Combining ability of some sorghum lines for dry lands and sub-humid environments of East Africa

1 School of Science, University of Eldoret. P. O. Box 1125, Kenya. 2 International Crops Research Institute for Semi Arid Tropics (ICRISAT), P. O. Box 39063, Nairobi, Kenya. 3 International Crops Research Institute for Semi Arid Tropics (ICRISAT), P. O. Box 502 324, Patancheru, India. 4 Mikocheni Agricultural Research Institute, P. O. Box 6226, Dar Es Salaam, Tanzania. 5 Rongo University College, P. O. Box 103-40404, Rongo, Kenya.


INTRODUCTION
Sorghum (Sorghum bicolor L. Moench) is a major staple crop grown in water stressed areas of the tropics (Abdulai et al., 2012), because of its resiliency.Lately, sorghum has received significant attention because of its multiple uses as food, feed, and raw material in brewing and biofuel industries (Paterson, 2008).According to FAO (2010), Africa contributes over 60% to the total land area dedicated to cultivation of sorghum.A report by Tanzania's Ministry of Agriculture Food Security and Cooperatives (MAFSC, 2012) indicates that, annual demand for white sorghum in Tanzania is 3,360 metric tonnes while the supply in the country during 2011/2012 was only 1,084 metric tonnes, indicating a significant difference between demand and supply.Further, demand for white sorghum in East Africa has increased dramatically after the East Africa Breweries Limited company started to use it for beer production.However, according to FAO (2010), sorghum productivity in Eastern Africa has been low (<1 t ha -1 ).Among the main causes for this low production level is the continuous use of low yielding landraces (Aruna and Audilakshmi, 2008) which could mainly be attributed to scarcity of adapted hybrids (Makanda et al., 2012).Deployment of adapted sorghum hybrids could be a practical and fast approach to boost productivity.Report by Makanda et al. (2012), Patil (2007) and Bantilan et al. (2004) indicates that sorghum hybrids can out yield non-hybrid cultivars by up to 60%.Despite all these benefits, most national sorghum breeding programs in the region have been focused on development of open pollinated varieties, with less emphasis on hybrids possibly due to lack of suitable parents for hybrid production and lack of means to buy seed every season.Sustainable sorghum hybrid program requires availability of locally adapted male sterile and restorer lines.The International Crops Research Institute for Semi Arid Tropics (ICRISAT) introduced new inbred lines from India and collections from various parts of East Africa but their combing ability has not been studied.Knowledge of general combining ability (GCA) and specific combing ability (SCA) is vital to start a hybrid program.The GCA assesses the average performance of an inbred line in hybrid combinations, while SCA identifies the crosses in which its combinations perform relatively better or worse than would be expected on the basis of GCA of the parents (Reddy et al., 2007).Theobjective of this study was to identify the best hybrids and their parents through estimation of GCA and SCA for yield and yield components of a comprehensive set of introduced inbred lines for sub-humid and dry low-lands of East Africa.

Description of experimental sites
Experiments were conducted in Tanzania (Ukiriguru and Miwaleni) and Kenya (Kiboko) locations respectively.Ukiriguru is found in sub-humid climate (ILCA, 1987) and is located at 2° 43' 0" S and 33° 1' 0" E on 1198 m above sea level.Temperatures vary from 18.3 to 29.6°C and annual rainfall of about 861 mm.Soil is mainly sandy loam.Miwaleni is located at 3° 25' 30" S and 37° 26' 45" E at 720 m above sea level.The soil types are reddish brown and the area experience tropical semi-arid climate.Temperatures range between 10 to 39°C and the annual rainfall ranging from 500 to 700 mm (John, 2010).Kiboko lies between 37°45'E and 2°15'S at 960 m above sea level and experiences a semi-arid tropical climate with a bimodal rainfall pattern.The annual rainfall is 655 mm (www.kari.org).The temperature varies from 13.7 to 24.7°C.The soil type at this location is sandy clay group.

Development, selection and evaluation of hybrid sorghum
A total of 121 sorghum lines including 36 pairs of male sterile (A, B lines) and 42 restorers (R-lines) were obtained from ICRISAT-Nairobi (Appendix 1) for evaluation and generating experimental hybrids.Production of the hybrids was conducted at Kiboko in 2010.Seed for all parents was hand planted in 2-m rows.Two rows of A-lines were grown parallel to 1 row of B-lines (for maintenance of A-lines and data collection on yield) alongside a block of R-lines.Each R-line occupied a single row.All plants were bagged before flowering to avoid cross pollination.Pollen was collected in paper bags from R-lines in morning (before 11:00) and dusted on to female panicles.Each single head of A-line was pollinated by single R-line and both bagged right after pollination.A total of 353 hybrids developed but only 121 had enough seed for multi-location testing to determine combining ability.These hybrids were sown in single, 4-m rows with 60 cm between rows and 50 cm between plants.A basal fertilizer application of 20 kg ha -1 (N/ha), and 20 kg ha -1 (P/ha) was applied during sowing.Five plants from each entry were selfed with pollination bags before flowering to determine the fertility status of the hybrid.Pollination bags were removed at the soft dough stage and the seed set on bagged heads was assessed visually using a scale of 0 to 100%; where 0% represented a completely sterile head without seed set, and 100% represented a completely fertile head with complete seed set.Thinning was done two weeks after emergence to 2 plants per hill.Top-dressing with urea, at the equivalent of 45 kg ha -1 was done at four weeks after emergence.Other agronomic practices including weeding and disease control was practiced as per requirements.Data were recorded for days to 50% flowering (whole-plot), plant height, tillers per plant, panicle length, panicle width, panicle exsertion, grain colour and grain yield using sorghum descriptors (IPGRI, 1993) on the five plants that were randomly selected and bagged before flowering.

Statistical analysis
The GCA and SCA effects were determined using SAS General Linear Model (GLM) procedure, (SAS Institute 2008, SAS V9.2).Both GCA and SCA effects were significantly different at P<0.05 and were calculated according to Kearsey and Pooni (1996) Where by: and Note: , = mean performance of female and male lines in crosses respectively; and = GCA for female and male parents respectively; µ = grand mean of all crosses.
where: = SCA effects of the two parents in the cross; = observed mean value of the cross; = expected value of the cross basing on the GCA effects of the two parents; and = GCA for female and male parents respectively and µ = grand mean of the crosses.

RESULTS AND DISCUSSION
Data on mean monthly temperature, rainfall and relative humidity from three locations are presented in Figures 1, 2 and 3 respectively.Ukiriguru experienced high relative humidity (77 to 79%) and temperatures (18.4 to 29.3°C)     Seed set percent range adopted from sorghum descriptors (IPGRI, 1993) especially during flowering (February).The mean monthly rainfall was lower (102 mm average) during the same period.Miwaleni location was characterised by relatively higher monthly rainfall (average of 156.2 mm), low temperatures (17.3 to 24.4°C) and low relative humidity (54 to 66.3%) during flowering (March).Kiboko experienced similar conditions to Miwaleni except that rainfall was relatively lower (114 mm) in March.Differences in grain yield and its associated traits between environments could be due to location's differences in weather during growing season and genetic potential of the specific cultivar.Significant variations in sorghum for yield and yield traits across environments have also been reported by Warkard et al. (2008).Kiboko location received relatively higher rainfall than other location resulting to overall high grain yield.Differences among crosses and among male lines were significant (P ≤ 0.05) for days to 50% flowering, productive tillers, plant height, panicle length, panicle width and yield (Table 1) indicating broad genetic diversity of sorghum materials used in this study.There was no significant difference between female parents.This could be due to the fact that, the female lines were purposely derived for developing hybrids suitable for dry lowlands and sub humid environments hence comparatively same background.Moreover, the differences recorded for parents and crosses imply that the materials are suitable for combining ability studies.The interaction between females and males were not significantly different for days to 50% flowering, plant height and panicle exsertion.The significant differences for Female × Male interaction for the productive tillers, panicle length, panicle width, panicle shape and grain yield indicate high contribution of SCA effects to those traits and, therefore, predominance of non-additive gene action.Similar results were reported by Vinaykumar et al. (2011).This necessitated testing the parents and hybrids for GCA and SCA effects across several environments and enable identification of outstanding cultivars for general and specific adaptation.
The summary of fertility restoration for experimental hybrids tested at Kiboko and Miwaleni is presented in Table 2.There was high difference in seed setting among the hybrids (Figure 4).Most of the test hybrids, 313 (93%) exhibited ≥ 80% seed set, with Kiboko registering higher values than Miwaleni.Only 110 (32%) of the hybrids had 100% restoration; among those, 64 were at Kiboko, and 46 at Miwaleni.One hundred and twenty hybrids (35.6%) did not produce seed at all in the bagged panicles in both locations.Three female lines A2DN55, ICSA479, ICSA469, consistently produced poor hybrids in terms of seed set irrespective of male parent used.A total of 171 hybrids were within the recommended fertility restoration range, 80 to 100%, for multi-location advanced trials.Due to seed availability, only 121 hybrids were tested in three sites alongside their parental lines for yield and its components and combining ability.There were significant differences observed in fertility restoration among hybrids and could be attributed to the specific interaction between the male and female parent genotypes and the environmental influences.Relatively lower mean temperatures at Ukiriguru and Miwaleni coupled with high relative humidity could have resulted in the low seed set.Effect of temperature and relative humidity has also been reported by Leland and House (1985).
The hybrids that failed to produce seed on the bagged panicles indicates that the corresponding male parents in such hybrid were non-restorers as also reported by Singh et al. (1997), and could serve as a source of A-lines.The hybrids that expressed full seed set in some bagged panicles but not others within and across environments were an indication that the male parents for such hybrids were segregating for fertility restoration, and cannot be used as they are in a breeding program (Murty et al., 1994).The A-lines A2DN55, ICSA479 and ICSA469 produced poor hybrids in terms of seed set irrespective of male parent could be due to the environmental effects and/or the genetic background of the A-line (Sleeper and Poehlman, 2006).Purification through recurrent backcrossing is recommended for these lines before used for hybrid production.Since these male sterile lines were recently introduced into Africa from different climatic conditions, some could be poorly suited for the new agroecologies.The temperature at the three locations ranged between 18 and 29.3°C which is within the optimum range for most sorghum cultivars (Reddy et al., 2007).
Negative GCA for plant height, days to flowering and positive GCA for yield and productive tillers is desired for a good genotype.This study found no parent that exhibited high and desired GCA for all traits evaluated including yield, plant height productive tillers (Table 3).The top 3 male sterile and restorer lines for early flowering were MB6, CK60B, ICSB11, and IESV 23010DL, S35, SP74279.Early maturing sorghum hybrids and parental lines could be favourable for semiarid areas because they can utilize the limited moisture available and hence escape terminal drought.The malesterile lines and restorer lines for plant height that expressed high and negative GCA were ICSB91002, ICSB89004 and ICSB90001; and ICSR24007, ICSR89001 and ICSR38.Negative GCA for plant height in sorghum is preferable as it is directly related to dwarfness, hence making plants less susceptible to lodging (Singh et al., 1997) and easier to handle for harvesting.Modification of plant height could be possible using the above lines as the height in those lines was determined by a relatively large proportion of additive genes, as shown by their significant GCA effect.The potential general combiners for productive tillers were ICSB654, ICSB687, and ICSB479 and ICSR153, Siaya#66-2, and IESV23011DL.A total of 14 male sterile parents revealed significantly negative (undesirable) GCA on productive tillers per plant of which SDSB4, ICSB366 and ICSB9 expressed highly negative significant effects.Tillering is generally among important traits affecting accumulation of biomass and ultimately grain yield in sorghum.Hammer et al. (1996) reported significant yield advantage of high-tillering sorghum types when water was plentiful, whereas such types incurred a significant disadvantage under water-limited circumstances.Generally, tillering is undesirable in sorghum male sterile lines as this give rise to a range in seed size and maturity in the field but it is desirable in pollen parent (restorers) as this gives a longer duration of pollen shed, as stated by Singh et al. (1997).
Panicle exsertion is an important attribute for clean seed in sorghum.The expression of GCA effects ranged from -7.1 (ICSB479) to 9.7 (MB6).Negative GCA for panicle exsertion is undesired (Dogget, 1988), because the leaf sheath provides favorable conditions for fungi and insects to develop at the base of the panicle hence extend to the whole panicle.The line MB6 is therefore the best source breeding material for well exerted-panicle sorghum hybrids.Positive and significant GCA effect on panicle width was recorded on 11 male sterile lines and 20 restorers.The male sterile lines ICSB687, ICSB88001 and ICSB293 were the best general combiners for panicle width.Basing on the same trait for the restorers, ICSR24008, IESV23011 and ICSR93034 had positive and significant GCA effects.Four lines; SDSB4, ICSB90001, ICSB88001 and ICSB89004 were best general combiners for panicle length across environments.The least general combiners for panicle length were ICSB479, MB6 and ICSB44 among the female lines.The best restorers for panicle length were ICSR89059, ICSR43 and ICSR89001.Panicle characteristics including length, width and shape is positively related to the final yield in sorghum as also reported by Can et al. (1997).Long, broad and compact panicles results into higher yields compared to their counterparts.
The best general combiners for grain yield were ICSB293, ICSB6, ICSB15 and BTX623, for female lines, and ICSR23019, Tegemeo, IESV91104DL and KARI MTAMA1 for restorers.In general, the means from all locations indicate that line ICSB687 expressed significant negative (desired) GCA effects for four traits viz days to 50% flowering, mature plant height, panicle length and panicle width.This parent could be utilized as a source of breeding lines for both dry lands and sub-humid areas.The potential combination for developing hybrids from the best parents basing on the GCA effects of the parents can be easily worked out and ranked (Table 4).The rank for the combination is obtained by taking combining ability as significant positive (high), non-significant (average) and significant negative (low).For days to 50%   dry lands environments.Restorer lines IESV91104DL, IESV91131DL, ICSR93034 were well suited to dry lands while KARI-MTAMA1 and IESV23019 were better adapted to sub-humid environments.These materials could be employed in hybrid program to produce high yielding, short and early maturing hybrids in East Africa and regions with similar condition.The information gathered is essential in selecting parental lines for producing suitable hybrid for particular agro-ecological zones of East Africa.

Figure 4 .
Figure 4. Fertility status of some hybrids tested at Kiboko and Miwaleni (a) fully restored (b) partially restored (c) extremely low restoration on bagged panicles indicated by arrows.

Table 1 .
Analysis of variance for some traits evaluated in sorghum across dry lands and sub-humid environments of Tanzania and Kenya.

Table 2 .
Rating scale and summary for seed set of sorghum evaluated at Kiboko and Miwaleni in 2011 season.

Table 3 .
Estimates of general combining ability (gca) for selected traits in some sorghum lines evaluated across 3 locations.

Table 5 .
Specific combining ability (sca) effects of sorghum hybrid parents for various traits across dry low land and sub-humid environments.