Leaf form and function

Una Smith smith-una at yale.edu
Mon Nov 2 11:24:33 EST 1992

Well, the thread has become pretty tangled, so I'll just cut through to
the (for me) more interesting bits.  If I've scrambled the attributions,
sorry, but the posted summary was a confusing mess!

Tony Travis (?):

>>>I've certainly never run across a shade leaf with a negative 24-hour
>>>net photosynthetic rate, and I would imagine that such a leaf would
>>>be quickly shed. [...] How does a reduced net assimilation rate in a
>>>shade leaf initiate abscission?

Bill (?):

>>Now, that is a VERY interesting question.  Frankly, it had never
>>occurred to me;  clearly the plant does a good job allocating its
>>resources (and de-allocating them), but exactly how this particular
>>feat is accomplished would be an extremely interesting story.

This issue has bothered me a great deal for several years now.  I 
have seen some research from La Selva in Costa Rica and Barro Colorado
Island in Panama showing that much of the (expensive) defense chemicals
produced by trees to ward off herbivores are transported out of leaves
before they are abscised, leaving behind the less expensive and less
transportable lignins.

Even more bizzare, I have seen some trees which cleanly abscise
entire branches, leaving a cone-shaped scar in the remaining branch,
and a matching cone-shaped stub on the fallen branch.  This shows 
with absolute clarity the distinction between cell lines derived from
lateral vs (more) terminal meristems.

>>>My point was that there is no advantage to optimising the water use
>>>efficiency of leaves that are contributing little photosynthetically
>>>(eg. older leaves lower down in the canopy receiving low PAR) because
>>>the cost in transpiration would be better utilised elsewhere in the
>>>photosynthetically active parts of the canopy.

>>Sure.  Before the leaf is kicked off, presumably its stomata would be
>>closed most or all of the time.

Low photosynthetic activity is not a good argument for the failure to
conserve water.  In an economic sense, once the leaf productivity falls
below some cost/benefit threshold, it makes sense to cut your losses
and discard the leaf.

The stomata of old (and by extrapolation "shaded") leaves may be closed
most of the time simply because either low photosynthetic rates and/or
low respiration rates and/or low ambient temperatures don't require high
rates of transpiration.

Tony, replying to Bill:

>This clearly does happen as the canopy develops.  Older sun leaves
>senesce and drop off as the upper canopy occludes them with new leaves.

Do old sun leaves drop off because they are old, or because they
have become shaded?  In temperate deciduous forests this is rather
difficult to tease apart, but in the tropics, where leaves are not
necessarily dropped each year, leaves can live for a very long time.
Phyllis Coley has been watching some marked leaves for over a decade
now.  I would expect a sun leaf that becomes shaded to be discarded,
while a shade leaf may be retained indefinitely.

>>>Shade leaves are a different matter, though, because they are
>>>characterised by a different anatomy to sun leaves and develop under
>>>low levels of illumination.  Presumably there is a selective advantage
>>>to the plant in developing shade leaves, but it is not necessarily
>>>their photosynthetic contribution to the rest of the plant.
>>What else?  It's hard for me to see what good a leaf is if not for
>>photosynthesis (except, of course, odd-ball cases like the bootstrap
>>epiphytes that make pots for themselves out of "leaves").

>[...shade leaves] may contribute to the burden of lifting a column of
>water up the vascular tissue to the canopy.

Here I am confused.  Tony, do you mean that shade leaves help in the
water transport problem, or are a burden to the plant (tree)?  Someone
asked me about the water transport problem just yesterday, in the
context of asking what determines the maximum height of a tree.  I 
could post my reply to that question here if there is any interest.

Personally, I see very large selective advantages to producing shade
leaves.  Competition for the full sun of the canopy is expensive in
terms of wood biomass, and full sun is a *lot* of radiation, most of
which is not usable and must be defended against.  Heat load is a 
severe problem for sun leaves in the tropics, and probably also in 
temperate regions at certain times of the year.


      Una Smith      Biology Department       smith-una at yale.edu
                     Yale University
                     New Haven, CT  06511

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