Effects of ozone on the production and utilization of assimilates in Plantago major

The impact of ozone (O3) on the production and utilization of photoassimilates was examined in Plantago major population ‘Valsain’. Plants were exposed in controlled environment chambers to either charcoal/Purafil-filtered air (CFA) or CFA plus O3 (15 nmol mol −1 O3 overnight rising to a maximum of 75 nmol mol −1 between 12:00 and 16:00 h). Measurements of leaf gas exchange and non-structural carbohydrate content were made on the youngest fully expanded leaf after 28 and 42 days of O3 exposure. Destructive harvests were made after 28 and 91 days. Impacts of O3 on reproductive structures were assessed at the final harvest. Daily carbon budgets constructed for the youngest fully expanded leaf revealed that at the younger growth stage (after 28 days exposure), net CO2 assimilation rate as well as the amount and relative proportion of newly fixed carbon exported in the light were reduced in O3-treated plants. As a result, non-structural carbohydrates (mainly starch) accumulated in source leaves during the day. ‘Costs’ associated with dark respiration were increased in O3-treated plants after 28 days fumigation. The effects of the pollutant on plant growth were consistent with shifts in leaf carbon metabolism; relative growth rate (R) between 0 and 28 days fumigation was decreased by O3, but the pollutant caused a stimulation in R over the subsequent 63 days of the experiment. Despite this compensatory growth response, plants exhibited a reduction in biomass and a decrease in the number of seeds produced per plant following exposure to environmentally relevant O3 concentrations. © 2000 Elsevier Science B.V. All rights reserved.