super-optimal stomatal conductance

Tony Travis ajt at
Mon Nov 16 19:22:50 EST 1992

In article <921116185833.MIN-LXADa00255.bionet-news at> you wrote:
: [...]
: You're suggesting, perhaps, that 'self stressing' may be a means of drought
: hardening?

Yes, but I was speculating about the selective advantage that losing
more water than was necessary for CO2 uptake may have.  A high rate of
transpiration does not, of course, imply water stress unless the supply
of water to the leaf is restricted.

I did some work (unpublished) in the early 80's where I grew plants at
high humidity then measured the sensitivity of stomata on isolated
epidermal strips.  Interestingly, the stomata on high humidity plants
opened wider, and appeared to be more sensitive to light and CO2.

There was speculation at the time that endogenous IAA levels were
higher in plants grown without water stress.  The role of IAA in
stomatal movements is not precisely understood, but the work of Marre
(1979) with the fungal toxin fusicoccin suggests that IAA may be
involved in the regulation of membrane bound ATPases involved in active
ion transport into guard cell vacuoles during stomatal movements.

: Its been suggested that low leaf water content or potential does not always
: indicate a 'stress', but may simply reflect high photosynthetic rate (via
: open stomata).  The argument is that stomata would close if the leaf were
: stressed.

Well, water only moves if a lower water potential exists in the leaf so
some degree of water stress is required as a basic component of the
solute transport mechanism.

However, as I said in a previous message there is evidence that a
feedback mechanism alone is not sufficient to explain the behaviour of
stomata (Cowan and Farquar, 1980).  Their evidence suggests that
stomata close _before_ the leaf becomes stressed (ie. feed-forward

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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