Agriculture Reference
In-Depth Information
D. cayenensis
, which originates from the West African
forest zone where the dry season is very short, shows
almost continuous vegetative growth. In contrast,
D. alata
and
D. rotundata
, originating respectively in Asia and
Africa, appear to be adapted to climates where there is a
longer dry season during which the plant survives as a
resting tuber.
Lower storage temperatures reduce the metabolic
activity of roots and tubers and prolong their dormancy.
Temperatures of 16° to 17°C have been used to prolong the
storage period for
D. alata
tubers for up to four months,
provided the tubers were properly cured prior to storage in
order to control infection by wound pathogens.
Sprout suppressants have almost all proved to be quite
ineffective on yams (Diop & Calverley 1998). This is
probably because yam tubers are unusual amongst plants
propagating vegetatively in not having pre-formed buds.
Many sprout suppressants, such as Isopropyl N-(3-
chlorophenyl) carbamate (CIPC) which is used extensively
on potato, affect the growing meristematic cells of the
sprouting loci. In yam sprout initials are formed only just
before the end of dormancy and then rise from beneath the
periderm. When a sprout inhibitor is applied on the yam
tuber just after harvest there are no sprout loci on which
the chemical can act. Once sprouting has been started, the
application of sprout suppressant may then inhibit further
growth of the sprout initials. Ile
et al
. (2006) have defined
three phases of dormancy, and postulate that chemicals
may only be effective once the meristem starts to form
foliar primordia; phase II by their definition. A number of
studies have looked at the concentrations of endogenous
plant growth regulators (PGRs) during the phases of dor-
mancy and the effects of exogenous PGRs (Ile
et al
. 2006).
Chloroethanol and thiourea can shorten dormancy, while
gibberellins prolong dormancy (e.g. Nnodu & Alozie
1992), (which is contrary to their effect on potato where
they promote sprouting). Tschannen (2003) carried out a
detailed study of Ga3 effects on
D. alta
and
D. cayenensis-
rotundata
. He showed that if applied immediately after
harvest Ga3 would prolong dormancy and also slow respi-
ration at the point of sprouting. It was postulated that its
action is actually to promote multiple sprouts over the
whole surface of the tuber. Tschannen devised treatment
methods using soil paste or gelatinised starch instead of
dipping. It is not clear whether this method is used practi-
cally as a strategy to extend dormancy at this time Attempts
to induce sprouting by the use of gibberellin inhibitors
(uniconazole-P, prohexadione-Calcium) has had some suc-
cess when inhibitors are applied as a foliar treatment prior
to harvest (Shiwachi
et al
. 2006).
STORAGE, HANDLING AND PROCESSING
OF CASSAVA
Cassava storage and handling
As described in previous sections cassava roots are very
perishable, as they are prone to post-harvest physiological
deterioration (PPD). Fresh cassava is most important in
regions of developing countries where resources are very
limited, so that in most cases appropriate handling and
storage methods must require few inputs. In-ground storage
where the crop is just left in the ground unharvested until
needed is one of the most practical storage methods used in
developing regions. This works particularly well for
cassava as it suffers from relatively few pests and diseases.
There is, however, a loss in quality as the roots lose starch
through metabolism and tend to become progressively
more fibrous after their optimum harvest date (Lancaster &
Coursey 1984). There is also the obvious disadvantage that
land is taken up by this strategy.
Traditional methods of post-harvest storage have been
reviewed by a number of authors such as Knoth (1993) and
Westby (2002). Methods include burying roots in the soil,
which is claimed to keep roots for months. In West Africa
and India harvested roots may be stored in heaps which are
kept moist by daily watering. Coating with loam paste is
said to extend shelf life by 4-6 days. Trench silos are claimed
to be the most successful traditional method of storage
(Westby 2002). For all these methods the quality of the roots
after harvest is critical. As most rots develop from the stem
end of the root, some people recommend leaving 2-3 cm of
stem attached (Diop & Calverley 1998). Pre-harvest pruning
(cutting of above ground stems) three weeks before harvest
has been shown to reduce harvest damage, although the
effect is not great (van Oirschot
et al
. 2000).
All the traditional methods of storage involve maintaining
a high humidity, suggesting that environments conducive to
wound-healing are important. This is confirmed by work
that has been carried out by the Centro Internacional de
Agricultura Tropical (CIAT) and the Natural Resources
Institute (NRI) since the 1980s to develop improved storage
methods. With appropriate handling, even in the absence of
temperature control it is possible to keep cassava for up to
4-8 weeks. This requires very careful harvesting, sorting to
remove damaged roots and a storage environment with high
humidity. Where roots are able to wound-heal, it has been
shown that this slows or stops the development of PPD
(Wenham 1995). This is confirmed by work in India where
it has been shown that cassava can be stored for two months
with less than 20% losses using pits with sand or soil at 15%
moisture content (Balagopalan 2000).
