Ozone pollution may limit the benefits of irrigation to wheat productivity in India

Ground-level ozone (O3) pollution and heat and water stress are recognised as key abiotic stresses which threaten the ability of wheat yields to meet the growing demand for food production in India. The magnitude and interplay of O3 and water stress effects are tightly coupled via stomatal conductance and the transpiration pathway. Existing modelling methods that assess the stress response as a function of O3 and water vapour stomatal flux are applied to assess O3's role in limiting the productivity afforded by irrigation. We investigate the effect of these stresses on the grain yield of older (HUW-234) vs. recently released (HD-3118) Indian wheat cultivars under recent-past and future climates and O3 precursor emission profiles (using RCP4.5 and RCP8.5 scenarios). Water stress in rainfed conditions was modelled to analyse the trade-off between O3-induced vs. water-stress-induced yield loss to quantify the extent to which water stress mitigates O3 stress via reduced stomatal conductance. Under rainfed conditions for the years 1996-2005, the mean water-stress-induced and O3-induced yield losses for HUW-234 were 13.3 % and 0.6 %, respectively. The latter was a significant decrease from the mean O3-induced yield loss of 10.6 % modelled under irrigated conditions (i.e. no water stress). Similarly, under the RCP4.5 and RCP8.5 scenarios for the mid-century, water-stress-induced yield losses under rainfed conditions were 10.1 % and 20.0 %, while mean O3-induced yield losses were only 1.0 % and 0.1 %, respectively. Under irrigation, O3-induced yield losses increased to 18.5 % and 13.7 %, suggesting that O3 stress will negate the beneficial effects of irrigation. The cultivar HD-3118 suffered, on average, 0.2 % greater O3 relative yield loss (O3-RYL) than HUW-234 across all scenarios. The O3-RYL increased with climate change by 7.9 % under the RCP4.5 scenario and by 3.0 % under the RCP8.5 scenario compared to the recent-past climate. Together, these findings suggest that O3 may continue to substantially limit the productivity benefits of the use of modern cultivars bred for high gas exchange under irrigated conditions in India.