Abstract: A study of the response of potato tuber yield and its component to GenotypexEnvironment (GxE) interaction was undertaken. Five cultivars of potato (Solanum tuberosum) including three new cultivars to Egypt (lady Belfour, Saxon and Bambino) were planted in order to test their stability. The evaluations were done under eight environments which consisted of 2 locationsx2 yearsx2 seasons treatment combinations. The locations were Abbies farm (Alexandria governorate- coarse clay soil) and Sadat Farm (Minufiya governorate-sandy soil). The experiment was conducted in two seasons (summer and winter seasons) of years 2007 and 2008. Combined analysis for obtained data was applied to reveal adaptability parameters and correlation coefficient among the investigated environments. All studied morphological and yield characteristics exhibited highly significant genotypic differences. The second order interaction (varietyxlocationxseason) reflected highly significant effect on all studied characters. It was obvious that the studied cultivars responded differently across environments, therefore, their ranks within environments indicated there specific adaptation which reflect the high magnitude of genotypexenvironment interaction. Data indicated that both linear (predictable) and non-linear (unpredictable) components significantly contributed to genotypexenvironment interaction observed for all studied characters. The study has identified high yielding and stable potato cultivars like Valour and lady Belfour.
INTRODUCTION
New potato cultivars generally need to be evaluated at different environments for several years before being released. To achieve this goal, Multi Environmental Trails (MET) from the core of varietals testing program should be conducted. Also, MET are important for testing general and specific cultivar adaptation. A cultivar grown in different environments will frequently show significant fluctuation in yield performance relative to other cultivars.
The effect of GxE interactions in breeding programs is to reduce correlations between phenotypic and genotype resulting in invalid or biased conclusions about genetic variance (Collins et al., 1987).
Genotypexenvironment interaction refers to as differential response of genotypes or cultivars to across a range of environments. Specific adaptations of genotypes to subsets of environments is a fundamental issue to be studied in plant breeding because one genotype may perform well under specific environmental conditions and may give a poor performance under other conditions (Yan et al., 2001).
Potato (Solanum tuberosum) are grown around the world in diverse environments and ranks the world’s fourth most important food crop after wheat, rice and maize (Manrique and Hermann, 2000). Many traits of interest to potato breeders have been shown to be sensitive to environmental changes as shown by previous GxE studies on several traits (Tai, 1971; Yildirim and Caliskan, 1985). It has been observed that the magnitude of the GxE interactions is a linear function of the environmental effects. Thus, differences in response by individual cultivars to a wide range of environments often, follow an orderly pattern which can be measured as differences between coefficients of linear regression of individual cultivars in the environments (Hill, 1975).
The regression technique for testing the genotypexenvironment interaction was
first suggested by Yates and Cochran (1938). This technique
was used and modified by Finlay and Wilkinson (1963)
to analyze the adaptation of number of barley cultivars, grown at different
environments. Eberhart and Russell (1966) proposed the
use of two statistical parameters, a regression coefficient (bi)
and the deviation from regression
The present study was carried out to achieve the following goals:
| • | Determining the magnitude of GxE interactions variation in potato regarding economical and quality characters |
| • | Determining adaptability and stability parameters of some potato cultivars under different environments of Egypt |
| • | Estimation of correlation coefficient among the different studied environments |
MATERIALS AND METHODS
Five new potato cultivars namely Lady Rosetta, Lady Belfour, Valour, Saxon and Bambino were planted under eight environments presented combinations of 2 locationsx2 seasonsx2 years Table 1 and 2.
| Table 1: | Description of the studied environments |
| Table 2: | Description of the studied locations |
| Table 3: | Some monthly meteorological data of the experimental locations during the two years (2008 and 2009) of study |
| T: Temperature, RH: Relative humidity, W: Winds speed, R: Rate of rains | |
A Randomized Complete Block Design (RCBD), with three replications was used. The experimental unit at Abbies farm consisted of 10 rows 0.75 m apart and 4 m long with a surface irrigation method. At Sadat city, the experimental unit consisted of 3 rows, 0.75 m apart and 10 m long under a drip irrigation system. Spacing between plants within rows was at 25 cm at the both locations.
The description of the eight experimental environments and their planting date are presented in Table 1 while, the description of the studied locations are presented in Table 2. Some monthly meteorological data of the experimental locations, during the four growing seasons at each location are listed in Table 3. The physical and chemical analysis of the soil of the two used experimental cities is presented in Table 4. All the agricultural practices used for potato production were carried out in all experiments in accordance with locally recommended practices.
Measured characters: Ten whole plant samples per plot were randomly used, 70 days after planting for the determination of the vegetative growth (plant height (m), number of branches, plant fresh weight (kg), tubers weight/plant (kg), tubers number/plant, average tuber weight (g), tuber diameter (cm)). Data were recorded for total yield/ plot at the end of the second season and was converted to ton/feddan (Feddan = 0.4 ha).
| Table 4: | Physical properties and chemical analyses of the experimental soils |
| EC: Electrical conductivity, OM: Organic matter | |
Statistical analysis: Analysis of variance at 0.05 and 0.01 test of significance for the data from each individual environment were analyzed, using proc ANOVA (SAS Institute, 2002) for the studied characters, according to Steel and Torrie (1980). Data were subjected to combined statistical analysis across environment using proc IML and proc Mixed (Littell et al., 1996).
Stability parameters: Stability analysis for studied characters was performed according to the following model of Eberhart and Russell (1966):
Yij = ui+BiIj+Sij |
| Where: | ||
| Yij | = | The mean of ith cultivar at the jth environment |
| ui | = | Mean of the ith cultivar over all environment |
| Bi | = | Regression coefficient for the response of the ith cultivar to varying environments |
| Ij | = | Environmental index obtained as the mean of all cultivars at the environment min as the grand mean |
| Sij | = | The deviation from regression of ith cultivar and jth environment |
Simple linear regression analysis, using Proc REG (SAS Institute, 2002) was used to estimate Eberhart and Russell (1966) parameters for genotypic stability of cultivars across environments.
From the regression analysis, the following four estimates of stability parameters were calculated linear regression coefficient, i.e., b, S2d (S2y.x-S2e) = Mean square of deviation from regression, R2 (determination coefficient) and the CV for each cultivar as a fourth measure of stability. The four estimates of stability in addition to the mean of the studied characters were included to determine the most stable cultivar. Finally, Spearman’s rank correlation coefficient between the different studied environments was calculated.
RESULTS AND DISCUSSION
Mean performance of potato cultivars over the different environments: The combined analysis of variance presented in Table 5 showed that all studied morphological and yield characteristics showed highly significant genotypic differences indicated that the evaluated cultivars differed in their genetic potentials concerning these characters.
| Table 5: | Combined analysis of variance for the studied morphological characters of potato |
| *,**Significant at 0.05 and 0.01 level of significant, respectively. Rep; replicates/season (error a), Feddan = 0.4 hectare | |
Most of the studied traits reflected clear significant effect for the environmental factors (locations and seasons), except plant height and average tuber weight, which indicated that there were some obvious fluctuations in the environmental conditions throughout the different experiments of the present study, variability among locations and years could mainly be related to differences in soil type, temperature and soil moisture conditions, during the various growing seasons which was also suggested by Ngeve (1991, 1993). The presence of highly significant of locationxseasons, suggested that climate was a significant factor in location differences affecting all studied characters from year to year. Similar explanation was also reached by Harris (1974), Gruneberg et al. (2005) and Claiskan et al. (2007). The first order interaction (LxV) and (SxV) appeared to be highly significant for most of the studied traits except plant height and number of branches (Table 5), indicating that the cultivars tended to rank differently when grown at different locations or at different years, as mentioned by Yildirim and Caliskan (1985) on potato and the experimental trials should be repeated over locations and seasons.
The second order interaction (VxLxS), which was considered as the genotypexenvironment interaction, reflected highly significant effect on all studied characters which mean that the evaluated cultivars showed different response when grown under variable environments and should be measured over multiple locations and seasons to separate cultivarxenvironment interaction component from total genotypic variance as stated also by Moussa et al. (2011) on sweet potato. Thus, the findings of the present study seem to fulfill the basic requirements for stability analysis, for the various studied characters.
| Table 6: | Mean performance of morphological yield and yield components characters of the five potato cultivars calculated over the environments |
| Values followed by the alphabetical in common, within a particular group of means in each character, do not significantly differ, using Revised LSD test at 0.05 level of probability | |
Data presented in Table 6 indicate clearly that all of studied morphological, yield and its component characters showed significant differences among the evaluated cultivars and reflect a large amount of variability. The cultivar Lady Belfouer produced the highest average fresh weight, number of branches, tuber weight/plant accompanied with the highest tuber number/plant, followed by the cultivar Valour and Saxon with significant difference. The cultivar Lady Rosetta possessed the highest average tuber weight while, Bambino showed the lowest value for average tuber weight and tuber diameter. Concerning tuber yield/fad, the highest average were obtained from Lady Belfouer followed by Valour with insignificant difference 11.52 and 10.86 ton fad.-1, respectively while lady Rosetta gave the lowest yield 6.54 ton fad.-1.
As for the average effect of the different environment on average tuber weight, average tuber number and total yield, data in Table 7 declare that the studied characters were highly affected by the different environment and appeared to be sensitive for their changes.
Total yield of cultivars ranged from 24.33 ton fad.-1 for Lady Belfouer at summer season of 2008 at Sadat, to 0.81 ton fad.-1 for Bambino at summer season of 2007 at Abbies, on the other hand. It was noticed also, that the summer season under Sadat location gave the highest total yield and tuber weight/plant E1 and E3, while, under Abbies location the winter season E6 and E8 gave a yield more than the summer season. The studied cultivars responded differently across environments. Therefore, their ranks within environments indicated their specific adaptation which reflect the highly magnitude of genotypexenvironment interaction.
For tuber number/plant, the values ranged from 7.33 for E3 to 4.63 E2 which reflect the highly effect of season summer and winter under Sadat location. While, under Abbies location (E5-E8) the fluctuations for total tuber number was stable to a great extent. Lady Belfouer at E3 gave the highest tuber number 10.4 while Lady Rosetta at E2 environment gave the lowest value being 2.83.
Stability parameters: Data of analyses of variance for estimating stability parameters of the studied yield characters are shown in Table 8.
| Table 7: | Mean performance of the five studied cultivars under different environment for total yield, tuber weight and tuber number |
The method described by Eberhart and Russell (1966) was used for estimating stability of the individual genotypes for the studied characters which already, exhibited significant difference for genotypexenvironment interaction. The differences among the evaluated cultivars and for the parameters environment (linear) were found high enough to reach the used significance level with respect to all studied characters.
The partitioning of mean square (environment+genotypexenvironment) showed that environments (linear) differed significantly and were quite diverse with respect to their effects on the performance of genotypes for potato yield and all yield components. Further, the higher magnitude of mean squares due to environment (linear) as compared to genotypexenvironment (linear) exhibited that linear response of environments accounted for the major part of total variation for all characters studied. Moussa et al. (2011) also, reported similar results and stated that the mean differences between seasonal effects and the effect of seasons on total yield of sweet potato were quite real in nature. The significance of mean squares due to genotypexenvironment (linear) component against pooled deviation for all studied characters subjected that the genotypes were diverse for their regression response to change with the environmental fluctuations. Similarly, the significant mean square due to pooled deviation observed for all the characters studied suggested that the deviation from linear regression also, contributed substantially towards the differences in stability of genotypes. Thus, both linear (predictable) and non-linear (un-predictable) components significantly contributed to genotypexenvironment interaction observed for all studied character. This suggested that predictable as well as unpredictable were involved in the differential response of stability but the major component of stability was due to the linear regression and not to the deviation from the linear function as illustrated by Claiskan et al. (2007).
| Table 8: | Analysis of variance for the estimated stability parameters for the morphological and yield component characters, calculated from the data averaged over all replications |
| *,**Significant at 0.05 and 0.01 level of significant, respectively | |
The hybrid is considered to be stable if its among environment variance is small, or if its response to environment is parallel to the mean response of all genotypes with in trial and the residual MS from the regression model on the environmental index (S2y.x) is small.
The modified model of Eberhart and Russell (1966) was used by several investigators.
A stable preferred hybrid would have approximately b = 1, S2d = 0, in addition to its superiority in yield. The regression coefficients (bi’s) were calculated and significantly tested Table 9.
To select certain tested cultivars based on its desirability, the following criteria were considered: The first criterion was the distance of each cross from the overall mean, using revised LSD value. The second criterion was the regression coefficients for each genotype (bi's). Variance of deviation from regression on environmental index (S2d) was the third criterion values insignificantly different from the pooled error, suggesting that they were considered stable. The fourth criterion was the Coefficient of Variation (CV) for each genotypes. The cultivars with the lowest CV value (<10.0) would be considered as stable. The CV% values can not statistically be compared, because it is a ratio between standard deviation and the mean. The genotypes with high values of R2 were also considered desirable. Lin et al. (1986) indicated that the b value was an indicator of the response of variety for predictable or macro-environmental features, while the S2d was an indicated for micro changes.
| Table 9: | Stability parameters of morphological and yield component characters of the five evaluated potato cultivars |
| *,**Significant at 0.05 and 0.01 level of significant, respectively | |
The results in Table 9 show that mean value for tuber weight/plant ranged from 0.40 for Bambino to 0.76 kg/plant for Lady Belfouer. The cultivar Lady Rosetta showed in stability for this trait because its S2d value were found significant and CV% value were high. The cultivars Lady Belfouer and Valour appeared to be desirable and stable cultivars because there high detected mean performance, bi was close to unity and insignificant value for S2d, while bambino was the least favorable cultivar because of its lower mean performance.
For tuber number/plant, the high yielding cultivar Lady Belfouer had a high bi value (2.02) indicating that this cultivar might reflect a higher response potential for this character under the more favorable environmental conditions, as illustrated by Habliza et al. (2009) on maize, by El-Sharkawy (2006) on summer squash and Naskar and Singh (1992) on sweet potato. The cultivar Valour proved to be the most desirable and stable cultivar followed by the cultivar Saxon.
The most stable cultivar for average tuber weight seemed to be Valour since it didn't posses any significant difference for the linear regression or the deviation from regression, the lowest value of CV% 8.31 and the higher value for R2 (0.93). On the other hand, Bambino appeared to be undesirable because of its lower mean performance (67.13).
Regarding total yield ton/fad., values of bi ranged from 0.84 to 1.13 with insignificant differences among the evaluated cultivars. The high yielding genotypes Valour produced mean yield 10.86 ton/fad., over all environments had regression coefficient close to unity and deviation from regression close to zero. This indicating its high yielding performance over all environments (and its high adaptability). Cultivar Lady Belfouer also produced high total yield over range of environments showed regression coefficient not significant from unity (1.13) and higher deviation from regression (2.03) indicating specific adaptability for this genotype at unfavorable environment or poor yielding environments. The cultivar Saxon considered unstable (S2d value is significant), while, Lady Rosetta seemed to be undesirable because of its lower production, S2d significant and low value of R2.
Data in Table 10 show the rank correlation among the studied eight environments for potato total yield of the five studied cultivars in order to exclude the highly correlated environment.
| Table 10: | Correlation coefficient among different studied eight environments for total yield character |
| *,**Significant at 0.05 and 0.01 level of probability, respectively | |
The results clearly illustrate the presence of highly positive correlation between E5 and E7 (0.98) which represent the summer season of 2007 and 2008 under Abbies location. The same trend was noticed between E6 and E8 (0.94) which represented the winter season of 2007 and 2008 under Abbies location. Meanwhile, this trend was not found under Sadat location. On the other hand, a highly positive correlation was found between E3 and E7 which represented the summer season of 2008 under Abbies and EL-Sadat respectively. The results suggested that these environments provide essentially the same information and consideration could be given to the elimination or relocation of one of these environments. These results are in agreement with those of Lynch and Kozub (1988).
CONCLUSION
In the light of results presented here, it can be concluded that the high yielding genotypes Valour had regression coefficient close to unity and deviation from regression close to zero this indicating its high adaptability over the studied environment. Lady Belfouer potato cultivar produced high total yield over range of environments since regression coefficient was not significant from unity (1.13) and higher deviation from regression (2.03) indicating specific adaptability for this genotype at unfavorable environment or poor yielding environment. Saxon and Bambino cultivars gave low yield and thereby these cultivars deserve further investigation to maximize their yield under the most proper environmental conditions.