Abstract
The effects of nitrogen (N) deposition, tropospheric ozone (O3) and their interaction were investigated in two Mediterranean tree species, Fraxinus ornus L. (deciduous) and Quercus ilex L. (evergreen), having different leaf habits and resource use strategies. An experiment was conducted under controlled condition to analyse how nitrogen deposition affects the ecophysiological and biochemical traits, and to explore how the nitrogen-induced changes influence the response to O3. For both factors we selected realistic exposures (20 kg N ha-1 yr-1 and 80 ppb h for nitrogen and O3, respectively), in order to elucidate the mechanisms implemented by the plants. Nitrogen addition resulted in higher nitrogen concentration at the leaf level in F. ornus, whereas a slight increase was detected in Q. ilex. Nitrogen enhanced the maximum rate of assimilation and ribulose 1,5-bisphosphate regeneration in both species, whereas it influenced the light harvesting complex only in the deciduous F. ornus that was also affected by O3 (reduced assimilation rate and accelerated senescence-related processes). Conversely, Q. ilex developed an avoidance mechanism to cope with O3, confirming a substantial O3 tolerance of this species. Nitrogen seemed to ameliorate the harmful effects of O3 in F. ornus: the hypothesized mechanism of action involved the production of nitrogen oxide as the first antioxidant barrier, followed by enzymatic antioxidant response. In Q. ilex, the interaction was not detected on gas exchange and photosystem functionality; however, in this species, nitrogen might stimulate an alternative antioxidant response such as the emission of volatile organic compounds. Antioxidant enzyme activity was lower in plants treated with both O3 and nitrogen even though reactive oxygen species production did not differ between the treatments.
Highlights
Mediterranean forests are subjected to challenging environmental conditions in the current global change context, since many stressors, individually or in combination, affect plant functionality simultaneously or successively over time [1]
After an acclimation period to nitrogen, when the cumulative dose was roughly equivalent to 14 kg N ha yr-1, which was within the threshold load considered as critical for Mediterranean vegetation, nitrogen concentration at the leaf level did not increase in both species, and no adverse effect was detected on photosynthetic traits
In the presence of high nitrogen availability, F. ornus can cope with incoming oxidative stress via the production of nitric oxide (NO), which owing to the rapid synthesis and prompt availability, allows prompt scavenging of free radicals and concomitantly increases the photosynthesis rate [83]
Summary
Mediterranean forests are subjected to challenging environmental conditions in the current global change context, since many stressors, individually or in combination, affect plant functionality simultaneously or successively over time [1]. Leaf gas exchange is affected by O3 through a direct impact on stomatal guard cell functionality [13,14] or stomatal number [15], as well as owing to a decrease in the photochemical and carboxylation efficiency [15,16,17,18] Leaf structural traits such as leaf mass area (LMA), in addition to leaf nitrogen and carbon concentrations, have been found to reveal ozone sensitivity and tolerance in different species [19,20], where species with low LMA and high leaf nitrogen concentration show higher O3 sensitivity [11,21]. O3 can adversely influence these functional traits, accelerating leaf senescence processes [22,23,24]
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