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  Re: [Aroid-l] Blanching Query
From: Ronmchatton at aol.com on 2006.06.29 at 19:31:37(14376)
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

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.

Ron McHatton

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