Greenhouse in the evening

Why light is important for plant growth?

Plants require light for optimum growth and development, but the three different aspects of light, quantity, quality and duration, also have a significant influence on growth.

A plant under natural conditions receives light from the sun; the amount, quality and duration greatly depend on the season of the year, hour of the day, geographical location and weather.

Below we will further explain the effect of each aspect on plant growth and development.

Light quantity

Plants use light as a source of energy for photosynthesis. The term photosynthesis refers to the reaction between carbon dioxide and water in the presence of light to produce carbohydrates and oxygen. The rate of this process is highly dependent on the light quantity; the photosynthesis rate is higher as the Photosynthetic Active Radiation (PAR) increases. 

The carbohydrates produced during photosynthesis are stored and used by the plant as a food source. Each plant species starts the process of photosynthesis at different light energy levels, which is called the light compensation point. This point starts when light energy is sufficient for photosynthetic activity to produce more oxygen than is required by the plant for respiration.

Likewise, the release of carbon dioxide through respiration by the plant must be less than the total carbon dioxide used by the plant for photosynthesis. Otherwise, the net photosynthesis will be null or negative.

Calculate the net photosynthesis

Net Photosynthesis = Photosynthesis – Respiration (1)

More light generally equates to higher levels of photosynthesis. However, as the light intensity increases, the photosynthetic rate eventually reaches a maximum point.

This point where the light intensity does not increase the photosynthesis rate is called the light saturation point. When this point is reached, the photosynthesis rate curve becomes flat (Figure 1).

Graph of the light compensation point and the light saturation point
Figure 1. Graph of the light compensation point and the light saturation point.

Light quality

Young plants purple light
Young plants in violet light.

Light quality refers to the color or wavelength. The sun emits wavelengths between 280 and 2800 nm (97% of total spectral distribution). They are divided into three regions: Ultraviolet (100-380 nm), visible light (380-780 nm) and infrared (700-3000 nm).

The highest energy corresponds to the lowest wavelengths; ultraviolet has higher energy than red. We as humans see wavelengths between 380-770 nm; this range is called visible light.

Visible light is divided into violet (380-430 nm), blue (430-500 nm), green (500-570 nm), yellow (570-590 nm), orange (590-630 nm) and red (630-770 nm). On the other hand, plants photosynthesize between 400-700 nm; this range is known as Photosynthetic Active Radiation (PAR).

Chlorophyll, the green pigment in leaves responsible for absorbing the PAR, has two peaks of absorption: blue and red light. Leaves absorb little green and reflect it; this is why we see the green color of the leaves.

What effects have different colors on plants?

  • Ultraviolet Light
    Ultraviolet light causes DNA damage, reduces photosynthesis rate, flowering and pollination decrease, and seed development is affected. Ultraviolet A (a subcategory of ultraviolet light) can cause plant elongation.
  • Blue Light
    It corresponds to one of the absorption peaks; therefore, the photosynthetic process is more efficient when there is blue light. Blue light is responsible for vegetative and leaf growth and is important for seedlings and young plants because it helps reduce plant stretching.
  • Red Light
    This is the other peak of light absorption by the leaves. Phytochrome (a photoreceptor) within the leaves is more sensitive to and responds to red light. This light is important in the regulation of flowering and fruiting. It also helps increase stem diameter and promotes branching.
  • Far-Red Light
    This light can cause plant elongation and trigger flowering in long-day plants.
  • Far-Red Light and Ratio
    Plant elongation results when this ratio is low. In other words, plants are more exposed to far-red than red. In nature, we see this phenomenon when plants are shaded by neighbouring plants; the shaded plants receive a higher ratio of far red light and tend to grow taller to reach more light. This can become a problem with greenhouse crops that are shaded by overhead baskets or are planted too close together.

How do different light sources distribute light differently?

  • Incandescent lamp
    It generates almost a linear trend with little light coming from the blue spectrum and the highest light coming from the red spectrum.
  • Fluorescent lamps
    They generate most of their light in the blue, green and red spectrum, with the highest light level coming from the blue spectrum.
  • High-pressure sodium lamp
    The highest peak is green closely followed by red.
  • Metal halide lamp
    The highest peak is in the green spectrum; the red spectrum has about half the energy peak as green followed by blue.
  • Light Emitting Diodes (LED)
    This type of light emits a specific wavelength. The manufacturer can produce these diodes in a specific color/wavelength (monochromatic) that a customer requires.
Hanging baskets in greenhouse
Hanging baskets in a greenhouse.

Light Duration or Photoperiod

The number of hours of daylight per day directly impacts flowering. Plants can be divided into three categories based on the required day length needed to trigger flowering:

  • Short-day plants
    These plants flower only when the day length is shorter than the night. They bloom in early spring or fall. When the day length exceeds a critical time, plants stop flowering and go into vegetative growth. Examples: chrysanthemum, cosmos, kalanchoe, poinsettia, zinnia, etc.
  • Long-day plants
    These plants flower when the day length is longer than the night. They flower in late spring to early summer. When the day length is shorter than a critical time, plants stop flowering and go into vegetative growth. Examples: carnation, dill, foxglove, petunia, snapdragon, etc.
  • Day-neutral plants
    These plants flower regardless of the day's length. Instead, they usually flower after reaching certain stages of development. Examples: celery, cucumber, hydrangea, pansy, pepper, tomato, etc.

In summary, the most important process triggered by light in plants is photosynthesis. Photosynthesis is a process used by the plant to produce food to help build more plant material. The faster the photosynthetic rate, the faster the plant grows.

The rate of photosynthesis is impacted mostly by the light intensity and quality. When it comes to flowering, the length of the day is important to know as it directly impacts the timing of flowering for many ornamental crops. For example, it is impossible to cause a short-day plant like poinsettia to bloom in the summer under natural conditions.