Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create energy in the form of carbohydrates. The rate of photosynthesis is directly related to the colors of light that a plant absorbs. Plants absorb all wavelengths of visible light (400-700 nm) but are most efficient with absorbing red and blue light. Understanding how light color affects photosynthesis can help improve growing conditions to optimize plant growth.
Overview of Photosynthesis
Photosynthesis takes place in plant cells, primarily within the leaves. The basic formula for photosynthesis is:
6CO2 + 6H2O + Light –> C6H12O6 + 6O2
This translates to carbon dioxide + water + light energy producing glucose sugar + oxygen. The glucose sugar serves as fuel for the plant while the oxygen is released as a byproduct.
There are two main stages of photosynthesis:
1. Light Dependent Reactions – This initial stage uses light energy to produce ATP energy and electrons. It takes place on the thylakoid membranes within the chloroplasts.
2. Light Independent Reactions (Calvin Cycle) – The ATP and electrons from light reactions provide energy to fix carbon into 3-carbon sugars. This takes place in the stroma of the chloroplasts.
For photosynthesis to occur, plants need sunlight, water, carbon dioxide, and chlorophyll. Chlorophyll gives plants their green color and absorbs light energy which drives the photosynthetic process. The rate of photosynthesis depends on having sufficient light energy to power these reactions.
How Light Wavelength Affects Photosynthesis
Visible light consists of different colors or wavelengths ranging from 400-700 nm. The particular color or wavelength of light impacts the photosynthetic rate. Here’s an overview of how different light colors affect photosynthesis:
Blue Light (400-500nm):
– Blue light provides more energy than longer wavelengths
– Maximizes chlorophyll absorption and efficiency of photosynthesis
– Optimizes photochemical reactions and electron transport rate
– Can promote vegetative leaf growth
Red Light (600-700nm):
– Also efficiently absorbed by chlorophyll, though less than blue light
– Drives photosynthesis and assimilation rates
– Increases starch accumulation in leaves
– Can stimulate flowering and fruit production
Green Light (500-600nm):
– Not efficiently absorbed by plant pigments
– Provides minimal impact on photosynthetic rates
– Reflected by plants, giving them their green color
Yellow & Orange Light (570-620nm):
– Also not efficiently absorbed by chlorophyll
– Generally a minimal effect on photosynthesis
– Can stimulate expansive growth and elongation
Far Red Light (700-800nm):
– Low energy wavelengths on the edge of infrared
– Has some photosynthetic effect, but much less than red/blue light
– Can stimulate stem elongation
So in summary, blue and red wavelengths are the most photosynthetically active, producing the highest rates of photosynthesis. Green and yellow/orange light have minimal impact as they are poorly absorbed. Far red light has a small effect.
Action Spectrum of Photosynthesis
The action spectrum of photosynthesis shows the relative effectiveness of different wavelengths of light at driving photosynthesis. It maps out the rate of photosynthesis at various light colors. Here is a graph of the action spectrum:
| Wavelength (nm) | 400 | 450 | 500 | 550 | 600 | 650 | 700 |
| Photosynthetic Rate | 0.4 | 0.8 | 0.9 | 0.5 | 0.7 | 0.4 | 0.1 |
Key Takeaways:
– Blue (400-500nm) and red (600-700nm) light have the highest photosynthetic rates
– Green light (500-570nm) has a lower rate
– Rates drop off on either end of visible spectrum
This aligns with the absorption spectra of chlorophyll a and b which peak strongly in the blue and red regions. The action spectrum underscores that plants maximize photosynthesis in blue and red light.
Factors Influencing Light Absorption
Several factors influence a plant’s ability to absorb and utilize light energy for photosynthesis:
Chlorophyll Content – Plants with higher concentrations of chlorophyll can absorb more light energy. Factors like water stress, low nitrogen, high CO2 can reduce chlorophyll content.
Leaf Anatomy – The anatomy and structure of leaves impacts light capture. Thicker leaves have more cells to absorb light. Palisade mesophyll cells are aligned for optimal light interception.
Leaf Area Index – Total leaf surface area affects total light absorption potential. So larger and more numerous leaves lead to more photosynthesis.
Leaf Orientation – The angle at which leaves are held influences light interception. Horizontal leaves maximize overhead light absorption.
Canopy Structure – In forests and crops, the overall canopy structure affects light penetration to lower leaves. More open canopies allow deeper light penetration.
Optimizing these factors allows plants to maximize light absorption and photosynthetic capacity.
Significance of Light Color for Crop Growth
The color or wavelength of light that crops receive can significantly impact their growth, development, and productivity. Here are some key effects:
Biomass Production – Blue and red light drive greater photosynthetic activity and biomass accumulation than green light. Blue light especially stimulates leaf growth.
Flowering & Fruiting – Blue light inhibits flowering in some plants, while red light promotes flowering and fruit production. Long dark periods also stimulate flowering.
Plant Morphology – Blue light encourages compact, leafy growth while far red light causes elongation and leggy growth.
Crop Quality – Blue light enhances leaf color, flavor, and nutritional quality in some crops. Red light can increase carotenoid and anthocyanin pigments.
Light Intensity – High light intensity fuels faster photosynthesis and growth. Low light causes excessive elongation and small leaves.
Optimizing light color and intensity throughout the crop growth cycle can improve yields and quality. LED grow lights allow specific light recipes to be tailored to plant needs.
Effects of Light Color on Selected Crops
Research studies have illuminated many crop-specific photomorphogenic effects. Here are light color impacts on a few economically important crops:
Tomatoes:
– Blue light boosts leaf growth and compact, bushy plants.
– Far red light causes leggy elongation of stems.
– Red light enhances flowering and fruit production.
– Green light reduces yields compared to red/blue light.
Lettuce:
– Blue light stimulates leaf expansion and biomass.
– Red light increases antioxidant phenolic compounds.
– Blue light gives leaves a dark green color.
– Mixing red and blue light optimizes yield and nutrition.
Cannabis:
– Blue light encourages strong vegetative growth.
– Red light stimulates flowering and boosts bud size/THC content.
– Far red light can extend flowering stage.
– Green light less effective than blue/red light.
Strawberries:
– Blue light enhances plant compactness and leaf color.
– Red light stimulates earlier flowering and fruiting.
– Light intensity affects fruit sugars and acids.
– High intensity blue light increases fruit yield.
Significance for Natural Ecosystems
Light color also impacts natural photosynthesis in wild plants and diverse ecosystems:
Forests – In forest canopies, blue and red light are absorbed by upper leaves while more green light penetrates to lower levels. Leaves acclimate to the light spectrum they receive.
Water – Water selectively absorbs longer wavelengths of light. So aquatic plants adapt to bluer light underwater.
Shade Light – Plants in shaded understories receive filtered light depleted in red wavelengths. They evolve to use the remaining light efficiently.
Light Fluctuations – Leaves can dynamically optimize photosynthesis as light shifts from sun to shade. Varying light keeps photosynthesis flexible.
Plant Communities – Species occupy their ecological niche based on light specialization. Canopy trees harness high light while understory plants use lower light.
Understanding light spectral impacts provides insights into the evolution and physiology of plants in diverse environments.
Conclusion
The color or wavelength of light has a significant influence over the process of photosynthesis in plants. Blue and red wavelengths are most efficiently absorbed by chlorophyll and show the highest photosynthetic rates. Green and yellow light are less effective. Optimizing light color along with intensity and duration can improve crop productivity, quality, and nutritional value. Insights into light spectral impacts informs both agriculture and the study of natural plant ecosystems. Continued research on crop photomorphogenesis will help refine specialty light recipes for superior plant growth.
