Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 215

Table 6.3 Calculation of potential root water extraction ( S p ), reduction factors for
water ( α rw ) and salt ( α rs ) stress and actual transpiration ( Σ S Δz ) according to the
macroscopic approach

α rw
(-)

α rs
(-)

S Δz
(cm d -1 )

Σ S Δz
(cm d -1 )

Depth
(cm)

h
(cm)

EC sw
(d S m -1 )

S p
(d -1 )

S
(d -1 )

0-20 -1200 2.0 0.0175 0.895 1.000 0.0157 0.314 0.314

20-40 -800 2.5 0.0125 0.947 1.000 0.0118 0.236 0.550

40-60 -300 5.5 0.0075 1.000 0.827 0.0062 0.124 0.674

60-80 -250 8.0 0.0025 1.000 0.655 0.0016 0.032 0.706

Details are in the text.

lists the h and EC sw values, which are measured during this day at different soil depths.
The question is which actual transpiration we may expect according to the described
macroscopic approach. The sensitivity of corn to water and salt stress is expressed
by h 1 = -15 cm, h 2 = -30 cm, h 3 = -400 cm, h 4 = -8000 cm, EC max = 3.0 d S m -1 , and
EC slope = 6.9% per d S m -1 .
First calculate the potential root water extraction rate S p , taking into account the
triangular distribution of the root density with depth ( Table 6.3 ). Next determine by
linear interpolation the reduction factors α rw and α rs according to Figures 6.7 and
6.8 . This gives the actual root water extraction rate S according to Eq. ( 6.17 ). Finally
we can calculate the actual transpiration rate by integrating S over the rooting depth
( Table 6.3 ). Note that, like in most irrigated soils, the water stress occurs in the top
part of the root zone, while the salt stress occurs in the bottom part of the root zone.
In this example the potential transpiration rate of 8.0 mm d -1 reduces to 7.1 mm d -1
because of water and salt stress.

Question 6.3: In the case of irrigated soils, why does the water stress often occur in the
top part of the root zone and the salinity stress in the bottom part of the root zone?

6.3 Water Flow within the Plant
After passing through the endodermis of the root, the water enters the stele
( Figure 6.1 ), where it is conducted through the cell tissue by osmosis, inally arriving
at the xylem strands. The xylem is composed of a few living parenchyma cells and
cells of large diameter that have lost their protoplasts. One of these types of cells is
called the trachea or the vessel member. Strung together these member form a vessel
( Figure 6.9 ). The vessels represent the 'hydraulic pipelines' for long-distance trans-
port of water within the plant. They permit rapid conduction from the root through
the stem axis to the leaf. The vessel members are joined together at their ends by open
perforations or perforation plates ( Figure 6.9 ), restricting to some degree the vertical