Many plants are selected for their natural red foliage, and many plants simply turn red. I won't talk about autumn leaves, because I know that's not what you're asking.

At low light levels, green leaves are most efficient at photosynthesis, but on sunny days, there is little or no difference between a green leaf's and a red leaf's ability to trap the sun's energy (carry on photosynthesis). Many pigments play some part in protecting various plants from sun's rays, but red anthocyanin is most effective at protecting plants from exposure to intense energy by absorbing ultraviolet light.

As light levels intensify in some plants, the levels of anthocyanin increases. This can be especially noticeable when sugars build up during periods of heavy photosynthesis while the growth rate is reduced (why bedding plants often turn purple in the chill of spring temps).

Often in young plants or in new growth, where chlorophyll and cuticular wax production (also protects from damaging light) has not yet begun and which would be unprotected from UV light, anthocyanin production increases. Parts or even the whole plant are colored by "juvenile anthocyanins," and thereby protected from damage. As soon as chlorophyll production begins, the production of the anthocyanin pigment is reduced.

The presence of noticeable amounts or a build-up of anthocyanin in plants is specific to the plant type, but can also depend in part on the soil conditions, light levels, temperature, pH and a few other cultural influences I'm sure I left out.

Al

Anthocyanins are also responsible for the reddish coloration of the underside of plant leaves that grow on the forest floor, as the far-red wavelengths are the only ones to make it to the floor (the lack of blue makes the red expression), and these pigments help the plant absorb those wavelengths (as well as reflect them). A fair rule of thumb with your houseplants with lots of red in the leaves means they are telling you they are lower-light plants.

Lettuce (cole crops) selections for color are, IIRC, ones that express traits via presence or absence of blue light and the light quality. I suspect that if you were to grow them under a shade cloth their coloration would be different. Certainly environmental stress wrt soil properties and temperature could likely alter color as well.

Dan

Thanks for the insight!

-V

Hi,

What has been written here is quite interesting, it seems that you do know a lot, however I am not total clear on what this all means.

Let me give you an example of what I am doing, I have LEDs lights and I am trying to find out what part of the light specifically will make a red lettuce red.

As it stands, I have one lettuce in a dark room with two red lights and one blue on them, the red lettuce has turned green and not red.

I have another in sunlight and that is growing as normal, however the redness of the lettuce is not as red as my partners red lettuce which has been growing in the sun at a higher lattitude. I am at 23deg North (in Taiwan) my friend is 38deg North (Japan).
The third experiment has two red light one blue and the sunshine, the redness of the lettuce is the same as the second experiment, however the lettuce is larger, and a slightly different shape, actually the leaves are flatter.

So my question is simple, given all this data, can anyone give me an insights or ideas as to why this has happened and how I should proceed in the future with the experiments.

I am a little new at all this and as so any and all thoughts are welcome. I will keep updating with results of the suggestions.

Thankyou,

David.

I noticed now (April 10, Zone 7) my indoor hydroponic red lettuce's color is fading into almost green but at the same time the same batch growing outdoor (lower temperature) in the sun is vibrant red. But my indoor lettuces are much bigger. Both groups have a same setup in terms of nutrient and their containers. So I conclude the sunlight and the low temperature must something to do with the redness. -- Xiao

The most common cause of expression of the purple pigment, anthocyanin (very common at this time of year), is a deficiency of phosphorus (P); this, because P is required to make ATP and ATP is needed to turn sugars to starch and to load sugar into phloem for transport. No P = no ATP = no move sugars/starches = anthocyanin (purple pigment) builds up = plants turn purple. The most frequent cause of a P deficiency is cold soils, which is why so many plants planted out too early turn purple.

Al