Published online 1 September 2007
Published in Crop Sci 47:1779-1786 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS
Maize Populations as Sources of Favorable Alleles to Improve Cold-Tolerant Hybrids
V. M. Rodríguez,
R. A. Malvar,
A. Butrón,
A. Ordás and
P. Revilla*
Misión Biológica de Galicia, Consejo Superior de Investigaciones Científicas (CSIC), Apartado 28, E-36080 Pontevedra, Spain. Research was supported by the National Plan for Research and Development of Spain (AGF01-3946 and AGF2004-06776), the Autonomous Government of Galicia (PGIDIT04RAG403006PR and PGIDIT02PXIC40301PN), and Excma. Diputación Provincial de Pontevedra. V.M. Rodríguez acknowledges a fellowship from the Ministry of Science and Technology from Spain
* Corresponding author (previlla{at}mbg.cesga.es).
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ABSTRACT
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Low spring temperatures are one of the main limiting factors for maize (Zea mays L.) growth along the European Atlantic coast. Several breeding programs have been performed to obtain cold-tolerant maize genotypes, but the few cold-tolerant inbreds and hybrids available need further improvements for cold tolerance and agronomic performance. The objective of this study was to identify cold-tolerant maize populations with favorable alleles to improve cold tolerance and agronomic performance in early sowing of two cold-tolerant hybrids. The parental inbred lines of two cold-tolerant hybrids were crossed to nine cold-tolerant populations. Tests were performed in a cold chamber and in the field for 2 yr at two locations. The populations Rojo de Tolosa and Puenteareas were the most promising sources of new favorable alleles for transferring cold tolerance to the hybrids EP80 x F7 and EP80 x Z78007, respectively. The populations Puenteareas and Silver King were the most outstanding donors to improve the agronomic performance of EP80 x F7 and EP80 x Z78007, respectively. Some improved versions of the cold-tolerant inbred parents could be developed from crosses between F7 or Z78007 and Puenteareas; alternatively, Rojo de Tolosa or Silver King could be used as donors of favorable alleles, but the simultaneous improvement of yield and cold tolerance is not straightforward.
Abbreviations: lplµ', relative number of loci containing favorable alleles in a population and absent in a hybrid • NI, net improvement • PNGceg, probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent
Maize Populations as Sources of Favorable Alleles to Improve Cold-Tolerant Hybrids
V. M. Rodríguez,
R. A. Malvar,
A. Butrón,
A. Ordás and
P. Revilla*
Misión Biológica de Galicia, Consejo Superior de Investigaciones Científicas (CSIC), Apartado 28, E-36080 Pontevedra, Spain. Research was supported by the National Plan for Research and Development of Spain (AGF01-3946 and AGF2004-06776), the Autonomous Government of Galicia (PGIDIT04RAG403006PR and PGIDIT02PXIC40301PN), and Excma. Diputación Provincial de Pontevedra. V.M. Rodríguez acknowledges a fellowship from the Ministry of Science and Technology from Spain
* Corresponding author (previlla{at}mbg.cesga.es).
Low spring temperatures are one of the main limiting factors for maize (Zea mays L.) growth along the European Atlantic coast. Several breeding programs have been performed to obtain cold-tolerant maize genotypes, but the few cold-tolerant inbreds and hybrids available need further improvements for cold tolerance and agronomic performance. The objective of this study was to identify cold-tolerant maize populations with favorable alleles to improve cold tolerance and agronomic performance in early sowing of two cold-tolerant hybrids. The parental inbred lines of two cold-tolerant hybrids were crossed to nine cold-tolerant populations. Tests were performed in a cold chamber and in the field for 2 yr at two locations. The populations Rojo de Tolosa and Puenteareas were the most promising sources of new favorable alleles for transferring cold tolerance to the hybrids EP80 x F7 and EP80 x Z78007, respectively. The populations Puenteareas and Silver King were the most outstanding donors to improve the agronomic performance of EP80 x F7 and EP80 x Z78007, respectively. Some improved versions of the cold-tolerant inbred parents could be developed from crosses between F7 or Z78007 and Puenteareas; alternatively, Rojo de Tolosa or Silver King could be used as donors of favorable alleles, but the simultaneous improvement of yield and cold tolerance is not straightforward.
Abbreviations: lplµ', relative number of loci containing favorable alleles in a population and absent in a hybrid • NI, net improvement • PNGceg, probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent
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INTRODUCTION
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COLD TEMPERATURE is one of the main stresses limiting maize performance in temperate regions. Chilling temperatures reduce emergence, seedling vigor, and early vigor, and produce chlorotic leaves (Miedema, 1982; Revilla et al., 2005). On the European Atlantic coast, maize cannot be sown before late May, because average and minimum temperatures are usually below 15 and 10°C, respectively.
Most studies on cold tolerance have been performed in a cold chamber or in the field, but seldom in both. Menkir and Larter (1985) reported that cold tolerance in the cold chamber is not correlated with performance in the field. Similar results were obtained by other researchers (Hope et al., 1992; Revilla et al., 2000; Rodríguez et al., 2007). These researchers recommended both methods of evaluation to select genotypes tolerant to low temperatures and with high agronomic performance. Certainly, evaluation of cold tolerance in the cold chamber allows precise control of cold temperatures, but does not predict plant growth under field conditions. Early sowing in the field is the only practical procedure to evaluate cold tolerance, but it does not guarantee cold conditions and suffers high climatic variability and strong fluctuations with time. Cold chamber evaluations could be used for screening for cold tolerance potential, while field evaluation provides information about the actual performance of the cold-tolerant genotypes in the field.
Many efforts have been dedicated to increasing cold tolerance, advancing sowing dates, and permitting cultivation of cultivars with longer cycles and higher yield (Revilla et al., 2003, 2005; Ordás et al., 2006). Mosely et al. (1984) achieved an improvement of 16.6% in the rate of germination of the population BS3 in early sowing under cold conditions. Landi et al. (1992) used full-sib divergent recurrent selection based on the highest and the lowest value of germination under cold and optimum conditions, and achieved an increase in germination percentage and a reduction in days to germination in one population of maize.
Few maize inbreds and populations have been identified as base germplasm for breeding for cold tolerance at the early stages of development (Hotchkiss et al., 1997; Revilla et al., 2005; Revilla et al., 2003; Ordás et al., 2006; Rodríguez et al., 2007). Revilla et al. (2000) found that the inbred line EP80 had a high percentage of emergence and rapid seedling growth, while F7 showed a high percentage of emergence. In previous unpublished studies performed in the cold chamber, these inbred lines, together with Z78007, were identified as cold tolerant at the early stages of development. The hybrids EP80 x F7, EP80 x Z78007, and especially F7 x Z78007, however, showed low yield at early sowing and a short growing cycle (Rodríguez et al., 2007). Cold-tolerant populations could be used as sources of favorable alleles for improving cold tolerance and agronomic performance of the current cold-tolerant inbred lines. The improved inbred lines that could result from those crosses would be subsequently used to produce cold-tolerant hybrids with higher yield, or to improve cold tolerance of elite inbred lines with longer growing cycles and higher yield.
Dudley (1987) proposed a method to identify populations with favorable alleles that are absent in an elite hybrid. This method is based on the fact that, for any pair of lines, four classes of loci could be considered: classes i, j, and k having favorable alleles for one or both parental lines, and class l carrying less favorable alleles for both lines. The statistic lplµ' was defined as an estimate of the product of the relative number of loci not containing at least one favorable allele in the hybrid and the average frequency of favorable alleles at such loci in the population. Several statistics were developed based on Dudley's theory; some of them offer specific contributions—particularly NI (net improvement) (Bernardo, 1990) is appropriate for choosing donor populations for direct production of improved inbreds, and PNGceg (probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent) (Metz, 1994) for nondominant gene effects. These statistics were widely used to estimate the potential improvement of field and sweet maize hybrids (Dudley, 1988; Cartea et al., 1996; Kraja and Dudley, 2000; Trifunovic et al., 2001). This method was originally proposed for using exotic populations as a source of new favorable alleles. Nevertheless, several studies have been performed involving adapted populations (Cartea et al., 1996; Ordás et al., 2006; Revilla et al., 1998; Taller and Bernardo, 2004). In different studies, the efficiency of these statistics has been compared (Hogan and Dudley, 1991; Pfarr and Lamkey, 1992; Revilla et al., 1998). High correlations were found among these statistics; however, each researcher proposed a different estimator as the most appropriate in each case.
Given the poor performance of the cold-tolerant inbreds available, their direct use as sources of favorable alleles for cold tolerance to improve elite hybrids with high yield is not recommended. Alternatively, cold-tolerant populations could be used as sources of favorable alleles for both yield and cold tolerance to improve cold-tolerant inbreds. This approach should produce enhanced cold-tolerant inbreds either for use as parents of improved hybrids or as enhanced donors of favorable alleles for cold tolerance to improve elite hybrids. The objective of the present study was to identify cold-tolerant maize populations with favorable alleles to improve cold tolerance and agronomic performance in early sowing of two cold-tolerant hybrids.
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MATERIALS AND METHODS
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Nine populations were evaluated as sources of favorable alleles for improving the cold tolerance and the agronomic performance of two cold-tolerant hybrids (EP80 x F7 and EP80 x Z78007) (Table 1
). In the cold chamber, the nine populations per se, 27 population x inbred line crosses, two cold-tolerant hybrids, and three parental inbred lines were grown using a randomized complete block design with four replications. Fifteen seeds of each genotype per replication were planted in 10-L trays with 6 L of sterilized and watered peat (Gramoflor GmbH & Co. KG, Vechta, Germany). Sowing depth was 2 cm and seeds were planted in rows spaced 5 cm apart with 2 cm between seeds. Conditions were set at 14 h with light (provided by seven very high output [VHO] fluorescent lamps with a photosynthetic photon flux of 228 µmol m–2 s–1) at 14°C, and 10 h without light at 8°C. All trays were watered again with 300 mL of water 20 d after planting, which was enough to keep the plants turgid. Field evaluations were performed at two locations in northwestern Spain, Pontevedra (42°24'N, 8°38'W, 20 m above sea level) and Pontecaldelas (42°23'N, 8°32'W, 300 m above sea level) and 2 yr, 2003 and 2004. Both locations have a humid climate with an annual rainfall of about 1600 mm and summer drought, and the soil is acid sandy loam. Agronomic handling was done according to the local practices, including irrigation of about 60 L m–2 at flowering. Minimum temperatures can fall below 0°C during the month following planting, the mean minimum temperature being between 5 and 8°C and the mean maximum temperature between 18 and 20°C.
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Table 1. Name, origin, and grain type of the maize populations and inbred lines evaluated as sources and recipients of favorable alleles for improving cold tolerance.
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Populations, population x inbred crosses, and hybrids were evaluated along with other genotypes not used for this study, following a 7 x 8 lattice design with three replications (Rodríguez et al., 2007). Each experimental plot consisted of two rows, with 17 hills per row and two seeds per hill. The three parental inbred lines were grown in a randomized complete-block design with four replications in an adjacent trial, to avoid interactions between genotypes, as the first trial included genotypes of similar heterozygosis and growing cycle, and the second trial included only the inbred lines. In the inbred trial, each experimental plot consisted of two rows with 15 hills per row and two seeds per hill. In both experiments, rows were spaced 0.80 m apart and hills were spaced 0.21 m apart. Hills were thinned to one plant with a final plant density of approximately 60,000 plants ha–1. Planting dates were 16 Apr. 2003 and 2004 in Pontevedra and 24 Apr. 2003 and 13 Apr. 2004 in Pontecaldelas.
Traits measured in the cold chamber were: seedling vigor (1 = weak to 9 = vigorous) defined as a general appearance of the plant at the end of the heterotrophic period (Revilla et al., 1999), days from emergence (days from the emergence of half of the plants in each plot to 30 d after sowing, which is the time when emergence had ended for all plots), emergence percentage (percentage of emerged plants), and survival percentage (percentage of final plants). Traits measured in the field included early vigor (at the beginning of the autotrophic stage), days from silking to 1 September, plant height (cm), grain yield (Mg ha–1), and grain dry matter concentration (g kg–1, calculated as the proportion of dry matter left after discounting moisture).
In the population and cross trials, means adjusted by lattice block effects were obtained using the LATTICE procedure of SAS (SAS Institute, 2000) and were used in the combined analysis across locations and years. Comparisons of means were performed for each trait using Fisher's protected LSD at P = 0.05 (Steel et al., 1997). Means of each trait were used to calculate three statistics to choose the population that had more favorable alleles not present in the hybrid. These statistics were lplµ' (Dudley, 1987), the original estimator of favorable alleles; NI, the most appropriate for short-term breeding (Bernardo, 1990); and PNGceg (net gain of favorable alleles if partial dominance or complementary epistasis is prevalent), suitable for traits not following a strict dominance model (Metz, 1994). The estimator lplµ' was calculated following Dudley (1987, 1988). Let I1 and I2 be inbred parents and Py the donor population. The NI was the larger of I1Py – I1I2 and I2Py – I1I2 (Bernardo, 1990). The PNGceg was computed as the larger of (I1Py – I1)/(2I1Py – I1 – Py) and (I2Py – I2)/(2I2Py – I2 – Py) (Metz, 1994).
The standard errors of each estimator were calculated as the square root of the variance of the linear function associated with each estimator. Estimators were considered different from zero if they exceeded twice their own standard error. Values for each estimator and within each hybrid were considered significantly different when the difference between a pair of estimates exceeded twice the standard error of the difference.
Dudley (1987) also proposed a method to determine the parental inbred line to be improved. Following this method, the expression I2Py – I1Py + 
(I1 – I2) was calculated. If this expression had a positive value, the population (Py) was crossed to inbred I1, and to I2 if the expression had negative value. In addition, estimates of jqjµ and kpkµ reported information about whether to self in F2 or to backcross before selfing. If I1 is being improved, when jqjµ = lplµ' selfing in the F2 would be the best approach, whereas backcrossing to I1 is advised when jqjµ > lplµ' and backcrossing to Py when jqjµ < lplµ'. If I2 is being improved, then the appropriate comparison is between kpkµ and lplµ'.
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RESULTS AND DISCUSSION
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Differences among inbred x population crosses were significant for seedling vigor and emergence percentage in the chamber, and early vigor, grain yield, grain dry matter, and silking days in the field (Table 2
). Therefore, statistics to identify donors of favorable alleles were computed only for those traits. The means used to calculate the statistics are shown in Table 2.
The populations evaluated as potential donors differed significantly only for seedling vigor in the cold chamber and for silking days in the field; the hybrids did not differ significantly for any trait; the inbred parents differed significantly for seedling vigor and chamber emergence percentage and for dry matter in the field, and the inbred x population crosses differed for seedling vigor, proportion of emergence, early vigor, grain yield, grain dry matter, and silking days (Table 2).
Early Stages of Development
High correlations between the estimators of favorable alleles evaluated in this study have been reported by many researchers (Pfarr and Lamkey, 1992; Cartea et al., 1996; Taller and Bernardo, 2004). Agreeing with those researchers, the populations chosen as best donors of favorable alleles based on different statistics were the same, except those selected on the basis of PNGceg, the only statistic that assumes partial dominance or epistasis.
Most populations could provide new favorable alleles for improving the cold tolerance of the hybrids. Differences among populations as donors of favorable alleles for EP80 x F7 were observed for seedling and early vigor (Table 3
). For these traits, only differences among PNGceg values of the donors were significant, which is in agreement with the results obtained by other researchers (Cartea et al., 1996; Malvar et al., 1997). According to the PNGceg statistic, Rojo de Tolosa had the highest value for seedling vigor, while Lalín, Rebordanes, and Silver King were the best donors of early vigor. For seedling vigor, six populations showed significant estimates for lplµ', but only three of them, Amarillo de Marañón, AS-B, and Rojo de Tolosa, also showed significant values for NI. Most populations could be used to improve seedling vigor of EP80 x F7, but the improvement would be more efficient with the populations that showed significant estimates for NI. On the other hand, none of the populations showed significant estimates of NI for early vigor. Differences among donor populations for improving the emergence percentage of the hybrid EP80 x F7 in the cold chamber were not significant for any statistic. Regarding the lplµ' values, most of the populations could be used for increasing emergence of this hybrid, except Lalín and Rebordanes. We would not expect, however, to obtain improved inbreds directly by self-pollinating the inbred parent x population donor because estimates for NI were not significant.
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Table 3. Estimates of the relative number of loci containing favorable alleles in a population and absent in a hybrid (lpl µ'), net improvement (NI), and the probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent (PNGceg) for nine populations evaluated as sources of favorable alleles to improve the first stages of development of the cold-tolerant single cross EP80 x F7 evaluated in both cold chamber and early planting in the field.
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The best donors of cold tolerance at the emergence stage (cold chamber evaluation) were Lalín and Rebordanes, but they were among the least appropriate as sources of cold tolerance at the postemergence stage (field evaluation). Rojo de Tolosa was the best potential donor for improving all traits in both the cold chamber and the field, as it had favorable alleles for improving the percentage of emergence in the cold chamber and early vigor in the field, and was the best donor for seedling vigor in the cold chamber.
For improving hybrid EP80 x Z78007, different populations were identified as the best donors of cold tolerance in the cold chamber and in the field (Table 4
). In the cold chamber, all populations except Lalín and Santiago showed significant values of lplµ' for seedling vigor. Only Amarillo de Marañón, AS-B, and Rojo de Tolosa, however, showed significant values for NI. Puenteareas was the best potential donor for improving early vigor, because it showed the highest values for lplµ' and PNGceg and a significant estimate for NI. Therefore, Puenteareas was the most promising donor for improving seedling and early vigor of EP80 x Z78007.
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Table 4. Estimates of the relative number of loci containing favorable alleles in a population and absent in a hybrid (lpl µ'), net improvement (NI), and the probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent (PNGceg) for nine populations evaluated as sources of favorable alleles to improve the first stages of development the cold-tolerant single cross of EP80 x Z78007 evaluated in both cold chamber and early planting in the field.
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Agronomic Performance in Early Sowing
For improving the yield of hybrid EP80 x F7, all donor populations showed positive and significant values of lplµ', indicating that donor populations had favorable alleles not present in the hybrid (Table 5
). Puenteareas was the best donor population, according to all statistics except PNGceg, followed by Silver King and Rebordanes. The statistic PNGceg was proposed by Metz (1994) as a useful statistic when partial dominance or epistasis are important, while the rest of the estimators assume complete dominance. For traits with important dominance effects, such as yield, PNGceg is not an appropriate estimator.
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Table 5. Estimates of the relative number of loci containing favorable alleles in a population and absent in a hybrid (lpl µ'), net improvement (NI), and the probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent (PNGceg) for nine populations evaluated as sources of favorable alleles to improve the agronomic performance in early sowing of the cold-tolerant single cross EP80 x F7.
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For grain dry matter concentration of hybrid EP80 x F7, no populations showed favorable alleles according to lplµ'. Only PNGceg showed positive and significant values for several populations. Nevertheless, when means of donor populations are lower than the hybrids, as in the present study (Table 2), the relative number of favorable alleles at class l can be negative and greater in absolute value than the relative number of favorable alleles at other classes of loci. In this situation, the interpretation of PNGceg would be erroneous (Cartea et al., 1996). Therefore, grain dry matter concentration is an important trait to be monitored in the breeding process because populations could provide unfavorable alleles for this trait.
Three populations, Puenteareas, Rebordanes, and Silver King, showed the best values of lplµ' for yield. For grain dry matter concentration, however, Silver King showed significant and negative values for lplµ'. Puenteareas was the only population with positive and significant estimates of NI for yield. This indicates that, using Puenteareas as the donor of favorable alleles for improving the hybrid EP80 x F7, the probability of gaining favorable alleles is higher than the probability of losing them (Bernardo, 1990). Therefore, Puenteareas is the best donor population to obtain new improved inbred lines. This population also could provide favorable alleles for improving days from silking, plant height, and most yield components (data not shown).
Few differences were observed among donor populations of agronomic performance for improving hybrid EP80 x Z78007 (Table 6
). Again, all estimators except PNGceg identified Puenteareas as the best donor population, along with Silver King. For dry grain matter concentration, most populations had estimates of qj and qk outside the limits of 0 and 1, so it was impossible to estimate lplµ' and PNGg. Dudley (1988) pointed out that when estimates of lplµ' could not be obtained, I1Py – I2Py values were larger than either I1I2 – I1 or I1I2 – I2, suggesting possible epistatic effects. When lplµ' could be calculated, donor populations showed negative and significant values, indicating that unfavorable alleles for dry grain matter concentration would be provided by these populations. Puenteareas had a negative and significant estimate of lplµ', so the population Silver King will be more appropriate for improving hybrid EP80 x Z78007. Besides, this population was the best donor population for all yield components (data not shown).
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Table 6. Estimates of the relative number of loci containing favorable alleles in a population and absent in a hybrid (lpl µ'), net improvement (NI), and the probability of a net gain of favorable alleles when partial dominance or complementary epistasis is prevalent (PNGceg) for nine populations evaluated as sources of favorable alleles to improve the agronomic performance in early sowing of cold-tolerant single cross EP80 x Z78007.
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To determine which parental inbred line must be crossed to the donor population, only yield was considered because heterotic patterns are based on grain yield. According to Dudley (1988), in a practical breeding program, the parent to be improved would be the most closely related to the population. In this study, populations are not significantly more related to one parental inbred line than to another, so the inbred line to be improved will, generally, be the one with the lowest performance. For improving agronomic performance of hybrid EP80 x F7, the population Puenteareas must be crossed to inbred line F7 and backcrossed with the donor population before self-pollination, because lplµ' was significantly higher than jqjµ. Simultaneously, for improving cold tolerance, the population Rojo de Tolosa could be crossed to inbred EP80. For improving yield of EP80 x Z78007, Silver King must be crossed to Z78007 and, again, backcrossing to the donor population will be needed before self-pollination. Alternatively, EP80 could be crossed to the population Puenteareas.
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CONCLUSIONS
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Most populations are potential donors of favorable alleles for improving cold-tolerant hybrids. The population Puenteareas was the best donor for the improvement of EP80 x F7, and F7 the best receptor; alternatively, the donor can be Rojo de Tolosa and the receptor EP80. The cross F7 x Puenteareas should be an appropriate source of inbreds with improved cold tolerance to combine with inbreds from EP80 x Rojo de Tolosa. For the improvement of hybrid EP80 x Z78007, the best donor of alleles for cold tolerance was Puenteares, with EP80 as receptor, or Silver King for the receptor Z78007. Puenteareas and Silver King were the best donors of cold tolerance and yield, respectively.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication January 9, 2007.
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ABSTRACT
INTRODUCTION
