Genetic Variation of Postharvest Physiological Deterioration ...

Published October 19, 2015

Research

Genetic Variation of Postharvest Physiological Deterioration Susceptibility in a Cassava Germplasm Kehinde O. Moyib,* Jonathan Mkumbira, Oyeronke A. Odunola, Alfred G. Dixon, Malachy O. Akoroda and Peter Kulakow

ABSTRACT Postharvest physiological deterioration (PPD) is an unresolved major stress in cassava (Manihot esculenta Crantz), which constrains the crop suitability for commercial and industrial purposes. The present study sourced for PPD tolerance from different genetic resources available within a cassava germplasm. Six hundred and twelve cassava accessions within International Institute of Tropical Agriculture (IITA)-Ibadan germplasm were evaluated 2, 4, 6, and 8 days after harvest (DAH) for PPD and rate of PPD per day (PPD d –1) over the evaluation dates. Sixteen cassava genotypes that had mean PPD d –1 of 0.0% d –1 were classified as non-deteriorators (NoDs) with possible delayed-PPD trait and nine genotypes with mean PPD d –1  70% d –1 as extra-super fast deteriorators (xSPDs) with possible early-PPD trait. The PPD was more pronounced at the proximal end of a root than at the middle and distal regions. Polyploid genotypes showed higher PPD tolerance than diploids but local varieties were more susceptible to PPD than the improved varieties. The study suggests that polyploidy could be an additional genetic resource for PPD tolerance in cassava. In addition, the knowledge of PPD status of a cassava germplasm allows its proper utilization. The NoD genotypes could sustain storage and transport within 8 d for commercial and industrial purposes and xSPD genotypes could be immediately processed into fermented foods and dried chips. The results presented in this study are explanatory in nature and could be explored further for integration into improvement programs in cassava.

J. Mkumbira, A.G. Dixon, and P. Kulakow, Cassava Breeding Unit, International Institute of Tropical Agriculture, PMB 5320, Oyo Rd., Ibadan Oyo State Nigeria; O.A. Odunola, Dep. of Biochemistry, Univ. of Ibadan, Ibadan, Nigeria; K.O. Moyib, Dep. of Chemical Sciences, Tai Solarin Univ. of Education, PMB 2118, Ijebu-Ode, Ogun State Nigeria; M.O. Akoroda, Cocoa Research Institute of Nigeria (CRIN), Km4 Ibadan-Ijebu-Ode express Road, Idiayunre, Ibadan, Nigeria. Received 3 Nov. 2014. Accepted 26 June 2015. *Corresponding author ([email protected], [email protected]). Abbreviations: DAH, day after harvest; Dev, deviation; Dl, distal; Freq., frequency; FST, fast deteriorator; GGC, genetic gain collection; Gn, grand mean; IITA, International Institute of Tropical Agriculture; Md, middle; MFD, moderately fast deteriorator; MSD, moderately slow deteriorator; N, number of genotypes; NMD, normal deteriorator; NOD, non-deteriorators; PPD, postharvest physiological deterioration; PPDdahn, postharvest physiological deterioration at day after harvest; PPD d–1, rate of postharvest physiological deterioration per day; PPDDl, postharvest physiological deterioration at the distal; PPDgen, postharvest physiological deterioration of a genotype; PPDMd, postharvest physiological deterioration at the middle; PPDPr, postharvest physiological deterioration at the proximal; PPDroot, postharvest physiological deterioration of a root; Pr, proximal; SAS, statistical analysis system; SLD, slow deteriorator; SPD, super deteriorator; Stdzd mean, standardized mean; Temp, temperature; UYT, uniform yield trial; xFTD, extra-fast deteriorator; Xn, rank; xSPD, extra-super fast deteriorators.

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assava is a starchy staple of low tropics for over half a billion people in the world. It is the seventh most important crop in order of world production and the second most important source of starch after maize and consequently, serves as a food security crop (Rosenthal and Ort, 2012; Stapleton, 2012). Cassava possesses thickened and knobby stems that reflect its artificial selection for vegetative propagation by stem cuttings. Vegetative propagation

Published in Crop Sci. 55:2701–2711 (2015). doi: 10.2135/cropsci2014.11.0749 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved.

crop science, vol. 55, november– december 2015

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helps to preserve clones with desirable characteristics and increase yields in cassava. It is also used to multiply superior clones generated from heterotic volunteer seedlings to avoid inbreeding depression and maintain genetic diversity (Rabbi et al., 2014). Some key characteristics of cassava that encourage its cultivation among African farmers are (i) it can grow and produce quality yields in soils where cereals and other food crops will not grow or produce well and (ii) it can tolerate drought and responds well to irrigation or higher rainfall conditions (Nweke et al., 2002). The IITA has a collection of more than 600 elite cassava clones with different characteristics accumulated over the last 40 yr. The genetic gain collection (GGC) was borne out of IITA-Ibadan’s response program to early challenges of cassava diseases and pests, which was launched in the early 1970s (IITA, 1976–1996; Okechukwu and Dixon, 2008). The IITA developed cassava mosaic disease (CMD)-resistant tropical manioc selection (TMS) varieties from a cross of Ceara rubber and a cassava hybrid, 58308. They also developed high yielding CMD-resistant varieties by crossing the generated CMDresistant varieties with many other high yielding varieties from West Africa and Brazil (IITA, 1976–1996). The continuous sourcing and selection for heterotic seedlings and superior clones for more important agronomic traits such as good root quality and stability of production led to an enriched GGC of IITA in the year 2000 (Okechukwu and Dixon, 2008). Also, included in the enriched GGC are the polyploid cassava clones that were generated using chemical-induced mutation to overcome fertility barrier encountered in their diploids and perhaps to enhance heterosis. Further development of more superior improved clones for increased protein, good starch quality, high  carotene, early maturity, and combined with accumulation of historical materials from diverse exotic sources led to a richer and broader GGC in the year 2005. The introduction and adoption of TMS varieties have provided food for about 50 million people in Nigeria and offered new markets to African farmers by the non-staple food industries (Okechukwu and Dixon, 2008). Cassava is presently a multipurpose crop with various end uses, as animal feed, food, feedstock for biofuel and starch industries (FAO, 1995; Rosenthal and Ort, 2012) and more recently, for bio-sorption of heavy metals in aqueous solution (unpublished data, 2014). However, commercial production advantages of cassava are offset by the rapid deterioration of its roots known as PPD, which can begin as quickly as 24 h after harvest and renders the roots unpalatable within 72 h after harvest (Reilly et al., 2007). The symptoms of PPD appear during the first 3 DAH as a blue–black discoloration of the xylem vessels, which later turns brown in the form of vascular streak known as vascular streaking. The initiation and subsequent degree of deterioration are reported 2702

to be closely related to the presence of mechanical damage that is unavoidable during harvest and transport of roots (Wheatley et al., 1985; Iyer et al., 2010). In village settings, African farmers practice periodic harvesting as would be required by immediate uses to avoid loss of roots to deterioration. Harvest and processing may be delayed until market or when other conditions are more favorable. However, PPD places serious constraints on cassava’s suitability for modern production, processing, and marketing and therefore has a negative impact on all levels of income generation from the crop (Wenham, 1995). Some postharvest management practices such as the immersion of roots in hot water, storage in a low-oxygen environment or an atmosphere of CO2, and covering with waxes were developed to overcome this shortcoming in cassava marketing. However, these practices were considered to be either technically or economically unsuitable for most marketing needs (FAO, 1995). Therefore, many research studies were directed towards the use of biotechnology tools to elucidate and provide solutions to the problem of PPD. The PPD has been linked to wound defense response of which the wound healing stage (for sealing of the wound) is impaired in detached cassava in contrast to other root crops such as yam (Dioscorea spp.) and potato (Solanum tuberosum L.) (Uritani, 1999). Many genes and gene products generated in the cascades of wound responses during PPD have been identified and characterized (Reilly et al., 2001, 2003, 2007). More recently, Iyer et al. (2010) reported a possible functional signaling role of acetone in PPD and Zidenga (2011) showed that oxidative burst in ruptured cassava is induced by cyanogens and accumulation of reactive oxygen species (ROS) is blocked by expression of Arabidopsis alternative oxidases gene (AOX1A) which resulted into 2 to 3 wk delayed of PPD. Considering the fact that numerous research studies have been performed on possible biochemical pathways leading to early PPD but its actual primary signaling events (except those of wounding) and candidate genes are yet to be discovered. Conventional breeding has been successfully used in cassava to increase yield and disease resistance (Jennings and Iglesias, 2002) but there are issues such as the multigenic nature of PPD, heterozygous nature of cassava, positive relationship between PPD and dry matter content that discourage breeding for PPD. However, damages and losses by PPD are many and cannot be overlooked. According to Rickard et al. (1992 cited by FAO 1995), PPD reduces income at all levels of market system with increasing number of days, which is very high at about 3 DAH. Therefore, PPD is a problem to be dealt with to increase the food security and the purchasing power of cassava growers and traders of which women’s participation is enormous. Reilly et al. (2003) suggested that screening the existing genetic diversity with respect to PPD within primary, secondary, and tertiary gene pools has the potential to

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provide useful information that is relevant to genetic modulation of PPD. Cassava researchers are therefore currently sourcing for PPD tolerance from diverse genetic resources. Despite the enriched nature of the IITA GGC, the status of its PPD is yet to be revealed as additional information to its database. Furthermore, knowledge of PPD status of any cassava germplasm allows its proper management and utilization, either as food or for commercial purposes, after harvest to maximize its genetic gain. In addition, more roles are expected of cassava growers with increasing population that will translate into more demands for cassava to satisfy its various end uses (Rosenthal and Ort, 2012). Therefore, research directed towards introducing resistance to PPD or delaying PPD response should be a priority to any cassava producing or utilizing nation and the world at large so as to maximize cassava viability as a food security and commercial crop. The present study was therefore designed to evaluate, screen, and identify genotypes with possible delayed PPD within IITA cassava germplasm.

MATERIALS AND METHODS Plant Materials The present study was conducted in 2006/2007 and 2007/2008 at the IITA, Ibadan, Nigeria, with an annual rainfall of 1305 mm; altitude 210 m; mean annual temperature of 20 to 34oC; situated at 7°31 N; 3o45 E with ferric Luvisol soil. The GGC of IITA cassava germplasm accumulated over 35 yr was selected for the present study, The collection comprises of about 597 elite cassava clones that are representing newly developed improved clones with diverse desirable agronomical traits such as high yielding combined with resistance to diseases and pests, high -carotene (yellow root), increased protein, good starch quality, and leaf retention ability (stay green) and historical landraces. In addition, eight uniform yield trials (UYTs) under various specific breeding objectives were also included in the study. Many clones in the UYTs were also part of the GGC. Two common checks, TMSI30572 and TMEB1 were used for all the trials and the GGC (except Eastern and Southern Africa Regional Center [ESARC]) while some trials had additional controls. Overall, a total of 612 cassava clones were selected for the study (Supplemental Table S1). Supplemental Table S2 lists the names of the eight UYTs with their respective number of clones and checks. Ridges of 1 m apart, 30 cm high, and 6 m long were made after harrowing. One decimeter length of fresh and healthy matured cassava stems of 12-mo old containing between five and seven nodes were planted at the beginning of Ibadan breeding seasons (May 2006/2007 and 2007/2008) when rain had stabilized for enough soil moisture and easy to till. The experimental design was a complete randomized block design with three replications for National Root Crop Program (NRCP) clones, four replications for the rest of the UYTs and two replications for GGC. The cassava cuttings were planted vertically at an angle, about 60°C to horizontal with two-thirds of the cuttings in the soil. Each plot consisted of 36 plants of each genotype in a 6 by 6 layout with spacing of 1 m between plants. Pre-emergence herbicide as recommended by the manufacturer (1% gramozone at 4 L ha–1) and hand weeding were used to control weeds. crop science, vol. 55, november– december 2015

Evaluation of Postharvest Physiological Deterioration and Rate of Postharvest Physiological Deterioration per Day Data were collected 12 mo after planting on per plot basis from inner 16 plants in 2007 and 2008 at the beginning of Ibadan raining season when the soil is softened enough (early April) to till. The cassava plants were harvested manually by cutting the stem a few centimeters above the ground and dug around the ridge using shovel to lose up the soil around the tuberous roots and then, pulled the stub of the stem to lift out the roots. Thereafter, remnant soils were shaken off the roots and the roots were immediately separated from the parent plants manually and meticulously to limit root’s bruises. Four healthy roots with no damage or bruise at harvest were selected from bulk per plant per replicate and kept at one side of the field. The PPD was evaluated 2, 4, 6, and 8 DAH using modified streaking-quantification method of Wheatley (1982). Roots were left Intact and kept under an opened shed on the field to allow natural deterioration as obtainable on farms. At the four evaluation dates, roots with intact ends were cut into three parts (proximal, Pr; middle, Md; and distal, Dl) and quantified for visual spread of streaking at each part (PPDPr/Md/ ) based on the proportion area of the root covered with streakDl ing (no streaking = 0.0; few streaking covering about a radius = 0.25; moderate streaking covering about a diameter = 0.5; streaking covering about three-quarter of root surface = 0.75; and full streaking covering entire root surface = 1). The average score of streaking over the three parts of a root multiplied by 100 was taken as PPD of the root at different DAHs. The rate of PPD per day (PPD d–1) for a root was estimated over the four evaluation dates and that of a cassava clone is the average obtained among the replicates of the same plant. The formulae equation for each of the collected parameters is as given below: PPDdahn = Average (PPDPr, PPDMd, PPDDI) ´ 100, where n = 2, 4, 6, or 8 PPDroot = Average (PPDdah2, PPDdah4, PPDdah6, PPDday8) PPDgen = Average (PPDrootn) where n = 1, 2, 3 or 4 (replicates) PPD d -1 = ([

å PPD

dahn

´ DAH n )] /[

å DAH

n

]) ,

where n = 2, 4, 6, 8 PPDMd, PPD at the middle; PPDPr, PPD at the proximal; PPDDl, PPD at the distal; PPDdahn, PPD at day after harvest (where n = 2, 4, 6, and 8 d) in percent; PPDroot, PPD of a root as average PPDdahn over the four dates in percent; PPDgen, PPD of a genotype as average PPDroot overall its replicates in percent; PPD d–1, rate of deterioration per day in percent for a root or genotype in percent per day (% d–1).

Data Analysis Cassava clones with completely full missing data were excluded from the statistical analysis. Therefore, a total of 595 clones were evaluated (Supplemental Table S1). The estimated PPD and

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PPD d–1 data were subjected to descriptive statistics and analysis of variance for effects of season, individual roots, genotypes parts, and length of day after harvest on PPD using statistical system of analysis (SAS Institute, 2002) software package.

Determination of Cassava Genotypes with Possible Delayed and Early Postharvest Physiological Deterioration The PPD and PPD d–1 data were also subjected to rank-sum analysis in SAS as described by Okechukwu and Dixon (2008) but with modifications to facilitate descriptions of PPD susceptibility among the cassava accessions. The syntax used for the generation of the PPD response classes based on rank-sum procedure is as follows: “if stdmean  –3 then class = NoD; else if ( > –3 stdmean  –1.2) then class = SLD; else if ( > –1.2 stdmean < 0.0) then class = MSD; else if (  0.0 stdmean  1.2) then class = NMD; else if (  1.2 stdmean < 2.4) then class = MFD; else if ( > 2.4  3) then class = FTD; else if ( > 3 stdmean  3.2), class xFTD; if ( > 3.2 stdmean < 3.4), class is SPD; and else if stdmean is  3 then class is xSPD” (where, NoD is non-deteriorator; SLD, slow deteriorator; MSD, moderately slow deteriorator; NMD, normal deteriorator; MFD, moderately fast deteriorator; FST, fast deteriorator; xFTD, extra-fast deteriorator; SPD, super deteriorator, and xSPD, extra-super deteriorator).

RESULTS AND DISCUSSION Variation in Reaction to Postharvest Physiological Deterioration and Postharvest Physiological Deterioration per Day within and among Parts of Cassava Roots A total of 2020 roots among 595 cassava genotypes were evaluated for deterioration at parts (Pr, Md, and Dl) 2, 4, 6, and 8 DAH. The descriptive statistics results showed that deterioration at each part among the roots ranged from 0.0 to 1.0 at each evaluation period. The obtained values indicate observation of no vascular streaking to moderate streaking that is covering as little as less than a radius to full streaking covering the entire surface of the roots. Table 1 presents a summary of the descriptive statistics of PPD values at different evaluation periods for the three parts. It is observed that PPD increases at parts as length of evaluation period increases from 2 to 8 DAH and with highest values at the proximal ends as shown in Fig. 1. Table 1 also provides summary of the ANOVA within and among parts at 2, 4, 6, and 8 DAH for PPD. Higher significant variations were observed within parts (P < 0.001) than among parts (P < 0.01). These effects were largely contributed by individual root and genotype. Genotypes showed higher significant variations at the Pr ends but with no significant difference between Md and Dl regions (Table 1). Therefore, Pr contributed immensely to high genotypic differences observed in PPD as earlier reported (Ekanayake and Lyasse, 2003; Salcedo et al., 2010). The highest deterioration observed at the proximal could be attributed to the unavoidable detachment of root at proximity to the parent stock, which caused wounding 2704

that initiated early PPD at the proximal (Wheatley, 1982; Reilly et al., 2001; Morante et al., 2010). On the other hand, Ekanayake and Lyasse (2003) observed more deterioration at the Dl ends. However, in support of these diverse reports, Wheatley et al. (1985) hypothesized that the Pr and Dl regions of cassava roots are more likely to suffer mechanical damage during harvest and are therefore, more prone to PPD than the middle.

Variation in Reaction of Roots and Genotypes to Postharvest Physiological Deterioration and Postharvest Physiological Deterioration per Day Postharvest physiological deterioration among roots at each evaluation date ranged from 0.00 to 100% with a mean PPDdahn of 14.0 ± 0.55, 23.0 ± 0.76, 32.0 ± 0.89, and 47.0 ± 1.04% for 2, 4, 6, and 8 DAH, respectively and an overall mean PPDroot of 28.7 ± 0.35%. The PPD d–1 among the roots at each storage date also ranged between 0.00 and 100% d–1 with a mean of 32.2 ± 0.38% d–1. The estimated PPD for genotypes (N = 595) over the four evaluation dates ranged between 0.00% and 80.2% with a mean of 22.7 ± 0.93% while PPD d–1 ranged from 0.00 to 82.1% d–1 with a mean of 25.7 ± 1.1% d–1 (Supplemental Table S1). These results indicate that no genotype had either an absolute mean PPD or PPD d–1 of 100% but roots and even parts from the same plant had PPD scores range of 100% (García et al., 2013; Chávez et al., 2005). This observation is pointing towards the high contribution of parts and individual roots to the genotypic differences of PPD in cassava. The present study also revealed the number of tolerant genotypes with zero susceptibility at each DAH as follows, PPDdah2 (370), PPDdah4 (227), PPDdah6 (151), PPDdah8 (105); and PPD d–1 (66), and thereby, supports an inverse relationship between length of storage and number of tolerant genotypes (Wheatley et al., 1985; Chávez et al., 2005; Salcedo et al., 2010). The present range and mean values of PPD among IITA-Ibadan cassava accessions are within earlier reported values but with a higher degree of susceptibility (Chávez et al., 2005; García et al., 2013; Morante et al., 2010; Salcedo et al., 2010; Wheatley and Gomez, 1985). However, the highest degree of tolerance observed so far was reported by Morante et al. (2010) for two amylose-free waxy starch mutants and a high carotene genotype that showed zero susceptibility till 40 d. This level of tolerance was possible because the assessed mutants were specifically developed from diverse exotic sources for PPD tolerance.

Identification of Genotypes for Possible Delayed and Early Postharvest Physiological Deterioration Trait Research efforts over the last decade have been geared towards sourcing for genotypes with delayed PPD in cassava. Some earlier reports have used categorical classification

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Table 1. Summary of descriptive statistics and analysis of variance for postharvest physiological deterioration (PPD) within and among root parts (proximal, middle, and distal) evaluated at different days after harvest (DAH) among 595 International Institute of Tropical Agriculture (IITA) cassava accessions assessed in 2007 and 2008. Analysis of variance Precision measures DAH 2

4

6

8

Grand

Part

Mean†

SE ±

CV

Proximal Middle Distal %PPDdah2 Proximal Middle Distal %PPDdah4 Proximal Middle Distal %PPDdah6 Proximal Middle Distal %PPDdah8 Mean Pr Mean Md Mean Dl % PPDroot PPD d –1

0.15 0.13 0.14 13.92 0.24 0.23 0.22 23.11 0.33 0.32 0.32 32.09 0.49 0.45 0.46 46.74 0.30 0.28 0.28 28.69 32.20

0.01 0.01 0.01 0.55 0.01 0.01 0.01 0.76 0.01 0.01 0.01 0.89 0.01 0.01 0.01 1.04 0.01 0.01 0.01 0.35 0.38

189.47 201.66 202.72 188.84 141.63 146.68 148.48 136.69 109.92 112.67 112.87 103.85 84.04 86.96 86.78 79.53 89.37 92.49 90/53 87.17 102.50

Max. 1 1 1 100 1 1 1 100 1 1 1 100 1 1 1 100 1 1 1 81.25 86.39

Source of Variation within part

Within parts

Among parts

P>F

P>F

P>F

SS %

P>F

SS %

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

– – – ** – – – ** – – – ns – – – *** – – – *** ***

*** ns‡ ns *** *** ** ** *** *** ns ns *** *** ns ns *** *** *** *** *** ***

66.2 27.3 26.2 258,499 125.1 57.6 55.5 513,635 116.6 62.4 63.09 538,210 134.4 60.1 70.8 548,051 31.41 30.5 30.6 861,333 951,325

*** *** *** *** *** *** *** *** *** *** *** *** *** * *** *** *** *** *** *** ***

4.7 2.4 1.9 20,311 8.2 2.7 2.6 30,228 10.1 2.6 2.9 31,986 7.5 1.3 2.8 23,653 4.57 2.51 3.05 109,518 74,391.2

Genotype

Replicate

* Significant at the 0.05 probability level. ** Significant at the 0.01 probability level. *** Significant at the 0.001 probability level. †

PPD at part was based on a score range of 0 to 1 while the average over the three sections in percent was taken as PPD at DAHn (PPDdahn, where n = 2, 4, 6, 8). Mean PPD over the four evaluation dates is taken as PPD for an individual root (PPDroot) in percent, and PPD d –1 is rate of PPD over time for root, expressed in percent per day.

‡

ns, nonsignificant at the 0.05 probability level.

Figure 1. Postharvest physiological deterioration (PPD) responses at parts for different day after harvest among 595 International Institute of Tropical Agriculture (IITA) cassava accessions assessed in 2007 and 2008. The PPD increased with increasing length of day after harvest and deterioration was more pronounced at the proximal than either the middle or distal end of the root. Equation y = 11.93x gives the linear relationship between PPD (y) and DAH (x), which was explained by a coefficient of multiple determination (R2) of 0.97. The mean PPDdah at each day after harvest is given as Xn where n = 2, 4, 6, or 8 d. Pr, proximal, Md, middle and Dl, distal end of cassava, respectively.

to describe different reactions of cassava to PPD susceptibility for easy description. For examples, five levels of PPD responses that ranged from very low to very high and five reaction types (RT 1–RT 5) have been described in cassava (Ekanayake and Lyasse, 2003; Morante et al., 2010). To crop science, vol. 55, november– december 2015

facilitate the description of PPD susceptibility among the presently evaluated cassava accessions, rank-sum analytical tool in SAS (SAS Institute, 2002) was used to rank the genotypes based on the deviation of their mean PPD/day from the average value. The Rank-sum procedure generated 10 classes

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Table 2. Precision measures and rank-sum standardized mean and grading limit for the nine classes of postharvest physiological deterioration (PPD) responses generated among 595 International Institute of Tropical Agriculture (IITA) cassava genotypes. Precision measures No. 1 2 3 4 5 6 7 8 9

Rank-sum‡

PPD class†

Mean PPD d –1

SD

SE ±

CV

Range

Min.

Max.

NoD SLD MSD NMD MFD FTD xFTD SPD xSPD

% d –1 0 4.92 14.62 24.22 33.81 45.73 53.97 63.19 75.26

0 1.60 1.15 1.38 1.68 2.83 2.44 2.73 4.70

0 0.19 0.11 0.13 0.17 0.42 0.42 0.56 1.41

0 32 7.86 5.71 4.97 6.18 4.44 4.32 6.24

0 8.34 9.44 9.86 10.0 9.45 9.84 8.08 12.36

0.0 0.83 10.0 19.58 29.58 39.58 49.58 59.63 69.72

0.0 9.17 19.44 29.44 39.44 49.03 59.42 67.71 82.08

Standardized PPD d –1 mean range grading limit  –3 > –3  –1.2 > –1.2 < 0.0  0.0  1.2  1.2 < 2.4 > 2.4  3 > 3  3.2 > 3.2 < 3.4 3.4

0.00–0.0 >1.0–9.0 10.0–19.0 20.0–29.0 30.0–39.0 40.0–49.0 50.0–59.0 60.0–69.0 70

†

NoD, non-deteriorator; SLD, slow deteriorator; MSD, moderate deteriorator; NMD, normal deteriorator; MFD, moderately fast deteriorator; FTD, fast deteriorator; xFTD, extra-fast deteriorator; SPD, superfast deteriorator, xSPD, extra-superfast deteriorator.

‡

Classes were generated based on standardized mean range for PPD d –1 and a range of 9.0% d –1 was used to modify classes generated by rank-sum procedure in SAS for easy presentation.

Figure 2. Distribution of the 595 International Institute of Tropical Agriculture (IITA) cassava accessions into nine different classes of postharvest physiological deterioration (PPD) responses as assessed in 2007 and 2008. The distribution of the genotypes was asymmetry towards the fast deterioratiors with a concentration at the slow deteriorators. NoD, non-deteriorator; SLD, slow deteriorator; MSD, moderate deteriorator; NMD, normal deteriorator; MFD, moderately fast deteriorator; FTD, fast deteriorator; xFTD, extra-fast deteriorator; SPD, superfast deteriorator, xSPD, extra-superfast deteriorator.

among the 595 genotypes. For easy presentation, the generated PPD classes were modified by using a grading limit of 9.0% d–1 in between classes (except for Non-Deteriorator, NoD class) and the 10th and ninth classes were merged into one for reasonable grading. Therefore, a total of nine PPD susceptibility classes were presented as shown in Table 2. Genotypes with many missing data were referred to as pseudo-members (Supplemental Table S1). The present study generated higher number of classes due to a higher degree of PPD susceptibility among the assessed genotypes (0.0 to 82.0%, N = 595) compared to Morante et al. (2010) report of 0.0 to 48.8% among 21 potential PPD-tolerant genotypes. The distribution of the assessed cassava genotypes was asymmetric towards the fast deteriorating classes with higher concentration of genotypes in the slow 2706

deteriorating classes (Fig. 2) as earlier reported (Ekanayake and Lyasse, 2003; Chávez et al., 2005). One hundred and seven (18.0%) cassava genotypes were classified as normal deteriorators (NMD). Sixty-nine genotypes were in the NoD class, out of which 16 were identified as genotypes with possible delayed-PPD trait. These 16 genotypes had a variance of 0.0, an average of 0.0% d–1 across and overall the four evaluation dates, complete replicates and no missing value. Genotypes in the slow deteriorating class (SLD = 11.6%) had PPD/day of 0.0% d–1 at least two evaluation dates. One hundred and twenty-three genotypes were in the fast deteriorating classes with a range of 40.0 to 82.0% d–1 and were observed to have a large variance in their PPD values. Thirteen genotypes among these classes had mean PPD/day of  70% d–1 and were

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Table 3. Postharvest physiological deterioration at different days after harvest (PPDDAH) and summary of rank-sum statistics in cassava genotypes selected for possible delayed and early PPD trait among 595 International Institute of Tropical Agriculture (IITA) cassava accessions. PPD DAH No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Genotype TMSI 010758 010903 011797 011818 9100438 9100455 9100089(3x) 930614(3x) 930639(3x) 930739(3x) 930832(4x) I000049 I000061 I000070 J920093 Z930151 011097 961672 996012 010046 961708 010265 9102312 940020 010045 010171 990554 010014 000388 O8400275¶ I960610¶ 92B0068 I960529¶ M980115 MM96JW1 MM972480 TME778 920325 TME785 Z950098 Mean PPDgen SE (±)

Rank-sum statistics

Freq†

2

4

6

8

PPD

PPD d –1

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 18 24 18 18 12 18 18 12 18 15 30 24 15 3 3 15 3 15 36 12 30 15 12 6

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.4 0.3 0.3 0.4 0.5 0.4 0.5 0.1 0.5 0.3 0.4 0.4 0.6 0.8 0 0.3 0.7 0.6 0.3 0.7 0.3 0.5 0.3 0.8 9.1 0.01

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.4 0.6 0.7 0.4 0.2 0.5 0.6 0.8 0.6 0.7 0.6 0.6 0.7 0.7 0.8 0.6 0.8 0.7 0.7 0.5 0.8 0.6 0.9 0.8 17.8 0.01

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0.7 0.7 0.8 0.5 0.7 0.6 0.5 0.6 0.7 0.6 0.4 0.7 0.8 0.8 0.8 0.8 0.7 0.7 0.9 0.9 0.9 0.9 0.9 25.8 0.01

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0.7 0.8 0.7 1.0 0.7 0.9 0.9 0.8 0.8 0.9 0.9 0.7 0.7 0.8 0.9 . 0.8 0.9 0.8 0.8 0.9 0.9 0.8 36.9 0.01

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 53.5 55.5 53.1 55.2 54.7 58.0 60.6 56.8 61.1 60.0 60.8 60.9 67.9 70.8 58.3 63.1 72.2 70.0 66.6 71.9 67.8 73.2 72.9 80.2 22.7 0.93

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 60.3 61.1 61.3 61.5 62.9 63.3 63.4 65.8 66.8 67.2 67.3 67.7 69.7 70 70.8 72.1 73.6 73.8 75.0 75.4 75.5 81.5 81.9 82.1 25.7 1.1

rank Xn 570 570 570 570 570 570 570 570 570 570 570 570 570 570 570 570 34 33 31.5 30 28 27 26 20.5 18 17 16 15 14 13 12 11 11 8 7 6 5 4 2.5 1 297.6 7.22

Dev stdmean‡ (Xn– Gn) (Dev/SD) 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 267.5 –268.5 –269.5 –271 –272.5 –274.5 –275.5 –276.5 –282 –284.5 –285.5 –286.5 –287.5 –288.5 –290.0 –291.0 –291.5 –293.5 –294.5 –295.5 –296.5 –297.5 –298.5 –300 –301.5 –4.89 7.22

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 –3 –3 –3.2 –3.2 –3.2 –3.2 –3.2 –3.2 –3.2 –3.2 –3.2 –3.2 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –3.4 –0.06 0.08

Class§ NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD NoD SPD SPD SPD SPD SPD SPD SPD SPD SPD SPD SPD SPD xSPD xSPD xSPD xSPD xSPD xSPD xSPD xSPD xSPD xSPD xSPD xSPD

†

There is variation in the number of replicates assessed among genotypes for some genotypes were present in more than one collection and some were used as checks in all the collections and replicate(s) with missing score were deleted.

‡

Xn, rank based on PPD d –1; Gn, the grand mean of the rank; dev, deviation of the rank from grand mean (Xn–Gn); SD, standard deviation of rank, and stdmean (standardized mean), deviation of rank over standard deviation as obtained in SAS (SAS Institute, 2002).

§

NoD, non-deteriorator were genotypes identified with possible delayed-PPD trait that could sustain storage for 8 d while extra-superfast deteriorator (xSPD), were genotypes with early PPD that could be channeled into immediate processing as fermented foods.

¶

Virtual-member, due to incomplete number of replicates

grouped as extra-super fast deteriorators (xSPD) that are highly prone to PPD but three of them had incomplete replicates. These extra-super fast deteriorators had a PPD range of 30 to 80% at 2 DAH, 50 to 100% at 4 DAH and 70 to 90% at both 6 and 8 DAH. The nine of them with complete replicates were regarded as the genotypes with crop science, vol. 55, november– december 2015

possible early-PPD trait. The 16 NoD genotypes represent the highest number of PPD-tolerant genotypes identified, so far, and the nine xSPD represent the worst clones for PPD tolerance. The summary of rank-sum statistics for the identified cassava clones with possible delayed and early PPD traits were presented in Table 3.

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Table 4. Mean level of postharvest physiological deterioration (PPD) and its rate over time (PPD d –1) at different parts, seasons, bio-status, and ploidy levels among 595 International Institute of Tropical Agriculture (IITA) cassava genotypes. PPD at different days after harvest N

†

Class

Part 1 Proximal 2 Middle 3 Distal Season 1 2007 2 2008 Bio-status 1 Improved 2 Landrace Ploidy level A 1 Diploid 2 Triploid 3 Tetraploid Ploidy level B 1 Diploid 2 Polyploid

†

Freq

2

4

6

8

2016 2020 2020

0.15 0.13 0.13

0.24 0.23 0.22

0.33 0.32 0.32

0.49 0.45 0.46

2400 3656

24.07 6.28

33.74 13.91

45.14 21.22

5718 338

13.62 22.11

22.54 31.22

5669 141 246

14.78 5.00 1.84

5669 387 Mean SD SE CV

14.78 3.10 13.92 26.29 0.64 188.84

PPD

PPD d –1

%

% d –1

30.0 28.0 28.0

34.0 31.0 31.0

64.92 30.43

41.39 18.34

47.48 20.29

32.09 38.93

46.08 57.93

28.12 35.44

31.88 39.16

24.04 2.78 9.19

33.78 11.93 12.94

48.50 20.60 14.58

29.71 9.37 9.25

33.57 11.83 11.20

24.04 6.64 23.11 31.64 0.77 136.87

33.78 12.54 32.09 33.87 0.85 105.55

48.50 16.84 46.74 37.49 0.99 80.2

29.71 9.30 28.69 25.03 0.67 85.02

33.57 11.45 32.2 27.81 0.38 86.39

N, number of genotypes; Freq, frequency.

A major obstacle in the use of cassava as a commercial crop is the transporting the harvested roots over a distance (Morante et al., 2010). Therefore, those genotypes in the NoD class could sustain storage and transport till 8 DAH for industrial and commercial uses while those in the xSPD could be directed into immediate processing as fermented foods, chips and starch within 24 h for home consumption.

Bio-status, Ploidy Level, and Seasonal Differences for Postharvest Physiological Deterioration and Postharvest Physiological Deterioration per Day and their Correlations The assessed cassava germplasm comprises many improved genotypes, few landraces, and some polyploid clones and was evaluated for variation of PPD and PPD/day at parts based on these diverse statuses at two breeding seasons. Levels of PPD at different evaluation dates and PPD/day for these diverse statuses are provided in Table 4. Overall, landraces were more susceptible to PPD (39.16 ± 2.07% d–1, N = 25) than the improved varieties (31.88 ± 0.38% d–1, N = 575), which is similar to the report of Wheatley and Gomez (1985) that the roots produced by a local cultivar were more susceptible to deterioration than the roots of the improved and hybrid cultivars. Noticeably, all the selected 16 genotypes for possible delayed PPD traits in the present study are improved varieties. However, there are exceptions in the present observations, two landraces, TMEB9 (OLEKANGA) and Ofege had a mean of 0.0% d–1 but with a missing score at 8 DAH, which led to their discard from 2708

the list of the 16 genotypes with possible early-PPD trait (Supplemental Table S1). In addition, low PPD d–1 values were also observed for some landraces such as Atu (3.3% d–1) and Obasanjo (4.16% d–1), which indicate the potential of landraces as resources for PPD tolerance, as earlier explored for cassava mosaic disease resistance (Akano et al., 2002). However, such opportunity may be rare for PPD tolerance using landraces due to their performance that is noticed to be environmental dependent (Egesi et al., 2007). Based on ploidy levels, diploids have higher PPD d–1 than polyploids in the following decreasing order, diploid (33.57 ± 0.38% d–1, N = 531) > triploid (11.83 ± 1.39% d–1, N = 25) > tetraploid (11.2 ± 1.09% d–1, N = 39) as observed in Table 4, signifying an inverse relationship between PPP and ploidy level. Table 5 enables comparison of PPD at ploidy level and indicates higher PPD tolerance in polypoids than their corresponding diploids. Triploids were excluded in this table due to the presence of many missing data in their corresponding diploids. Five triploids and a tetraploid were among the selected 16 genotypes with possible delayed-PPD trait, despite their low frequency among the 595 cassava accessions evaluated. The higher PPD tolerance observed in polypoids could be explained by changes in physiology and increased genetic buffering provided by their extra genome copies (Hegarty and Hiscock, 2007). Other reported features of polyploids are enhanced vigor, well-developed organs and tissues, higher tolerance to nutrient stress, enhanced resistance to environmental stresses and presence of novel phenotypes

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Table 5. Postharvest physiological deterioration at different day after harvest (PPDDAH) and its rate over time (PPD d –1) among diploids and their corresponding tetraploids† with their respective PPD class. PPD DAH N

‡

Genotype TMSI

Freq

2

4

6

8

PPD%

PPD d –1

PPD class§

12 12 12 12 18 12 54 12 63 12 12 6 12 12 12 12 18 12 18 12 27 6 54 12 18 12 15 12 6

0.00 0.17 0.21 0.38 0.25 0.00 0.18 0.00 0.18 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.13 0.00 0.39 0.00 0.06 0.00 0.43 0.00 0.14 0.00 0.17 0.00 0.00

0.38 0.17 0.38 0.38 0.60 0.00 0.37 0.00 0.25 0.00 0.08 0.00 0.21 0.00 0.25 0.17 0.07 0.25 0.50 0.29 0.41 0.25 0.49 0.00 0.38 0.21 0.29 0.00 0.00

0.54 0.00 0.71 0.38 0.65 0.10 0.41 0.75 0.43 0.00 0.21 0.00 0.04 0.00 0.33 0.00 0.53 0.00 0.69 0.25 0.23 0.25 0.55 0.13 0.45 0.00 0.00 0.25 0.00

0.42 0.04 0.04 0.42 0.64 0.00 0.72 0.00 0.70 0.46 0.58 0.00 0.46 0.21 0.75 0.00 0.63 0.13 0.63 0.04 0.61 0.00 0.69 0.33 0.68 0.42 0.33 0.33 0.25

32.29 9.38 33.33 38.54 49.31 2.60 40.66 18.75 37.76 11.46 14.58 0.00 17.71 5.21 34.72 4.17 31.37 9.38 55.21 14.58 30.63 12.50 52.51 11.46 43.40 15.63 19.91 10.42 6.25

%d 41.25 6.67 32.50 39.17 54.86 3.13 46.40 22.50 45.34 18.33 19.58 0.00 23.75 8.33 40.00 3.33 40.28 10.00 59.72 15.00 37.22 12.50 56.60 17.08 51.81 20.83 19.44 14.17 10.00

FTD SLD MFD MFD xFTD SLD FTD NMD FTD MSD NMD NoD NMD SLD FTD SLD FTD MSD SPD MSD MFD MSD xFTD MSD xFTD NMD MSD MSD MSD

6 6 6 90 6

0.00 0.00 0.00 0.17 0.00

0.00 0.00 0.00 0.19 0.00

0.00 0.08 0.00 0.31 0.00

0.00 0.00 0.00 0.58 0.00

0.00 2.78 0.00 29.63 0.00

0.00 4.17 0.00 32.62 0.00

NoD SLD NoD MFD NoD

–1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

30001 30001(4x) 30555 30555(4x) 91934 91934(4x) 4(2)1425 4(2)1425(4x) 8200058 8200058(4x) 9101730 9101730(4x) ¶ 9102316 9102316(4x) 9102325 9102325(4x) 9102327 9102327(4x) 920057 920057(4x) 920067 920067(4x) 920326 920326(4x) 920342 920342(4x) 920427 920427(4x) TMEB117 TMEB117(4X)§

31 32 33 34

OBASANJO OBASANJO(4X) TMEB1 TMEB1(4X)

†

There was lack of corresponding diploids for the assessed triploids and so were not included in the table.

‡

N, number of genotypes; Freq, frequency.

§

FTD, fast deteriorator; SLD, slow deteriorator; MFD, moderately fast deteriorator; xFTD, extra-fast deteriorator; NMD, normal deteriorator; MSD, moderate deteriorator; NoD, non-deteriorator; SPD, superfast deteriorator.

¶

Not among the selected 16 NoD genotypes due to some missing scores in replicates.

absent in diploids (Fawcett et al., 2009). However, the findings of Caruso (2010) showed that not all morphoanatomical characters respond to increasing changes in chromosome number but most responses to polyploidization are species or genotype dependent. Table 4 indicates that breeding season two (2007/2008) had lower mean PPD d–1 (20.29% d–1) than breeding season one (2006/2007) (47.48% d–1), which signifies higher PPD susceptibility at breeding season one. Season showed a high variation for both PPD and PPD d–1 (Table 6), which corroborates some earlier reports on evaluation of PPD under diverse environments, weather conditions, and seasons in crop science, vol. 55, november– december 2015

cassava (Ekanayake and Lyasse, 2003; Reilly et al., 2003; Chávez et al., 2005). Table 6 shows a high interaction of season ´ genotype for PPD and therefore, any breeding objectives for PPD tolerance should take environmental conditions into consideration. Pearson correlation analysis was used to observe relationships among pairs of bio-status, ploidy level, season, part and PPD and PPD d–1. The results revealed that PPD at each evaluation date and PPD d–1 had weak significant relationships with part but showed inverse relationships with bio-status, ploidy level, and breeding season (Table 6). The positive relationship obtained between PPD and part (in

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Table 6. Analysis of variance for postharvest physiological deterioration (PPD) and its rate over time (PPD d –1) as affected by replicate, genotype, season, part, ploidy level, bio-status and their pair wise relationships based on Pearson Correlations. Analysis of variance PPD d , % d –1

PPD

–1

df

SS

Mean square

2920 2531

3457903 758727.5

1184.2 299.8

R2

CV

Root MSE

Mean

0.82

53.8

17.3

32.2

Type III SS

Mean square

Source Model Error

Source

df

Replicate 3 Genotype 610 Breeding season 1 Part 2 Ploidy level 2 Rep ´ Genotype 710 124 Genotype ´ Season Bio-status

74097.9 24699.3 956890 1568.672 205074.9 205074.9 4903.952 2451.976 6270 3135 739764.8 1041.922 204753.5 1651.238

1

7342

F value

P>F

3.95

F

Source

df

82.39 5.23 684.1 8.18 6.99 3.48 5.51