Experimental assessment of ozone risk on ecotypes of the tropical tree Moringa oleifera

Ozone (O₃) is an oxidative air pollutant that affects plant growth. Moringa oleifera is a tree species distributed in the tropical and subtropical regions. This species presents high morphological plasticity, which increases its ability to tolerate stressful conditions, but with no O₃ risk assessment calculated so far. The present study assessed the O₃ risk to different M. oleifera ecotypes using exposure-based index (AOT40) or flux-based index (PODy - where y is a threshold of O₃ uptake). PODy considers the O₃ uptake through the stomata and the consequence of environmental climate conditions on stomatal conductance (g_sto); thus, it is efficient in assessing O₃ risk. Five M. oleifera ecotypes were subjected to ambient (Amb.); middle (Mid. X1.5), and High (x2.0) O₃ concentrations for 77 days in a free-air controlled exposure facility (FACE). Leaf biomass (LB) was evaluated, and the biomass loss was projected assuming a clean atmosphere (10 ppb as 24 h O₃ average). The g_sto parameterization was calculated using the Jarvis-type multiplicative algorithm considering several climate factors, i.e., light intensity, air temperature, air vapor pressure deficit, and AOT40. Ozone exposure harmed the LB of all ecotypes. The high g_sto (~559 mmol H₂O m^-2 s^-1) can be considered the reason for the species' O₃ sensitivity. M. oleifera is adapted to hot climate conditions, and g_sto was restricted with air temperature (T_min) below ~ 9 °C. As expected, the PODy index performed better than the AOT40 for estimating the O₃ effect on biomass losses. We recommend a y threshold of 4 nmol m^-2 s^-1 to incorporate O₃ effects on M. oleifera LB. To not exceed a 4% reduction of LB for any M. oleifera genotype, we recommend the critical levels of 1.1 mmol m^-2 POD₄.