Even on the hottest days many people around the world wake up to a fresh cup of Joe.

However, heat waves, even for a short duration, may be preventing Coffea Arabica plants from producing flowers and fruit, according to new research.

A study by Oregon State University’s College of Forestry showed that the world’s most dominant coffee-plant species—which accounts for 65 percent of the commercial production of the nearly 20 billion pounds of coffee consumed globally each year— is at risk under high temperature stress.

The researchers investigated how leaf age and heat duration affected C. arabica’s recovery from heat stress during greenhouse testing. They found that the younger, “expanding” leaves were particularly slow to recover compared to mature leaves, and that none of the plants that endured the simulated heat waves produced any flowers or fruit.

“This emphasizes how sensitive Coffea arabica is to temperature,” lead author Danielle Marias, a plant physiologist with OSU’s Department of Forest Ecosystems and society, said in a statement. “No flowering means no reproduction which means no beans, and that could be devastating for a coffee farmer facing crop failure.

“Heat is very stressful to the plants and is often associated with drought,” she added. “However, in regions where coffee is grown, it may not just be hotter and drier, it could be hotter and wetter, so in this research we wanted to isolate the effects of heat.”

During the study, C. Arabica plants were heated in a growth chamber at 120 degrees Fahrenheit for either 45 or 90 minutes. 

The expanding leaves subjected to the 90-minute treatment took longer to recover physiologically as measure by photosynthesis—chlorophyll fluorescence, an indicator of photosynthetic energy conversion and the presence of nonstructural carbohydrates, which include starch and free sugars involved in growth, reproduction and other functions.

“In both treatments, photosynthesis of expanding leaves recovered more slowly than in mature leaves, and stomatal conductance of expanding leaves was reduced in both heat treatments,” Marias said.  “Based on the leaf energy balance model, the inhibited stomatal conductance reduces evaporative cooling of leaves, which could further increase leaf temperatures, exacerbating the aftereffects of heat stress under both full and partial sunlight conditions, where C. arabica is often grown.”

No matter the leaf age, the longer heat treatment resulted in decreased water-use efficiency, which could also worsen the effects of heat stress, particularly during drought.

The study was published in Ecology and Evolution.