Color is the spice of green
We expect green from plants, so it is no surprise that when bold colors make their appearance on a plant, the spectacle grabs our attention. For example, look at the thumbnail picture to the right, showing Alocasia macrorrhizos 'Lutea'. If this plant were all green, would it attract your eye as much as this one does? Chances are, it wouldn't, unless you are partial to large leaved aroids (as I am!). What I wish to share here, though, is the why of leaf colors, or what is most accurately called variegation
In my view, variegation consists of four main types, chimeric, structural. or anatomical, genetic, and viral. Here I'll focus on chimeric variegation, which includes whites, yellows, oranges, pinks, reds, and purples in splotchy or random patterns on the leaves.
Two in One
The word "chimera" (from which the word "chimeric" is derived) refers to a mythological beast that consisted of multiple animals combined into one monstrous creature. Chimeric variegation refers to a plant consisting of two genetically distinct types of cells, yielding random areas of coloration on an otherwise green plant. By contrast, non-chimeric plants have cells of identical genetic constitution. What is relevant here is that the different cells are the ones showing the coloration, while the normal cells are the ones that appear as green or as whatever uniform normal coloration the leaves have. Chimeric variegation can show as white, yellow, orange, pink, red, and variations or combinations of those colors.
For example, take a plant whose uniform normal leaves are reddish overall. This plant is not considered to be variegated just because it has reddish leaves. Next, imagine that this normally red-leaved plant is transformed into a white variegated chimera. What color are the light areas on the leaves going to be? If you thought "white", think again, because the presence of the red pigmentation will make all "white" zones appear pink! The reason for this is because white variegation is caused by the absence of three pigments; chlorophyll (green), carotenoids (orange) and xanthophylls (yellow). None of these three produce the red coloration in the leaves. Furthermore, the oil-soluble green, yellow and orange colors are found in the plastids, or photosynthetic organs, a different location than the water-soluble red, blue and purple colors, which are found in the vacuoles, or water-filled sacs within each cell. The differing gene yielding white or yellow variegation affects pigment content in the plastids, not the vacuole. Red chimeras do exist, but chlorophyll is not absent in these plants, even in the colored zones.
The Pattern is not Full!
Why is the coloration so often irregular on leaves like these? One reason has to do with the differing rates of cell division amongst the colored cells versus the green cells while the leaf is being formed. Nutrient availability (particularly phosphorus) and environmental factors exert strong influences on this, so the net effect is that no two leaves will look alike. This is also one reason why I don't consider chimeric variegation to be genetic, even though the difference in genetic constitution is what makes this variegation happen. What I consider to be true genetic variegation is coded for in every cell of the plant, not just certain cells, as is the case in chimeric variegation. Tissue taken from either variegated or non-variegated parts of a plant whose variegation is genetic will grow into plants that look just like the plant from which the tissue came. This is not true with chimeras; green cells taken will result in an all-green plant, while cells lacking in the three plastid pigments will yield a completely albino plant, unable to survive.
Another reason is that the actual patterns shown on particular leaves are governed by where in the growing point, or meristem, the chimeric cells are located. Chimeric cells may arise from one or more of three different layers which determine the constitution of the different layers of cells in each leaf. Using a yellow variegated chimera as an example, if areas of variegation coincide in all three layers, you have pure yellow on the leaf. If in either the upper, middle, or lower layer, you have different shades of lighter green or yellow-green appearing. Think of it like this: with three layers, you can have [g-g-g] (all green), [y-y-y] (all yellow), [g-y-g], [y-g-y], [y-y-g], [y-g-g], [g-y-y], and [g-g-y], all of which show as different shades of lighter green or yellow-green (see photo, left). Each of the mixed combinations will look different when viewing the leaf from the upper surface, leading to the distinctive appearance of these variegated plants. These varied combinations also result in a different pattern on the underside of the leaf than you see on the upper side. If you could see the inner layer of the leaf, the pattern there would differ from both the upper and lower surfaces! This shows that many, many possibilities can manifest, and no two of them will be the same.
How does a plant become a variegated chimeric?
Some reason that the cause is a benign viral infection that simply shuts down the gene for plastid pigment or vacuole pigment production. Evidence favoring this hypothesis, at least in my own observation, is that if you have a quantity of variegated plants in your garden or collection, the likelihood of seeing a new variegated form of one of your all-green plants springing up is higher. I've even seen variegated weeds spring up. This phenomenon argues for both the viral hypothesis and for the agent of transfer being some insect. Presumably, a sucking insect could take up the piece of DNA from one chimeric cell and transmit it in meristem cells on another non-variegated plant. The result would be a leaf or branch showing up unexpectedly variegated on an otherwise green plant. I have actually observed this myself. This hypothesis could be tested by using aphids or mealybugs in a trial, allowing them to feed on variegated leaf tissue, then transferring them to new green tip growth of another plant and seeing what the result turns out to be.
Another hypothesis is that of a random mutation occurring in meristematic cells, but this would not account for the higher probability of spontaneous variegated plants coming up when in the presence of other variegated plants. Whatever the cause, the result is very ornamental and quite sought after for our gardens.
Photo credit: LariAnn Garner, Aroidia Research.