Agriculture Reference
In-Depth Information
(b)
Corm
structure
Head
(a)
Periderm
Middle
Cortex
Meristematic layer
Tail
Cortex
Figure 18.4 Structure of the yam tuber. (Adapted from Diop and Calverley 1998, with permission from the FAO).
D. dumentorum (Kunth.) Pax. (trifoliate yam), D. esculenta
(Lour.) Burk (chinese yam) and D. rotundata Poir (white
yam). D. cayenensis and D. rotundata are sometimes
pooled into the D. cayenensis-rotundata complex due to
their morphological and molecular relatedness. Also of
importance are D. batatas Decne, D. bulbifera L (aerial
yam) and D. trifida Lam (cush-cush yam). Yams have
many varieties compared to other crops, with some having
slight morphological differences generally recognized only
by farmers. There is therefore much confusion in naming
the cultivated forms, which in most parts of the world have
never been properly scientifically classified.
Unlike other tropical root and tuber crops yams normally
produce a single large tuber each year, often weighing 2-10 kg
The tuber shape and size can vary greatly due to genetic and
environmental factors. However, cultivated forms of yam gen-
erally produce tubers that are more or less cylindrical in shape
and 3-5 kg in weight with a dry matter content ranging from
20-40%. The yam tuber grows from a corm-like structure at
the base of the vine. Occasionally this corm remains attached
to the tuber after harvest, in which case when sprouting occurs,
the sprouts will develop from it. More normally, when the
corm separates from the tuber, sprouting occurs from the tuber
near to the point at which the corm was attached.
The structure of the tuber is illustrated in Figure 18.4.
The outermost layer is a corky periderm which is a thick
Table 18.5 Tuber Composition for a Range of
Species of Yams.
% fresh weight
Moisture
content
Crude
Protein
Species
Carbohydrate
Fats
D. alata
65-73
22-29
0.1-0.3
1.1-2.8
D. rotunda
58-80
15-23
0.1-0.2
1.1-2.0
D. cayenensis
D. esculenta
67-81
17-25
0.1-0.3
1.3-1.9
D. bulbifera
63-67
27-33
0.1
1.1-1.5
Source: Information from Knoth (1993).
layer of cork cells, which although often cracked provides
a barrier against water loss and pathogen invasion.
Immediately below the periderm is the cortex, which is a
layer of thin-walled cells only a few millimetres thick, with
very little stored starch. Below this is a meristematic layer,
of elongated thin-walled cells. Sprouts are initiated from
this later. The central portion of the tuber is the paren-
chyma, composed of thick-walled starchy cells, with large
inter-cellular spaces, interspersed with vascular bundles
(Degras 1993, Diop & Calverley 1998).
The composition of the main yam species is summarised
in Table 18.5.
Search WWH ::




Custom Search