An assessment of ozone risk for date palm suggests that phytotoxic ozone dose nonlinearly affects carbon gain.

Tropospheric ozone (O ₃ ) is a significant phytotoxic air pollutant that has a negative impact on plant carbon gain. Although date palm (Phoenix dactylifera L.) is a globally important crop in arid or semi-arid regions, so far O ₃ risk assessment for this species has not been reported. This study estimated leaf- and plant-level photosynthetic CO ₂ uptake for understanding how elevated levels of O ₃ affects date palm biomass growth. Ozone risks to date palm plants were assessed based on exposure- (AOT40) or flux-based indices (Phytotoxic Ozone Dose, POD _y , where y is a threshold of uptake). For this purpose, plants were exposed to three levels of O ₃ [ambient air, AA (45 ppb as daily average); 1.5 × AA; 2.0 × AA] for 92 days in an O ₃ Free-Air Controlled Exposure facility. According to the model simulations, the negative effects of O ₃ on plant-level net photosynthetic CO ₂ uptake were attributed to reduced gross photosynthetic carbon gain and increased respiratory carbon loss. Season-long O ₃ exposure and elevated temperatures promoted the negative O ₃ effect because of a further increase of respiratory carbon loss, which was caused by increased leaf temperature due to stomatal closure. POD ₁ nonlinearly affected the photosynthetic CO ₂ uptake, which was closely related to the variation of dry mass increment during the experiment. Although the dose-response relationship suggested that a low O ₃ dose (POD ₁  < 5.2 mmol m ^-2 ) may even positively affect photosynthetic CO ₂ uptake in date palms, stomatal O ₃ uptake at the current ambient O ₃ levels has potentially a negative impact on date palm growth. The results indicate 5.8 mmol m ^-2 POD ₁ or 21.1 ppm h AOT40 as critical levels corresponding to a 4% reduction of net CO ₂ uptake for date palm, suggesting that this species can be identified as a species moderately sensitive to O ₃ .
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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