Re: [Aroid-l] Blanching Query

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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.

 

Ron McHatton

Central Florida

Zone 9