SPECIAL EDITION: Farina - White Powder on Leaves
The Perle von Nurnberg (left) has farina. The same Perle von Nurnberg (right) does not. On this SPECIAL EDITION of Succulent Science Sunday, we will explore the question: What is Farina?
Farina is that white-powedery stuff on your succulent's leaves.
You may recognize farina as looking like "farine" which is flour in French. It comes from the Latin word for grain. All you need to know is that this white powdery dusting is called farine, or epicuticular wax, or glaucous leaves.
Epicuticular: epi means above (think epicenter) + cuticular (like cuticle) means the outermost layer and comes form an Old English word for skin.
Glaucous: a word for the color of grayish green or grayish blue. It comes from Greek glaukos for bluish-green.
Farina is found on many different plants and fruits. Think grapes and blueberries:
Or even on other plants, like roses:
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| farina wax crystals on the underside of a rose leaf |
Scientists have found, using scanning electron microscopy, that epicuticular wax crystals or farina crystals are made of hydrophobic, organic molecules that form nanotubes and nanosheets. Asymmetric secondary alcohols and β-diketones form waxy nanotubes, while symmetrical secondary alcohols and primary alcohols form nanosheets.
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| A simple β-diketone called diacetyl |
Farina protects our succulents
Think of the surface of your succulent leaf as a multilayer dip. The top layer is coated in farina crystals, the second layer beneath is the farina film, and more layers down is when you hit the plant cell walls (the plant cells you probably learned about from biology class).
Scientists have found that the farina helps the plant prevent water loss from transpiration (this is evaporation for plants), protects the plant from pests and diseases, and acts in a very sophisticated way to create a "self-cleaning" surface, which prevents dust and debris from blocking much needed sunlight. It can even screen from too much sunlight by blocking out upwards of 83% of UV-B rays (sunscreen for succs!). [UV-B can harm the DNA of succulents.]
Lotus Effect
Water makes beads because of cohesion (one of the many special properties of water.) But why does the leaf seem to repel the water? It is because the farina is made of hydrophobic, aliphatic organic fats. Hydro meaning water, phobic as in fear. It means that there are nonpolar compounds (think oil) that repel water. In fact, oil, which is made of fats is a nonpolar, hydrophobic compound. Succulents have ultrahydrophobicity or superhydrophobicity, so they can really get the water off them. Another factor that contributes to this ultrahydrophobicity is the farina nanocrystals. Nanocrystals are tiny structures on the surface of the leaf that manage to trap air underneath them. This keeps the water from ever touching the leaf, even when submerged! This self-cleaning effect also helps to keep pests off your succulent, making them happier and healthier.
How farina changes the color of your succulent's foliage
These farina crystals are tiny tiny structures that are nanometers big (sometimes even micrometers) that are both 2D and 3D. This affects how light interacts at the leaves, affecting the appearance of the leaves like color and shade. However, this isn't a phenomenon exclusive to succulents. It is a common biological adaptation referred to as "structural color." Structural color is present in peacock feathers and butterfly wings to give off a unique color-iridescent effect.
These tiny crystals can reflect or scatter light in a way that constructively or destructively interferes at different frequencies of light. You may have experienced how light can play tricks on our eyes when you look at a glass of water. Think how a straw in a glass appears differently in the water vs. in the air (more here). It's about the speed of light traveling through different media (water vs. air) and this is often referred to as the index of refraction. But let's just resume our conversation about color.
The human eye detects colors of light that are "visible light" and each color has a distinct wavelength (longer wavelength is red and shorter wavelengths are blue) on the electromagnetic spectrum. By changing the tiny little structures on the surface of the leaves, it affects how the light scatters, reflects, and interferes with other wavelengths. In other words, when the powder is off, the leaves appear dark green, powder on: light green.
So really...my Perle von Nurnberg was always that ugly red color...it was just hiding under a robe of farina powder that bent light into becoming the lovely pink it was. Good news: if you ever touch your plant or buy one with fingerprints all over it, the new leaves grow back with farina intact.
Farina on Opuntia
This article: Microscale Mechanism of Age Dependent Wetting Properties
of Prickly Pear Cacti (Opuntia) found here talks about the age-dependent properties of farina epicuticular wax relating to Opuntia cacti. (I'm more of an Echeveria person myself, but this is interesting!). Scientists found that older parts of the cactus allowed water to touch them, while younger parts were fully water repellent. They put the farina wax through NMR analysis (nuclear magnetic resonance aka what you may know as an MRI) and found that the chemical composition of the farina wax was different in newer and older cladodes (cladodes are flattened leaflike stems, or the paddles of the cactus). Scientists think that as the plant grows, their movement creates tiny cracks in the wax surface, allowing water to enter into the lower hydrophilic (water-loving) layers.
Parting Word
While unbroken farina is a much coveted feature of a quality succulent, broken farina can look quite beautiful too. Here's my Echeveria Morning Beauty that has some broken farina. It looks like a painter's canvas, one of a kind, and beautiful.
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References:
3. Nanostructure on Taro Leaves Resists Fouling by Colloids and Bacteria under Submerged ConditionsJianwei Ma, Yuekai Sun, Karla Gleichauf, Jun Lou, and Qilin Li
Langmuir 2011 27 (16), 10035-10040
DOI: 10.1021/la2010024
8. Hallam, ND (1967) An electron microscope study of the leaf waxes of the genus Eucalyptus L'Heritier, PhD thesis, University of Melbourne
9. Jeffree, CE, Baker, EA, and Holloway, PJ (1975) Ultrastructure and recrystallisation of plant epicuticular waxes. New Phytologist, 75, 539–549
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