Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 220

Figure 6.11 Schematic view of a stomate: top view of a closed and an open stomate
(top) and side view of a stomate in a leaf (bottom). (After Konrad et al., 2008 )

Water vaporization occurs within the leaf, namely in the intercellular spaces. From
there the water vapour has to move within the leaf, leave the leaf through the stomate
and inally has to escape from the air layer directly adjacent to the leaf, viz. the leaf
boundary layer. If the vapour lux is formulated in terms of a potential difference and
a resistance (compare Chapter 3 ), two resistances can be identiied on this route: the
variable stomatal resistance r s and the boundary-layer resistance r b . The latter is not
only important as an obstacle to transport, but it also provides the link between the
conditions in the air within the canopy and the conditions at the leaf surface (e.g.,
temperature, CO 2 concentration) as they are experienced by, and relevant for, the leaf
(Goudriaan and Van Laar, 1978; Collatz et al., 1991 ). The boundary-layer resistance
is dealt with in Section 6.6.4 .
If we focus on the transport through the stomate (see Figure 6.12 ), the water vapour
lux (transpiration) can be expressed as:

qq
r

−

T =− ρ e

i

(6.20)

s