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Inst. of Plant Sciences, ETH Zentrum, CH-8092 Zurich, Switzerland
* Corresponding author (peter.stamp{at}ipw.agrl.ethz.ch)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ASI, anthesis–silking interval • CMS, cytoplasmic male sterile/sterility • GPA, general pollinator ability • KN, kernel number per square meter • KW, kernel weight • masl, m above sea level
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In summary, previous studies have shown that the effect of male sterility on grain yield seemed to be modified by natural and agronomic stresses, heterotic groups, and genotypes. In recent decades, genetics has brought about considerable improvements in terms of yield potential (Russell, 1991). Therefore, the impact of male sterility on the grain yield and yield components should be reevaluated for today's hybrids.
Xenia refers to the immediate effects of a foreign pollen parent on nonmaternal tissue of the kernel (Kiesselbach, 1960). The embryo receives one half and the endosperm one third of its genome from the sperm; the former contributes 11% and the latter 83% to the dry weight of the kernel (Tollenaar and Dwyer, 1999). Thus, the potential influence of xenia on grain yield is obvious. Nevertheless, studies of the responses of grain yield to cross-pollination gave inconsistent results. Tsai and Tsai (1990) showed that the grain yield of the hybrid Pi3732 increased significantly when cross-pollinated by B73 x Mo17, whereas this effect was much less strong when B73 x Mo17 was cross-pollinated by Pi3732. Weiland (1992) was unable to reproduce these findings but found that pollen from LH146 x LH82 significantly increased the yields of B73Ht x LH156. Therefore, Weiland suggested that xenia is strongly affected by the environment. Hoekstra et al. (1985) found that the average yields of mixtures of hybrids under severe moisture stress were 6% higher than expected when based on the yields of pure stands. The reason for the higher yields is often reported to be an increase in the single-kernel weight after cross-pollination (Kiesselbach and Cook, 1924; Odhiambo and Compton, 1987; Seka and Cross, 1995; Letchworth and Lambert, 1998). Xenia on single-kernel weight was associated with changes in the rate of kernel growth (Seka and Cross, 1995), duration of grain filling (Poneleit and Egli, 1983; Bulant and Gallais, 1998), or both (Odhiambo and Compton, 1987). According to Hallauer and Miranda (1981), the genetic correlation between KW and grain yield of maize is low. Hence, the most important cause of yield increases is the implementation of the genetic potential of the hybrids to produce more kernels per unit field area (Russell, 1991). However, studies of xenia on grain yield and kernel number are scarce. To date, no work has been published, which concentrates on the combined effects of male sterility and xenia and their contribution to changes in grain yield and yield components. The aim of our study is to quantify the changes in grain yield as a result of the combined effects of CMS and xenia.
We used high-yielding commercial hybrids from central Europe and their nonrestored counterparts converted to T-cytoplasm. A reliable testing system was developed to investigate the combined effects of male sterility and xenia, referred to as the plus-hybrid effect. Finally, the GPA of male-fertile hybrids is proposed as a new selection criterion.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Three different experimental sites were selected each year to represent a wide range of environments. Eschikon and Lindau, at a relatively high altitude [550 m above sea level (masl)], are considered to be marginal areas for the production of grain maize. The temperature there, averaged across decades, tends to be rather low for plants to reach maturity. Rickenbach (440 masl) and Dätwil (370 masl) are more favorable, and St. Aubin (440 masl) and Delley (500 masl) are very favorable sites for the production of grain maize. Despite a relatively dry spring in 1998 and a long period of heavy rainfall in May 1999, both years were advantageous for the production of grain maize. The water supply during flowering was sufficient, and there was no heat or cold stress. Grain yields at all locations were higher than the average of the previous 5 yr. The agricultural practices were the same as those implemented locally. All the fields were plowed in autumn with the exception of the field in Eschikon, which was plowed in spring. Sowing dates were 4 May in Lindau, 7 May in Rickenbach and Dätwil, 11 May in St. Aubin, 12 May in Eschikon, and 26 May in Delley. The amounts of the P2O5 and K2O fertilizers that were broadcast before sowing were adjusted according to the quantity of those nutrients available in the fields. Nitrogen was applied three times: 50 kg 2 wk after sowing, 50 kg at the fifth leaf stage, and 80 kg 3 wk before flowering. The germination rate was normal at all locations and stands were free of weeds and diseases.
Pollination Control and Investigated Parameters
A white-grain experimental hybrid, DSP17007, was used as a pollinator at all locations. In the subplot with this male-fertile hybrid, all the plants in the two rows were detasseled. The plants produced yellow kernels when pollinated by yellow-grain genotypes outside the DSP17007 pollinator block (see Fig. 1). The number of yellow grains on the ears of 10 plants was determined in each subplot with detasseled DSP17007; the rate of unwanted pollination within this test design was calculated.
The first 20 plants in one row per subplot were used to determine the days from planting to anthesis, when 50% of the plants shed pollen on the main branch and on at least two of the side branches of the tassel, and from planting to silking when 50% of the plants showed silk. The difference between the anthesis and silking date was taken as the anthesis–silking interval (ASI). At physiological maturity (black layer formation), ears from the central 3 m2 of each subplot, that is, from 
35 plants, were harvested manually and dried for 2 d to a grain moisture content of 60 g H2O kg-1. The ears were shelled, and a 500-g subsample was dried at 65°C to constant weight to determine the grain yield on a dry-weight basis. A sample of 200 kernels was taken from each subplot to determine the 100-kernel weight. The KN was calculated by dividing the grain yield by the KW. The gravimetric content of grain moisture at harvest was determined at St.Aubin and Delley.
Calculation of Effects and Statistics
Male sterility effect.
The grain yield and its components (KW and KN) of an isogenically pollinated CMS hybrid and of the corresponding male-fertile version of every replication were compared. Isogenically pollinated means that the male-sterile hybrid was pollinated by the same genotype. Thus, changes in grain yield, KW, and KN relative to the male-fertile hybrid were considered to be an effect of male sterility.
Xenia.
The grain yield and its components of a nonisogenically pollinated and of the same isogenically pollinated CMS hybrid were compared. Nonisogenically pollinated means that the male-sterile hybrid was pollinated by an unrelated genotype. Values relative to the isogenically pollinated CMS hybrid were calculated; the variations in yield were considered to be the result of xenia, that is, of cross-pollination.
Plus-hybrid effect.
When the yield of a cross-pollinated CMS hybrid was compared with the yield of the isogenic fertile hybrid, a combined effect of male sterility and cross-pollination was calculated.
General pollinator ability.
In accordance with the concept of GCA and specific combining ability (SCA) (Sprague and Tatum, 1942), the average performance of a pollinator in combination with a broad set of CMS hybrids was characterized as the GPA. Specific combining ability was used in our study to designate those cases, in which certain combinations of CMS hybrid x pollinator do relatively better or worse than would be expected based on the average performance of the two hybrids involved. General pollinator ability, GCA, and SCA were estimated according to Becker (1993).
Analysis of variance and mean comparisons for absolute and relative grain yields, KW and KN, were conducted (PROC GLM, SAS Institute, 1998). Plant density in the subplots at harvest was used as a covariance factor. Environments were combinations of years and locations and were treated as random factors. Sources of variation and appropriate F-ratios were applied according to McIntosh (1983). The comparison of means was performed with the Student-Newman-Keuls test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Effect of Male-Sterility on Grain Yield and Yield Components
The three CMS hybrids, tested in all six environments, had 7.4% higher grain yields than their male-fertile counterparts (Table 2) . The cytoplasm x genotype and cytoplasm x environment interactions were not significant across the six environments, indicating that the effect of male-sterility on grain yield was relatively consistent across hybrids and environments (Table 3)
. The average grain yields were 1000 g dry wt. m-2, which is equivalent to 11.8 Mg ha-1 at a moisture content of 150 g H2O kg-1. This is approximately the yield level of well-managed conventional Swiss farms. Three of the four CMS hybrids tested in three environments showed a trend toward higher yields; this trend was, however, insignificant (Table 2). The cytoplasm x genotype and cytoplasm x environment interactions of these hybrids were not significant. A clear tendency was not detected for KW. Of the three hybrids tested in six environments, only the KW of CORSOms was significantly different from the KW of its fertile counterpart; this difference was negative. Of the four hybrids tested in three environments, DSP16391A6ms had a lower, and GOLDENSOms a higher KW than its fertile counterpart (Table 2). The average KN of both sets of hybrids tested in three and in six environments was higher for the male-sterile version. Considering the minor changes in KW, the observed increases in the yield of the CMS hybrids were due to the significant increases in KN (Table 3). For two environments and three hybrids, a slight tendency of CMS hybrids to dry faster than their male-fertile counterparts was observed (+0.4% dry matter content, P < 0.10).
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General Pollinator Ability
The six pollinators tested in six environments showed significantly different ability to influence the grain yields of the three CMS hybrids (Tables 7 and 8)
. DELPRIM had the highest GPA for grain yield and KN. BANGUY had a high, positive GPA for grain yield. Only DELPRIM and BANGUY showed a positive GPA for KW. A positive but small GPA for grain yield and KN was observed for PACTOL and CORSO. DSP17007 and SILPRO exhibited a negative GPA for grain yield and yield components.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The physiological explanation for the increase in the yield of CMS hybrids is also unclear. In our study, no comparisons have been made of CMS hybrids with and without fertility restoration. Thus, it is unknown to what extent pollen sterility and the male-sterile cytoplasm per se contributed to grain yield differences between the CMS hybrid and the male-fertile hybrid. In addition, it can be argued that observed yield increases from the selfed male-fertile to the sibbed male-sterile treatment may be due to the effect of sib-pollination rather than to the effect of male-sterile cytoplasm per se. To separate the effect of male-sterile cytoplasm from the effect of sib-pollination, it would have been necessary to combine both types of cytoplasm (male-sterile and male-fertile) with both types of pollination (self- and sib-). Self-pollination with nonrestored male-sterile cytoplasm is not possible, and we did not in particular investigate the difference between sibbed and selfed male-fertile cytoplasm because, under open pollinating field conditions, maize is naturally sib-fertilized at a rate of 85 to 99% (Feil and Schmid, 2002). Therefore, the effect of male sterility in our study is the sum of effects resulting from male-sterile cytoplasm, pollen sterility, and 100% sib-pollination, just as would be the case in the field with a mixture of isogenic male-sterile and male-fertile plants.
The effect of pollen sterility per se can be explained by the fact that fertile pollen is a powerful sink for mineral nutrients such as N. The N concentration in the pollen is much higher than in most of the plant organs. According to Hess (1990), dry pollen contains between 16 and 30% protein, and the fresh weight per pollen grain is 2.47 x 10-7 g. The moisture content in fertile pollen at anthesis is 58% (Kerhoas et al., 1987), there are 14 to 50 million pollen grains per plant (Feil and Schmid, 2002), the N to protein ratio is 1:6.25, and the density in our experiments was 11.5 plants m-2. Thus, male-sterile hybrids could save 10 to 30 kg N ha-1. In addition, the developmental stage of the plant, at which the demand for N of male-fertile and male-sterile plants differs, may play a key role. The realization of the potential yield is strongly affected by a shortage of nutrients, especially during the flowering period (Uhart and Andrade, 1995). Nitrogen, which is not used at this time for pollen production, may be used in the female organs to increase the yield potential (Sanford et al., 1965). Physiologically this may be due to a reduced activity of the CMS-plants to channel N into the topmost male flowering organs, comparable with a diminished apical dominance.
The advantage of CMS-plants is not due to N alone. Interactions of different metabolic processes with demands for energy and water during the preanthesis and anthesis periods may give an advantage to CMS-plants over fully fertile plants (Criswell et al., 1974). Thus, kernel primordia which are likely to be aborted in the male-fertile hybrids may have better access to nutrients and water in male-sterile hybrids and may survive and become real kernels. This hypothesis is supported by our findings that yield increases due to CMS were associated with a higher KN across both sets of hybrids. As expected, the higher KN was slightly correlated with a lower KW, but the negative compensation was not significant. Thus, similar to our previous study (Stamp et al., 2000), the higher KN (+8.0%) was primarily responsible for the higher grain yields (+7.4%) across six environments.
For many years, breeders have selected maize hybrids for smaller tassels (Duvick, 1997), which may have reduced the demand for mineral nutrients by male flowering organs as well. In addition to reducing the demands for N and energy, small tassels also reduce the shading of the uppermost leaves and may contribute to higher yields (Duncan et al. 1967). Such positive effects are even more pronounced in male-sterile hybrids. In our field trials, the upper leaves of male-fertile hybrids were covered with a large number of detached anthers and by a thin layer of pollen grains during the grain-filling period. The uppermost leaves of male-sterile plants were much cleaner, contributing to a realization of their full photosynthetic potential.
The contribution of the yield components KW and KN to grain yield seemed to depend on the environment and on the genotype x environment interaction. In contrast, the cytoplasm x genotype and cytoplasm x environment interactions were not significant for yield and yield components, and the hybrids in our study tended to have higher yields as a result of CMS. This indicates that the potential yield advantage of CMS hybrids may higher yields (Duncan et al. 1967). Such positive effects germplasm and in a wide range of environments. It seems that this chance to achieve higher grain yields has not been exploited completely and that grain maize production systems, at least those using European germplasm, may be able to attain higher grain yields if a large proportion of the crop stand would consist of CMS-plants and only a small number of male-fertile plants would be distributed in the field to ensure complete fertilization.
Xenia and Plus-Hybrid Effects
In the early 1900s, several authors considered exploiting xenia to enhance maize yields (Collins and Kempton, 1913; Carrier, 1919; East and Jones, 1920; Kiesselbach and Cook, 1924; Kiesselbach and Leonard, 1932). In the past decade, other researchers suggested improving the performance of grain maize by making use of the benefits of xenia (Seka and Cross, 1995; Bulant and Gallais, 1998). In our study, the cross-pollination of CMS-hybrids resulted in substantial and statistically significant yield gains as a result of xenia. These yield advantages can be ascribed both to a higher KW and a higher KN after cross-pollination. According to Bulant and Gallais (1998), the advantage of xenia is related to the genetic distance. With conventional single-cross hybrids, maximum heterosis is intrinsic to the hybrid seed and to the vegetative parts of the resulting plant. However, grains produced on such self- or rather sib-pollinated single-cross hybrids are the F2 generation, and the grain yield potential may be limited by early effects of inbreeding. We did not investigate genetic distance between the CMS hybrid and the pollinator, but information about the pedigree of the lines was used to select the single-cross hybrids. Bulant et al. (2000) investigated the enzyme activity in cross-fertilized kernels and found that differences in the enzyme activity are expressed soon after fertilization but are insufficient to explain the xenia effects on KW. It is known from the TopCross (DuPont Specialty Grains, Johnston, IA) method (Lambert et al., 1998) that paternal genes can influence the quality of the kernel, that is, the oil concentration in the endosperm and the embryo. Not only the prevalence of quality traits but also grain yield may increase after cross-pollination, which principally increases the level of heterozygosity of the kernel, also referred to as sink strength (Bulant and Gallais, 1998).
Compared with their normal fertile version, the total yield gains of CMS hybrids after nonisogenic pollination were due to the combined effect of male sterility and xenia (= plus-hybrid effect) and were larger than yield gains resulting from the effect of male sterility or xenia alone. From a biological point of view, partitioning plus-hybrid effects in effects of male sterility and xenia is, to some extent, artificial; this was done from a theoretical point of view only. Nevertheless, it seems to be applicable, since yield alterations resulting from both effects can be added. For instance, when SILPROms was pollinated nonisogenically, an increase by 15.8% in the grain yield across five pollinators was found as a result of the plus-hybrid effect. Of this increase, 8.2% resulted from the effect of male sterility and 8.0% from xenia.
The 9.1% increase in the yield of plus-hybrids across all environments and all pollinators suggests that, in a plus-hybrid system, the grain biomass of high yielding hybrids may be increased even further; an application would be similar to the TopCross system where 90% CMS hybrids are mixed with 10% pollinators. In addition, our results may also be important for test procedures in cultivar trials. As early as 1919, Carrier made the remarkable comment: "We still find agronomists conducting variety and ear-to-row tests where no provision is made for preventing cross-pollination". Kiesselbach (1960) warned that xenia may be a possible source of errors in comparative yield tests. According to our study and that of Bulant and Gallais (1998), xenia must indeed be considered to be responsible for some of the variation in grain yield, especially in trials in which only one or two rows of unharvested plants form the border zones.
General Pollinator Ability
The pollinator genotype x CMS hybrid interaction was not significant. Thus, the effect of the pollinator on grain yield can be discriminated from the general yield capacity of the CMS hybrid. General pollinator ability may be a valuable tool for identifying suitable pollinators for a plus-hybrid system. According to Becker (1993), it should be possible to preselect suitable cross-parents with the aid of GCA; this is a more reliable method than the selection based on the performance of the individual genotype. Likewise, preselection using GPA may be useful in the plus-hybrid system where two hybrids are combined. For instance, across six environments, male-fertile SILPRO had the highest 100-kernel weight (30.1 g), while male-fertile DELPRIM had one of the lowest (26.2 g). In selecting for a high KW, male-fertile DELPRIM would be an unsuitable cross-partner, since its performance for this trait is relatively low. However, when using GPA to select for KW, male-fertile DELPRIM ranked highest, whereas male-fertile SILPRO showed a negative GPA. In line with Bulant and Gallais (1998), there was a weak correlation between the KW of a genotype used as pollinator and its GPA for KW. In contrast, Odhiambo and Compton (1987) found a significant increase in the KW (+4.7 g per 1000 kernels) of the Krug yellow dent cultivar after one cycle when selecting both the seed parent and the pollen parent for a high KW. Seka and Cross (1995) reported heavier kernels (+3.9%) after pollination by a large-grain hybrid than by a small-grain hybrid. Furthermore, our findings seem to contradict other findings with regard to quality traits such as oil content. Lambert et al. (1998) found higher oil concentration in kernels of CMS hybrids after cross-pollination by a pollinator with a high oil concentration in its own kernels. Thus, for practical application, breeders may decide to look for easily measurable traits that are correlated with a positive GPA for grain yield.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication August 20, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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