The AMMI analyses of Variance
A combined analysis of variance for marketable tuber yield of the 13 tested potato genotypes across 6 environments is presented in Table 3. The AMMI analysis of variance [additive main effects] showed that genotype, environment, and their GEI significantly affected marketable tuber yield. From the total variance, the environment (E)main effect alone accounted for 40.2%, whereas genotype (G) and GEI effects accounted 33.8% and 24.3%, respectively. The result indicated that the environmental main effect was the predominant source of variation, followed by the Genotype and GEI effect. A previous report on potato also indicated that the environmental effect accounted for the largest part of the total variation [17, 18]. The amount of variance contributed by genotype by environment interaction indicates that there was a noticeable G × E interaction effect in these potato genotypes’ multi-environment data, leading to substantial differences in genotypic responses across the test environments.
Table 3
AMMI analysis of variance of marketable tuber yield of 13 potato genotypes
Source | Df | Sum of squares (SS) | Mean square (MS) | % SS explained | Proportion % |
ENV | 5 | 3930.7 | 785.6** | 40.2 | |
REP(ENV) | 12 | 173.2 | 14.49ns | 1.7 | |
GEN | 12 | 3304.1 | 275.36** | 33.8 | |
GEN*ENV | 60 | 2373.5 | 39.56** | 24.3 | |
PC1 | 16 | 1180.9 | 73.81** | | 49.8 |
PC2 | 14 | 700.3 | 50.02** | | 29.5 |
PC3 | 12 | 297.1 | 24.76 | | 12.5 |
PC4 | 10 | 137.8 | 13.78 | | 5.8 |
PC5 | 8 | 57.3 | 7.17 | | 2.4 |
Residuals | 144 | 1861.9 | 12.93 | | |
Total | 293 | 14017.8 | 47.84 | | |
R2 = 84% | CV = 13.95 | G. Mean = 25.75 | |
ENV = Environment, REP = Replication, GEN = Genotype, PC = principal component |
AMMI multiplicative component further partitioned the GEI into five interaction principal component axes (IPCAs). The first two principal components cumulatively explained a larger portion of the variation and contribute more to the accurate prediction of the genotype by environment interaction than the remaining PCAs which does not help in the accurate prediction rather they may contribute to noise [23]. Furthermore, the first two PCA axes showed a significant contribution to the GEI in the AMMI model. Therefore, the dataset obtained from the interaction of 13 potato genotypes tested at 6 environments was best predicted by the first two PCAs.
GGE Biplot analysis
Ranking of Genotypes Based on Yield and Stability Performance
The yield adaptability and stability of the potato genotypes were visually assessed using the GGE biplot analysis. The biplot created from the two PCAs, PCA1 plotted on the x-axis and PCA2 on the y-axis, provides a graphical representation to summarize information on the main effects and interactions effect of both genotypes and environments simultaneously. The best genotype can be defined as the one with the highest yield and stable across environments. A stability study helps to identify the ability of genotypes to avoid critical fluctuation in yield over a range of environmental settings. The stability of the genotype is explained by the arrowed line which passes through the biplot origin and is perpendicular to the average Environmental coordinate (AEC) which is explained by the average scores of the two principal components of all environments [24]. This single arrow line is used to indicate the mean yield performance of the genotypes. The arrow sign on the average Environmental coordinate abscissa line directed the ranking of genotypes in ascending order with greater mean performance for a trait [25]. Genotypes located near the origin were the more stable and not responsive to environments that would rank the same in all environments [26].
Genotypes or Environments placed on the right side of the midpoint of the perpendicular line have higher yields than those placed on the left side of the perpendicular line which is a grand mean value, reflecting better adaptation to that environment. The current result [Figure 1] showed that Genotype Belete produced a higher marketable tuber yield followed by Gera, Guassa, and Jalenie. Genotypes Ararsa, Local, Bubu, Dagim, Zengena, Motie and Hundie had lower mean marketable yield than the grand mean.
The most stable genotypes across environments were Belete, Dagim, Local, Guassa and Motie since they have the shortest distance from the average environment abscissa [22]. Genotypes Jalenie, Gudenie, Gera, Ararsa and Bubu were relatively stable genotypes. Genotype Gorebella, Hundie and Zengena had the longest distance from the average environment abscissa indicating their large contribution to the genotype-by-environment interaction and they were unstable across environments.
Simultaneous consideration of both the mean yield and stability performance is crucial to select the best variety for production. Hence, based on the biplot result, Belete and Guassa were the most stable and high-yielder genotypes while Gorebella was the least stable genotype with intermediate mean yield performance. Gera, Jalenie and Gudenie were the among moderately stable and good yielder genotypes whereas Ararsa and Local exhibited low tuber yield but relatively stable genotypes.
The polygon view of GGE biplot “Which -won- Where”,
The polygon view of the GGE biplot is the best way for the identification of winning genotypes by visualizing the patterns of genotypes and environments interaction and helps to show the mega environments present in the target environment [24; 27]. In the GGE biplot, a polygon was formed by linking the vertex genotypes with straight lines and the remaining genotypes were confined within the polygon and had shorter vectors, suggesting that relatively they are less responsive to interaction with the environment. The equality lines, which originate from the center of the biplot and are perpendicular to the sides of the polygon, divide the graph into different sectors.
According to Yan and Kang [24], the vertex genotypes having the longest distance from the center are the best or poorest in some or all environments and are more responsive to environmental change, and are considered as especially adapted genotypes. Hence, the current result (Fig. 2) showed, genotypes fell into five sections and Belete, Gorebella, Local, Hundie and Ararsa were vertex genotypes. These genotypes were superior in the environment lying within their respective sector or the most inferior genotypes in some of the environments because they were farthest from the origin of the biplot [27]. However, all test environments were grouped into one mega-environment with the winner genotype Belete and a mega-environment is defined as a group of locations that consistently share the best set of genotypes [13]. Such type of result was reported previously on the yield of potato genotypes [28].
Ranking the Genotype
AMMI stability value (ASV) and GGE biplot were used to determine the ideal Genotype. An ideal genotype should have both high mean tuber yield performance and high stability across environments [22, 24]. The genotype with lower AMMI stability value (ASV) and Yield stability index (YSI) values is considered more stable. However, the most stable genotypes would not necessarily give the best yield performance [19], hence there is a need to consider both mean yield and stability value/index.
In the GGE biplot, an ideal genotype is always placed at the center of the concentric circles and relatively nearer the AEA arrowhead pointed to the positive direction. It has the greatest vector length, the highest mean performance, and the highest stability [29]. The arrow sign on the AEA line directed the ranking of genotypes in increasing order with a greater value of traits evaluated. The ideal genotype can be used as a benchmark and the genotypes located near the ideal genotype are highly desirable compared to the other genotypes. Thus, the genotypes were ranked as follows: Be > Gu > Ge > Ja > Go > Gd > Hu > Da > Za > Mo > Bu > Lo > Ar.
On the other hand, the YSI and ASV (Table 4) showed that the first-ranked genotype was Belete based on YSI followed by Guassa, Gera and Jalenie. Based on the ASV Guassa was the first-ranked genotype followed by Motie, Local, Jalenie Bubu and Belete. The mean marketable tuber yield value of genotypes indicate that the Local genotype (20.07 t.ha− 1) was the lowest yielder while Belete (33.13t.ha− 1, which is 165% of the local genotype) was the maximum yielder followed by Guassa (30.07 t.ha− 1) and Gera (30.01 t.ha− 1).
Table 4
AMMI stability value, Yield stability index and Mean marketable tuber yield (MTY) of 13 potato genotypes
Genotype | YSI | rYSI | ASV | rASV | MTY |
Belete | 7 | 1 | 2.02 | 6 | 33.13a |
Guassa | 3 | 2 | 0.48 | 1 | 30.07b |
Gera | 12 | 3 | 2.54 | 9 | 30.01b |
Jalenie | 8 | 4 | 1.85 | 4 | 27.97bc |
Gorebella | 18 | 5 | 3.86 | 13 | 27cd |
Gudenie | 16 | 6 | 2.89 | 10 | 26.77cde |
Hundie | 18 | 7 | 2.89 | 11 | 25.13def |
Dagim | 20 | 8 | 3.02 | 12 | 24.56efg |
Zengena | 17 | 9 | 2.54 | 8 | 23.87fg |
Motie | 12 | 10 | 0.85 | 2 | 23.41fg |
BUBU | 16 | 11 | 1.98 | 5 | 22.6g |
Ararsa | 19 | 12 | 2.21 | 7 | 20.12h |
Local | 16 | 13 | 0.89 | 3 | 20.07h |
LSD value | | | | | 2.3679 |
YSI = Yield stability index, ASV = AMMI stability value, rASV = Rank of AMMI stability value, rYSI = Rank of yield stability index, means = average genotype by environment |
Therefore, genotype Belete can be regarded as an ideal genotype for the tested environments based on the present GGE Biplot, mean performance, YSI and ASV. The genotypes Guassa and Gera could be noted as the leading desirable genotype followed by Jalene since they are located near to the ideal genotype Belete. The lowest yielding varieties (Local and Ararsa) were not desirable because they are located far from the ideal variety. Previous research has reported that tuber yield was used to rank potato genotypes using AMMI stability value/index and GGE Biplot [17, 18, 28].
Discriminativeness vs. representativeness pattern of GGE biplot.
The determination of a best suited test environment is crucial. Some environments may not provide unique information, as they are always similar to some other environment(s) in separating and ranking the genotypes. PC1 and PC2 were used to obtain the ideal test environments. The ideal location should both be able to differentiate the genotypes and represent the target location [22]. Discriminativeness is the capacity of the environment to differentiate the genotypes while representativeness is the ability of an environment to represent the conditions of all other evaluated environments [29, 30].
The environment having a smaller angle with the AEA abscissa is more representative of other test environments than those that have larger angles. Higher vector length, which is proportional to the standard deviation within the respective environment, indicates a greater discriminating ability of the environment. A short vector means the environment is not well represented by PC1 and PC2. Thus, environments with representativeness and discriminating ability are good for selecting generally adapted genotypes; while the environment with discriminating ability, but not representativeness, is good for selecting specifically adapted genotypes [22, 31].
In the current result (Figure. 4) the ranking order of 6 testing environments in terms of being the most representative environment (based on the angle between the environment vector and AEC) was Dt2 (rank of 1) followed by W1(rank 2), Dk2 (rank 3), W2 (rank 4), Dk1 (rank 5) and Dt1 (rank 6). On the other hand, the ranking order of environments in terms of their ability to discriminate genotypes (based on the length of their vectors) was Dk1 (rank of 1) followed by Dt2 (rank 2), Dk2 (rank 3), Dt1 (rank 4), W1 (rank 5) and W2 (rank 6).
These 6 environments may be classified into one of three groups. The short vector [W2], category-1 environment, provided little or no information on genotypes. Dt2, W1 and Dk2 were designated as the category-2, environments with long vectors and small angles with the AEA abscissa, indicating that these environments were ideal for promising genotype selection because of their notable representativeness and discriminating power. Yan and Rajcan [13] noted that a long environmental vector with a short angle is a model environment, which is effective and productive to consider assessing the test environment. If budgetary constraints allow only a few test environments, group 2 test environments can be the first choice [22]. The 3rd environmental category includes environment Dt1 and Dk1 which had long vectors and large angles with the AEA abscissa; they cannot be used in selecting superior genotypes, but are useful for detecting and culling unstable genotypes [22].
Ranking of genotypes relative to the ideal environment
Figure 6 illustrates a graphic comparison of the relative performance of all varieties relative to the Dt2. The Dt2 axis, which is the line that passes through point Dt2 and the origin of the biplot, is called the axis of this environment [31] and along it is the ranking of genotypes. The genotypes were ranked based on their projections onto the Dt2-axis, with rank increasing in the direction toward the positive end [14]. Accordingly, Gorebella, Gudenie, Jalenie, Guassa, Gera, and Belete had higher marketable yield performance than the average yield in increasing order while the remaining 7 genotypes had lower than the average yield. Hundie genotype had a yield near the average yield while Belete and Ararsa were the highest and the lowest yielding genotypes, respectively.