Early events in Populus hybrid and Fagus sylvatica leaves exposed to ozone.

This paper aims to investigate early responses to ozone in leaves of Fagus sylvatica (beech) and Populus maximowiczii x Populus berolinensis (poplar). The experimental setup consisted of four open-air (OA) plots, four charcoal-filtered (CF) open-top chambers (OTCs), and four nonfiltered (NF) OTCs. Qualitative and quantitative analyses were carried out on nonsymptomatic (CF) and symptomatic (NF and OA) leaves of both species. Qualitative analyses were performed applying microscopic techniques: Evans blue staining for detection of cell viability, CeCl3 staining of transmission electron microscope (TEM) samples to detect the accumulation of H2O2, and multispectral fluorescence microimaging and microspectrofluorometry to investigate the accumulation of fluorescent phenolic compounds in the walls of the damaged cells. Quantitative analyses consisted of the analysis of the chlorophyll a fluorescence transients (fast kinetics). The early responses to ozone were demonstrated by the Evans blue and CeCl3 staining techniques that provided evidence of plant responses in both species 1 month before foliar symptoms became visible. The fluorescence transients analysis, too, demonstrated the breakdown of the oxygen evolving system and the inactivation of the end receptors of electrons at a very early stage, both in poplar and in beech. The accumulation of phenolic compounds in the cell walls, on the other hand, was a species-specific response detected in poplar, but not in beech. Evans blue and CeCl3 staining, as well as the multispectral fluorescence microimaging and microspectrofluorometry, can be used to support the field diagnosis of ozone injury, whereas the fast kinetics of chlorophyll fluorescence provides evidence of early physiological responses.

Validation of the stomatal flux approach for the assessment of ozone visible injury in young forest trees. Results from the TOP (transboundary ozone pollution) experiment at Curno, Italy.

This paper summarises some of the main results of a two-year experiment carried out in an Open-Top Chambers facility in Northern Italy. Seedlings of Populus nigra, Fagus sylvatica, Quercus robur and Fraxinus excelsior have been subjected to different ozone treatments (charcoal-filtered and non-filtered air) and soil moisture regimes (irrigated and non-irrigated plots). Stomatal conductance models were applied and parameterised under South Alpine environmental conditions and stomatal ozone fluxes have been calculated. The flux-based approach provided a better performance than AOT40 in predicting the onset of foliar visible injuries. Critical flux levels, related to visible leaf injury, are proposed for P. nigra and F. sylvatica (ranging between 30 and 33 mmol O(3) m(-2)). Soil water stress delayed visible injury appearance and development by limiting ozone uptake. Data from charcoal-filtered treatments suggest the existence of an hourly flux threshold, below which may occur a complete ozone detoxification.

Visible leaf injury in young trees of Fagus sylvatica L. and Quercus robur L. in relation to ozone uptake and ozone exposure. An Open-Top Chambers experiment in South Alpine environmental conditions.

An Open-Top Chambers experiment on Fagus sylvatica and Quercus robur seedlings was conducted in order to compare the performance of an exposure-based (AOT40) and a flux-based approaches in predicting the appearance of ozone visible injuries on leaves. Three different ozone treatments (charcoal-filtered; non-filtered; and open plots) and two soil moisture treatments (watered and non-watered plots) were performed. A Jarvisian stomatal conductance model was drawn up and parameterised for both species and typical South Alpine environmental conditions, thus allowing the calculation of ozone stomatal fluxes for every treatment. A critical ozone flux level for the onset of leaf visible injury in beech was clearly identified between 32.6 and 33.6 mmolO3 m(-2). In contrast, it was not possible to identify an exposure critical level using the AOT40 index. Water stress delayed the onset of the leaf visible injuries, but the flux-based approach was able to take it into account accurately.