However, even during seed sink initiation, roots
greater soil volume (Harris 1977). Main axis root
do not completely stop growing. Root length
elongation rates of forage legumes were signifi-
density generally decreases with increased soil
cantly correlated with seed weight (Brar et al.
profile depth.
1990a). Cultivars within the species had signifi-
Seminal and crown roots are different morpho-
cant impact on root elongation rates. Similarly,
logically and functionally. For instance, in wheat
root growth also varied greatly among plant spe-
and grasses, seminal roots go deeper, are thinner
cies (Stone and Taylor 1983, McMichael et al.
in size, and are more branched compared to
1985, Brar and Palazzo 1995). The rate of root
elongation may range from 0.5 to 3 cm day1
crown roots (MacKey 1973). The crown root sys-
(May et al. 1965) or more (Taylor and Ratliff 1969).
tem is more flexible regarding development un-
Elongation rates of main root axes, primary later-
der changed environmental conditions (Tennant
als, and secondary laterals of small grain cereals
1976).
grown in a relatively favorable laboratory envi-
ronment were 20, 0.5, and 0.1 cm/day, respective-
Dicots
ly (Lungley 1973). The root extension rates de-
The diameter of dicot roots decreases with in-
creased with increased plant age and were
crease in order (Rendig and Taylor 1989). The av-
negligible in perennial crops (Hillel and Talpaz
erage functioning life of higher-order roots may
1976).
be 10 to 20 days. Higher-order roots form when
the environmental conditions are favorable and
die when water and nutrients are depleted in
ROOTSHOOT RELATIONSHIPS
their immediate vicinity. When soil water poten-
Root and shoots are strongly interdependent.
tial decreases below 0.2 MPa for 7 to 10 days,
The roots receive photosynthates and growth
about 80% of the roots can become senescent.
hormones from shoots and in return furnish wa-
Time and environmental conditions substan-
ter and nutrients to shoots (Taylor and Arkin
tially influence the initiation rates of new roots
1981). After seed germination, plants generally
and death of old roots. Therefore, the root length
contribute the major portion of the metabolites
density and root length per plant fluctuate during
towards root expansion. For instance, the radicle
the season. As observed in monocots, the root
grows faster than the plumule of wheat after ger-
growth of dicots continues during the reproduc-
mination; however, the trend is reversed at later
tive phase.
stages of growth. Root:shoot ratios differ depend-
ing on environmental conditions around the
ROOT GROWTH AND DEVELOPMENT
plant.
Relationships of root and shoot growth for 22
Roots grow because new cells are formed in
winter wheat crops grown in the United King-
the meristematic tissue near the root tip, and
dom are illustrated in Fig. 3. Dry matter is equally
these newly formed cells increase in volume,
partitioned between roots and shoots in the early
pushing the root tip forward if growth conditions
stages of wheat growth, but at anthesis the root-
are satisfactory (Taylor and Brar 1991). Before the
per-plant ratio has fallen to 0.1 (Barraclough et al.
cell starts expanding, the water potential inside
the cell, ψinside, is equal to the water potential out-
1991). The total shoot dry matter of 18 t ha1 was
side, ψoutside, and ψinside = ψp + ψo, where ψp is
accompanied by a root dry matter of 1.5 t ha1.
turgor potential and ψo is the osmotic potential.
Winter crops with longer growing seasons pro-
Processes responsible for cell expansion are de-
duced more shoot dry matter compared with
scribed by Hsiao and Bradford (1983) as
spring crops, and similar trends were observed
1. Cell-wall loosening or relaxation, i.e.,
for the root system. A well-developed root sys-
ψoutside > ψinside;
tem was observed for winter wheat compared
2. Water flow into the cell, diluting its solutes;
with arable crops (Table 1).
3. Water influx causing the cell wall to expand
Soil physical factors such as temperature and
until ψinside and ψoutside are nearly equal.
soil water potential affected the root:shoot ratio
These processes occur simultaneously and con-
of sorghum seedlings (Brar et al. 1992). The ratio
tinuously in the plant root system.
significantly increased with increases in soil tem-
perature from 15.9 to 25.2C and declined at tem-
Rapid elongation of primary roots, combined
peratures of 30.2 and 35.8C (Table 2). Further-
with well-developed secondary roots, allows the
plants to exploit moisture and nutrients from
more, the ratio was greater with 0.1 than 0.03
3
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