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Optimizing Your LED Spectrum for Leaf Surface Temperature

By Andrew Myers
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Every detail counts at an indoor grow facility. Indoor growers have complete control over nearly every aspect of their crop, ranging from light intensity to air circulation. Among the most important factors to regulate is temperature. While ambient air temperature is critical, growers will also want to measure leaf surface temperature (LST).

To illustrate, let’s say you keep your living room at a cozy 76 degrees. Then, if you place a thermometer under your tongue – your body is (hopefully) not at 76 degrees but is likely between a healthy temperature of 97 to 99 degrees.

A similar story can be told for cannabis plants grown indoors. A grow facility’s ambient air is often different than the plants’ LST. Finding an ideal LST for plant growth can be complex, but modern technology, including spectrally tunable LED grow lights, can simplify monitoring and maintaining this critical aspect.

Why Should Growers Care About LST?

Temperature plays a pivotal role in plant health. Many biochemical reactions contributing to growth and survival only occur within an ideal temperature range. If temperatures dip or spike dramatically, growers may witness inhibited growth, plant stress or irreversible damage to their crops.

The leaf is among the most important plant structures as it’s where most metabolic processes happen. Therefore, finding an optimum LST can improve growth rate and the production of metabolites such as pigments, terpenes, resins and vitamins.

Because many plants rely on their leaves for survival, it makes sense that leaves have their own temperature regulation system. Evaporation through pores in the leaf – known as stomata – can cool the plant through a process called transpiration. Up to 90% of water absorbed is used for transpiration, while 10% is used for growth.

The efficacy of transpiration is determined by the vapor pressure deficit (VPD), which refers to the relative humidity in the ambient air compared to the relative humidity in the leaf. If relative humidity is low, the VPD can be too high, which may cause plants to have withered, leathery leaves and stunted growth. On the other hand, a low VPD correlates to high relative humidity, and can quickly result in disease and mineral deficiencies. Higher humidity often results in a higher LST as transpiration may not be as effective.

When it comes to LST, growers should follow these basic guidelines:

  • Most cannabis plants’ LST should fall between 72 and 86 degrees – generally warmer than the ambient air.
  • LST varies depending on individual cultivar. For example, plants that have evolved in colder climates can generally tolerate cooler temperatures. The same can be said for those evolved in equatorial or temperate climates.
  • CO2 availability also plays a role in LST; CO2 generally raises the target temperature for photosynthesis.

How Does Light Spectrum Affect LST?

We know that CO2 concentration, specific genetic markers and ambient temperature all play an important role in moderating LST. But another important factor at an indoor grow is light spectrum – especially for those using spectrally tunable LEDs. Growers will want to optimize their light spectrum to provide their crop with ideal conditions.

A combination of red and blue wavelengths is shown to have the greatest impact on photosynthesis and, thus, LST. Photons found along the green and yellow wavelengths may not be absorbed as efficiently and instead create heat.

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Indoor cultivator facilities often use high powered lights that can give off heat

Optimized light spectrums – those with an appropriate balance between red and blue light – create more chemical energy instead of heat, thereby resulting in a lower LST. Using fixtures that are not spectrally tuned for plant growth, on the other hand, can waste energy and ultimately contribute to a higher LST and ambient temperature, negatively affecting plant growth. Consequently, measuring LST doesn’t only indicate ideal growing conditions but also indirectly illustrates the efficiency of your grow lights.

LED fixtures already run at a lower temperature than other lighting technologies, so indoor growers may need to raise the ambient temperature at their grow facilities to maintain ideal LST. Switching to spectrally tuned LEDs may help growers cut down on cooling and dehumidifying costs, while simultaneously improving crop health and productivity.

What’s the Best Way to Measure LST?

There are several tools available for growers to measure LST, ranging from advanced probes to specialty cameras. However, many of these tools provide a reading at a specific point, rather than the whole leaf, leading to some inaccuracies. Temperature can dramatically vary across the leaf, depending if parts are fully exposed to the light or in the shadows.

Investing in a forward-looking infrared camera (FLIR) gives indoor growers a more accurate picture of LST and light efficiency. That being said, growers should not only measure leaves at the top of the plant, but across the middle and bottom of the plant as well. That way, growers receive a complete snapshot of growing conditions and can make changes as needed.

At an indoor grow facility, it’s not enough to only measure ambient room temperature. Of course, this aspect is important, but it will paint an incomplete picture of plant health. Measuring LST gives growers nuanced insights as to how plants respond to their environment and how they can better encourage resilient, healthy growth.

Using spectrally tunable LEDs makes achieving LST easier and more cost-effective. Lights with optimized spectrums for plant growth ensure no energy is wasted – resulting in superior performance and efficiency.

Applications for Tissue Culture in Cannabis Growing: Part 3

By Aaron G. Biros
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In the first part of this series, we introduced some relevant terms and principles to tissue culture micropropagation and reviewed Dr. Hope Jones’ background in the science of it. In the second part, we went into the advantages and disadvantages of using mother plants to clone and why tissue culture could help growers scale up. In the third part of this series, we are going to examine the five steps that Dr. Jones lays out to successfully micropropagate cannabis plants from tissue cultures.

Cleaning – Stage 0

Explant cuttings are obtained from mother plants. The cuttings are further separated into smaller stem pieces with a single node.

Micropropagation includes 5 stages. “Stage 0 is the preparation of mother plants and harvest of cuttings for the explant material,” says Dr. Jones. “To ensure the best chance of growing well in culture, those ladies [the mom’s] should be cleaned up and at their best. And hopefully not stressed by insects or pathogens.” She says growers should also make sure the plants are properly fertilized and watered before harvesting explants. “Obtaining the explants is done with a clean technique using new disposable blades and gloves,” says Dr. Jones. “Young shoot tips are harvested and placed in labeled, large Ziploc bags with a small amount of dilute bleach and surfactant solution, then placed in a cooler and taken to the lab.” This is a process that could be documented with record keeping and data logs to ensure the same care is taken for every explant. “Once in the lab, working in the sterile environment of the transfer hood, the cuttings are sterilized, typically with bleach and a little surfactant, and then rinsed several times with sterile water,” says Dr. Jones. Once they reach the sterile environment, Dr. Jones removes the leaves and cuts the stem down to individual nodes.

Establishment – Stage 1

Established explants propagating shoots

Establishment essentially means waiting for the shoots to develop. Establishing the culture requires an absolutely sterile environment, which is why the first step is so important. “Proper explant disinfection is equally as important is the control parameters of the facility itself,” says Dr. Jones. Mother plants are not grown in sterile facilities, but in an environment that is invariably contaminated with dust, which harbors micro-organisms, insects and other potential sources of contamination, including human handling. We discussed some of this in Part 2.

Explants, once sterilized and placed in the culture vessel, must establish to the new aseptic conditions. “Basically Stage 0 ends when the explants are cleaned and placed in the vessel. Stage 1 begins on the shelf while we patiently sit, watch and wait for the shoot growth,” says Dr. Jones. “Successful establishment means we properly disinfected the explants because the cultures do not become contaminated with bacteria or fungi and new shoot growth emerges.”

Multiplication – Stage 2

Stage 2 involves subculturing an explant to produce new shoots

This stage is rather self-explanatory as multiplication simplified means generating many more shoots per explant. In order to create a large number of plants needed for meeting the demand of weekly clone orders, Dr. Jones can break up, or subculture, one explant that contains multiple numerous new shoots. “Let’s say one vessel, which originally started with 4 explants each developed four new shoots. Working in the hood, I remove each explant from the vessel and place it on a sterile petri dish. Now I can divide each explant into 4 new explants and then place the four new explant cuttings into their own vessel. In this example, we started with one vessel with 4 explants,” says Dr. Jones. “Which when subcultured 4-6 weeks later, we now have 4 vessels with 16 plants.” This is instrumental in understanding how tissue culture micropropagation can help growers scale without the need for a ton of space and maintenance. From a single explant, you can potentially generate 70,000 plants after 48 weeks, according to Dr. Jones. “Starting with not 1, but 10 or 20 explants would significantly speed up multiplication.” Using tissue culture effectively, one can see how a grower can exponentially increase their production.

Rooting – Stage 3

“When the decision is made to move cultures to the rooting stage, we typically need to subculture the plantlets to a different media formulated to induce rooting,” says Dr. Jones. “In some instances, the media is very dark, and that’s because of the addition of activated charcoal.” Using activated charcoal, according to Dr. Jones, helps darken the rooting environment, which closely mimics a normal rooting environment. “It helps remove high levels of cytokinin and other possible inhibitory compounds,” says Dr. Jones. Cytokinins are a type of plant growth hormone commonly used to promote healthy shoot growth, but it is important to make sure the culture contains the right ratio of hormones, including cytokinin and auxin for maximum root and shoot development. Dr. Jones suggests that growers research their own media formulation to ensure nice, healthy roots develop and that no tissue dies in the process. “With everything I grow in culture, when it comes to media, in any stage and with all new strains, I run some simple experiments in order to refine the media used,” says Dr. Jones. She puts a special focus on the concentrations and ratios of plant hormones in formulating her medias.

After harvesting and multiplying, these explants are ready for rooting

“We commonly think of auxin’s role in rooting, but it’s also important in leaves and acts as a regulator of apical shoot dominance,” says Dr. Jones. “So having no auxin may not be ideal for the shooting media used in Stages 1 and 2.” Auxin is a plant hormone that can help promote the elongation of cells, an important step in any plant’s growth. “And cytokinins are typically synthesized in the root and moves through xylem to shoots to regulate mitosis as well as inducing lateral bud branching, so again finding that nice balance between these two hormones is key.”

Acclimation & Hardening Off – Stage 4

“When plants have developed good looking healthy roots, it’s time to pop the top,” says Dr. Jones. This means opening the vessel, another risk for contamination, which is why having a clean environment is so crucial. “The location of these vessels needs to be tightly controlled for light, relative humidity, temperature and cleanliness.” In the culture, sugar is a main ingredient in the medium, because the growing explants are not very photosynthetically active. “By opening the lid of the vessel, carbon dioxide is introduced to the environment, which promotes and enhances photosynthesis, really getting the plants ready for cultivation.”

Harvesting explant material from mother plants

The very final step in tissue culture micropropagation is hardening, which involves the formation of the waxy cuticle on the leaves of the plant, according to Dr. Jones. This is what preps the plant to actually survive in an unsterile environment. “The rooted plants are removed from the culture vessel, the media washed off and placed in a potting mix/matrix or plug and kept in high humidity and low light,” says Dr. Jones. “Now that there is no sugar, contamination is no longer a threat, and these plants can be moved to the grow facility.” She says conditioning these plants can take one or two weeks. Over that time, growers should gradually increase light intensity and bring down the relative humidity to normal growing conditions.

Overall, this process, if done efficiently, can take roughly eleven weeks from prepping the explants to acclimation and hardening. If growers perform all the steps correctly and with extra care to reduce risks of contamination, one can produce thousands of plants in a matter of weeks.

In the fourth and final part of this series, we are going to dive into implementation. In that piece, we will discuss design principles for tissue culture facilities, equipment and instrumentation and some real-world case studies of tissue culture micropropagation.