Crop Science 43:777-781 (2003)
© 2003 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Effect of Cytoplasm on the Agronomic Performance of Grain Sorghum Hybrids
J. L. Moran and
W. L. Rooney*
Dep of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843-2474
* Corresponding author (wlr{at}tamu.edu)
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ABSTRACT
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Hybrid sorghum [Sorghum bicolor (L.) Moench] seed production relies exclusively on cytoplasmic male sterility (CMS) systems and almost all hybrid sorghum seed is produced using the A1 CMS system. However, the reliance on a single CMS system increases the vulnerability of the crop to diseases and stresses that may attack that particular CMS system. Alternative CMS systems have been described and even used on a limited basis for hybrid seed production, but a direct comparison of the agronomic effects of different cytoplasms has not been possible because male-sterile lines with a common genetic background (and different cytoplasm) were not available. The recent development of isocytoplasmic A-lines allows more direct comparison of cytoplasmic effect on agronomic performance. The purpose of this study was to determine by means of a set of isocytoplasmic hybrids if cytoplasm per se influences agronomic performance. Twelve hybrid genotypes were created in three different cytoplasms (A1, A2, and A3 for a total of 36 hybrids), and they were evaluated for plant height, days to anthesis, and grain yield at Weslaco and College Station, TX, in 1998 and 1999. As expected, significant differences existed among hybrids for plant height, days to anthesis, and grain yield. Cytoplasm type had no effect on plant height and was of minimal practical effect on days to anthesis, but a significant reduction in yield was observed in A3 cytoplasm hybrids as compared with A1 and A2 cytoplasm hybrids. The specific reason for the reduced yield of A3 hybrids is not known, but seed set data indicated that it was not associated with fertility restoration. The results indicate that hybrids created in A2 cytoplasm yield comparably to the commonly used A1 cytoplasm and therefore, the A2 system will provide a suitable alternative for hybrid seed production should problems be encountered in the A1 CMS system.
Abbreviations: CMS, cytoplasmic-genetic male sterility • DTA, days-to-anthesis
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INTRODUCTION
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THE DISCOVERY of cytoplasmic male sterility in grain sorghum led directly to the development of the hybrid grain sorghum industry (Stephens and Holland, 1954). This particular source of male sterility, designated as the A1 CMS system, has been the predominant source of male sterility in hybrid sorghum breeding programs for the past 40 yr.
In 1970, the Southern Leaf Blight epidemic [caused by Cochliobolus heterostrophus (Drechs.) Drechs. (anamorph: Bipolaris maydis (Nisikado & Miyake) Shoemaker = Helminthosporium maydis (Nisikado & Miyake)] in corn (Zea mays L.) devastated corn hybrids possessing T cytoplasm (Tatum, 1971). To avoid similar problems in sorghum, additional CMS systems were identified and developed for use in sorghum hybrid breeding programs. The A2 system was described by Schertz (1977), followed by the A3 system by Schertz (1984), and the A4 system by Worstell et al. (1984). In addition, several additional systems are known to exist but the research to classify and characterize these systems has not been completed (Schertz et al., 1997).
From these systems, sorghum breeders have released improved lines for use in sorghum breeding programs. Miller (1986) released A2Tx632, followed soon after by A2Tx636 and A2Tx637 (Miller et al., 1992a). These A/B lines are agronomically adapted male steriles that possess the A2 CMS system. Miller (1984b) released RTx432 and this line is a fertility restorer for the A2 sterility system. In addition, several researchers have released male sterile lines using the A3 CMS system for use in breeding programs (Pederson, 1997; Pederson et al., 1997; Schertz et al., 1997), but the utilization of A3 CMS has been limited because of the paucity of elite male restorers for this system.
Concurrent with the development of this germplasm, it was necessary to determine the effect of cytoplasm on agronomic performance. However, studies evaluating the agronomic potential of different CMS systems are inconsistent in their results. Kishan and Borikar (1988) reported that A2 is superior to A1 for grain size and yield while Maves and Atkins (1988) observed a yield decrease in A2 hybrids compared with A1 hybrids. Secrist and Atkins (1989) who found no significant differences (P > 0.05) in grain yield between A1 and A2 hybrid, but they reported a 6% reduction in grain yield in A3 hybrids compared to A1 hybrids. The combined results from these studies indicate that there is no clear effect of cytoplasm on the agronomic performance of the hybrids.
In all of the previously described studies, the genotypes of the hybrids differ among the cytoplasm systems. It is possible that the variable results seen in these studies may be due to differences in agronomic adaptability among the genotypes evaluated. To determine the effect of cytoplasm on the agronomic performance of sorghum hybrids, the hybrids should be genetically similar, differing only for cytoplasm.
Sorghum breeders and geneticists have developed isocytoplasmic versions of several elite sorghum parental inbred lines. The term isocytoplasmic indicates that these parental lines are genetically similar and phenotypically identical but they differ for specific sterility-inducing cytoplasms. This germplasm provides an opportunity to determine the effect of cytoplasm on performance of genetically similar hybrids. The objective of this research is to evaluate a set of isocytoplasmic hybrids to determine the effect of cytoplasm on agronomic performance.
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MATERIALS AND METHODS
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Four female parental lines and three male parental lines were selected for this research. The four female parental lines used in this study were A/BTx378, A/BTx623, A/BTx626, and A/BTx631 (Stephens and Karper, 1965; Miller, 1986). Each of the female parental lines was originally developed and released in the A1 CMS system. A2 and A3 isocytoplasmic versions of each line were developed through backcrossing by the methodology described by Miller et al. (1999). The male parental lines used in this study were RTx430, RTx432, and RTx436 (Miller 1984a,b; Miller et al., 1992b). The Texas Agricultural Experiment Station released all these parental lines and they all have been or are currently used for the production of commercial hybrid sorghums.
Using the parental lines, we created 12 hybrid genotypes in three different cytoplasms for a total of 36 hybrids. Each male parent was hybridized to each isocytoplasmic version (A1, A2, and A3) of each female parent. During the summer of 1997, hybrid seed for all 36 hybrids was produced via hand pollination in a crossing block at the Texas Agricultural Experiment Station farm located near College Station, TX.
The set of 36 hybrids was evaluated at Weslaco and College Station, TX, in 1998 and 1999. The tests were grown under the agronomic practices standard for each location. To ensure maximum yield expression, supplemental irrigation was applied as needed to avoid drought stress. In each environment, the hybrids were arranged in a randomized complete block design with three replications. Each experimental unit was composed of two rows 5 m long with row spacing of 0.76 m. Because the male parental lines RTx430 and RTx436 do not restore fertility in the A2 and A3 CMS system, and RTx432 does not restore fertility in the A3 CMS system, pollinator rows were interspersed throughout the test at regular intervals to ensure that pollen was available for pollination of the male-sterile hybrids. These pollinator rows were composed of male fertile hybrids of variable maturity to ensure that pollen was shed throughout the duration of anthesis (Lee et al., 1992). In addition, 50 panicles from both male-fertile and male-sterile hybrids in each environment were visually inspected at the soft dough stage of maturity to determine the frequency of unfertilized florets. This information was used to determine if any differences in seed set existed between male-sterile and male-fertile hybrids.
In each environment, days to anthesis (DTA), plant height and grain yield were measured. Days to anthesis was recorded as the number of days from planting required for 50% of the plants in a plot to reach midanthesis. Plant height was recorded as the height of the plant from the ground to the tip of the main tiller. Grain yield was measured by harvesting the whole plot using a MF8 plot combine. Grain yields were adjusted to 130 g kg-1 moisture.
Data were analyzed following a factorial design. Each environment was analyzed separately for factor significance, and because cytoplasm per se or cytoplasm interactions had limited significance in individual environments, the environments were combined for analysis. Bartlett's test for heterogeneity of error variances (Steel and Torrie, 1980) indicated that the error variances across environments were heterogeneous. However, further analysis of the data failed to reveal a distributional pattern that could be eliminated by data transformation. Therefore, combined analyses of variance were computed on untransformed data. Appropriate F tests were based on expected mean squares where the effects due to cytoplasm, female parent, and male parent were considered as fixed while effects due to environment were considered random. If a respective effect (male, female, or hybrid) was significant, mean separations were performed following the Student-Newman-Keuls (SNK) procedure with a probability level of 0.05.
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RESULTS
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As expected, the combined analysis of grain yield revealed significant differences across environments, parents, and hybrids as well as significant interactions between genotypes and environments. Cytoplasm had a significant effect on yield, and interactions involving cytoplasm, parents, and hybrids were also detected (Table 1). Interactions between cytoplasm and environment were minimal. Across all environments hybrids with A3 cytoplasm yielded significantly less than the hybrids with A1 and A2 cytoplasm (Table 2). When each hybrid was analyzed independently, differences among isocytoplasmic versions of the same hybrid were detected for only three of the 12 hybrids. In each of these three hybrids, the A3 version of the hybrid was the lowest yielding (Table 2). The male x cytoplasm interaction was significant (Table 1), but there were no major shifts in ranks when hybrids within male and cytoplasm were compared (Table 3). The female x cytoplasm interaction was highly significant (Table 1) primarily because ATx631 hybrids did not show a statistical difference in yield while the hybrids of other females did have a significant reduction in yield in either A2 or A3 cytoplasm (Table 3). Because seed set is crucial to grain yield, it is important to note that conditions for pollination were very good in all four environments. Seed set in the primary florets of all hybrids was consistently near 100% and no visual differences in seed set were detected between male-fertile and male-sterile hybrids in any environment.
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Table 1. Combined analysis of variance for grain yield (kg/ha), days-to-anthesis, and plant height (m) for 36 hybrids evaluated at College Station and Weslaco, TX, in 1998 and 1999.
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Table 2. Comparison of average grain yield (103 kg/ha) of 36 hybrids evaluated at College Station, TX, and Weslaco, TX, during 1998 and 1999. Statistical differences among cytoplasms for each hybrid and when combined are designated by letters in each row.
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Table 3. Comparison of average grain yield (103 kg/ha) of four female and three male parental lines in hybrid combinations evaluated at College Station and Weslaco during 1998 and 1999. Statistical differences among cytoplasms within a line and among lines within a cytoplasm are designated by letters and numbers, respectively.
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Several factors affected the DTA behavior of the hybrids, including environments, cytoplasm, male and female parents, and several interactions between these factors (Table 1). While there was a significant effect of cytoplasm on DTA, there were no detectable interactions involving both cytoplasm and environment. When combined, hybrids with A1, A2, and A3 cytoplasms averaged 70, 70, and 71 DTA, respectively (Table 4). While this was statistically significant, differences of only 1 d are of minimal practical importance. In addition, there was a marginal significance (P < 0.035) in the DTA within the different cytoplasms of the same hybrid and differences among isocytoplasmic versions of the same hybrid were detected for only three of the 12 hybrids (Table 4). In these three hybrids, the differences among cytoplasm types were at most one d and differences of this magnitude are of little practical significance. A male x cytoplasm interaction was not detected, indicating that the male parent did not influence days to flowering of a particular cytoplasm. A female x cytoplasm interaction was detected although no important shift in ranks or reactions were observed among the four females (Table 5).
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Table 4. Mean comparisons for average days to anthesis of 36 hybrids evaluated at College Station, TX, and Weslaco, TX, during 1998 and 1999. Statistical differences among cytoplasms for each hybrid and when combined are designated by letters in each row.
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Table 5. Comparison of average days-to-anthesis of four female and three male parental lines evaluated in hybrid combination at College Station and Weslaco during 1998 and 1999. Statistical differences among cytoplasms within a line and among lines within a cytoplasm are designated by letters and numbers, respectively.
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The combined analysis of plant height revealed significant differences across environments and parents, as well as significant interactions between genotypes and environments. However, as a main effect cytoplasm did not influence plant height (Table 1). Surprisingly, differences among isocytoplasmic versions of the same hybrid were detected for five of the 12 hybrids (Table 6). Differences in height in these five hybrids were variable and no specific trend of height and cytoplasm could be detected (Table 6). A male x cytoplasm interaction was not detected, indicating that the male parent did not influence plant height of a particular cytoplasm, but a significant cytoplasm x female parent interaction was detected (Table 1). For example, hybrids of Tx378 with A3 cytoplasm were significantly reduced in height compared with the same hybrid in A1 and A2 cytoplasm while Tx631 hybrids in A1 cytoplasm were reduced in height relative to the same hybrids in A2 and A3 cytoplasm (Table 7).
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Table 6. Mean comparsions for average plant height (meters) of 36 hybrids evaluated at College Station and Weslaco, TX, in 1998 and 1999. Statistical differences among cytoplasms for each hybrid and when combined are designated by letters in each row.
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Table 7. Comparison of average plant height (m) of four female and three male parental lines evaluated in hybrid combination at College Station and Weslaco during 1998 and 1999. Statistical differences among cytoplasms within a line and among lines within a cytoplasm are designated by letters and numbers, respectively.
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DISCUSSION
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In the current study, hybrids with A1 and A2 cytoplasm had similar yield potential and both yielded significantly greater than hybrids with A3 cytoplasm. Kishan and Borikar (1988) reported that A2 cytoplasm was superior to A1 cytoplasm for grain size and yield. Maves and Atkins (1988) observed a decrease in yield of A2 hybrids compared with A1 hybrids. However, Secrist and Atkins (1989) found no significant differences (P > 0.05) in grain yield between A1 and A2 hybrids. Secrist and Atkins (1989) also reported a 6% reduction in grain yield in A3 hybrids compared to A1 hybrids. Our results are consistent with those of Secrist and Atkins (1989) because we observed a 5% reduction in grain yield of hybrids with A3 cytoplasm when compared to hybrids with A1 and A2 cytoplasm. Lee et al. (1992) reported no difference in yield in the combined analysis among 22 hybrids evaluated in both A1 and A3 cytoplasm, but in pairwise comparisons of the individual hybrids differences in grain yield were detected in three hybrids. In each of these cases, the hybrid with A3 cytoplasm was lower in yield. With regard to comparisons within an individual hybrid, the results of Lee et al. (1992) are similar to those found in the current study (Table 2).
Some authors suggest that differences observed in grain yield among A1, A2, and A3 hybrids are due to the lack of fertility that results in lower seed set (Maves and Atkins, 1988). In the current study, seed set in all hybrids was excellent and no differences in seed set among the different hybrids (whether fertile or sterile) were detected in this test. In addition, yield data seem to confirm that the lack of fertility restoration was not responsible for the drop in yield in A3 cytoplasm. For example, RTx436 hybrids in A1 and A2 cytoplasm were equal in yield and significantly reduced in A3 even though RTx436 does not restore fertility in either A2 or A3 cytoplasms (Table 3).
Given these facts, the differences in performance between the cytoplasms must be due to the presence of nuclear–cytoplasmic interactions that are affecting the expression of agronomic characters. These interactions do not involve only the female and the cytoplasm, as indicated by the numerical reduction in yield of Tx631 hybrids in A1 cytoplasm, but they involve the male parent as well, since the performance of RTx436 was affected primarily in A3 cytoplasm (Table 3). These results differ from those of Lenz and Atkins (1981), who reported effects of milo (S. bicolor) and KS cytoplasms were not influenced by the male parent. In the current study, cytoplasm had no practical effect on DTA and plant height.
Several authors have discussed the importance of broadening the range of cytoplasms employed in the production of commercial sorghum hybrids, to increase genetic variability and reduce the risk of disease epidemics directly related to cytoplasmic-genetic male-sterility systems (Ross and Kofoid, 1979; Lenz and Atkins, 1981; Kishan and Borikar, 1988; Secrist and Atkins, 1989). We agree with this approach, but variability should be sought without compromising agronomic performance. Our data indicate that hybrids made with A2 cytoplasm yield comparably to hybrids made with A1 cytoplasm, and that A2 CMS provides a useful secondary CMS system in case it is needed.
Received for publication April 29, 2001.
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REFERENCES
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- Secrist, R.E., and R.E. Atkins. 1989. Pollen fertility and aronomic performance of sorghum hybrids with different male-sterility-inducing cytoplams. J. Iowa. Acad. Sci. 96:99–103.
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ABSTRACT
INTRODUCTION
