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Combining ability of introduced sorghum parental lines for major morpho-agronomic traits Taye Tadesse1*, Tesfaye Tesso1 and Gebisa Ejeta2 1. Ethiopian Institute of Agricultural Research, Melkassa Research Center, PO Box 436, Nazareth, Ethiopia 2. Purdue University, Department of Agronomy, West Lafayette, IN 47907, USA *Corresponding author: [email protected]

Citation: Tadesse T, Tesso T and Ejeta G.. 2008. Combining ability of introduced sorghum parental lines for major morpho-agronomic traits. Journal of SAT Agricultural Research 6.

Abstract

Introduction

Sorghum (Sorghum bicolor) is the dominant crop in the arid and semi-arid tropics, where drought seriously affects its production. The use of improved cultivars, particularly hybrids, was found to be the major component of the integrated approach of mitigating the drastic effect of drought. Thus information on the combining ability of different lines is very pertinent to choose the best parents and to set the suitable breeding method. Fifty-four F1 hybrids were developed by crossing 18 pollinator parents with three standard females following the Design II mating scheme. The hybrids were evaluated at two drought prone locations, Melkassa and Shoa Robit in Ethiopia, using randomized complete block design. This research was conducted with the objectives of estimating the combining ability of the parents with respect to yield and yield-related parameters linked to drought tolerance, and to determine the mode of gene action for drought tolerance traits. The Statistical Analysis Systems (SAS) was used to analyze the data. Combined analysis of variance revealed significant variation among entries for panicle length, seed weight, plant height, time to maturity and panicle exsertion. The entry × location effect was significant for the other observed traits thus signifying differential response of the genotypes in each of the locations. General combining ability (GCA) for plant height, panicle exsertion, panicle length, grain yield and seed weight was significant among male parents indicating the prevalence of additive gene action in determining these traits. The GCA for females was not significant for any of the observed traits except panicle exsertion. However, P9517A appeared to be marginally superior to the other two females in producing high-yielding hybrids having relatively higher panicle weights and panicle yields as well as in possessing larger numbers of green leaves at maturity, a trait termed as stay-green. The male parents exhibited considerable variability for most of the traits considered. Specific combining ability (SCA) effects were not significant for any traits observed.

Drought is perhaps the most prevalent abiotic stress affecting plant growth, survival and productivity in the world (Boyer 1982, Bohnet and Jensen 1996). The effect of drought is more pronounced in the semi-arid tropics (SAT), where rainfall is generally low and erratic in distribution. The drastic effect of drought can be overcome by growing crops that have the desired traits, using reliable soil and water conservation practices. Growing crops that withstand moisture stress better is considered the most effective method to enhance crop production under sub-optimal moisture conditions; also crop growth duration should match the rainfall pattern (Tuinstra et al. 1996). Drought tolerance is a phenotypic expression of a number of morphological and physiological mechanisms, including dehydration avoidance and dehydration tolerance (Levitt 1972, Ludlow 1993). Efforts have been underway to determine the genetic and physiological mechanisms that condition the expression of drought tolerance in crop plants. Sorghum (Sorghum bicolor) is one of the most important cereal crops grown in arid and semi-arid parts of the world, evolved in semi-arid tropical Africa where it is still used as a major food grain. In Eastern Africa, more than 70% of sorghum is cultivated in the dry and hot lowlands where serious water deficit is the major production constraint (Mukuru 1993). In Ethiopia, sorghum is grown as one of the major food cereals. Annually 1.3 million ha of land is allotted for sorghum production and 1.7 million t of grain is produced in the country (CSA 2005). Sorghum is grown in different agro-ecological zones, but predominantly cultivated in dry areas that cover nearly 66% of the total area of the country (Geremew et al. 2004). Nevertheless, crop productivity is estimated at 1400 kg ha-1 (CSA 2005) which is considerably lower than experimental yield that reaches up to 3500 kg ha-1 on farmers’ fields in major sorghum growing regions of the country (Geremew et al. 2004). This still is very low when compared with the yield of 7000 to 9000 kg ha-1

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obtained under intensive management, indicating that drought is one of the prime factors reducing sorghum yield in semi-arid regions (House 1985). Besides the direct effect on yield, drought also predisposes the crop to other yield limiting factors such as pests and diseases (McBee 1984). Wide genetic variations exist among sorghum germplasm for tolerance to drought indicating the potential to develop new sorghum cultivars that may be better adapted to drought conditions. After the discovery of cytoplasmic-genetic male sterility (Stephens and Holland 1954), hybrid development has been getting more emphasis as hybrids give better yields, are more tolerant to moisture stress, and are earlier maturing than their parental lines. Since the early 1960s, hybrid sorghums have been in production dramatically increasing sorghum yields. In many African countries yield increment was obtained as a result of area expansion. However, in India while there is 37% reduction in land allotment for sorghum production, yield increased by 80% (USDA 1997) due to concerted effort in the development and expansion of rainy season adapted sorghum hybrids. Besides the high yield performance, hybrids were found stable under wide range of environmental conditions (Haussmann et al. 1999). Since the commencement of sorghum hybrid development research activities in Ethiopia several parental lines were introduced and evaluated for their adaptability and performance in different sorghum growing areas. Over the years considerable achievements were obtained in identifying suitable parental lines for the hybrid development endeavors of the national research program. However, information related to the combining ability of the introduced parental lines, particularly related to traits associated to drought tolerance is very limited. This information is pertinent for the development of superior genotypes and required to devise a sound breeding approach for the improvement of this trait. Therefore, this study was conducted (1) to estimate the combining ability of selected R-lines for tolerance to drought using yield and yield-related traits as indirect indicators; and (2) to determine the mode of gene action for drought tolerance traits for the pollinator and seed parents.

sterile lines were bred at Purdue University and released for commercial production in the United States. The pollinators are from different genetic backgrounds with different levels of tolerance to drought stress. All three female parents are selections derived from different families of the same population. Description of the study site. The testing sites are situated in the dryland areas of Ethiopia characterized by low amounts of rainfall with erratic distribution. Melkassa Research Center, where the national sorghum improvement program is based, is located at 1550 m above sea level and receives average annual rainfall of 800 mm; the temperature ranges from 15 to 34°C. The dominant soil texture of this area is Andosol. The second testing site was at the prison farm of Shoa Robit district located at 1400 m above sea level with temperature ranging from 17 to 35°C. The dominant soil type of the area is Vertisol. Field operations. The entries were laid out in randomized complete block design (RCBD) with three replications. During sowing, the seeds were manually drilled into 2row plots of 5 m length, spaced 0.75 m apart. At approximately 20 days after sowing the seedlings were thinned to 0.2 m between plants. Data were collected from 0.75 m2 plot area for panicle yield (g), panicle weight (g), grain yield (kg ha-1), 1000-seed weight (g), above ground biomass (kg ha-1) and harvest index (%); and randomly selected five plants were used for plant height (cm), panicle exsertion (cm), number of green leaves 95 days after planting (Haussmann et al. 1999) and panicle length (cm); the whole plot area was considered for seedling vigor (scored on a 1–5 scale, where 1 = highly vigorous and 5 = very low vigor), time to 50% flowering (days) and time to maturity (days). Phosphorus and nitrogen fertilizers were applied at the recommended rates of 46 kg P2O5 ha-1 and 54 kg nitrogen ha-1 in the form of diammonium phosphate and urea, respectively. The plots were weeded as frequently as needed. Statistical analysis. Data were analyzed using statistical analysis systems (SAS 1989) for both the individual locations and for the combined analysis over locations. The combined analysis of data over environments was performed by considering the location and block effects as random and the rest of the variables as fixed. Significance of all sources of variance was tested as per their estimated expected mean squares. Design II fixed model (Model I) of Hallauer and Miranda (1988) was used to obtain independent estimates of the general combining ability (GCA) for male and female parents as well as the specific combining ability (SCA) effect. The GCA for individual parental lines was calculated as the

Materials and methods Genetic materials. Fifty-four sorghum hybrids derived from three commercial seed parents and 18 pollinators were evaluated in two dryland environments, Melkassa and Shoa Robit, during the 2005 main season. The test entries were constituted using a Design II mating scheme where every male parent was crossed with every female parent. A locally adapted improved variety and an advanced hybrid were included as checks. The male

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difference between the grand mean of the hybrids and the marginal means for each male and female parent. Then the significance of the effect was tested using a two-tailed t-test procedure in SAS after rearranging the data set. The result was confirmed by manually computing standard error for GCA for both male and female parents and SCA for hybrid crosses following the procedure outlined by Cox and Frey (1984).

reasonably good yield performance under moisture stress conditions. The entry effect was further partitioned into checks and hybrids. Variation among hybrids followed similar trend as that of test entries indicating the possibility to select suitable hybrids adaptable to the target area with the desired traits. Although there was high entry and hybrid × environment interactions, the discrepancy in variation for time to maturity for both entry and hybrid effects indicates the presence of inherent differences in grain-fill duration among test entries and hybrids. Selection of early-maturing varieties possessing better yield performance has been considered as a possible option to mitigate the drastic effect of terminal stress (House 1995) provided that this does not result in exposure of grain-filling stages to rainfall that will favor grain mold development. The variation in grain-fill duration has also important agronomic implications in that it is associated with principal yield components notably the seed weight and some important nutritional parameters such as protein and metabolizable energy content (Hicks et al. 2002, Kriegshauser et al. 2006). Partitioning of the hybrid effect into male and female components and their interaction showed that much of the significant variation among the hybrids was primarily contributed by the male effects as expected since the three female parents used were related by descent. Accordingly, the male effect was significant for plant height, panicle exsertion, panicle length, grain yield and 1000-seed weight (Table 1), indicating contribution of additive genetic effects in determining these traits. In contrast, variation was not significant for all the traits measured except panicle exsertion for female and male × female interaction effect (Table 1). The high variability of the male parents for the indicated traits may be because they were developed from relatively broader genetic base and perhaps possessed the desired gene for drought tolerance as compared to the standard seed parents. Hybrid × location interaction effect and the interaction of male, female and male × female components with location were significant for all growth and phenological parameters except panicle exsertion for one or more than one of the interaction components. Unlike the main hybrid effect where male parents contributed too much of the variation, the female × location interaction made a greater contribution to variation in hybrid × location interaction effect than did male × location interaction. The interaction effects, however, involved changes in degree rather than change of ranks. Previous reports have showed the variable expression of drought tolerance traits in relation to the environment considered (Jowett 1972, Clarke et al. 1992, Ejeta et al. 1997, Haussmann et al. 1999), which is the common phenomenon in the SAT. Such a problem under experimental conditions might be

Results and discussion The analysis of variance for individual locations for most of the main effects revealed the existence of variation among tested genotypes for the major traits considered (data not presented). The combined analysis of variance for phenological and yield and yield-related parameters is presented in Table 1. A significant location effect was observed for all phenological traits. This result agreed with the established fact that drought hardy crops like sorghum adjust their phenological growth in response to the prevailing climatic conditions. In spite of the higher mean value, significant location effect was obtained only for 1000-seed weight and harvest index from yield and yield-related traits. Entry × location interaction was significant for plant height, time to maturity, panicle exsertion, panicle length and 1000-seed weight. Though not statistically significant, higher mean values were also recorded for other traits, which is indicative of the differential response of the genotypes over the two testing sites. Regardless of the significant entry × location interaction, significant variation among entries was observed for plant height, time to maturity, panicle exsertion, panicle length and 1000-seed weight. Despite the high mean square value for rest of the traits considered, variation among entries was not significant perhaps because the high mean square for the interaction of location with the main effect was used as a denominator. Seedling vigor and number of green leaves, two of the traits associated with drought tolerance, suggested the existence of variability among entries though this was not statistically significant. Early season plant vigor may be considered a pre-flowering drought tolerance trait (Ludlow and Muchow 1990, Cisse and Ejeta 2003). Such genotypes may grow fast and complete their life cycle before the onset of drought, a mechanism known as drought escape (Stout et al. 1978). The effect of seedling vigor is expressed on plant characters such as seed weight, germination and emergence (Cisse and Ejeta 2003). However, those genotypes with low early season vigor may not be necessarily susceptible to drought. Certain genotypes tend to reduce their growth rate in response to moisture stress to save resources for later use, and hence may appear less vigorous, while having

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1 4 55 1 53 17 2 34 1 55 1 53 17 2 34 1 212 220

df

3420.19** 55.22** 30.73 8.33 30.8 40.7 289.3 10.6 0.67 20.32** 5.33 20.40** 30.20** 93.90** 11.3 28.89 7.8 7.66

TF (days) 7028.0** 36.7 616.72** 3816.3** 301.50* 682.10* 1030.3 68.3 14122.5** 120.09** 192 114.80** 241.20** 274.10** 42.3 328 52.4 56.82

PH (cm) 1560.25** 60.0** 33.16** 18.75 32.28* 30.47 402.3 11.4 94.11* 7.64** 14.08 17.80** 27.80* 31.69 11.9 14.28 10 10.08

TM (days) 911.97 166.22** 53.12** 0.48 48.62** 110.93** 35.57** 18.23 344.20** 17.05 0.013 17.52 21.72 0.2 16.44 9.53 15.1 15.07

PEX (cm) 6.03** 1.38** 0.43 0.08 0.44 0.76 0.15 0.3 0.44 0.75** 0.08 0.75** 0.72 4.09** 0.57* 0.99 0.34 0.35

SV 8.28 14.60** 2.55 15.41* 2.26 1.57 19.39 1.61 4.57 1.67 0.21 1.73* 1.51 9.80** 1.37 0.04 1.2 1.23

NGL 142.87 3.97 30.79** 239.94** 22.31** 57.85** 21.94 4.57 271.45** 9.59** 0.02 9.70** 16.19** 41.23** 4.61 12.97 4.62 4.49

PLE (cm) 6021.76 378.61* 380.07 82.68 382.71 316.49 2039.09 318.38 537.64 491.12** 1.26 492.91** 309.66 6722.60** 218.07* 886.5 128.88 135.03

PW (g) 4616.45 287.29* 275.3 17.52 282.65 298.12 583.24 257.23 144.53 329.13** 0.02 336.39** 199.18 4293.94** 172.19* 274.09 107.76 113.21

PY (g) 2123.5 329.2** 143.5 1.31 143 199.1* 288.8 106.4 22.4 151.1** 0.14 154.5** 72.69 124.6** 131.2** 8.9 61.8 63.1

GY (kg ha-1)

2110.89** 28.36** 29.00** 96.33** 28.05** 62.07** 50.97 9.69 12.37 8.21* 16.33 8.17* 11.03 5.13 6.93 1.72 5.73 5.59

TSW (g)

-4−0.210 −1.800 2.030 NS

TM (days) 0.60** −0.06 −0.54** 1.02

PEX (cm)

0.04 −0.04 0.00 NS

SV

0.127 −0.470 0.350 NS

NGL

−0.40 −0.08 0.48 NS

PLE (cm)

−1.96 −3.01 4.98 NS

PW (g)

−1.24 −1.41 2.69 NS

PY (g)

−211 55.99 154.6 NS

GY (kg ha-1)

−0.80 0.40 0.40 NS

TSW (g)

** = Statistically significant at P ≤0.01 level of probability; NS = Not significant. TF = Time to 50% flowering; PH = Plant height; TM = Time to maturity; PEX = Panicle exsertion; SV = Seedling vigor (scored on a 1–5 scale where 1 = highly vigorous and 5 = very low vigor); NGL = Number of green leaves 95 days after sowing; PLE = Panicle length; PW = Panicle weight; PY = Panicle yield; GY = Grain yield; TSW = 1000-seed weight.

−3.56 2.02 1.54 NS

−0.333 −1.400 1.780 NS

P9501A P9514A P9517A LSD

1.

PH (cm)

TF (days)

Line

Table 2. General combining ability (GCA) estimates for phenological and yield-related parameters of female parental lines used in the development of sorghum hybrids tested at Melkassa and Shoa Robit in 2005/06 crop season1.

1. * = significant and ** = highly significant at P ≤0.01 level of probability. TF = Time to 50% flowering; PH = Plant height; TM = Time to maturity; PEX = Panicle exsertion; SV = Seedling vigor (scored on a 1–5 scale where 1 = highly vigorous and 5 = very low vigor); NGL = Number of green leaves 95 days after sowing; PLE = Panicle length; PW = Panicle weight; PY = Panicle yield; GY = Grain yield; TSW = 1000-seed weight. 2. Nested effect of replication over location.

Location (L) Loc (rep)2 Entry Check (C) Hybrid (H) Male parent (M) Female parent (F) M×F H×C Entry × L C×L H×L M×L F×L M×F×L H×C×L Error (a) Error (b)

Source

Mean squares1 ________________________________________________________________________________________________________________________________________

Table 1. Combined analysis of variance for phenological and yield-related parameters of sorghum genotypes tested at Melkassa and Shoa Robit in 2005/06 crop season1.

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1.41 −1.2 −0.54 1.63 1.74 0.69 −1.04 0.13 −1.81 −3 0.79 0.46 2.19 −1.59 1.29 1.07 −0.2 −1.98 NS

P89003 P89005 P89006 P89007 P89009 PU301 PU302 PU303 PU304 Su Cr57 1/1 PDL7 PDL16 P1170 P1177 P1178 P1212 P1214 P3912 LSD

TM (days) 1.89 −1.27 −0.94 1.56 1.62 0.18 −1.27 0.18 −1.71 −2.49 0.56 −0.10 1.18 −0.27 1.68 0.84 −0.55 −1.10 NS

PH (cm)

−2.29 −4.29 −4.73* −1.01 −0.56 1.99 2.38 −2.29 −4.23 −1.06 −0.79 5.38* −7.34** 13.4** −11.3** 5.82** 11.8** −0.84 6.79

−2.04 −0.17 1.59 2.93** −3.00** −0.93 0.77 −2.42* −2.92** 4.45** −2.89** 3.46** −2.16* 2.83** −0.58 −1.29 3.06** −0.73 2.48

PEX (cm) 0.11 −0.11 −0.28 −0.01 0.056 0.056 0.112 −0.39 −0.277 0.056 0.278 0.334 0.334 0.112 −0.222 −0.055 0.00 −0.055 NS

SV −0.19 0.416 −0.451 0.344 −0.045 −0.362 −0.106 0.183 −0.162 −0.351 −0.045 0.449 0.505 0.244 −0.14 0.088 −0.106 −0.273 NS

NGL 0.5 −0.99 −0.85 −1.00 2.6** 0.59 1.02 0.61 0.97 −4.60* 1.17* −0.65 −0.65 3.4** −2.3** 1.02 −0.63 −0.32 1.33

PLE (cm) 3.89 −1.17 2.07 1.12 −3.14 1.05 3.65 −5.13 −5.6 −6.76 −5.42 2.38 −0.9 1.05 4.28 −2.65 9.01 2.27 NS

PW (g) 1.25 −2.28 3.69 3.04 −4.98 2.13 3.27 −4.52 −3.8 −4.05 −6.52 4.05 −1.82 1.38 3.97 −4.15 7.79 1.51 NS

PY (g) 509.2* −90.6 442.6* 325.3 −246.6 −81.33 553.2* −229.3 −159.7 −480* −735** 121.3 −323.9 −262.6 450.6* −330.6 458.6* 78.65 435.9

GY (kg ha-1)

−4.50** −1.14 0.63 2.02* −0.31 −2.20* −2.20* −1.03 0.18 0.13 0.91 1.13 −0.14 2.36** −0.81 2.13* 2.91** −0.03 1.34

TSW (g)

1. * = significant and ** = highly significant at P ≤0.01 level of probability; NS = Not significant. TF = Time to 50% flowering; PH = Plant height; TM = Time to maturity; PEX = Panicle exsertion; SV = Seedling vigor (scored on a 1–5 scale where 1 = highly vigorous and 5 = very low vigor); NGL = Number of green leaves 95 days after sowing; PLE = Panicle length; PW = Panicle weight; PY = Panicle yield; GY = Grain yield; TSW = 1000-seed weight.

TF (days)

Line

Table 3. General combining ability (GCA) estimates for phonological and yield-related parameters of male parental lines used in the development of sorghum hybrids tested at Melkassa and Shoa Robit in 2005/06 crop season1.

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minimized through conducting multilocational experiments (Romagosa and Fox 1993). Significant GCA effects were observed for panicle exsertion for female lines (Table 2); plant height, panicle exsertion, panicle length, grain yield and 1000-seed weight for male lines (Table 3). This indicated the importance of additive genetic variance for the expression of the indicated variability. In line with this result, additive gene action controlling plant height was reported (Toure et al. 1996) for both plant height and inflorescence length (Kenga et al. 2004). The male × female interaction and SCA effect for all of the parameters considered was not significant indicating the little importance of non-additive genetic effects in the expression of those traits. Regardless of its large mean square values, variation for the other traits was not significant due to inflated mean square values of the corresponding location interaction effects. The means for panicle length for the male parent ranged from 23 cm in Su Cr57 1/1 to 31 cm in P1177 with the corresponding significant GCAs of −4.60 to 3.45, respectively. Even though the values were not significant the highest positive GCA was recorded in the line P1214 for panicle weight (9.01) and panicle yield (7.79). The negative GCA for panicle weight was −6.76 in Su Cr57 1/1 and −6.52 for panicle yield in PDL7. Variation in mean grain yield and 1000-seed weight among male parents was significant with the highest mean yield being recorded for the line PU302 and the least yield in line PDL7. Lines with significant positive GCA for yield include PU302, P89003, P1214, P1178 and P89006, whereas the significant negative GCA values were recorded for this trait in lines PDL7 and Su Cr57 1/1. Male line PU302 consistently had higher GCA values of both yield components and phenological and growth parameters. Though many other lines had higher values of the different traits considered, the performance of lines differ from one trait to another. Comparison of individual GCA for plant height indicated that male lines P1178, P1170 and P89006 had the most significant negative GCA for plant height, while male lines P1177, P1214, P1212 and PDL16 had significant positive GCA for this trait. As to which height group is preferred will depend on the relative height of seed parents selected for hybrid production. Depending on the interest of the growers modification of plant height could be possible in both ways using the above lines as the height in those lines was determined by relatively large proportion of additive genes as shown by their significant GCA effect (Table 3). The GCA of individual males for panicle exsertion shows that lines Su Cr57 1/1, P1214, PDL16, P89007 and P1177 had significant and better exsertion as compared to the rest of the lines. These lines, provided that other traits are acceptable, may

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be either used in breeding programs to further improve the trait or directly used in the production of hybrid cultivars. Though not statistically significant, some of these lines also had early maturity, early season plant vigor, and larger number of green leaves, important traits associated with drought tolerance in sorghum; eg, Su Cr57 1/1 (early flowering/maturity and excellent exsertion) and PDL16 (large number of green leaves and excellent plant vigor). But lines P89009, PU304, PDL7, PU303 and P1170 had notable poor panicle exsertion indicating that the trait in these lines needs to be improved. This may not be a critical problem for subsistence farmers where mechanical harvesting is not very common. The results from this study indicated that while attention for high mean values of yield components is very important, one may also need to make logical assessment of stability of genotypes with respect to values of different yield-related traits, and where possible consider those that have above average performance of all relevant traits than one that is on the top for only one or two traits. Since grain yield is a function of linear additive effects of various yield components, it has always been important to pay close attention to these traits. The relative contribution of the traits may vary also depending upon the environment and crop management conditions where the crop is grown. Acknowledgments. The first author gratefully thanks the Ethiopian Agricultural Research Institute for giving the chance to pursue MSc study and also Prof Gebisa Ejeta for the financial and technical support through INTSORMIL. Sincere appreciation is also extended to Dr Tesfaye Tesso for his unreserved advice in all circumstances.

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December 2008 ⏐ Volume 6