Climate change is causing more extreme weather conditions. Paradoxical as it may sound, the future will be both warmer, colder, wetter and drier.

That's why researchers around the world are working hard to find varieties and species of agricultural crops that are more resilient and can survive the changing weather conditions.

A part of this research is finding out how plants respond to different types of stress.

Several different types of stress

At the Department of Food Science at Aarhus University, one of the country's leading researchers in stress and multi-stress in plants is walking around between greenhouses and climate chambers. Here, different plants such as lupins, chilies and tomatoes are placed under different climatic conditions and measured, weighed and analysed from top shoot to root system.

The professor's name is Carl-Otto Ottosen, and he has been working with plant cultivation in both the field and in greenhouses for almost 40 years. Most of the latest projects he has been involved in have been about how plants cope with stress.

Just like humans, plants can be exposed to many different stressors. These can be factors such as drought, heat, flooding, and salt from seawater that all can affect plant growth. Some plants are naturally better at tolerating certain stresses.

Most of the research regarding stress has been done on one type of stress at a time, but plants react differently to e.g. heat and drought, that often coincide. And this poses a challenge.

How does stress affect plants?

One of the things typically measured, when measuring stress in plants, is how the stomata - the pores in the leaves - react. The stomata typically open to evaporate water when the temperature rises, but when the plants dry, the stomata gradually close.

Some more tolerant varieties try to keep the stomata open under stress, allowing the leaves to stay green and photosynthesise instead of wilting.

You can measure how open the stomata are, by placing the plant in a climate chamber and measuring the gas exchange to show whether the plant takes up or releases carbon dioxide (CO2). By doing this it is possible to tell whether the stomata are open.

Another way of measuring a plant’s stress response is by looking at what is called chlorophyll fluorescence. When a plant’s photosynthesis is under stress, the plant releases heat or a weak red light that is measurable.

Measurements from both flowers and leaves

In recent experiments, the researchers divided tomato plants into four groups - a control group that was not exposed to any type of stress, a group that was exposed to heat stress, a group that was exposed to drought stress, and a group that was exposed to both heat and drought stress.

Normally, when you measure plant stress, you measure the plant's leaves. However, in this study, which was a part of Nikolaj Bjerring Jensens PhD project, the sepals (the green leaves that surround the flower) were also measured to see if the stomata in the two different types of leaves reacted differently.

"In the experiment, we chose to look at the regulation of stomata in leaves and flowers of tomatoes, which has not been studied before. The plants were exposed to heat or drought stress and a combination of the two stresses, and then we also looked at what happened to them when the stress disappeared," says the professor.

The plants that were subjected to drought stress developed leaves with fewer stomata that were more closed, resulting in lowered photosynthesis and warmer leaves. But the exact opposite happened when the researchers measured the sepals of the flowers. When the leaves became warmer, the flowers stayed cooler.

"The study clearly shows that when the plant is stressed and has to prioritise, it is the flower that it spends energy on. It doesn't matter as much if a leaf is damaged, the flower is protected to secure the seed production and the survival of the species," says Carl-Otto Ottosen.

Even when the stress disappeared, the plants retained the same patterns in the stomata. The study furthermore showed that evaporation in flowers is regulated by temperature, but evaporation in leaves is regulated by water availability.

The eternal trade-off

The study is an important step towards finding out which varieties are most resistant to climate change.

But more research is needed on multi-stress, and resilience is not straightforward, says the professor:

"In this experiment, we looked at the combination of heat and drought stress, but what if there is a week of frost in late Spring. The unpredictability of the weather is clearly one of the biggest challenges for the future of food cultivation," says Carl-Otto Ottosen.

He would also like to see us become more sustainable and to produce more of our food locally so that we are less dependent on transport and imports from other countries. But in order for this to be possible, the willingness to pay must follow suit and we need more robust and tolerant crops.

"Basically, it's about ensuring security of supply locally, so we're not so sensitive in the supply chain. The more local we can eat, the better," he says.