Geoscience Reference
In-Depth Information
6
Vegetation: Transport Processes Inside
and Outside of Plants
Plants serve as an intermediary between the atmosphere and the soil: they eficiently
transport soil moisture into the air and at the same time ingest atmospheric CO
2
for
their growth. This chapter deals with the transport of water inside plants (from the
root to the stomata), the link between water uptake and dry matter production and the
modiication of the near-surface atmosphere by vegetation, including microclimate,
dew and rainfall interception.
6.1 Functions of Water in the Plant
Water performs many essential functions within plants. As a chemical agent, water
facilitates many chemical reactions, for instance in assimilation and respiration. Water
is a solvent and a transporter of salts and assimilates within the plants. Water enables
the regulatory system of the plant, as it carries the hormones and substances that are
required for plant growth and functioning. Water confers shape and solidity to plant
tissues. If the water supply is insuficient, herbaceous plants and plant organs that lack
supporting tissue will lose their strength and wilt. The hydrostatic pressure in cells
depends on their water content and permits cell enlargement against pressure from
outside, which originates either from the tension of the surrounding tissue or from
surrounding soil. The large heat capacity of water greatly dampens the daily luctua-
tions in temperature that a plant leaf may undergo, due to the considerable amount of
energy required to raise the temperature of water. Energy is also required to convert
liquid water to vapour that transpires from leaves, causing cooling due to evaporation.
Without these temperature compensating effects, plants would warm up much more
and eventually die from overheating. Owing to these effects, transpiration rates can
be estimated from surface temperatures, obtained by infrared thermography using
remote sensing from aeroplanes or satellites (Ehlers and Goss,
2003
).
In addition, in quantity, water is an important constituent of plants. The com-
position of roots, stems and leaves of herbaceous plants is 70-95% water. In con-
trast, water comprises only 50% of ligneous tissues, and dormant seeds contain only
200
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