Night-time heat is killing crops. Scientists are rushing to find resilient plants

As the sun sets outside with temperatures in the high 80s, where they’ll stay most of the night, several varieties of potted rice plants grow in two sections of a greenhouse on the roof of the Arkansas State University Biosciences Institute.

In one section, the greenhouse temperature is about the same as the temperature outside; in the other, it has been raised by 4C. Here, a dry, brittle flower droops from one of the plants, its development stunted by the heat.

This is the most important part of the day for Argelia Lorence, a professor of metabolic engineering, and her team’s research project. Because while the climate crisis is pushing daytime temperatures to record highs, those at night are rising significantly faster. This is a big problem for humans and animals, who struggle to cool their bodies at night. But it’s also a crisis for plants, which have fewer defense mechanisms available at night, posing a huge threat to the global food system.

“Because of how photosynthesis works, plants need cooler temperatures at night. And there are processes that are now being disturbed,” says Lorence.

Every 1C rise in night-time temperatures could cause wheat yields to drop by 6% and rice yields by as much as 10%. Hotter nights can also affect quality, making the rice chalky and less palatable and can even change its nutritional composition.

Lorence and her team at Arkansas State University Biosciences Institute are part of a race to figure out how to create varieties of rice – the main food source for billions of people and a vital crop for farmers around the world – that can withstand the impacts of a fast-changing climate.

In 2019, they launched a two-year experiment in rice breeder RiceTec’s experimental fields in Harrisburg, Arkansas, applying heat stress to different varieties of rice crops. The team built six customizable greenhouses that allow them to grow rice in field conditions during the day, and create higher temperatures at night. The greenhouses were built from a kit, assembled from parts “like Lego pieces”, including walls that slide back and forth and a roof that rolls back to expose plants to the air.

Researchers planted 1,920 seed packets by hand – 320 varieties of rice in each greenhouse – and waited for the plants to flower, which Lorence called “the most sensitive developmental stage in a rice plant”.

At night, during two weeks of the flowering stage, an automatic system raised the temperature in three of the greenhouses by 4C relative to the temperature in the other three control greenhouses – which matched the outside. This warming was hot enough to have a clear effect on the plants, but not so overwhelming as to kill them all outright.

The researchers traveled to and from the site daily during the experiment, manually opening the plastic roofs in the morning and closing them in the evening. At the end of summer, in August and September, the team harvested 30,000 plants, which were then taken back to campus for researchers to analyze which varieties fared better and why.

Lorence’s lab at Arkansas State, which butts up against the Mississippi River, the heart of the state’s $6bn rice industry, is part of a wider project to understand the impacts of nocturnal temperatures on crops called the Wheat and Rice Center for Heat Resilience (WRCHR), a collaboration which also includes the University of Nebraska-Lincoln and Kansas State University.

Krishna Jagadish, a founding researcher on the WRCHR project when at Kansas State and now a professor at Texas Tech, said one reason the rise in night-time temperatures can have a worse impact on crops is that plants don’t have as many defense mechanisms at night.

In the daytime, they can use their stomata – small pores on the leaf’s surface – to guard against the heat, but most of these are closed at night, said Jagadish. Plants also have nowhere to hide from night-time heat, unlike during the day when shade from crop canopies allow parts of a plant to escape the heat.

The research collaboration is helping develop new varieties of wheat and rice. “We can make these available for breeders. And then breeders can make that available for farmers to start plugging into the soil,” said agronomist Harakamal Walia, who is leading the project at the University of Nebraska-Lincoln.

His lab took images of 400 varieties of rice – grown individually in a hi-tech greenhouse – at every stage of development and analyzed the images to identify changes caused by temperature stress. They have already identified one gene that regulates grain width in rice: fie1. Varieties with this gene were less sensitive to high night-time temperatures. “They were able to sustain the grain weight even when the nights were warmer,” said Walia.

The process is a little trickier in wheat, which is harder to analyze and sequence because it has six separate sets of chromosomes compared with the two in rice. There’s been less research to determine the impact of high night-time temperatures on wheat and what characteristics might lead to greater resilience, Jagadish said. His former lab at Kansas State attempted to fill this gap.

Kansas State, which first developed the greenhouses used by Lorence’s team, used them to run similar experiments on wheat in 2019. The research team found that for field-grown wheat, high night-time temperature stress caused lower yields, lower grain weight, and a decrease in starch and protein.

In Arkansas, Lorence’s team is now deep in the laborious process of analyzing two years of data from their greenhouses. On this late July afternoon, her lab is crowded with a dozen researchers, busy placing granules of rice in trays for analysis, writing and running scripts for image analysis, or watering plants on the rooftop.

Lorence, originally from Mexico, has put together an international team of scientists. Crop physiologist and PhD student Cherryl Quiñones, originally from the Philippines, is studying how vitamin C could help plants be more resilient. Kharla Mendez, a PhD student who, like Quiñones, came to Arkansas State from her native Philippines, is examining how sugar and starch might also offer plants increased protection. And Karina Medina, a biotechnology post-doc from Mexico and the lab manager, is using robotic imaging on rice seeds to understand how higher night-time temperatures affect their weight, size and quality.

As her researchers gather their data, Lorence sends it to Gota Morota, a geneticist at Virginia Tech, who is running analyses to isolate the genes that seem to correlate with rice’s resilience to high night-time temperature.

The WRCHR hopes that the varieties, genes and phenotypes it isolates can be used by rice and wheat breeders to create new varieties of crops, which can then be passed on to farmers to keep their fields producing at the highest capacity, even as temperatures rise.

Breeders, the researchers hope, will be able to combine genes that are resilient to a number of stressors – not just high night-time temperatures but also drought, salinity and others – to create varieties of rice and wheat that could keep the world’s food supply consistent as the effects of the climate crisis continue to worsen.

The effects of high night-time temperatures on rice are already here. An intense heatwave that swept Bangladesh in April 2021 ruined tens of thousands of hectares of rice crops, affecting 300,000 farmers and causing losses of nearly $40m.

“When we look into parts of Bangladesh or eastern India, these combinations of stressors and [high night-time temperatures] alone have been happening for well over one and a half decades,” said Nese Sreenivasulu, researcher at the International Rice Research Institute, which also researches the effects of high night-time temperature on rice. Sreenivasulu’s research found that Bangladesh and southern and eastern India are already vulnerable to rising night – and daytime – temperatures.

“These stresses that affect crop yield and quality are going to become more and more frequent,” said Medina, the lab manager at Arkansas State. “We need to act now to develop the varieties that we’re going to need in the future.”

The work to understand and develop these heat-resilient crops is still just beginning. “We are just starting to unravel which genes can help us develop better crops in the future,” Lorence said. “There are a lot of genes still to be discovered, a lot of mechanisms still to be understood.”