Geoscience Reference
In-Depth Information
10.3.2
Transpiration
The long-lasting existence of bare soil is rare owing to the management of various
types of landscapes. In agricultural regions of crop production, bare soils exist only
after harvest, and even there, the plant residues are frequently left on the surface as
a protection against excessive loss of soil water by evaporation as seen above in
Fig.
10.9
. Although we are not speaking about deserts and semideserts, in such
regions we often observe irrigated crop production where bare soil may also exist.
Water conducted by the irrigation system to the plants usually wets the soil with a
portion of the applied water lost by evaporation from channels, furrows, and bare
soil. Recent irrigation systems, particularly those called drip systems, are the most
effi cient and reduce water losses to a practical minimum. Water, conducted just to
the plants by a system of tubing, is dripped directly to specifi c locations within the
root zone of plants at selected times and rates of application.
We now return to geographical regions supplied by rainfall in a more or less suf-
fi cient way. Cultivated plants shadowing the soil surface of agricultural fi elds reduce
evaporation from the bare soil surface. As the crop root system pumps water out of
the soil, the thickness of soil layer “donating” water increases as plants grow and
their roots more thoroughly penetrate the soil profi le. Although nearly all of the
water sucked up by roots fl ows through a plant to its leaves to participate in the
formation of new tissues by photosynthesis in the presence of the sun's radiation,
the vast majority of the water moving through the plant simply evaporates directly
into the atmosphere from its leaf surfaces (Fig.
10.10
). We easily compare this ener-
gized photosynthetic activity to an industrial process with evaporation serving as
the cooling process within the factory. As in most simplifying analogies, the entire
process is more complicated and requires potential gradients to enable the upward
fl ow of water. This form of evaporation from plants is called transpiration.
The ratio of the mass of water extracted by a plant from the soil in order to pro-
duce dry organic matter of the plant is denoted as the
transpiration coeffi cient
(or
ratio
); it is simply expressed as grams of water to produce 1 g of plant organic
matter. Plants need much more water than animals for production of 1 g of their
bodies.
The transpiration coeffi cient depends on climatic and soil conditions and on the
species of plant. Its value ranges from 300 to 900 for cultivated plants and from 150
to 700 for trees of the temperate zone. Water is the main constituent of plant tissues.
Although the percentage of water in hydrophytes (aquatic plants growing only in
water) is about 80-85 % and a little less in cultivated plants (70-80 %), the water
content of individual cells or particular plant parts deviates greatly. The metaboli-
cally most active plant parts and young tissues contain about 90 % water in relation
to dry organic matter, leaves of cultivated plants 70-90 %, wood of trees about
50-60 %, and seeds as little as 5-10 %. About 95 % or even more of the water
extracted from soil is transpired and fl ows through the soil-root-stem-leave-atmo-
sphere system. Almost all of it winds up in the atmosphere by escaping through an
exorbitant number of microscopic windows in the leaves called stomata. A very
