If the production time for cucumbers is to be reduced, then it is worth looking at the lighting to which the plants are exposed - especially when germinating in the "vertical farms" of the future. This is the conclusion of a new study by researchers from the Department of Food Science at Aarhus University in collaboration with the University of Copenhagen and Örebro University in Sweden. The study is, among others, funded by CiFOOD (Aarhus University Centre for Innovative Food Research).

More specifically, the combination of ultraviolet radiation (UV-B) and the monochromatic light, i.e. light with only one wavelength, has been the focus of the study. This is an area in which there has been very little knowledge so far - but one with great potential thanks to new LED technology.

The energy-rich UV-B radiation affects the plant's metabolism and morphology - the plant's shape and structure. The morphological reaction can be positive by strengthening the growth of the plant, but it can also be negative. Large amounts of UV-B can stress the plant, which respond by producting free radicals, that can damage the plant's DNA - and inhibit the plant growth.

Professor Carl-Otto Ottosen, Department of Food Science at Aarhus University and one of the researchers behind the study, elaborates:

- How the plant reacts to UV-B depends on whether the plant in question has previously been exposed to radiation, and thereby acclimated. In addition, the photosynthetic pigments and the chemical compounds with phenol, both found in the outer cell layers of the leaves, are among the factors that play an important role. One of the things that is very important is what plant species it is.

A sensitive plant

It is therefore no coincidence that the choice has fallen on the cucumber, in latin Cucumis sativus L. The cucumber is not only an important crop, but also one that grows quickly and is very sensitive to the composition of the light that it is exposed to.

In the experiment, cucumbers have been exposed to a low radiation of UV-B, and this has happened in connection with four different light treatments: a monochromatic blue, green and red as well as a broad-spectrum white light. Previous studies have often been based on a red-blue (almost purple) or white light, but there are a number of studies, which indicate that different monochromatic light conditions affect plant growth differently - and this explains the choice on the four color treatments.

In each of the four light treatments, the researchers looked at the plant's morphology and the plant's photosynthesis, such as the leaves' gas exchange, but also the plant's ability to adapt to light stress.

Carolina Palma from the Department of Food Science at Aarhus University has done the study af part of her PhD, and she has found that one of the four colors has a positive effect on the plant:

- When red light and UV-B go hand in hand, it has a positive effect on the formation of antioxidants in the cucumber plant, which makes the plant better able to defend itself against stress and diseases. We can also see that precisely the right combination of light colors can even reduce the negative effects of light on photosynthesis - in fact, the maximum speed of photosynthesis increases significantly, and so does the plant's respiration in the dark. However, green light also works well - photosynthesis is the same and the leaves become larger but thinner.

Important knowledge for vertical farming

With the development of LED technology, knowledge of the most optimal lighting conditions has become even more relevant. LED has a much higher energy efficiency, and LED lamps emit much less heat. Therefore, they can be placed close to the plant. This is particularly relevant for the vertical farms, which may have a greater significance in the food supply of the future - especially in the beginning of the production:

- In the vertical farms, the plants grow in many layers, where the sunlight is completely replaced with LED lights. The vertical farms have the advantage that more plants can grow on a smaller area, and it is therefore a relevant method for germination of plants and the first part of the growth. Thus, a light treatment with UV-B and monochromatic light in vertical farming can be an effective method to control growth, says Carl-Otto Ottosen, who is in no doubt about the many possibilities that the new knowledge offers:

- We see that UV-B can improve plant growth and make the plants more robust, but not only that, we also know that UV-B and monochromatic light can affect antioxidants and other ingredients, so the plants grown indoors can have more taste than they otherwise would have. The right light composition offers many opportunities to improve the food of the future with less resource consumption and less waste.