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Chlorophyll and Sunlight

Dear List,

I am always tempted to move certain of my plants outdoors for the
summer. My wife refers to this as having my plants go to summer camp.
Here in the North (Michigan) many plants weaken if they live for years
on a window ledge. My typical thinking is that a short season bathed
in sunlight in a natural setting will re-invigorate a plant. This
theory is sort of correct. The plants do derive a dose of vigor once
they overcome the shock of the change.

The problem comes with the transition from one to another condition,
especially going from indoors to out. I see this occasionally
described as “sun scald”, which entails a kind of sunburn, perhaps
coupled with desiccation. What this means in practice is a loss in
appearance, often including the functional killing of the existing
foliage. While this is usually overcome by the growth of new, tougher
leaves, there is no doubt that sun scald is a setback.

Some experimentation will show that sun scald is more than simple
overexposure to intense sun. It will also happen if the plants are set
out in shady areas, even in what is known as “deep” shade. One can
also mitigate water loss as a cause by sheltering and application of
frequent mist. Scalding still occurs.

What is it about the outdoors that causes sun scald?

Here is an example of one of the most severe cases. For the past few
years I have rejuvenated my Pistia (aroids) with a summer vacation
every year. By September, helped by additions of fertilizer, my Pistia
are big and bountiful, harboring an abundance of small flowers, which
indicate horticultural contentment. Over the winter the refreshed
individuals regress anew and are ready for summer camp once again.

When the plants are moved outside, they turn white within a day or two
of the new harshness. This is sun scald with a vengeance. To be sure,
tiny kernels of life remain at the plants’ centers and new growth
starts at once. But the bulk of the leaves have not only been scalded,
they have been bleached white. Within a week the former green leaves
have been converted to a minor remnant of white mush. The loss of
color indicates to me that the chlorophyll has been destroyed. It is
always surprising that the plants are not killed and that they are
bigger and more vigorous than ever within a month.

I have heard the theory that summer leaves grown outdoors have a
tougher “cuticle”, referring to a transparent layer that shields the
inner goodies from harshness. I have also heard the theory that the
leaves arising in outdoor conditions have fewer stomata on the leaf
undersides. It seems there are hardened leaves produced outside and
“hothouse” leaves produced under protected conditions, presumably
allowing readier access to what little light there is indoors compared
with what is received from nature directly.

But is also seems as though the hardened leaves also have a degree of
screening power against ultraviolet (UV) rays. I assume here that UV
is what attacks the chlorophyll.

Do we have any authorities on these maters tuning into the list that
could comment and clear up these matters for me? I hold out hope that
there are one or two.

Best regards,

Ted Held
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