This is, in effect, sunburn. Leaves produced inside are much larger to maximize the amount of light that they can absorb. When taken outside the dramatically increased light levels simply destroy the chlorophyll faster than it can be produced hence the blanching. In the worst case, the leave tissue overheats, dies and blackens resulting in necrosis. While a bright window inside the home may look very bright it's really very dim in comparison to outside light levels. For instance, the typical office environment is kept at about 35-50 footcandles and light levels above that are quite glaring on white paper. At about noon in mid-June with no cloud cover, natural sunlight is about 11,800 footcandles give or take a bit depending on latitude. Every plant species has a natural level of light that it will take. For instance, Phalaenopsis plants are completely burned to a crisp at levels of about 3000 footcandles and even lower if the transition is fast enough. Cymbidiums however, easily adapt to 4500 footcandles without damage.
The opposite phenomenon occurs coming inside but the old leaves don't die because the lower light levels inside don't result in chlorophyll bleaching or over exposure. The plants are slowed down simply because they can no longer get as much light as they did outside with the existing leave surface. Eventually new leaves are produced but they are much larger. Think of leaves as solar collectors. Solar flux available is determined by surface area and light level. Higher light levels....smaller surface area and vice versa.
The crowd I hang around with has always called this "sunburn." Moving a plant outdoors into the sun does not have to result in leaf drop (or permanent scarring or death in the case of cacti and other stem succulents) if you introduce them to ever increasing amounts of sun over a period of weeks.
Start with dappled shade, after a week or two more sun, etc. Soon enough you can have them in full sun with no ill effects. Sorry I can't give you the scientific reason why...
Dear aroid horticulturists,
Here I am again being curious.
What is happening when an indoor plant is moved outside and experiences massive blanching? This seems pretty universal for plants, not just for aroids. If the weather is clement, new leaves will eventually appear that are hearty enough to be fully green and lush without any protection. Sometimes those leaves are smaller or more intensely green than the indoor versions. But it means the plant is capable of receiving a full dose of weather and sunlight. It is as though leaves with different endurance characteristics are produced to match a given climate situation. It does not even help if the transition is made very gradually.
I have heard before that this involves a cuticle layer that either inhibits desiccation or not depending on whether or not it is present. But the blanching I refer to does not seem to involve desiccation - except if it is so severe that the leaf dies and the desiccation is associated with necrosis. Plants kept moist and in humid conditions will still blanch.
The reverse is also true - sort of. Plants with "outside" leaves are stalled when brought indoors. Most of the time the outside leaves stay, but when new ones come on they are now of the weaker "indoor" variety. Soon enough the outside leaves fall off, apparently ill-equipped to function in the new climate.
It is not always the case that outdoor leaves die when brought inside. But it is almost always the case for the reverse.
Do we have different chlorophyll types, or internal shading or illuminating structures?
I know somebody knows the answers.
Thanks for indulging me.