For a good part of the year, the level of natural light is below optimal for most crops. Any additional light that then enters the greenhouse contributes to improved production and quality, says Anne Elings, Plant Physiologist at Wageningen University & Research.
Photosynthesis is the name for a series of complex processes that can, however, be easily summarised as "the plant manufactures sugars from CO2 and water." These sugars subsequently serve as nutrients for the plant. This process takes energy, and sunlight and/or artificial light supplies this energy.
The question then becomes, which of these three is the limiting factor in this process. Elings: "Generally, there is sufficient water in a greenhouse. This means that the CO2 content and light level are the determining factors. The principle that applies to both these factors is that more is better, but only up to a certain level. Furthermore, the additional yield levels off before this upper limit is reached."
More light in the greenhouse
During low-light periods – late autumn, winter, early spring – light is the limiting factor. Adding more light to the greenhouse then is almost always beneficial. This can be achieved through assimilation lighting or by treating the greenhouse cover with coatings that increase light incidence.
"The condition is that the greenhouse air must contain sufficient CO2. When the vent windows are open, some is lost, and the CO2 content may then drop to such a low concentration that it becomes the limiting factor. This is the reason why more light does not always have the desired effect. The CO2 dosing and the light level must always be in balance," says Elings.
Balance between light and temperature
Furthermore, there is one other key factor at play here: temperature. "The light level determines the quantity of sugars that can be manufactured. The temperature determines the demand for sugars in all growing plants. Supply and demand must be in balance. For example, if production of sugars becomes too high in relation to demand, this can slow down photosynthesis," says the researcher.
When there is more light, the temperature must be increased, but not too much, otherwise the plant’s respiration rate increases too much. Respiration is the process in which part of the sugars are ‘burned up’ to obtain energy for a wide range of recovery processes. This is necessary to repair damage and aging. When the temperature is too high in comparison to the light level, you are actually losing potential production. Controlling the balance of light and temperature, however, is far from easy, and although calculation models can provide an indication, it is part of the grower’s professional craftsmanship.
By contrast, the humidity in the greenhouse air does not affect photosynthesis all that much. "Under very dry or very humid conditions, the skin pores tend to close. As a result, the plant absorbs increasingly less CO2. It is seldom that the humidity in the greenhouse is so high that the plant is no longer capable of evaporation and, therefore, closes its skin pores. Even when the humidity is very high, the humidity within the leaf’s hollow cavities is higher still, and the difference between the two is the driving factor behind the evaporation. The threat of high humidity to photosynthesis often is less than thought," he says. For an active climate in the greenhouse, as a grower, you may be forced to open the vent windows in order to get rid of some moisture. We will always have to search for the optimal balance between greenhouse climate, light, and CO2.
The question is whether a little bit of additional light in winter makes a real difference. Elings: "That’s definitely the case. For a traditional planting date for fruiting vegetables late in the autumn, you’ll see that growth only really starts up once the light increases towards spring. All of the additional sunlight is welcome when light levels are low. And besides, it’s free."