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Published online 30 July 2007
Published in Crop Sci 47:1467-1474 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS

Combining Maize Base Germplasm for Cold Tolerance Breeding

V. M. Rodríguez, A. Butrón, G. Sandoya, 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 Spanish National Plan for Research and Development (AGF01-3946 and AGF2004-06776), the Autonomous Government of Galicia (PGIDIT04RAG403006PR), and the Excma. Diputación Provincial de Pontevedra. V.M. Rodríguez and G. Sandoya acknowledge their fellowships from the Ministry of Science and Technology from Spain

* Corresponding author (previlla{at}mbg.cesga.es).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Early planting can contribute to increased grain yield of maize (Zea mays L.), but it requires cold tolerance. A limited number of cold-tolerant maize genotypes have been reported. The objectives of this study were to test a new strategy to improve cold tolerance in maize searching for broad x narrow genetic combinations that may be useful as base populations for breeding programs, to compare genotype performance under cold-controlled and field conditions, and to establish the major genetic effects involved in crosses between cold-tolerant inbred lines and populations. Nine cold-tolerant populations were crossed to five inbred lines and evaluated in a cold chamber and in the field. Most inbred line x population crosses performed better than populations per se or hybrids used as checks, both in the cold chamber and in the field, suggesting that broad x narrow genetic combination could be a suitable start point for further breeding programs for cold tolerance. The crosses between the inbred line EP80 and northwestern Spanish populations are the most promising base germplasm. In particular, EP80 x Puenteareas showed the greatest yield and good performance at the first stages of development under cold conditions. In addition, EP80 and Puenteareas showed favorable general combining ability for most traits. Early vigor rating would be the most suitable trait to select maize genotypes with superior cold tolerance during emergence and postemergence stages, because it was the only trait for which differences among genotypes were observed in both the cold chamber and the field. Although evaluation under controlled conditions is essential to test cold tolerance, field evaluations are complementary because no association was found between traits evaluated in both conditions.

Abbreviations: GCA, general combining ability • SCA, specific combining ability


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN AREAS WITH COLD SPRINGS, maize sowing must be delayed until late May, and only genotypes of medium or short cycles can be sown, which results in reduction of potential grain yield. Early sowings allow growing later genotypes, which have more yield potential than early ones (Shawn, 1988; Lauer et al., 1999). Moreover, there is an increase of yield in early sowing due to the coincidence of grain-filling stage with the period of potential maximum photosynthesis (Mock and Pearce, 1975). Gupta (1985) reported that early sowing is highly recommended in temperate areas because some field-drying of maize can occur, allowing a greater profit margin. Revilla et al. (1999, 2000) concluded that breeding for cold tolerance should be a priority to improve maize yields in areas with short growing seasons, leading to the development of longer cycle genotypes with cold tolerance.

Most breeding studies on cold tolerance of maize have focused on germination and crop establishment under cold conditions. Cold-tolerant genotypes at the first stages of development are required for early sowing since these stages are more sensitive to low temperatures than mature stages (Greaves, 1996). Several studies have been performed to select cold-tolerant genotypes in the laboratory (Semuguruka et al., 1981; Lee et al., 2002; Revilla et al., 2003) or field (Mosely et al., 1984; Verheul et al., 1996) conditions, but few studies included both (Revilla et al., 2000). Menkir and Larter (1985) pointed out that emergence-related traits determined under controlled environment conditions were not correlated with those recorded in the field. Therefore, both the laboratory and the field evaluations are necessary for choosing the best genotypes, because cold can be guaranteed in a cold chamber, while field trials provide the real conditions (Revilla et al., 2005).

A limited number of reports have presented cold-tolerant maize inbred lines and populations around the world, of which few field corn inbred lines and populations could be adapted to the European Atlantic conditions (Revilla et al., 2005). The collection preserved at the Misión Biológica de Galicia (CSIC) contains only three inbred lines, belonging to the European flint group, and nine populations with superior cold tolerance, although all of them with low agronomic performance. Base populations are often made by crossing inbred lines or open-pollinated populations. The use of open-pollinated populations as base germplasm strongly limits the potential gain through selection and reduces the chances to obtain elite genotypes. Crossing inbred lines results in narrow genetic base populations that limit the gain per cycle of selection, particularly when few or related inbreds are available. The amount of variability obtainable from crosses among inbred lines is limited and depends on the genetic relationship among inbred lines (Butrón et al., 2003). Tabanao and Bernardo (2005) concluded that elite maize inbred lines supply nonadditive gene effects valuable for maintaining genetic variation and that the use of multiple parents is important for retaining genetic variability during selection. Inbred x open-pollinated populations crosses capitalize on the advantages of broad-low performing and narrow-high performing combinations. Genes involved in cold tolerance have additive effects (Eagles and Hardacre, 1979; Eagles, 1982; Mahajan et al., 1993; Revilla et al., 2000), but hybrid performance under cold conditions could not always be predicted from the performance of its parental inbreds (Aidun et al., 1991). Genetics involved in the potential breeding populations derived from crosses between cold-tolerant inbreds and populations are unknown, and they should be elucidated to design the most adequate breeding program. The objectives of this study were to test a new strategy to improve cold tolerance in maize searching for broad x narrow genetic base combinations which may be useful for further breeding programs, to compare genotype performance under cold-controlled and field conditions, and to establish the major genetic effects involved in crosses between cold-tolerant inbred lines and populations.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nine cold-tolerant populations were crossed to five inbred lines, of which three were regarded as cold tolerant and two functioned as testers for combining ability (Table 1). The nine populations and the three inbred lines were identified as cold tolerant in previous unpublished evaluations. Each population was crossed to each inbred in 2002. Seed was produced for all genotypes in 1 yr and in one location to reduce environmental effects on seed quality. Populations were used as males and pollen from a minimum of 40 tassels in each population was bulked and used to pollinate each inbred line. Three cold-tolerant hybrids (used as checks) were produced and the nine populations were multiplied in the same environment.


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  		Table 1. Name, origin, and grain type of the maize germplasm evaluated in cold chamber and in field trials at early planting.

 
Growth Chamber Trial
The 45 crosses, nine populations, five inbred lines, and three cold-tolerant hybrids were planted in trays filled with 6 L of sterilized peat and 1.2 L of water and arranged in a cold chamber following a randomized complete block design with four replications. The cold chamber had four pairs of shelves, each pair at a different height. Trays containing the same replication were placed at the same height. The experimental plot consisted of a row of 15 seeds. 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 at 14°C and 10 h without light at 8°C. All trays were watered with 300 mL of water 20 d after planting. Data were recorded on five traits related to early development in a cold environment: color (1 = albino to 9 = dark green) and early vigor rating (1 = weak to 9 = vigorous) as visual ratings, days to emergence (days from planting to 50% emergence), proportion of emergence (% of emerged plants), and proportion of survival (% of plants alive). Analyses of variance and mean comparisons were performed for all traits. Sources of variation were genotypes and replications. Genotypes were considered as fixed effects. Sum of squares due to genotypes was divided into crosses, populations, inbred lines, and hybrids. Sum of squares due to crosses was divided into populations, inbred lines, and population x inbred line interaction. The sum of squares for populations and inbred lines correspond to the general combining ability (GCA) and sum of squares for population x inbred line interaction to the specific combining ability (SCA). GCA and SCA were calculated according to Falconer and Mackay (1997). Comparisons of means were performed for each trait using the Fisher's protected least significant difference (LSD) at P = 0.05 (Steel et al., 1997). Analyses were made using the GLM procedure of SAS (SAS Institute, 2000).

There was a gradient of air flow in the cold chamber that resulted in differences among trays within each replication for peat humidity. To minimize the effect of these differences in plant development data, covariance analyses were performed using peat humidity as covariate. A sample of peat from each tray was weighed at the end of the experiment and after a period of 48 h at 80°C. Peat humidity was calculated as [(fresh weight – dry weight)/dry weight]. When the covariate was significant, means were adjusted by peat humidity.

Field Trial
Crosses, populations, and cold-tolerant hybrids were evaluated during 2 yr (2003 and 2004) in two locations, 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). Both locations have a humid climate with an annual rainfall of about 1600 mm. In both locations, mean minimum temperatures between 10 April and 15 May were between 5° and 8°C, reaching, in 2004, absolute minimum temperatures below 0°C. Genotypes were evaluated in a 7 by 8 lattice design with three replications. Each experimental plot consisted of two rows with 17 hills per row and two seeds per hill. Rows were spaced 0.80 m apart and hills were spaced 0.21 m apart. Hills were thinned to one plant achieving a final plant density of approximately 60000 plants ha–1. Standard management and cultural practices were used in all trials. Traits measured were days to emergence, proportion of emergence, early vigor rating, plant height, days to silking, grain yield (Mg ha–1), grain moisture (g kg–1), ear and seed lengths, seed width, ear rows, and 100 seed weight. 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. Sources of variation were genotypes, locations, years, and their interactions. Genotypes were considered as fixed and locations, years, and all possible interactions were considered random effects. The pooled error mean square was calculated as reported by Cochran and Cox (1957). Sum of squares due to genotypes were divided into crosses, populations, and hybrids, while sum of squares due to crosses were divided into GCA of populations, GCA of inbred lines, and SCA.

Field trials were planted on 16 Apr. 2003 and 2004 in Pontevedra and on 24 Apr. 2003 and 13 Apr. 2004 in Pontecaldelas. Comparisons of means were performed for each trait using Fisher's protected LSD at P = 0.05 (Steel et al., 1997). Analyses were made using the GLM procedure of SAS (SAS Institute, 2000). Estimates of GCA and SCA were calculated according to Falconer and Mackay (1997). For each trait related to the first stages of development (days to emergence, proportion of emergence, and early vigor rating), simple correlation coefficients were calculated between the values recorded in the cold chamber and in the field trials.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Cold Tolerance at Germination and Early Development
In the cold chamber, the covariate peat humidity was significant for color, early vigor rating, and days to emergence (data not shown). Genotypes were significantly different for all traits, except for proportion of survival (Table 2). Both inbred lines and crosses showed significant differences for color, early vigor rating, and proportion of emergence, while populations were only significantly different for color. No differences among cold-tolerant check hybrids were observed. In the field, genotypes were only significantly different for early vigor rating and differences were due to the variability among crosses (Table 2). There were significant differences among GCA effects of inbreds for color, days to emergence, percentage of emergence, and early vigor rating in cold chamber, while GCA effects of populations were significantly different for color and early vigor rating both in the cold chamber and in the field trials. The SCA effects were only significant for the percentage of emergence. Most interactions were not significant.


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  		Table 2. Means of cold-tolerant inbred line x populations crosses, populations, inbred lines, and hybrids evaluated in the cold chamber and in the field at early planting for different traits related to early development.

 
As expected, inbred lines were more sensitive to cold temperature than populations and crosses. Populations were significantly different for color. In general, the performance of the cold-tolerant inbred line x population crosses was comparable or even better than the performance of hybrids. Crosses between populations and inbred lines A661 and A641 had poor color and proportion of emergence (Table 2) as expected, since A661 and A641 are not cold tolerant. Among the cold-tolerant inbred line x population crosses, EP80 x Rebordanes, F7 x Lalín, Z78007 x Amarillo de Marañón, and Z78007 x Puenteareas showed the best performance in both the cold chamber and field at the first stages of development.

Menkir and Larter (1985) found significant and positive correlations between postemergence seedling growth in controlled and field environments, but not for emergence. In the present study, significant correlations were not observed between performance in the cold chamber and in the field (Table 3). Nevertheless, a positive and significant correlation (0.56, P < 0.01) was observed between early vigor rating evaluated in the field and an index computed with the vigor and color evaluated in the cold chamber (1/2 color + 1/2 vigor). Early vigor rating was recorded 30 d after sowing in both the field and the cold chamber, but plant development stage was different. Because of higher temperatures after the first week, plant development was faster in the field than in the cold chamber. Therefore, 30 d after sowing, plants were at three- to four-leaf stage in the cold chamber while, in the field, plants were at five- to six-leaf stage. Cooper and Macdonal (1970) stated that at approximately the three- to four-leaf stage, the seed reserves are exhausted and photosynthesis begins. So, we can considerer early vigor rating in the cold chamber as an emergence trait and early vigor rating in field as a postemergence trait. The lack of correlation between traits recorded in the cold chamber and in the field supports the hypothesis of different genetic factors controlling both stages emergence and postemergence (Hodges et al., 1997), although color and early vigor ratings recorded at emergence could affect, to some extent, early vigor rating at five- to six-leaf stage.


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  		Table 3. Simple coefficients of correlation between traits recorded in the cold chamber and early field sowing.{dagger}

 
The inbred EP80 showed favorable GCA for color and early vigor rating in the cold chamber, while F7 and Z78007 only had favorable GCA for color (Table 4). In the field, differences among the GCA values of inbred lines for early vigor rating were not significant. Puenteareas, Rebordanes, and Lalín had the best GCA for early vigor rating in the field and did not differ from the population with the most favorable GCA for color and early vigor rating in the cold chamber. Puenteareas, Rebordanes, and Lalín come from northwestern Spain and were, in general, the most cold tolerant. These results agree with previous evaluations of the European Union Landrace Core Collection (unpublished data, 2006). However, Revilla et al. (1998) found that the place of origin of a variety is not a sufficient guarantee for possessing high cold tolerance because genotypes with a short growing cycle escape cold temperatures when planted late. Malvar et al. (2005) pointed out that natural selection would favor cold tolerance in the full season materials, since this germplasm is typically planted earlier.


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  		Table 4. Estimates of general combining ability of five inbred lines and nine maize populations for seven traits related to early development evaluated in cold chamber and in early field planting.

 
Significant SCA effects were not detected for any trait, except proportion of emergence. Previous work pointed out that many genes implicated in maize tolerance to low temperatures have important additive effects (Eagles and Hardacre, 1979; Eagles, 1982; Mahajan et al., 1993; Revilla et al., 2000). However, nonadditive genetic effects could be important in some genetic backgrounds (Aidun et al., 1991; Revilla et al., 2000). In the present study, inbred lines and populations from two different germplasm groups, European and U.S. Corn Belt, were included, but few crosses showed significant SCA. Therefore, additive effects appeared to be larger than nonadditive effects, encouraging recurrent selection on new populations derived from crosses between cold-tolerant inbreds and populations with favorable GCA effects.

Agronomic Performance in Early Sowing
Most populations included in this study belong to the flint European germplasm group and performed better in crosses to A641 than to A661 (Table 5). The inbred line A641 is derived from Reid germplasm and several studies reported high heterosis in crosses between flint European maize and Reid germplasm (Moreno-González, 1988; Soengas et al., 2003). Four populations (Amarillo de Marañon, Gallego/Hembrilla norteño, Rebordanes, and Silver King) showed higher yields in crosses to A641 than per se, while only Amarillo de Marañon showed higher yield in crosses to A661 than per se. The other populations did not show differences between per se performance and in crosses with tester inbred lines. Therefore, the heterotic group should not be an important criterion in the election of the best cold-tolerant inbred x population cross as the base population for further breeding programs. Moreover, a Reid tester would be preferable for improving SCA of the resulting base population.


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  		Table 5. Means of maize inbred line x populations crosses, evaluated for yield and other agronomic traits in early sowing.

 
The high potential of the populations for improving the agronomic performance of the existing cold-tolerant hybrids was demonstrated, since hybrid yields were significantly surpassed by the yield of some crosses (Table 5). In general, crosses among EP80 and northwestern Spanish populations were among the most productive. Among cold-tolerant inbred x population crosses, EP80 x Puenteareas showed the largest yield (7.4 Mg ha–1), along with five crosses involving EP80 and two involving F7 (Table 5). In addition, EP80 x Puenteareas showed tall plants, late flowering, and low grain moisture. Although late genotypes are more productive than early ones (Shawn, 1988; Lauer et al., 1999), the genotypes with late flowering are usually more affected by stresses. Nevertheless, the cross EP80 x Puenteareas showed low grain moisture content at harvest, which reduces damage caused by pests and fungal infections (Cartea et al., 1994; Munkvold, 2003) and problems with storage. The cross EP80 x Silver King had high productivity and showed the best values for yield components (data not shown); nevertheless it showed the highest grain moisture.

The GCA for traits related to earliness (grain moisture and silking days) and plant height, were, in general, significant for populations and for inbred lines (Table 6), agreeing with results reported by other authors in optimum conditions (Vasal et al., 1992; Eyhérabide and Gonzalez, 1997; Hede et al., 1999). These authors also found significant GCA for yield, which was not observed in the present study. No differences among SCA for any trait were found. The increase of experimental errors due to evaluation in stress conditions could be partially responsible for this lack of significance.


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  		Table 6. Estimates of general combining ability of five inbred lines and nine populations for yield and other agronomic traits in field early sowing.

 
The inbred line EP80 showed high values for GCA for silking days and plant height, both traits being positively correlated with yield (Hallauer and Miranda, 1988), while crosses to the inbred lines F7 and Z78008 had the earliest and shortest plants. The population Silver King showed, in general, favorable GCA for all traits, except for grain moisture. This population presented the worst value of GCA for grain moisture, which is contrary to the observation that dent maize generally has lower grain moisture than flint maize at harvest time (Hunter et al., 1979). This disadvantage showed by the crosses between Silver King and European inbred lines discourages the use of this population for further breeding programs. On the other hand, Puenteareas showed the most favorable values of GCA for grain moisture and plant height.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
None of the maize inbred lines or populations could be considered completely cold tolerant and the genetic base of cold-tolerant inbred lines is not large enough to produce superior second-cycle inbred lines. In most cases, the performance of cold-tolerant inbred line x population crosses was better than populations per se or hybrids, suggesting that inbred x population crosses are advisable as base populations for cold tolerance breeding programs. The crosses between the inbred line EP80 and northwestern Spanish populations are the most promising base germplasm for further breeding programs for cold tolerance. Particularly, EP80 x Puenteareas showed the highest yield and good performance at the first stages of development under cold conditions, either in the cold chamber or in the field. Early vigor rating was the only cold-related trait for which differences among genotypes were observed in both cold chamber and field. The results of our work suggest that early vigor rating would be the most suitable trait to select maize genotypes with superior cold tolerance during emergence and postemergence stages. Additive effects appeared to be larger than nonadditive, encouraging recurrent selection on new populations derived from crosses between cold-tolerant inbreds and populations with favorable GCA effects.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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 October 10, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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V. M. Rodriguez, R. A. Malvar, A. Butron, A. Ordas, and P. Revilla
Maize Populations as Sources of Favorable Alleles to Improve Cold-Tolerant Hybrids
Crop Sci., September 1, 2007; 47(5): 1779 - 1786.
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