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  Blanching Query
From: ted.held at us.henkel.com on 2006.06.29 at 09:54:58(14372)
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.

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From: Ronmchatton at aol.com on 2006.06.29 at 12:31:37(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
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

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From: Ken Mosher ken at spatulacity.com> on 2006.06.29 at 21:30:07(14378)
Ted,

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...

-Ken

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From: Steve Marak samarak at gizmoworks.com> on 2006.06.29 at 23:50:08(14379)
I'm pretty sure I've read that one of the fascinating adaptations of plants is
the repositioning of chloroplasts within the cells to adjust to different light
conditions.

I seem to remember that this works both ways - in low light conditions, they
will be positioned to maximize light absorption, and in in very high light
conditions, the sort that can cause leaf damage (in the same way that it can
damage human skin cells), they're arranged to reduce absorption.

What I haven't seen is anything specifically connecting this up with the
adaptation process Ken mentions, but I've always thought it made sense both
ways. If you adjust the light levels gradually, there's time for such a
mechanism to work; if the change is too abrupt, it may be more economical for
the plant to cut its losses, ditch the poorly adapted/damaged leaf, and produce
another which is better suited for that light level. But that's me speculating.

I read this long ago, before there was much of a web. Probably there's lots of
information on it out there now (if I'm more or less on track, that is).

Steve

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From: "Agoston Janos" agoston.janos at citromail.hu> on 2006.07.01 at 11:09:42(14381)
Dear All,

We laerned, that some of the herbicides are destructing the
pigments or stop their biosinthesis.

Some of them are destructing the chlorophylls (ABC), but some
of them destructing carotionoids as Carotine, Xantophyl. The carotionoids main
profit is that they are collecting the harmful or too much sunlight (foton) so
the chlorophylls can work as they should.

If you spray plants with carotinoid destructing herbicides in
one or two days they seem to be sunburned, and they die in it, becouse the
harmful waves (e.g. UV) destroy the chlorophylls. So if your plant is sunburnt
(and not sprayed with herbicide by one of your "favourite" neighbour) the
carotinoids are not sufficient in the leaves and stems, than the
chlorophyll is destructed too.

Bye,

Jani

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