Lol, ok, I kinda get purple stems almost everytime, although intensity will vary. After some research and asking around, I'm still puzzled. I like to think it's because of high temperature differential with lights on/off, as I can't identify other symptoms.
Can I have your opinion on this?
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You wont find much on it and of course what is around on the subject leaves everryone puzzled. Its like the freaking 64 gazillion dollar question which i as many others have debated heatedly for quite a long time and there are a lot who are not willing to be open to the subject citeing either genetics, cold, overwatering or phosphorous.
If you take any of my advice your a FOOL i tell ya and i cant offer any but merely touch on the topic of 'Purple on a grern plant'.
1.) Purple is anthrocyanin which i think is a flavanoid or somthing like that. It produces energy the plant can use from light but it is no where near as efficient as chlorophyll (the reason plants are green) so is not desirable for growth generally.
2.) Because it has light absorbing properties (in the green and blue wavelengths i think) it can be usefull to the plant when there is too much light, chillipepper growers refer to it as 'having a sun tan' as the anthocyanin can absorb the extra light thus taking some of the workload of the chlorophyl (that green stuff again) hence by protecting it. Every chloroplast in the chlorophyll gets damaged eventually from light photons and the plant has a very good repair system for that, the purple anthocyanin gives the plant some breathing space to get on with repairs (which even on a healthy plant happen all the time, the 'PS II system if memory serves me right).
3.) Stems, petioles, veins and roots are non carbohydrate producing areas, any kind of nutrient deficiency, too much light (Photoinhibition), coldness, nutrient toxicity etc etc could cause the plant to quickly colour these areas purple. The reason being that anthrocyanin is an antioxidant (Thinks they are still debating this one in the plant sciency laboritories around the world at the moment but...) these areas not being carbohydrate producing areas can produce a lot of free radicles or oxygen (bit complex) so the anthocyanin scavenges up these toxic particles and elements thus saving the day for the plant as it nay wants that shit in its system.
4.) At some point your going to realise purple is different for different situation. Henceforth the leaf has a few different purple patches or A.) That purple shit on top of the leaves. B.)That really annoying purple crap underneath the leaves. C.) C is not an obvious one but its the very dark colour of leaves because anthrocyanin can make a leaf look darker with no purple because there is an abundance of chlorophyll which hides the purple colour but the both together make a dark shade of green.
The purple on top of the leaves is very often Phosphorus deficiency or a nutrient/system related to Phosphorous. The reason it shows on the upper part of the leaf is because the mesophyll (not the spongy mesophyll below the mesophyll, although similar) have a lot to do with phosphate and hence any shortage and its anthrocyanin to the rescue to slow down photosynthesis or scavenge bad stuff or what ever their needed for. So thats one type of purple but under the leaf purple presents in different situations (possibly some of the same as the upper leaf purple, who knows?). The thing with the under leaf purp is thats in the spongy mesophyll area (not to be confused with the mesophyll layer above it). This is the underside area where theres air spaces so the plant can absorb Co2 for oxygen, any kind of excessive fast growth of the plant like what you see in seedlings placed too close too that damn light or flourescent t5!!!!! which with too much light forces the chlorophyll to work faster and harder and in turn the only system that cant keep up or the first system that fails is its ability to suck up enough Co2 (Carbon Dioxide which it splits for its source of carbon, you might have heard that most living organisms are carbon based). So angain the plant sends its little purple headed warriors to sort shit out or try and do their damage control stuff.
So theres a distinct difference between plant area and could all happen to you at once, your fucking up! Some purple is livable and most growers will get it in some form or another and still the plant will grow and yeild big no problems but in excess its a sign of underlying problems.
5.) Cold and genetics kind of go hand in hand, the plants responce to cold is linked to stems petioles and non carbohydrate producing areas but also its lack of absorbing Phosphorus from the roots due to its imobility or some crap i need to look up but cannot be bothered to (Sorry....).
Heres the crux about the genetics part because weed grown in colder latitudes has always expressed purple more than warmer climates (Generally). Its more in their genes/genetics, i mean all weed plants have the ability to produce anthrocyanin just some more than others ( We all decended from one African but although were all basically the same as well as atanomically some who lived in different climate lost or gained cetain abilities and we evolved Chinese, European, Middle eastern etc etc). So this ability is able to breed on in some strains meaning the colder it is the more purple it displays and the bettet equipped it is to deal with the cold.
As said before anthocyanin is inefficient compared to green chlorophyl and purely because we have a simple liking (nothing else) of weed with purple colours.
Obviously weed makes flowers and can genetically have purple buds but this isnt the leaves and stem its just the flowers or bud. Check out 'Purple Bud' by Sensi Seeds which shows just a purple bud on a pretty much 100% green plant with no purple stems, leaves or petioles.
Genetics is probably the most misunderstood part when it comes to purple, no seed breder produced genetically purple seedlings igts just a sign of somthing else, in the proper enviroment all seedlings and pla.ts should be green. So many get it we dont pay it much attention and just palm it off, some problems can take years to iron out it easy to ignore them if the plant still grows. I see a lot of Nitroven Toxicity pictures that look similar to your picture but in reality this aint the plant hospital and if youve read the above youll realise its way past my pay grade to give advice on topics that have confused me since i first put seed to soil years ago.
I dont think anything of your leaf or have any advice except it dosent look too bad, keep up the good work