SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

50
• l:' effect it may have on the gas phase mainly closure of stomata or stornatal
resistance.
THE SOIL-PLANT-AIR CONTINUUM (SPAC)
In the past 30 years the water flow through the system (F) has been
represented by the van den Honert (1948) principle:
F = ?/Js - ?/Jr =?/Jr - ?/J1 = ?/J1 - ?/Ja
R s R p Ra
where ?/J s ' ?/Jr' ?/JI and ?/Ja are the total water potential of soil, at the root
surface, at the evaporating surfaces of the leaf, and of the bulk air,
respectively. R s ' R p and Ra are the resistances of the soil, plant, and
vapour pathways, respectively. While the potential drop in the two liquid
phases of the pathway is normally no greater than 3 MPa, the drop in the
water potential between leaf and bulk air (?/JI - ?/Ja) can be 100 MPa or
greater and emphasizes the significance of sto mata in controlling the flow
of water through the system.
However, caution is needed when attempting to extrapolate from eq. (7).
Flow through the soil, plant and atmosphere are coupled but separate
systems, the uptake and redistribution of water from the soil within the
plant occur in the liquid phase, while loss of water from the evaporating
sites in the leaf is in the vapour phase. A calculation using gradients of
water potential as the driving force willlead to a substantial overestimation
of the magnitude of the resistances of the vapour phase due to the
logarithmic relationship between water potential and vapour pressure
(eq. 2). Also, the van den Honert model assumes a steady-state situation
which sel dom exists within the plant. Finally, tissues of plants with
significant storage capacity can be regarded as alternative sources of water
linked in parallei with the soil.
The above concept of van den Honert (eq. 7) treating the soil-plantatmosphere
water movement in analogy with Ohm's law has stimulated
much useful research since its adoption. The recognition of the stornatal
resistance being the crucial resistance regulating water vapour flux out of
the plant, and the controi of stornatal closure under water stress by
decrease of turgor pressure of stornatal cells, caused by decrease of leaf
water potential, are similarly well recognized.
When the water status of a plant starts to fall, which it will if the
evaporative loss from the leaves exceeds the influx of water from the roots,
a threshold behavior is of ten observed; Le. stomata close when a certain
threshold of ?/J 1 is reached, of ten from - 1.2 to - 1.6 MPa for field grown
crops. The threshold behavior act s as an overriding protective mechanism
against excessive leaf water deficit (Hsiao and Bradford, 1983).
(7)