6. Chapter Six: Genotype by environment interaction and yield stability of sweetpotato
6.2 Materials and Methods
6.2.1 Plant materials
A total of 45 clones were included in the present study. Thirty two were newly bred and candidate clones (Table 6.1). The candidate clones were selected based on their relatively high storage root yields and dry matter content of storage roots and drought tolerance.
Twelve parental clones used in developing the new clones and a control variety ‘Mugange’
were included in the study. The description of the plant materials is given in Table 6.1.
143 Table 6.1: Description of sweetpotato clones and varieties used in this study based on previous study
ID Pedigree/Name Origin Key traits Skin
color
Flesh color 21 4-160 x 2005-020 Newly bred High yield and drought tolerance Purple Cream
22 4-160 x 2005-020 Newly bred Drought tolerance Cream White
37 Kwezikumwe x 2005-020 Newly bred Drought tolerant Cream Cream
42 Kwezikumwe x 2005-020 Newly bred High DMC and drought tolerance Pink White 46 Kwezikumwe x 2005-020 Newly bred High yield and drought tolerance Cream Yellow
62 Otada 24 x 2005-020 Newly bred Drought tolerant Purple Yellow
81 Purple 4419 x 2005-020 Newly bred High yield and DMC and drought tolerance Cream Cream 88 Purple 4419 x 2005-020 Newly bred High yield and drought tolerance Cream Cream
103 SPK004 x 2005-020 Newly bred Drought tolerant Cream Cream
137 K513261 x 2005-034 Newly bred High yield and DMC and drought tolerance Purple White 210 8-1038 x 4-160 Newly bred High yield and DMC and drought tolerance Pink White 249 2005-034 x 8-1038 Newly bred High yield and DMC and drought tolerance Cream White 259 2005-034 x 8-1038 Newly bred High yield and DMC and drought tolerance Cream Orange
321 SPK004 x 8-1038 Newly bred High DMC Purple White
358 Ukerewe x 8-1038 Newly bred High DMC and drought tolerance Purple White 381 2005-034 x K513261 Newly bred High DMC and drought tolerance White Yellow 442 Purple 4419 x K513261 Newly bred High DMC and drought tolerance Pink Orange
455 SPK004 x K513261 Newly bred High DMC and drought tolerance Pink White
456 SPK004 x K513261 Newly bred High DMC and drought tolerance Purple Yellow 460 Ukerewe x K513261 Newly bred High DMC and drought tolerance Purple Cream 488 Nsasagatebo x Kwezikumwe Newly bred High DMC and drought tolerance Cream Yellow
509 Ukerewe x Kwezikumwe Newly bred High DMC White Cream
577 Nsasagatebo x Otada 24 Newly bred High DMC and drought tolerance Cream Cream 613 Otada 24 x Purple 4419 Newly bred High DMC and drought tolerance White White
639 K513261 x SPK004 Newlybred High DMC and drought tolerance Purple Yellow
641 K513261 x SPK004 Newly bred High DMC and drought tolerance White Orange
661 2005-020 x Ukerewe Newly bred High DMC and drought tolerance Cream Purple
700 8-1038 x Ukerewe Newly bred High DMC and drought tolerance Pink White
721 Otada 24 x Ukerewe Newly bred High DMC and drought tolerance Pink Yellow
733 Purple 4419 x Ukerewe Newly bred High DMC Cream Yellow
744 Purple 4419 x Ukerewe Newly bred High DMC Purple Yellow
746 Purple 4419 x Ukerewe Newly bred High yield Cream Yellow
P1 2005-020 NARO High yield White White
P2 2005-034 NARO High DMC White Orange
P3 2005-110 NARO High DMC Yellow Yellow
P4 4-160 ISAR Drought tolerant White White
P5 8-1038 ISAR Drought tolerant Red White
P6 K513261 IITA High yield Red White
P7 Kwezikumwe ISAR High yield Yellow Yellow
Control Mugange Local variety Adopted and adapted Red White
144
ID Pedigree/Name Origin Key traits Skin
color
Flesh color
P8 Nsasagatebo Landrace Drought tolerant White White
P9 Otada 24 NARO High yield Red White
P10 Purple 4419 ISAR Drought tolerant Red Orange
P11 SPK004 KARI High DMC Pink Orange
P12 Ukerewe CIP High DMC Red Orange
ID: clone identification number, P1, P2 … P12 represent parents such as Parent 1 (P1), DMC: Dry matter content, ISAR: Institut des Sciences Agronomiques du Rwanda, KARI: Kenya Agriculture Research Institute, NARO: National Agricultural Research Organisation/Uganda, CIP: International Potato Center.
6.2.2 Description of the study sites
The study was conducted in two growing seasons (season A: September 2014 - February 2015 and season B: March - August 2015) each at Karama, Masoro and Rubona Research Stations of the Rwanda Agriculture Board (RAB), respectively, providing a total of six environments (Table 6.2). The study sites represent the major sweetpotato growing agro- ecology in Rwanda. Agro-climatic and geographic descriptions of the study sites are presented in Table 6.2. In general, soil, climatic, and biological conditions of the study sites vary considerably. Season A is the main growing season with extended and heavy rainfalls, while Season B is the short growing season with reduced rainfall.
Table 6.2: Description of Karama, Masoro and Rubona research sites of the Rwanda Agriculture Board where the present study was conducted
Environment
code Site Season
Geographic position
Annual rainfall‡
(mm)
Temperature (oC)
Soil type Longitude Latitude Altitude
†(m.a.s.l.) Min. Max.
E1 Karama A
(September 2014 - February
2015)
E030o16’06.2’’ S02o16’46.5’’ 1330 567.9 17.2 28.4 Sandy and clay soils E2 Masoro E030o10’04.0’’ S01o55’40.0’’ 1482 722.4 15.7 27.1 Clay and kaolin soils E3 Rubona E029o45’58.2’’ S02o29’03.2’’ 1673 804.3 13.4 26.9 Clay and kaolin soils
E4 Karama
B (March - August
2015)
_ _ _ 351.8 22.1 32.2 _
E5 Masoro _ _ _ 407.3 19.5 31.4 _
E6 Rubona _ _ _ 461.7 17.6 28.5 _
† (m.a.s.l.): meters above sea level, ‡: Rainfall during growing season.
145 6.2.3 Experimental design and field establishment
Clones were established using a 5×9 alpha-lattice experimental design with three replications at each location. Vine cuttings of five nodes were prepared for field planting.
Cuttings were planted on ridges with inter-row spacing of 80 cm and intra-row spacing of 50 cm. The experimental plot consists of three rows consisting of 30 plants. Experimental plots were bordered by growing two rows of a sweetpotato variety Ukerewe. Weeding was carried out as required and no fertilisers and pesticides were applied. Harvesting was carried out 135 days after planting.
6.2.4 Data collection
The following data were collected: weight of storage root, weight of vine and dry matter content of storage root. Data on yield and yield components were recorded on the inner row of experimental plots. Six plants were harvested and the weight of storage roots and vines were determined using a field balance. The recorded yields were converted to t ha-1. The dry matter content was determined after modifying the methods described by Carey and Reynoso (1996). Fresh root samples were collected from five healthy, big roots from each plant within the harvested plot. About 100 g of fresh weight were excised on each root and a composite of 500 g were prepared in which 100g were sampled and kept in a paper bag prior to drying. Samples were dried in an oven at 70oC for 72 hours. Dried samples were weighed with a sensitive balance and the dry matter content was determined using the formula: Dry matter content (DM) expressed in % = ((Dry weight/Fresh weight) x 100).
6.2.5 Data analysis
6.2.5.1 Analysis of variance
A combined analysis of variance across seasons and sites was carried out to identify the effects of genotypes, sites, seasons and their interactions on yield of storage root and vines, total biomass and dry matter content of storage root using GenStat 14th edition (Payne et al.
2011). The clones were treated as fixed factor, while environments (both spatial and temporal), replications within environments and blocks within replications were random factors. The additive main effects and multiplicative interaction (AMMI) and genotype and
146 genotype x environment (GGE) biplot models were computed sequentially to analyze G x E interaction and yield stability of genotypes. The AMMI model as formulated by Gauch and Zobel (1996) is:
= + + +
Σ
+ + ,Where : the observed mean yield of genotype i in environment j, : the grand mean, : the genotype main effect, : the environment main effect, : the eigenvalues of the interaction principal component analysis (IPCA), and : the genotype and environment scores for the IPCA axis, : the interaction residual, N: the number of IPCA retained in the model and : the random error term. The ANOVA and ranking of best performing sweetpotato clones in each environment were performed with AMMI. Further, AMMI’s stability value (ASV) was determined to rank tested sweetpotato clones based on their stability using the formulae of Purchase (1997):
ASV=
Where, SS: sum of squares; IPCA1: interaction principal component analysis axis 1, IPCA2:
interaction principal component analysis axis 2.
6.2.5.2 GGE biplot
The GGE biplot analysis allows to identify the relationship among sweetpotato clones and the environments. The variations caused by genotype and genotype x environment interactions were explored using GGE biplot based on the principal component analysis (PCA) of environment-centred data (Yan et al. 2000; Yan and Kang 2002). The GGE biplot was analysed using Genstat 14th edition (Payne et al. 2011). The basic model for a GGE biplot as described by Yan et al. (2001) is:
Yij= bj + bjαi +λlζilηjl+Ɛij,
Where Yij: Average yield of i genotype in the environment j, bj: the average yield of all genotypes in environment j, αi: the main effect of genotype i, λn: the singular value for principal component PCn, ζil and ηjl: scores for genotype i and environment j on PCn, respectively, and Ɛij: the residual associated with i genotype and j environment. The which- won-where polygon view pattern of GGE biplot was used to identify high yielding clones in a
147 specific environment and mega-environments in tested environments (Yan et al. 2000; Yan and Kang 2002). The GGE biplots based on average environment coordination (AEC) (Yan and Kang 2002) was used to determine the discriminating power of each test environment.