Agriculture Reference
In-Depth Information
POST-HARVEST PHYSIOLOGY OF FRESH
ROOT CROPS
Although cassava, sweet potato and yams are all
subterranean storage organs which accumulate starch, they
have different botanical origins. A yam is a tuber, but
although the harvested part of cassava and sweet potato are
also often referred to as tubers, they are in fact lateral roots
in which starch has accumulated.
Harvested root crops are living parts of the plant that
continue to metabolise and respire after harvest. Cassava
roots are used by the plant to store energy while sweet
potato roots and the yam tuber not only act as an energy
store, but are also reproductive organs. Despite their
agronomic advantages over grains, which are the other
main staple food crops, root crops are far more perishable.
Out of the ground, and at ambient temperatures these root
crops have shelf lives that range from a couple of days for
cassava (Wenham 1995), two to four weeks for sweet
potato, to between four and 18 weeks for the natural
dormancy of yams (Knoth 1993). There are two main
approaches to overcoming this problem of perishability:
the breeding of varieties with longer shelf-lives; and the
use of improved storage techniques to optimise storage
environment. Breeding is a long-term strategy, whereas
improved storage is likely to have a more immediate
impact, although in this case the extent of the improvement
will still be limited by the roots inherent perishability.
The ability of plant tissues to heal wounds is important
to prevent excessive water loss and pathogen invasion.
Thus the ability of roots and tubers to wound heal has
important implications for shelf-life, and is usually
exploited by a process termed curing, in which they are
placed in an environment to promote healing of wounds
incurred during harvesting and handling. The ability of
each of the major root crops to wound-heal will be
discussed within the individual sections below.
Figure 18.1 Cassava roots being transported by
truck in West Africa.
Periderm
Cortex
Peelings
Starchy flesh,
parenchyma
Central vascular tissue
Figure 18.2 Cross section of cassava root. (Adapted
from Diop and Calverley 1998, with permission from
the FAO.)
germplasm collections are maintained by the International
Center for Tropical Agriculture (CIAT) in Columbia and
the Empresa Brasileira de Pesquis a Agropecuaria
(EMBRAPA), Brazil.
The harvested cassava roots are generally adventitious
roots that have developed into storage roots, although if
propagated from seed, the tap root can also become a stor-
age root. Unlike sweet potato, the cassava root is a 'true'
root rather than a tuberous root, and as such cannot be used
for vegetative propagation (Alves 2002).
The root structure (Figure 18.2) consists of the outer
periderm, which is a few cells thick and constitutes about
3% of the total root weight. Underneath this is the cortex,
which consists of the sclerenchyma, cortical parenchyma
BOTANY AND PHYSIOLOGY OF CASSAVA
Cassava is known by many names, including tapioca, man-
ioc, mandioca and yuca. Although the genus Manihot has
about one hundred species, Manihot esculenta is the only
one cultivated (Alves 2002) (Figure 18.1). It is grown in
tropical and sub-tropical regions. There are two plant
types; erect and spreading. Harvest time ranges from six to
24 months, and roots can be left in the ground until needed,
making cassava a very useful food security crop (Cardosa &
Souza 1999). As a perennial shrub, the plant can grow
indefinitely, alternating vegetative growth with periods of
carbohydrate accumulation and near dormancy at times of
stress. Cassava germplasm is very variable; the largest
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