New Method Measures Ozone Stress in Soybeans

Researchers have used SIF to measure the effects of elevated ozone (O3) on soybean plants.

Over the last decade, agriculture has relied on solar-induced chlorophyll fluorescence (SIF) measurements to detect plant stress. Researchers need methods to examine how air pollutants damage crops as the globe focuses on not just fixing the climate problem but also sustaining the world's food supply.

Plants absorb sunlight to power photosynthesis, and the unused energy is expelled as heat and a small glow undetectable to human eyes, a phenomenon known as fluorescence. SIF has been used by researchers to study photosynthetic dynamics since the first global SIF map was created in 2011. It has been used, for example, to evaluate how high amounts of carbon dioxide (CO2) or excessive temperature alter the attributes of plants.

SIF has now been utilised by a team from the University of Illinois Urbana-Champaign and the USDA Agricultural Research Service to assess the impact of high ozone (O3) on soybean plants. The findings were published in the Journal of Experimental Botany

"Researchers have found SIF to be a faster, safer, and noninvasive way to study photosynthesis," noted Genghong Wu, the work's first author and a former PhD student at the Department of Natural Resources and Environmental Sciences, under the supervision of Prof. Kaiyu Guan, the senior author of the work. "That is why it has become so popular. The novelty of this study is that for the first time, SIF was used to measure elevated ozone stress on soybeans in the field."

Ozone is a damaging air pollutant that is costly to farmers. The SoyFACE facility provides a testbed for studying the effects of ozone pollution in the field. It is managed by USDA ARS scientist and Prof. Lisa Ainsworth. For the current study, she designed the elevated O3 experiment with four plots as a control and the other four plots with higher amounts of O3.

The team used a portable spectroscopic system placed about half a meter above the plant canopy to take its measurements on both control and elevated O3 plots. They found that increased O3 levels resulted in a decrease in SIF, by as much as 36 per cent during the late growing season.

Other processes associated with photosynthesis, such as electron transport and leaf-gas exchange, were simultaneously measured along with SIF. "As we observed those levels decrease with higher ozone levels, it confirmed to us that a decrease in SIF is a sign of stress," Ainsworth said.

Although SIF is directly related to photosynthesis -- the process by which plants absorb light and turn it into chemical energy -- it isn't the only factor to influence SIF. But Wu notes that plant photosynthesis, combined with measures of the size of the plants[MAD3], can give farmers a good estimate of yield.

One of the advantages of SIF is that it is scalable. Wu is currently studying in Germany with colleagues, who use aircraft flying 1 kilometre off the ground to evaluate SIF's effects on an entire field. Alongside Prof. Kaiyu Guan, the Founding Director of the Agroecosystem Sustainability Center and a fellow investigator on this study, Wu hopes to use the method to track photosynthesis in regions around the world from a satellite orbiting the Earth.

"We want to use SIF to estimate or to monitor the dynamics of photosynthesis on a regional or global scale," Wu reiterated. "To do that, we need to also further understand the mechanistic relationship between SIF and photosynthesis."