Why is only chlorophyll important for sunburn or frost? Frost affects leaves if they freeze, which is more a function of whether they have water in them and what the freezing point depression effect of the sap salts is.

Alpine plants usually have blue leaves.

In fact red leaves indicate high levels of anthocyanins which are antioxidants which protect the leaves (or fruit) from various damage. There was an interesting catalyst show on this a few years back. blueberries are meant to be very good foryou as a result.

There is a lot of scientific work being done in this area for a number of reasons. One is the theory that the presence of carotenoids and anthocyanins (red, yellow and brown colours)in plants are a form of UV protection for the plant surface. It is particularly apparent on leaves of rainforest plants such as Lilly pillies and Eupomatias when they are exposed to strong sunlight. It can also be seen on ripening fruit such as some apples, where the fruit turns a colour when it is exposed to the sun.

Carotenoids sort of carry away the extra sunlight energy in the form of unwanted electrons, from locations that could damage plant tissues and photosynthesis. Carotenoids are in carrots naturally, but how the sun is responsible for a root vegetable turning yellow/orange, is another story.

As for blue leaves in Alpine plants, the colour tends also to be a silver-grey. The lighter colour reflects some of the intense light. In Alpine situations the plant is much more exposed to the sun's harmful rays. Waxes on the leaf surface deflect ultra-violet rays etc., away from the leaf. Fine hairs also do the same as well as protecting against excessive water loss and also help insulate plants against frost/snow damage. The stomata tend to be fewer and more hidden to handle the harsh winds and temperatures. They are often found on the underside of the leaf. These factors will effect the colour of the leaves. Just look at heathland plants in any exposed coastal plant community and the leaf colour is pretty well the same as in the Alpine heathlands. Photosynthesis has to be minimised because of the exposed conditions. The leaves tend to be much much smaller as well.

It is all a case of the plant adapting to the environmental situation. Fascinating!!!

Interesting. The sunburn thing seems to make sense, but I don't know about the frost. Perhaps the anthocyanin has an anti-freeze effect? Frost is so rare in my garden after many years of micro-climate modification, that I can't easily find examples of red and green leaves side by side to look at in frost conditions.
Trish