ABSTRACT
The present investigation was undertaken in F1 population of 48 crosses, developed by crossing 16 lines (8 gynoecious) and 3 testers during the year 2011. All the parental lines and their F1 hybrids were evaluated in randomised complete block design for yield and its contributing traits during the year 2012. Experimental results revealed that parental lines LC-1-1, CGN-20953, CGN-19533, Gyne-5, LC-15-5 and testers Japanese Long Green and K-75 were found superior on the basis of mean performance and general combining ability effects. The cross combinations LC-1-1 × K-75 (monoecious), CGN-19533 × K-75 (gynoecious), CGN-20953 × Poinsette (gynoecious), Gyne-5 × K-75 (gynoecious) and LC-3-3 × Poinsette (monoecious) excelled based on per se performance, specific combining ability and heterosis studies. Further, performance of top 10 heterotic hybrids illustrated the presence over dominance effects in all the crosses except in one cross, where no dominance was observed. Gene action studies indicated that non-additive gene action governed all the traits under study, suggesting the importance of heterosis breeding for the development of high yielding stable parthenocarpic gynoecious hybrids in cucumber.
Introduction
Cucumber (Cucumis sativus L.) cultivation in open-field conditions of Northern India during winter season is extremely difficult; however, its cultivation under protected structures provides protection from the excessive cold as well as round the year supply of fresh produce during off-season; which can fetch very high remunerative prices (Yadav et al. Citation2014). However, cultivation of cucumber under protected conditions in India is restricted due to non-availability of suitable parthenocarpic gynoecious varieties/hybrids from public sector and high cost of the hybrid seeds developed by the private sector (Kumar et al. Citation2016). Moreover, parthenocarpic gynoecious varieties/hybrids available in the country usually become unstable, that is, gynoecism breaks down under high temperature conditions of protected structures (Cantliffe Citation1981). Consequently, development of high yielding stable parthenocarpic gynoecious varieties/hybrids for protected cultivation is immensely needed.
Heterosis breeding provides an opportunity for achieving unique improvement in yield and other desirable attributes in one generation that would be more time consuming and difficult with other conventional breeding methods (Sherpa et al. Citation2014). Since cucumber is a monoecious and cross-pollinated crop and has appreciable number of seeds per fruit, so it provides enough scope for the exploitation of hybrid vigour (Bairagi et al. Citation2002). Further, use of gynoecious lines in hybrid development will not only enhance the chances of getting high yielding hybrids, but also reduce the cost of hybrid seed production significantly as compared to monoecious hybrids (Rai & Rai Citation2006). This view also lends credence from the findings of Wehner and Miller (Citation1985) and Sharma (Citation2010), who had reported that gynoecious × monoecious hybrids were significantly high yielding than monoecious × monoecious hybrids. More (Citation2002) had developed two gynoecious tropical hybrids, namely Phule Champa and Phule Prachi, involving GYC-2 and GYC-4 as female parent, respectively. At present, these hybrids have become obsolete and it is the need of the hour to develop comparatively more yielding parthenocarpic gynoecious hybrids through heterosis breeding by exploring new gynocious lines of cucumber from different indigenous and exotic sources. But, before the exploitation of gynoecy in heterosis, choice of suitable parental lines for hybrid development is of utmost importance. The combining ability studies are used to identifying suitable parental lines with good general combining ability (GCA) and their performance in specific cross combinations. Further, knowledge of nature and magnitude of gene action controlling the inheritance of yield and its contributing traits in F1 hybrids would facilitate the choice of efficient breeding method and suitable parental lines for genetic improvement of any crop (Rattan & Chadha Citation2009). Additionally, potence ratio is useful to determine the nature of dominance and its direction. However, till date very meagre information is available in the literature pertaining to estimation of combining ability, gene action, heterosis and potence ratio for yield and its contributing traits, using gynoecious lines of cucumber. Therefore, present study was aimed to develop high yielding stable parthenocarpic gynoecious hybrids based on the knowledge of combining ability, gene action, heterotic potential and potence ratio in cucumber.
Materials and methods
Experimental site and layout plan
The present investigation was carried out during kharif (June–September) 2011 and 2012 at Research Farm of the Department of Vegetable Science, Dr Y.S. Parmar University of Horticulture and Forestry, Nauni, Solan (HP), India located at an altitude of 1270 metres above mean sea level lying between 35.5° North latitude and 77.8° East longitude. The experimental material comprised of 16 lines (females) (8 parthenocarpic gynoecious) and 3 testers (males) (). Crosses were made during 2011 as per the line × tester design given by Kempthorne (Citation1957). The F1 population of 48 crosses along with their parents were planted in a randomised complete block design with three replications during the year 2012. During both the years of study, the experimental field was disked and levelled. About 10 Mt ha−1 of well decomposed farm yard manure was mixed in the soil at the time of field preparation. The recommended fertiliser dose of N: P2O5: K2O (400, 315 and 100 kg ha−1 as calcium ammonium nitrate, single-superphosphate and muriate of potash, respectively) was applied in two equal split doses at the time of sowing and commencement of flowering (Directorate Citation2012). Seeds were directly sown in the field in the month of May 2011 and 2012. Three to four seeds per basin were sown at a spacing of 100 × 75 cm in a plot having size of 4.0 × 3.0 m2, accommodating 16 plants per plot. The crop was trained on the trellises and all the cultural practices as recommended in the package of practices for vegetable crops (Directorate Citation2012) have been followed to raise the healthy crop stand.
Table 1. List of the cucumber genotypes used in the hybridisation programme.
Data recording and statistical analysis
The observations were recorded on five randomly selected plants in each entry (genotype/hybrid) over the replications for all the traits under study, namely days to first female flower appearance, node number bearing first female flower, days to marketable maturity, fruit length (cm), fruit breadth (cm), average fruit weight (g), number of marketable fruits per plant, harvest duration (days) and marketable yield per plant (kg). The perusal of data recorded on the 48 crosses and their 19 parents were subjected to analysis of variance manually in MS Excel-2007 worksheet following Panse and Sukhatme (Citation1967). For line × tester analysis, data were analysed as per the model suggested by Kempthorne (Citation1957) through OPSTAT software (Sheoran et al. Citation1998). Further, additive and dominance components of variance were computed by using the following formulae (Dabholkar Citation1992; Singh & Chaudhary Citation1997):
where F is the Inbreeding coefficient (F = 0, since cucumber is cross-pollinated in nature), σ2 A the additive variance and σ2 D the dominance variance.
The estimates of heterosis over mid-parent (MPH) and better parent (BPH), were calculated manually in MS Excel-2007 worksheet by using the following formulae (Singh Citation1973): MPH = and BPH =
.
Further, statistical significance of both the estimates of heterosis was assessed through the t-test given by Wynne et al. (Citation1970).
The nature of dominance was determined by calculating the potence ratio (P) using the following equation given by Smith (Citation1952):
where P is the relative potence of gene set, F1 the first generation mean, P1 the mean of lower parent, P2 the mean of higher parent and MP the mid-parents value. Complete dominance was indicated when P = ±1; while partial dominance is indicated when ‘P’ is between (−1 and +1), except the value zero, which indicates absence of dominance. Over dominance was considered when potence ratio exceeds +1. The positive and negative signs indicate the direction of dominance of either parent.
Results and discussion
Mean performance
Significant differences were recorded among the parental lines and their hybrids for all the traits under study (). Substantial variations were observed among the parents and hybrids for the traits determining the earliness of a variety/hybrid, namely, days to first female flower appearance (parents = 50.17–62.33 and hybrids = 47.20–63.77), node number bearing female flower (parents = 3.23–9.87 and hybrids = 2.73–10.95) and days to marketable maturity (parents = 58.13–70.33 and hybrids = 55.00–72.40). The genotype CGN-20953 followed by CGN-19533 and CGN-20969 among the parents and the cross combination CGN-20953 × Poinsette followed by LC-1-1 × K-75 and LC-2-2 × Poinsette among all the hybrid combinations were found superior for the traits determining earliness in cucumber. Ample variations with respect to earliness were also reported by Bairagi et al. (Citation2005), Munshi et al. (Citation2007), Hanchinamani et al. (Citation2008), Yadav et al. (Citation2009) and Kumar et al. (Citation2013) by using monoecious cultivars of cucumber. All the parents and hybrids also revealed wide variations with respect to yield and yield contributing traits, namely fruit length (parents = 12.20–24.17 and hybrids = 14.80–24.60 cm) and breadth (parents = 3.40–5.60 and hybrids = 3.53–6.23 cm), average fruit weight (parents = 167.30–348.63 and hybrids = 214.33–369.60 g), number of marketable fruits per plant (parents = 3.77–8.67 and hybrids = 4.02–12.03), harvest duration (parents = 14.55–28.17 and hybrids = 14.25–36.69 days) and marketable yield per plant (parents = 0.95–2.29 and hybrids = 1.06–4.21 kg, respectively). The genotype, Japanese Long Green and cross combination LC-25-7 × Japanese Long Green recoded the longest fruits, while maximum fruit breadth was observed in the genotype CGN-20515 and the hybrid combination LC-25-7 × Japanese Long Green. The highest average fruit weight was recorded in the genotype Japanese Long Green and the cross combination LC-25-7 × Japanese Long Green followed by LC-1-1 × K-75. The genotype, Gyne-5 recorded maximum number of marketable fruits per plant, followed by LC-1-1 and CGN-20515; while among the hybrids, maximum number of marketable fruits per plant were obtained in the cross combination CGN-20953 × Poinsette followed by LC-1-1 × K-75, CGN-19533 × K-75, LC-2-2 × Poinsette and Gyne-5 × K-75. The longest harvest duration was recorded in the genotype LC-1-1 followed by Poinsette, LC-2-2 and Gyne-5, while among the hybrid combinations harvest duration was observed maximum in the cross combination CGN-20953 × Poinsette followed by LC-1-1 × K-75 and CGN-19533 × K-75. Among the parents, the highest yield per plant was observed in the genotype LC-1-1, which was followed by CGN-20953, while among the hybrids LC-1-1 × K-75 followed by CGN-19533 × K-75 and CGN-20953 × Poinsette recorded highest yield per plant. In overall, the genotype Japanese Long Green, CGN-20515, Gyne-5, LC-1-1, CGN-20953 and the cross combination LC-25-7 × Japanese Long Green, LC-25-7 × Japanese Long Green, CGN-20953 × Poinsette, LC-1-1 × K-75 and CGN-19533 × K-75 were found to be most promising for yield and yield contributing traits. Substantial variations with respect to yield and yield contributing traits in monoecious cultivars of cucumber have also been reported earlier by Munshi et al. (Citation2007), Kumar et al. (Citation2008), Hossain et al. (Citation2010), Golabadi et al. (Citation2012) and Ranjan et al. (Citation2015). The general approach of selecting parental lines based on mean performance does not necessarily give fruitful results (Allard Citation1960). Therefore, before drawing any conclusion, we have determined combining, gene action, heterotic potential and potence ratio for all the traits under study.
Table 2. Top five parents and cross combinations identified on the basis of mean performance for different yield and its contributing traits in cucumber.
Combining ability
The combining ability analysis is an imperative method to recognise the genetic potential of parental lines and their hybrids. GCA is the average performance of a line in a series of crosses, which is governed by additive gene action and is fixable. While, specific combining ability (SCA) is performance of these parental lines in specific crosses, which is due to the non-additive gene action (dominance or epistasis or both) and is non-fixable. The experimental results pertaining to significant desirable GCA effects of 19 parental lines for different traits under study have been presented in the . A perusal of GCA effects for the traits related to earliness revealed that parents LC-1-1, CGN-20953, CGN-19533, Gyne-5, LC-2-2 and Poinsette were found the best combiners due to their significant negative GCA effects. Different parents expressing negative GCA effects for earliness were also reported earlier by different workers (Munshi et al. Citation2006; Yadav et al. Citation2007; Dogra & Kanwar Citation2011; Kumar et al. Citation2011) in cucumber. For marketable yield per plant, LC-1-1, CGN-20953, CGN-19533, Gyne-5 and K-75 exhibited the highest positive GCA effects. Besides this, most of these parents also exhibited significant positive GCA effects for other yield contributing traits as well, thereby suggesting close association between GCA of these parents for yield with other yield contributing traits. The present findings are in line with those of Singh and Sharma (Citation2006), Yadav et al. (Citation2007), Dogra and Kanwar (Citation2011), Kushwaha et al. (Citation2011) and Kumar et al. (Citation2011), who had also reported significant positive GCA effects of different parental lines for yield and yield contributing traits in cucumber.
Table 3. Estimates of GCA and SCA effects for different yield and its contributing traits in cucumber.
The significant desirable SCA effects of top five cross combinations for different traits under study as presented in revealed that no single hybrid combination exhibited the significant SCA effects for all the traits under study. The cross combinations, CGN-20953 × Poinsette, CGN-19533 × K-75, LC-1-1 × K-75, Gyne-5 × K-75 and LC-2-2 × Poinsette due to their significant negative SCA effects were found the best specific combiners for earliness. All of these crosses involved the parents with good × good GCA effect, indicating additive × additive type of gene interactions operating for earliness. Therefore, these crosses can be exploited to isolate transgressive segregants in F2 and later segregating generations. Kushwaha et al. (Citation2011) had also reported that additive × additive components had predominant role in influencing the earliness in cucumber. Hence, developing superior lines from such crosses will be useful to had earliness in cucumber. SCA effects for marketable yield per plant were found significantly high for CGN-19533 × K-75 (good × good), LC-25-7 × Japanese Long Green (average × poor), LC-1-1 × K-75 (good × good), CGN-20953 × Poinsette (good × good) and Gyne-5 × K-75 (good × good). Beside this, most of these cross combinations also revealed significant SCA effects for other yield contributing traits under study. The crosses which involved good × good general combiners include positive alleles from both the parents and thus can be fixed in the subsequent generations for effective selection of desirable lines, if no repulsion phase linkage is present, whereas crosses which involved average × poor combiners may be used for exploitation of heterosis in F1 generations. Different crosses expressing high desirable SCA effects with respect to yield and its contributing traits in cucumber had also been reported by Sharma et al. (Citation2000); Singh and Sharma (Citation2006); Kushwaha et al. (Citation2011) and Dogra and Kanwar (Citation2011).
Gene action
The relative estimates of components of genetic variance, namely additive and non-additive are essential for successful crop improvement programmes. The data presented in indicate that the estimates of σ2SCA were higher in magnitude as compared to σ2GCA (average) for all the traits under study, thereby indicating predominant role of non-additive gene action governing these traits. Thus, hybrid breeding could better be exploited for genetic improvement of these traits. Similar results have also been reported earlier by Singh et al. (Citation1973) and Bhateria et al. (Citation1995). Earlier, workers like Singh et al. (Citation2011) had made known the significance of variance ratio (σ2 g/σ2 s) for gene action studies in cabbage. Baker (Citation1978) had also recommended that progeny performance should be evaluated by estimating the components of variance and expressing them in predictability ratio, 2 σ2 g/(2σ2 g + σ2 s). The closer this ratio to unity, greater the predictability based on GCA alone. In present investigation, variance ratio was found less than one for all the traits under study. Again it confirmed the role of non-additive gene action governing different traits in cucumber. The results of present investigation are in line with the earlier workers for the traits related to earliness, fruit length, fruit breadth and average fruit weight (Munshi et al. Citation2006; Dogra & Kanwar Citation2011; Kumar et al. Citation2011), number of fruits per plant and marketable yield (Singh & Sharma Citation2006; Yadav et al. Citation2007; Dogra & Kanwar Citation2011; Kumar et al. Citation2011) in cucumber. Here, gene action studies revealed the importance of non-additive gene action in the expression of different traits under study; hence hybrid breeding could be exploited for the improvement in yield and its contributing traits in cucumber.
Table 4. Estimates of genetic components of variance for yield and its contributing traits in cucumber.
Heterosis and potence ratio
Heterosis breeding is one of the most important tools to exploit genetic diversity in cucumber (Mohanty & Mishra Citation1999). Days to first female flower appearance (Miller & Quisenberry Citation1976), node number bearing first female flower (El-Shawaf & Baker Citation1981a) and days to marketable maturity (Kumar et al. Citation2013) are considered as good indices for earliness in cucumber. In the present studies, the cross combination CGN-19533 × K-75, CGN-20953 × Poinsette, LC-1-1 × K-75, LC-2-2 × K-75, LC-2-2 × Poinsette, LC-21-6 × K-75, LC-28-8 × K-75 and Gyne-5 × K-75 revealed significant negative values of MPH and BPH for the traits related to earliness (). Here, it is interesting to note that hybrid combinations produced form gynoecious × monoecious parents are more heterotic than produced form monoecious × monoecious parental lines. Gynoecious lines are said to be early in maturity than the normal monoecious types (Peterson Citation1960) and thus ensure early picking. El-Shawaf and Baker (Citation1981b) also recorded hybrid vigour for earlier flowering and lowest pistillate node in a study of 20 F1 hybrids created by crossing four gynoecious lines with five hermaphrodite lines. Early flowering, fruit maturity and harvest may also be attributed to quicker establishment of hybrid plants and their faster growth and development. Earlier researchers, namely Kumbhar et al. Citation2005; Yadav et al. Citation2008; Kumar et al. Citation2010; Kushwaha et al. Citation2011 had also reported the importance of heterosis for earliness using monoecious cultivars of cucumber. Further, estimated values for potence ratio of top 10 heterotic hybrids () illustrated that all the hybrid combinations have positive nature for all the traits related to earliness. These results reflected over dominance in nine crosses towards lower number of days to first female flower appearance, node number bearing first female flower and days to marketable maturity. On the other hand, absence of dominance was found only in one cross combination, namely LC-28-8 × K-75 for all the traits related to earliness. In contrary, El-Tahawey et al. (Citation2015) had reported negative estimates of potence ratio for number of nodes to the first female flower and number of days to the first female flower in number of crosses of pumpkin.
Table 5. Estimates of heterosis and potence ratio for yield and its contributing traits in cucumber.
Top ten cross combinations exhibiting significant positive heterosis for fruit size (fruit length and breadth) and average fruit weight have been presented in the . The cross combinations, Gyne-5 × K-75, LC-1-1 × K-75 and CGN-19533 × K-75 for fruit length; LC-1-1 × K-75, CGN-20953 × K-75 and CGN-20515 × Japanese Long Green for fruit breadth and Gyne-5 × K-75, LC-1-1 × K-75 and CGN-19533 × K-75 for average fruit weight were found best heterotic crosses; which recorded significant positive values for both the estimates of heterosis. The present findings are in conformity with Sudhakar et al. (Citation2005), Kumar et al. (Citation2010) and Singh et al. (Citation2012). But, these results are in discrepancy with the earlier findings, that is, Kartalov (Citation1966) reported that hybrids of cucumber were intermediate in fruit length; while Singh et al. (Citation1970) observed that all the F1 hybrids produced smaller fruits as compared to their respective mean of parents and standard check cultivar. The deviation could be on account of the variation in genotypes used in hybrid combinations and also in environments under which these were evaluated. Further, potence ratio of top 10 heterotic hybrids for fruit length, breadth and average fruit weight as presented in revealed positive nature; which reflected over dominance towards longer fruit length, higher fruit breadth and average fruit weight. Abd-Rabou and Zaid (Citation2013) indicated that potence ratio of seven cucumber hybrids was higher than one, indicating over dominance of this trait towards the heavy parent. On the contrary, two hybrids showed over dominance and one revealed partial dominance towards the lighter parent. In pumpkin, El-Tahawey et al. (Citation2015) had reported positive estimates of potence ratio in most of the hybrids for average fruit weight.
Estimates of MPH and BPH of top 10 hybrids for number of marketable fruits per plant, harvest duration and marketable yield per plant () revealed that cross combinations, CGN-19533 × K-75, CGN-20953 × Poinsette, LC-1-1 × K-75, LC-3-3 × Poinsette and Gyne-5 × K-75 had significant positive values for both the estimates of heterosis, hence designated as best heterotic crosses among all the hybrids under study. The high consistent performance of these hybrids for marketable yield may be attributed to their hybrid vigour for increased fruit size, weight and number recorded in the present study. Significant heterosis for all these traits in monoecious cultivars of cucumber were reported earlier by Bairagi et al. (Citation2005), Yadav et al. (Citation2008), Singh et al. (Citation2010) and Kushwaha et al. (Citation2011). The estimated values of potence ratio of top 10 heterotic hybrids for number of fruits per plant, harvest duration and marketable yield per plant was found positive in nature. These results reflected over dominance towards higher number of fruits per plant, longer harvest duration and higher marketable yield per plant. Abd-Rabou and Zaid (Citation2013) had reported that potence ratio in 10 hybrid combinations of cucumber for number of fruits per plant and marketable yield per plant exhibited over dominance towards the higher parent in three and five hybrids, respectively.
Conclusion
The present investigation concluded that among the parental lines LC-1-1, CGN-20953, CGN-19533, Gyne-5, LC-15-5 and testers Japanese Long Green & K-75 were found superior on the basis of per se performance and GCA effects for different yield and its contributing traits. While, cross combinations LC-1-1 × K-75 (monoecious), CGN-19533 × K-75 (gynoecious), CGN-20953 × Poinsette (gynoecious), Gyne-5 × K-75 (gynoecious) and LC-3-3 × Poinsette (monoecious) were found outstanding based on mean performance, SCA and heterosis for yield and its contributing traits in cucumber. Among these outstanding crosses, all the gynoecious hybrids were found stable under open-field conditions. These hybrids can be released for commercial cultivation in different parts of the country after multilocation testing. Besides this, three gynoecious lines, namely CGN-19533, CGN-20953 and Gyne-5 result in the development of stable parthenocarpic gynoecious hybrids; hence these lines can be utilised by other researchers across the world for high yielding stable parthenocarpic gynoecious hybrid development in cucumber.
Acknowledgements
The authors are highly thankful to Centre for Crop Genetic Resources, the Netherlands for providing cucumber germplasm to conduct present investigation.
Disclosure statement
No potential conflict of interest was reported by the authors.
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