repeatedly, observations of different species of plants that have white leaves turn up in Inat. Almost as frequently, the question about the cause is asked. Does somebody has an explanation for this phenomenon? Is it some genetic defect that inhibits chlorophyll formation?
It is called variegation, and it can have different causes. White tissue has no chlorophyll, whereas abnormal shades of green usually means there’s multiple layers of tissue, some of which have chlorophyll and some of which don’t. Other colors like pink or yellow are caused by the presence of different pigments that chlorophyll would normally be masking.
So what causes it? Unfortunately my college plant morphology class, despite having an entire project centered around variegated leaves, was much more focused on cell division and the layers revealed, and not the underlying cause. So uh, “it can have different causes” is all I know really.
Cool thing is, a plant showing stable variegation can produce 3 kinds of seedlings: white, green, and variegated, depending on whether the seed cells had normal chlorophyll, no chlorophyll,
or some cells of each. Chlorophyll is only inherited from the seed parent (not the pollen parent) because the genetic code for it is contained in the chloroplast organelles, and pollen cells don’t have those organelles.
Variegation like this can be caused by a number of factors. Usually it’s caused by an infection (e.g. a mosaic virus) that results in chlorosis. Sometimes it’s chimeric (different genotypes within the same plant; some produce no chlorophyll). Some plants are completely unable to produce chlorophyll, but usually they don’t survive unless they are supported by other plants. For example, “albino redwoods” produce no chlorophyll but they can survive because they can take sugars from the fungal network. If the deficient genome of an albino redwood is combined with a redwood that can produce chlorophyll, the resultant chimera may look variegated.
Chimeric variegated redwood:
Thank you, both, for the very helpful answers! In most of the cases linked above, it is only a single plant within a local population where all other plants are normal. This single plant has only around half of its leaves partly or completely white, while other leaves are totally normal. Example: https://www.inaturalist.org/observations/49742220
So, do I understand correctly that this particular case could be either a virus infecting some of the leaves (but not the whole plant), or a genetic mutation? I doubt that there was an albino parent plant in these cases.
I am confident that every observation linked in this thread shows variegation, and in particular, variegation caused by a mutation that interrupts the process of chlorophyll synthesis. In these cases there is no reason to suspect the influence of an infection.
With these examples, it doesn’t look like a virus is causing the variegation because the chlorosis is so distinct and uniform, whereas viral variegation usually causes splotchy chlorosis that looks more like yellow or light green freckles rather than a section of solid white. It is possible for a plant to develop genetic chimerism due to a mutation or misdivision, even if there are no possible parent plants.
I don’t know if it would even be possible for an “albino” parent plant to produce viable chimeric offspring. Usually they don’t survive very long; plants have chlorophyll for a reason, so without chlorophyll they need to get energy from somewhere else. Even if they did survive because of support from mycorrhizae and other plants, they may not be able to reproduce. I know that, in the case of albino redwoods, most can’t even produce cones, but I don’t know how this could affect angiosperms. Chromosomal changes often prevent reproduction entirely.
I should add that the seedlings I mentioned are of cultivated plants, so humans are providing them extra care. Certainly in the wild this is a weakness evolution would select against, but humans think it is pretty and like to keep it going.
For for seedlings completely without chlorophyll, they starve to death if not grafted onto a green plant host. If you have ever seen those bizarre “moon cactus” where the top is bright pink or orange or yellow, that is what was done–the colorful part, a seedling (or cutting) without chlorophyll, was grafted onto the green part. For other cultivated plants with both white and green tissue, they can survive, although they grow more slowly, and propegation by cuttings is most reliable to make sure the clones look just like the original. Even so they sometimes spontaneously revert to natural all green; if they can generate a normal bud then the normal branch growing from it has competitive advantage, grows faster and comes to dominate the organism. Gardeners are instructed to cut off solid green branches to prevent that from happening.
As far as I understand it, it’s generally a genetic defect. Many cultivated plant varieties are specifically bred for partially white leaves, because it makes interesting-looking garden specimens. It definitely makes them a bit weaker, but in cared-for garden setting it’s not really an issue.
You’ll find completely albino plants surprisingly often, usually if you look at seedlings of large-seeded plant species. The small seeded species use up all the energy in the seed before they get big enough to get noticed, and then die. But things like California Buckeye, for example, can get 6-8 inches tall before their seed is depleted and they die, so they’re more likely to be noticed.