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

The impact of ozone (O-3) on the production and utilization of photoassimil ates was examined in Plantago major population 'Valsain'. Plants were expos ed in controlled environment chambers to either charcoal/Purafil(R)-filtere d air (CFA) or CFA plus O-3 (15 nmol mol(-1) O-3 overnight rising to a maxi mum 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 O-3 exposure. Destructive harv ests were made after 28 and 91 days. Impacts of O-3 on reproductive structu res were assessed at the final harvest. Daily carbon budgets constructed fo r the youngest fully expanded leaf revealed that at the younger growth stag e (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 r educed in O-3-treated plants. As a result, non-structural carbohydrates (ma inly starch) accumulated in source leaves during the day. 'Costs' associate d with dark respiration were increased in O-3-treated plants after 28 days fumigation. The effects of the pollutant on plant growth were consistent wi th shifts in leaf carbon metabolism; relative growth rate (li) between 0 an d 28 days fumigation was decreased by O-3, but the pollutant caused a stimu lation in R over the subsequent 63 days of the experiment. Despite this com pensatory growth response, plants exhibited a reduction in biomass and a de crease in the number of seeds produced per plant following exposure to envi ronmentally relevant O-3 concentrations. (C) 2000 Elsevier Science B.V. All rights reserved.