PLANT LIGHT SCIENCEUNDERSTANDING THE SCIENCE BEHIND WHAT MAKES PLANTS GROW
PLANT LIGHTING FUNDAMENTALS – MICROMOLES (PHOTONS)
- MICROMOLES– Intensity of a light is measured over a fixed area to understand the ability of a light to cover an area with that intensity (coverage). Micromoles of photons per square meter per second (µmol/m2/s) measures the intensity by taking measurements in a consistent 4 x 4 grid, measures the coverage of that intensity–commonly known as PPFD per 4 x 4 grid. Rather than measuring light intensity by lumens per watt with a lux meter, growers should use a quantum PAR meter to measure the micromoles of photons per square meter per second (µmol/m2/s) at the canopy level. To best calculate PAR light intensity coverage using PAR mapping 4 x 4 grids, be sure and measure at various heights. Spot readings (PPF) metrics are misleading. We recommend growers use PPFD per square meter to accurately measure intensity and coverage of that intensity.
- SPECTRUM WAVELENGTH – In addition to PAR light intensity, plants need a tailored spectrum designed to maximize photosynthesis. Cultivated plants require high intensity and broad but balanced spectrum. Two grow cycles (veg/cloning and flowering/bloom) require different spectra to maximize results. It’s important to use a light that concentrates energy on the blue part of the spectrum for growth and the red part for flowering. However, smaller amounts of yellow and green wavelengths are also needed for optimal growth. In the end, it’s all about intensity, micromole levels at the canopy and providing plants the proper spectrum for target plant species. New spectrum combinations are sure to be on the horizon as growers experiment more.
- DAILY LIGHT INTEGRAL (DLI) – DLI is defined as the amount of photosynthetically active radiation (PAR) delivered over a 24-hour period. It is measured as the number of moles (particles of light) per day (mol.m-2.d-1 ), and often abbreviated to “moles/day” (or m/d) in trade journals. DLI is the total quantity of micromoles of photons per square meter per second delivered over the course of an entire day. Plants have an optimal DLI for various growth and flowering stages which drive light recipes (intensity and spectrum) and lighting duration plans.
UNDERSTANDING PAR LIGHT
The McCree Curve represents the average photosynthetic response of plants to light energy. The McCree Curve, also known as the Plant Sensitivity Curve, begins at 360nm and extends to 760nm. This curve can be placed over a spectral distribution chart to see how well a light source can affect plant growth. The quantum response begins at 400nm and extends to 700nm.
UNDERSTANDING CHLOROPHYLL A AND B
Structure – Chlorophyll A and B differ in structure only at the third carbon position. Chlorophyll B has an aldehyde (-CHO) side chain at this carbon position as compared to the methyl group (-CH3) for chlorophyll A. This difference in structure contributes to their varying light absorption properties.
Chlorophyll A – Chlorophyll A is the most commonly used photosynthetic pigment and absorbs blue, red and violet wavelengths in the visible spectrum. It participates mainly in oxygenic photosynthesis in which oxygen is the main by-product of the process. All oxygenic photosynthetic organisms contain this type of chlorophyll and include almost all plants and most bacteria.
Chlorophyll B – Chlorophyll B primarily absorbs blue light and is used to complement the absorption spectrum of chlorophyll A by extending the range of light wavelengths a photosynthetic organism is able to absorb. Both of these types of chlorophyll work in concert to allow maximum absorption of light in the blue to red spectrum; however, not all photosynthetic organisms have the chlorophyll B pigment.
Accessory pigments, the carotenoids, present in many photosynthetic organisms, increase absorbance of green light.
Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the sun, into chemical energy that can be later released to fuel the organisms’ activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis. In most cases, oxygen is also released as a waste product. Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth.
Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centers that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances such as water, producing oxygen gas. Furthermore, two further compounds are generated: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the “energy currency” of cells.
TWO STAGES OF PHOTOSYNTHESIS
Certain spikes in the green and yellow wavelengths are essential for chlorophyll A and B development as well as the formation of carotenoids, which aid in the process of photosynthesis. During this process, the plant produces energy from light itself. The leaves of each plant possess many functions that all work together to create the energy that is needed to sustain life.
Within the leaves’ cells is energy producing factories called chloroplasts that hold all the chlorophyll, which are the light absorbing pigments. There are two types of chlorophyll which absorb different spectrums of light: Chlorophyll A absorbs the violet, blue, orange and red spectrum the most. Chlorophyll B absorbs the violet, blue, orange and red as well but absorbs more of the blue that’s closer to the green spectrum. Carotenoids are also light absorbing pigments in the plant, but don’t produce energy themselves. They must pass that energy onto the chlorophyll which then can utilize that energy to create carbohydrates. Carotenoids also provide protective properties to plant cells that help protect the plant from elements such as ultra-violet light.
Because light is made up of tiny packets of energy called photons, that energy hits the chlorophyll and is immediately absorbed. This boosts the electrons of the pigment to higher energy levels. Those extra electrons are passed on to molecules of NADP (Nicotine adenine dinucleotide phosphate) and hydrogen to form NADPH, which is used later during the photosynthesis process. However, during this process chlorophyll gives away too many electrons. To regain balance, the plant breaks apart water into hydrogen and oxygen stripping electrons from the molecules in the process. Oxygen is then released into the atmosphere as waste and the hydrogen becomes ionized and is pushed through a proton pump where it is used in the bonding of ADP (Adenosine diphosphate) and a phosphate molecule. ADP is essentially the backbone molecule of all metabolisms or the flow of energy in a cell. After the bonding process the new molecules are ATP (Adenosine triphosphate) which is the main transport of chemical energy. This entire process is called the light reaction.
In addition to the light reaction, there is the dark reaction, also referred to as light-independent reaction because this process can happen with or without light. This interaction is referred to as the Calvin Cycle. During this process the ribulose phosphate, a five carbon molecule, is stimulated by enzymes to convince ATP to give up one of its phosphate groups. Once ATP gives up the phosphate group it becomes ADP, which ultimately will be recharged back into ATP during the light reaction process. That phosphate group is combined with ribulose phosphate to produce ribulose biphosphate, which then joins with water and carbon dioxide. After the joining of water and carbon dioxide, the molecules become phosphoglyceric acid after it breaks into two identical molecules. Phosphoglyceric acid receives another phosphate from ATP to become biphosphoglyceric acid. NADPH and the hydrogen ion which made earlier are then used to remove the phosphate to provide the energy and hydrogen needed to create phosphoglycric acid or PGAL. PGAL is used to make sugar and replenish the ribulose phosphate stores so this reaction can continue to happen again.
PAR Light – Photosynthetically Active Radiation
- Light between 400 and 700 angstroms (nanometers, 10^-8 meters)
- PAR is for plants, lumens is for humans
Measuring PAR – Photosynthetic Photon Flux (PPF)
- Measured as micromoles (µ, or 10^-6) )of photons emitted per second
- One Mole = 6.02 * 10^23 (Avagadro’s Constant)
Photosynthetic Photon Flux Density (PPFD)
- Micromoles of photons emitted per square meter per second
- Way more important to growers than PPF
- An HID light might have huge PPF numbers at the bulb, but inverse square law of light means that the number of photons hitting a given area (PPFD) declines exponentially as the distance between the light source and the plant surface increases.
Daily Light Integral (DLI)
- DLI = PPFD over the course of a light cycle
- Measured as moles of PAR photons per day
- Example: a light emitting PPFD = 1000 running 12 hours emits a DLI = 43.2 moles/day
- Computed as ((60 sec/min)*(60 min/hr)*(12 hrs/day)*(PPFD))/1 million
- Current research indicates Cannabis grows best at PPFD = 1500 (DLI = 65) and temperature = 30 degrees C
Lighting Efficiency = PPFD/joule or PPFD/input watt
- Measure of how well a light converts electrons to photons
- Lights requiring supplemental AC or cooling fans will have very low numbers
- Can also look at both purchase price per PPFD and five year cost per PPFD
How to Play Games With the Numbers
- Report PPF (photons emitted by the bulb) rather than PPFD (photons hitting the plant)
- Use lights that emit intense PPF over a small area
- Put reflective walls around the light to reflect the light back into the measurement area
- Report overall PPFD for only a small portion of the growing area (Hot Spot)
- Use inconsistent grid sizes