How light affects plant growth

Solar light and plants

All plants that live on Earth need sunlight to germinate, grow and bear fruit. The Sun emits radiation with wavelengths of between 280 and 2800 nm. This radiation splits into three regions: ultraviolet light (100 to 380 nm), visible light (380 to 780 nm) and infrared light (700 to 3000 nm).

In order to be able to see and regulate our biological clock, humans need radiation known as ‘visible radiation’. As living things, plants also need light, and it is fundamental for their survival. When exposed to light, plants perform photosynthesis, a process whereby they absorb CO₂ and water, transform these into carbohydrates, and release oxygen.

In the same way we ‘see’ this radiation in a particular way, plants also have their particular way of ‘seeing’ light. They perform photosynthesis when they receive radiation with wavelengths between 400 and 700 nm, which is known as photosynthetically active radiation (PAR).

Clorophyll, the green pigment in leaves that absorbs the energy from the PAR, principally reacts to red and blue light. The leaves absorb little green and reflect it back, which is why we see that colour.

Light measurements and plants

It is important to note that the photometric quantities used in traditional lighting projects are not compatible with plant crops. Luminous flux, which is measured in lumens; light intensity, measured in candelas (cd); illuminance (lux) and luminance (cd/m2) all refer to human vision.

Plants do not have the same sensitivity as the human eye, which is why ‘classical’ quantities cannot be used. Therefore, when we talk about light and plants, what we want to know is how many photons we are going to supply to achieve optimal energy conditions. Botanists have agreed that this measurement should be the micromole (μmol) for counting photons.

As with luminous flux, using micromoles we can measure flux and density. We can work out its density because it is measured per square metre per second, while the amount will depend on the distance of the light source from the plant, and the light distribution of the lamp. In this way, micromoles behave like lux.  The closer to the plant the light source is, the more micromoles per unit of illuminated surface it will receive.

So, the unit of measurement of light for plants is the micromole per second per square metre (µmol·s-1·m2).

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The lighting needs of plants depending on their stage of development

The technological advances that artificial lighting has undergone with the arrival of LED and electronic controls has meant environments can be appropriately lit to grow plants in any conditions. The option of regulating light quantity, distribution of luminous flux, colour and spectrum, allow us to adapt and stimulate the different phases of plant growth and development.

There are certain aspects we need to verify in order to guarantee a suitable environment, as the type of light supplied in controlled crops can modify their growth and quality. Therefore, the chosen lighting system has to provide conditions that take into account the following aspects:

• Level of light

The appropriate amount of light for each type of crop and the growth phase of the plant.

• Uniformity

The distribution of the light must be even, so that the light reaches all the growing area equally. This is achieved with the correct choice of photometry and distribution.

• Distribución espectral

For the growth phase, the right amount of radiation must be supplied to guarantee the plants receive light with the necessary characteristics to boost growth.

Artificial lighting systems for cultivation

When considering the advantage of one light source over another, we must ask the question: Which light source provides the micromoles of photons my plant needs? The answer is complicated, as it will depend on the lamp’s wattage at a specific height to cover the number of plants, with minimum cost and the most even light distribution possible. As there is no standard answer, the best thing to do is ask an expert.

Another aspect to consider is whether there is going to be any natural light available, or whether only artificial light is going to be used. If there is natural light available, the artificial light will act as back-up, while if there is no natural light, all light will have to come from the artificial light installation.

Below we take a look at some of the recommendations for providing light for crops.

The lighting needs of plants depending on their phase of development

The development of almost all crops can be divided into three phases: Germination, rapid growth and flowering. In each of these phases, the plants have different requirements regarding light, water, space, type of attention and work needed to keep them flourishing.

Germination phase

For plants cultivated from seeds, the establishment phase is defined as beginning when the seeds are sown, and continuing through germination, emergence and the development of the first leaves.

In this phase, the seedlings require blue light to start germinating and to sprout.  If there is sunlight, by reinforcing it with blue light (75%) and red light (25%), we speed up the process of photsynthesis.

When there is no sunlight, the artificial light should emit the full spectrum of light, but using the same ratio of blue light (35%), red light (25%), far-red light (25%) and white light (4000K, CRI70, 15%).

Growth phase

In this phase, the rapid growth of the plant needs to be prioritised by boosting the process of photosynthesis.  If there is natural light available, the recommendations for stimulating this phase will be to provide red light (90%) and blue light to a lesser degree. If solely artificial light is used, the recommended proportions will be white light (4000K, CRI70, 80%) and red light (20%).

Flowering phase

In this phase, when there is sunlight, we need to boost it with red light (60%) and far-red light (20%), with some blue light (20%); while if only artificial light is used, white light (4000K, CRI70, 60%), red light (20%) and far-red light (20%) are recommended.

Fruiting phase

When there is sunlight, the recommendation is to provide white light (4000K, CRI70, 20%), red light (70%) and far-red light (10%). With only artificial light, the recommendation is to use white light (4000K, CRI70, 60%), red light (30%) and far-red light (10%).

Types of growing facilities

Different methods of cultivation are used depending on the type of plant, the space available and the phase that needs stimulating.

• Seedbeds: This is a space that has been specially prepared and adapted for planted seeds to germinate in optimal conditions. They allow seedlings to grow without impediment until they are ready to be transplanted.

• Vertical gardens: These are living walls of plants that are created on different buildings, both inside and out. They have arisen as a new concept for injecting greenery into urban spaces, while maximising the use of the most precious thing in our towns and cities: space.
• Greenhouses: These are walk-in closed spaces for growing both ornamental and edible plants, while protecting them from the cold at certain times of year.  They usually have a translucent outer cover of plastic glass, which allows temperature, humidity and other environmental factors to be controlled.
• Indoor plants: These plants are used as decorative elements indoors. They help purify the air and create a relaxing atmosphere, making spaces attractive and welcoming.

Lighting applications for cultivation

• Top lighting: This consists of illuminating the plants from above, in a similar way to the sun. The aim is to supplement the natural light to optimise growth in greenhouses.
• Inter lighting: This is a lighting system that supplements natural light. It consists of placing artificial light between the plant and the leaves so that no shadows appear and growth optimal is achieved.
• Vertical farms: The plants are arranged in vertical layers, one above the other, with the aim of optimising space. This system uses light between each layer. The lights are close to the plants, which means it is important they do not generate heat. It also needs to ensure an even distribution of the light over the growing area, so that the plants grow evenly.

Effects of radiation on plants

Different types of radiation generally have different effects on plants. This is why it is important to understand them when choosing the ideal types of light for each development phase.

• Infrared radiation: This stimulates plant growth, slows branching, and encourages large, thin leaves, as well as flowering and fruiting.

• Ultraviolet light: This slows the speed of photosyntehsis, reduces flowering and pollination and affects seed development. Although excessive exposure to ultraviolet light is dangerous for flora, small amounts of UV light can have beneficial effects. Ultraviolet light is often a very important factor in colour, taste and aroma.
• Blue light: This radiation is responsible for vegetative growth and leaf growth. It is important for seeds and young plants because it helps reduce plant stretch. It stimulates photosynthesis, improves the plant’s nutrition, boosts chlorophyll and chloroplasts, opens the stomata and gives the plant a compact structure with small, thick leaves.
• Red light: This is another critical point of light absorption for the leaves. Red light is important in the regulation of flowering and fruit production. Moreover, it helps increase the diameter of the stalk and encourages branching. It stimulates photosynthesis, slows plant growth and generates smaller, thicker leaves.
• Far-red light: This light can cause the plant to elongate and triggers flowering in long-day plants.
• Red/far-red ratio: When the ratio is low it causes the elongation of the plant. In nature we see this phenomenon when plants in the shade receive a greater amount of far-red light and tend to grow taller to reach more light.

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