I think looking over some of other postings here can give you an idea of the
"dogma" I was referring to. Referring to it as dogma is not to suggest that it is all
baloney. But I am wondering about mechanism and I'm wondering if there ever any
exceptions. The way I understand it--from the textbooks--is that a young enlarging
leaf takes up C from the plant via the phloem until it achieves a net positive C
balance (more photosynthesis than respiration). Now this leaf becomes a net exporter
of photosynthetically derived carbohydrate. And when it begins to senesce it continues
to function as a source but now it is exporting amino acids that are the product of
regulated leaf proteolysis. This basic idea makes sense but I can imagine conditions
where it might not work that way. Three examples are outlined here: (1) Sudden
overshading of a mature leaf, (2) Seasonal shifts in physiology in a long-lived leaf,
and (3) insect or microbe mediated gall formation on a mature leaf.
Imagine a leaf becomes heavily shaded. It may be that the leaf "knows" this
and is able to begin digesting itself in a regulated way that provides energy for the
leaf to undergo the normal senescence routine and export resources to other parts of
the plant. Or it may be that the sudden drop in light results in autoconsumption of
the leaf (starvation?) and there is no energy available for exporting amino acids and
other leaf resources. Perhaps both of these happen, depending upon many things
including the developmental stage of the leaf when it becomes shaded. But if the leaf
"knows" it has become shaded and engages the normal senescence program how does it
know? And if it undergoes "unregulated starvation" is there an anatomical or
biochemical or molecular explanantion as to why this shaded leaf can't take up C from
the phloem stream? And if this happened would it keep the leaf alive long enough for
it to export some of the other goodies?
And what happens in leaves of evergreen species in seasonal climates? This is
one Anna Schoettle may be able to help with. Photosynthetic capacity in needles of
Loblolly pine drops in the winter and, depending upon the exposure and the severity of
the winter, the needles will lose chlorophyll too. I assume they will also lose
soluble protein too. But the following spring the photosynthetic capacity and
chlorophyll (and protein?) levels in these same old needles increase again. Is this
all xylem? Or do these old needles become phloem sinks again?
Likewise when a gall forms on a fully developed oak leaf does that shift the
sink/source balance so that the leaf becomes a net sink (some galls can be pretty big
and they can grow quickly)? If so, is this a passive response or is the insect/microbe
manipulating hormone levels to convert the leaf back to a net sink?
And this is taken from a letter Doug sent to me which speaks to the same issue...
"We are always given such a simple view of phloem movement in courses, but it must be
more complex. Maybe the leaf is simply one way or the other. But I have seen cites
about transport in two different directions within a stem. These strands could be
quite isolated from each other however. What I find so interesting about this whole
idea though is that with a source only leaf, the only way the rest of the plant could
communicate with that leaf is through the xylem stream from the roots, so any shoot
stress that needs to be communicated will have to be transmitted through the roots.
You know the other issue that everyone takes for granted is the root shoot interface.
A lot of phloem mixing is postulated to take place here and there should be some
bidirectional transport I would think."