Light for plants: how to optimize its use in greenhouses

The light is a fundamental factor for the life and the growth of plants, especially in greenhouses. Optimizing the use of the light for plants in greenhouses is essential, not only to obtain more quality but also to produce vegetables with healthy properties thanks to some compounds produced by plants.

In some ways, light can be considered a production factor for plants and there is not the habit to take it into account. Otherwise, for protected crops in greenhouses, the light can be a key factor to maximize the production and the quality. Let’s see how!

Importance of the light for the photosynthesis

The light supplies the energy for the reaction of the photosynthesis. In addition to nitrogen, phosphorus, potassium etc. plants need also for about 90-95% hydrogen, carbon, and oxygen. The reaction of the photosynthesis depends on these elements. The light is essential to supply the energy required to the reaction of the photosynthesis with the purpose to produce glucose.

Through the chemical process of the photosynthesis, plants convert the carbon dioxide (CO₂) and the water (H₂O) into plant material, thanks to the use of the light energy coming from the sun. In this way the energy can be preserved as organic matter (glucose). During the process of the photosynthesis, 6 molecules of CO₂ and 6 molecules of H₂O are absorbed, resulting in the production of 6 molecules of oxygen. The plant material is used from the plant and from animals for energetic purposes, through combustion or digestion. Thanks to the chemical process of the breathing, plants produce the energy useful for all the activities regarding the growth, the development, and the production of biomass of vegetable organisms. Carbon dioxide and water return in these processes too.

In a simple and general way, we can write down the reaction of the photosynthesis with the following calculation:

6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂

Like all the living beings, also plants breath. The process works as the opposite from the one regarding the chlorophyll photosynthesis where energy is released from 1 mole of glucose and 6 moles of oxygen. This is the energy that the plant needs. As a “residue” we have 6 moles of carbon dioxide and 6 moles of water.

The result of the photosynthetic reaction is glucose, while oxygen is considered as waste product, even though it is fundamental for life on earth.

Natural light and wavelength

In the optical wavelength, light is essential for plants. The light, which can come from the sun or from a lamp in the greenhouse, from a physical point of view follows the electromagnetic undulatory theory or the corpuscular theory (the electromagnetic radiation as a sequence of packages of energy called photons). In the first case, the energy level of the light is related to its wavelength: the shorter the wavelength the higher the energy level.

In the following chart you can see the partition of the light based on different wavelength. Plants use the light with wavelengths ranging from 400 and 700 nanometers as source of energy for photosynthesis process. The red light (650-700 nm) and the blue light (460-480 nm) are the most efficient wavelengths for the process.

The sunlight which reaches the earth represents 1.800-2.000 μmoli/m2 per second. The Photosynthetic Active Radiation (PAR) or photosynthetically active light has a value of wavelength equal to 400-700 nm which means about 300-400 μ moli/m2/sec.

Optimize the us2

e of the light in greenhouses

For greenhouse crops it is important to have a cautious management of the system (structure and roofing of the greenhouse) to optimize the use of the entering light. Only for the roofing and the structure you can suffer from a loss of transmissivity of about 30-40% of the total light coming from the outside. Protected agriculture uses greenhouse effect to accelerate the growth process of plants, thanks to the fact that in greenhouses, compared to the outside, there is an optimal medium temperature for plants. This is possible by entrapping the light (as sun energy) to avoid that the heated air mixes with the outside environment and with the colder layers of the atmosphere.

In spring or summer with sunny days, the temperature in greenhouse can be too high, for this reason it is necessary to lower them by using the ventilation. Proceed with various air exchange or reduce the energy of the sun with shadowing.

Besides the fact that you shall pay close attention to how plants are exposed to sunlight, it is absolutely necessary to choose wisely the covering material to be applied in greenhouses as well as a good maintenance and cleaning. In case of new constructions, you shall not omit the choice of light structures which minimize the formation of shadows.

In modern greenhouses with advanced technology, after supplying plants with water and nutrients through fertigation, you shall keep an adequate temperature. In winter you can use the heating by keeping the greenhouse closed to save the heat. This closure brings the environment of the greenhouse to a decrease of carbon dioxide by reaching a level which blocks the process of the photosynthesis. If the CO₂ becomes a limiting factor for the production, the fertigation, and the heating result useless. According to this, you shall use the carbon fertilization with the spread of artificial CO₂ in the greenhouse coming from tanks or from the combustion of the methane used for the heating.

At this point you can control all the production factors: the light becomes the limiting factor. During the winter season, days are shorter, and the weather is almost always cloudy. These lower levels of light and sun radiation represent a strong limitation for plants growth. To supply light to the plants, it is necessary to use artificial systems through sodium lamps or LED lamps.

What amount of light plants need? When is it necessary?

In the last few years the use of light for artificial plants together with sunlight has become a very popular technique in Holland and it is gaining importance also in Italy. Besides the advantages here above described, lamps can support a continuous production with a more constant quality throughout the year. Basically, the artificial light improves the bargaining power of the manufacturer.

Artificial light increases the production and offers more possibility of planning. It is undeniable the fact that, the main reason behind the success of artificial light were political incentives of cogeneration, that is the installation of machines with continuous production of electricity and heat. The greenhouse grower is a producer of electricity owner of the two sub-products: heat and carbon dioxide.

An example in Italy can be found in high-value productions, for example nurseries for seedlings production (both horticultural and floricultural) in which there is the necessity to produce also in period characterized by low light. For example, to produce tomato seedlings ready for sale you shall accumulate at least 250-300 of mol/m² light. For every 8 hours of starting of a system of lamps with 150 μmol/m²∙s (high pressure sodium or LED) the period useful to produce plants with seeding in November shortens by a day, while with seeding in March it shortens by less time. Obviously, it is up to the entrepreneur to consider costs and benefits. In similar cases, it is useful to proceed with an economic analysis.

A research: the use of red spectrum LED light to reduce nitrate of rocket

The University of Bari (Italy) together with the University of Ghent (Belgium) has carried out a recent study on a specific vegetable: the rocket. Researchers carried out some tests by applying LED lights to lower the content of nitrate in the rocket. This one accumulates lots of these substances, especially when it grows wild.

Then they applied LED light in blue and red spectrum and the results have shown that the red LED is very useful to reduce nitrate. The red spectrum can reach the limit of 4.200 mg/kg of fresh product, against the 7.100 mg/kg of the blue one. The study has shown that the red light produces more glucosinolates (substances which can prevent and contrast degenerative diseases like cancer). The results of this study have been published on the magazine “Frontiers in Plant Science”.

This project is part of the PAC standards (“Politica Agricola Comune” translated into Common Agricultural Politic) and aims to the reduction in the use of pesticides by 50% and of fertilizers by 20% in the agricultural sector by year 2030.

CONCLUSION

Light for plants is a very important aspect. If outdoor the rays of the sun have an important role in the development of the plant, in greenhouses you shall work for an integration of artificial and solar light to maximize the production.

A good use of the light in greenhouses shall always be followed by a previous process of soil preparation which shall be carried out in an optimal way. This is possible with the use of specific, compact, and efficient machineries which shall be applied without any difficulty even in limited areas like greenhouses.

From almost 50 years, Forigo Roter Italia develops and manufactures agricultural and horticultural machineries aimed to satisfy various needs. If you want to know more about it and find the best solution for you, do not hesitate to contact us: the team Forigo is ready to satisfy every answer and curiosity.