Heritability and Correlation among Sugarcane (Saccharum spp.) Yield and Some Agronomic and Sugar Quality Traits in Ethiopia

DOI: 10.4236/ajps.2016.710139   PDF   HTML   XML   2,027 Downloads   2,938 Views   Citations

Abstract

To assess broad sense heritability and phenotypic and genetic correlations among sugarcane yield components, an experiment was conducted at Wonji and Metehara Sugar Estates of Sugar Corporation of Ethiopia during 2012/2013. High broad sense heritability (h2) was detected for stalk diameter (0.730), single cane weight (0.672), millable cane number (0.624), stalk height (0.624) and pol % (0.608), indicating that these traits could be selected for easily. Expected genetic gain of the yield components was moderate to high. All traits had low to high genetic correlations (rg = -0.005 to 0.884) with cane yield and (rg = 0.027 to 0.999) with sugar yield. On average genetic correlations were higher than phenotypic correlations. High Genotypic Coefficient of Variation (GCV), broad sense heritability and expected genetic advance were recorded for stalk diameter, single cane weight and millable cane number. A selection strategy based on these traits could lead to improvement in cane and sugar yield.

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Tena, E. , Mekbib, F. and Ayana, A. (2016) Heritability and Correlation among Sugarcane (Saccharum spp.) Yield and Some Agronomic and Sugar Quality Traits in Ethiopia. American Journal of Plant Sciences, 7, 1453-1477. doi: 10.4236/ajps.2016.710139.

Received 5 March 2016; accepted 19 July 2016; published 22 July 2016

1. Introduction

Sugarcane varieties in commercial cultivation are complex polyploid. The heterozygous and polyploid nature of this crop has resulted in generation of greater genetic variability. The information on the nature and the magnitude of variability present in the genetic material is of prime importance for a breeder to initiate any effective selection program.

In Ethiopia, as in other tropical counties, sugarcane is the major raw material used for sugar production. Sugarcane has been cultivated in the country mainly for chewing purpose in the backyards of small holder farmers since ancient times. Commercial sugarcane cultivation in Ethiopia started in 1954 at Wonji on 5000 ha. At present sugarcane is cultivated on 37,000 ha and the four sugar mills in different parts of the country produce about 300,000 ton sugar per annum. However, this production does not satisfy the domestic consumption, and thus the deficit is being offset by importing sugar from abroad. To bridge the gap between supply and demand as well as to exploit the international market, besides expanding the existing ones, Ethiopia is in the course of establishing new sugar factories with large tract of sugarcane plantation with the aim of attaining production of 2.25 million tons sugar and 181 million litres of ethanol at the end of the year 2014/15 [1] .

Since the start the sugar industry of Ethiopia has been relying on importation of sugarcane varieties from many source countries to satisfy the varietal requirements of the sugarcane plantations. So far more than 300 sugarcane varieties has been imported and preserved in germplasm conservation garden located at Wonji. Importing variety per se is not an easy task. Moreover, all the introduced varieties may not become successful commercial cultivars. There is luck of information on these imported varieties vis a vis pedigree, identity of the varieties, etc., which is very difficult to trace as many of the clones are of old generation and significant number are of unknown sources. In spite of a long history of the varieties since introduced no systematic effort has been made to understand the morphogenetic behaviour of traits of these cultivars.

To alleviate the problem of varietal requirement of the industry the Ethiopian Sugar Corporation is now determined to establish sugarcane breeding program to develop its own sugarcane varieties. In any breeding program collection of germplasm is always the first step as it provides plant breeders with sources of useful traits. Especially collecting local germplasm and land races would be crucial as they provide locally adapted genes for better crop improvement. Towards this effort, exploration and collection of local sugarcane germplasm in different geographic regions of the country has been conducted and more than 200 materials were collected and preserved [2] .

Information about the nature and the magnitude of variability present in these germplasm collections (introduced and locals) is of paramount importance for their efficient management and effective utilization in the future sugarcane breeding program of Ethiopia.

In sugarcane, the cane and sugar yields are considered to be the complex characters. The information on the phenotypic and genotypic interrelationship of cane yield and sugar yield with their component characters would be of immense help to the sugarcane breeder. Understanding the associations between traits is of great importance in breeding and selection studies especially for low heritability or difficulty in measuring traits [3] [4] . Consideration of genetic relationships between important attributes in exploiting genetic populations through breeding and directed selection is essential, primarily to understand how changes made by selecting one character may cause changes in others [5] [6] . This knowledge can be used when devising appropriate selection strategies for particular traits in a sugarcane breeding program [7] . Number of millable stalks, stalk height and stalk diameter were reported to be positively associated with cane yield [8] [9] . Reference [10] studied phenotypic associations between yield and its components in sugarcane and concluded that selecting for stalk number, diameter and length should be emphasized in sugarcane variety development programs where high cane yield was the primary goal.

The breeder also requires information on the nature and magnitude of genetic variability in the material available. Heritability estimates, together with expected genetic gain, are more useful than the heritability values alone in predicting the effects of selecting the best genotypes. Reference [11] reported high heritability and genetic gain for single cane weight followed by number of millable cane indicating substantial scope for cane yield improvement. On the other hand, sucrose content recorded low heritability and genetic gain suggesting little scope for improvement in this character [12] . In their program, [13] found moderate heritability estimates for length of stalk (0.41), diameter of stalk (0.51) and number of millable stalks (0.53) and significant positive genetic correlations between yield and the three traits. However, it should be remembered that the magnitude of heritability and association among traits is peculiar to the type of population and environments in which they are evaluated [14] .

Genotypic Coefficient of Variation (GCV) is another measure of relative genetic variation of a trait in a population [15] . Traits exhibiting relatively high GCV estimates may respond favorably to selection. Reference [16] reported high GCV for single stalk weight and millable cane.

Genotype × Environment (G × E) interactions are a serious concern in breeding programs as they affect selection decisions. When the rank of a genotype changes across environments it necessitates evaluation of genotypes across the environments to determine their real value [17] . Studies in various sugarcane breeding programs have reported significant G × E interactions for cane and sugar yield [17] - [19] .

Since breeding of sugarcane in Ethiopia is in its inception, information on genetic parameters and the relationships among cane yield and its components is crucial for effective selection strategy. The study was therefore conducted to estimate heritability of sugarcane yield and some of its components, and to determine phenotypic and genetic correlations among sugarcane yield components.

2. Materials and Methods

2.1. Description of the Study Sites and Plant Material

2.1.1. Site description

The experiment was conducted at Wonji and Metehara Sugar Estates during 2012/2013 cropping season.

Wonji

Wonji Sugar Estate is located in Oromia Regional Government State, Eastern Shewa Zone, Adama Woreda, About 110 km from Addis Ababa and about 10 km south of Adama Town with latitude 8˚31'N and longitude 39˚12'E with elevation of 1550 masl. The average annual rainfall is 800 mm with maximum and minimum temperatures 26.9˚C and 15.3˚C respectively.

Metehara

Metehara sugar Estate is located in Oromia Regional Government State, Eastern Shewa Zone about 200 Km from Addis Ababa and about 8 km south of Metahara Town with latitude and longitude 8˚51'N and 39˚52'E respectively and with elevation of 950 masl. Annual rainfall is 554 mm with temperature maximum and minimum of 32.6˚C and 17.5˚C respectively.

2.1.2. Plant Materials

The plant materials for this study consisted of a total of 400 accessions of which 174 were local sugarcane genotypes (Appendix 1) collected from different regional states of Ethiopia and 226 were introduced sugarcane variety collections (Appendix 2) maintained at conservation garden found at Wonji, Research and Training, Sugar Corporation of Ethiopia. Selection among the local genotypes was made based on geographical regions where the materials were collected and the morphological variations noted during the collection work and when the varieties were quarantined in their collection areas for one year. In exotic/introduced genotypes selection was made taking into consideration the variation in place of origin i.e. source countries and different periods of introductions to the country.

2.2. Experimental Design and Field Layout

The experiment was laid out in 20 × 20 partial balanced lattice design with two replications. Canes were cut into three budded sets and planted in single row plots of 5 m × 1.45 m and 20 cm between plants within a row. Uniform crop management practices were applied to all entries in the trial as recommended for the areas.

2.3. Data Collected

Data on quantitative stalk characters and juice quality parameters were recorded vis sprout count 1 and 2 months after planting (SPC1MAP and SPC2MAP), tiller counts 4 and 5 month after planting (TC4MAP and TC5MAP), stalk count 10 months after planting (STC10MAP), millable stalk count per hectare (MSCHA), single cane weight (SCW), number of internodes (NOI), internodes length (IL), stalk height (SH), stalk diameter (SD), leaf length (LL), leaf width (LW), leaf area (LA), cane yield quintal per hectare (CYHA), brix percent (brix %), pol percent (pol %), purity percent (purity %), sugar percent (SR %) and sugar yield quintal per hectare (SY). For every accession, ten individuals were used for recording data which were recorded on plot basis. Count data and cane yield were recorded considering all cane stalks from the whole plot.

2.4. Statistical Analysis

Combined analysis of variance over two locations was conducted using the general linear model (GLM) procedure of SAS version 9. Estimates of genetic, genotype by environment and error variance components were computed using the VARCOMP procedure of SAS. These components were used to estimate broad-sense heritability [20] [21] .

Heritability on genotype mean basis (2 replicates and 2 locations):

where, = genotype, = genotype × location = error variances, r = number of replications and l = number of locations.

All variance components were converted to their respective coefficients of variation to allow direct comparisons between traits. Genetic coefficients of variation provide a unit less measure of a trait’s genetic variance relative to its mean and permit comparisons among traits with different units and scales and give perspective to available variability to be potentially exploited for genetic gain [8] . The phenotypic coefficient of variation (%) was calculated as PCV = 100σp/phenotypic mean of a trait, and genotypic coefficient of variation as GCV = 100σg/phenotypic mean of a trait.

Expected genetic advance (GA) for each trait was calculated as a proportion of the general mean to allow comparison among traits for potential improvement through selection [8] [22] thus:

where i = selection intensity, σp = phenotypic standard deviation of trait, h2 = heritability.

Genetic (rg) and phenotypic (rp) correlation coefficients and their standard errors were obtained among all the traits by estimating genetic, genotype by environment and error covariances combined across locations using version 9 of SAS Proc Mixed and the REML analysis method based on the variance and covariance components according to [23] as:

where rij = phenotypic or genetic correlation coefficient between trait i and trait j; σij = genetic or phenotypic covariances between trait i and trait j; σi and σj are phenotypic or genetic standard deviations of trait i and trait j respectively. Genetic and phenotypic correlations were considered significant if their absolute value was higher than 1.96 times their standard error [24] [25] .

3. Results and Discussion

Significant (P < 0.01) differences were observed among the genotypes for cane yield, related traits and sugar quality parameters revealing a high level of genetic diversity among them (Table 1). The relatively large genotypic mean squares indicated that clones differed in their potential for the traits. This result indicated that there is significant amount of phenotypic variability and all the genotypes vary each other with regard to the characters that opened a way to proceed for further improvement through simple selection [26] [27] . Genotype × location interactions were significant for all traits except internodes length. The amount of variation accounted for, ranged from moderate (R2 = 0.59) for internodes length to high (R2 = 0.81) for cane yield. Studying on thirteen sugarcane clones [11] on the other hand reported moderate R2 value for cane yield. This variation might be due to the large number of genotypes considered in this study. Highly significant genotype × location interactions for most of the traits revealed that mean performances of the genotypes were influenced by the locations. This interaction was largely due to changes in the relative ranking of the genotypes across the locations [11] . The relatively higher CV% for some of the traits was due to this higher variability of genotype X location in the performance of the genotypes. This suggests that at this stage evaluating sugarcane genotypes in more locations rather than one may be satisfactory [27] [28] .

Table 1. Analysis of variance for twenty stalk characters in 400 sugarcane genotypes grown at Wonji and Metehara during 2012/2013.

SPC1MAP and SPC2MAP = Sprout count 1 and 2 months after planting; TC4MAP and TC5MAP = Tiller counts 4 and 5 months after planting; STC10MAP = Stalk count 10 months after planting; MSCHA = Millable stalk count per hectare; SCW = Single cane weight (Kg); NOI = Number of internodes; IL = Internodes length (cm); SH = Stalk height (cm); SD = Stalk diameter (cm); LL = Leaf length (cm); LW = Leaf width (cm); LA = Leaf area (cm2); CYHA = Cane yield (qt/ha); Brix% = Brix percent; Pol% = Pol percent; Purity% = Purity percent; SR% = Sugar percent; SY = Sugar yield (qt/ha); **P < 0.01; *P = 0.05; ns = nonsignificant; numbers in parenthesis are degrees of freedom.

3.1. Estimation of Genotypic and Phenotypic Coefficients of Variation

The variance components were used to compute heritability estimates in Table 2 [29] . Genetic variance is important as it describes the amount of genetic variation present for the trait. The genetic variance component for all traits exceeded the genotype × location (Table 2). After partitioning phenotypic variance, it was found that genotypic variance was lower than the environmental one for most of the characters studied except single cane weight and stalk diameter. Genetic and environment variances were similar for single cane weight and stalk diameter. Congruent with the present study [11] studying on fourteen sugarcane clones also reported similar genetic and environment variances for single cane weight and stalk diameter combined over locations and plant and first ratoon crop. These results indicated that a significant role was played by the environmental factors in the inheritance of the characters in sugarcane.

The magnitude of genetic variance was the highest in number of millable canes (= 838,822,000.00, = 525,333,966.000) followed by cane yield (= 261,988.00, = 163,306.500), sugar yield (= 4884.90, = 5604.40) and leaf area (= 3274.40, = 9815.80). The high genotypic variance for millable cane num- ber was reported also by other researchers [30] [31] . In contrast to the present study [16] studying on 32 sugarcane

Table 2. Combined components of variances, coefficients of variation, heritability and genetic advance for twenty characters in 400 sugarcane genotypes grown at Wonji and Metehara in 2012/2013.

= Genotypic variance; = Genotype × Location variance; = Environment variance; PCV = Phenotypic coefficient of variation; GCV = Genotypic coefficient of variation; h2 = Heritability percentage; GA = Genetic advance in percent of mean *Character abbreviations as given in Table 1.

genotypes in Nepal in one environment reported higher genotypic variance over environmental one for cane yield, millable cane number, single cane weight, stalk diameter, stalk length and sucrose percent. The reason for this variation could be the large number of genotypes and more than one environment considered in the present study. Reference [11] also observed in plant cane that the highest magnitude of genetic variance relative to environmental variance was exhibited by number of internodes (151.23%), millable cane (143.84%) and stalk weight (116.31%) indicating that environmental factors influenced their expression less than the other traits.

Genotypic coefficient of variation (GCV) is another measure of relative genetic variation of a trait in a population [15] . Traits exhibiting relatively high GCV estimates may respond favourably to selection. The estimates for phenotypic coefficient of variation (PCV) were higher than for genotypic coefficient of variation (GCV) in all the traits (Table 2), suggesting that the apparent variation is not only due to genetics but also due to environmental influences. Reference [32] also found higher PCV over GCV for number of millable cane, stalk height, stalk diameter, single cane weight, brix %, pol % and cane yield.

The highest phenotypic coefficient of variation were observed for sprout count 1 month after planting (PCV = 52.683) followed by sugar yield (PCV = 48.927) and cane yield (PCV = 47.472). On the other hand the highest genotypic coefficient of variation were observed for sugar yield (GCV = 39.070) followed by cane yield (GCV = 37.628) and number of millable cane (GCV = 32.688). High genotypic and phenotypic coefficients of variation for number millable cane were reported earlier [33] . The GCV values for cane yield and its components like single cane weight, number of millable canes, stalk height and diameter were larger than the values for sucrose content (pol %) and juice brix. Large amount of genetic variation for stalk height and diameter, number of millable canes and hand rifractometer brix reading at active growth stage was reported and concluded that progress in breeding for higher sucrose yield can be made by emphasizing selection for high sucrose content at early ripening stage along with higher cane yield [34] [35] .

3.2. Heritability

The success of a variety improvement programme depends largely on the amount of genetic variability present in the population. Genotypic coefficient of variation is not a correct measure to know the heritable variation present and should be considered together with heritability estimates. Genetic coefficients of variation along with heritability estimates give a better indication of the amount of genetic variation for a trait than either parameter alone.

In the present study, high heritability estimates were recorded for stalk diameter (0.730), single cane weight (0.672), sugar yield (0.638), cane yield (0.628), millable cane number (0.624), stalk height (0.624) and pol % (0.608) (Table 2).

Moderate broad sense heritability estimates ranging from 0.599 - 0.480 were found in sugar %, brix%, leaf width, stalk count 10 months after planting, leaf area, tiller count 4 months after planting and number of internodes. This suggests that a considerable proportion of the total variance is heritable and selection of these traits would be effective. High heritability estimate for millable cane number (0.88), stalk diameter (0.85) and single cane weight (0.84) was reported [16] . Similarly [11] also reported high heritability for stalk diameter (0.928), number of millable canes (0.912), single cane weight (0.907), number of internodes (0.907) and moderate heritability for cane yield (0.515). However, the heritability values were relatively higher than the present study. The probable cause of the disparity could be due to the fact that the heritability of a given trait refers to a particular population under a particular condition or environment. Moreover the study by [16] was conducted in single environment and considered only 32 sugarcane genotypes. Similarly [11] included only 14 genotypes in their study. High heritability estimate was also reported elsewhere for single cane weight [31] [36] . Low heritability estimates were observed in sprout count 1 and 2 months after planting, tiller count 5 month after planting, internodes length, leaf length and purity %. Selections might be considerably difficult or virtually impractical for a character with low heritability (less than 0.4) due to the masking effect of environment on genotypic effects [37] [38] . Generally the heritability values for the important stalk characters studied were high to moderate paving the way for improvement of these characters through simple selection. Knowledge of variability and heritability of characters is essential for identifying those amenable to genetic improvement through selection [39] . Results of the current study indicate that use of the traits with high heritability as selection criteria together with cane yield could lead to genetic improvement in cane and sugar yield. Under the conditions of this study stalk diameter, single cane weight, sugar yield, cane yield, millable cane number, stalk height and pol % were reliable selection parameters.

3.3. Genetic Advance

The effectiveness of selection depends not only on heritability but also on genetic advance [29] [40] . Heritability estimates along with expected genetic gain is more useful than the heritability value alone in predicting the resultant effect for selecting the best genotypes [41] . As presented in Table 2, maximum genetic gain (as percent of mean) was observed for sugar yield (64.3%) followed by cane yield (61.4%), millable cane number (53.2%), stalk count 10 months after planting (42.0%), tiller count 4 months after planting (41.5%) and single cane weight (32.5%). Greatest genetic advance is also expected in leaf area (20.3%), leaf width (18.4%), stalk diameter (16.2%) and stalk height (15.3%). The results suggest existence of considerable scope for improvement of these characters by adopting suitable breeding strategy. High genetic advance has also been reported elsewhere for single stalk weight and number of millable cane [42] - [44] . The high genetic gain of these characters was the result of high broad sense heritability and high GCV for these traits [45] . The high broad sense heritability coupled with high genetic advance for these characters indicates these traits are under the control of additive genetic effects and highlights the usefulness of selection based on phenotypic performance [22] . High genetic advance (as percent of mean) for millable cane number was also reported by [16] and for cane yield [11] . Moderate heritability with low genetic advance for sugar quality parameters indicate presence of non-additive gene action and therefore simple selection on phenotypic performance may not be effective [22] . Similarly, low genetic advance with low heritability was recorded for sprout count one month after planting, leaf length, tiller count 5 months after planting, number of internodes and inter node length.

The results suggested that selection should be practiced on the basis of stalk diameter, single cane weight and millable cane number for higher cane and sugar yield. These were followed by stalk height, stalk count 10 months after planting, tiller count 4 months after planting, leaf width and leaf area. The same result for single stalk weight followed by number of millable cane and stalk diameter was reported [11] . Similarly, [16] also reported selection should be practiced on the basis of single cane weight and millable cane number for higher cane yield. Reference [46] showed that plant height, cane diameter, leaf area and intermodal distance had positive and significant correlation with millable cane weight that in turn showed a major contribution towards the final cane yield, however cane diameter, height and internode length can be exploited successfully for further and future cane improvement program. They concluded improvement in these traits would lead to a significant improvement in yield in limited selection cycles. Moderate heritability accompanied by low genetic advance for sugar quality parameters indicated that there is little hope for improvement of these traits by simple selection. Similar results for juice quality characters were obtained [16] [42] .

The higher broad sense heritability, expected genetic advance and GCV for cane yield and sugar yield indicated that direct selection for these traits seems plausible. Reference [47] have also reported high estimates of broad sense heritability and expected genetic advance for cane and sugar yield.

3.4. Genotypic and Phenotypic Correlations

Genotypic and phenotypic correlation matrixes are presented in (Table 3) and (Table 4) respectively. Millable stalk count showed highly significant genetic correlations with tiller count 5 months after planting, stalk count 10 month after planting, single cane weight, stalk diameter, brix percent and significant association with sprout count 2 months after planting. Non significant positive correlation with cane yield, sugar quality parameters and sugar yield was also observed. Similarly, [48] reported significant genetic correlations of number of millable cane with sprout counts, single cane weight and cane yield.

Cane yield had highly significant strong positive genetic correlation with number of internodes, leaf length and width and leaf area and negative significant association with sprout count 1 month after planting and negative non significant correlations with tiller counts and single cane weight. Genetic correlation of cane yield with stalk height, stalk diameter, single cane weight, number of millable stalks and sugar quality traits was positive and non significant. Reference [11] also reported significant positive genetic correlation of cane yield with number of internodes and positive non significant correlation with millable stalk number but in contrast to the present study they found negative and non significant correlation of cane yield with stalk height. Reference [48] also found positive and highly significant genotypic associations of cane yield with single cane weight, number of millable cane per plot, leaf area and germination percent. On the other hand, [13] found in their program significant positive genetic correlations of cane yield with stalk height, stalk diameter and number of millable cane. However, it should be remembered that the magnitude of heritability and association among traits is peculiar to the type of population and environments in which they are evaluated [14] . Moreover the number of genotypes considered in their study and the present work largely varies. More number of genotypes was considered in the current study.

Sugar yield was mainly dependent on number of internodes, stalk diameter, leaf length, pol %, purity % and sugar %. The association with cane yield was positive but nonsignificant. Positive and significant association of sugar yield with cane yield, pol % and sugar % was reported [49] .

Single cane weight exhibited highly significant strong positive genetic correlation with sprout count 2 months after planting, tiller count 5 month after planting, stalk count 10 month after planting, stalk diameter, number of internodes and brix percent and non significant positive association with cane yield. Negative non significant genetic correlation with internodes length and non significant positive association with other parameters was observed for this trait. Strong positive genetic correlations of single cane weight with number of internodes, stalk diameter and stalk height and non significant positive association with cane yield was reported [11] .

Stalk diameter showed highly significant genetic correlation with tiller count 5 month after planting, stalk count 10 month after planting, number of millable cane, single cane weight, number of internodes and brix percent. It

Table 3. Genetic correlations among 21 phenotypic characters measured on 400 sugarcane genotypes.

A = Sprout count 1 month after planting; B = Sprout count 2 months after planting; C = Tiller count 4 months after planting; D = Tiller count 5 months after planting; E = Millable stalk count 10 months after planting; F = Millable stalk count per hectare at harvest; G = Single cane weight (kg); H = Number of internodes; I = Internodes length (cm); J = Stalk height (cm); K = Stalk diameter (cm); L = Leaf length (cm); M = Leaf width (cm); N = Leaf area (cm2); O = Cane yield (qt/ha); P = Laboratory brix%; Q = Pol%; R = Purity%; S = Sugar %; T = Sugar yield (qt/ha); ** and * = highly significant at P < 0.01 and P = 0.05 respectively.

had also significant positive genetic correlations with all sugar parameters and sugar yield but showed nonsignificant correlation with sugar percent. In conformity with this finding [11] also reported significant positive genetic correlation of stalk diameter with number of internodes and single cane weight and non significant association with cane yield. In the present investigation, it is interesting that stem diameter had significant positive association with sugar yield and sugar quality characters indicating the significance of this trait in improving sugar quality traits and sugar yield. Similar reports have been made by earlier sugarcane workers [48] [50] for sugar yield.

Number of internodes had highly significant positive genetic correlations with stalk diameter, single cane weight, leaf length, leaf area, cane yield, brix percent, pol percent, purity percent, and sugar yield while it had significant positive correlation with leaf width and sugar percent and positive non significant association with number of millable cane. Similarly, [11] reported significant positive genetic correlation of number of internodes with stalk diameter and single cane weight and non significant positive association with cane yield. Association with other characters was positive and non significant. Significant association of number of internodes with brix percent, sugar percent and sugar yield was also reported by [48] .

Leaf area exhibited highly significant strong positive genetic correlation with number of internodes, leaf length and cane yield and significant correlation with sugar percent while association with other characters was nonsignificant. Reference [48] also found highly significant correlation of this trait with cane yield and sugar percent.

Table 4. Phenotypic correlations among 21 phenotypic characters measured on 400 sugarcane genotypes.

A = Sprout count 1 month after planting; B = Sprout count 2 months after planting; C = Tiller count 4 months after planting; D = Tiller count 5 months after planting; E = Millable stalk count 10 months after planting; F = Millable stalk count per hectare at harvest; G = Single cane weight (kg); H = Number of internodes; I = Internodes length (cm); J = Stalk height (cm); K = Stalk diameter (cm); L = Leaf length (cm); M = Leaf width (cm); N = Leaf area (cm2); O = Cane yield (qt/ha); P = Laboratory brix%; Q = Pol%; R = Purity%; S = Sugar %; T = Sugar yield (qt/ha); ** and * = highly significant at P < 0.01 and P = 0.05 respectively.

On the other hand [46] reported significant genetic correlation of leaf area with stalk height, stalk diameter and internodes length.

Sugar quality parameters showed highly significant strong positive genetic correlations with each other and with sugar yield revealing that any of these juice quality traits could be considered for selection leading to the simultaneous improvement in the remaining quality traits and sugar yield [48] [51] .

Moderate to high highly significant positive phenotypic correlations with most of the traits were observed for number of millable cane and cane yield. The phenotypic correlations of sugar quality parameters and sugar yield with number of millable cane were positive and highly significant. Highly significant negative correlations with number of millable cane were observed for internode length, leaf length and leaf area and non significant positive correlation with cane yield. It was demonstrated by [11] that phenotypic correlations between cane yield and single cane weight, number of internodes, stalk diameter, stalk height and number of millable canes were positive and significant. They further indicated that number of millable canes was negatively correlated with all other yield components.

Phenotypic significant and positive association of cane yield with single cane weight, number of internodes, stalk height and diameter, leaf length and width, leaf area, brix% and sugar yield was recorded. Association with pol% and purity percent was negative and non significant while positive and non significant with millable cane number and sugar%. Negative correlation of pol % with cane yield was also found by [49] in their study. In conformity with the present study significant positive phenotypic correlation of cane yield with number of internodes, stalk height and diameter and millable cane number [11] and with sugar yield [52] was reported.

Sugar yield was observed to have positive significant phenotypic correlation with sprout count 1 month after planting, tiller count 5 months after planting, number of millable cane, single cane weight, number of internodes, stalk diameter, leaf length, cane yield, brix%, pol%, purity% and sugar%. Sugar yield mainly depends on number of tillers, cane weight, pol%, sugar% and purity% [49] . The negative correlation of pol% with cane yield and positive correlation with sugar yield is one of the major constraints in the improvement of sugarcane [49] . Reference [52] reported strong positive phenotypic correlation of sugar yield with number of millable cane, single cane weight, stalk height, cane yield and non significant positive association with stalk diameter.

Single cane weight, number of internodes, stalk diameter and leaf length also showed highly significant positive phenotypic correlations with most of the traits including sugar quality parameters, cane and sugar yield. This is corroborated by the findings of [11] who reported significant positive correlation of single cane weight with number of internodes, stalk diameter, stalk height and cane yield. It is amazing to notice in the current study that single cane weight, number of internodes, stalk diameter and leaf length had significant positive association with both cane and sugar yield indicating the significance of these traits in improving both cane and sugar yields.

Stalk height demonstrated highly significant positive phenotypic correlations with millable cane number, cane yield, brix percent and most of the yield components but the association with sugar yield was non-significant.

Sugar quality parameters showed highly significant moderate to high positive phenotypic correlations with each other and with sugar yield.

In general, genotypic correlation coefficients were higher than their corresponding phenotypic correlation coefficients indicating a fairly strong inherent relationship among the traits [11] .

The lower estimates of phenotypic correlation indicated that the relationships were affected by environment at phenotypic level [49] . Such environmental influence in reducing the correlation coefficients in rice was also reported by [53] . Correlations among phenotypic traits may reflect biological processes that are of considerable evolutionary interest and can be the result of genetic, functional and physiological or developmental nature [54] [55] .

In this study, most of the important yield components had positive genetic and phenotypic association with cane yield (Table 3 and Table 4). The strong genetic correlation of single cane weight, stalk diameter and number of internodes with other agronomic traits suggests that selection of these traits could simultaneously improve the other traits. On the other hand, though nonsignificant, the negative genetic association of number of millable cane with internodes length, leaf length, leaf area and sugar percent indicates that improvement in the former could result in decrease in the latter traits. These results together with the information obtained on heritability and genetic advance indicated that single cane weight, stalk diameter and millable cane number are the key component characters of cane yield. Sugarcane genotypes with high cane yield have been selected on the basis of stalk number and single cane weight [56] .

4. Conclusions

This study revealed that cane yield was associated with its various components, sugar quality traits and sugar yield genetically and phenotypically in various magnitudes. Further, the study has indicated the magnitude of the correlations among cane yield traits, their heritability, expected genetic advance and genotype × environment interactions that could be encountered within the sugarcane breeding programme and demonstrated differential responses of different sugarcane clones to various environmental conditions.

The results suggest that evaluation of sugarcane clones for cane and sugar yield in many locations should identify superior clones. This testing approach coupled with a selection strategy based on single cane weight, stalk diameter and number of millable canes per unit area might result in significant genetic improvement in cane and sugar yield. This study also reveals that higher number of tillers, number of millable cane, single cane weight, number of internodes, stalk diameter, leaf length endowed with better pol%, purity% and sugar% are the important characters which should be considered while selection to be made for higher sugar yield in sugarcane genotypes.

Acknowledgements

The authors are grateful to the financial grant of the Sugar Corporation of Ethiopia. Many thanks are forwarded to Ethiopian Institute of Biodiversity for its technical advice during collection of the local sugarcane genotypes. The staff and laboratory technicians at Wonji and Metehara, Sugar Corporation, Research and Training are duly acknowledged for their supports.

Appendixes

Appendix 1.Passport data of local sugarcane genotypes collected during 2010/11.

Appendix 2.Introduced sugarcane varieties in Ethiopia used for the study.

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NOTES

*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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