Agriculture Reference
In-Depth Information
Active absorption refers to the selective accumulation of ions and is generally more
important than passive absorption. It is an energy-consuming process which may be
limited when energy reserves are depleted. Active absorption is only possible after
negative potential and pH gradients have been created by the hydrolysis of ATP by
ATPases. Some ions, such as are absorbed by passive absorption while others,
such as are only absorbed by active processes. The selectivity of absorption
processes is still poorly understood.
3.2.1.3
Dynamics of absorption
Plant growth is more likely to be limited by the availability of solutes rather than by
the intrinsic absorption capacity of roots. In general, as little as one fifth of the root
system immersed in a suitable nutrient solution may provide sufficient root area for
adequate nutrition. For example, maize plants 45-75 days old have a root biomass of
ca.
500 g although only 30 to 35 g of this is necessary to adequately supply the plant
(Nye and Tinker, 1978; Callot
et al.,
1982).
Under natural conditions, low nutrient availability and metabolic limitations in the
utilization of absorbed nutrients necessitate the existence of a well-developed root
system and its further extension with mycorrhizal mycelia.
In laboratory experiments, all parts of the root system of maize plants appear to have
equal absorptive capacities, even those parts called coronary roots specialised for
anchoring. Nonetheless, absorption is usually highest near the apex of the young
unsuberised roots and by mycorrhizae (Bowen and Theodorou, 1973). When the plant
is immersed in a nutrient solution, branching does not appear to affect absorption (Callot
et al.,
1982). Similarly, age also has little influence although the relative positions of
absorbing roots are important; when two roots are left, absorption is greater if they are
diametrically opposed rather than adjacent. Genotypic variation may also be important.
In seven genotypes of corn, nitrate uptake by six-day-old root systems varied from 44 to
86 f wt during an 8 h period; differences in lateral root proliferation
and root elongation were considered to explain these differences (Pan
et al.,
1985).
Finally, the importance of the volume of the above ground part of the plant influences
the rate of uptake by roots through a 'sink' effect. As a result, excised roots have lower
rates of uptake than similar amounts of roots with their above-ground parts still attached
(Maertens and Clauzel, 1980).
3.2.2
RESISTANCE TO WATER STRESS
The uptake of water by roots is closely linked to that of ions. The major mechanism is
evapotranspiration of leaves and shoots which creates a net flow of water and ions from
the soil solution to the roots.
As with most soil organisms, roots are very sensitive to water stress and have devel-
oped both short and medium-term mechanisms of coping with it.
