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

Tony Travis ajt at
Thu Oct 29 18:32:53 EST 1992

In article <921029215003.MIN-LPIAa00274.bionet-news at> you wrote:
: [...]
: I think the significance lies in, not larger guard cells in shade leaves,
: but smaller ones in sun leaves.  I've only seen one reference on this, but
: it seems that CO2 tends to spill into the stomate over the rim of the guard
: cells, whereas H20 tends to leave the leaf via a chimney effect as a column

I think you are describing the hypothesis of molecular 'distillation'
that was popular some years ago.  The surface of a leaf normally has a
'boundary' layer of air next to it that can present a considerable
resistance to the diffusion of CO2 and water vapour.  Someone (sorry I
have forgotten who proposed the theory) suggested that CO2 and water
vapour pass through this layer a different rates.

: throught the center of the stomate.  Having smaller guard cells reduces the
: aperature area of each stomate, and with more stomata per unit leaf area the
: result is less total aperture area with more total aperture circumference.
: The result is greater water use efficiency, or CO2 gained over H20 lost.  I
: don't think this idea is very well accepted because I haven't seen it
: modeled based on the diffusion characteristics of gasses and the heat
: exchange of the leaves.  Nevertheless, it was discussed briefly in R.G.S.
: Bidwell's book Plant Physiology (1974) Macmillan Publishing Co., New York.

If this purely physical effect really exists, it should be possible to
produce a membrane with stoma-sized holes in it that will accumulate CO2
on one side provided that an unlimited supply of water exists on that side
and that drier, CO2 containing air is present on the other side.  The energy
needed to create such a CO2 concentration gradient would be the latent heat
of vaporisation of water on the CO2 accumulating side of the membane.

: I should also point out that these differences in guard cell lenght are not
: allways seen between sun and shade leaves, whereas differences in stomatal
: density are much more consistent.

There is little point in optimising water use efficiency in leaves that
are not photosynthetically active and I guess that 'shade' leaves are
contributing little (or are actually sinks) in which case it would be a
selective advantage to have fewer stomata on them.

Are shade leaves actually an adaptation to low light, or an adaptation
to exploit opportunistic phenomena such as sun-flecking or breaks in
the canopy?  If opportunistic, they would be adapted to bursts of
photosynthesis (rather than sustained rates) and lower levels of gas
exchange would be needed.

Dr. A.J.Travis,                       |  Tony Travis
Rowett Research Institute,            |  JANET: <ajt at>
Greenburn Road, Bucksburn,            |  other: <ajt at>
Aberdeen, AB2 9SB. UK.                |  phone: 0224-712751

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