Holy crop! What’s happening with these soybeans will plough through your mind.
Actual Title: Current and future consequences of tropospheric ozone on soybean biochemistry, physiology and yield
Crop losses to the damaging effects of tropospheric ozone in the United States are estimated to cost $1-3 billion annually. One of the world’s most important oilseed crops, soybean (Glycine max [L.] Merr.), is particularly sensitive to O3 with current estimated losses of 8.5-14% depending on genotype and environmental conditions. Zea mays (maize) is the most important food crop globally in terms of production, and has previously been classified as moderately sensitive to O3. In the United States, soybeans and maize are commonly grown in a crop rotation with each other. The Midwestern United States “Corn Belt” produces 38%% of the world’s maize and 34%% of the world’s soybean crops (USDA FAS), and currently experiences O3 concentrations that are high enough to negatively impact yields. In my dissertation research I approached the problem of crop loss to O3 in three ways. Since soybean sensitivity to O3 has already been demonstrated I first tested the hypothesis that there is cultivar variation in the antioxidant, photosynthetic, and yield responses of soybean to growth at ambient and double ambient [O3] under field conditions. Ten cultivars of soybean were grown at elevated [O3] from germination through maturity at a fully open-air agricultural field location in the Soybean Free Air Concentration Enrichment (SoyFACE) facility in 2007, and six of those were grown again in 2008. In order to determine what parameters could be used to predict the sensitivity of seed yield to elevated [O3], photosynthetic gas exchange, fluorescence, chlorophyll content, antioxidant capacity, and leaf area index were monitored. Doubling the [O3] over ambient in those years decreased soybean yields by an average of 17%, with a range of 8-37% depending on cultivar and year. Chlorophyll content and photosynthetic parameters were positively correlated with seed yield, while antioxidant capacity was negatively correlated with photosynthesis and seed yield, suggesting a possible shift in the carbon balance between antioxidant metabolism and carbon gain. Exposure-response curves derived from these results indicate that breeding has not inadvertently selected for O3 tolerance. While I calculated an exposure response of different genotypes to O3 from different years of treatment at SoyFACE, variation in temperature, moisture availability and planting date could potentially interact with O3 to alter the response of soybean to the pollutant. Therefore, the experiments present in Chapter 3 are for soybean exposed to nine different concentrations of [O3] (38 ppb to 120 ppb) in each of two growing seasons, in order to measure the physiological and agronomic O3 dose response. All genotypes responded similarly with O3 exposure causing a linear decrease in leaf area, light absorption, specific leaf mass, primary metabolites, seed yield, and harvest index, while antioxidant capacity linearly increased. Although the two growing seasons experienced different temperature and rainfall patterns, there was a robust linear seed yield decrease of 37-39 kg ha-1 per ppb of cumulative O3 exposure over 40 ppb (AOT40). The existence of immediate effects of O3 exposure on photosynthesis, stomatal conductance, and photosynthetic transcript abundance before and after initiation and termination of fumigation of O3 fumigation were concurrently assessed, but there was no evidence of an instantaneous photosynthetic response. Growing season-long O3 exposure, however, negatively impacted the ability of the soybean canopy to intercept radiation, the efficiency of photosynthesis, and harvest index, suggesting that there are multiple targets for improving soybean responses to this damaging air pollutant. To further explore the exposure-response of soybean, and to better understand how it is affecting yields in the Midwest United States, a region which produces nearly 40% of the world’s soybean and ~36% of maize crops, the study in Chapter 4 approached yield responses on a larger scale, using time series modeling to determine the O3 response of soybean and maize. Time series models are commonly used to predict the potential effects of climate change on crop yields on a large scale, in an agronomic setting, over many years and growing conditions using historical observations of seed yield and measurements of weather. In this study temperature and O3 were negatively correlated with soybean and maize yield, while water availability was positively correlated. Accounting for the colinearity in weather variables, O3 significantly decreased maize yield by 163 kg ha-1 and soybean yields by 55 kg ha-1 for every 1 ppm h increase in AOT40 over the growing season. In this dissertation I demonstrate significant intraspecific variability of soybean yield response to doubled ambient [O3], correlate the yield response to physiological and biochemical parameters measured late in the growing season, and discuss potential ways to screen germplasm for tolerance to O3. I then develop an O3 exposure-response for soybean, and estimate a loss of 37-39 kg ha-1 per ppm h AOT40 for field-grown soybean. I quantitatively parse the yield loss into decreases in the efficiencies of light interception, solar energy conversion into biomass, and partitioning efficiency, further supporting the conclusion that there are multiple opportunities for selection of soybean tolerance to this harmful pollutant. Finally, I utilize historical yield, O3, and meteorological data to show that the five greatest soybean and maize producing states are currently suffering significant yield losses due to O3 air pollution at a rate of 55 kg ha-1 for soybean and 163 kg ha-1 for maize per ppm h over AOT40. These effects of O3 estimated from data collected over the last quarter century underscore the importance of including an O3 exposure term in statistical models of crop and ecosystem responses to global climate change. They further suggest that developing O3 tolerance in maize, as well as soybean, should be a target for improving current and future crop production.