Agriculture Reference
In-Depth Information
However the calculations indicate an unlikely lack of margin for error in the root
length. A possible explanation is that N species other than NH
4
+
are also being
absorbed—such as NO
3
−
or amino acids—as discussed in Section 6.3.
Transport of NH
4
+
to the roots in Kirk and Solivas' experiment was mainly
by diffusion. The additional transport resulting from mass flow of soil solution
in the transpiration stream would have increased the influx across the roots by
about 100
av
a
/
0
.
5
bD
%where
v
a
is the water flux (Tinker and Nye, 2000, pp.
146-148), or about 4% in Kirk and Solivas' experiment. A sensitivity analysis
showed that rates of diffusion will generally not limit uptake in well-puddled
soils, but they may greatly limit uptake in puddled soils that have been drained
and re-flooded and in unpuddled flooded soils.
Note that the above conclusions refer to uptake of soil N by the main body
of the rice root system in the anoxic soil beneath the soil-floodwater interface.
Uptake of fertilizer N broadcast into ricefield floodwater and absorbed by roots
in the floodwater or soil near the floodwater is not likely to be limited by root
uptake properties or transport (Kirk and Solivas, 1997).
6.3 NUTRIENT ABSORPTION PROPERTIES OF WETLAND
PLANT ROOTS
6.3.1
ION TRANSPORT IN ROOTS
Taiz and Zeiger (2002) give a full account of this topic. Mineral ions absorbed
from solution outside the root surface must be transported across the root to
the main long-distance transport vessels in the xylem, through which they reach
the shoot. This process is highly specific for different ions and molecules and
is closely regulated. The regulation is in part a function of the anatomy of the
various root tissues and in part a function of active transport processes in root
cells. The pathways and transport processes are affected by root adaptations
to anoxia.
Molecules and ions move across the root through both extracellular and intra-
cellular pathways (Figure 6.9). The extracellular route exists because all cells
have walls containing solution separated from the cytosol by plasma membranes,
and there is therefore a continuous 'apoplastic' pathway through which solutes
can diffuse from one cell wall to the next without crossing a plasma membrane.
There is also a continuous intracellular 'symplastic' pathway because the cytosols
of neighbouring cells are connected by cylindrical pores called plasmodesmata,
20-60 nm in diameter, through which ions and molecules that have been taken
up into the cytosol may diffuse. In tissues where significant intracellular transport
occurs there may be up to 15 plasmodesmata per square
µ
m of cross-section.
An ion entering a root may immediately enter the symplast by crossing the
plasma membrane of an epidermal cell, or it may remain in the apoplasm and
diffuse through cell walls. It may subsequently enter the symplasm by crossing
