Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 204

soil particles. The slimy material protects the tip from desiccation. It supports the col-
onization of the immediate surroundings of the tip by microorganisms, forming the
rhizosphere. The cells of the cap sense gravity, thus enabling the root to grow verti-
cally downward (Ehlers and Goss, 2003 ).
The meristematic cells at the upper side generate embryonic cells backwards. Thus
the root cap is being moved forwards through the soil as new cells are formed and
then expand. It is in the zone of elongation that these cells expand, particularly grow-
ing in length, allowing the root system grow to depth. Thereafter the enlarged cells
turn into the maturation zone, also named the root hair zone. Here the cells become
specialized into tissues of different function. The epidermis and beneath it the cortex
are formed, and inside is the stele, the vascular tissue ( Figure 6.3 ).
6.2.3 Physiology of Root Water Uptake
Within root cells, the concentration of many solutes is greater than in the solution
outside the root. Hence these nutrients have to be transported against an existing
concentration gradient. Also in some cases ions are selectively excluded from cells.
Therefore nutrient uptake is an active and energy-consuming process. The energy
required is generated by cell metabolism.
On the other hand, the uptake of water by roots and the conduction within the plant
does not rely on energy consumption. Water lows from sites with higher hydraulic
head to sites with lower hydraulic head. Differences in hydraulic head cause water
low within the plant, from the root surface to the xylem in the central stele, from the
stele to the various organs of the plant and inally from the leaves to the atmosphere
(Ehlers and Goss, 2003 ).
The hydraulic head declines from the bulk soil to the root surface and drives the
water towards the root. The higher the hydraulic head difference, the higher is the
water uptake rate. Hainsworth and Aylmore ( 1986 ) measured for the irst time
the water content distribution around a single root by computer-aided tomography
( Figure 6.2 ). The lower water contents measured near the root surface correspond to
a lower hydraulic head.
Without hairs, roots may lose hydraulic contact with the soil. Either the root shrinks
when the plant experiences water shortage or the soil contracts and separates from the
root due to drying, a feature particularly associated with clayey soils. But also with-
out any shrinkage of root or soil, the hydraulic contact will decrease, as the soil dries
owing to root uptake and the wetted contact area between soil and root diminishes.
Therefore root hairs are important for bridging the gap. In the language of hydraulics:
root hairs lower the hydraulic resistance.
Water can enter the root interior through two pathways: via the root hairs of the
rhizodermis cells (cellular pathway) or by entering into passage cells of the exodermis
(apoplast pathway). Water arriving at the innermost concentric cellular layer of the