Unpacking Photobleaching With Dr. David Hawley
Courtesy of Fluence

Unpacking Photobleaching With Dr. David Hawley

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

May 25, 2022

It’s no secret that higher light intensities equate to higher yields. But for many indoor growers, bud-boosting light intensity and unwelcome photobleaching go hand in hand.

David Hawley, Ph.D., principal scientist for Fluence, explains that doesn’t have to be the case. By understanding the mechanism behind photobleaching and how light intensity and light quality relate, you can avoid photobleaching—even under high-intensity lighting.

What Is Photobleaching?

Hawley explains that photobleaching is simply what it seems: the literal bleaching of floral bud.

“When you look at the top of the cannabis canopy, you'll see that all of the upper inflorescences or colas or floral bud—whatever you like to call it—will be bleached white. They won't look green like you might expect them to,” he says.

The cause of that superficial bleaching lies underneath, where chlorophyll and other plant pigments have broken down. Hawley compares it to cell-damaging reactive oxygen species in humans—the reason behind urgings to consume antioxidants for health.

“It's actually a very similar mechanism as to what's happening with these chlorophylls in the plants,” he says.

With photobleaching, through several very specific steps, reactive oxygen species generated in the cannabis plant pull electrons off chlorophyll. “When that happens, chlorophyll no longer has the means to hold itself together, so it basically disintegrates.” Hawley says. “Chlorophyll is what makes plants look green, so if it goes away, you're just left with white floral bud tissue.”

What Clues Reveal Photobleaching Has Started?

Hawley explains that photobleaching doesn’t announce itself with early morphological cues in the leaves. Instead, you may go into your grow and notice the upper inflorescences look a bit pale, or you may discover the very tips of plants are totally white. “Those are signs of things to come,” he says.

Although an inexperienced grower could possibly misdiagnose photobleaching as a nutrient deficiency, Hawley says the two look noticeably different.

And, while chlorosis is still a breakdown of chlorophyll, it works through a different means.

“[Chlorosis] is basically chlorophyll being scavenged and reallocated in the plant,” he explains. “You'd probably see that more in the leafy tissue and it would be a little bit more gradual. You wouldn't see such a stark, bleached floral bud.”

What Light Intensities Trigger Photobleaching?

With the right light quality, Hawley explains that you can avoid photobleaching even at light intensities reaching 2,500 PPFD (photosynthetic photon flux density, measured in micromoles per meter squared, per second). “We haven't tested higher than that just because the sheer commercial practicality of putting any more light on the canopy than that becomes pretty unreasonable,” he says. “Economically, it stops making sense.”

However, the Fluence team has seen photobleaching occur at 800 micromoles or lower with lighting rich in red light.

 

Courtesy of Fluence
 
“We’ve even had the very rare cannabis cultivar that would bleach around 800 micromoles with white light, but that's the exception. Most cannabis cultivars do appear to tolerate white light without problem,” he adds.

Hawley says most growers run around 1,300 to 1,500 micromoles without problems. But higher intensities bring higher yields.

“The sweet spot, if you have the infrastructure to support it, that we see is around 1,850 to 1,900,” he says.

But be mindful of the cultivar caveat, he stresses. Certain cultivars may bleach much, much lower.

Fluence research has shown that light quality near 60% to 65% red is enough to induce bleaching, even at conventional intensities such as 1,300 PPFD. “Honestly, it's pretty surprising to me, because when we pull [red light] down to the 40-something percent range, it’s a non-issue,” Hawley says.

He explains that the relative fraction of red light—not the absolute quantity of red light—seems to make the difference.

With light quality in the 40% red range, as with Fluence’s broad spectrum R4 lighting, photobleaching doesn’t seem to occur even at light intensities hitting 2,500 PPFD. “It does seem to be something about the ratio and balance of red amongst everything else,” Hawley says.

The most important point to understand is that photobleaching doesn’t occur with a more balanced broad spectrum. “I theorize that that's because we're balancing the energy across the entire range of PAR or photosynthetically active radiation. We not focusing that energy into a narrow peak,” he adds.

How Does Photobleaching Impact Crop Yield and Quality?

For growers struggling with photobleaching, impacts on yield and quality, particularly secondary metabolites, are primary concerns. “Honestly, photobleaching shouldn't have an enormous impact on either of these things,” Hawley says.

Yield is similar to a tomato, he explains. Cannabis flower, like the tomato fruit, doesn’t rely on high levels of local photosynthesis to put on mass.

“The buds do so little of that, that it doesn't really impact the yield all that much,” he says.

With quality, separating correlation and causation complicates things. Hawley has seen cases where there is a correlation between photobleaching and lower concentrations of cannabinoids and terpenes. But he’s also seen photobleached plants with cannabinoid and terpene concentrations equal to plants grown with no photobleaching under white light.

“It's really tough to say that photobleaching causes a decrease in cannabinoid and terpene concentration, but there may be a correlation. But it's also probably fair to say, photobleaching aside, a more red light quality induces a lower potency—and disentangling those two things is pretty tough,” Hawley says.

How Can Growers Avoid Photobleaching Altogether?

If photobleaching hits your grow in the middle of a flower cycle, Hawley says reducing light intensity is the only way to stop what’s underway. But a better approach is to use light quality that avoids photobleaching even at high light intensities.

“The biggest thing is to use the right light quality. If you're using the right light quality, you can go basically to the highest intensity you can achieve in your facility with very, very little risk of photobleaching. So that's the very best thing you can do,” Hawley says.

He adds that proper light quality enables growers to push to higher intensities, while light quality more rich in red demands reduced intensity instead. “But when you reduce intensity, you're also reducing yield. Most cannabis producers are in the business of making money. They would like yield to be high,” Hawley says. “And to get the highest yields, they have to use a more balanced, more white spectrum with less red.”