Molecular distillation [was: Re: HELP: humongous leaves!]

Bill Williams bwilliam at
Mon Nov 2 08:43:13 EST 1992

Tony noted that our illuminating discussion on the adaptive advantage
of something or other
had become a two-person E-mail exchange, so I am uploading it to
bionet.plants.  Surely SOMEBODY
must be interested?

Bill Williams said:
> >In article
<921030204554.MIN-LFQAa00274.bionet-news at> you wrote:
> >: [...]
> >: I've certainly never run across a shade leaf with a negative
> >: net photosynthetic rate, and I would imagine that such a leaf
would be
> >: quickly shed. My own experience, and the literature that I'm
> >
> >How does a reduced net assimilation rate in a shade leaf initiate
> >abscission?
> >
> >Leaf senescence and abscission is under hormonal control, so what
is the
> >mechanism linking photosynthetic activity to the shedding of leaves?
> >
> >Redistribution of assimilates occurs in accordance with the
> >relationships between different parts of the plant and older leaves
> >become sinks.
> >
> Now, that is a VERY interesting question.  Frankly, it had never
> to me;  clearly the plant does a good job allocating its resources
> de-allocating them), but exactly how this particular feat is
> would be an extremely interesting story.

Aaaargh! - this *always* happens: just when an interesting thread
develops on bionet.plants it becomes a two-way email discussion :-(

Don't get me wrong, Bill.  I'm delighted to have the opportunity of
chatting to you about shade leaves etc. via email, but I'd like it a
whole lot more if more people would join in on bionet.plants.

In fact, I've had some really interesting email exchanges since
bionet.plants began, but people seem reluctant to state their views in
public because of the criticism that often follows.

> >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)
> >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.

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

> >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
> >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").

Well, for example, they affect the microclimate around the tree and may
contribute to a reduction in transpirational losses from
photosynthetically active parts of the canopy elsewhere.

In addition to this effect on microclimate, they may contribute to the
burden of lifting a column of water up the vascular tissue to the

They may also contribute to a reduction in PAR reaching the ground and
suppress competition for water and nutrients from ground cover or lower
strata species.


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