Exposure- and flux-based assessment of ozone risk to sugarcane plants. (March 2018) - Moura, Bárbara Baêsso
Abstract: Ozone (O3 ) is a toxic oxidative air pollutant, with significant detrimental effects on crops. Sugarcane ( Saccharum spp.) is an important crop with no O3 risk assessment performed so far. This study aimed to assess O3 risk to sugarcane plants by using exposure-based indices (AOT40 and W126) based on O3 concentrations in the air, and the flux-based index (PODy, where y is a threshold of uptake) that considers leaf O3 uptake and the influence of environmental conditions on stomatal conductance ( g sto ). Two sugarcane genotypes (IACSP94-2094 and IACSP95-5000) were subjected to a 90-day Free-Air Controlled Experiment (FACE) exposure at three levels of O3 concentrations: ambient (Amb); Amb x1.2; and Amb x1.4. Total above-ground biomass (AGB), stalk biomass (SB) and leaf biomass (LB) were evaluated and the potential biomass production in a clean air was estimated by assuming a theoretical clean atmosphere at 10 ppb as 24 h O3 average. The Jarvis-type multiplicative algorithm was used to parametrize g sto including environmental factors i.e. air temperature, light intensity, air vapor pressure deficit, and minimum night-time temperature. Ozone exposure caused a negative impact on AGB, SB and LB. The O3 sensitivity of sugarcane may be related to its high g sto (∼535 mmol H2 O m −2 s −1 ). As sugarcane is adapted to hot climate conditions, g sto was restricted when the current minimum air temperature (Tmin ) was below ∼14 °C and the minimum night-time air temperature of the previous day (Tnmin ) was below ∼7.5 °C. The flux-based index (PODy ) performed better than the exposure-based indices in estimating O3 effect on biomass losses. We recommend a y threshold of 2 nmol m −2 s −1 to incorporate O3 effects on both AGB and SB and 1 nmol m −2 s −1 on LB. In order not to exceed 4% reduction in the growth of these two sugarcane genotypes, we recommend the following critical levels: 1.09 and 1.04 mmol m −2 POD2 for AGB, 0.91 and 0.96 mmol m −2 POD2 for SB, and 3.00 and 2.36 mmol m −2 POD1 for LB of IACSP95-5000 and IACSP94-2094, respectively. Highlights: Free-air O3 exposure negatively impacted sugarcane biomass. Stomatal conductance was parameterized and O3 uptake by sugarcane leaves was estimated for the first time. Flux- (PODy ) and exposure-based (AOT40 and W126) functions were tested. The flux-based PODy was the best approach for assessing O3 risk in sugarcane. A critical level of 1.04 mmol m −2 POD2 is recommended for these two sugarcane cultivars.