Young poplar trees (Populus tremula Michx. x Populus alba L. clone INRA 717-1B4) were subjected to 120 ppb of ozone for 35 days in phytotronic chambers. Treated trees displayed precocious leaf senescence and visible symptoms of injury (dark brown/black upper surface stippling) exclusively observed on fully expanded leaves. In these leaves, ozone reduced parameters related to photochemistry (Chl content and maximum rate of photosynthetic electron transport) and photosynthetic CO(2) fixation [net CO(2) assimilation, Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) activity and maximum velocity of Rubisco for carboxylation]. In fully expanded leaves, the rate of photorespiration as estimated from Chl fluorescence was markedly impaired by the ozone treatment together with the activity of photorespiratory enzymes (Rubisco and glycolate oxidase). Immunoblot analysis revealed a decrease in the content of serine hydroxymethyltransferase in treated mature leaves, while the content of the H subunit of the glycine decarboxylase complex was not modified. Leaves in the early period of expansion were exempt from visible symptoms of injury and remained unaffected as regards all measured parameters. Leaves reaching full expansion under ozone exposure showed potential responses of protection (stimulation of mitochondrial respiration and transitory stomatal closure). Our data underline the major role of leaf phenology in ozone sensitivity of photosynthetic processes and reveal a marked ozone-induced inhibition of photorespiration.
Young trees of European beech (Fagus syl7atica L.) were phate carboxylase/oxygenase activity, while the summer and exposed in a phytotron to different levels of ozone and CO 2 early autumn harvests showed strong reductions in these under the climatic simulation of one vegetation period. High parameters. Only phosphoenolpyruvate carboxylase (PEPozone levels were simulated similar to high ozone concentra-case) activity remained higher under high ozone. The effects of high CO 2 appeared in general as a small stimulation in tion in the field (up to 110 ppb), while CO 2 was added as 300 ppm to the present level of ca 380 ppm. Our study describes enzyme activity like PEPcase in spring. However, with indifferent aspects of photosynthesis from the leaf level to the creasing time of fumigation, reductions of all parameters were observed. Especially chlorophylls showed strong reduc-reactions of selected thylakoid components at different hartions under high CO 2 . The combined treatment with high vest times during growth of the beech trees under the different fumigation regimes. Ozone effects appeared in the first weeks ozone plus high CO 2 resulted mostly in an amelioration of the of the treatment as a stimulation of chlorophyll fluorescence negative ozone effects, although control levels were not (F v /F m ), in oxygen production and in ribulose-1,5-bisphos-reached.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.