Genotypic variations in physiological deterioration of cassava (Manihot esculenta Crantz) storage roots under inland valley conditions

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Food, Agriculture & Environment Vol.1(2) : 108-111. 2003 www.world-food.net
Genotypic variations in physiological deterioration of cassava (Manihot esculenta Crantz)
storage roots under inland valley conditions
Indira J. Ekanayake* and Omar Lyasse
International Institute of Tropical Agriculture (IITA), c/o Lambourn & Co., Carolyn House, 26 Dingwall Road,
Croydon CR9 3EE, London. *e-mail: iekanayake@yahoo.com
Received 5 January 2003, accepted 18 April 2003.
Abstract
Cassava (Manihot esculenta Crantz) is an important starchy food crop in the inland valley ecosystem areas in West Africa where harvesting and marketing
infrastructure is not optimal and postharvest losses are high. An attempt was therefore made to assess the genotypic variability of physiological deterioration
(PD) of storage roots obtained from plants grown under inland valley field conditions. Cassava storage roots showed rapid PD within 48 hours after
harvesting due to wounding related discoloration. After 6-months field growth under hydro-morphic conditions during the dry season, roots were harvested
and the root PD scores obtained visually at 24 and 48 h after was compared among 60 genotypes. The differences in root PD reaction between these
genotypes were highly significant (P<0.001) at 24 h after harvest. Genotypic differences in root PD (at 24 h after) were expressed more in the distal than
the proximal region of roots. At 48 h after harvest, however the genotypic differences became apparent only at proximal ends of roots. Location of the field
with respect to water regime significantly (P<0.001) affected the root PD response 24 h after harvest. Rapid PD test was appropriate to differentiate the
genotypic differences 24 and 48 h after root harvest. This study provides an early visual scoring method on cassava root quality and thereby aids to detect
a key part of postharvest deterioration losses incurred.
Key words: Manihot esculenta Crantz, visual assays, wounding, physiological deterioration, water stress, inland valley.
Introduction
genotypes and methods to reduce post harvest losses has been
The storage roots of cassava (Manihot esculenta Crantz) provide
reported else where 4,10,11. This study was therefore conducted to
a cheap source of food in tropical Africa. Fresh cassava roots
examine the usefulness of a quick, cheap approach based on a vi-
consumed boiled or fresh are common in some parts of West
sual evaluation method to differentiate root PD at the early stage
Africa, Andean Latin America and Paraguay, and in ‘Kerala’ state
of preparation of the product ready for the market. To this end
of India 1,2. Subsistence farmers and rural consumers rely on cas-
genotypic variation of 60 cassava clones of the African Manihot
sava roots for food substitution notably in marginal lands and un-
genotypes grown under hydro-morphic field conditions in an in-
der adverse environmental conditions such as prolonged drought.
land valley was evaluated for its root PD response from harvest to
In West Africa, cassava is cultivated during the dry season, or af-
48 h after harvest.
ter a rice crop, in hydro-morphic or seasonally dry paddy soils 3,4.
Cassava cultivated in hydro-morphic soils are exposed to lack of
nutrients during the flooded period 5 causing in-ground deteriora-
Materials and Methods
tion of roots and a reduction of root quality 4 that further influ-
Field trials and design: Field experiments were conducted dur-
ence post harvest behavior of roots. Production and ecological
ing the dry season in a hydro-morphic soil at a research site lo-
aspects concerning changes in root quality traits such as cyano-
cated in the inland valley (IV) area at the International Institute of
genic potential in the cassava grown in hydro-morphic soils 6 or
Tropical Agriculture (IITA) farm, Ibadan, Nigeria (latitude 70030’N,
root rotting due to microbes 7 have also been described. Cassava
longitude 30054’E, altitude 210 m above sea level). Annual rain-
storage root deterioration is the result of two causal and linked
fall was 1253 mm, with a mean maximum temperature of 270C
processes: physiological and microbiological 8. The primary PD
and a mean minimum temperature of 210C. Soils were sandy loam
of cassava roots is physiological in nature and occurs when the
classified as Aeric Tropaquents. The experimental design was a
storage roots are exposed to air and when detached from mother
factorial design with four replications, where the four water re-
stem at harvest or after harvest, during washing to remove soil
gimes were based on the changes in seasonal water table depth: a)
and prepare for marketing. PD is characterized by a bluish-brown
dry at early (December – February) and late (March – June) sea-
or bluish-black discoloration of tissues due to leuco-anthocya-
sons (dry/dry), b) dry at early season and wet at late season (dry/
nin pigment production 9. This happens along the peripheral vas-
wet), c) wet at early season and dry at late season (wet/dry), and d)
cular bundles of roots, and spread over to adjacent root paren-
wet at both seasons (wet/wet). Water table depth was monitored
chyma, and is termed as “vascular streaking”. It is a humidity-sen-
during the crop season using piezometers installed at various depths
sitive wound response that depends upon the degree of mechani-
in each replication and each moisture level treatment. Preparing
cal damage encountered by roots 8. Reducing above described
high ridges, which thereafter dried due to natural drainage and
postharvest root quality losses associated with PD and increasing
evapo-transpiration, imposed moisture treatments. The rate of
market value of the roots can aid to improve the status of poor
drying depended upon slope and proximity to water table. As the
involved with cassava cultivation in inland valley ecosystem of
dry season progressed, with the gradual drying of ridged soil the
West African countries where cassava is a major income generat-
following ranking of plots by moisture stress was created: dry/
ing crop. Greater perishability of cassava roots in such stressful
environments (water stress) and the need to use better adapted
dry > dry/wet > wet/dry > wet/wet.
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a)
b)
c)
Figure 1. Frequency distribution (%) of genotypes with varying physiological deterioration (PD) scores of the roots a) at proximal, middle and distal ends of
roots evaluated at 24 h b) at proximal, middle and distal ends of roots evaluated at 48 h after harvesting, and c) mean root PD scores at 24h and 48 h after
harvest.
Table 1. Scoring system for evaluating the physiological deterioration
Plant culture: Sixty cassava genotypes that represented the West
(PD) reaction of harvested cassava storage roots.
African germplasm maintained at IITA composed of both farm-
ers’ landraces and some elite improved lines were used. Cassava
PD score
Qualitative
Percentage of vascular
stem cuttings were planted on 50 cm ridge of topsoil, at 1 m x
evaluation term
streaking and storage root
0.75-m spacing. Each treatment per replicated plot was a single
color change (%)
row of 10 plants. Cuttings were planted at the end of the previ-
1
Very low
0-19
ous rainy season and mature plants were harvested 6 months af-
2
Low
20-39
ter planting.
3
Intermediate
40-59
4
High
60-79
Tuberous root sampling and quality analysis protocols. Stand-
5
Very high
80-100
ard plant sampling procedure 12 was followed and care was taken
to separate roots at the plant base. All roots were placed in paper
bags and taken to laboratory for further analysis. Individual tu-
berous roots were cut to inflict wounding patterns similar to slic-
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Table 2. Genotypes identified with relatively low tuberous
Table 3. Mean physiological deterioration (PD) scores of cassava roots
root physiological deterioration (PD) responses 24 h and 48 h after
at 24 h and 48 h after harvest as influenced by water regime during the
harvest based on three positions (proximal, middle, and distal ends)
preharvest growing season.
of storage roots of cassava.
Field location
Proximal
Middle
Distal end Root section
Proximal end
Middle of root
Distal end
Root section
& water regime end of root
of root
of root
mean
of root
of root
average
24 h after harvest of roots
After 24 h
Genotypes with very low PD scores (1.51 - 1.75)
Dry/dry
2.7a
2.7a
2.7a
2.7a
90059
-
-
-
Dry/wet
2.8a
2.7a
2.6ab
2.6a
83672
-
-
-
Wet/dry
2.1c
2.1c
2.2c
2.1b
82/00249
-
-
-
Genotypes with low PD scores (1.76 – 2)
Wet/wet
2.4b
2.4b
2.4b
2.1b
M86/00009
M86/00009
83672
TME 1
Significance of
***
***
***
***
M85/00665
305555 P3-2
30555 P3-2
30001
F-statistic
M86/00106
M86/00106
M86/00106
M85/00665
After 48 h
M6298
M82/0249
M86/00009
81/00016
Dry/dry
2.9
3.1b
3.2
2.9c
82/00249
M6298
M85/00665
M86/00106
Dry/wet
2.8
3.3b
3.3
3.2b
087/00401
M85/00313
M85/00313
M85/00313
Wet/dry
2.5
3.9a
3.4
3.9a
M41627
87/00691
82/00422
81/00942
Wet/wet
2.8
3.2b
3.2
2.8c
81/00942
Bida Local
087/00401
087/00691
Significance of
ns
*
ns
*
-
M6298
087/00691
M86/00009
-
M41627
M41627
M41627
F-statistic
-
81/00942
M6298
-
Means in each column followed by the same letter were not significantly different at P<0.05 by DMRT
ns, * and *** denote non significance and significance at P<0.05 and P<0.001 levels, respectively.
-
-
M85/00313
-
-
-
81/00942
-
-
-
087/00691
-
ends rather than at distal ends; differences among genotypes were
48h after harvest of roots
significant but at a lower probability level (P<0.05). Postharvest
Genotypes with low PD scores (1.76 – 2)
losses at the farm gates are generally concentrated at proximal
82/00422
M85/00665
-
-
and distal root extremes, which are often discarded in the prepa-
M86/00080
-
-
-
ration of food 13. Genotypes evaluated 24 h after harvest was
90029
-
-
-
skewed to the right for PD scores (Fig 1a). The PD score distri-
bution pattern at 24 h was not affected by the root section (Fig.
ing and 2 cm root sections were selected from the proximal, mid-
1c). No genotypes with PD scores at the lower end (scores less
dle, and distal ends of each root. These sections were used for
than 2) were observed after 48 h. Genotype frequency distribu-
evaluation. Physiological deterioration (PD) evaluations were
tion was normal but with different distribution patterns at the sec-
made at 24 and 48 h after harvest. The root samples were stored
ond evaluation (Fig. 1b and c). More genotypes had PD scores
under ambient conditions (260C and 65% relative humidity and
between 2.5 and 3.75 irrespective of the root section. The me-
dian score for genotypic distribution at 24 hours which was 2.25
artificial light). PD of roots was evaluated using a visual score
moved upwards to a PD score of 2.5, 48 h after. Interestingly, few
(Table 1) modified from the original scale 13. Original evaluation
of the tested genotypes recorded lower PD scores 48 h after har-
procedures advocated for PD involves scoring 1.5-cm long root
vest (Table 2) suggesting the availability of some genotypes with
pieces after 3 days of storage, with a large number of replicates.
less deterioration that could be recommended for growing under
In our tests, for PD evaluations during storage, coefficient of vari-
inland valley conditions. Genotypes 90059, 8367, and 8/0049 had
ation was relatively high at 25 to 31.5% but was acceptable for the
very low scores at 24 h after harvest. While genotypes 82/00422,
objectives of this study. The use of a minimum of three tuberous
90029, M86/00080, and M85/00665 had lower scores 48 h after
roots per plant helped maintain CV at or below this level.
wounding. Genotype 90029 was the best in terms of maintaining
Statistical analysis: All data were analyzed using SAS package 14
low PD levels.
and means and variances were computed. Significance was deter-
Effect of preharvest water stress on PD. Location of the field
mined at P < 0.05. Frequency distribution of these genotypes by
with respect to water regime significantly (P<0.001) affected the
PD class was determined based on mean scores.
root PD response 24 h after harvest (Table 3). The location effect
was highly significant at all 3 root sections but the sectional ef-
fect changed after 48 h. Roots harvested from very wet soils and
Results and Discussion
wet to dry transition soils gave a delayed PD reaction 24 h after
Genotypic variability for cassava root PD. Differences in PD
harvest than those harvested from dry soils and soils with dry to
scores at 24 h after harvest were highly significant (P<0.001) for
wet seasonal transition. Root PD response was less in those har-
the 60 tested genotypes. Mean root deterioration score was 2.47
vested from wet soils after 48 h. Therefore PD response was
with 29.4% vascular streaking 24 h after harvest among all tested
clearly affected by preharvest environmental factors, such as flood-
genotypes. At this time the differences were mainly expressed at
ing or dry soils, and post harvest conditions such as length of stor-
the distal end of roots as compared to the proximal end of the
age period.The rapid screening test was useful in distinguishing
roots. Forty-eight hours after harvest, mean PD reached 55% and
genotypic differences for PD on the first day of harvest and wound-
the genotypic differences were more prominent at proximal root
ing and clearly detected the changes occurring as a response to
110
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wounding within the initial 24h period. After 48 h, the visual screen-
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Acknowledgement
The International Institute of Tropical Agriculture (IITA), Ibadan,
Nigeria, supported this research work. Authors are grateful to
A.G.O. Dixon, cassava breeder at IITA-Nigeria for providing the
source genotypes for this study.
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