They'll turn colourful in full sun as a response to increased light, for one reason, rather than sunburn. It's also a response to stress, whether not enough water/soil.

Look up CAM - Crassulaic Acid Metabolism, where you can learn about cyanins and such.

Mostly to protect themselfs. Also in cool, cold climate they may turn red to protect themselves. My Crassula certainly do. I have 'Ruby Jade' it turns red in the winter, with no water during the rainy season. It still thinks it's in Africa when they are supposed to be dormant and flowering, It's a good time to take cuttings after they flower and trim while doing so. One of my C. ovata , plants are already setting flowers. There rainy season is over in Africa and it's going into Spring at this time. 'Ruby' must be stressed before it will turn red. Norma

So this is what I figure: In full sun there is less of a need for chlorophyl so it becomes more dispersed. A plant is only going to grow as much as it is geneticaly told to. Once it reaches this "critical mass" it pulls chlorophyl from the leaves to keep the balance between food and growth. The chloroplast captures mainly red light to use in creating food. What Im thinking is that since the chloroplast are so spread out that red light is reflected back since it is not needed, giving us a beautiful display of color. I have yet to find any info on this. These are just my thoughts. If anyone knows the reason I would love to know. Thank you!!
Steve

I just came from a program on plants from Mexico. Echeveria, Sedum, cactus some living next to each other, some were red others green, so why? Norma

The following are guesses/opinion on my part:

Cactus that turn red in stress or winter - I would guess that it would be a breakdown of chlorophyll (if the cactus is not metabolyzing at a high rate it would have less need of chlorophyll).

For plants that are red during the growing season/full sun -- I would guess that there is not any less chlorophyll, but just an increase in cyanins that mask the chlorophyll. I would guess this is to help protect the plant from excess sun.*

Most plants that have varying levels of reddish color, are more red in in bright light than low light, and tend to be more red in spring when the leaves are young and more vulnerable to damage (One common example I'm thinking of is Japanese Maple (excluding those varieties that have been bred to be red constantly.)

*As to why some plants in the same space are green and some not. Different species are going to respond to the amount of light differently, and different varieties within a species are going to have different capabilities to produce cyanin. Some cacti (it is believed) have evolved to be spiny or hairy not to protect them from predation as much as to filter the light reaching the stem.

Thank you everyone for your responses. And some good questions have been raised. I agree with mauch1 it must be the production of cyanin. I feel much better on this now. It is such a strange phenomonon that I had to get to the bottom of it. My curiousity is satisfied...for now :)
thank you
Steve

If memory serves correctly, the red pigments are upregulated during high sun and UV intensity. Since chlorophyll does not absorb the yellow and green portion of the spectrum, the added pigments help absorb some excess light energy (photons) to help "shield" the chlorophyll.

I would imagine that during the stress of cold or drought, the pigmentation has the same function - limiting excess energy and protecting chlorophyll, because metabolic activity is reduced and the photons are not needed.

Hope this helps.

By the way, Norma, you keep mentioning things about your plants thinking they are in South Africa and supposed to be dormant; what is the point you trying to communicate? I don't understand what you're getting at...

x

Thank you X!!! I have a scientist's mind. That is the exact answer I was looking for. It has helped a great deal. I love breaking things down and then watching the whole thing work together. It helps me to understand and troubleshoot ideas or inventions. Or to just better know my suroundings(now that everything is inside lol). Thank you everybody for your responses!
steve

will the plant turn back to green if moved out of direct sunlight?

...yes. Depending how much less light, it could really 'bleach out', to color it isn't supposed to be.

Could it be possible that the scion of a red cacti can be able to have it own root? Or only those cacti that have atleast little amount of green pigment in it will have its own root? Beacuse as you have judt discussed, that cacti will turn reds only to protect the chlorophyll from excess sunlight, so maybe it could he possible to root a red cacti somehow. Thank you for answering.

Could it be possible that the scion of a red cacti can be able to have it own root? Or only those cacti that have atleast little amount of green pigment in it will have its own root? Beacuse as you have judt discussed, that cacti will turn reds only to protect the chlorophyll from excess sunlight, so maybe it could he possible to root a red cacti somehow. Thank you for answering.