Exposure- and flux-based assessment of ozone risk to sugarcane plants

Ozone (O-3) is a toxic oxidative air pollutant, with significant detrimental effects on crops. Sugarcane (Saccharurn spp.) is an important crop with no O-3 risk assessment performed so far. This study aimed to assess O-3 risk to sugarcane plants by using exposure-based indices (AOT40 and W126) based on O-3 concentrations in the air, and the flux-based index (PODy, where y is a threshold of uptake) that considers leaf O-3 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 O-3 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 O-3 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 O-3 sensitivity of sugarcane may be related to its high g(sto) (535 mmol H2O m(-2) S-1). As sugarcane is adapted to hot climate conditions, g(sto) restricted when the current minimum air temperature (T-min) was below similar to 14 degrees C and the minimum night-time air temperature of the previous day (T-nmin) was below similar to 7.5 degrees C. The flux-based index (PODy) performed better than the exposure-based indices in estimating O-3 effect on biomass losses. We recommend a y threshold of 2 nmol m(-2) s(-1) to incorporate O-3 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.