Soil Regulates Circulation of Water on the Planet Earth - Soil: The Skin of the Planet Earth - page 150

Fig. 10.13 The dependence of the ratio of actual to potential transpiration rate T RA /T RP plotted
against the relative soil water content (actual water content divided by saturated soil water con-
tent). The curve denoted by 1 T RP shows the conditions at very low potential transpiration, while
3 T RP represents conditions at very high transpiration. And 2 T RP is the potential transpiration roughly
in the middle of two extremes

wilting in Fig. 10.13 . It is worth mentioning that at soil water contents very close to
saturation the actual transpiration can also sink below the potential value. During
such times, oxidation-reduction conditions in the soil reduce the root activity of
cultivated plants and transpiration.
Up to now we have dealt only with average values of transpiration. However, as
was already mentioned earlier in the section on evaporation, the evaporative demand
of the atmosphere manifests large diurnal amplitudes. Such fl uctuations of the
atmosphere occurring during the day and night cause important fl uctuations of leaf
water potential; see Fig. 10.14 . Compared to those of the leaf, the amplitudes of the
water potential in the root are smaller, occur later, and manifest a lag or phase shift.
In the root zone, diurnal amplitudes of soil water potential driven by atmospheric
demand are negligible, except perhaps at shallow soil depths where large daily tem-
perature fl uctuations occur.

10.3.3 Wilting Point

We have already shown that if the rate of soil water extraction by plant roots is
insuffi cient to meet a transpiration rate reduced to its minimum, the plant is wilting.
A deeper knowledge about plant physiology and transpiration will allow us now to
better understand how wilting point was derived. If the plant does not regenerate its
turgor and biological activity after a rewetting of the soil, we defi ne such soil water