old leaves as carbon sinks?

Mark E. Kubiske mkubiske at cfr.msstate.edu
Wed Nov 19 09:32:47 EST 1997

John,  these are some intriguing questions.

> 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.

Of course, this isn't EXACTLY right -- leaves don't instantaneously shift
from a net importer of C to a net exporter.  In Populus, leaves become
generally self-sustaining at about 1/2 full expansion at which time they
neither import nor export.  Photosynthate manufactured locally is used for
further leaf development.  The rate of CHO export then probably gradually
increases as the leaf approaches maturity.

>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

It seems to me that one needs to be very clear and consistent when talking
about source-sink relations:  source/sink of which resource?  If the issue
is leaf senescence induced by C imbalance, then clearly the pertinent issue
is source/sink relations of C.I also think its inappropriate to use the
term "source" in reference to the remobilization of nitrogenous compounds
from senescing leaves.  The reason is that, while many of these compounds
are synthesized in the leaf, the raw materials are not assimilated by the
leaf.  In this sense, a source could be narrowly defined as the
assimilatory organs.  (On the other hand, we often refer to roots as CHO
sources following dormancy, so maybe I'm full of ... dogma).

But I digress from your main point, that the energy for retranslocation of,
mostly nitrogenous compounds, must come from somewhere, and what is the
transducer that turns on the mobilization and retranslocation process?

>        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
>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

Ah, now I see what you meant previously by "normal" senescence.  Does a
deeply shaded leaf undergo the same translocation/abscission sequence as
when  a plant is entering dormancy or during leaf shedding in evergreens.
A simple test of N and non-structural CHO should at least indicate if they

These molecular mechanisms are not something that I've studied, so the
least I can do is embarass myself with speculation.  Perhaps the molecular
cue lies somewhere in sucrose synthesis.  We know that restriction of C
sinks results in feedback on photosynthetic capacity via inhibition of
sucrose synthesis (see Stitt 1991, PC&E 14:741-762).  Perhaps there is some
mechanism in which the reverse is also true:  that inhibition of
photosynthetic capacity triggers the retranslocation/senescence sequence,
assuming that it occurs in shaded leaves.

Dr. Mark E. Kubiske                     Phone: 601-325-3550
Department of Forestry            Fax:   601-325-8726
Box 9681                                   Email: mkubiske at cfr.msstate.edu
Mississippi State University
Mississippi State, MS  39762-9681

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