Agriculture Reference
In-Depth Information
Aerial plant parts
including flowers,
leaves and stems
Pencil
root
Fibrous root
Underground plant
parts including a
range of root types
Storage root
String root
Figure 18.3
Morphology of the sweet potato plant. In practice, the proportion of foliage to root is somewhat
greater than that shown here. (Adapted from Woolfe 1992.)
Sweet potatoes are grown from 40°N to 32°S of the
equator. On the equator they are grown from sea level to
3000 m. Growth is best at or above 24°C and when
temperatures fall below 10°C it is severely retarded. The
crop is damaged by frost and this restricts its cultivation in
temperate regions to areas with a minimum frost free
period of 4 to 6 months (Onwueme 1978).
Like cassava, the storage organ of the sweet potato is a
root, although unlike cassava it can be used for vegetative
propagation (Figure 18.3). This is in contrast to potatoes,
which are tubers originated from the stem (Kays 1985).
Sweet potato is a perennial plant, but it is normally
grown as an annual. Under cultivation it is usually propa-
gated from vine cuttings, although in some countries such
as the USA it is common to obtain growing material from
roots. The growth occurs in three more or less distinct
phases. In the first phase fibrous roots grow extensively and
there is only moderate growth of vines. In the second phase
the vines grow, the leaf area increases and the growth of
storage roots is initiated. In the third phase the bulking of
the storage roots takes place (Kays 1985).
Sweet potato storage roots are ready to harvest after 4 to
5 months, but sometimes, depending on the cultivar, this can
be longer. The number of storage roots produced per plant
varies but is generally from three to ten roots (Woolfe 1992).
Sweet potato roots vary enormously in characteristics
among cultivars. The basic structure consists of starch stor-
ing parenchyma, an outer cortex, and the surface periderm.
The colour of the flesh can be uniform or multicoloured,
with colours ranging from white, cream, yellow, orange
and even purple. The colour of the skin is likewise very
variable; cream, orange, dark red.
Sweet potato root composition
Typical ranges in composition are given in Table 18.4.
Sweet potato is high in carbohydrates and can produce
more edible energy per hectare per day than wheat, rice or
cassava. The protein content (generally about 5% by dry
weight) is too low to provide a balanced diet on its own,
and is lower than found in staples such as potato, yam and
cereals, but higher than crops such as cassava and plantain.
More than 80% of the protein is the storage protein
sporamin (A and B) (Maeshima
et al
. 1985). Digestibility
of this protein by humans is not high, suggesting that there
is some component within sweet potato that affects protein
digestion (Woolfe 1992).
Some varieties with orange flesh are particularly high in
pro-vitamin A, and have been the subject of a range of
initiatives to combat vitamin A deficiency focused on
sub-Saharan Africa (Tomlins
et al
. 2010). While most sweet
