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DIGGIN' IN CLIMATE | By Carrie H. Wohleb Impacts of wildfire smoke on potatoes Seeing through the Haze The policy over much of the last century to quickly suppress wildfires no matter where they occur has led to a buildup of fuels in forests. This, coupled with some hot and dry conditions, has resulted in more frequent and intense wildfires across much of the U.S. and Canada. As many growers can attest, this increase in wildfires has had impacts on agriculture. What's in Wildfire Smoke? Wildfires discharge large amounts of carbon dioxide, black and brown carbon particles, and ozone precursors like hydrocarbons and nitrogen oxides that can form ozone when they mix with air. Many of these things are toxic pollutants that can be harmful to plants, people and animals when exposed to them in concentration. These emissions also modify solar radiation and other things that change the weather we experience at the ground level. Ozone Injury Related to Wildfires Ozone is one of the most damaging air pollutants for plants. It enters the leaf through stomata (pores on the surface of the leaf) during normal gas exchange processes. Once inside the plant, it forms reactive oxygen species that can damage cell membranes and cell contents. Exposure to high concentrations of ozone for several hours (acute exposure) can result in visible injury to leaf tissues. Symptoms may include stippling (small, brown- black spots), flecking (tiny, light-colored spots), bronzing and reddening. These symptoms appear within a few hours or days of exposure. With continued exposure to high concentrations of ozone, symptoms can progress to chlorosis, necrosis and early plant senescence. Exposure to more moderate levels of ozone for prolonged periods does not usually cause visible symptoms, but it can impair plant functions and limit growth. Ozone injury symptoms are occasionally seen on potatoes, but we don't see them every time there is smoke because ozone originating from wildfires is highly variable. Despite the presence of ozone precursors in smoke plumes, ozone doesn't always form. If it does form, it may not stick around for long. Cool and cloudy conditions inhibit ozone production, and windy conditions tend to dissipate ozone when it does form. This explains why I have only observed acute ozone injury symptoms on potatoes (and other plants) when wildfires were very close to the field, it was very hot and sunny, and air inversions kept the smoke low to the ground and limited its dispersal. How Does Smoke Affect Photosynthesis? Some potato growers have speculated that wildfire smoke is limiting yields by slowing photosynthesis. They may be right; however, quantifying the effects of smoke on photosynthesis is complex and depends on many details (plant species involved, quality of the smoke, etc.). It is more practical to consider its effects in general terms. The main factors that affect the rate of photosynthesis are light intensity, carbon dioxide concentration and temperature; whichever of these factors is most limiting will determine the rate. This is called the law of limiting factors. Smoke can affect these limiting factors in different ways, some positive for plant productivity and some negative. • Light intensity is reduced by smoke, but smoke almost never completely blocks the sun's rays. How dark it gets depends on the components of smoke and how much they are absorbing light or scattering it. Smoke with a lot of soot tends to absorb light, while smoke with ash and other light- colored particles tends to reflect it like 28 POTATO GROWER | APRIL 2022