Agriculture Reference
In-Depth Information
Root diameter is also an important parameter. Thick suberised roots have a clearly
different function than fine, unsuberised pale-coloured roots. An important step towards
a functional interpretation of root morphology was made when coarse and fine roots
were distinguished, depending whether their diameter was more or less than an arbitrary
limit of 2 mm. Jenik (1978) uses the terms 'proximal' and 'distal' roots to define coarse
and fine roots while Lyford and Wilson (1964) proposed the terms 'macrorrhizae' and
'brachyrrizhae' to qualify the two above-mentioned categories.
Coarse roots
are generally highly suberised and lignified. They support the mass of
the plant and conduct water and nutrients from the soil to its aerial parts, and soluble
carbohydrates formed in the leaves to the root extremities and out into the soil.
Coarse roots are perennial and form the framework on which new root growth occurs
(Bowen, 1984). These roots are longeval and may persist throughout the life of the plant.
Fine roots
are specialised for the assimilation of water and nutrients. They are neither
lignified nor suberised and are generally pale-coloured and covered with hairs, which
increase their absorptive surface area. Their life-spans are generally of the order of months
but may vary from a few days for non-mycorrhizal roots to several years when they are
infected with mycorrhizae. A number of other biotic and abiotic factors may determine
root lifespan such as soil fertility, root herbivory, climate and competition with other
plants (Eissenstat and Yanai, 1997). Fine roots will therefore be replaced many times
during the life of a perennial plant and may be considered the endogeic analogues of
leaves. Annual or ephemeral plants produce only fine roots and, in perennial grasses, it
is difficult to differentiate between coarse and fine roots. Root hairs that grow on fine
roots have even shorter life spans, of the order of a few days (see,
e.g.,
Fusseder, 1987)
Specific root length
(SRL), the ratio of total root length to weight, is a useful index
for estimating the relative importance of fine and coarse roots.
3.1.2
ROOTING 'STRATEGIES'
The importance of the soil-root contact may be evaluated by the overall surface area of
roots (Callot
et al.,
1982). This surface is relatively small, and part of it may actually not
be in contact with the soil solution when roots follow old termite or earthworm galleries
or cross soil macropores. The root surface of a mature wheat plant is about 1 whereas
the overall surface of soil particles in the volume of soil exploited by these roots may be
close to 10,000 On average, the ratio of the total surface of soil particles to roots is
approximately or including the surface area of the absorbing hairs. In contrast,
the diffusion of water and nutrients in soil is relatively slow and may be quite limited in
amount [see Chapter I]. In consequence, the total mass of roots produced, and their
horizontal and vertical distributions within the soil, are particularly important factors
in plant fitness.
Rooting strategies may be defined as the way roots exploit soil nutrient and water
resources. This, together with the energy that plants allocate to growing roots, may be
described in terms of root topology, biomass, root:shoot ratio (above-ground to below-
ground biomass), specific root length, horizontal and vertical distribution and concen-
tration in fertile patches of the soil. Several factors may influence these parameters,
including nutrient and water supply and soil physical characteristics.
