By studying the effect of light on the growth of plants, there have been some terms and definitions used in describing certain processes and phases of development:

Radiometry: deals with the detection and measurement of electromagnetic radiation across the total spectrum

Photometry: subfield of radiometry; radiometric power scaled by the spectral response of the human eye

Photon Flux: number of photons in a spectral range per unit time. When limited to the range 400 -700 nm, it is termed Photosynthetic Photon Flux.

Mol/mol/µmol: In chemistry, a unit of measurement counting the number of atoms/molecules/electrons/etc. in substance (for horticulture, photons) By definition, the number of photons in a mol is 6.022 x 10 23 (Avogadro's number).

Photon: Discrete bundle (quantum) of electromagnetic radiation (light). Can be considered to be a particle (although it displays properties of waves as well). The energy of a photon depends upon its wavelength. Conversely, if the energy & wavelength are known, the number of photons can be calculated

Photosynthetically Active Radiation (PAR): Radiation between 400 nm and 700 nm. Spectral region most useful to plants for photosynthesis

Photosynthetic Photon Flux Density (PPFD): Radiation between 400 nm and 700 nm. Radiation hitting a surface

Photosynthesis: A process used by plants and other organisms to convert light energy into chemical energy that can be later released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water.

Germination: Germination is the process by which a plant grows from a seed. It is also known as sprouting of a seedling from a seed.

Vegetative Growth: Vegetative Growth is the period between germination and flowering. It is also known as vegetative phase of the plant development. During this phase the plants are performing photosysthesis and accumulating resources which will be used for the flowering and reproduction in the later stage.

Photomorphogenesis: Because light is the energy source for plant growth, plants have evolved highly sensitive mechanisms for perceiving light and using that information for regulating development changes to help maximize light utilization for photosynthesis. The process by which plant development is controlled by light is called photomorphogenesis. Typically, photomorphogenic responses are most obvious in germinating seedlings but light affects plant development in many ways throughout all stages of development.

Flowering: The transition to flowering is one of the major phase changes a plant makes during its life cycle. The transition must take place at a time that is favorable for fertilization and the formation of seeds. The right photoperiod is essential for the flowering.

Etiolatio: Abnormal shape of plants due to significantly accelerated length growths caused by insuficient illumination which can be used for photosynthesis.

Photosynthesis is an important biochemical process in which plants, algae and some bacteria use the energy of solar radiation as a source of energy for the synthesis of food.

Photosynthesis is an important biochemical process in which plants, algae and some bacteria use the energy of solar radiation as a source of energy for the synthesis of food. Then, from the simple inorganic material (carbon (IV) -oxide and water), sugar-monosaccharides are synthesized. Synthetically synthesized organic matter is a source of food and energy both to the plants in which they are synthesized, and to other organisms on Earth, which makes this process crucial for the survival of life on Earth. Photosynthesis is also responsible for the constant production of oxygen. Organisms that produce energy with photosynthesis are called phototrophs (figure 1).

Photosynthesis is a basic process in nature because it provides organic matter for all living organisms. All other syntheses in living entities continue on photosynthesis. It runs through two phases: light for which light is needed, and dark, for whose lightning is not necessary.

Figure 1: Photosynthesis

Figure 2 shows the entire spectrum of light. The wavelength range that can be used for photosynthesis is between 400-750 nm.

Figure 2: Absorption Spectrum of all pigments


For green plants, the most important maxima of absorbed light are :

  1. Chlorophyll-a: 430 nm / 662 nm
  2. Chlorophyll-b: 453 nm / 642 nm
  3. Carotenoids: 449 nm / 475 nm

Figure 3: Wavelength of light, which influence photosynthesis


The number of photons in the light energy unit is directly proportional to its wavelength. That's why red light is almost 1.5 times more efficient than blue. The extent of blue light is important because of the stretching of the stem. Less than 10% of blue light can lead to problems in the development of some plants. Light near UV radiation affects the biosynthesis of compounds, such as compounds that give taste to some fruit.

In the biology that studies the development, photomorphogenesis (figure 4) is the development of light, where the growth phases of the plants respond to the spectrum of light. This process is completely different from photosynthesis, where light is used as a source of energy. Photomorphogenesis of plants is often studied using a light source of strictly controlled frequencies. There are at least three phases of plant development during photomorphogenesis: germination, seedlings development and the transition from the vegetative stage to the flowering stage.

Figure 4: Photomorphogenesis


Figure 5: Photoreceptors


The development of plants depends on ambient conditions. Because light is a source of energy for plant growth, plants have developed a very sensitive mechanism for receiving light and its use to regulate the development phases so that light can be maximized. The influence of light is most noticeable during germination, but light affects the development of plants in many ways during all stages of development.

Figure 6: Responses to Light

The life cycle of plants (figure 7) or ontogenesis is the individual development of the plant, that is, the process of morphological, physiological and biochemical transformations of the organism from fertilization to the end of life. It consists of two phases:

  • vegetative stages - includes embryo development, seeding, juvenile periods and development of vegetative organs
  • Reproductive stages - includes maturity of the plant (adult period), development of flowers, pollination and fertilization, development of fruits and seeds, aging and death individuals.

Figure 7: Life cycle of plants


Their duration varies depending on the type of plant and is largely conditioned by the length of the life cycle. In the case of annual plants, both phases are short, while for many years they are longer.

Plant growth is directly dependent on the color, strength and duration of the light that the plant receives. The required light intensity for photosynthesis is about 80W / m2, which means in practice that the 400W bulb is sufficient for about 6 m2 of surface covered with plants (600W lamp for about 9m2). .

The best growth and development of plants ( figure 8) can be achieved if they have a light about 16 hours a day. During flowering, plants need light from 12 to 14 hours, then a period of twelve hours of darkness without interruption. In practice, it has been shown that flowers work very well with a period of 12 hours of light and 12 hours of darkness.

Figure 8: Daily cycle of plants


The photosynthesis process is best done with a light red spectrum (600-680nm) and a blue spectrum (380-480nm) emitting light bulbs for production in horticulture. Metal halogen lamps emit a blue / white spectrum which is the primary source of light (replacement for natural light) and allows a compact vegetative growth of the plant. High-pressure sodium emit red light spectrum and stimulates blooming or fetching fruits.


Bellow pictures shows spectral distribution of HPS and Metal halide lamps.

Picture 9: Spectral distribution for HPS

Figure 10: HPS vs METAL HALIDE

The blue spectrum of light stimulates growth-boosting hormones and controls the rest period until favorable conditions for seed germination are obtained. The plant grows towards the light source. MH lamps emit high light from the blue spectrum, so they are good at the initial stages of the development of the plant. The blue spectrum of light allows for good growth and proper branching of the plant. The red light spectrum enhances the photosynthesis process, helps seed germination, enhances pigmentation and causes and helps bloom.

Lighting in the cultivation of plants plays a major role due to the possibility of shortening the time required for the growth and development of the plant.