by Ed Rosenthal
PHOTOSYNTHESIS
Photosynthesis is the process in which plants capture the energy from light and use it to power a series of biochemical reactions. Carbon dioxide from air and water are combined to produce sugar and release oxygen to the atmosphere. Sugar is used by the plant as a tissue building block to power metabolism—i.e., energy for life processes.
Photosynthesis started between 3.5 and 2.5 billion years ago when cyanobacteria, also known as blue-green algae, first evolved the use of photoreceptors to capture and utilize the energy from sunlight. The oxygen generated by this process radically altered the early atmosphere of the Earth, raising the concentration of oxygen from an estimated 1% to today’s 21%, and changing the composition of the dissolved solutes in the world’s oceans.
The biochemical process of photosynthesis in green plants takes place in an inner cellular organelle called the chloroplast, which captures the energy of sunlight and converts it to electrical charges used to make sugar. It is thought that chloroplasts which developed as cyanobacteria established an endosymbiotic relationship with their hosts.
According to this theory, chloroplasts were separate organisms that were taken inside the plant cell, just as proteobacteria were incorporated to become mitochondria, the cells’ energy center. In fact, chloroplasts, like their counterparts the endosymbiotic mitochondria, still maintain enough of their original DNA genome to code for around 200 different proteins. These genes are inherited separately from the cell genome. But photosynthesis requires an additional 1,000 compounds, blueprints which are encoded in the DNA of the plant’s nucleus.
The chloroplast is surrounded by an outer, permeable membrane and a second, relatively impermeable inner membrane. The stroma are inside this double barrier. Another membrane holds the thylakoid membranes that surround the innermost compartment, the lumen. The light-harvesting mechanism for photosynthesis is found in the area where the thylakoid membrane separates the stroma from the lumen.
Plants capture the energy from light and use it to fuel a complex set of processes in which they capture carbon dioxide (CO2) from the air, break apart water (H2O), and then attach the hydrogen (H) atoms that are released to the CO2 to form sugar. The overall formula is: Light + 6(H2O) + 6(CO2) =C6H12O6 +6O2.
Plants absorb or reflect most wavelengths of light. Far-Red light passes through; therefore there is a higher ratio of Far-Red to Red light in the shade. Plants use the Far-Red ratio to detect being in the shade; they grow longer stems to try to reach the light. Notice the peak of green in the visible spectrum of the canopy light; since green light more than other visible colors is reflected or passes through leaves, plants appear green.
Chloroplasts contain a number of photoreceptors, including the principal pigment chlorophyll, as well as beta carotene. These pigments allow the chloroplast to absorb light over most of the visible spectrum, though most of these receptors absorb more red and blue light and reflect the green, which gives plants their characteristic color. These pigment molecules are arranged in large symmetrical protein structures called light-harvesting complexes. These complexes serve as antennae and channel the light-excited electrons to a chlorophyll molecule at the main reaction center, where photosynthesis occurs.
The “light” reactions take place in two distinct stages in protein complexes associated with the thylakoid membrane: Photosystems I and II. (Because the photosystems were named in their order of discovery, not function, the first set of photosynthesis reactions takes place in Photosystem II, not I.)
PIGMENT ABSORPTION SPECTRUM (top): The three pigments that capture most of the light used for photosynthesis are two forms of chlorophyll, A and B and Beta-Carotene. They are most efficient at capturing light in various wavelengths of the red and blue bands. They are not efficient at using green and yellow light.
ACTIVE PHOTOSYNTHESIS SPECTRUMS (bottom): The active photosynthetic spectrum gets most of its energy in the red and blue light. However the drop-off of efficiency of use of light for photosynthesis in the orange, yellow and green bands is not as great as would be expected if only chlorophyll and carotene were considered. Light in these wavelengths is harvested by other pigments (called accessory pigments) that transfer the energy to chlorophyll.
Chloroplasts contain the pigment chlorophyll. They are held in structures called thylakoids. They are extremely efficient at converting captured light to energy, which powers photosynthesis. Each photosystem contains hundreds of chlorophyll A, chlorophyll B and carotenoid molecules packed together and integrated into the thylakoid membrane. There are hundreds of these photosystems in each chloroplast and there are dozens of chloroplasts per cell and hundreds, if not thousands, of cells per leaf. The chances for absorption of photons is enormous. The absorption spectra for purified chlorophylls A and B (in vitro), have peaks only in the red and blue portions of the spectrum. In the leaf the dense packing of photo-active molecules and the transfer of electrons between the molecules allows photosynthesis to function across the spectrum, including in the green portion, where absorption is minimal. It also explains how minimally absorbed wavelengths are able to affect photosynthesis. Far-red light between 700 and 800nm is able to enhance the rates of photosynthesis under low light conditions by altering the distribution of PSI and PS2 leading to increased grana stacking and a closer association between the photosystems.
The plant stomata regulate the exchange of gasses and liquids to and from the leaf. They function sort of like human pores. In their open position they absorb carbon dioxide as well as moisture and nutrients. In the closed position they retain water. Plants regulate water content and temperature using the stomata. When they transpire water, it cools the plant in much the same way that sweating helps us.
Photosynthesis relies on the microscopic photosystems found in plant leaves. Each photosystem contains hundreds of chlorophyll A, chlorophyll B and carotenoid molecules, which are packed together in the thylakoid membrane of the leaf. Since there are hundreds of these photosystems in each chloroplast, dozens of chloroplasts per cell, and hundreds if not thousands of cells per leaf, plants have an enormous number of chances to absorb light photons. The quantity of light chlorophylls A and B will absorb in laboratory tests peaks only in the red and blue portions of the spectrum, but the dense packing of photo-active molecules in the leaf allows photosynthesis to function across the spectrum, even at wavelengths that are only minimally absorbed. For instance, far-red light between 700 and 800nm enhances the rates of photosynthesis under low light conditions by altering the distribution of Photosystem I and Photosystem 2 to increase thylakoid stacks in chloroplasts and more closely associate the photosystems.
When Photosystem II absorbs light, it passes high-energy electrons along to the next stage, but it then needs additional electrons to return to its previous state. These electrons are supplied by the oxygen-evolving center (OEC) which separates electrons from water molecules in a process that leaves behind protons and oxygen. This very rapid reaction can produce as many as 50 oxygen molecules per second for every Photosystem II complex. This generates most of the Earth’s breathable oxygen.
After the OEC has separated oxygen and protons from the water molecule, the extra electron is transferred in a process that removes protons from the stroma and adds them to the lumen. The electron is then passed to a water-soluble protein that delivers it to Photosystem I.
Photosystem I uses a higher wavelength of light than Photosystem II to re-excite the electron. It is used to produce ATP (adenosine triphosphate).
The energy generated by the “light” reactions power the “dark” reactions of photosynthesis that take place in the chloroplast stroma. There, an enzyme complex known as RUBISCO (ribulose bisphosphate carboxylase/oxygenase) uses CO2 as the base to combine with hydrogen to make starches and sugars, including amino acids and carbohydrates. The sugars generated this way are processed further and stored in the plant as glucose polymers called starches for use later. They become the building blocks for tissue building and
are used to power metabolism.
THE LIMITING FACTORS
Marijuana plants are dependent on their environment for materials and energy. There are five essential factors that affect marijuana growth: light, carbon dioxide (CO2), nutrients, water, and temperature.
Each of these inputs is required for photosynthesis and growth, and they must all be available in adequate amounts for a plant to reach its potential. For example, as the intensity of light increases, the plant’s ability to utilize it depends upon the availability of the other four factors. For this reason the five are called “limiting factors”.
FIVE LIMITING FACTORS: NECESSARY COMPONENTS IN PLANT GROWTH
A deficiency of any single factor limits growth to the level that factor supplies. No matter how well other factors are supplied they cannot be utilized. For example, insufficient amounts of CO2 limit the use of light, water, nutrients and temperature/humidity. If light is limited, lower the nutrient, CO2 and temperature levels
The limiting factor—that is, the factor that is not supplied adequately—determines the rate of growth. Insufficient supplies of any one factor slows or stops growth.
It is unlikely that either water or nutrients are limiting factors in an indoor garden, since they are easily supplied. Oxygen, which is required by the roots and sometimes absorbed by the leaves, comprises 21% of the air, so leaves have easy access. Oxygenating the water and using porous mediums keep the roots supplied. That leaves three factors that are likely to limit plant growth: light, CO2, and temperature.
Cannabis’ metabolic rate—how fast it functions on a cellular level—is determined by temperature. Warm-blooded animals, such as humans, maintain a steady metabolic rate by regulating their temperature internally. Almost all other life forms’ metabolic rates are dependent on their environment. In cool weather they function slowly, and their metabolism speeds up as it warms.
For this reason, all the factors in the garden must be considered in relation to each other. As the amount (intensity) of light increases, cannabis requires more CO2 to use as raw material for photosynthesis.
LIGHT
Plants use light for several purposes, including the regulation of life processes such as the initiation of flowering. But the most amazing thing that they do with light is photosynthesis, the process that provides the foundation for most of life on Earth. Plants use photosynthesis to power the process of making sugar (C6H12O6) from water (H2O) and carbon dioxide (CO2). Plants also use it to convert the sugars they make into starches and then into complex molecules such as cellulose. Add some nitrogen atoms, and you get nucleic acids and amino acids, the building blocks of all proteins.
Plants draw the energy they need from light across a spectrum broader than the human eye can see, from 400 nm (blue light) to 730 nm (red). Plants do different things with different wavelengths of light. Understanding the differences can help the careful cultivator ensure that the plants are getting everything they need to thrive.
For photosynthesis, light energy is captured by chlorophylls A and B primarily from the red and blue portion of the spectrums. Light absorption by chlorophyll A peaks at 430 nm in the blue band and 662nm in the red, and chlorophyll B peaks at 453 nm in the blue and 642 nm in the orange-red bands. Chlorophyll synthesis peaks at 435 nm and 445 nm in the blue spectrum and 640 and 675 nm in the red wavelengths.
Chlorophyll is not the only light-sensitive part of the plant. Carotenoids, are a group of orange pigments that capture light in the blue portion of the spectrum, primarily at about 450 nm in the blue spectrum and 475 nm in the blue-green range. Carotenoids not only contribute to photosynthesis but also protect the chlorophyll frm excess light that could have destructive effects.
Anthocyanin and other flavinoid pigments also absorb blue and UV light to protect chlorophyll from photo-destruction.
Another pigment that appears to play a role in plant health is xanthophyll. This yellow pigment captures light in the range from 400-530 nm, but is usually hidden from our view by the green of chlorophyll. If a leaf loses its chlorophyll—because of a nitrogen deficiency, for instance—xanthophyll’s bright yellow color becomes apparent. Xanthophyll has several functions. First, it acts as a light and heat regulator. At dawn, it is in its low-energy form, violaxanthin, which has peak reactions to light at 480 nm and 648 nm. As the light increases to levels that might hurt the thylakoids and lead to photo-oxidation of the chlorophyll molecules, violaxanthin siphons off the excess energy of photons, using them to create its high-energy form, zeaxanthin. When light intensity decreases, the zeaxanthin returns to its low energy state, violaxanthin, in a cycle that can take anywhere from a few minutes to several hours. These chemical processes enable plants to cool themselves during lighted periods and to stay warm during cool nights. Plants bank energy during the day and release it at night by shifting xanthophyll to its low-energy form, releasing heat. During the day, some of the light energy may also be transferred to chlorophyll by releasing an electron to be used for photosynthesis. Other plant pigments also gather energy from spectrums not used by chlorophyll. Neoxanthin, lutein, and zeaxanthin each transfer more than half the energy they gather to chlorophyll.
ELECTROMAGNETIC RADIATION SPECTRUM
Visible light is a small portion of the electromagnetic radiation reaching Earth. Plants use a portion of that light to power photosynthesis.
VISIBLE & NEAR-VISIBLE LIGHT SPECTRUM
Plants use light from the visible spectrum for photosynthesis. They use red and blue light most efficiently, but use light from other spectrums as well. Blue light contains more energy than red light and more electricity is used to produce it than red light. However, plants obtain the same amount of energy no matter the spectrum. This is one reason why it is more cost effective to provide plants with mostly red rather than blue light.
MEASURING LIGHT
When considering lighting for your garden, you will see various terms used to describe the output of different lamps. Understanding the relationship between such things as a lamp’s wattage and its output in lumens can help make meaningful comparisons between products.
The basic principles of how light is measured are also helpful for planning how to deploy lamps for maximum effectiveness and checking how much light your plants are getting. As anyone who has purchased a lightbulb is aware, bulbs are sold by wattage. But a quick comparison between fluorescent and incandescent bulbs, for instance, reveals that a 20w fluorescent may produce as much light as a 75w incandescent.
Watts only measure how much power a given bulb draws; output depends on the bulb technology. Light output or intensity is measured in several different units, including candelas, foot-candles, lumens, lux, and moles. Many of these units are based on each other, and some are more commonly used than others. They measure three basic things:
•the amount of visible light emitted (candelas, lumens);
•the amount of light that reaches a defined area (foot-candles, lux); and
•the total number of light particles (moles).
The candela (or candle or candle-power) is the most basic international unit for measuring emitted light and is defined as the illumination created by a common candle. While that once meant an actual candle with a burning wick, scientists devised more precise and replicable standards over the years, but all aimed at maintaining the basic unit.
Lumens (lm) measure the visible light or “luminous flux” emitted in a defined beam. A single candela light source that radiates equally in all directions produces exactly 4π (12.6) lumens; a 23w compact fluorescent emits about 1600 lumens.
The foot-candle (fc) is a closely linked unit of measure, defined as the amount of light at a distance of one foot from a single-candle light source.
The lux is similar to the foot-candle in that it measures the visible light intensity (luminous flux) that reaches a particular area, defined as one lumen per square meter. So 100 lumens concentrated in an area of one square meter equals 100 lux; if that same 100 lumens is spread
over a space ten square meters, you have 10 lux.
Moles: All of these ways of quantifying light intensity measure the amount of light at one instant in time. When measuring the light reaching a garden, growers are concerned with the amount of light the plants get over the entire lit period. For indoor setups where the light output is constant, knowing a lamp’s lumens or the lux at the canopy works well enough because the total for the lit period is easily calculated. But for outside or in greenhouses, the light intensity varies depending on the time of day and season of the year. For them, an integrated light measure that adds up the amount of light throughout the day is more meaningful, and moles per day is a common way of expressing that.
Not to be confused with skin spots or burrowing rodents, moles measure the number of light photons (one mole = 6.02 x 1023 photons) and are frequently combined with units of area and time to give you moles per meter per day.
An advantage of using moles or micromoles (millionths of a mole) is that the light quanta measured is not just the spectrum of light visible to the human eye, which is all candelas, foot-candles, lumens, and lux measure. Plants use far more of the light spectrum for photosynthesis than humans can see. In fact, the light we see best is some of the least useful to plants.
PAR: The light that plants use is known as Photosynthetically Active Radiation, or PAR. Humans see light best in the the yellow-green wavelengths around 550 nm, but PAR ranges from 400 to 700 nm, and plants make most use of light in the red and blue spectrums. Since PAR is usually expressed in moles, which measure light quanta (photons), PAR is also called quantum light. Measuring quantum light is the only way to be certain your plants are getting all the usable light they need.