Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 85

Water. Green plants consist of approximately 70 to 90 % of water. This water
content changes with type and age of the plant organ. It has very important func-
tions within the plant, among others transporting dissolved substances and main-
taining hydrostatic pressure keeping the tissue firm. Water is also an important
raw material for all metabolic processes such as photosynthesis. Furthermore,
most bio-chemical reactions take place in aqueous solution.
Water balance within plants is defined by water absorption, mainly through the
root, and the release of water. The latter mainly takes place through transpiration
by the leaves. A water deficit occurs if the amount of released water exceeds the
absorbed amount. This can happen at a high level of transpiration, low water
availability in the ground or an inhibited metabolism in the root. The root absorbs
water from the ground by means of the suction power of the root cells. The water
absorption capacity stops at the wilting point, where the water content in the
ground is so low that the water retaining capacity of the ground exceeds the suc-
tion power of the roots.
The plant biomass production is directly linked to their water supply. Each
plant requires a specific amount of water to produce organic mass. The transpira-
tion coefficient describes the amount of water required by the plant to produce
1 kg of dry mass. With 220 to 350 l/kg, C 4 -plants, such as maize and miscanthus,
use water most efficiently and thus show the lowest transpiration coefficients.
This is, among other factors, due to their closely arranged photosynthetically ac-
tive cells and the thus lower transpiration loss. C 3 -plants such as wheat and the
fast-growing willow require between 500 and 700 l/kg. In general, increased wa-
ter supply increases the potential biomass productivity of a site (Fig. 2:50, left).

30 30

25 25

20 20

15 15

10 10

5 5

0 1,000 2,000 3,000 4,000
Mean annual precipitation in mm

0

-10 0 10 20 30

Mean annual temperature in °C

Fig. 2.50 Net productivity of forests depending on the mean annual precipitation (left) and
mean annual temperature (right) (see /2-30/)

Temperature. Temperature affects all living processes. This is particularly the
case for photosynthesis, respiration and transpiration. In their activity, plants show