Light is a form of electromagnetic energy that moves as waves. Visible light has wavelengths between 390-700 nanometers that appear as different colors when separated with a prism.
Plants respond to different spectrums of light by secreting growth hormones that direct them toward particular directions of growth, making them phototropic and having an impactful influence on their morphology.
Photosynthesis
Plants draw their energy from the sun. Before plants can utilize this solar energy for growth and other metabolic processes, however, it must first be converted to chemical energy through photosynthesis. This chemical reaction combines 12 molecules of H2O with six molecules of carbon dioxide to form glucose; during which process oxygen is also released as a byproduct. Plants then send this sugar through tiny pores on their leaves known as stomata to other parts of their bodies for use as growth energy or other metabolic processes.
Photosynthetic plants utilize the subpart of electromagnetic radiation spectrum known as PAR (Photosynthetic Active Radiation). This includes wavelengths that are visible to human eyes such as blue and red light; but it also includes far red photons which cannot be seen directly but interact with the other wavelengths to increase photosynthesis efficiency.
Key components of photosynthesis for plants include the pigments and proteins contained within their chloroplast. This collection of chemicals, known as reaction centers, convert sunlight to chemical energy in the form of electrons. Chlorophyll is one of these reaction centers and gives plants their characteristic green hue by absorbing blue and red light waves into its structure allowing it to capture photons from solar energy into its chloroplast.
Energy from photosynthesis is then transferred to various pigments and proteins in the chloroplast’s thylakoid membranes, where they combine with sugar molecules and organic compounds produced from them to form sugar molecules and other organic compounds that fuel plant growth, flowering, fruiting, etc. Seasonal variations affect photosynthesis rates; during summer and spring growth is prioritized more while as light decreases in autumn plants move toward conserving energy by slowing their metabolism and conserving energy reserves.
Chlorophyll
Chlorophyll is the main force behind green plants’ vibrant hue in gardens. As an essential component of photosynthesis – which converts light energy to food for plants via photosynthesis – chlorophyll plays an essential role. Plants use light energy along with water and carbon dioxide to make glucose, an energy source which fuels all cells while providing fuel for growth. Chlorophyll along with other light-absorbing pigments is responsible for harnessing solar energy and turning it into sustenance for their cells.
Chlorophyll takes in light at various wavelengths and absorbs them with different molecules absorbing various colors of wavelengths, giving rise to various hues that reflect back. Most commonly found is chlorophyll A; commonly seen among green plants and some cyanobacteria; chlorophyll B is more abundant among cyanobacteria; while rare types c and d can sometimes be found among golden algae; and in rare instances some photosynthetic bacteria also contain it as part of their composition.
While chlorophyll chemical structures vary slightly, they all share one thing in common: a porphyrin (reduced porphyrin) ring system with a sequestered magnesium atom at its center – similar to hemoglobin’s heme molecule. Thus the name “chlorophyll” derives from Greek for green.
Chlorophyll serves an essential purpose: it absorbs sunlight and transfers its energy directly to a plant through its chloroplasts, where photosynthesis occurs to convert carbon dioxide and water into glucose for growth.
As sunlight shifts across the seasons, plants adjust their growth habits accordingly to take advantage of abundant energy during summer and spring months. When daylight decreases further in autumn and winter months, plants adjust by conserving energy and slowing their growth rate to conserve resources as winter sets in.
Leaf Absorption
Light that strikes a leaf’s surface may either reflect off it, transmit through it or be absorbed by it depending on factors like photosynthetic pigments and mesophyll morphology; diffuse reflection can occur depending on roughness scale of surface roughness of leaf surface; light is also often diffusely scattered due to diffuse scattering from mesophyll cells within its cell walls and cell walls; light reflection patterns vary with roughness level on leaves’ surfaces.
As light passes into a leaf’s interior, it may be dispersed into multiple directions by different wavelength velocities between mesophyll cells and air. This phenomenon is known as diffuse or multiply scattered reflectance and has an impactful impact on how much light escapes through its surface.
Light entering a leaf is captured by chloroplasts and bacteriorhodopsin pigments, which transform photons of blue and red light into energy for photosynthesis. Unlike most pigments, bacteriorhodopsin absorbs all visible and near-infrared wavelengths making it the dominant light absorber among green plants.
Visible light reflected and transmitted back through leaves includes the green portion of its spectrum that gives leaves their characteristic visual color, though how much light reflects and transmits depends on factors like leaf shape, surface roughness and mesophyll density.
Light reflecting off a leaf depends on its structure and refractive index of its material composition; however, biochemical composition plays a key role in how much light a plant absorbs. An analysis of scattering and absorption patterns on leaves can provide clues to their biochemistry allowing scientists to distinguish related plants while their spectral fingerprint can also help detect contaminants within food chains.
Leaf Reflection
Light passing through an epidermal leaf surface or mesophyll cells will be scattered in new directions depending on differences in wavelength velocities through various media; this phenomenon is governed by Snell-Descartes law with effective refractive index values dependent upon biochemistry of each leaf.
Roughness of surfaces where light reflects is also key, with smooth surfaces reflecting specularly while rough ones diffusely scatter it. Scientists can use spectral measurements to identify individual plant species or even leaves within that species through these variations in surface properties.
Green leaves typically reflect more of the visible portion of the spectrum than blue or red ones due to chlorophyll’s ability to reflect shorter wavelengths while absorbing longer ones. Furthermore, mature leaves may contain substances which enhance its reflection and transmission capabilities for specific wavelengths.
Mildew-infected Populus alba leaves increase their reflection by nearly 100% while anthocyanins increase transmission of light into them. Focused light transmission into leaves allows plants to alter their internal lighting environment to optimize photosynthetic performance in various habitats.
Researchers have used fiber optics to measure light transmission through individual leaves and canopies, as well as their internal structure in order to understand how photosynthesis influences them. Researchers discovered that the ratio of direct to diffuse lighting influences photosynthesis rates across species. As more direct light dominates over diffuse lighting sources, its relative abundance increases or decreases photosynthesis rates accordingly. Reasons for this phenomenon lie not so much with diffuse lighting itself but in its impact on chloroplast movements between and within mesophyll cells in leaves (Misson et al. 2005, Brodersen & Vogelmann 2007). Diffuse light often penetrates deeper than direct sunlight into leaves and canopies, increasing time spent within stomata while decreasing photosynthesis activity while direct sunlight promotes high photosynthetic rates.
Seasonal Changes
Plant growth depends upon both the quantity and quality of light that it receives. As seasons shift, daylight hours vary and intensity varies with regards to sunlight; plants have evolved to take advantage of these fluctuations when possible, prioritizing leaf development, flower production, fruit bearing as much as possible; when darkness falls they conserve energy by slowing their growth rates in preparation for winter.
Horticulturist Matthew Kispert of CropKing notes, as seasons shift and light levels increase, plants will absorb different nutrients than during times of rapid vegetative growth. Kispert advises growers to focus on irrigation first in order to avoid issues like deficiency caused by an increase in light levels; also, as seasons progress he advises decreasing EC levels of their nutrient solutions as the seasons advance.
Though most research on seasonal changes focuses on how plants know when it’s time for them to flower, Yale University researchers have discovered that plants can detect daylength of each season as well. Their team studied Arabidopsis plants under various amounts of light and temperature conditions before tagging specific genes within these plants to observe when expression changed and indicated what season the plant thought it was in.
Plants not only absorb visible light but can also take advantage of ultraviolet (UV) wavelengths to convert energy from it into usable form for growth and reproduction. Although UV wavelengths cannot be seen directly by humans, plants possess special receptors in their cell walls that capture it for energy use and use it as energy for photosynthesis – without this ability, leaves can turn yellow or brown; in extreme cases this lack of UV can even kill off plants entirely.
