Agriculture Reference
In-Depth Information
(Beeching 2001, Han
et al
. 2001, Huang
et al
. 2001, Reilly
et al
. 2001, Taylor
et al
. 2001a.)
Beeching and co-workers postulate that in common with
many stress defence pathways in plant tissues (Scott
et al
.
1999) reactive oxygen species are central to the PPD
response. Thus PPD is associated with peaks of reactive
oxygen species and increased activity of enzymes that
modulate reactive oxygen species. (Reilly
et al
. 2001, 2003)
as well as the accumulation of secondary metabolites, some
of which show antioxidant properties, and the altered
regulation of genes related to the modulation of reactive
oxygen stress. This is consistent with the observations that
decreasing oxygen levels by controlled atmospheres,
modified atmospheres or waxing, all slow the PPD response.
It has also been observed that the content of the carotenoids,
which have antioxidant activity, above a certain threshold
(5 mg/Kg fresh weight) reduces PPD response (Chavez
et al
. 2000, Sanchez
et al
. 2006). Both β-carotene and
ascorbate levels decrease during PPD.
It is known that cassava cultivars differ in their
susceptibility to PPD, opening the possibility of breeding
for reduced PPD either through conventional methods or
by transformation. This is a major objective for the Cassava
Biotechnology Network, organised through CIAT. It has
been shown that there is a genetic contribution, (PPD is a
complex multigenic trait), but that there is also a strong
environmental interaction (Cortes
et al
. 2002). Molecular
information can provide valuable information and tools
such as quantitative trait loci (QTL) mapping and
marker-assisted selection could assist and accelerate
breeding programmes. Several transformation systems are
now available and have been applied for research purposes
(e.g. Taylor
et al
. 2001b).
The biochemical events during deterioration in a range of
cassava varieties showing differential deterioration responses
have been studied, as well as the phenomenon that cassava
shows less PPD when plants are pruned a few days before
harvesting (van Oirschot
et al
. 2000). Low PPD by variety
and due to pruning have been shown to be associated with
increased catalase activity. Less susceptible cultivars or
pruned plants may efficiently utilise catalase to scavenge
H
2
O
2
produced after wounding, resulting in the formation of
O
2
and H
2
O (Reilly
et al
. 2003). Where catalase levels are
relatively lower and peroxidase levels relatively higher, a
significant proportion of H
2
O
2
scavenging could occur via
peroxidase mediated reactions requiring the participation of
cellular components including scopoletin, as an electron
donor. Thus relatively higher levels of peroxidase would
lead to increased oxidation of scopoletin observed as
vascular streaking. In support of this hypothesis, pruned
cassava with lower PPD show higher levels of the MecCAT1
transcript suggesting higher catalase levels.
Wound-healing of cassava roots
Cassava roots are able to wound-heal, but less efficiently
than the other tropical roots crops. At relative humidities of
around 80% to 90% periderm formation occurs in seven to
nine days at 35°C and 10 to 14 days at 25°C (Rickard 1985).
Cyanogenic compounds
Cassava contains cyanogenic glucosides, which together
with their breakdown products (cyanohydrins and free
HCN) formed during processing, can cause health problems.
Acute intoxication, manifested as vomiting, dizziness or
even death can occur under rare conditions. Such poisoning
occurs when food shortage and social instability induce
shortcuts in established processing methods or when high
cyanogen varieties are introduced into an area lacking
appropriate processing techniques (Bokanga
et al
. 1994). It
is well established that thiocyanate resulting from dietary
cyanide exposure can aggravate iodine exposure deficiency
expressed as goitre and cretinism (Bokanga
et al
. 1994).
There is also strong evidence for a causal link between
cyanide and the paralytic disease konzo (Tylleskar 1994)
and tropical ataxic neuropathy (Osuntakun 1994).
The cyanogenic glucosides present in fresh cassava
roots are linamarin (93%) and lotaustralin (7%) (Nartey
1978). These are hydrolysed to the corresponding ketone
(cyanohydrin) and glucose by the endogenous enzyme,
linamarase, when cellular damage occurs (de Bruijn 1973,
Nartey 1978). Cyanohydrins breakdown non-enzymically
at a rate dependent upon pH and temperature (Cooke 1978)
with their stability increasing at acidic pH values. The
removal of cyanogens from cassava during processing is
achieved through this chain of reactions. The development
of an assay method for the different cyanogenic compounds
(Cooke 1978, modified by O'Brien
et al
. 1991) allowed
scientists to start to understand the mechanisms of
cyanogen reduction during processing. (See section on
cassava processing).
BOTANY AND PHYSIOLOGY OF SWEET
POTATO
Sweet potato is a dicotyledonous plant belonging to the
Convulvulaceae family. A large number of sweet potato
cultivars exists, with a variability that is greater than for
most other tropical root crops. Cultivars have arisen through
systematic breeding as well as natural hybridization and
mutations. In East Africa alone, 2000 clones have been
identified (Jana 1982).
