Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 201

5-15% water. The daily transpiration of plants is large compared to the amount of
water contained in plants. For example, consider a mid-season wheat crop with a dry
weight of 4 t ha -1 . During a dry and windy summer day, the crop may lose 6 mm of
transpiration, which has to be extracted from soil moisture. This 6 mm of water cor-
responds to 60 t ha -1 , or 15 times the dry weight of the crop! If we assume that the
plant tissues consist on average of 85% water, the wheat crop contained 85/15 × 4 =
22.67 t water ha -1 . Compared with this store of water in the plant tissues, 60/22.67 =
2.6 times more water was extracted from the soil and passed on to the atmosphere
within one day. The amount of water that is transpired daily by plants is generally
1-10 times more than the water stored in them. Therefore each day plants should
extract about the same amount of water from the soil as they lose by transpiration to
the atmosphere. Compared with the amount needed for cell division and cell enlarge-
ment, the transpiration amount is 10-100 times more, and compared to the needs for
photosynthesis it is even 100-1000 times greater (Ehlers and Goss, 2003 )!
6.2 Root Water Uptake
6.2.1 Functions of Roots
Roots grow into the soil, anchoring the plant and nourishing the growing shoot by pro-
viding water and mineral nutrients. Water is transported from soil to root by mass low,
driven by a difference in hydraulic head between the root surface and the surrounding
soil. The hydraulic head gradient generates the convective low of water towards the
root surface. Those plant nutrients that are dissolved in the soil solution are inevitably
drawn to the root with the convective low of water. Roots may not take up some solutes
as quickly as they arrive with the water. Consequently the soil in the vicinity of the roots
will become enriched with nutrients such as calcium and magnesium (Jungk, 2002 ).
Besides mass low, nutrients will be transported to the root surface by the process
of diffusion. Transport by diffusion is triggered by nutrient uptake into the plant itself,
which lowers the nutrient concentrations at or near the root surface. Diffusion is the
more important transport process for nutrients such as phosphorus and potassium,
which are present at only small concentrations in the soil solutions and for which the
convective low is insuficient (Jungk and Claassen, 1989 ).
Roots are not only effective in removing those nutrients from soil solution that are in
an available form, but it is signiicant that they are also able to secrete protons, organic
acids and chelating agents. Using these materials they can modify the availability of
certain nutrients for themselves. These root exudates stimulate the release of ions from
soil minerals, and therefore the bioavailability of macronutrients, such as phosphorus,
and some micronutrients that normally are only sparingly soluble (Jungk, 2002 ).
Most of the material secreted by plant roots is in the form of mucilage, much of it
originating in the root cap cells. Root mucilage provides an attractive environment for
microorganisms that use components as a substrate for their growth, and in doing so