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

Ozone (O-3) 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 O-3 risk assessment calculated so far. The present study assessed the O-3 risk to different M. oleifera ecotypes using exposure-based index (AOT40) or flux-based index (PODy -where y is a threshold of O-3 uptake). PODy considers the O-3 uptake through the stomata and the consequence of environmental climate conditions on stomatal conductance (g(sto)); thus, it is efficient in assessing O-3 risk. Five M. oleifera ecotypes were subjected to ambient (Amb.); middle (Mid. X1.5), and High (x2.0) O-3 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 O-3 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) (similar to 559 mmol H2O m(-2) s(-1)) can be considered the reason for the species' O-3 sensitivity. M. oleifera is adapted to hot climate conditions, and g(sto) was restricted with air temperature (T-min) below similar to 9 degrees C. As expected, the PODy index performed better than the AOT40 for estimating the O-3 effect on biomass losses. We recommend a y threshold of 4 nmol m(-2) s(-1) to incorporate O-3 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.

Automated summary

The study evaluated the sensitivity of different M. oleifera ecotypes to ozone risk, with high stomatal conductance being identified as a possible reason for the species' sensitivity to ozone.