Agriculture Reference
In-Depth Information
2002, 2006). Low dry matter content is associated with
longer shelf life and more efficient healing at low humidity.
In order to understand the basis for this association, van
Oirschot
et al
. (2002) carried out a screening of a wide
range of sweet potato germplasm originating from many
regions of the world. Cultivars from different regions tended
to cluster both by wound-healing efficiency and dry matter
content. Thus African cultivars tended to be both poor
healers and have high dry matter content. Evidence has
been obtained suggesting that there is a direct link between
carbohydrate levels (sugar and starch) and sweet potato
tissue response to water stress, which thus affects wound
healing efficiency (Rees
et al
. 2008). However, there appear
to be additional cultivar factors controlling wound-healing
efficiency and more work is needed if it is to be possible to
breed for better storing cultivars.
plant growth regulators has not been studied as extensively
as potato, but appears to have certain similarities. Thus
sprout growth is stimulated by gibberellin and inhibited by
continuous ethylene (Cheema
et al
. 2010).
Sprout production is an important economic consideration
in commercial sweet potato cropping. At planting time, vig-
orous and plentiful sprout production is required to minimise
the cost of propagation material. However, sprout growth
decreases the quality and value of roots for fresh market
sales. Sprouting is generally controlled by manipulating the
temperature and humidity under which the crop is stored.
Sprout suppression would be useful for produce in transit
as export to the northern hemisphere requires shipping
through equatorial conditions that promote sprouting.
Control of sprout production in sweet potatoes has been
examined using a number of treatments. For sweet potato
roots, isopropyl N-(3-chlorophenyl) carbamate (CIPC)
(Kushman & Wrights 1969), gamma irradiation (Bonsi &
Loretan 1988), naphthalene acetic acid (Paton & Scriven
1989) all suppressed sprouting to some degree.
Respiration and chilling injury in sweet potato
During long-term storage, respiration rates can be a good
indication of storage life, and respiration can contribute
significantly to weight loss in sweet potato roots. High rates
of metabolism, can be detrimental to quality, by changing
the carbohydrate composition, or in the extreme case, by
metabolising so much starch that air spaces form, and the
texture of the root becomes spongy. Most work on carbohy-
drate metabolism and respiration has been carried out on
North American or Japanese cultivars under the temperature
regimes used in refrigerated stores (typically 13-15°C)
(e.g. Woolfe 1992 and references therein, Takahata
et al
.
1992). These have generally shown that sugar levels increase
during storage (Woolfe 1992). However, the metabolic rate
is temperature dependent, and the cultivars can vary
significantly in their metabolic characteristics (e.g. Ahn
et
al
. 1980), so that there is a need to determine how cultivars
behave under tropical conditions. Storage roots are more
sensitive to temperature than the rest of the plant. Whereas
the Q10 (increase in respiration rate over 10 degrees) of
whole plant is 1.6, that of storage roots is 2.5 (Kays 1985).
Sweet potato roots are sensitive to chilling injury below
about 12.5°C. Symptoms of damage include flesh discol-
ouration, internal breakdown, increased decay, off-flavours,
hard core when cooked (Wang 1990).
BOTANY AND PHYSIOLOGY OF YAM
The yams are members of the genus
Dioscorea
in the sec-
tion Enantiophyllum. They belong to the monocotyledonous
plant class Liliposida, the subclass Liliidae that comprise
the orders Orchidales, Pandanales, Liliales and
Dioscoreales (Ayensu & Coursey 1972). Members of the
genus
Dioscorea
have an underground tuber which is an
annually renewed organ as in the case of all edible yams, or
may be perennial, becoming larger and progressively more
lignified from year to year to form a rhizome. In certain
species, bulbils (aerial tubers) are formed in the leaf axils.
The plants are dioecious with white, green, or red flowers
arranged in clusters or spikes. The aerial stem may be
smooth, thorny or hairy and may be round or square in section
with alternate or opposite leaves which are usually heart
shaped and may be smooth or hairy. Many of the cultivated
forms have become sterile as a result of centuries of
vegetative propagation. In the southern United States the
name yam is used for sweet potato (
Ipomea batatas
,
L. Poir) and in other places the edible tubers of the aroids
but more generally and in this chapter the term yam is
confined to plants of the genus
Dioscorea
.
Yams are predominately grown in three regions; West
Africa, South America and Asia. About 600 species has
been identified world-wide (Burkill 1960) of which only
20 are consumed with eight specifically regarded as a
source of staple carbohydrate and medicinal compounds
(Degras 1993). Pre-eminent among these are:
D. alata
L.
(water yam),
D. cayenensis
Lam. (yellow or guinea yam),
The control of sprouting of sweet potato roots
The sweet potato storage root can be used for vegetative
reproduction, and thus it will sprout easily. There appears to
be no dormancy period, so that harvested roots can gener-
ally be induced to sprout by being placed under appropriate
conditions (20°C and above, and high humidity). There is no
evidence of preformed eyes, as found in potato. The role of
