The Structure and Function of Stomata
Stomata are microscopic holes found on the undersides of plant leaves. They play an important role in evapotranspiration and carbon dioxide uptake. For marijuana growers, that’s relevant because factors like moisture loss, CO2 exposure, and photosynthesis all impact yields and proper regulation of these essential processes is dependent on the opening and closing of the stomata.
Composition of the Stomata
These foundational components are composed of guard cells and auxiliary cells. The purpose of the guard cells is to allow each stoma to open or close in response to environmental conditions. They do this by leveraging the power of osmosis, a process of absorption by which a liquid with low concentrations of dissolved solids is able to travel across a cell membrane to an area with a higher level of dissolved solids.
Guard cells accumulate these dissolved solids from potassium, sugars, and other nutrients, which they gather through specialized channels in the stomata’s auxiliary cells. Since guard cells can absorb water faster than they can release it, plants can close their stomata faster than they can open them. They do this when exposed to a hormone called abscisic acid, which is generated internally whenever the plant is in a low-moisture state.
How Stomata Function
Marijuana plants use a vacuum system to open and close their stomata. Whenever relevant environmental stimuli change, this system kicks in. The guard cells shed or accumulate more water, and the stomata open or close.
When stomata open, they allow plants to absorb CO2, one of the atmospheric components required to perform photosynthesis. They also release oxygen and water, though. While oxygen is a waste product for plants, water is an absolute necessity. That’s why plants need to maintain a careful balance between CO2 absorption and moisture loss.
If the stomata stay closed for too long, the plant can’t get the CO2 it needs to convert sunlight into energy and grow. If they remain open, the plant loses internal moisture, which will also negatively affect its health. Thankfully, growers can optimize their plants’ stomata behavior by altering the grow environment.
Optimizing Light Cycles
Stomata open and close according to solar cycles, just like people operate according to circadian rhythms. They usually open during the day to absorb CO2 to use for photosynthesis, then close at night to retain more moisture. Other factors, such as light intensity, also affect the function of stomata.
Outdoor growers have little control over how much light their plants receive beyond cutting down trees that would otherwise shade their gardens. There’s no way to turn the sun up on a rainy day or create extra hours of daylight, but for indoor growers, altering light intensity and light cycles are no problem.
Increasing light intensity will cause plants to create wider stomal openings. The biological mechanism behind this change is fairly simple. Intense light decreases concentrations of CO2 in the plants’ leaves, which triggers the guard cells to open up, at least during daylight hours, to absorb more of it.
Optimizing CO2 Levels
Indoor growers can also alter CO2 levels in the air. Since plants need CO2 to photosynthesize and grow, infusing extra carbon dioxide into the air can boost yields. Growers should note that there’s such a thing as too much CO2, though. If external CO2 levels are too high, it causes the plants not to open their stomata as wide, which can lead to overheating.
Most experts believe that keeping CO2 levels in the grow room at around 1,500 ppm offers the perfect balance. Growers who supplement their CO2 must provide plenty of light so that the plants will use it up quickly and ensure adequate airflow at all times to prevent overheating. Look for signs of overheating like yellowing leaves when supplementing CO2.
To increase carbon dioxide levels in their grow rooms, growers can purchase either CO2 generators or compressed CO2 canisters. Compressed CO2 canisters are best for small grows since they don’t produce heat. CO2 generators are better suited to larger indoor grows equipped with climate control.
Growers should note that carbon dioxide isn’t the only gas plants absorb through their stomata. If a grow room contains airborne contaminants like carbon monoxide or ethylene, these gases can also pass through the guard cell’s membrane and enter the plants’ tissues. Test the air quality in the grow room periodically to ensure that the plants aren’t being exposed to potentially harmful airborne contaminants.
Optimizing the Light Spectrum
Plants absorb some wavelengths of light more efficiently than others. They require more light from the blue end of the spectrum than the red. When growers supplement blue light, the plants’ stomal openings will grow larger, introducing more CO2 and facilitating increased growth.
Be cautious when manipulating the light spectrum. It’s never wise to use colored LEDs to alter the light spectrum, as this can alter the plant’s ability to regulate CO2 absorption and water retention. Purchase HPI or HPS lamps optimized to provide balanced lighting instead.
Optimizing Moisture Levels
Marijuana plant’s stomal responses to changes in CO2 and light levels are comparatively straightforward, but their responses to internal and external moisture are more complex. Anyone who has ever so much as taken care of a houseplant knows that all plants require adequate access to water, not just to grow, but to survive. Optimizing stomal opening and closing in response to moisture levels requires at least a basic understanding of how plants absorb and release water.
Most growers realize that plants absorb water through their roots. The water then moves through the plant, moving nutrients and minerals to different areas and maintaining a state of homeostasis throughout their various parts. Excess water is then released through the stoma, allowing the plant to cool itself in the process.
When a marijuana plant’s internal moisture level drops too low, its stomata will close, regardless of light conditions and CO2 levels. This stops the plant from photosynthesizing, interrupting its growth, but in the wild, it could also prevent it from drying out during periods of drought. The same is true in poorly tended marijuana gardens.
Closing the stoma does more than stunt a plant’s growth. It also inhibits evapotranspiration, which makes plants more prone to overheating. When the stomata remain closed during daylight hours, the leaves get hotter, which forces some water out even when these apertures are fully closed. This exacerbates the plant’s problem with insufficient internal moisture.
To make matters even worse, inhibited evapotranspiration reduces the external humidity of the air surrounding the plants. This creates an even greater disparity between external humidity and internal moisture levels. The result will be that the plants lose more water, become more prone to overheating, absorb less CO2, and experience greater reductions in growth and yield.
The only way to prevent this vicious cycle from occurring is to ensure that the plants have access to plenty of water. In soil-based systems, that means setting up irrigation or watering the plants daily. In hydroponic systems, it means ensuring that the grow media don’t dry out. Either way, it’s one of the most important things growers can do to ensure not just optimal stomal operation, but also optimal plant health, more generally.
Like all plants, marijuana plants evolved to be adaptive to their environments. Since they evolved in arid climates, cannabis plants don’t have appropriate mechanisms for regulating their stomata when exposed to constant high humidity.
In theory, there should be no need for them to close their stomata when the humidity levels remain high. Recent research performed at the University of Wageningen studied plants’ responses to high humidity and provided evidence for their lack of adaptive mechanisms.
The takeaway from this study is that marijuana plants aren’t always equipped to respond properly to environmental stressors. That’s why it’s so important for growers to ensure optimal light, CO2, moisture, and humidity levels, especially when they grow indoors.
As a general rule, cannabis plants grow best when the relative humidity level is between 65 and 80 percent during the seedling stage, between 55 and 70 percent during the vegetative stage, and between 40 and 50 percent during the flowering stage. In hot environments, marijuana plants benefit from higher relative humidity levels. In cooler environments, growers should aim for the lower end of the spectrum.
The easiest way to reduce the humidity in a grow room is to install intake and exhaust fans and a dehumidifier and to decrease the temperature, especially at night. To increase relative humidity levels, turn down the exhaust fan, increase the temperature, and mist plants in the seedling and vegetative stages. Never mist plants in the flowering stage, though, as this can cause bud rot.
The Bottom Line
All terrestrial plants use stomata to regulate CO2 levels and internal moisture. Growers should now have a basic understanding of the biological mechanisms at work behind the scenes and have all the knowledge they need to manipulate indoor grow environments. This will help their plants optimize photosynthesis and evapotranspiration, leading to increased growth and improved yields.
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