It won't matter how much Oxygen you give these stems..
Here's my theory:
From the plant's perspective, mature leaf is a finished job and should contribute to the well-being of the plant. It should generate and provide:
1. sugars, from photosynthesis,
2. oxygen (the lower/older the leaves, the more they contribute to oxygen flow to the roots, while upper leaves to the apex "under construction"), and
3. nutrients.
A mature leaf should never be a liability to the plant as a whole, always an asset, or (!) neutral. At the same time, a mature leaf needs some amount of above mentioned 'goods' for itself - for routine respiration and for repairs.
A leaf is in negative numbers when it requires the plant to supply sugars, oxygen, or nutrients. Of these, demand for nutrients is small, the leaf is already build-up and not much nutrients are required for repairs. As for sugars, I think (and observed accordingly) that even shaded leaves do perform some photosynthesis, and supposedly create enough sugars for the leaf's needs (if not seriously damaged or in some trouble). But oxygen, oxygen is needed all the time, respiration runs permanently and cannot be delayed, and there is very limited storage capacity for oxygen within a leaf.
That is why I believe that a leaf is in the greatest danger of falling into red numbers in connection with oxygen. If its oxygen balance is negative, the plant sheds it.
I do not insist that my theory is flawless and complete. And I admit that observation often does not look like in accordance with it. But there are many troubles which may happen, and I think at the core of such troubles is oxygen, because increased respiration results from all kinds of stress.
photos.app.goo.gl
