What is photobleaching?

With proper light quality, you can avoid photobleaching at all light intensities

As a grower, you have many things to worry about. Plant life cycles, positioning of light, heat, light spectrum, space you have, are all extremely important factors to consider when setting up a a greenhouse or growing space. It's no secret that higher light intensities mean higher yields. But for many indoor gardeners, bud-boosting light intensity and unwanted photobleaching go hand in hand.

Cannabis buds discolored by light can sometimes turn white

Your cannabis plant can only withstand a certain amount of light. Beyond a certain stage, your cannabis will start to turn yellow or show signs of stress on the leaves near your sources of light and/or heat.

This is how you get “albino” or white buds. Light bleaching is more common with high power LEDs and HPS lamps, as they can have higher light intensities in sunlight. Basically, bud discoloration is what happens when your buds get too much light, much like hair fading if you spend a lot of time in the sun. Except that a "sunburned" bud is less noticeable and it may have lost its "cannabis" smell. »

Dr David Hawley, professor at Fluence and creator of lighting solutions, explains that this is not inevitable. By understanding the mechanism of photobleaching and its relationship between light intensity and quality, it is possible to avoid the problem even under high intensity lighting.

What is Photoelectric Bleaching?

Hawley explains that photobleaching is just what it sounds like: literal bleaching of the flower bud.

“If you look at the top of the cannabis canopy, you will see that all the upper inflorescences, colas or flower buds – whatever you want to call them – will be bleached. They won't have the green look you might expect,” he explains.

https://www.cannabisbusinesstimes.com/article/photobleaching-fluence-david-hawley/

The causes of this superficial bleaching are found below, where chlorophyll and other plant pigments break down. Hawley compares this to parts of cells in men that react to oxygen and damage them, often leading to the consumption of antioxidants to improve one's health.

"It's actually a very similar mechanism to what happens with chlorophylls in plants," he explains.

With photobleaching, through several very specific steps, the oxygen-reactive parts generated in the cannabis plant strip electrons from the chlorophyll. "When this happens, the chlorophyll no longer has the means to maintain itself, so it essentially decays." Hawkey said. "Chlorophyll is what gives plants their green appearance, so if it disappears, you are left with just the white tissue of flower buds."

What clues show that photobleaching has started?

Hawley explains that photobleaching is not announced by early morphological clues in the leaves. Instead, you may go into your grow and notice that the upper inflorescences look a little pale, or you may find that the tips of the plants are totally white. " These are signs of what's to comer,” he says.

Although an inexperienced grower might confuse photobleaching with nutrient deficiency, Hawley says the two phenomena are significantly different. And, although the chlorosis is always a degradation of chlorophyll, it works by a different means.

“Chlorosis is essentially due to the fact that chlorophyll is eliminated and reallocated in the plant,” he explains. “We would probably see it more in the leaf tissue and it would be a bit more gradual. You wouldn't see such an austere, bleached flower bud. »

However, the Fluence team found that photobleaching occurred at 800 micromoles or less under high red light illumination.

What light intensities trigger photobleaching?

With the right quality of light, Hawley explains, you can avoid photobleaching even at light intensities up to 2500 PPFD (photosynthetic photon flux density, measured in micromoles per square meter, per second). "We didn't test higher intensities because the business aspect of applying more light to the canopy becomes rather unreasonable," he explains. "From an economic point of view, it no longer makes sense."

"We've even had a very rare cannabis cultivar that bleached around 800 micromoles in white light, but that's the exception. Most cannabis cultivars seem to tolerate white light without a problem,” he adds.

According to Hawley, most growers use between 1300 and 1500 micromoles without a problem. But higher intensities allow for better yields.

“The sweet spot, if you have the infrastructure, is between 1850 and 1900 micromoles“, He said.

But pay attention to the caveat about the cultivar, he points out. Some cultivars can bleach much, much less. Fluence research has shown that a light quality close to 60% to 65% red is sufficient to induce whitening, even at conventional intensities such as 1300 PPFD. "Honestly, that's kind of surprising to me, because when we get [the red light] down to around 40%, it's not a problem," says Hawley.

He explains that it is the relative fraction of red light, not the absolute amount of red light, that seems to make the difference.

With light quality in the 40% red range, as with Fluence's broad-spectrum R4 illumination, photobleaching does not appear to occur even at light intensities up to 2500 PPFD. “It seems to be a matter of ratio and balance between the red and the rest,” says Hawley.

The most important point to understand is that photobleaching does not occur with a more balanced broad spectrum. “I think it's because we balance the energy across the entire photosynthetic active radiation (PAR) range. We don't concentrate that energy into a narrow peak,” he adds.

What is the impact of photobleaching on crop yield and quality?

For growers faced with photobleaching, the impact on yield and quality, especially on secondary metabolites, is a major concern. "Honestly, photobleaching shouldn't have a huge impact on either of those things," says Hawley.

The yield is similar to that of a tomato, he explains. The cannabis flower, like the fruit of the tomato, does not depend on high levels of local photosynthesis to gain mass.

"The buds make so little that it doesn't really impact the yield," he says.

When it comes to quality, separating correlation and causation complicates things. Hawley has observed cases where there is a correlation between photobleaching and lower concentrations of cannabinoids and terpenes. But he also saw photo-bleached plants with concentrations of cannabinoids and terpenes equal to plants grown without photo-bleaching under white light.

How can growers avoid photobleaching?

If photobleaching hits your crop in the middle of a flowering cycle, Hawley says reducing light intensity is the only way to stop what's going on. But a better approach is to use a light quality that avoids photobleaching even at high light intensities.

The most important thing is to use the right amount of light. If you use the right amount of light, you can go all the way to the highest light intensity you can achieve with your setup with very, very low risk of photobleaching. So it's the best thing to do,” says Hawley.

He adds that proper light quality allows growers to achieve higher intensities, while more red-rich light quality requires lower intensity instead. “But when you reduce the intensity, you also reduce the output. Most cannabis growers aim to make money. They would like the return to be high,” says Hawley. “And to get the best yields, they need to use a more balanced, whiter and less red spectrum. »