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

Bill Williams bwilliam at
Fri Oct 30 14:01:20 EST 1992

In article <1992Oct29.233303.4935 at> Tony Travis,
ajt at writes:
>There is little point in optimising water use efficiency in leaves
>are not photosynthetically active and I guess that 'shade' leaves are 
>contributing little (or are actually sinks) in which case it would be
>selective advantage to have fewer stomata on them.

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. My own experience, and the literature that I'm familiar
with, suggests that the typical shade adaptations -- lower
concentrations of Rubisco, larger photosynthetic units (i.e., more
antenna per reaction center), and consequent general thinness and lack
of stomata (since conductance is generally correlated with
photosynthetic rate) -- are  to allow positive net photosynthesis
in more-or-less constant low-light conditions.  Indeed, these
adaptations are generally incompatible with good performance in high
light (e.g., sun flecks), and most shade leafs develop significant
photoinhibition even at quite moderate light levels.  That this may be
a problem without an obvious physiological solution is illustrated by
Oxalis oreganum, which has very low-light adapted leaves but manages to
survive intense sunflecks by folding its leafs so that they're parallel
with the rays in the fleck, thus greatly reducing absorbed intensity.
William E. Williams, bwilliam at
Divison of Natural Sciences and Mathematics
St. Mary's College of Maryland
St. Mary's City, MD 20686

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