Pruinescence in succulents

Have you eaten grapes and noticed the whitish, powdery coating on the skins? That is called the pruina. Studies on its purpose appear to be few or lacking altogether, but because it is prevalent on plants that receive high amounts of solar exposure, the pruina then can be construed as an adaptation for the purpose of bouncing off elevated light levels and heat, thus preventing the occurrence of sunburn. Additionally, the pruina may help in the regulation of transpiration. An interesting adaptation seen in a number of plants from different families with known representatives inhabiting very dry localities is the so-called Crassulacean Acid Metabolism (CAM). Most plants transpire, or breath, day and night which allows them to release unwanted gasses and also moisture. However, in dry habitats, moisture is a valuable resource and releasing it during the heat of the day can expose the plants to risk of dehydration. And so in plants that have resorted to the CAM, transpiration is only done at night, when temperatures are distinctly lower, to better manage moisture loss. The pruina possibly aids in the closing of the stomata during the daytime. Pretty funky, isn't it? Now, imagine yourself breathing only at night.

When I mentioned the CAM, the bit that said 'Crassulacean' probably struck a chord among you succulentophilics. And that is for good reason, because the adaptation is most widely known among the family Crassulaceae, which includes Crassula, Echeveria, Kalanchoe, Orostachys, and Sedum, among others. Not surprisingly, many species from the family exhibit the glaucous (bluish-gray) bloom, such as on this Orostachys and echeverias:

The blue-gray cast is evident on this Kalanchoe humilis. The well-known Mother-of-millions (K. daigremontiana) is another good example of a pruinescent Kalanchoe.

Even from other families, the pruinose covering can also be found, though not as extensively as in the Crassulaceae. Within the stapeliads, it can be found on truly arid-loving carallumas, such as Caralluma speciosa and its allies. The genus Euphorbia contains around 2000 extant species, but relatively few are pruinose, among them, this E. greenwayi:

The family Bromeliaceae also contain entities with the same waxy covering, and it can be found on a number of genera, including Aechmea. But perhaps the most well-known in this regard is Tillandsia. However, in this family the waxy layer has underwent development as being more scaly than powdery, and assists in the capture of moisture and nutrients from the air; this is best exemplified in Tillandsia where their roots mostly serve the purpose of an attachment mechanism rather than for the acquisition of water and nutrients.

Some of the more extreme examples can be found on species such as T. tectorum, wherein the protective layer has taken the appearance of fluff. I wish I could photograph one for you, but my plant died years ago.

Nevertheless, the pruina is not the sole property of these highly specialized plants. Take the leaf of this Strelitzia reginae, the Bird of paradise, as an example. Incidentally, strelitzias are also found in often very dry habitats and their roots have taken the function of being water storage systems.

I was once asked if the pruina, which she called the 'farina', will return on a plant once it is inadvertently rubbed (or purposely, in the case of very curious and nosy humans), and the simple answer is 'no'. However, one can promote its presence if we grow our plants hard, that is, if they are exposed to high light levels for a sufficient number of hours. Remember, a pruina's purpose is to shield the plant from light and heat extremes. If you are pampering your plants by putting it under some form of shade, then there would be no reason for those plants to protect themselves, or if they do, only quite minimally. You don't open your umbrellas in the shade, do you?