Variegation

By Ashely Gaden, Master Gardener Intern

Roses are red
Violets are blue
Plant leaves are green
And other colours, too?

Even from school age, we’re taught that plants photosynthesize sunlight to produce energy for themselves. Plants rely on the chlorophyll inside their leaves for this chemical reaction. The chlorophyll absorbs most of the light spectrum but reflects the green wavelengths, which is why we see leaves as green.

But some plants have foliage of two or more colours. Leaves can have spots, stripes, mottling, or venal or bordered leaf patterns. This phenomenon is called variegation. Contrasting colours are often a welcome addition in the home or outdoor spaces. Some variegated tropical plants are highly desirable and expensive, such as variegated Rhaphidophora tetrasperma or Monstera deliciosa ‘Albo-Variegata’, which can fetch prices in the three to five digits. There are many types of variegated plant varieties in our gardens from hostas and irises to dogwoods and lilacs.

Non-variegated and variegated peace lilies (Spathiphylum sp.) (photo by A. Gaden)

Fishbone prayer plant (Ctenanthe burle-marxii) (photo by A. Gaden)

Shady corner with variegated hostas, pulmonaria and lamium (photo by Lynne McCarthy)

Variegation occurs due to two main mechanisms. The first is related to pigment. Pigmental variegation may result from (1) natural regulation of pigments or pattern genes, (2) more than one genotype due to a mutation in the growing shoot tips, (3) pathogens (e.g., virus), and (4) artificial induction. The second main mechanism is related to leaf structure.

In natural variegation, colouration traits are built into the plant’s DNA and are reproducible by seed or vegetative propagation. In some zones of plant leaves, different cells can have other pigments such as carotenoids (yellow to red) and anthocyanins (red, blue, purple) in addition to chlorophyll, or little to no chlorophyll at all, which ends up looking white, yellow, or light green.

However, there are also instances in which random mutations in the apical shoot meristem (growing tips) produce cells incapable of producing chlorophyll, yielding zones of variegation. These variegated plants are known as chimeras because they have two or more genotypes in the shoot tips. Depending on which layer of cells in the apical meristem the mutation occurs, chimeral variegation is either stable or unstable. Because variegated plants have less chlorophyll, unstable variegated plants may revert to fully green foliage to help them survive. To maintain the variegated foliage, ensure the correct level of brightness needed for the plant variety (often bright direct light or bright indirect light), and prune emerging green shoots.

Examples of unstable chimeral variegation are the variegated Rhaphidophora tetrasperma and Monstera deliciosa ‘Albo-Variegata’, both species of the Araceae family.  Because there is no guarantee that propagation will yield variegated offspring, the plants are rare, which explains the high price tags often associated with them.


Random variegation in unidentified species of the Araceae family. Photo green plants on white background,
Karolina Grabowska, Pexels, cc license

Some plant viruses cause symptoms of variegation. A well known example is the hosta virus X, which causes hosta leaves to become irregularly mottled along the leaf veins and eventually results in stunted or disfigured leaves. The virus can remain in the sap of a plant for many years, slowly killing the leaf cells, without actually killing the plant. Once infected, a hosta will have the virus for life. Infected plants are still on the market, so inspect specimens for discolouration and mottling before purchasing.

Plants can also be artificially induced with chemicals to create temporary, unnaturally bright colours. An example of an artificially variegated plant is the Philodendron “Pink Congo”, which has bright, pink new leaves (check out this article by Avery Rowe (2021) for pictures and more information about this plant).

Moving on from pigmentation, the other main mechanism for variegation is leaf structure. Leaves with tiny pockets of air under their epidermis (skin) reflect some of the incoming light. The resulting lighter-coloured areas are referred to as ‘bubble’ or ‘reflective’ variegation, and they are only present on the adaxial (top) leaf surface. Test this out for yourself by examining the top and bottom leaf surfaces of a variegated begonia, lungwort, or Philodendron. Structural variegation is also the result of light reflection from plant hairs or differences in epidermal cell shape or thickness.

Reflective variegation in Begonia rex (photo credit: Begonia ‘Escargot’, the justified sinner, flickr, CC BY-NC-SA

Why does variegation exist? Although the scientific jury is out, some studies have suggested variegation is a defensive mechanism by making plants unattractive to herbivores and leaf-egg laying insects, and providing some photoprotection. Regardless of why variegation occurs in nature, as long as humans have a fondness for this colouration trait, variegated plants will continue to thrive in households and gardens.

References
Geneve et al. https://propg.ifas.ufl.edu/03-genetic-selection/04-genetic-chimera.html

Newton. 2018. Hosta Virus X. The 2018 Prairie Garden. Sable, L. (Ed). pp. 48-50.

Pao et al. 2020.  Click Here

Porter. 2021. https://gardenprofessors.com/houseplant-hubub-the-rage-about-variegation/

Sheue et al. 2012.  Click Here

The Royal Horticultural Society https://www.rhs.org.uk/problems/reversion

Xu. 2007. https://www.extension.iastate.edu/news/2007/feb/071502.htm

Published:  March 2023