Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 218

4

2

4. dry soil, high transpiration rate
3. dry soil, moderate transpiration rate
2. wet soil, high transpiration rate
1. wet soil, moderate transpiration rate

3
1

-5000

-4000
-3000
-2000

-1000

-250

-200

4

-150

3

-100

2

-50

1

0

A B C D E

soil root stem leaf atmosphere

Figure 6.10 The potential decline in the soil-plant-atmosphere continuum at two
soil moisture levels and slower and faster transpiration rates. (After Hillel, 1980 )

an identical soil water hydraulic head, so that the critical hydraulic head within the
leaf is almost attained. There is a similar situation just before wilting in case 3, when
the available soil moisture content has been greatly depleted, the leaf hydraulic head
is close to -200 m in some of the mesophyll cells, and the transpiration rate is also
small. But inally, if the transpiration rate increases when soil moisture is in short sup-
ply, which is the situation in case 4, the leaf hydraulic head will fall below the critical
level and the plant wilts.
Within the xylem sap a large negative pressure head of -150 < h < -50 m may
exist. Because of wall thickening ( Figure 6.9 ) the vessels will not collapse, but can
withstand the pressure difference between inside and outside. Normal tissue cells
with more elastic and unreinforced walls would break down more easily with the
application of such pressures.
There is another problem with water transport through the plant. Air, dissolved in
water, is released when the pressure head of the water becomes more negative. The
gas forms bubbles, a process called cavitation . The bubbles will interrupt the water
transport through the vessel ( Figure 6.9 ). Water molecules in the liquid state are linked