"Lettuce on water 'is spoiled' so less equipped for stress"

Growing on water is done in greenhouses for years, usually in a closed system in which water with nutrients circulates. Norwegian growers cultivate, among others, curly lettuce this way. 

The plant's roots grow and take up the nutrition they need. The unused nutrients are stored in large tanks. There, nutrients are added and harmful substances are filtered out, after which the water goes to the plants again. That cycle repeats about once every hour, so the need for water and nutrients is always covered. 

The method does often affect the plants in such a way that the leaf tips of older, and the leaf edges of younger plants are damaged. Now researchers have reason to believe there might be a connection between stress in plants and damage to the edges of the leafs. 

Stress in lettuce's relaxed life
Before, the damage was mainly attributed to a lack of calcium, but plant professor Sissel Torre of the NMBU (Environmental and Bio scientific University of Norway) is now working on a different theory: she thinks the lack of calcium is just one of the factors. Torre thinks that the optimized growing environment and the relaxed life of the lettuce may be the cause. For that research, she stresses the plants everyday by changing different climate factors to be able to study the plants' reaction. 

The lettuce is a little spoiled
What strikes the most, is that the damage occurs more often in lettuce grown on water, while the other circumstances are the same as plants grown in soil. 

Torre: "When plants grow in soil, the amount of water and nutrients varies, but when they're grown on water, water and nutrients are available all the time. The plants are less able to handle changing circumstances because they are used to an easy life."

Especially stress in spring
"The plants can't protect themselves well against fast changes like varying light circumstances, when the sun gets behind a cloud on a sunny day. According to Torre, the most damage occurs in spring, and in summer the littlest. In the summer, the circumstances are heavy the whole time, even in a greenhouse, because the plants have to endure strong sunlight during their whole development. In the Norwegian greenhouses, curly lettuce is grown in winter as well, but with the help of artificial light, and when the circumstances are stable and constant, the damage is the worst."

"Damage at the leaf edges can, in the worst case, lead to a harvest loss of up to 20 percent. That can lead to delivery problems for growers. Because the damage can be so big, the industry is working hard on finding solutions", the research reads.

Optimal growth gives less stress tolerance
"Growers want the plants to grow fast, because of energy usage, among other things, but the downside to a quick growth is that the plants are less capable to handle stress and can't manage little changes without developing damage. Finding the balance is a challenge for the growth on water." Torre: "We are busy trying to understand how plants take stress, and what we can do to develop more stress-resistant plants for water cultivation."

At the NMBU, curly lettuce is grown in a small system, based on growth in greenhouses. Here, researchers and students test various strategies to induce stress in plants and they can monitor the reaction. There's multiple ways to create this stress, like changing the humidity, temperature, amount and type of light, or multiple changes at a time. 

The optimized circumstances for a quick growth contribute to the fact that plants take longer to adapt to a change. The plants aren't trained to respond to quick changes. According to Torre there is no good solution available to completely prevent the damage, but the results seem to show that plants can handle more if during the day, quick changes in the climate are happening. 

A lot of advantages in water cultivation
As described before, plants grown in soil have to work a little harder to get their nutrients and water, which may cause them to grow slower. When grown in water they never suffer drought or other deficiencies and grow optimally. 

The water in the system is used multiple times and nothing flows away. Thus, less water and nutrients are used per grown head of lettuce. Many plants are suited for cultivation on water, think tomato, cucumber, bell pepper, strawberry, raspberry. 

Torre: "Cultivation on water is increasingly popular, both in commercial cultivation and at home. There are also a lot of developments in for instance technology. There's also more research to be able to make plants suitable for new systems."

Water cultivation can also be used in vertical farming and Torre adds that the method could be applied everywhere, from the desert up to space, as long as the plants have their water, light and nutrition. Torre is convinced that cultivation on water will be used more and more, especially in places where water or workable soil is hard to access. 

The cultivation project is a collaboration between, among others, the NMBU and five growing companies, and is financed by, among others, Grofondet (with Bama, Norgesgruppen and Gartnerhallen) and the Norwegian Research council.

Source: forskning.no

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