, 1990;Farage et al, 1991), but our resources did not allow us to ascertain whether stomatai closure was the result of, or the cause of, reduced photosynthesis. Assimilated carbon may have been partitioned differently during tbe ozone exposure and this may have accounted for some portion of the response.…”

The influence of CO2 and O3, singly and in combination, on gas exchange, growth and nutrient status of radish (Raphanus sativus L.)

“…1). Recent reports show that this decline in photosynthesis is primarily due to a decrease in carboxylation efficiency (Farage et al, 1991), but other factors also contribute, including increased stomatal limitation, reduced photochemical efficiency of PS II and a decline in chlorophyll content. The marked reduction in photosynthesis in Og-treated leaves overshadowed marginal effects on E achieved through a sligbt reduction in stomatal conductance, so there was a pronounced decline in WUE,.…”

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV‐B

“…Measurements of CO2 assimilation by gas analysis on attached leaves were performed as described previously (Farage et al 1991) but with a combined CO2 and H2O analyser (LI-6262, LICOR, Lincoln, NE, USA). The inlet CO2 concentration (Q) was controlled by a gas-dilutor (GD-600, ADC Ltd, Hoddesdon, UK) and leaf temperature was maintained at 23 ± 0-5 °C.…”