Photosynthesis
Photosynthesis
Photosynthesis
Controlling plant growth using LED Lights
Plants have developed the ability to use light energy from the sun to produce a food source through the process of photosynthesis. Although the process is quite complex, a somewhat simplified definition of photosynthesis is: light energy is used to split water (HO2) and fix carbon dioxide (CO2) to form carbohydrates (CH2O) and oxygen (O2):
Instead of focusing on the biochemical light reactions of photosynthesis, we look at the different properties of light and their influence on photosynthesis. Quality (spectrum), quantity (intensity), and duration (photoperiod) are separate but related light properties that influence photosynthesis in horticultural processes:
SPECTRAL LIGHT QUALITY
Photosynthetically Active Radiation (PAR) is the major driver of photosynthesis in plants. However, not all wavelengths of light are equally efficient at driving photosynthesis. McCree's research resulted in the creation of a photosynthetic response curve, the McCree curve.
The image below shows that red light (600-700 nm) is almost twice as effective as blue light (400-500 nm) at driving photosynthesis, with green (500-600 nm) light in between.
Prior to this there was a misconception that since chlorophyll absorbs light mainly in the red and blue parts of the visible light spectrum (leading to the green color of plant leaves) that green light was not used by plants for photosynthesis.
However, higher plants have evolved both biochemical and biophysical solutions, pigments, to utilize green light. These accessory pigments (mainly carotenoids) can be thought of as storage molecules for photons that are not directly absorbed by chlorophyll.
Spectral light quality is a key component that goes into the design of horticulture lighting systems, especially in sole-source (absence of sunlight) lighting applications. Traditional High-Intensity Discharge (HID) lighting systems (high-pressure sodium and metal halide) have always had a limitation when it came to modifying the spectral light quality.
Using Light-Emitting Diodes (LEDs) for horticulture lighting systems allow manufacturers the ability to create custom spectral light qualities along with many other advantages over conventional lighting systems, including: high photoelectric conversion efficiencies, low thermal output, and adjustable light intensities.
Light quality not only influences photosynthesis, but it also influences the morphology of plants, which is known as photo-morphogenesis.
LIGHT INTENSITY
The number of photons that are absorbed by specialized photo-receptors known as chloroplasts directly influences the rate of photosynthesis. As light intensity (PPFD) increases, so does the rate of photosynthesis, until a saturation point is reached. Every plant species has a different light saturation point where photosynthetic levels plateau based on the light environment they evolved in.
Light saturation occurs at less intensities in plants evolving in shade conditions than under full sun conditions. However, light saturation in sun plants often occurs when some other factor (e.g. CO2) is limited.
There is also the light compensation point. Plants have a minimum light intensity, required to maintain growth and keep plants alive. The light compensation point occurs at higher light intensities for sun-plants than shade-plants. Adequate light intensities with the correct spectral light quality is critical to promote new plant growth.
Horticulture lighting systems can be used in two ways to increase light intensities to promote photosynthesis:
One of the key benefits of using LEDs for either application is the low thermal output at the surface of the diode, wheras HID lights need to be kept further from the crop canopy, because they emit much energy as infrared (IR) light. IR light is not photosynthetically active and significantly increases plant temperatures.
However, increasing the distance between the light and the crop canopy, will result in decreased light intensities and is limited to facilities with tall ceilings.
Since LEDs usually dissipate most of their heat from the back side of the diode, these lighting fixtures can be placed much closer to the crop canopy, allowing very high PPFD levels (≥ 1000 µmol/m²/s) to plants.
LIGHT DURATION
Light duration is the length of time a plant is exposed to light during its 24-hour cycle. This length can influence the overall light intensity that a plant receives in 24 hrs, which in turn influences overall growth.
This is described as Daily Light Integral (DLI), which is defined as the cumulative PPFD delivered during 24 hours, and is expressed in mol/m²/d.
This light duration also influences the transition from vegetative to reproductive growth in several plant species. However, it is actually the dark-period, not the light-period, that determines when certain species will transition to reproductive growth.
The photo-receptor phytochrome is mainly responsible for signaling the transition to reproductive growth in light-periodic crops.
Long-night plants flower when the phytochrome perceives an uninterrupted long-night (≥ 12 hrs). Short-night plants flower during short-nights (≤ 12 hrs). Alternatively, several plant species are day-neutral, where the light-period does not influence flowering.
Horticulture lighting systems can be used to provide light-periodic light to extend the day to either promote flowering of long-day plants or suppress flowering of short-day plants regardless of the season or climate.
Traditionally, HID, incandescent, or fluorescent lights have been used to provide photoperiodic lighting in greenhouses. However, these technologies are relatively inefficient at converting electrical energy into PAR.
CONCLUSION
The study of photo-biology is still in its infancy. Rapid advances in LED technologies allow researchers and growers to further explore the interaction between life and light.