Ozone uptake at night is more damaging to plants than equivalent day-time flux

Main conclusion Plants exposed to equivalent ozone fluxes administered during day-time versus night-time exhibited greater losses in biomass at night and this finding is attributed to night-time depletion of cell wall-localised ascorbate. Abstract The present study employed Lactuca sativa and its closest wild relative, L. serriola, to explore the relative sensitivity of plants to ozone-induced oxidative stress during day-time versus night-time. By controlling atmospheric ozone concentration and measuring stomatal conductance, equivalent ozone uptake into leaves was engineered during day and night, and consequences on productivity and net CO2 assimilation rate were determined. Biomass losses attributable to ozone were significantly greater when an equivalent dose of ozone was taken-up by foliage at night compared to the day. Linkages between ozone impacts and ascorbic acid (AA) content, redox status and cellular compartmentation were probed in both species. Leaf AA pools were depleted by exposure of plants to darkness, and then AA levels in the apoplast and symplast were monitored on subsequent transfer of plants to the light. Apoplast AA appeared to be more affected by light–dark transition than the symplast pool. Moreover, equivalent ozone fluxes administered to leaves with contrasting AA levels resulted in contrasting effects on the light-saturated rate of CO2 assimilation (Asat) in both species. Once apoplast AA content recovered to pre-treatment levels, the same ozone flux resulted in no impacts on Asat. The results of the present investigation reveal that plants are significantly more sensitive to equivalent ozone fluxes taken-up at night compared with those during the day and were consistent with diel shifts in apoplast AA content and/or redox status. Furthermore, findings suggest that some thought should be given to weighing regional models of ozone impacts for extraordinary night-time ozone impacts.

Characterisation of ozone deposition to a mixed oak-hornbeam forest. Flux measurements at 5 levels above and inside the canopy and their interactions with nitric oxide

In the framework of the European FP7 project ECLAIRE a joint field campaign was run in Marmirolo, in the northern part of the Italy, one of the most polluted areas in Europe due to intense industrial activities and peculiar climate conditions promoting high ozone formation by photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds. The studied ecosystem is a mixed oak-hornbeam forest and the aim of this field campaign was to investigate the processes regulating the gas exchange between the forest and the atmosphere with a focus on ozone flux measurements and the interaction with reactive gases. Measurements were run on a 40 m tower equipped with sonic anemometers and fast ozone analyzers for eddy covariance at five different heights: one at canopy height, two above canopy and two below canopy. NO fluxes were measured above canopy and NOx fluxes were measured with a dynamic chamber system at the soil level. Ozone fluxes measured at different levels above the canopy showed a good agreement between each other, while fluxes at 24 m were surprisingly higher than the above ones. In this paper we discuss the possible reasons for this discrepancies shedding light on the role of NOx and of the coupling between forest and atmosphere will be explained. A partition of the ozone fluxes will be shown too to identify the most relevant sinks in the soil-plant continuum.