Here's a great article from our friends at Seed Mine.
By Burny Canterbury
All plants--well, all medical marijuana strains--are purple. Before you throw your bong at the screen, let me backtrack with a little precursor botany lesson. When chlorophyll works and captures sunlight, it's green. When zanaphyll captures sunlight, it's purple. All cannabis plants have zanaphyll in them. The zanaphyll resides just underneath the chlorophyll. So to recap: all cannabis plants are purple--they're just covered up by green chlorophyll.
Now, in some plants, through evolution and breeding and mendelian tricks, we've bred the chlorophyll to die, so its death naturally occurs at the end of a life cycle or through response to cold or any other external factors. But in some cannabis seeds we've bred naturally, the chlorophyll just dies at the end anyway. Any strain can be purpled up by getting the right DIF (short for differential), which is your day to night temperential temperature difference; you'll ideally get your temperatures to have about 30 degrees difference. So, if you're running your daytime temps at about 85, then you want your nighttime temps to be about 55. And that'll purple up just about any plant. You can go down to as low as about 50 degrees Fahrenheit.
So all you're really basically doing is wiping the green away. And what you're looking at is the emergency lights in the submarine and it's a secondary back up system that supplies about 30% of the plants energy. But it's never to the plants benefit. Purple cannabis like purple kush seeds could have been 20% stronger if it could have stayed green all the way to the bitter end because the green is that much more efficient at supplying energy and energy gets turned into resin, which is how plants get THC. And as you probably know, THC is what gives you your buzz.
As you've probably realized by now, the purple phenomenon has become a marketing gimmick But it's still worth buying and growing because the general public are too ignorant and misinformed to realize this blatant fact.
This piece is from the Seed Mine Blog and republished with permission.