Abstract
Several studies have highlighted the negative effects of ozone (O3) on wheat development and productivity. The negative effects of O3 are mediated by changes in photosynthetic carbon and nitrogen metabolism, which are difficult and time-consuming to assess and are thus only measured sporadically throughout the plant cycle. Stable isotope measurements in grains can help integrate the effects of chronic O3 exposure over the lifespan of the plant. This particular study focuses on the extent to which the stomatal conductance and productivity of Mediterranean wheat are related to carbon and nitrogen isotopic signatures under chronic O3 exposure. An open top chamber experiment was designed to analyse the effects of the pollutant on 12 Spanish wheat genotypes, which included modern cultivars, old cultivars and landraces. Four O3 treatments were considered. Stomatal conductance (gs) measurements were carried out during anthesis, and yield and nitrogen content parameters were taken at maturity, along with the carbon (δ13C) and nitrogen (δ15N) isotopic composition measured in grains. Modern and old cultivars responded similarly to O3 and were sensitive to the pollutant regarding yield parameters and gs, while landraces were more O3-tolerant. Grain δ13C had a strong negative correlation with grain yield and stomatal conductance across genotypes and O3 conditions, and increased under higher O3 concentrations, showing its capacity to integrate O3 stress throughout the wheat cycle. Meanwhile, a higher nitrogen concentration in grains, coupled with smaller grains, led to an overall decreased grain nitrogen yield under higher O3 concentrations. This nitrogen concentration effect within the grain differed among genotypes bred at different ages, following their respective O3-sensitivity. δ15N showed a possible indirect effect of O3 on nitrogen redistribution, particularly under the highest O3 concentration. The correlations of δ15N and δ13C to the usual effects of ozone on the plant suggest their potential as indicators of chronic ozone exposure.
Highlights
Even though tropospheric ozone (O3) is both a naturally occurring and anthropogenic gas, current global levels are mainly caused by photochemical reactions, due to precursors from industrial and urban emissions [1,2]
Mean Filtered Air (FA) values ranged between 20–35 nL L−1, well reproducing the natural background levels currently found in the area during the winter and early fall seasons
Non-filtered air (NFA) represents the common chronic high spring-summer levels of rural areas in the Central Iberian Peninsula, with values at the central hours of the day ranging between 50–60 nL L−1
Summary
Even though tropospheric ozone (O3) is both a naturally occurring and anthropogenic gas, current global levels are mainly caused by photochemical reactions, due to precursors from industrial and urban emissions [1,2]. Local and regional air masses can transport precursors throughout long distances before leading to ozone production. When high ozone precursors and O3-favorable meteorological conditions for atmospheric chemistry coincide, pollutant formation increases, causing chronic and extensive O3-pollution problems in rural and forested areas, far away from the source of emissions [3]. This, along with the complexity of atmospheric O3 photochemistry, coupled with high NOx levels, explains the difficulty in maintaining O3 levels under safe values for crops and ecosystems [2,4]. Background O3 levels still do not present a clear decreasing trend [5,6], and safe levels are not expected to be reached by the year 2030 [2]
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