Wheat yield losses in India due to ozone and aerosol pollution and their alleviation: A critical review

This review examines estimations made over the last two decades of the wheat yield losses in India due to elevated tropospheric ozone (O-3) and to aerosol pollution. Notwithstanding the poor record of O-3 and solar radiation measurement in wheat regions, a reasonable estimate can be made yield losses around 2010. From an average of the more credible studies, this is around 30% compared to the yield which would have been achieved with 1970 solar radiation and O-3 assumed to be <40 ppb (parts per billion by volume). The results suggest that O-3 causes two-thirds of the loss and reduced total solar radiation due to aerosols (but corrected for increased diffuse radiation) one-third. Ozone, in particular, appears to be still increasing. It is likely that increased stomatal conductance (e.g. due to more irrigation (97% of production now irrigated) and the unwitting consequence of breeding for increased potential yield) has added to current O-3 losses. Much is known about the physiology of O-3 damage in the wheat leaf and plant. What is lacking is a thorough search for genetic tolerance independent of stomatal conductance and potential yield penalties, and this requires, at least initially, many chamber and/or FACE (free-air CO2 enhancement) experiments under typical Indian field conditions. Genetic engineering could offer a solution for O-3 but only in the long term. Other solutions to pollution damage are very limited, but earlier planting should help wheat escape the rising O-3 level in the spring. The 40 million tonnes of wheat currently foregone annually by India due to these two pollutants, even if an overestimate, far exceeds estimates of wheat yield loss to date due to climate change and demands concerted action to reduce the sources of this pollution, bringing added direct advantages for other crops and for human health.