How can we make lettuce more resistant to the next heatwave?

For the past five years, PhD candidate Esther van den Bergh investigated the effect of light and temperature on bolting in lettuce. Here she explains how insights from her research can be used to make lettuce more resistant to the changing climate. Van den Bergh will defend her dissertation on 10 July.

“With the high temperatures of last week, it is better to find shaded place or stay inside and cool. Plants, however, have no option of going to a cooler place and will have to face any outdoor conditions that come to them.

The recent heatwave is not a one-time event, and warmer temperatures are predicted with the changing climate we are in. This also affects the crops we grow locally and eat, as they are currently not designed for warmer climates.

Even though wild lettuce is not commonly consumed, we can investigate how it reacts to heat, and if it has certain traits that we can use for our edible lettuce.

Stressed, bitter plants

For lettuce, warmer temperatures make the plant bolt, as the stressed plant wants to elongate its stem, make flowers and seeds as fast as possible. Bolting affects the marketability of the plant, as the leaves become bitter during this process.

To make local lettuce crops suitable for a changing climate, one can take different roads. We can make the current lettuce crops more resilient to the heat, so that they don't bolt as fast. Alternatively, we could also consider growing plants in indoor systems as vertical farms, as the climate can be regulated inside. However, indoor systems also have flaws, such as that space is limited and plants preferably are grown at high densities. And under high densities, lettuce plants tend to elongate their stem, again affecting the yield that can be achieved per square meter.

To make lettuce plants that don't elongate as quickly and that are therefore better suited for higher temperatures and high-density systems, the first step is to investigate how these processes happen in detail.

Crops and wild lettuce

So that is what I set out to do in my five years working with lettuce in my PhD. From the start of my PhD, we already knew that some lettuce variants react differentially to heat than others, and don't elongate as quickly. Variants grown in the south of Europe for example, are more suited for the heat than local variants.

Aside from variation that is available in crops that are being sold in the supermarket, we also had wild lettuce variants available from Europe and Asia. Wild lettuce or prickly lettuce (Lactuca serriola) does not resemble any lettuce you can buy in the supermarket but instead can just be found outside anywhere in the Netherlands. It is a bitter weed with spines on its leaves and stem. Centuries of domestication altered this plant in the common lettuce (Lactuca sativa) we have today. Even though wild lettuce is not commonly consumed, we can investigate how it reacts to heat, and if it has certain traits, like bolting less fast, that we can use for our edible lettuce.

It is of essential value that these fundamental studies are done, as they lay the basis for the future of our crops.

A changing climate, a changing lettuce

In my PhD, we grew 400 edible and prickly lettuces in a field in Limburg and assessed how quickly they bolted. By looking at the DNA of these lettuces and their responsiveness, we could pinpoint multiple parts of the DNA that were always present in those that bolted early.

Next, we wanted to investigate these areas more in depth, by doing a study in a climate-controlled room. In this room we can make the lettuce bolt by giving them more hours of light a day and by increasing the temperature. Not all parts of the DNA are "on" all the time. Since we can make the lettuce bolt when we want, we can then see which parts of the DNA are active during bolting, to get another insight into which parts of the DNA are important in this process.

A new world: indoor farming

Stem elongation in lettuce does not only affect yield in the field, but also in indoor farming, a production method that is projected to increase significantly in the coming decades. Lettuce has been optimized for growing outside, where there is a lot of light. In indoor systems other factors come to play, as the plants receive less light and plants are often grown close to each other. Low light conditions make lettuce elongate its stem, resulting in a spindly plant that cannot stand upright.

To gain more insight into this process, we grew 200 edible lettuces under low light. While the plants reacted very differently, it was not easy to map a DNA region to the process. This was likely the result of the process being regulated by many DNA regions at the same time. Through additional experiments, we found out that a specific plant growth hormone plays an important role in this process as well as a light receptor.

Lettuce look into the future

The identified parts of the DNA that are important to make lettuce bolt less quickly in the heat and more suitable for high-density indoor farming are a first step. Further research will continue the findings of this thesis to learn more about these DNA parts and to implement changes in the lettuce we eat. It easily takes ten to fifteen years of research before a crop reaches the supermarket. But it is of essential value that these fundamental studies are done, as they lay the basis for the future of our crops.”