Air pollution is a major problem in much of the world, with direct implications for crop yields. Studies have shown that soybeans, wheat, maize and other staple crops have reduced yields due to ozone pollution in particular – which is expected to keep rising in East and South Asia, Southern Europe and other regions until at least 2030.
A collaborative study involving scientists at the Centre for Ecology and Hydrology (CEH), Bangor, and SEI’s York Centre offers a new cause for concern: Soy plants are becoming more sensitive to ozone – particularly newer, high-yielding varieties. Steph Osborne, a PhD student with CEH and SEI York, explains below.
Q: What led you to do this study?
SO: Food systems are already under a great deal of pressure from climate change, soil degradation and population growth, and there is strong evidence that air pollution is a serious threat as well. We wanted to know whether soybean varieties growing in different parts of the world have different sensitivity to similar levels of ozone pollution. We were also intrigued by recent studies showing that modern wheat varieties are more sensitive to ozone pollution than historical ones. The authors of these papers hypothesized that this might be the result of crop breeding, so we wondered if we might see the same effect in soybean.
Q: How much research was there on that already?
SO: As far as we know, this is the most comprehensive study looking at how soybean yield responds to ozone pollution. We compared Asian and U.S. soybean data, which is not new, but we are the first to show an increase in ozone sensitivity over time. We also show the strongest evidence to date that there are differences across continents. This is important because experimental data from the U.S. tend to dominate the literature, and when applied to Asia, U.S. dose-response data may result in an under-estimate of the impact of ozone pollution on soybean crops.
Q: How much does ozone sensitivity vary among cultivars?
SO: We found that it varies substantially, which is important because it suggests there is plenty of scope for breeding varieties with higher tolerance to ozone pollution. We also found patterns. Sensitivity increased between the 1950s and 2000, and we also found that reported sensitivity was higher in India and China compared with the U.S.
Q: Is this sensitivity an unintended consequence of breeding for maximum yields?
SO: It could be. One way to increase yields is to select for higher photosynthetic rates, which requires an ability to absorb large amounts of carbon dioxide through the plant pores on the leaf surface (stomata). The problem is that these plants also absorb pollutants such as ozone at higher rates. So crop breeders have developed cultivars that grow faster but also are potentially more vulnerable to ozone.
Q: Do you have any sense of why soybean crops in India and China were more sensitive to ozone than those in the U.S.?
SO: We can’t really say for sure. It may be related to the biology of the varieties, or to the local conditions such as soil characteristics, temperature, humidity, or the presence of other air pollutants (e.g. nitrous oxide, sulphur dioxide).
In a humid environment, plants will lose less water to the atmosphere, so they tend to open their stomata wider and for longer to maximize carbon dioxide uptake to increase photosynthesis. If ozone is present in the atmosphere, this would mean they would also be take up more ozone. So in regions with high humidity during the soybean growing season, you might expect more ozone uptake and greater yield reductions.
Temperature could also play a role, because plant pores also respond to temperature – if it gets too hot or too cold, the stomata will close. But the higher sensitivity we observed in varieties from India and China might also be as a result of intrinsic characteristics specific to those varieties – for example, less ability to detoxify ozone, more stomata on the leaf surface, or a higher maximum rate of gas uptake.
Q: What insights do you think soybean producers and policy-makers should take from your research?
SO: We found that 17% of potential soybean yield is currently being lost as a result of ozone, so it’s definitely in the interest of plant breeders to try to breed more resilient soybean varieties. One way of doing this would be to breed varieties with a lower maximum rate of gas uptake through their stomata. There is a trade-off involved with this approach, because it would also reduce CO2 uptake and thus productivity.
In theory, these varieties might produce lower yields in “clean” years (because of reduced CO2 uptake), but might perform better in years with high levels of ozone pollution – so in places where pollution is common, the average yield over a number of years would rise. Such a strategy could be refined by growing ozone-tolerant varieties primarily in regions where high ozone levels are frequently reported, and non-tolerant varieties elsewhere.
I should note that we did not look at the role of climate change as a stressor, but it does need to be thought about in assessments of future ozone impacts, because of the potential for high ozone to co-occur with extreme climate events such as droughts and heat waves. At the moment we don’t know much about how these different stresses might interact with one another, and what the consequences might be for food security.
Ozone is also a greenhouse gas and a pollutant that harms human health, so reducing emissions of ozone precursors (nitrogen oxides and volatile organic compounds, including methane) is perhaps the most obvious response to the problem.
Q: What are your next steps?
SO: One of the main unanswered questions from this study is why the varieties from India and China showed higher sensitivity to ozone. For the last few years we have been working on a project which aims to find out more about exactly how exposure to ozone affects crop varieties from different world regions. This has involved carrying out large scale experiments at the Centre for Ecology and Hydrology solardomes in North Wales, where different varieties of soybean and wheat have been artificially exposed to ozone pollution to measure their physiological responses.
I also recently took part in experiments in India measuring the response of an Indian crop variety to ozone pollution within the local climate and environment. These experiments, part of an international research project that SEI York is involved with, may enable us to understand how climate extremes might influence crop sensitivity to ozone, and help us develop more sophisticated ways to predict ozone effects on crop yields.