Geoscience Reference
In-Depth Information
Fig. 10.14
Diurnal changes in leaf and root water potential are distinct, while the soil water poten-
tial of the root zone is smoothly sinking during rainless summer days. The changes in leaf and root
water potentials are responsible for the diurnal variation in transpiration
content as the soil water content of permanent wilting. Resuming the discussion of
the fl ow of water within plants, we recognize that the soil water content of wilting
cannot be the same for different plants. Even if we restrict our consideration to a
particular cultivar, the soil water content of wilting depends upon the developmental
stage of the plant, its water stress susceptibility, and the evaporative conditions of
the atmosphere. Moreover, the composition and concentration of the soil solution
plays a role. An increased soil solution concentration increases the soil water con-
tent of wilting through a decrease of the total soil water potential. The increased
value of hydraulic conductivity owing to an increased soil solution concentration
may also infl uence the value of the soil water content of wilting provided that there
are no other concentration effects linked to soil structure and transport across cell
membranes.
Because the soil water content of permanent wilting separates soil water into two
categories (one useful and available to plants and one that is not), practitioners
introduced the concept of the “wilting point” especially for the production of culti-
vated crops. The “point” is indeed nothing more than an average of soil water con-
tent values measured at various times when plants on a given soil were observed to
be permanently wilting.
Wilting point is now defi ned in soil science as the soil water content at a soil
water potential of −1.5 MPa (also expressed as −15 bar, −15,000 cm, or a pF of 4.18
which is the common logarithm of the absolute value of the soil water pressure head
expressed in cm). The permanent wilting for individual plants and atmospheric con-
ditions usually differ, since the abovementioned value is the result of convention
and should be understood in a way as the average of measured data on permanent
