Estimation of variability parameters within
`Mysore' banana clones and their implication
for crop improvement
J.A. Sirisena
a,*, S.G.J.N. Senanayake
ba
Regional Agricultural Research Centre, Department of Agriculture, Bandarawela, Sri Lanka
b
Department of Agricultural Biology, Faculty of Agriculture, University of Ruhuna, Mapalana, Kamburupitiya, Sri Lanka
Received 14 October 1998; received in revised form 16 March 1999; accepted 16 July 1999
Abstract
A study was conducted with diverse accessions ofMusacv Mysore for a three year production period to investigate the possibilities for genetic improvement through within-clone selection. Thus, the phenotypic and genotypic variability, broad sense heritability (h2), phenotypic coef®cient of variation (pcv), genotypic coef®cient of variation (gcv), expected genetic advance (ega) and phenotypic and genotypic correlations were studied on economically important characters of banana cv Mysore. Also the direct and indirect effects of some selected characters on yield were studied.
From the pcv, gcv, h2, ega and genotypic and phenotypic correlations, it was found that the pseudostem girth, fruit maturity period, bunch weight, total fruit weight, average fruit weight and fruit circumference in the second comb had high genotypic variation and genotypic correlations which would be bene®cial for crop yield improvement for banana cv Mysore through within-clone selection. Fruit maturity period had a signi®cant negative correlations with yield and yield components.
High levels of correlated responses in improvement of bunch weight could be obtained when selection was made for average fruit weight and pseudostem girth. Selection for average fruit weight was also likely to improve total fruit weight. Selection for a short fruit maturity period was found bene®cial since fruit maturity period had negative correlated responses for improving bunch weight and its components. The correlated response of the selected characters on improvement of average fruit weight was very low. Selection for total fruit weight had a high response in improvement of fruit circumference in the second comb.
*
Corresponding author. Tel.:94-5722499; fax:94-5722520.
E-mail address: ddrbwela@sri.lanka.net (J.A. Sirisena).
Path analysis revealed that average fruit weight had a high positive direct effect on bunch weight while fruit circumference in the second comb, pseudostem girth and fruit maturity period had high indirect effects on bunch weight via average fruit weight. Thus, a useful path diagram to show the relationship of average fruit weight, pseudostem girth, fruit circumference in the second comb and fruit maturity period to bunch weight has been proposed.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Genetic variation; Genetic correlation;Musaspp.; Mysore; Path-analysis
1. Introduction
Bananas (Musa spp.) are native to Southeast Asia and make a most important contribution to the international fruit industry in South and Southeast Asia (Valmayor et al., 1991). Also there is no other fruit in the world, which surpasses banana and plantains either in production tonnage or in trade volume in fresh form (FAO, 1985). From its centre of origin in Southeast Asia, the banana was introduced to all tropical and subtropical regions of the world where it gained great importance and popularity (Simmonds and Shepherd, 1955). Banana is an extremely frost-sensitive perennial with long life under proper farm management techniques (Valmayor et al., 1991). The banana plant itself emerges above ground from its rhizome in the form of overlapping long sheaths, which appear to form the stem or trunk of the plant and are called a pseudostem. The plants emerge continuously from a single rhizome. Therefore, a banana clump consists of several plants in different growth stages on the one rhizome. The true stem of banana plant emerges in the growth cycle through these overlapping leaf sheaths, bearing an in¯orescence at its top. The in¯orescence drops `bracts' one by one over a period of 5±7 days. As each `bract' falls from the stem it reveals a double row of banana known as a `comb'. The bunch consists of several combs attached to the peduncle, the distal part of the true stem. The time required from emergence of sucker to harvesting varies with the cultivar and ranges from 1 to 1.5 years.
et al., 1995). However, their high capital involvement and time-consumption limit the usefulness of these methods. Large numbers of banana and plantain cultivars such as Mysore. Silk, Pisang ambon and Pisang awak (cooking type) have been identi®ed based on existing natural variability (Simmonds, 1966). Also the genetic variability between these cultivars has been extensively studied (Sree Rangaswamy et al., 1980; Rosamma and Namboodiri, 1990; Rekha and Prasad, 1993). However, the information generated from these studies cannot be used with respect to the within cultivar genetic improvement. In other words, the existing genetic diversity between `Mysore' and `Silk' could not be used to improve `Mysore' or `Silk' through selection using natural genetic variability. This shows the importance of the exploitation of within clone genetic variability to improve particular banana clone through selection.
The natural variability in existing populations of clones may occur due to spontaneous mutations followed by natural selection as a response to climatic stresses and to different environments (Wright, 1931). This variability may be found in physiological and/or morphological characters of banana (Simmonds, 1966). Spontaneous mutations are important in banana improvement because it is the only genetic variability that occurs naturally within the banana clones creating new sub-clones. Spontaneous mutants have been reported inMusawith respect to agronomic, bunch and fruit characters (Simmonds, 1966); i.e. off-types in `Gros Michel' banana in Jamaica (Larter, 1934) and the dwarf plants of wild bananas (Richardson, 1961). The `Dwarf Cavendish' banana is a mutant of an unknown clone (Gross and Simmonds, 1954). The number of mutants that exists in a population is proportional to the population size and the period of cultivation (Simmonds, 1966), but in excess of the evolutionary needs (Wright, 1921), occurring at a frequency of about 10ÿ4 per generation (Watson, 1970).
However, there is apparently no work reported on genetic improvement of banana using variability and character association within a particular clone. Thus, the objectives of the present study were to estimate the (1) phenotypic and genotypic variability, (2) phenotypic and genotypic correlation between the characters with signi®cant genetic variation, including yield with respect to correlated response to selection, (3) direct and indirect effects of important characters on yield using path coef®cient analysis and to (4) suggest a proper path diagram between bunch yield and related characters.
2. Materials and methods
2.1. Planting materials
Out of 200 banana accessions from Musa cv Mysore (`Embul' AAB group) which had been collected from different agro±ecological zones representing a major portion of the island of Sri Lanka, 37 banana accessions showing diversity were included in the study. These banana accessions were multiplied in the ®eld using naturally emerging suckers, in order to obtain suf®cient amount of planting materials uniform in size and age.
2.2. Experimental design
Banana accessions were grown in a randomised complete block design with three replications. Two clumps from each accession were maintained in each block. The number of clumps per block was 74.
2.3. Crop management
Sword suckers, uniform in size (1.5±2.0 m long) and age (212±3 months) were used as planting materials. Blades of large leaves of suckers and all the dead portions of the rhizomes were removed. The rhizomes were treated with `Dithane' fungicide mixed with dry wood-ash. Treated suckers were kept under shade for about 24 h before planting. The suckers were planted in July 1992 in planting holes (45 cm45 cm45 cm) with both within and between row distance of 3 m. Each planting hole was ®lled with well-decomposed cattle manure mixed with topsoil. Dolomite was applied as a calcium and magnesium source at the rate of 600 g per planting hole as recommended by the Department of Agriculture, Sri Lanka. Banana cultivar Pisang awak (dessert type) plants were established around the experimental area to avoid border effects and wind damage.
Carbofuran (3%) granules was applied at the rate of 15 g per planting hole at the time of planting to control banana nematode Radopholus similis (Cobb) Thone. When no rainfall for more than 10 days the soil was irrigated every 10 days so as to bring it to ®eld capacity to a depth of 75 cm. Fertiliser mixture of 12 : 8 : 34 (N : P2O5: K2O) recommended by the Department of Agriculture was
plant until four months after planting, two plants from 4 to 9 months after planting and three plants at 12 months after planting. Thereafter, three plants in a clump with an age difference of 3±4 months were maintained. Harvesting was done when one fruit of the ®rst or second comb of the bunch turned yellow. The ®rst year harvests were taken from the original plant and from a sucker that emerged subsequently in each clump. Two harvests per clump were then taken from two different suckers in each subsequent harvest year.
3. Data collection
Data were collected from the crop for three years in the present study. Two harvests per clump were taken in each year from two different suckers. It has also been observed that the maximum harvest potential in the clump of `Mysore' banana was obtained in the ®rst harvest year itself (the second year of the crop), the harvest per clump did not vary signi®cantly in the second or the third year over the ®rst year (Sirisena and Senanayake, 1997). Therefore, data collection was performed from the ®rst harvest year onwards.
3.1. Growth characters
The following growth characters were recorded: (1) Leaf blade length was measured as a direct measurement from leaf blade base to the tip of the leaf at the time of ¯owering. (2) Leaf blade breath was measured at the point where the maximum breath exists in the leaf at the time of ¯owering. (3) Pseudostem girth was measured 10 cm above the ground level at the time of harvesting. (4) Pseudostem height was measured from the ground level to the base of the leaf petioles at the time of harvesting. These measurements were recorded from the two suckers per clump in each year from 1993 to 1995.
3.2. Fruit characters
second comb taken after removing fruit stalk and the bottle-neck part in the distal end of the fruit. Fruit measurements were taken from three fruits per comb. These measurements were recorded at each harvest of the clump during the three years from 1993 to 1995.
4. Statistical procedures
4.1. Phenotypic and genotypic components of the variability
The following statistical model was used in the analysis
Pijkmgiyjbk gyijeijk; (1)
where Pijk is the measured value for the ith accession in the jth year in the kth
block;mis the population mean;giis the effect of theith accession;yjis the effect
of thejth year;bkis the effect of thekth block; (gy)ijis the interaction between the
ith accession and the jth year; ejik is the random error component (includes
interaction effect of block with accessions, year and accessionyear).
4.2. Phenotypic and genotypic variances
The analysis of variance and the expected mean squares (EMS) performed as described by Hanson et al. (1956) and Singh and Choudhry (1985) are presented in Table 1. The phenotypic variance s2p of a character comprises the genotypic
variance (s2g) and the environment variance (s2e). This relationship could be
expressed symbolically as follows:
s2ps2gs2e; (2)
Table 1
Analysis of variance and the expected mean squares for the modelPijkmgiyjbk gyij
eijk: (r, a and y symbolise numbers of replicates, accessions and years, respectively; s
2 e:
environmental variance;s2y: variance due to year;s2a: variance due to accessions;s2ay: variance due
to AXY interaction effect)
Source df Expected mean squares
Blocks rÿ1 ±
Treatments tÿ1 ±
Accessions (A) aÿ1 s2ers2ayr:ys2a
Years (Y) yÿ1 s2ers2ayr:as2y
AY (aÿ1)(yÿ1) s2ers2ay
wheres2e and s2g were estimated using EMS as follows
EMSaccessionsÿEMSaccessionyearrys2g
s2ers2ayrys2a ÿ se2rs2ay rys2a rys2g; where,r3 and y3; thens2g could be computed.
Sinces2eEMS for error, phenotypic variance was computed as follows:
s2ps2gEMS: (3)
4.3. Estimation of broad sense heritability (h2), phenotypic coef®cient of
variation (pcv), genotypic coef®cient of variation (gcv) and expected genetic advance (ega)
In this studyh2 was calculated as the ratio of s2g and s2p. The ega of a given
character was estimated considering 5% selection from the parent population by the method of Falconer (1976) and Singh and Choudhry (1985). The gcv and pcv were computed for each character as described by Johnson et al. (1955) in order to make comparisons between different characters.
4.4. Phenotypic and genotypic correlation
The covariance analyses were performed between the characters which showed signi®cant variations among banana accessions. The covariance analyses were performed as suggested by Singh and Choudhry (1985) and they are presented in Table 2.
Estimates of covariance for environment (cove) and covariance for accessions
(covg) were obtained from the analysis. The phenotypic covariance (covp) was
expressed as covecovgand computed accordingly. The phenotypic correlation
Table 2
Analysis of covariance and the expected mean product for the model Pijk mgiyjbk
gyijeijk(r,aandysymbolise numbers of replicates, accessions and year, respectively; cove: environmental covariance; covy: covariance due to year; cova: covariance due to accessions; covay: covariance due to AXY interaction)
Source of variation df Expected mean products (MP)
Blocks rÿ1 ±
Treatments tÿ1 ±
Accessions (A) aÿ1 cover.covyr.y cova
Years (Y) yÿ1 cover.covayr.a covy
AY (aÿ1)(yÿ1) cover.covay
and genotypic correlation were estimated using following equations as described by Falconer (1976);
Genotypic-correlation n covg
s2Gx1s2Gx2
q ; (4)
Phenotypic-correlation n covp
s2Px
1s 2 Px2
q ; (5)
wherex1andx2are the two characters between which correlation was measured.
The expected change in one character as a result of selecting for another was estimated in the following manner as described by Singh and Choudhry (1985);
Ry ihxhyrgxyspy; (6)
where,Ryis the expected change inxby selectingy;iis the selection intensity;hx
is the heritability ofx;hyis the heritability ofy;rgxyis the genotypic correlation
coef®cient betweenx andy;spy is the phenotypic standard deviation ofy.
4.5. Path analysis.
The path-coef®cients for direct and indirect effects were estimated for four important characters with bunch yield. The path-coef®cient for direct and indirect effects ofx1 on bunch yield (y) is as follows (Singh and Choudhry, 1985)
rx1y arx1x2brx1x3crx1x4d; (7)
where,rx1yis the genetic correlation between the character and bunch yield;a,b,
candd, are the direct effects of characterx1,x2,x3, andx4, respectively on bunch
yield;yis the bunch yield;rx1x2:b,rx1x3:candrx1x4:d, are the indirect effects ofx1 andy viax2, x3 andx4, respectively.
Three more equations could be written as above for the direct and indirect effects ofx2, x3 and x4 on y. Solving the four equations, a, b, c and d could be
calculated. Sincervalues are already known, The indirect effect can be calculated. Residual effects can be calculated as follows (Singh and Choudhry, 1985);
Residual-effectp1ÿabcd: (8)
5. Results
5.1. Phenotypic and genotypic variability
fruit length in the second comb showed signi®cant interaction between accessions and year despite the signi®cant difference between accessions. Pseudostem height, number of combs per bunch and number of fruits per bunch did not show signi®cant difference between accessions. The rest of the nine characters showed signi®cant difference between accessions but their accessionsyear interaction was not signi®cant (Table 3).
5.2. pcv, gcv, h2 and ega
The genetic analysis made for the nine characters which showed stable differences between accessions is presented in Table 4. The gcv, pcv,h2and ega values are presented for the characters investigated (Table 4). The range of phenotypic values in comparison to the general mean showed wide variability for most of the characters studied (Table 4). A very low heritability value was computed for number of fruits per comb (4%). The rest of the characters showed heritability estimates of 13% or more. The expected genetic advance (ega) was low for the number of fruits per comb (0.7%), bunch maturity period (1.3%) and pseudostem girth (3.8%). The rest of the six characters showed ega estimates more than 5% (Table 4).
Table 3
Analysis of variance Ð mean squares for analysis of 14 characters inMusacv Mysore
Character Mean squares
Year Accession Accessionyear
Weight of second comb (kg) 0.03b 0.71a 0.17a Fruit Circumference in second comb (cm) 2.70a 2.36a 0.79b
Bunch weight (kg) 52.80a 33.80a 11.83b
Total fruit weight (kg) 36.20a 22.20a 7.61b Bunch maturity period (d) 188.0a 82.00a 32.80b Pseudostem girth (cm) 4538.00a 112.00a 54.00b Average fruit weight (g) 3034.00a 413.00a 231.00b F. length in second comb (cm) 2.78b 4.12a 2.19a No. of fruits per comb 54.10a 4.21a 300b Leaf length (cm) 834.35a 4200.00a 228.40b
Leaf width (cm) 39.85a 128.58a 5.18b
Pseudostem height (cm) 277 661.00a 2097.00b 1023.00b No. of combs per bunch 69.90a 2.68b 2.25b No. of fruits per bunch 40 769.00a 1430.00b 1370.00b
a
Table 4
Phenotypic and genotypic components of variability and coef®cients of variation, broad sense heritability and expected genetic advance in growth and bunch characters ofMusacv Mysore
Character Value Variance Coefficient Heritability (%) egac(%)
Mean Range Phenotypic Genotypic pcva(%) gcvb(%)
Leaf width (cm) 71.20 60.0±84.0 29.72 13.70 7 5 46 5.8
Leaf length (cm) 222.00 147.0±270.0 983.40 421.50 14 9 43 8.0
F. circumference in second comb (cm) 11.50 9.0±14.2 0.62 0.17 6 3 27 4.3
Bunch weight (kg) 12.66 5.5±24.0 9.84 2.38 24 12 24 14.5
Total fruit weight (kg) 10.70 4.7±20.2 7.53 1.82 25 12 24 13.7
Bunch maturity period (d) 103.60 70.0±118.0 39.00 5.49 6 2 14 1.3
Pseudostem girth (cm) 66.10 43.0±93.0 48.60 6.4 10 4 13 3.8
Average fruit weight (g) 60.30 33.0±150.0 147.20 20.19 20 7 13 7.2
No. of fruits per comb 15.28 8.0±20.0 2.61 0.11 11 8 4 0.7
a
Phenotypic coef®cient of variation. bGenotypic coef®cient of variation. c
Expected genetic advance at 5% selection.
J.A.
Sirisen
a,
S.G.J
.N.
Senanayake
/
Scientia
Horticultu
rae
84
(2000)
5.3. Phenotypic and genotypic correlations
The phenotypic and genotypic correlation coef®cients of the six important characters are presented in Table 5. The number of fruits per comb, leaf length and width did not have signi®cant genetic correlation with the rest of the characters, so they were not presented in Table 5. Fruit maturity period had negative correlation with the bunch weight and its components. The positive and high correlations were estimated between bunch weight and its component characters.
Estimates of expected progress in improving bunch weight by selecting for other characters (expressed in percentage of the progress expected from selecting for bunch weight itself) are presented in Table 6. It shows that selection for other
Table 5
Phenotypic and genotypic correlation coef®cients among six characters of Musa cv Mysore (`Embul' banana)
Indicates signi®cant at 5% probability level. b
Indicates phenotypic correlation coef®cients given in parenthesis.
Table 6
Progress expected in bunch weight resulting from selection for other characters expressed as a percentage of the change expected when selection was done for bunch weight itself
Character on which the selection is performed Progress expected in bunch weight (%)
Total fruit weight 40.0
Fruit Circumference in second comb 14.6
Average fruit weight 200.0
Pseudostem girth 91.0
characters particularly the average fruit weight and pseudostem girth has signi®cant impact on the improvement of bunch weight. Selection for fruit circumference in the second comb had a very low impact on the bunch improvement. Estimates of expected progress in improving total fruit weight by selecting for other characters (expressed in percentage of the progress expected from selecting for total fruit weight itself) are presented in Table 7. Selection in favour of average fruit weight increases the total fruit weight more than the selection for bunch weight or direct selection for total fruit weight itself. Selection for high pseudostem girth had a considerable correlated response on improvement of total fruit weight. Estimates of expected progress in improving average fruit weight by selecting for other characters (expressed as a percentage of the change expected when selection was done for average fruit weight itself) are presented in Table 8. It appears that the correlated response to increase average fruit weight as a result of selecting for bunch weight and some of its component characters was low (Table 8). Estimates of expected progress in improving fruit circumference in the second comb by selecting for other charcters (expressed as a percentage of the change expected when selection was done for fruit circumference in the second comb itself) are presented in Table 9. Selection in favour of total fruit weight increases the fruit circumference in the second comb as much as 1.8 times that of direct selection for fruit circumference itself.
Table 7
Progress expected in total fruit weight resulting from selection for other characters expressed as a percentage of the change expected when selection was for total fruit weight itself
Character on which the selection is performed Progress expected in bunch weight (%)
Bunch weight 47.0
Fruit Circumference in second comb 14.5
Average fruit weight 113.0
Pseudostem girth 51.8
Fruit maturity period ÿ30.0
Tabel 8
Progress expected in average fruit weight resulting from selection for other characters Ð expressed as a percentage of the change expected when selection was done for average fruit weight itself
Character on which the selection is performed Progress expected in average fruit weight (%)
Bunch weight 16.6
Total fruit weight 14.5
Fruit Circumference in second comb 3.0
Pseudostem girth 17.6
However, selection for fruit circumference in the second comb did not improve bunch weight (Table 6), total fruit weight (Table 7) or average fruit weight (Table 8). Moreover, selection for long fruit maturity period had a high negative response (±84%) on the fruit circumference in the second comb.
5.4. Path coef®cients for direct and indirect effects
The direct and indirect effects of average fruit weight, fruit circumference in the second comb, pseudostem girth and fruit maturity period on the bunch weight are presented in Table 10. The average fruit weight had a very high direct effect (1.50) on bunch weight. The pseudostem girth had a negative direct effect despite the high positive correlation coef®cient with bunch weight. However, pseudostem girth had a high indirect effect with bunch weight via average fruit weight. Fruit circumference in the second comb showed a low positive direct effect on bunch yield, low positive indirect effect on bunch yield fruit maturity period but very high positive indirect effect via average fruit weight (Table 10). The direct and
Table 9
Progress expected in fruit circumference in second comb resulting from selection for other characters expressed as a percentage of the change expected when selection was done for fruit circumference in second comb itself
Character on which the selection is performed Progress expected in fruit circumference in second comb (%)
Bunch weight 22
Total fruit weight 180
Pseudostem girth 6
Average fruit weight 15
Fruit maturity period ÿ84
Table 10
Path coef®cients for direct and indirect effects of some of the important characters on bunch yield of banana cv Mysore estimated through path coef®cient analysis
Character Genotypic correlation with bunch yield
Effects on the bunch yielda
Direct effect
Indirect effect via other characters
X1 X2 X3 X4
Average fruit weight (X1) 0.99 1.50 ± 0.47 ÿ0.59 0.85 F. Cir. in second comb (X2) 0.99 0.63 1.12 ± ÿ0.35 0.62 Pseudostem girth (X3) 0.77 ÿ0.68 1.30 0.32 ± 0.24 F. maturity period (X4) ÿ0.55 ÿ0.87 ÿ1.47 ÿ0.45 0.19 ±
aResidual effect
indirect effects of fruit maturity period were generally negative with the bunch weight.
6. Discussion
6.1. Phenotypic and genotypic components of the variability
With respect to the characters with signi®cant interaction effects of accessionsyear, the differences between accessions varied over years so that selection of accessions for these characters would be dif®cult due to the lack of stability of the average effects over years (Table 3).
Among the 12 characters with no signi®cant accessionyear interaction, main effects for accessions were found to be useful only in 9 characters while leaving the rest of the characters unimportant for genetic variability among accessions (Table 3). Among the nine characters, bunch weight, average fruit weight and total fruit weight had a considerable pcv higher than 15% (Table 4). Only bunch weight and total fruit weight had a gcv more than 10% indicating some promise for genetic improvement. Sree Rangaswamy et al. (1980) estimated pcv and gcv as 29% and 25%, respectively, for bunch weight, and 14% and 13%, respectively, for stem girth in a range of dessert banana cultivars. The low pcv and gcv estimated in the present study may be either due to the occurrence of low spontaneous mutation rates (Wright, 1931) or to the small population size or to both reasons. Out of the nine characters on which genetic analysis was done, only bunch weight, leaf length and average fruit weight had moderateh2(13±43%) and ega (7.2±14.5%) showing promise for genetic improvement (Table 4). Higher heritability (75%) and ega (75%) were estimated for bunch weight and average fruit weight in a large number of banana cultivars (Rekha and Prasad, 1993) may be due to wide genetic variability of the germplasm belonging to different genomic groups. It has also been reported that heritability estimates would be reliable if accompanied by a high ega (Singh and Choudhry, 1985).
6.2. Phenotypic and genotypic correlations and correlated responses
Although the population size is comparatively low, considerables2pands2gwere
correlated response to yield of a character is very important for crop yield improvement in addition to the other variability parameters. However, the pre-requisites for a character to have a correlated response to yield are its highs2g;h
2
and high genetic correlation with yield. Surprisingly, as a correlated response to bunch weight, the selection for high average fruit weight resulted in an increase in bunch weight that was twice as great as of selection for bunch weight itself (Table 6). This is because the genotypic correlation between average fruit weight and bunch weight was very high despite the moderate values of genetic variability,h2 and ega computed for average fruit weight (Table 5). However, low genetic correlation (0.56) than that in the present study (0.77) was estimated between pseudostem girth and the bunch weight in a large number of banana cultivars (Rosamma and Namboodiri, 1990). Apparently, there are no reports on the quanti®cation of correlated responses in banana for the comparison.
Selection for high fruit circumference in the second comb did not improve bunch weight (Table 6) or average fruit weight (Table 8) as a correlated response because phenotypic and genotypic variances of fruit circumference in the second comb were very low (Table 4). Only a low progress could be made in average fruit weight as a result of selecting for even bunch weight (16.6%), and pseudostem girth (17.6%) (Table 8). Thus, average fruit weight can only be improved through direct selection. Low expected progress (6%) in pseudostem girth as a result of selecting in favour of fruit circumference in the second comb may be due to low variances in fruit circumference of the second comb and the low genetic correlation between the two characters. Rosamma and Namboodiri (1990) were unable to observe a signi®cant genetic correlation between fruit circumference and pseudostem girth in a range of banana cultivars.
typical environmental conditions prevailing during 1992±1995 at the Regional Agricultural Research and Development Centre, Angunakolapelessa, Sri Lanka and some of the climatic parameters during this period have been reported previously (Sirisena and Senanayake, 1997). Improvement of fruit size increases the unit price of the produce in banana due to the higher fruit grade. The high bunch yield increases the productivity of the crop. Mannion et al. (1992) estimated the average yield of the banana cv Mysore in a range of 12±16 t per hectare per year while average yield in the present study being 25 t per hectare per year (with the 1000 clumps per hectare and two harvests per clump per year). This could be due to high management under the experimental conditions. However, the bunch weight in the different accessions used in this study ranged from 5.5 to 24 kg projecting the yield range of 11±48 t per hectare per year. This shows the potential for increasing bunch yield by selecting the superior banana accessions with respect to the characters identi®ed in this study.
Based on s2g, h2, ga and genotypic correlations and correlated responses, the
average fruit weight, total fruit weight, fruit circumference in the second comb, pseudostem girth and fruit maturity period were identi®ed as important traits in a selection program to improve bunch yield of Musa cv Mysore through within-clone selection. Also studies on direct and indirect effects of those characters on bunch yield are important for an ef®cient selection program.
6.3. Path coef®cients for direct and indirect effects
The higher path coef®cient for average fruit weight is due to its direct effect on bunch yield. On the other hand, for all the other characters, the highest path coef®cients were recorded always via average fruit weight. Therefore, in the path diagram the effect of average fruit weight on bunch weight was basically direct while the effects of the other three characters on bunch weight were basically indirect via average fruit weight. Thus, the suggested path diagram is given in (Fig. 1).
Acknowledgements
The authors acknowledge the Council for Agricultural Research Policy, Sri Lanka for providing ®nancial support. We are very much thankful to Dr Sumith Abesiriwardena for his technical guidance in preparing the paper.
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