What’s really going on inside your garden plants as they soak up sun and water?
This illustration shows how photosynthesis works. Plants take water and carbon dioxide in, add some sun, make sugars and release oxygen.
Photo by Fotolia
While planting the first crops of spring, you may envision the luscious products of photosynthesis, but you likely won’t ponder the process responsible for your anticipated harvest. Sure, you know that most fruits and vegetables need at least six hours of sunlight each day, preferably more, but understanding the how and why of the sun’s effects on plant physiology can improve your garden’s production — or your over-the-fence garden chats.
Blue Light Special
So let us start with the source — the sun. Sunlight comes in a variety of wavelengths, only some of which are useful to plants — some more useful than others. Plant pigments, located in the chloroplasts of plant cells, absorb energy from wavelengths that make up the visible spectrum of sunlight — red, orange, yellow, green, blue, indigo and violet. The most abundant plant pigment, chlorophyll, absorbs energy from red and blue wavelengths and is responsible for most of the plant’s energy absorption. Most of the green wavelength is reflected rather than absorbed, which is why leaves appear predominantly green. Some pigments pick up other wavelengths, including green, to enhance energy absorption. Come fall, when the chlorophyll begins to degrade, these other pigments shine through, absorbing the green wavelengths and reflecting the yellows, reds and oranges we “ooh and ahh” over. Chlorophyll’s affinity for red and blue wavelengths is one of the reasons growers use fluorescent lights for starting plants indoors. Your everyday incandescent light produces plenty of the red and infrared (heat) wavelengths, but fluorescent lights produce more blue wavelengths, which increase vegetative growth and are cooler. LED (light emitting diode) lights, however, are all the rage. LEDs are not only more energy-efficient, they can also be tuned to specific wavelengths — customized sunlight for each crop! In fact, designers of large, futuristic urban and suburban farms speak of glowing pink towers where growers tend plants indoors under an exclusive LED mix of blue and red light.
Whether from fancy fuchsia LEDs or the sun, what happens when light falls on leaves? The packets of energy, or photons, that make up wavelengths of light excite electrons in the chlorophyll molecule, which kicks off a fascinating — if complex — chain of events that results in a simple sugar and our ability to breathe.
Here is the basic biology class formula for how photosynthesis works:
6H2O (water) + 6CO2 (carbon dioxide) → sunlight → C6H12O6 (sugar) + 6O2 (oxygen)
Plants take water and carbon dioxide in, add some sun, make sugars and release oxygen — light energy is converted to chemical energy. Water, then, is the source of the oxygen we breathe and carbon dioxide is the source of carbon in the carbohydrates we eat. The food-making process happens in two phases within the plants. The first phase, the light-dependent reaction, uses water and the sun’s energy to make molecules. Those molecules then power the second phase, which is a light-independent reaction (the Calvin cycle) that makes sugar molecules.That’s photosynthesis in an edamame pod, but understanding the process at a deeper level will help explain why plants die if they aren’t watered correctly and why insufficient levels of carbon dioxide can reduce your produce haul. (For now, hold that thought about increasing atmospheric carbon dioxide levels!)
Water is integral to both parts of the food-making process within plants. During daylight hours, light hits chlorophyll pigments that then set off a chain reaction that requires the components held within water molecules to complete its cycle. The cycle’s end products are energy — in the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) — and oxygen, which is diffused as a “waste product” out of the leaf.
The light-independent Calvin cycle begins when carbon dioxide enters the leaf through pores, called “stomata.” Water, also essential in this phase, splits CO2 and other molecules until it creates a simple sugar. All this rearranging of molecules takes energy, which has conveniently been provided by the light-dependent reaction (ATP and NADPH). The sugars produced by the Calvin cycle are then used for a variety of plant functions or stored as starch. We know these stored carbohydrates by a whole smorgasbord of names — beets, carrots, kale, lettuce, potatoes or, just generally, food. To get into the fascinating, nitty-gritty details of how electrons are transferred and molecules are divided, see “Full Scientific Explanation” at the end of this article.
Carbon Dioxide and You
You, the gardener, absolutely influence the quantity of carbon dioxide your plants get when you water them. Overwatering and underwatering will cause a plant’s stomata (the pores by which CO2 enters the plant to start the Calvin cycle) to close in an attempt to conserve water. If CO2 can’t enter the leaf, oxygen can’t depart. When this happens, the now-burgeoning internal oxygen supply is captured inside the plant instead — a process called “photorespiration.” Under these conditions, photosynthesis becomes highly inefficient and produces fewer carbohydrates. Photorespiration can also occur when carbon dioxide is depleted in the plant’s immediate microclimate, which can happen under hot, dry, crowded conditions with little air circulation (think August on the prairie). Greenhouse owners might want to take note, as photorespiration can occur during winter in tightly weatherized greenhouses, because a lack of circulating air (venting) can deplete the amount of carbon dioxide available to the plants. Try composting crop residues and manure in your greenhouse to compensate, or try keeping chickens and rabbits (watch the lettuce) in the greenhouse to boost carbon dioxide levels. Those furry or feathered CO2 generators won’t help, however, if the greenhouse is vented or if they only hang out in the greenhouse at night when plants are not photosynthesizing. (Remember: the initial phase of the process requires light.)
So, you may wonder, “If low carbon dioxide levels limit photosynthesis, will rising levels of atmospheric carbon dioxide increase photosynthesis?” Yes, but here is where climate change ho-hummers conveniently forget the rest of the equation — it will only increase photosynthesis to a certain point. The photosynthetic capacity of plants — their ability to convert sunlight into food — is not just tied to CO2, but is also dependent upon their ability to acquire water, nutrients and light, and to grow in their optimum temperature range. Increasing temperatures (associated with escalating CO2 levels) also enhance photosynthesis, but again, only to a point. Not only do high temperatures demand more water, they also cause enzymes in the photosynthetic process to break down, and increase the respiratory rate of the plant — using all those valuable carbohydrates to survive instead of storing them for us to eat. The optimal temperature for maximum photosynthesis varies from crop to crop and can change across a growing season for an individual plant. The same goes for the effect of light intensity on photosynthesis. As light levels increase, the light-dependent reaction makes more energy available to the Calvin cycle, but photosynthesis is then limited by the second part of the cycle’s physical ability to process CO2 and water into sugars.
Trellis systems, which expose greater leaf area to sunlight, are one way you can increase photosynthetic activity in your plants. The relationship between increasing availability of a resource and higher-but-plateauing rates of photosynthesis also exists for nitrogen and carbon dioxide itself. Nearly 50 percent of the nitrogen in a leaf is devoted to photosynthesis. Some studies show that as carbon dioxide increases, nitrogen in leaves declines, so in low-nutrient situations, photosynthetic gains from higher concentrations of carbon dioxide are limited.
Sometimes we are so focused on improving conditions for our crops that we forget about unintended consequences. If a higher level of carbon dioxide is good for our pampered edibles, it’s also good for those “other plants” — weeds. And while we covet larger, more productive plants, those kinds of plants covet more nutrients and water. At some point, the positive relationship between productivity and increasing levels of CO2, light, nutrients and temperature begins to level off, a point that varies from crop to crop.
Getting It Right, or at Least Having Fun Trying
Plants generally respond positively to increasing levels of light, carbon dioxide, nitrogen and temperature if given sufficient water. In the garden, you control the light, nitrogen and water, and, indirectly, the carbon dioxide. Getting everything right to maximize net photosynthetic production — which equates later to loads of tomatoes and beans — has always been a challenging trial-and-error game. Too much or too little of any factor will reduce your bounty, and this balancing act changes during the current growing season and across all growing seasons. It’s enough to put off any beginning gardener, if it wasn’t so much fun. All the grumbling, commiserating and tip-trading is really all about experimentation — the blending of art and science in the garden. Experienced gardeners have been running their own experiments for years — moving perennials around the yard, placing new garden beds, trying new (or old) tomato varieties — essentially tweaking the combination of nutrients, light and water. Horticultural researchers may retain hordes of graduate students who tend row after row of veggies and fruit and measure photosynthetic activity leaf by leaf by leaf. Certainly their research is most helpful, but you possess your own biology lab right outside your back door, and with your own over-the-fence co-investigators.
There’s More Than One Way to Get Your Sugar Fix
Yes, there is more than one way to make food from sunlight. Take corn, for example. Corn, as well as sorghum, millet, and sugar cane, bypass the problem of photorespiration by tweaking the basic photosynthetic pathway (the two-step process, which is called the “C3 pathway”). Essentially, these C4 plants, as they are called, store carbon dioxide to maintain an internal, readily available supply for conversion into sugars. C4 plants incorporate a middle step between the light-dependent reaction and the Calvin cycle in which CO2 is first captured by a molecule other than RuBP, and then shuttled for storage to a separate plant cell called the “bundle sheath cell.” Carbon dioxide then enters the Calvin cycle, just as it does in the C3 photosynthetic pathway. Additionally, the enzyme responsible for initially sequestering the CO2 is much more finicky than RuBP and won’t use oxygen as an easy substitute. What does all this mean? It means that corn is more photosynthetically active at higher temperatures than your squash or beans, because when the corn’s stomata are closed and the plant is conserving water, corn can still produce sugars with stored carbon dioxide. This neat C4 trick plays out right before your eyes in lawns each summer — as the season becomes drier and hotter, the C4 crabgrass starts spoiling the C3 fescue pallette.
If you’re lucky enough to tackle growing pineapples, you can further expand your edible garden collection of photosynthetic pathways. Pineapples, and many desert plants, use a photosynthetic pathway similar to the C4 plants, called “crassulacean acid metabolism” (CAM). Instead of CO2 accumulating in another cell, as happens in C4 plants, the CO2 in a CAM plant accumulates and is stored at night while the stomata are open. Come daylight, the light-dependent reaction kicks in, supplying the energy necessary to run the Calvin cycle without the need for open stomata and dehydration.
Full Scientific Explanation of How Photosynthesis Works
When light hits chlorophyll pigments, the pigments give up electrons, or negatively charged sub-atomic particles. (Protons are positively charged sub-atomic particles, and an ion is an atom or group of atoms that has a positive or negative electric charge from losing or gaining one or more electrons.) Electron-carrying molecules then pass these jolted electrons from one pigment to another, each electron eventually used to create an energy molecule known by a ridiculously long but meaningful scientific moniker, nicotinamide adenine dinucleotide phosphate (NADPH). The chlorophyll pigment is now missing an electron that must be replaced if photosynthesis is to continue. Water, split into its component parts (electrons, hydrogen ions, and oxygen), supplies the replacement. While the water’s electrons pass to the chlorophyll pigments, the hydrogen ions build up inside the thylakoid and move out into the interior of the chloroplast, moving through specialized proteins that use the protons to create the other energy molecule, adenosine triphosphate (ATP). The oxygen on which life depends is a “waste product” that diffuses out of the leaf.
The Calvin cycle begins when CO2 enters the leaf through pores called “stomata” and is captured by a molecule with the shorthand name RuBP (ribulose-1,5-bisphosphate). Water, also essential in this phase, splits the now-linked molecules of CO2 and RuBP into smaller molecules that are then further rearranged, eventually ending up as regenerated RuBP and a simple sugar. All this rearranging of molecules takes energy, which has conveniently been provided by the light reaction in the form of ATP and NADPH. The sugars produced by the Calvin cycle are then used for a variety of plant functions or stored as starch. We know these stored carbohydrates by a whole smorgasbord of names — beets, carrots, kale, lettuce, potatoes or, just generally, food.
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