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Genetic parameters and character association for clonalselection in potato breeding programmes

Jai Gopal

To cite this version:Jai Gopal. Genetic parameters and character association for clonal selection in potato breeding pro-grammes. Agronomie, EDP Sciences, 1999, 19 (6), pp.531-539. �hal-00885951�

Original article

Genetic parameters and character associationfor clonal selection in potato breeding programmes

Jai Gopal

Division of Genetics and Plant Breeding, Central Potato Research Institute, Shimla-171 001, HP, India

(Received 26 April 1999; accepted 1 June 1999)

Abstract - In order to plan a strategy for clonal selection in potato, the behaviour of genetic parameters, namely coeffi-cient of variation (both phenotypic and genotypic), heritability (broad sense) and genetic advance (as percentage ofmean), and correlation coefficients (both phenotypic and genotypic) for 12 characters in 22 genotypes were studiedunder diverse seasons (two autumn and two spring) in the sub-tropical north-western plains of India. All parameterswere affected by season as well as by year. Based on the magnitude and consistency of parameters for various charac-ters it was concluded that simultaneous selection for plant vigour, average tuber weight and tuber yield would be effec-tive in improving tuber yield in potato. Selection for breeder’s preference score at harvest should also be effective inimproving the overall worth of a clone. (© Inra/Elsevier, Paris.)

potato breeding / heritability / genetic advance / correlations / selection procedure

Résumé - Paramètres génétiques et associations de caractères pour la sélection clonale dans les programmesd’amélioration de la pomme de terre. Dans le but d’établir une stratégie pour la sélection clonale de la Pomme deterre, le comportement d’un certain nombre de caractères, en particulier le coefficient de variation (phénotypique etgénotypique), l’héritabilité au sens large, le progrès génétique (en % de la moyenne), et les corrélations (phénotypiqueset génotypiques) ont été étudiés pour 12 caractères chez 22 génotypes à différentes saisons (deux automnes et deuxprintemps) dans le climat subtropical des plaines du Nord-Ouest de l’Inde. Tous les paramètres sont affectés par la sai-son et par l’année. En fonction de l’ampleur et de la cohérence des paramètres de ces caractères, on peut conclure que lasélection simultanée pour la vigueur de la plante, le poids moyen de tubercule et le rendement en tubercules pourraits’avérer efficace pour améliorer le rendement. La sélection pour un score « de préférence » du sélectionneur à la récoltepourrait également être efficace pour améliorer la valeur globale d’un clone. (© Inra/Elsevier, Paris.)

pomme de terre / amélioration / héritabilité / progrès génétique / corrélations / sélection

Communicated by Mervyn Humphreys (Aberystwyth, UK)

* Correspondence and reprintsjai_gopal@cpri.hp.nic.in

1. Introduction

Studies on genetic parameters and characterassociations provide information about the expect-ed response of various characters to selection and

help in developing optimum breeding procedures.However, such studies in potato (Solanum tubero-sum L.) have not led to any consensus. Dayal et al.[5], Gaur et al. [8], Sidhu and Pandita [17] andGarg and Bhutani [7] reported that selection fortuber yield should produce a higher geneticadvance than selection on its components tubernumber and average tuber weight, whereasChaudhary and Sharma [4] and Birhman and Kaul[2] reported higher genetic advance for tuber num-ber than for tuber yield and average tuber weight.Tai [21], Killick [13], Thompson et al. [24] andGopal et al. [12] found higher heritability for aver-age tuber weight than for tuber number, whereasMaris [14] found little difference in the heritabilityvalues of these characters.

Similarly, there is no consensus with regard tocharacter association in potato. Singh and Singh[20] and Maris [15] reported a significant associa-tion between plant height and tuber yield, whereasPatel et al. [16], Sidhu et al. [18] and Desai andJaimini [6] reported that plant height was of littleimportance for tuber yield. Among tuber yieldcomponents, Maris [14] and Gopal et al. [12]reported that both average tuber weight and tubernumber are associated with tuber yield but averagetuber weight has a stonger correlation with tuberyield. However, Thompson and Mendoza [23] andMaris [15] reported that tuber number is moreimportant than average tuber weight, althoughGaur et al. [9] found no relationship between tubernumber and tuber yield. Desai and Jaimini [6]found no relationship between average tuberweight and tuber yield, but Tarn and Tai [22] foundsimilar correlation coefficients between tuber num-ber and tuber yield and between average tuberweight and tuber yield in Tuberosum families. Therelationship between tuber number and averagetuber weight has been reported to be non-signifi-cant [6, 17, 23] as well as highly significant andnegative [9, 16, 24]. Many such variations havebeen reported for other characters as well.

These variations may be due to the type of mate-rial used and the environmental conditions towhich genotypes were subjected. Further, most ofthese studies were based on only 1 year’s data.Thus, the need remains to elucidate and develop abroad strategy for clonal selection aimed at

improving tuber yield and other agronomic charac-ters in potato breeding programmes. That wasattempted in this investigation by studying geneticparameters and character associations in thediverse agroclimates of autumn and spring for2 years in the north-western plains of India. In thisregion, the main potato crop is taken during theautumn season (October-January) and a supple-mentary crop during the spring season

(January/February-April/May). The autumn seasonhas short days (10-11 h) and low temperatures(max. temp.: mean = 24.3 °C, range =

19.0-32.5 °C and min. temp.: mean = 10.0 °C,range = 4.0-17.3 °C). These conditions are morefavourable to tuberization than to foliage growth.The spring season has long days (14-15 h) andhigh temperatures (max temp.: mean = 29.7 °C,range = 19.0-39.6 °C and min. temp.:mean = 15.8 °C, range = 7.0-23.7 °C ). These con-ditions are more conducive to foliage growth thanto tuberization. This region thus offers an ideal sit-uation for investigating the behaviour of geneticparameters and character associations in potatounder diverse conditions.

2. Materials and methods

A random sample of 22 potato genotypes was used inthis study. Eighteen of these genotypes, viz. AB455,E4451, F1277, JE812, JH222, JN1501, JR465,JTH/C107, MS78-46, MS78-56, MS79-34, MS80-758,MS81-152, MS82-638, MS84-1169, PJ376, RG1197and SLB/K23, were clones from Indian potato breedingprogrammes and four genotypes, namely CP1710,CP2132, EX/A680-16 and EX/A723, were germplasmaccessions. These were evaluated in two autumn sea-sons during 1994-1995 and 1995-1996 and two springseasons during 1994 and 1995 at the PunjabAgricultural University, Ludhiana (31°N, 75°E, 230 mabove sea level). All field experiments were designed ina completely randomized block with three replications

in short rows of five tubers each (15 tubers per clone perplot) at the recommended intra- and inter-row distanceof 20 and 60 cm, respectively. Normal manurial andcultural schedules were followed. Haulms cutting wascarried out at maturity. The dates of sowing, haulmscutting and harvesting were as given below:

2.1. Characters recorded

Data for 12 characters were recorded. Foliage charac-ters were recorded at full growth (80 days after sowing)on five competitive plants (i.e. the best growing plants)per plot and tuber characters on a plot basis at maturity.The characters were: plant vigour (score 1-5: 1 = veryhigh, 5 = very poor); plant posture (score 1-3: 1 = erect,3 = prostrate); number of main stems per plant, plantheight (cm), number of nodes per plant (on the longeststem only), internode length (cm), maturity (days tosenescence of 50 % foliage), tuber yield per plant (g),tuber number per plant, average tuber weight (g), num-ber of eyes per tuber and breeder’s preference scorebased on overall tuber characters (score: 1-5: 1 = veryhigh, 5 = very low).

2.2. Statistical methods

Data were subjected to analyses of variance andcovariance [19]. For each character the following para-meters were computed:

phenotypic coefficient of variation: 100 √σ2p/xgenotypic coefficient of variation: 100 √σ2g/xheritability in broad sense: σ2g/σ2px

genetic advance (as % of mean): (S. σ2g/√σ2p) 100/xwhere σ2p is genotype mean square/number ofreplications, σ2g is (genotype mean square - error meansquare)/number of replications, x is the populationmean, S is selection differential for 5 % selection

(= 2.06).

Phenotypic and genotypic correlation coefficientsbetween characters in all combinations were computedas:

phenotypic correlation coefficient: σ2p (c1c2)/√σ2p (c1)σ2p (c2)

genotypic correlation coefficient: σ2g (c1c2)/√σ2g (c1)σ2g (c2)where σ2p(c1,c2) is the genotype mean product of charac-ters c1 and c2/number of replications, σ2g (c1,c2) is (geno-type mean product of characters c 1 and c2 - error meanproduct)/number of replications.

Computer software SPAR-1 (IASRI, New Delhi) wasused for these computations.

3. Results

Analysis of variance showed highly significant(P < 0.01) genotypic differences for all charactersin all the trials.

3.1. Coefficient of variation

Phenotypic coefficients of variation (PCV) werehigher than the corresponding genotypic coeffi-cients of variation (GCV) for all characters.Sufficient variation was present at the phenotypicas well as the genotypic level for selection to beeffective for various characters (table I). The high-est coefficient of variation (both PCV and GCV)was observed for tuber number in the spring seasonand for plant height in the autumn season (averageover years). Average coefficient of variation in theautumn season was higher than in the spring sea-son for plant height, number of nodes and inter-node length, whereas a reverse trend was observedfor tuber yield, tuber number and number of eyes.Such marked differences were, however, notobserved for other characters. The most consistentcoefficients of variation (both PCV and GCV) overseasons and years were for average tuber weight

and breeder’s preference score. The most inconsis-tent characters were tuber number and tuber yield.Number of eyes had a similar coefficient of varia-tion in the 2 years within a season.

3.2. Heritability and genetic advance

Heritability values for various characters variedbetween seasons as well as between years within aseason (table II) but ranged between moderate(&ap; 0.50-0.75) to high (> 0.75). Plant height, num-ber of eyes, plant vigour, maturity and breeder’spreference score, in general, had high heritabilitiesin both seasons, whereas number of nodes, numberof stems, tuber yield, tuber number, average tuberweight and plant posture had moderate heritabili-ties. Heritability for internode length was moderatein the spring season and high in the autumn season.

Expected genetic advance was converted intopercentage of clones’ mean so that comparisoncould be made among various characters which

had different units of measurement. These valuesvaried with character, season and year. The maxi-mum fluctuation in genetic advance was for tubernumber and ranged between 24.57 % in autumn1994-1995 and 104.46 % in spring 1994. Genetic

advance for tuber yield also showed big fluctua-tions (31.55 % in autumn 1994-1995 to 89.70 % inspring 1994). In contrast, genetic advance of aver-age tuber weight, breeder’s preference score andplant vigour moved in a narrow range of moderatevalues, i.e. 39.52-58.69, 34.89-44.91 and52.82-65.63 %, respectively. Number of nodes,number of eyes and plant posture, in general, hadlow values of genetic advance ranging between18.09 and 35.24, 26.83 and 32.78, and 27.03 and49.65 %, respectively. Plant height and maturityhad moderate to high values of genetic advance,whereas the values for internode length and num-ber of stems were of low to moderate magnitude(table II). Genetic advance was higher in theautumn than in the spring season for plant height,internode length, number of stems and averagetuber weight. For other characters no consistentpattern was observed in this respect.

3.3. Character association

Most of the correlation coefficients were non-

significant. Significant ones are presented intable III. Genotypic correlation coefficients (rg)were, in general, higher than the phenotypic corre-

lation coefficients (rp). Most of these are of moder-ate magnitude (&ap; 0.50-0.75). However, correlationcoefficients between plant height and internodelength in autumn seasons were of high magnitude(rp = 0.87; rg = 0.90-0.93).

Correlation coefficients (both phenotypic andgenotypic) which were significant in both the sea-sons and in both the years in a season were between:

plant height and number of nodes (rp = 0.43-0.77;

rg = 0.43-0.81), plant height and internode length(rp = 0.64-0.87; rg = 0.75-0.93), tuber yield andaverage tuber weight (rp = 0.43-0.73; rg = 0.44-0.78)and tuber yield and plant vigour (rp = -0.43 to

- 0.61; rg = -0.64 to -0.91).

The correlation coefficients which were signifi-cant in both years for the spring season only werebetween: plant height and breeder’s preferencescore (rp = 0.43-0.49; rg = 0.46-0.62), plant heightand maturity (rp = 0.45-0.49; rg = 0.51-0.54),number of nodes and plant vigour (rp = -0.43 to

- 0.49; rg = -0.44 to -0.75), internode length andmaturity (rp = 0.43-0.44; rg = 0.51-0.57) andinternode length and breeder’s preference score(rp = 0.49-0.61; rg = 0.69-0.85). On the otherhand, the correlation coefficients which were sig-nificant in both years for the autumn season only

were between: plant height and plant posture(rp = 0.54-0.62; rg = 0.70-0.82), internode lengthand plant posture (rp = 0.51-0.67; rg = 0.71-0.87)and average tuber weight and breeder’s preferencescore (rp = -0.43 to -0.45; rg = -0.68 to -0.71 ).

Some correlations were significant in only oneyear for only one or both the seasons (table III).The important ones among these were: number ofstems with tuber number in spring 1994 (rp = 0.53;rg = 0.57) and autumn 1995-1996 (rp = 0.55;

rg = 0.81), tuber yield with tuber number in spring1994 (rp = 0.69; rg = 0.69) and autumn 1995-1996

(rp = 0.50; rg = 0.55), tuber number with averagetuber weight in spring 1995 (rp = -0.52; rg = -0.52)and autumn 1994-1995 (rp = -0.61; rg = -0.69) andplant posture with breeder’s preference score inspring 1994 (rp = 0.44; rg = 0.64) and autumn1994-1995 (rp = 0.50; rg = 0.71).

For some character combinations only genotypiccorrelation coefficients were significant in one orboth years. These combinations were: tuber yieldwith maturity and number of nodes; internodelength with tuber number and plant vigour; numberof nodes with tuber number and plant posture; andplant vigour with maturity and breeder’s preferencescore.

4. Discussion

Individual clonal selection is known to be ineffi-cient in early generations of potato breeding pro-grammes [1, 3, 11, 15], mainly because of largeerror and sampling variations due to inaccuracy ofselecting on small plots (initially often individualplants). In these generations only negative selec-tion is recommended for various agronomic char-acters such as tuber yield and its components [11,15]. The present study was based on advancedstage clones and some germplasm accessionswhere material was not limited and statisticallysound replicated trials could be conducted. Theresults based on this study can thus be reliablyused in advance stages of potato breeding pro-grammes where positive selection is practised forvarious characters. The highly significant genotyp-ic differences observed for various characters indi-cate the suitability of this material for the presentstudy. The choice of the various characters studiedwas based on their reported importance to tuberyield. In addition, plant posture, number of eyes,maturity and breeder’s preference score were alsostudied as there was little previous informationavailable on these characters.

Higher values for phenotypic coefficients ofvariation than genotypic coefficients of variationindicate the presence of environmental variationwhich also resulted in lower phenotypic correlationcoefficients compared to corresponding genotypiccorrelation coefficients between various characters.

However, moderate to high values of heritabilityobserved for various characters indicate that envi-ronmental variation could be limited by the designused. Heritability is, in reality, a measure of theefficiency of an evaluation system in separatinggenotypes. The introduction of clonal replicationinto the system obviously increased the size of theheritability values.

Higher coefficients of variation for plant height,number of nodes and internode length in theautumn than in the spring season, and the reversepattern for tuber yield and tuber number, suggestthat under conditions adverse to the expression of acharacter, genotypic differences are enlarged for

that character. This supports our earlier observation[10] where genotypic differences under in vitroconditions were higher as compared to in vivo con-ditions.

Genetic parameters as well as correlation coeffi-cients were consistent over seasons and years forsome characters, whereas for others these varied.From a breeding point of view, a character which isless affected by change of season and/or year iseasier to handle. Among tuber yield, tuber numberand average tuber weight, the last character was themost consistent with regard to coefficient of varia-tion as well as genetic advance. Tuber yield andnumber of tubers tended to be the most inconsis-tent among all the characters studied. Correlationcoefficients between average tuber weight andtuber yield were moderate, but significant over allyears and seasons. On the other hand, correlationcoefficients between tuber number and tuber yieldwere highly inconsistent and varied even in differ-ent years of the same season. So, it would not beadvisable to use tuber number as a selection para-meter to improve tuber yield. Rather, average tuberweight would be a better choice. Tuber yield hadgood variation, reasonable heritability and goodgenetic advance which were comparable to or evenhigher than those for average tuber weight in cer-tain years. So combined selection for tuber yieldand average tuber weight should be a better strate-gy for improving tuber yield. Tuber number wasnegatively associated with average tuber weight intwo of the four environments studied. Thus, toavoid the risk of selecting clones with fewer, over-sized tubers, it is suggested that a standard may befixed for the minimum number of tubers requiredin the selected types before employing selectionfor tuber yield and average tuber weight. Thesefindings endorse our earlier recommendation [12].

Among the foliage characters, consistent corre-lation coefficients were found between plant heightand number of nodes and plant height and intern-ode length. The positive and moderate to high cor-relation coefficients between these pairs indicatethat a change in plant height is accompanied by acorresponding change in its components, i.e. num-ber of nodes and internode length. There was noassociation between number of nodes and intern-

ode length indicating that plant height and its com-ponents could be improved simultaneously.Improvement in these characters should not be dif-ficult as they had reasonably good variation andmoderate to high heritability and genetic advance.However, these characters are of little value for

improving tuber yield as correlation coefficientsbetween these characters and tuber yield or itscomponents were, in general, non-significant. Inthis regard our findings differ from those of Singhand Singh [20] and Maris [15] who reported a sig-nificant association between plant height and tuberyield. Our findings, however, are in agreement withthose of Patel et al. [16], Sidhu et al. [18] andDesai and Jaimini [6]. Dayal et al. [5] reported asignificant association between number of nodesand tuber yield.Among the other foliage characters, namely

number of stems, plant posture, maturity and plantvigour, only plant vigour had a consistently signifi-cant association with tuber yield. The moderateand negative associations between these traits indi-cate that vigorous clones (i.e. those with lowerscores in the present study) are expected to have ahigher tuber yield. Reasonably good variation andconsistently high heritability and genetic advancefor plant vigour suggest that selection for plantvigour should be effective.

Maris [15] found a significant relationshipbetween tuber yield and maturity indicating thathigh yielding genotypes may be late in maturity.However, no such consistent relationship wasobserved in the present as well as in some otherstudies [6, 13, 24]. Furthermore, plant vigour andaverage tuber weight which were related to tuberyield, in general, had no association with maturity.Thus, the clones selected for plant vigour, averagetuber weight and tuber yield are not expected tocomprise only late types.

Breeder’s preference score was based on all thevisual characters at harvest, i.e. tuber yield, tubernumber, average tuber weight, tuber colour, tubershape, eye depth, tuber cracking, uniformity intuber size, etc. In general, non-significant correla-tion coefficients of breeder’s preference score withtuber yield and its components suggest that whilerecording this trait, no undue preference was given

to tuber yield and its components. Breeder’s prefer-ence score had a high heritability in both seasonsand years within a season. It also had a consistentlyuniform coefficient of variation. Thus, selection forthis trait should be effective in identifying cloneswith superior overall worth.

Some character associations were significant inonly the autumn or spring season. These were,however, not of any importance to tuber yield(table III). Some associations were significant inonly one year for the autumn and/or spring seasonindicating that they cannot be relied upon for usein breeding programmes. This, however, stressesthe need for conducting such studies over yearsand seasons and also explains the variationobserved among results obtained in earlier studies.

5. Conclusion

Simultaneous selection for plant vigour, averagetuber weight and tuber yield should be effective inimproving overall yield in the potato. Breeder’spreference score at harvest should be a good guideto improve the overall worth of a clone for variousagronomic characters of economic importance.This strategy could be adopted for potato breedingprogrammes in both the autumn and spring seasonsin the sub-tropical north-western plains of India.

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