Geoscience Reference
In-Depth Information
Fig. 10.10
Rate of transpiration and rate of evaporation directly from the soil surface measured in
a rape fi eld during the vegetation season. Except during the fi rst 3-4 weeks after planting, transpi-
ration exceeds evaporation from the soil surface because the rape canopy shades the soil surface
small portion of the water moving within plant stems also escapes into the atmo-
sphere through a few stomata as well as other pores called lenticels that serve as
windows in the external stem tissue. Because stomata can be opened, half closed, or
completely closed like real windows, plants are able to regulate their transpiration.
The driving force of the water transport is the gradient of the water potential.
Resistances at the soil-root and leaf-atmosphere interfaces and those inside the
plant, soil, and atmosphere infl uence the actual fl uxes; see Fig.
10.11
. The water
potential in plant tissues is defi ned analogously to that in soils. The total plant water
potential is usually partitioned into two components: (1) turgor or turgor pressure
identifi ed with a pressure soil potential and (2) osmotic pressure.
An increase of turgor is accompanied by an increase of cell volume and a
decrease of osmotic pressure to zero at full turgor. A decrease of turgor owing to
dehydration causes the cells to shrink and at a certain threshold value we observe
the wilting of leaves. Complete wilting occurs at osmotic pressures between 0.5 and
20 MPa. Roughly one century ago, the fi rst generation of soil physicists started to
use the average value and called this characteristic the wilting point, as we men-
tioned already in Sect.
9.1
.
We describe its main mechanism here with an under-
standing of plant physiology.
