Re: Chlorophyll and Sunlight

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Greetings, Ted!

 

Your plants want bright indirect light. They sunburn, just like we do. I would keep them in the shade, but with some sort of semireflective surface nearby that reflects sunlight to them without the direct burning rays touching their fair skin. I do this with many of my hardier plants.

 

Happy days,

 

D. Christopher Rogers

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-----Original Message-----
From: aroid-l-bounces@gizmoworks.com [mailto:aroid-l-bounces@gizmoworks.com] On Behalf Of Theodore Held
Sent: Tuesday, June 16, 2009 11:42 AM
To: aroid-l@gizmoworks.com
Subject: [Aroid-l] 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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