wind and transpiration

Tom Buckley buckley at BIOSERVER.VSB.USU.EDU
Tue Feb 23 10:26:49 EST 1999

The student is correct, in principle, though the effect is probably
insignificant in the big picture.  First, Dr. Cramer made the key point
- that wind speed (and leaf size and surface features) primarily affects
gas exchange via the boundary layer.  Wind facilitates gas exchange by
decreasing the distance across which the diffusion gradient occurs.  (In
exactly the same way, it speeds up heat exchange between a human body
(or a leaf) and the air - wind chill).  Boundary layer effects are very
important in ecophysiology.

RE the Venturi effect... The decrease in air pressure at the pore caused
by moving air can't induce a continuous flow of air out of the leaf
(otherwise, the leaf would deflate!)  Instead, this lower pressure will
quickly equilibrate with the air inside the leaf by inducing a small
bulk flow out of the leaf - lowering the pressure and volume inside the
leaf.   I suspect these decreases are very small indeed.

See if the student can track her/his insight down to this logical
conclusion!  I remember making the same point to my ecophys. instructor
years ago, and it took me a while to figure it out.

Beyond that, the decreased air pressure at the pore and within the leaf
may speed up diffusion a tiny bit by decreasing the interfering effect
of gas components moving in opposite directions.  Water vapor and CO2
generally diffuse in the opposite direction through stomata, and their
interactions can slow diffusion measurably (enough that they need to be
taken into account for accurate modeling).  These interactions are a
function of the partial pressures of water vapor and CO2, which decrease
with total pressure, so the Venturi effect may reduce the interactions.
The effect is probably not *significant*, but the principle holds.


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