Geoscience Reference
In-Depth Information
stop even if the stomata close completely. And if transpiration exceeds the infl ux of
water into the roots, plant tissues lose both water and turgor. If the loss of turgor
continues beyond a critical threshold value, wilting occurs. Wilting can occur even
if the soil is fully moist when the evaporative demand of the atmosphere is large and
the infl ux to the roots is lower than the transpiration loss. Often during a hot summer
midday, plants having large leaf areas may temporarily wilt. Subsequently, in the
evening hours when the extreme air conditions cease and the transpiration rate
decreases, turgor in the leaf tissues returns. When extreme air conditions prevail or
when the soil remains relatively dry for several days, the loss of turgor in cells
accompanied by irreversible changes in the plant fi nally causes permanent wilting.
Stomatal apertures also provide pathways into the plant for CO
2
needed for photo-
synthesis and the growth of plant tissues. Hence, a correlation between transpiration
and crop yields exist and has been frequently documented. Some authors claim that
the priority for receiving CO
2
necessary for photosynthesis indeed regulates stoma-
tal aperture.
Transpiration rates of a certain type of plant and its variety depend widely upon
soil water content and upon weather conditions, such as temperature, humidity, and
sunlight intensity. Analogous to potential evaporation, the potential transpiration is
defi ned as the loss of water from plant tissues to the atmosphere according to the
evaporative demands of the atmosphere with the stomata fully opened. Additionally,
it is understood that water movement from the soil to the plant roots does not limit
the process. With atmospheric conditions including the energy source and radiation
controlling the phase change of water, energy regulates the process for a given sto-
matal density. The term unstressed transpiration is also used for potential
transpiration.
Let us now assume that the soil water content suddenly decreases. Concomitantly,
values of soil water potential and unsaturated hydraulic conductivity both drasti-
cally decrease. In such a case, the evaporative fl ux is maintained by the increase of
the gradient of the decreasing soil water potential. As a consequence the cell water
potential continually decreases and is accompanied by a loss of cell turgor and at its
critical value, the stomata start to close. This critical value also depends upon plant
type, its variety, its susceptibility to water stress, and local environmental condi-
tions - quality and intensity of light, CO
2
concentration, and surface temperature of
leaves. We must also remember that transpiration has an important cooling effect on
the plant. If all conditions remain constant except the evaporative demand of the
atmosphere, the critical value of turgor pressure starts to decrease due to the closing
of the stomatal openings. Owing to this increasing stomatal resistance, the rate of
transpiration is reduced and causes the actual transpiration to be less than the poten-
tial transpiration. With further extraction of soil water, the unsaturated hydraulic
conductivity decreases substantially and the compensating increased soil water
potential gradient causes a drastic decrease of plant water potential. This decrease is
accompanied by a decrease of turgor that closes virtually all stomata. Nevertheless,
water vapor still continues to be lost through the plant leaves but at an extremely
reduced rate. The greatly reduced transpiration rate allows the cell turgor in the
leaves to decrease to such an extent that wilting is clearly evident; see the range of
