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In search of optimal LED cultivation: 'Plant does not cool by evaporation alone'

In the switch to (full)LED cultivation, tomato growers are busy optimizing their crops. Moisture and evaporation are key concerns. In the search for solutions to cultivation problems, sometimes resulting in 'discolored tomatoes,' much attention is paid to evaporation. Gerrit Vermeer, former cherry tomato grower and now PUM advisor in tropical countries, points out that plants manage energy even without evaporation. They do that themselves, in growth. "You can smell it as a grower, the smell of growth is in the water vapor," he says.

As growers, the key is to understand how plants do that manage growth. With that knowledge, they can make decisions in their climate management. Knowledge of a whole range of balances helps with this, Gerrit is convinced. Although, he also knows that it is "quite a complex whole" that still needs a lot of research to learn how the balances (Gerrit mentions thirty of them in an extensive document he wrote himself, link at the bottom of this article) interact.

If the balance is affected by climate or human action, it almost always causes a lower optimum of the later harvest. "This is because the plant always adjusts its metabolism to the new reality," explains the former tomato grower. "Look at cacti in the desert or curly leaves in other crops."

Smart plant
According to Gerrit, a plant functions optimally if it can also convert the energy it receives into growing elements, i.e., roots, stems, and fruits. "Plants have a mechanism to properly convert energy coming from sunlight into growth even at high humidity," he says. If a plant fails to convert that energy into growth, it makes adjustments. "And these are irreversible." Resulting in undesirable results for growers, including, perhaps, discolored tomatoes.

Gerrit suspects that the latter, a plant that cannot convert energy into growth, also occurs in tomato LED cultivation. "The importance of infrared light is underexposed in the Western world," he says. The former tomato grower believes that the energy required in evaporation via reflective infrared radiation plays an underestimated role in cultivation. "During condensation, latent energy is largely released in the form of infrared radiation, which is then mainly absorbed and emitted by water vapor molecules in the atmosphere."

Conversely, infrared radiation will activate evaporation. Infrared radiation reduces the binding energy in water molecules. "Indeed, we see that when condensation occurs on a blade of grass, it does not heat up via the latent energy released in the process. If it did, the condensation process would stop immediately at the first condensation. So the latent energy must be released in some other way," Gerrit concludes.

In the tropics, he sees crops doing well even in humid, cloudy weather. "Cold LED light has no infrared radiation that activates evaporation or the growth process. HPS lights and sunlight clearly do. With only LED light, we need to figure out how to use infrared radiation, via a different route, to stimulate process energy."

Infrared radiation
Even in the days of the rise of HPS lighting, growers were looking at infrared radiation. "Certain varieties could not be grown well under HPS lights at that time. That was a big problem." Gradually, growers managed to overcome these problems.

With LED lighting, conditions change, and growers have to find the balance all over again. The translation of lessons from the transition to HPS may now have to be made to LED cultivation, it was suggested at the meeting early this summer. Pay attention to heat radiation, advises Gerrit, who also took note of the meeting. "That heat radiation is crucial, in combination with evaporation, it keeps the plant in balance."

In nature, it can be seen that when relative humidity (RH) is high, above-ground growth increases. "Plants become vegetative and weak if this goes on for too long," the grower knows. "You can see in the tropics, with lots of light, that in tomatoes, bunch formation then becomes weaker just like with us in winter."

The current theory is that moisture deficit and convection cause evaporation. "At Plant Empowerment, they point out that planting depends mainly on evaporation for cooling. I have a slightly different view on that. You can also see through growth that a plant processes its energy and thus brings about cooling. Every grower knows that a well-growing crop determines the climate. We see in summer that the temperature then drops far below the outside temperature in the greenhouse."

The experienced grower raises the question of how latent heat is released during phase change. He advocates more research into the role of infrared radiation in this. "Without water vapor in the air, there is no growth. Water vapor is a greater growth gas for the plant than CO2, mainly because it allows the plant to balance itself. A plant recognizes this and has it genetically built-in, just like the principle of balance. Nature can recognize the complexity in this."

This winter, growers are going to gain a lot of new knowledge again. What works for one person will not necessarily work for another. Even with the increase in knowledge, one growing recipe for every grower with LED will not just emerge. "Per variety, the way of growing needed for optimal results can already be different," the former tomato grower knows from his own experience. "A cherry tomato has a completely different conversion process of energy than, say, a beef tomato." He calls for learning to understand especially well what the plant itself can already do. Knowledge of a whole range of balances can help, as detailed in this paper by Gerrit (NL).

For more information:
Gerrit Vermeer
[email protected]

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