Ornamental plants for the aquarium and pond are beloved by many, but you may be curious about the beautiful patterns of white and green on some plants that are normally all green.
This phenomenon is an example of variegation. Whenever a part of a plant has more colors than it ordinarily should, we call it variegated.
Typically, this appears as differently-colored zones in a section of the plant and is most common in leaves or fruits. Flowers are commonly excluded from the definition of ‘variegation.’ Here we’ll mainly focus on foliage.
There are three main types of zoned discolorations that you’re likely to encounter, so understanding each of them will help you get the best results out of your plants.
Rather than trying to classify all possible patterns that can occur on leaves, we’ll instead focus on some distinct ones with clear causes and then mention the underlying processes for these discolorations in general.
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Naturally variegated plants and mutants
Some plants naturally grow pretty patterns on their leaves.
Take, for example, Nymphaea lotus, the tiger lotus: its underwater leaves feature black spots. Or look at the V-shaped leaf markings on red clover (Trifolium pratense).
These aberrations appear because the plants either mask the leaves’ green color with naturally-produced pigments or don’t produce green chlorophyll in certain regions of their structure.
Besides unaltered wild plants making patterns, mutants can occasionally occur.
A color mutation happens in regions of the genome that regulate pigment production for a given structure in the plant.
For example, the wild Shinnersia rivularis is an all-green plant. However, the cultivar Shinnersia rivularis ‘White-green’ has a mutation that inhibits chlorophyll production near the veins.
Since the scope of potential mutations is nearly limitless, “naturally variegated” is a way to categorize any variegation with an unclear cause.
On the upside, they’re fairly easy to propagate. You can reproduce species that are variegated in the wild any way you like.
Special mutants may inherit traits in specific ways, but you can reproduce them by the seed as well.
Chimeras
A ‘chimera’ was a beast from Greek mythology – a lion which has a goat protruding from its back and a snake instead of a tail. Obviously, lions, goats, and snakes are all very different types of animals, yet in the myth, they’re combined into one organism.
Unlike with animals, making plant chimeras is not too hard.
For example, an apple twig grafted onto a pear-tree is considered a chimera, as there are tissues from different species functioning as one organism.
On a less extreme example, chimeras can also occur between members of the same species, the common example being a normal green plant and a white mutant of the same species.
Plants, however, have an interesting property: they grow continuously, and the shoot tip generates the leaf and the stem.
So, what would happen if a part of one shoot tip were to be swapped out with the shoot tip of something else? What pattern would the plant generate?
As it turns out, partially replacing shoot tips is not too difficult with a little dedication or luck, and you’ll find many such chimeras in your aquarium plant shop.
But first, we should examine the shoot tip a bit closer.
The shoot tip
The portion of the shoot tip that’s actually made up of blank cells that can become the plant’s various structures is called the meristem.
Here we will focus specifically on the shoot apical meristems, or shoot tips, of dicots (also known as broad-leafed plants).
Other plants may differ in the details and between different orders, families, or genera, but the broad principles hold true. The shoot apical meristem is a small dome on top of the stem from which the leaves emerge.
This little dome is made up of layers of cells that do not grow into each other.
In the top two layers, the cells only divide to the sides. These two layers (L1 and L2) are called tunica.
In the third layer (L3) the cells divide in all directions, creating a region called the corpus.
In dicots – broad-leaf plants – the first layer (L1) of the tunica makes the clear epidermis that coats the whole plant above ground (a.k.a. the skin layer).
Layer two (L2) produces the leaf margin and flower parts as well as the outer portion (excluding the epidermis made by L1) of the stem. The third layer (L3) produces the center of the leaf as well as the center of the stem.
Beyond the dicots it gets stranger: monocots, such as grasses, often have three layers in their meristem, but examples with two or just one layer are not unheard of. In those that have three, L1 also makes up the leaf margin while L2 produces a strip between the margin and the center of the leaves.
Submersed growth is a special case: the epidermis is normally a clear layer that prevents the water inside the plants from leaking out or evaporating.
Underwater, however, there is no risk of drying out, so the underwater portions of aquatic and semi-aquatic plants have no epidermis. Instead, they feature a less remarkable layer of cells in its place.
Thus, the construction of the leaves also changes. Layer two no longer produces the leaf margins, but instead a flat sheet of cells forms across the whole leaf.
Mericlinal chimeras
We call a chimera mericlinal when a portion of a layer is substituted.
A mericlinal chimera develops naturally when a mutation starts near the sides of a meristem layer: as the cells divide the mutation will take up more of the layer but will not extend through the whole layer.
This arrangement is not very stable; the mutation may eventually be expelled as the remaining normal cells push the mutated ones further and further out.
If you’re lucky, the meristem may produce a lateral bud (a second bud from the same meristem) that contains a more stable arrangement. This can happen because it does not take the whole meristem to make the lateral bud, but only a sector.
There are three types of mericlinal chimeras based on which layer (L1, L2, or L3) is substituted with mutant cells.
Once the plant is fully grown, we’ll see a portion that displays the mutated trait. This portion, naturally, is the section that grew from the mutated meristem.
The diagram shows an example of a mericlinal chimera where layer two is partially substituted. Thus, we would expect to see the different traits only on one leaf edge.
Since this type isn’t entirely stable, they’re generally not sold in stores. But if you have the fortune of finding a chimera of your own, chances are good that it is a mericlinal chimera.
Periclinal chimera
The periclinal chimera is the most stable type. In this case a whole layer is replaced. This type can also happen naturally if you are lucky. If the mutation occurs right in the center of a layer, it will comprise more and more of the layer as the cells divide, and may eventually push the “normal” cells away. This type is commonly found in stores in the form of white-green cultivars. Typically, either layer two or layer three has been replaced by a mutation that doesn’t produce chlorophyll, so it is white.
Acorus gramineus ‘variegatus’ is an example of a periclinal chimera among pond plants, and shows white tissue on the edges of its leaves. In the aquarium hobby there are typically only two periclinal chimeras available: Ludwigia inclinata var. verticillata ‘White’ and Hygrophila lancea ‘Chai.’ Both are cases of layer two being substituted, and their over-the-waterline forms clearly show white leaf edges. Underwater that growth transitions into vibrant white-green and pink-green growth as the tissue there forms more than just the leaf edge.
As with the mericlinal case, there are three types of periclinal chimeras in plants with three layers. Since layer one only produces the transparent epidermis, color changes may be difficult to find on a mutated L1. But if, for example, you have a blackberry plant with an epidermis that features prickles (that is, sharp points sprouting from the epidermis) you may be able to find a mutation that inhibits prickles in L1. Mutations to layers two or three work in a similar way as they do in mericlinal cases.
Sectorial chimera
Sectorial chimeras are mutants in which the same region in every layer is replaced. These types are not stable because, as was the case in the mericlinal chimera, the mutated section may or may not grow out. Since the layers don’t frequently mix on their own, this type of chimera rarely forms in normal shoot-tip growth. However, they can form from wounded tissue or be intentionally created by cutting two plants along the stem and grafting them together.
One may also find sectorial chimeras in plant organs that arise from a single cell if a mutation happened early during division. That can occur, for example, with fruits or flowers. As such, you’ll find that the replaced tissue seems as an easily-divisible fraction – it may replace exactly half (½) or a fourth (¼) or an eighth (1/8), etc.
Since these are not stable, you generally don’t find them sold intentionally. If you look long enough, though, you may find a lucky example in a fruit or flower display simply due to the sheer numbers.
Marbled chimera
Here is the second stable type: a wild, random mix between the normal plant cells and the substitution. To push out of the pattern, one would need to un-mix the mixed pattern, which is very unlikely to happen on its own. But if the substituted tissue is one that divides slowly, the ratio of normal to substituted cells may gradually shift towards the healthier tissue.
Here it should be noted that the meristem itself is generally shadowed by leaves, so will stay in relative darkness. It gets its nutrients from the existing mature leaves, so the amount of resources it produces on its own pales in comparison to the amount it receives from elsewhere in the plant. Thus, a white mutation that doesn’t produce chlorophyll on its own divides at the same speed as a healthy region and doesn’t get outcompeted during this growth.
Since the patterning is random, there are effectively infinite types or marbled chimeras. Further, the ratio between substituted and normal tissue can fall anywhere on the continuum from 0% to 100%. Finally, the size of the replacements can also vary – the substitutions can range from individual cells to much larger sectors. The marbled type is also called the non-patterened sectorial chimera. There are a few marbled chimeras found in the aquarium hobby: Anubias barteri var. nana ‘Pinto’ is a marbled chimera between the normal A. barteri var. nana and a chlorophyll-less mutation thereof.
Deficiencies and diseases
Since our definition of ‘variegation’ generally includes all kinds if discolorations within the same part, we also have to talk about deficiencies and diseases. Much like animals, plants can become ill. We can roughly classify the issues that cause discoloration into two categories: those that arise from wrong care and those from external sources.
Deficiencies
Plants need nutrients. Plant care does not only mean watering, but also maintaining a soil with the right amount of nutrients and occasionally fertilizing, even if that only includes traces in the water you use to water your plants. Plant growth can become unexpectedly slow, and in much the same way a human can starve, plants can become deficient. In plants, the lack of a certain nutrient generally manifests in a well-understood response, although the full extent of every response is beyond the scope of this article.
Chlorosis. The word ‘chlorosis’ generally describes plant tissue that is brighter than it should be. You’ll most often see deficiency-related chlorosis as interveinal chlorosis – that is, only the regions of the leaf between the veins gets paler. There is no type of chimera that only produces paler webbing between the leaf veins, so if you see a plant with dark or normally-colored veins and unhealthy, pale webbing, it may be a deficiency.
Nitrogen deficiency, magnesium deficiency, or a deficiency of the trace elements iron or zinc are possible candidates for a deficient, chlorotic plant. Nitrogen deficiency may be a result of sandy or nutrient-poor soil. Magnesium is generally part of tap and/or ground water, so a deficiency is fairly rare. Iron and zinc are trace elements and if you suspect a deficiency, a fertilizer with trace elements will help.
Diseases
The history of plant diseases is as long as the history of plants themselves, and plant pathology is an immensely complex field. Diseases in plants differ from ornamental variegation mainly in its consistency; if the whole plant has a consistent pattern, it may not be a disease.
There is a notable exception, however, in the so-called mosaic viruses: These are viral infections that produce random, marble-like patterns that are not unlike those of sectorial chimeras with larger sectors. However, a chimera is made entirely of cells that distinctly differ from one another, so you won’t see blurred edges in a chimera. Mosaic viruses do produce blurred edges and often deformities as well, so a close look can help you differentiate them.
How to Care for Your Variegated Plant
Naturally-variegated plants only exist in the variegated form, so you care for them as you would any other. If your plant is deficient or diseased, you must treat the plant so it can return to health.
Most of the variegated cultivars you’ll find in your local stores are chimeras. Periclinal chimeras with the second layer replaced by a white mutation without chlorophyll are the most common type. If the plant in question has brown or red leaves rather than green, the mutation may not affect the non-green pigments and the ‘white’ tissue may come out pink. The white/pink tissue is dead weight for the plant – it produces no sugars due to the lack of chlorophyll for photosynthesis, but still requires resources to stay alive. An all-white plant cannot live on its own. Thus, chimeral variegated plants are generally slower growing and more fickle than their non-variegated counterparts.
Chimeras made through grafting can break apart into the plants they are made from. Occasionally a stray cell does manage to jump layers in the shoot tip even in periclinal chimeras and may divide there, resulting in a fully green or fully white shoot emerging on the plant. Since these shoot tips are entirely made from one type of tissue, they won’t return back into a chimera and will need to pruned away. (Note, this of course ignores the tiny possibility of another random chimera by luck appearing in exactly that stray shoot).
Chimeras cannot be reproduced by seed. In dicots (broad-leafed plants), the second layer of the shoot tip produces the organs for sexual reproduction, so all seeds inherit the genetic traits of only the second layer. If you have a periclinal chimera with white tissue in the second layer, this gives you the option to explore how the white trait is inherited. To actually reproduce the variegated plant, you need to clone it while conserving the structure of the shoot tip. The easiest way to do this is by taking shoot cuttings, but you can also graft a lateral bud onto another plant so it may sprout there. Root cuttings or leaf cuttings do not contain a shoot tip, so they won’t grow an identical chimera. That said, leaves do contain both tissues, so a newly-emerging shoot from a leaf cutting may be a different type of chimera if you’re lucky.
How to Make a Variegated Plant
The long and short of creating variegation is that you have to get lucky. This luck generally takes one of two forms. One is that you have the incredible fortune to find a mutation that generates a pattern, such as turning off chlorophyll development in veins. Systematically searching for such a specific pretty mutation is not very reasonable for a hobbyist. Chimeras are a bit easier, but you still need a good amount of luck. For white tissue to occur, something has to go wrong at a genetic level with the chloroplasts, the cell organelles that contain chlorophyll. For example, during cell division a cell may not get chloroplasts and so you end up with a white cell. Or inside the chloroplast itself, something may go wrong such that you get white ones, possibly to the extent that the entire cell is white, and contains only defective chloroplasts.
Either way, ‘something went wrong with the leaves’ green’ is much more likely to happen than a single specific attractive mutation. You also have to hope that when things went wrong, they went wrong inside the shoot tip and not in a part of an existing stem or leaf. If you happen to have a few hundred plants of a species prone to mutations, you may actually find a chimera if you specifically look for a one. It takes a keen eye and a lot of patience but keeping this thought in the back of your mind may be enough to find something now and then.
Once you have found your starter, you need to stabilize it. You can try to grow shoots from buds where you think the plant may have produced variegated lateral buds. You can try to reproduce the plant from a leaf cutting of that variegated leaf. There are also other options if you have skill in tissue culture: you can graft a plant lengthways to make a sectorial chimera and then select a periclinal from buds, or graft wound tissue/callus together and regenerate shoots near the graft section, with the hope they result in chimeras.
Summary and Remarks
To summarize, there are three types of variegated plants: naturally variegated plants, chimeras, and diseased/nutrient-deficient plants.
Chimeras are effectively two plants in one organism. Grafts are good examples, but some are more clever: the section replaced is actually a part of the shoot tip, the section that grows to become the whole plant.
The shoot tip has layers that don’t mix. Layer one makes the clear skin, layer two the leaf edge, and layer three the leaf center.
Some chimeras replace a part of a single layer (not stable), a whole layer (stable), the same region of all layers (not stable), or random bits throughout (stable).
Trim away non-variegated parts of variegated plants. If you’re lucky and select well, you too can make a variegated cultivar.
Further Reading
- More pictures for the mericlinal, periclinal, and sectorial chimeras:
Plant Propagation glossary by Hudson et al.: http://irrecenvhort.ifas.ufl.edu/plant-prop-glossary/03-genetic-selection/04-genetic-chimera.html
- An article with various sectorial chimeras in fruits and flowers:
‘Mutation examples and how they happen’ by Ronald Goldy: https://www.canr.msu.edu/news/mutation-examples-and-how-they-happen
- An in-depth-review of plant chimeras:
Margaret H. Frank and Daniel H. Chitwood: ‘Plant chimeras: The good, the bad, and the ‘Bizzaria’ ‘ in Developmental Biology 419 (2016) page 41– 53
Sources
Image: Diagram of the SAM: Wikimedia, https://en.wikipedia.org/wiki/Meristem#/media/File:M%C3%A9rist%C3%A8me_couches.png by dakdada
Image: Chimera types: Edits of above.