Is this light bleaching?

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Delps8

Well-Known Member
cdgmoney250 said:
The simple observation that every time the light was increased the plants became more stressed.

And if you read again what I wrote: “looks like the plants are receiving too much light for the environmental conditions, likely a VPD/transpiration issue.”

Hopefully that clears that up.
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I fully understood what you wrote the first time.

"The simple observation that every time the light was increased the plants became more stressed."

I'm a strong believer that correlation is not causation. Symptoms of excess light are photoavoidance and, in extreme cases, destruction of leaf tissue. I'm not seeing either of those.

cdgmoney250 said:
Could possibly be over-watering, but to me it looks more like the plant just isn’t drinking properly. Soil doesn’t look soggy, and the plants are large enough to snap out of an over-water situation quickly if the environment is in order. Thats why I recommended letting the soil dry out a bit before the next irrigation cycle.



You’ll note that I didn’t recommend adjusting the light levels. I recommended adjusting the temperature higher into the mid 80s and adjusting humidity higher. I also mentioned looking into VPD, because I personally have found that it works in helping alleviate certain transpiration issues, including overwatering, and other abiotic stresses. The warmer temps cause more water to evaporate from the soil, increases leaf temperature, and plants generally photosynthesize more efficiently at warmer temps (to a point).



Phosphorus deficiency typically causes Purpling in stems, petioles, and leaf veins when it is not an inherent trait of the strain. Phosphorus demand can outpace uptake during certain stresses.

Phosphorus deficiency typically causes Purpling in stems, petioles, and leaf veins when it is not an inherent trait of the strain. Phosphorus demand can outpace uptake during certain stresses.

VPD being out of range can cause transpiration issues, affecting stomatal opening, lowering the rate of photosynthesis, which can reduce water uptake making the soil take longer to dry out between irrigation cycles. When the rate of photosynthesis is lowered, but photons are still raining down on the canopy, the plant has to put energy into dealing with any surplus photons. This will increase nutrient demand. If demand can’t be met, deficiency is increased with higher light loads.

I’m not saying all P deficiency or purpling of stems are related to transpiration issues by any means. I’m just stating that transpiration issues can be a factor in those symptoms. Based on OP’s environmental conditions, I’d bet on it.
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No argument there.
 
cdgmoney250

cdgmoney250

Well-Known Member
Delps8 said:
Symptoms of excess light are photoavoidance and, in extreme cases, destruction of leaf tissue. I'm not seeing either of those.
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The destruction of leaf tissue you are describing is photooxidation, also described as photo damage or solarization. The light avoidance you are describing is physiological leaf roll, which is a heat/drought/high light response. Most leaves can reposition their chlorophyll pigments, sometimes they reposition the entire leaf. See links below to see the correlation between these processes, and how they can be caused by photoinhibition.

Photoinhibition, in my opinion, is what OPs plants are suffering from. Photoinhibition occurs at all light intensities and at all wavelengths. It happens on a molecular level. It only becomes visible when the rate of damage exceeds the rate of repair. By the time you SEE the damage, it may be irreversible, but it had already been happening and reducing photosynthesis.

Here are a few links on the matter:


en.m.wikipedia.org

Photoinhibition - Wikipedia

en.m.wikipedia.org en.m.wikipedia.org



Here’s a quote and link from an article talking about the relationship of Vapor Pressure Deficit and rate of photosynthesis. This is concerning my comments and recommendations about OP’s environment and WHY it is relevant to light intensity.

“High VPD typically causes plants to close their stomata to minimize water loss and avoid critical water tension within the xylem (Running, 1976), which occurs at the cost of reduced photosynthesis. Simultaneously, transpiration rate increases with high VPD up to a point, after which it either remains high or starts decreasing (Franks et al., 1997), resulting in a further exacerbation of plant water stress.”


If you end up reading through all that and still disagree, then we can agree to disagree :peace:
 
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bignugs68

Active Member
PXL_20240401_143344670.jpg

Not to interrupt debates, but they're doing great lol :D. I did feed them some tap water with Epsom salts. Tap is 7.8ph roughly and need magnesium - Epsom salts. If you look closely, got some dark veining+light green between veins.
 
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Delps8

Well-Known Member
cdgmoney250 said:
The destruction of leaf tissue you are describing is photooxidation, also described as photo damage or solarization. The light avoidance you are describing is physiological leaf roll, which is a heat/drought/high light response. Most leaves can reposition their chlorophyll pigments, sometimes they reposition the entire leaf. See links below to see the correlation between these processes, and how they can be caused by photoinhibition.

Photoinhibition, in my opinion, is what OPs plants are suffering from. Photoinhibition occurs at all light intensities and at all wavelengths. It happens on a molecular level. It only becomes visible when the rate of damage exceeds the rate of repair. By the time you SEE the damage, it may be irreversible, but it had already been happening and reducing photosynthesis.

Here are a few links on the matter:


en.m.wikipedia.org

Photoinhibition - Wikipedia

en.m.wikipedia.org en.m.wikipedia.org



Here’s a quote and link from an article talking about the relationship of Vapor Pressure Deficit and rate of photosynthesis. This is concerning my comments and recommendations about OP’s environment and WHY it is relevant to light intensity.

“High VPD typically causes plants to close their stomata to minimize water loss and avoid critical water tension within the xylem (Running, 1976), which occurs at the cost of reduced photosynthesis. Simultaneously, transpiration rate increases with high VPD up to a point, after which it either remains high or starts decreasing (Franks et al., 1997), resulting in a further exacerbation of plant water stress.”


If you end up reading through all that and still disagree, then we can agree to disagree :peace:
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I'll read it, thank you for posting the links and that's exactly the kind of answer I'm happy to get. Thank you.

I realize that photo inhibition (I've referred to it as avoidance) is a function of movement of hormones but what I have read (so far) is that they migrate away from the source, thus causing the impacted leaf material to curl up. That reduces the area exposed to the light. Those are the "classic symptoms". What we're seeing is leaves turning down. My thinking is that was a watering/turgor issue.

I've caused photo-inhibition a few times and the leaves tacoed or rotated around the petiole. In the case of one large cola, it got 1250µmol by mistake and the top of the cola bent at about a 30° angle. The leaves re-oriented; the cola stayed bent.

Again, thank you for the links. I do respect your taking the time to dredge it up and post it. Kudos.
 
cdgmoney250

cdgmoney250

Well-Known Member
Delps8 said:
I'll read it, thank you for posting the links and that's exactly the kind of answer I'm happy to get. Thank you.

I realize that photo inhibition (I've referred to it as avoidance) is a function of movement of hormones but what I have read (so far) is that they migrate away from the source, thus causing the impacted leaf material to curl up. That reduces the area exposed to the light. Those are the "classic symptoms". What we're seeing is leaves turning down. My thinking is that was a watering/turgor issue.

I've caused photo-inhibition a few times and the leaves tacoed or rotated around the petiole. In the case of one large cola, it got 1250µmol by mistake and the top of the cola bent at about a 30° angle. The leaves re-oriented; the cola stayed bent.

Again, thank you for the links. I do respect your taking the time to dredge it up and post it. Kudos.
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You’re very welcome grower. I’ve also caused photoinhibition a number of times, and had to do a whole lot of reading and sort my problems out the hard way. I just try to remember that everything about growing is super dynamic and interconnected, especially when trying to diagnose issues or come up with solutions.
 
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