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CROP BREEDING, GENETICS & CYTOLOGY

Genotype x Environment Interactions in Maize Hybrids from Temperate or Highland Tropical Origin

^a Unité de génétique et d'amélioration des plantes, Institut National de la Recherche Agronomique, Domaine Brunehaut, F-80200 Estrées-Mons, France
^b Cargill Hybrid Seeds, P.O. Box 701, Kaunakakai, HI 96748 USA
^c Unité de génétique et d'amélioration des plantes, Institut National de la Recherche Agronomique, Domaine de Melgueil, F-34130 Mauguio, France

giauffre{at}mons.inra.fr

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Exotic germplasm is often used in maize (Zea mays L.) breeding programs. However, the occurrence of genotype x environment interactions (GEIs) may mask the potential utility of exotic material. Our objective was to understand better GEIs for temperate, temperate x highland, and highland tropical maize genotypes cultivated under temperate or highland tropical conditions. We related yield instability to GEIs observed for vegetative or flowering traits, and determined the response of these vegetative or flowering traits to temperature and photoperiod. Forty-one hybrids grown in four environments were observed for several pre-flowering, flowering, and yield traits. Grain yield variation was related to two major adaptation factors: disease resistance and planting-to-silking duration (PSD). Yield variations were also related to variation in traits measured before flowering, such as emergence duration or early growth. For emergence duration, leaf stage at a given date, or leaf size, one simple temperature covariate accounted for more than half of the interaction sum of squares. Temperate x highland tropical hybrids had an intermediate behavior and were more stable than the pure temperate or pure highland tropical hybrids. For total number of leaves, photoperiod at tassel initiation explained a higher proportion of the interaction sum of squares than temperature. The response of highland tropical hybrids to photoperiod was larger than for the temperate hybrids. We recommend breeders who wish to introduce exotic material into adapted material utilize mass selection and/or advanced backcrossing, with marker assisted selection for specific traits with low heritability, such as cold tolerance.

Abbreviations: CIMMYT, International Maize and Wheat Improvement Center • EH, ear height • ELN, ear leaf number • GDD, growing degree days • GEI, genotype x environment interaction • INRA, Institut National de la Recherche Agronomique • LL6, length of Leaf 6 • LL8, length of Leaf 8 • MASL, meters above sea level • MLn, mature leaf number at the nth counting • PED, planting-to-emergence duration • PH, plant height • PSD, planting-to-silking duration • TLN, total leaf number • VLn, visible leaf number at the nth counting • WL6, width of Leaf 6 • WL8, width of Leaf 8

	INTRODUCTION

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NORTHERN TEMPERATE REGIONS (e.g., Northern Europe, Canada) and cool highland tropical zones (e.g., central Mexico, Andean highlands) are considered fringe areas for maize production. Early flowering, early maturity, and cold tolerance are needed to ensure adaptation in these fringe zones. Temperate breeding programs for these regions may use highland tropical material as a source of cold tolerance. Highland tropical programs may introduce temperate germplasm to improve agronomic traits. Germplasm from such crosses may be less photoperiod sensitive than pure highland tropical material. Defining the main biotic and abiotic stresses specific to a given environment leads to the definition of pertinent selection criteria. For some traits (i.e., root lodging), it is possible to screen the material in a single location that is favorable for expression of the trait. But, for many others traits, GEIs may mask the potential utility of exotic material in a given environment. In most GEI studies, maize was cultivated in similar geographical areas (Matzinger et al., 1959; Lonnquist and Gardner, 1961; Francis and Kannenberg, 1978; Vincourt et al., 1984, Argillier et al., 1994); however, international trials were also analyzed (Crossa et al., 1990; Bonhomme et al., 1994; Byrne et al., 1995). The traits usually studied are biomass yield (Argillier et al., 1994) or grain yield, but some studies also include plant height, anthesis-silking interval (Byrne et al., 1995), ear moisture loss rate (Magari et al., 1997), or root characteristics (Hébert et al., 1995). Some authors attempted to relate GEI to genotypic or environmental variables (Argillier et al., 1994; Hébert et al., 1995; Magari et al., 1997). But, to our knowledge, no one has examined hybrids from various origins, cultivated under very different environments, for traits related to leaf area establishment and flowering.

This project was designed to provide a better understanding of GEI for germplasm of temperate, temperate x highland tropical, and highland tropical origin cultivated under temperate or highland tropical conditions. Our objectives were to quantify GEI, to relate yield instability to GEIs observed for vegetative or flowering traits, and to determine the response of these vegetative or flowering traits to temperature and photoperiod.

	Materials and methods

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Experimental Data
Five temperate inbred lines (F2, F252, F564, W117, MBS847) representing different heterotic groups from the USA or Europe, and five inbred lines (CML242, CML246, CML245, C4, C5) selected in highland tropical environments at CIMMYT's experiment stations in Mexico were chosen as parents for a half-diallel mating design experiment (Table 1) . F2 and F252 are cold tolerant, early-maturing inbreds of European origin. F2 has yellow flint kernels, while F252 has yellow dent kernels. F564 is a mid-to-late maturity inbred with flint kernels but rather cold susceptible. MBS847 is a dent mid-to-late maturity inbred belonging to the Iodent heterotic group. W117 is a cold susceptible early-maturing inbred derived from Minnesota 13. Two lines (C4, C5) are experimental CIMMYT inbreds adapted to very cold highland tropical areas (< 30°N and S latitudes, 2600 meters above sea level [MASL]). They are of intermediate-to-full-season maturity in their zone of adaptation with white semi-dent kernels. C5 comes from CIMMYT Population 900 that is adapted to very cold zones (12.5–15°C). CML242 is an early-maturing inbred derived from CIMMYT Population 85 with white semi-flint kernels. CML245 is an early-maturing inbred derived from CIMMYT Population 86 with yellow semi-flint kernels. CML242 and CML245 are adapted to warmer highland areas with mean growing season temperatures of 15 to 17°C. CML246 is an intermediate-maturity inbred derived from CIMMYT Population 800 with white semi-dent kernels. It was selected in the tropical highlands, but targeted for the Himalayan region where mean temperatures are between 15 and 20°C. Two commercial single-cross hybrids were included as checks: DEA is adapted to Northern Europe, while H33 is adapted to highland tropical zones.

The experimental design was a randomized complete block with three replications. To minimize competition due to differences in plant height and to allow for different harvesting dates, the hybrids were grouped according to the origin of their parents: temperate x temperate, temperate x highland tropical, highland tropical x highland tropical. Groups were randomized within blocks and hybrids were randomized within each group. The two checks were included in each group of each block. They did not show any difference among groups within each block. Twelve variables were defined (Table 2) . Four-row plots were used at El Batán, Toluca, and Montpellier, and six-row plots were used at Mons. Measurements prior to flowering time were taken on one of the center rows. The following traits were measured on 15 plants per plot: number of leaves observed at five different dates, length and width of the sixth and eighth fully expanded leaves, ear bearing node, total number of leaves, silking dates, and ear and plant height (Table 1). The length of the planting to silking period was evaluated in growing degree days with a base temperature of 6 °C (GDD) (Derieux and Bonhomme, 1982). Grain yield and grain moisture data were determined from the two center rows. Ears were hand-harvested at Toluca and El Batán. They were mechanically harvested at Mons and Montpellier. At these locations, harvest dates were different for each type of hybrid. At Montpellier, the highland tropical hybrids were harvested 6 wk later. The checks replicated in each group showed that the fresh grain weight did not change between the two harvest dates, but the grain moisture decreased by 90 g kg^-1. Then, moisture data were corrected (+90 g kg^-1) for each highland tropical hybrid. At Mons, the temperate x highland tropical hybrids were harvested 3 wk later, but no significant difference in fresh grain weight or grain moisture was observed between the two harvesting dates on the checks, and no correction was done. The grain of the highland tropical hybrids did not mature and were not harvested at Mons. In highland tropical environments, susceptibility to rust (Puccinia sorghi Schwein.) was evaluated by a visual rating scale (1 = highly resistant, 5 = highly susceptible). In temperate environments, the number of diseased ears and stems was counted and susceptibility to smut—Ustilago zeae (Beckm.) Unger [ = Ustilago maydis (DC.) Corda]—was determined as follows:

The model including a hybrid x location interaction was written as follow:

(1)

where Y_ijk is the mean for the ith hybrid, in the kth block of the jth location; µ is the grand mean; _i is the mean deviation for the ith hybrid; ß_j is the mean deviation for the jth location; _ij is the interaction term for the combination of the ith hybrid with the jth location; _jk is the mean deviation for the kth block of the jth location; _ijk is the residual. Model 1 was used for grain yield and for all vegetative and flowering traits. The model ignores the group and the group x block source of variation which was not significant for any of the traits.

In order to explain the hybrid x location interaction for grain yield, we used the following covariance model:

(2)

where C_ij is the covariate; is the mean regression coefficient on the covariate; _i is the deviation of the regression coefficient for the ith hybrid; _j is the deviation of the regression coefficient for the jth location; and ^'_ij is the remaining interaction. This model was used only for grain yield. The interaction parameter (_ij) estimated from Model 1 for the different vegetative and flowering traits served as the covariate (C_ij).

Factorial regression models (Wood, 1976; Denis, 1988; Van Eeuwijk et al., 1996) provide a biological interpretation of the GEI. In order to describe the variation for vegetative and flowering traits, we used a factorial regression model with one environmental covariate (Table 3):

(3)

where is the mean genotypic regression coefficient on the environmental covariate C_j; ß^''_j is the remaining location effect; _i is the deviation of the regression coefficient due to the ith hybrid; and ^''_ij is the remaining interaction. Both location main effects ß_j and interaction effects _ij from Model 1 were thus modeled for each of the vegetative and flowering traits.

	Results

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There were large differences among hybrids for grain yield and maturity (data not shown). Hybrids yielded between 5.2 and 9.6 Mg ha^-1 at 150 g kg^-1 moisture. Grain moisture ranged between 220 and 400 g kg^-1. The earlier-maturing hybrids (lower harvest grain moisture) were of temperate origin while the later-maturing hybrids were of highland tropical or mixed origin. Mean yields across locations of temperate and highland tropical material were similar. The best yields were obtained with mixed hybrids. The maturity of these hybrids was generally intermediate, except for four hybrids with grain moistures over 350 g kg^-1. Four hybrids, all involving the Iodent line MBS847, were identified as the best overall hybrids having both high yield and acceptable maturity: MBS847 x CML245, MBS847 x C4, MBS847 x C5, and F564 x MBS847. These hybrids yielded over 8 Mg ha^-1, with grain moistures between 300 and 350 g kg^-1.

The hybrid x location interaction for yield was highly significant (57% of the total sum of squares) and must be taken into account when characterizing hybrid performances. The group of hybrids x location effect explained 80% of this interaction with only 6 degrees of freedom. The relative contribution of each hybrid to the hybrid x location interaction sum of squares (ecovalence) (Wricke, 1962) provided a measure of its stability in our experimental conditions (Fig. 1) . Highland tropical hybrids were the most unstable (ecovalence higher than 4%), temperate hybrids were relatively stable (ecovalence between 2 and 4%), while, with two exceptions, mixed combinations had ecovalences below 2%.

View larger version (15K): [in this window] [in a new window]   Fig. 1 Relationship between mean grain yield over four environments and percentage of total hybrid x location sum of squares, a measure of yield stability

View larger version (19K): [in this window] [in a new window]   Fig. 2 (a) Rust susceptibility and mean grain yield of 41 maize hybrids and two commercial checks in two tropical highland environments, 1993. Disease rating scale 1 to 5, where 1 = highly resistant and 5 = highly susceptible. (b) Smut susceptibility and mean grain yield of 41 maize hybrids and two commercial checks in two temperate environments, 1993. Disease rating scale 0 to 3, where 0 = no symptoms and 3 = all plants with symptoms on the ear and on the stem

View larger version (22K): [in this window] [in a new window]   Fig. 3 (a) Planting-to-silking duration and mean grain yield of 41 maize hybrids and two commercial checks in two tropical highland environments, 1993. (b) Planting-to-silking duration and mean grain yield of 41 maize hybrids and two commercial checks in two temperate environments, 1993

Partitioning of the Hybrid x Location Interaction for Grain Yield
Interaction effects were highly significant for all pre-flowering or flowering variables, except for ML2. These interaction effects were introduced as a covariate to explain the interaction sum of squares observed for grain yield (Model 2). The interaction parameter of any vegetative or flowering trait explained at least 40% of the interaction sum of squares observed for grain yield (Fig. 4) . All regression terms (mean regression, deviation due to the hybrid, deviation due to the location) were significant at the 0.01 level. The interaction parameter of PSD explained 90% of the yield interaction. Most of it was directly explained by the mean regression, which means that the relationship varied little among hybrids or locations (Fig. 5a) . Yields were lower than expected when flowering was delayed more than expected based on the mean performance of the location or the hybrid. This was especially true for the highland tropical group of hybrids in the temperate environments. The interaction parameter for ear height (EH) explained 62% of the yield interaction (Fig. 4), but the relationship was different depending on the genotype (Fig. 5b). This difference appeared among groups of hybrids. The relationship was loose for the mixed hybrids, but was clearly negative for the temperate and highland tropical hybrids. The amount of the yield interaction explained by the interaction parameter of WL6 was 59% (Fig. 4). This relationship depended on the hybrid, but no general trend was found for each group of hybrids. For planting-to-emergence duration (PED), highly significant differences were found among locations. The relationship between the interaction parameter for yield and the interaction parameter for PED was negative for three locations, but positive for El Batán (Fig. 5c). This means that, in El Batán, those hybrids that emerged more rapidly were lower yielding. No explanation exists for this particular behavior.

View larger version (77K): [in this window] [in a new window]   Fig. 4 Partitioning of the hybrid x location interaction sum of squares for grain yield. The partitioning is based on a covariate effect. The covariates are the interaction parameters estimated for different traits (see abbreviations in Table 1). The first interaction term with the covariate to be introduced was the one that explained the highest amount of the variation

View larger version (20K): [in this window] [in a new window]   Fig. 5 Relationship and regression coefficients (b) between grain yield interaction parameter estimates and interaction parameter estimates for (a) planting-to-silking duration, (b) ear height, and (c) planting-to-emergence duration

Temperature and Photoperiod Response of Vegetative and Flowering Traits
The hybrid x location interaction effect for all pre-flowering or flowering variables (Table 2) except LL8 was significantly explained by the group of hybrid x location effect. But the amount of interaction explained was different depending on the variable: more than 50% for PED, and PSD; between 25 and 50% for VL2, VL4, EH, and ear leaf number (ELN); less than 25% for ML2, ML4, LL6, WL6, LL8, WL8, plant height (PH), and total leaf number (TLN).

Factorial regression models were tested. These models utilized only one covariate each time, among the 10 covariates previously defined (Table 3). For each trait, we retained as the best covariate the covariate that accounted for the highest proportion of the hybrid x location interaction (Table 4) . The best covariate for PED, MLn, or VLn was the temperature from planting to emergence. This implies that any developmental advantage that a hybrid had at emergence time was maintained up to flowering time. For leaf characteristics (LL6, WL6, LL8, WL8), the best covariate was the mean temperature from emergence to the fourth leaf stage. This interval corresponds to the formation of the photosynthetic apparatus. This could explain why temperature during that interval greatly influenced the size of the future leaves. The temperature between the twelfth and the fourteenth leaf stage appeared to determine plant height. This was the period of active elongation of the stem. For traits related to flowering (PSD, EH, PH, ELN, and TLN), we also tested the photoperiod covariate that corresponded to the period of tassel initiation. Photoperiod was a covariate that explained a greater amount of the interaction sum of squares than temperature for ELN and TLN, but not for PSD, EH, or PH.

For each of these appropriate covariates, and for each hybrid, the regression coefficient of mean performance on covariate across locations was estimated. The mean of these regression coefficients for the three groups of hybrids (Table 4) gives an idea of the response of the hybrids to the covariate. For PED, the response of the temperate hybrids to temperature was of greater magnitude than the response of the highland tropical hybrids. Temperate hybrids were more affected by the low temperatures in the coldest environment (Toluca). For MLn and VLn, and for traits related to leaf size (LL6, WL6, LL8, WL8), a higher response for temperate hybrids was also observed. For all traits related to flowering (PSD, EH, PH, ELN, TLN), temperate hybrids had lower slopes and were less sensitive to covariates than the mixed or tropical hybrids. This is likely an indirect effect of photoperiod response. Photoperiod was correlated with temperature; temperatures were higher and day lengths longer in the temperate environments. The effect of photoperiod on leaf number (ELN, TLN) at flowering was clearly demonstrated (50% of the interaction sum of squares). The response of the highland tropical hybrids in this case was three times higher than the response of the temperate hybrids.

Finally, for almost all traits and all covariates, the intermediate group of hybrids had intermediate behavior. This suggested that the additive adaptation genes combined in these hybrids improved stability across environments.

	Discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Genotype x environment interaction for grain yield was explained by variation observed for different vegetative or flowering traits. Generally, grain yields were reduced when emergence or silking was delayed. In maize, Argillier et al. (1994) showed that biomass yield was related to silking date, but that the relationship depended on the location; early cultivars had an advantage in some locations, but not in others. In others species, some authors succeeded in determining key stages of yield elaboration. Biarnès-Dumoulin et al. (1996) demonstrated the role of the node number of the first flower and the mean number of reproductive nodes in the GEI for grain yield in dry pea (Pisum sativum L.). In most cases, the genotypic covariates that were used were the genotypic mean value or the additive genotypic effect observed for the explanatory trait. Baril (1992) observed a GEI for the 1000-kernel weight in wheat (Triticum aestivum L. em Thell.) and therefore used a multiplicative genotypic parameter derived from a multiplicative model as a covariate. The interpretation of the multiplicative genotypic parameter is difficult because it cannot be done separately from the interpretation of the multiplicative environmental parameter. Therefore, we used the interaction parameter as a covariate.

In maize, some authors who have attempted to relate GEI to environmental variables succeeded in accounting for the total GEI sum of squares with only one to three covariates (Argillier et al., 1994; Hébert et al., 1995). In our case, the remaining GEI sum of squares was still significant. It remained significant even when we tested both photoperiod and temperature covariates (results not shown). This might be due to the differences among our environments that were not measured (solar radiation, rainfall, irrigation, soil type, plant density, etc.). Nevertheless, a single covariate accounted for more than half of the interaction sum of squares in many instances. Kang and Gorman (1989) found that environmental covariates removed a small amount of the heterogeneity due to GEI for grain yield. Our better results in explaining GEI are probably due to the fact that the analyzed traits were not grain yield, but simpler traits that were directly influenced by photoperiod and temperature. Magari et al. (1997) also gave a satisfactory explanation of GEI for a simpler trait, e.g., ear moisture loss rate.

With the statistical models used, it was possible to relate yield instability of hybrids from temperate or highland tropical origin evaluated across temperate or highland tropical environments to major adaptation factors of disease resistance, temperature, and photoperiod insensitivity. These adaptation factors were also pointed out by Eagles and Lothrop (1994). We found that PSD and TLN were related to photoperiod around the time of tassel initiation. This photoperiod sensitive period for flowering and leaf number was clearly identified in maize by Coligado and Brown (1975), Tollenaar and Hunter (1983), and Ellis et al. (1992). Yield instability was related to factors occurring before flowering. Planting-to-emergence duration, leaf stage, leaf size, and plant height were clearly affected by temperature. The influence of temperature on germination has long been known (Lehenbauer, 1914). The optimum temperature is around 30°C, whereas the minimum temperature is below 10°C. Leaf stages directly result from the leaf appearance rate. Many authors have studied the relationship between leaf appearance rate and temperature (Brouwer et al., 1973; Tollenaar et al., 1979; Warrington and Kanemasu, 1983; Giauffret et al., 1995). Hesketh and Warrington (1989) showed that maximum leaf length was less affected by temperature than leaf area and maximum leaf width.

Highland tropical and temperate materials behaved quite differently. Highland tropical genotypes emerged rapidly under cold conditions. This was also observed by Eagles and Brooking (1981), Brooking (1990), and Giauffret and Derieux (1991). It is a general characteristic of highland tropical maize, and it is often associated with an ability to emerge from planting depths of up to 25 cm because of mesocotyl elongation (Eagles and Lothrop, 1994). Stamp (1984) also observed that highland tropical types developed faster than temperate types under cool planting conditions. In controlled environment studies, Hardacre and Eagles (1986) concluded that hybrids containing highland tropical germplasm tolerate low temperature better than temperate hybrids. Bonhomme et al. (1994) compared the response to photoperiod of highland tropical material vs. temperate material. They found that highland tropical cultivars were photoperiod sensitive (although less so than lowland tropical cultivars), while temperate cultivars are insensitive. We demonstrated that temperate x highland tropical hybrids were intermediate compared with the parental groups for photoperiod and temperature sensitivity. This was also true for disease resistance. It reveals that these traits were, to a large extent, controlled by additive gene action.

All the results were obtained after a 1-yr experiment with four locations. They could therefore be attributed to particular events, with no general meaning. However, we did not identify any such specific event. The temperature and photoperiod differences among the locations were so large and in accordance with the general trend for temperate and highland tropical locations that the results should remain useful to the breeder. To our knowledge, no other experiment involved 41 single-cross hybrids from highland tropical, temperate, or mixed origin evaluated at many different developmental stages in such contrasting locations.

In conclusion, we suggest some selection methods for breeders who would like to introduce exotic material into adapted material to improve cold tolerance or photoperiod insensitivity. Because of the magnitude of the GEI for yield, it is not possible to conduct a good yield evaluation of pure exotic material. Therefore, it would be necessary to improve the general adaptation of the material before yield evaluation. The main traits to improve are, for highland tropical material in temperate environments, smut resistance and earliness through a reduction in photoperiod sensitivity; and for temperate material in highland tropical locations, rust resistance and cold tolerance during germination and emergence (continuing up to flowering). This improvement could be made by mass selection during several short cycles. This method was successfully used to improve the earliness of a lowland tropical population and adapt that population to Corn Belt conditions (A.R. Hallauer, 1998, personal communication). The improvement could also be done by crossing the exotic with adapted germplasm. Tanksley and Nelson (1996) proposed an advanced backcross method to introduced valuable traits of unadapted germplasm into elite cultivars. These authors recommend eliminating the non-desirable traits by phenotypic selection during the formation of the back-cross population. Again, the traits that could be improved in maize would be disease resistance, earliness, and duration to emergence. Then, after these first steps of selection, the use of molecular markers for quantitative trait loci mapping would be useful to improve quantitative traits with low heritability (Kang, 1998).

This study also suggests ideas on the choice of locations for the breeding procedure. Because of the magnitude of the GEI, it will not be necessary to have different locations for the first steps of the breeding. One location in the target area should be sufficient to improve disease resistance and earliness. In order to improve cold tolerance during the early stages, it might be useful to test emergence and early development in two temperature regimes (two planting dates for example). This test would be useful for both highland tropical or temperate target areas. For a highland tropical area, it will eliminate all temperate material that develops too slowly, and for a temperate area, it will ensure that the highland tropical material that is introduced will improve the cold tolerance of the adapted material. A selection method using two stress levels in the same location was successfully applied by Byrne et al. (1995) to improve drought resistance in lowland tropical maize.SAS Institute 1989

Received for publication January 26, 1999.

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