Crop Science 43:483-488 (2003)
© 2003 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Genetics of Suicidal Germination of Striga hermonthica (Del.) Benth by Cotton
C. J. Botanga*,a,
S. O. Alabib,
C. A. Echekwub and
S. T. O. Lagokec
a Dep. of Biology, Gilmer Hall, Univ. of Virginia, Charlottesville, VA 22903
b Dep. of Plant Science, Ahmadu Bello Univ., Zaria, Nigeria
c Univ. of Agriculture, Abeokuta, Nigeria
* Corresponding author (cjb2v{at}virginia.edu)
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ABSTRACT
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The germination of giant witchweed [Striga hermonthica (Del.) Benth], a noxious root parasite of many cereal crops, is stimulated by exudates from the roots of both host and non-host trap plants. Forty genotypes of cotton (Gossypium hirsutum L. and G. barbadense L.), a trap crop, were screened in the laboratory, using the cut-root technique, to investigate the variability among these genotypes for their ability to stimulate suicidal germination of S. hermonthica and to determine the inheritance of the trait. The genotypes exhibited significant differences for the trait. S. hermonthica seed germination percentages ranged from 13.3 to 50.0% for the cotton genotypes compared with 47.3% for the susceptible sorghum [Sorghum bicolor L. Moench)] cultivar CK60B. Three cotton genotypes were selected based on their S. hermonthica seed germination stimulation and used as parents in crosses of the combination low x high S. hermonthica seed germination stimulation. The F1s, F2s, and parents of the crosses, RASA(78)11b x ‘SAMCOT-10’ and RASA(78)11b x TX-CABS-1-83 were evaluated in separate experiments in batches of 12 entries. Broadsense heritability estimates for the trait ranged from 71.8 to 78.5%. This was reflected in the discrete frequency distribution of the F2 populations into two classes of high and low S. hermonthica seed germination stimulation, fitting a classical 3:1 phenotypic ratio. These results suggest that S. hermonthica seed germination stimulation by cotton is a qualitatively inherited trait, and that the gene controlling this trait is monogenic and simply inherited, with high S. hermonthica seed germination stimulation dominant over low S. hermonthica seed germination stimulation. It should be possible to select and breed cotton genotypes that produce highly active germination stimulants in large amounts, while maintaining or improving other agronomic attributes.
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INTRODUCTION
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THE GENUS Striga of the family Scrophulariaceae is composed of some 50 species, all holoparasites of tropical cereals and legumes (Butler, 1995). Striga species (witchweeds) are noxious parasitic weeds that attack the roots of cereals and legumes, causing serious yield loss. Striga and related parasitic weeds constitute the most important biotic constraint to food crop production in sub-Saharan Africa, causing between 10 and 100% loss in crop yield, while the rate of spread to areas not hitherto infested is alarming (Lagoke et al., 1995). In northern Cameroon and other areas in the Sahelian zone, Striga species cause great losses in the staple foods, sorghum and millet [Pennisetum typhoides (Burm. f.) Stapf & CE Hubb.] (Pieterse, 1985). In addition, Striga species in West Africa attack upland rice (Oryza sativa L.), maize (Zea mays L.), sugarcane (Saccharum officinarum L.), and cowpea [Vigna unguiculata (L.) Walp]. In northern Cameroon, two-thirds of the 600 000 ha of cultivated land is severely infested by Striga (Njinyam, 1985). A severe infestation can result in complete loss of the crop and abandonment of otherwise productive fields (Butler, 1995). Well over a decade ago, a conservative estimate of crop losses due to Striga in Africa was 40%, representing an annual loss of cereal worth U.S. $7 billion (M'boob, 1988). There are indications that these figures are much higher today.
Of the 23 species of Striga identified in Africa, S. hermonthica (giant witchweed) is the most ubiquitous species, causing much of the problems in the Sahelian areas (Pieterse, 1985). Striga hermonthica and S. asiatica (L.) Kuntze are the species which cause the most economically significant damage to cereals. Striga gesnerioides (Willd.) Vatke is the species most serious on cowpea and tobacco (Nicotiana tabacum L.) (Butler, 1995).
There is no single effective and economically feasible Striga control method available to the small scale African farmer (Lagoke et al., 1991; Butler, 1995). The control methods identified include land preparation, hand pulling and hoe weeding, use of trap and catch crops, use of nitrogen fertilizer, seed treatment, chemical stimulants, biological control, herbicide application, and the use of resistant cultivars. Of these methods, the two most promising and culturally acceptable ones are suicidal germination (trap cropping) and the use of resistant cultivars, both of which are primarily concerned with manipulating Striga seed germination and seedling establishment (Sahai and Shivanna, 1982). It would appear that the major problem associated with the use of resistant cultivars is the lack of universal resistance. This is probably due to the existence of different biotypes of Striga (Efron et al., 1988). Striga hermonthica is cross pollinated, and thus generates a great amount of variability through genetic recombination, making improvement of host resistance against the weed a difficult task (Olaniyan et al., 1993). In a study of S. hermonthica populations from West and East Africa using isozyme and RAPD analyses, Koyama (2000) observed significant variation within and between the populations, while Roman et al. (2001) using RAPD markers, made similar observations on populations of Orobanche crenata (Forst.), a related parasite to Striga species. Ejeta et al. (1992) pointed out that genetic differences among host germplasm may be obscured by diverse and shifting population of the parasite, while Berner and Williams (1998) showed that variability in response to germination stimulants exist in S. gesnerioides. These variations have serious consequences for the breeding of broad and durable host resistance.
Trap crops are those crops that induce germination of Striga seeds but are not parasitized, and consequently result in suicidal germination of Striga seeds. Cowpea, pigeon pea [Cajanus cajan (L.) Millsp], cotton, soybean [Glycine max (L.) Merr], and groundnut (Arachis hypogaea L.) when grown in rotation with a susceptible host or as an intercrop, have been reported to induce abortive germination of Striga seeds, with a consequent reduction in infestation (Carson, 1985; Parkinson et al., 1988). Cowpea is, however, susceptible to S. gesnerioides, but is a trap crop to S. hermonthica. The first indication of the possible use of trap crops was the synthesis of a potent germination stimulant, strigol (Cook et al., 1972), a compound extracted from the roots of cotton. In surveys conducted in the Republic of Benin and Togo (West Africa), it was reported that the level of infestation was always lower in cereal crops planted after cotton (Parkinson, 1985).
Variability for traits or characters of interest remains the workhorse of the conventional plant breeder, and more often than not there is the need to introgress trait(s) from a less desirable background to more desirable agronomic backgrounds or improve a trait in a background of interest. The objectives of this study were to (i) investigate the variability among selected cotton genotypes with respect to their ability to stimulate germination of S. hermonthica seeds so as to assess the possibility of improving this crop for the trait and (ii) ascertain the genetic control and mode of inheritance of S. hermonthica seed germination stimulation.
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MATERIALS AND METHODS
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Origin and Description of Genetic Materials
Forty genotypes of cotton obtained from the Fiber Research Program of the Institute for Agricultural Research (I.A.R), Ahmadu Bello University, Zaria, Nigeria were screened in the Weed Science laboratory of the Department of Agronomy (commencing in January of 1996) for their ability to stimulate the germination of S. hermonthica seeds. The 40 genotypes consisted of seven breeding lines of G. barbadense and 33 genotypes of G. hirsutum including eight commercial cultivars, 10 eastern zone breeding lines, 10 northern zone breeding lines, and five exotic (not bred in Africa) breeding lines.
Three genotypes of cotton were then selected on the basis of their performance with respect to their ability to stimulate S. hermonthica seed germination and used as parents in the generation of genetic populations. These genotypes were RASA(78)11b (P1), for low S. hermonthica seed germination stimulation, and SAMCOT-10 (P2) and TX-CABS-1-83 (P3), for high S. hermonthica germination stimulation. Crosses were made in each case between the low germination stimulant producer and the high germination stimulant producer; RASA(78)11b (P1) x SAMCOT-10 (P2) and RASA(78)11b (P1) x TX-CABS-1-83 (P3). The F1s resulting from these crosses were later advanced to the F2 generation through controlled self-pollination. At the same time, crosses were made to obtain fresh F1s.
Screening Technique
Forty genotypes of cotton were screened in the Weed Science Laboratory of the Department of Agronomy by means of the cut root assay developed by Van Mele et al. (1992), with minor modifications. The seeds of each cotton genotype were delinted with concentrated sulphuric acid, rinsed several times with tap water and then sun-dried. The seeds were surface sterilized by soaking in 1% (v/v) sodium hypochloride (NaOCl) solution for 5 min and then rinsed several times with distilled water. The seeds were then placed in sterile Petri dishes lined with moistened filter paper. The Petri dishes were wrapped in aluminum foil and incubated at 28°C for 48 h to germinate. The germinating seeds were planted in sandy soils contained in small plastic pots (625 mL by volume) and grown in two sets (duplicate) for 14 or 21 d. Moist glass microfiber filters (5.5 mm in diameter) were placed in Petri dishes lined with a double layer moistened Whatman No. 2 filter paper of 9-cm diam. Striga hermonthica seeds, collected in 1991 at Hayan Ojo (Kaduna State, Northern Guinea savanna- Nigeria), were surface sterilized in 1% NaOCl for 3 min, rinsed several times with distilled water and then air-dried at room temperature. Striga hermonthica seeds were then spread one layer thick on the moist glass microfiber filter disks, wrapped with aluminum foil, and incubated at 28°C on the same day that the germinating cotton seeds were planted into small plastic pots. At either 14 or 21 d after sowing, roots were obtained from the plants, washed free of soil, and 1.0 g of root pieces were placed in a cone (made of aluminum foil) at the center of the Petri dishes lined with filter paper. Twenty disks of glass microfiber filters (5 by 4 disks) with preconditioned S. hermonthica seeds were placed in a diverging manner from the central root core in the Petri dishes (Fig. 1). This assay was set up in three repetitions for each cotton genotype and thus functioned as replication in this experiment. The Petri dishes were covered and sealed with tape, wrapped in aluminum foil, and incubated at 28°C for 3 d, after which germinated and ungerminated S. hermonthica seeds were counted with the aid of a microscope. Susceptible sorghum cultivars SAR35 and IS1260 were used as controls in the preliminary trial aimed at establishing the level of variability among selected cotton genotypes (from different genetic backgrounds) for their ability to stimulate S. hermonthica seed germination. A confirmatory trial to check for consistency of results in potential parents for use in a genetic study was conducted with 12 genotypes of cotton whose selection was based on the results of the preliminary trial. A susceptible sorghum cultivar, CK60B, was used as the control in this experiment. The protocol for this confirmatory trial was the same as that described for the preliminary trial.
Because of the laborious procedure required in the evaluation for the trait, the parents, F1, and F2 were evaluated in separate experiments in batches of 12 individual plants in each trial. Twelve plants each for the parents and F1s, and 50 F2 plants for each cross were evaluated, with CK60B as the control.
Statistical Analyses
Data analyses for both the preliminary and the confirmatory experiments involved the analysis of variance (ANOVA) as a completely randomized experimental design. Separation of treatment means was done by the Duncan Multiple Range Test (DMRT). The statistical model used in the analysis of the crosses was:
where µ = population mean, Gi = genetic effect of the ith genotype and G is assumed to be normally and independently distributed (NID) (µ = 0, 
2G), and Eij = random error and this is also NID (µ = 0, 2E) i = 1,2,... n genotypes and j = 1,2,... n replicates.
Segregation in the F2 population was tested by Chi-square (
2) for goodness of fit to a 3:1 phenotypic ratio. Genotypes with means not significantly different from the high parent (based on DMRT) were considered to be high, while those not significantly different from the low parent were considered to be low. Broadsense heritability and genetic components were estimated by means of variances obtained from the ANOVA with one-way classification in a completely randomized design. Broadsense heritability was also estimated by means of Weber and Moorthy (1952). An estimate of the minimum number of genes controlling S. hermonthica germination stimulant production in cotton was obtained on the basis of Wright (1921) and Castle and Wright (1921). The phenotypic and genotypic coefficients of variation for S. hermonthica germination stimulant production in the F2 populations were calculated by the formula suggested by Burton (1952).
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RESULTS AND DISCUSSION
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S. hermonthica Seed Germination Stimulation by Cotton Genotypes
In general, higher S. hermonthica seed germination was recorded with roots obtained from 3-wk-old plants and S. hermonthica seeds preconditioned for the same period, than the corresponding 2-wk crop growth stage and preconditioning period (Table 1 and Table 2). The mean percent germination at 2- and 3-wk crop growth stages were 25.2 and 32.5%, respectively, in the preliminary screening, and 31.4 and 36.2% for the confirmatory trial conducted with some selected genotypes (Table 2). This is an indication that S. hermonthica germination increased with a combined increase in the stage of crop growth and the length of preconditioning period. We made similar observations while evaluating some genotypes of cowpea, soybean, and groundnut for their possible use as trap crops to S. hermonthica (C. Botanga, and S. Lagoke, unpublished data, 1997). Our result is consistent with reports of increased sensitivity of Striga seeds to germination stimulants with length of preconditioning period (Worsham, 1987; Babiker et al., 1993). Babiker et al. (1993) also pointed out that a major step in the preconditioning process is the release of a restriction on ethylene biosynthesis from the precursor 1-aminocyclopropane-1-carboxylic acid (ACC).
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Table 1. Germination of S. hermonthica seeds induced by some cotton genotypes at 14 and 21 d growth stages (preliminary trial).
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Table 2. Germination of S. hermonthica seeds induced by some cotton genotypes at 14 and 21 d (confirmatory trial).
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The results indicated wide variability among cotton genotypes for their ability to stimulate S. hermonthica germination. Butler (1995) observed that sorghum genotypes showed wide differences in their capacity to produce germination stimulant, but relatively little variability in their capacity to produce haustorial initiation factor, and that the inheritance of these traits was completely independent. Generally, the eastern zone breeding lines and the exotic cultivars stimulated higher percentages of S. hermonthica seed germination compared with the northern zone breeding lines (Table 1). In the preliminary trial, Bar14/25(81)16, BarXL7(79)35, Bar14/25(81)14, ‘SAMCOT-8’, RSA(79)4a, ASA(78)17b, ASA(74)80c, Y1422(79)30c, ACSA(79)5f, Y1422(79)19b, and ‘TAMCOT sp. 21S’ stimulated S. hermonthica seed germination comparable to the maximum of 41.7% obtained with TX-CDP37HH-1-83 when cotton was planted for 14 d and S. hermonthica seeds incubated for the same period (Table 1). A number of cotton genotypes and the two sorghum cultivars (SAR 35 and IS1260) were not significantly different from the lowest (13.3%) observed, ASA(75)13b (Table 1). In a combination of 3-wk-old plants and S. hermonthica seeds preconditioned for a corresponding period, TX-CABS-1-83 recorded the highest S. hermonthica seed germination percentage (50.0%) which was comparable to that of SAMCOT-10, and TX-CDP37HH-1-83 (Table 1). RASA(78)11b and the two sorghum cultivars were not significantly different from Y1422(70)19 g which had the lowest percentage germination (13.3%) (Table 1). The two sorghum cultivars, SAR 35 and IS1260, gave relatively poor results at 2 and 3 weeks suggesting that they only promote the germination and subsequent emergence of S. hermonthica at an advanced stage under field conditions. This probably explains the tolerance of IS1260 to S. hermonthica. These cultivars were selected for use as controls based on their support of high emergence of S. hermonthica in the field, usually at 9 to 12 wk after sowing (C. Botanga, and S. Lagoke, unpublished data, 1997). A susceptible cultivar, CK60B, however stimulated a high level of germination in the laboratory when used later as the control (Table 2). It was therefore a more suitable control in the confirmatory trial. All the other genotypes recorded a moderate S. hermonthica seed germination (>20.0%). ACSA(79)8c, ASA(78)17b, SAMCOT-10, and TX-CABS-1-83 stimulated S. hermonthica seed germination comparable to that of the susceptible sorghum control (Table 2). Moderate levels of germination (>35.7%) were recorded by RSA(79)4a, ACSA(79)5f and ‘TAMCOT-CAMD-E’. SAMCOT-9 and RASA(78)11b recorded a low germination of 24.3 and 19.0%, respectively, with 19.0% being the lowest observed in the trial. At the 2-wk crop growth stage and S. hermonthica seeds preconditioned for a corresponding period, CK60B out-performed all the cotton genotypes. ASA(78)17b, SAMCOT-10, TX-CABS-1-83, and TX-CDP37HH-1-83 were superior to all the other cotton genotypes except ACSA(79)5f. SAMCOT-9 and RASA(78)11b induced a relatively low S. hermonthica seed germination with 21.7 and 22.3%, respectively. The wide variability observed among cotton genotypes with respect to their ability to stimulate S. hermonthica seed germination is an indication of the enormous potential for improving the crop for this trait. Dejongh et al. (1993) reported variabilities in Striga seed germination stimulation among crop cultivars and types including cotton (Gossypium spp.). The variability observed in these genotypes could be attributed to inherent genetic differences among the genotypes for the trait in question, as reflected in the background of the genotypes. Aggarwal (1988) reported that genetic control varied with parental source background and the mechanism of resistance in cowpea.
Generation Means, Segregation Pattern, and Genetics of Suicidal Germination
The mean performance of RASA(78)11b (P1), SAMCOT-10 (P2), TX-CABS-1-83 (P3), F1, and F2 generations resulting from the crosses P1 x P2, P1 x P3 (low x high) are presented in Table 3. The mean of the F1s resulting from the cross, P1 x P2 is very close to that of the high S. hermonthica germination stimulating parent, while that resulting from the cross, P1 x P3 is almost exactly equal to that of the high germination stimulating parent. In both crosses, all the F1s were high S. hermonthica seed germination stimulating plants (Table 3). While the differences within the parentals and F1s where not significant (on the basis of DMRT), the differences among the F2 genotypes were highly significant. The F2 mean for each of the crosses was about equal to the mid-parent value in each case (Table 3). The closeness of F1 means to the high parents, and the fact that all the F1s were high germination stimulant producers suggests that high S. hermonthica germination stimulation is dominant over low S. hermonthica germination stimulation (Table 3). The dominance of high S. hermonthica germination stimulation over low S. hermonthica germination stimulation production was also reflected in the means of the F2 generation for both crosses, which are very close (P1 x P2) to the mid-parent value or slightly above the mid-parent value (P1 x P3) (Table 3).
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Table 3. Germination of S. hermonthica seeds induced by genetic populations (parents, F1 and F2) of cotton from the crosses RASA(78)11b x SAMCOT-10 (P1 x P2) and RASA(78)11b x TX-CABS-1-83 (P1 x P3).
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The F2 distribution fell into two distinct phenotypic classes of high and low S. hermonthica seed germination stimulation. At least 64.0% of the plants were high stimulant producers in each of the crosses, while 36.0% were low stimulant producers, with 14 and 10 plants being lower than the low parent in the cross P1 x P2 and P1 x P3 (Table 3), respectively. The rest of the plants falling within the low germination stimulating groups were either equal to, or slightly higher than the low stimulant producing parent. The 2 test for goodness of fit to classical 3:1 phenotypic ratio indicated that the pattern of segregation in the F2 fits a ratio of 3 high: 1 low S. hermonthica germination stimulating plants for each of the crosses. The goodness of fit for the cross P1 x P3 was closer than that for the cross P1 x P2. This difference may be attributed to inherent genetic differences between the two high S. hermonthica germination stimulating parents, as they differ in their genetic background. The dominance of high S. hermonthica germination stimulant production in cotton is supported by earlier reports that high germination stimulant production was dominant in sorghum (Hess and Ejeta, 1992). The crosses used in this study did not involve any reciprocals because of the large number of genotypes to be evaluated. Consequently the possibility of maternal effects or cytoplasmic inheritance on the control of this trait (S. hermonthica seed germination stimulation) cannot be ruled out.
Heritability Estimates and Gene Numbers
Broadsense heritability estimates determined on the basis of genetic components resolved from the ANOVA were 74.6 and 78.5% for P1 x P2 and P1 x P3, respectively. The estimates obtained by means of Weber and Moorthy (1952) formula were slightly lower, with values of 71.8 and 72.2% for P1 x P2 and P1 x P3, respectively. This discrepancy is a result of the larger error variance obtained by the Weber and Moorthy (1952) formula that incorporates the nonsegregating population variance as an estimate for the environmental variance. In each case, these values suggest the trait is under genetic control.
Estimates of the minimum number of genes controlling Striga germination stimulation in the crosses P1 x P2 and P1 x P3 were less than one, as determined on the basis of either the Wright (1921) or Wright and Castle (1921) formula. Using Wright's (1921) formula, we obtained values of 0.72 and 0.67 for P1 x P2 and P1 x P3, respectively. The estimates determined on the basis of Wright and Castle (1921) were slightly lower for the cross P1 x P2 (0.68 gene), but gave a similar value (0.67 gene) for the cross P1 x P3. In either case, these estimates suggest that S. hermonthica germination stimulation is monogenically controlled, and that it is a qualitatively inherited trait. This finding is also reflected by segregation in the F2 generation into two distinct classes of segregates. It is worth noting here that the materials used in this study did not meet all the assumptions that go with the formulae used in estimating the number of genes. For instance, Castle and Wright (1921) assumed that both parents are homozygous and that there is no dominance, with each gene having an equal effect. The estimates obtained by means of the Wright (1921) formula are more acceptable since the materials used in the study appear relatively close to meeting the assumptions that go with this formula.
Although this study did not set out to investigate the mechanism of suicidal germination, it was observed that cotton genotypes have the ability to induce haustoria formation in S. hermonthica seeds. It would appear therefore that cotton produces the second chemical required for haustoria initiation. Thus, the failure of this crop to support the growth of S. hermonthica is likely not a result of the absence of the haustoria initiation chemical. Consequently, there is a need to investigate the mechanism of suicidal germination in the cotton plant.
The high heritability and simple inheritance of S. hermonthica suicidal germination indicates that the trait should be easily incorporated into cultivars with good agronomic attributes for use in Striga infested areas where the crop can be grown.
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ACKNOWLEDGMENTS
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This work was supported in part by the Food and Agricultural Organization (FAO) through the Pan African Striga Control Network (PASCON). The Authors wish to thank Mr. Moses I. Aboh, of the Weed Science Laboratory, Department of Agronomy, Ahmadu Bello University Zaria (Nigeria), for his assistance during the laboratory aspect of this study.
Received for publication March 28, 2001.
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ABSTRACT
INTRODUCTION
