Influence of Light on Crop Growth
Tuesday, September 29, 2020 | Jose Chen Lopez
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.
Quantity of light:
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. In other words, the net photosynthesis (Eq. 1) is zero:
Net Photosynthesis = Photosynthesis – Respiration (1)
Light saturation point. 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).
"Figure 1. Graph of the light compensation point and the light saturation point."
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). 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 back; this is why we see the green color of the leaves.
In general, different colors have different effects 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.
- Red: Far Red 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.
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.
Light Duration or Photoperiod:
The number of hours of day light 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 include: 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 include: carnation, dill, foxglove, petunia, snapdragon, etc.
- Day-neutral plants: These plants flower regardless of the day length. Instead, they usually flower after reaching certain stages of development. Examples include: 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.
For more information, contact your Premier Tech Horticulture Grower Services Representative:
Jose Chen Lopez
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