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Dep. of Plant Agriculture, Crop Science Building, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
* Corresponding author (mtollena{at}uoguelph.ca)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DMA, dry matter accumulation • G x E, genotype-by-environment interactions • LAI, leaf area index • r, phenotypic correlations • rG, genetic correlations
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Although several economically important crops benefit from the manifestation of heterosis, both the genetic and physiological mechanisms underlying this phenomenon are still unexplained. Two major hypotheses have been proposed regarding the genetics underlying heterosis: (i) dominance hypothesis and (ii) overdominance hypothesis. The dominance hypothesis attributes heterosis to the accumulation of favorable dominant genes or masking of deleterious recessives in the hybrid. This theory is consistent with recent genomic evidence of differences in genic content between maize inbred lines (Fu and Dooner, 2002), and has been demonstrated as the underlying cause of a heterotic response for grain yield in a quantitative trait locus (QTL) mapping study (Graham et al., 1997). The other hypothesis, overdominance, argues that the heterozygous combination of the alleles at a single locus is superior to either of the homozygous combinations. There is no direct evidence in support of this hypothesis in the literature; however, it has not been rejected as a genetic cause. Several attempts also have been made to explain the physiological basis of heterosis for grain yield in maize. In one of the first attempts at explaining hybrid vigor, Ashby (1930) suggested that the larger stature of the hybrids compared with their parents was associated with greater embryo size in hybrids. However, no consistent relationship was found between hybrid vigor and the embryo size by other workers (Whaley, 1950; Voldeng and Blackman, 1973), and this theory was abandoned. Phenotypic expression of heterosis for grain yield in maize is simply the discrepancy in grain yield between hybrids and their parents.
Williams (1959) stated that heterosis is the property of quantitative characters, which are complex interactions between simpler processes, and "many of the difficulties that are encountered in the interpretation of heterosis arise out of the failure to recognize the component parts of complex expressions." Grain yield is influenced by the interaction of numerous physiological processes and the environment throughout the life cycle. Heterosis has been discussed in terms of an absolute or static attribute, but heterosis should be regarded as a dynamic attribute influenced both by the environment and by the stage of development. To overcome many of the difficulties that are encountered in the interpretation of heterosis for complex traits, component-analysis approaches have been used to study the effect of heterosis on grain yield (cf., Sinha and Khanna, 1975). Grain yield has been subdivided, for instance, into ear number, kernel number, and kernel weight in an attempt to understand how heterosis influences grain yield (cf., Sinha and Khanna, 1975). These grain yield components, however, are static attributes that do not lend themselves to a process-based analysis of grain yield formation. An alternative approach is to quantify grain yield in terms of the product of total aboveground DMA at maturity and the proportion of the aboveground dry matter that is partitioned to the grain (i.e., harvest index). Heterosis for DMA at maturity and harvest index can subsequently be analyzed in terms of their underlying physiological processes at various levels of organization. For instance, DMA at maturity is the result of rate of DMA throughout the growing season, and the two processes underlying rate of DMA are light interception by the canopy and leaf photosynthesis. Harvest index is associated with the capacity of the reproductive sink to accommodate assimilate, for example, kernel number and kernel weight, and the assimilate supply for grain DMA during the grain-filling period.
The objective of the present study was to quantify the heterotic response observed in the physiological processes contributing to grain yield, within a set of 12 F1 hybrids and their seven parental inbred lines. The physiological basis of heterosis for grain yield will be discussed at two levels of organization. First, we will discuss heterosis for grain yield in terms of heterosis for its two component processes, DMA at maturity and harvest index. At the second level of organization, we will discuss heterosis for grain yield in terms of heterosis for the processes underlying the two component processes of grain yield.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Experimental Procedures
Studies were conducted at the Elora Research Station, ON, Canada (43°38'N, 80°25'W, 380 m above sea level) in 2000, 2001, and 2002. To avoid competition effects between inbred lines and hybrids, the 12 hybrids and their seven parental inbred lines were planted in adjacent blocks. The blocks of hybrids and inbred lines were treated as two separate experiments. Between 1 May and 10 October, the average precipitation in the region is approximately 400 mm and average seasonal heat unit accumulation is around 2650 Ontario Crop Heat Units. The soil type is a London loam (fine-loamy, mixed, semiactive, mesic Typic Hapludult).
Maize was sown with hand planters at two seeds per hill and thinned to one plant per hill at the seedling stage for a final plant density of 70000 plants ha–1. Before seeding, 600 kg ha–1 of 20–20–10 (N–P–K) fertilizer and 3 L ha–1 of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) were applied to the soil. Weed control was completed by applying 0.28 kg ha–1 bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) before Leaf 6 was fully expanded and by manual weeding. In addition, 100 kg N ha–1 as ammonium nitrate (34–0–0) was applied at approximately the 12-leaf stage (i.e., leaf tips). The experiment was arranged in a randomized complete block design with four replications in each year. Each plot consisted of a 12-m long, six-row plot with 76-cm interrow spacing and the two middle rows of each plot were used to measure the following characters in hybrids and their parental inbred lines.
Dry Matter Accumulation
Dry matter accumulation was determined at the six-leaf stage (i.e., leaf tips), the 14-leaf stage, silking, and maturity by destructive whole-plant sampling during the 2001 and 2002 growing seasons; DMA was determined only at silking and maturity in 2000. Dry matter at the six-leaf stage was determined for each plot by taking a random sample of 10 plants in two middle rows from those pulled out at thinning at the six-leaf stage. Plant samples were dried at 80°C until constant weight. Three sample areas (each 3.04 m2) were used to determine DMA of each plot at the other three stages of development. Sample area was separated on each end by a 2-m long internal border and two border rows on each side. For the determination of DMA at the 14-leaf stage and at silking, all plants in the sample area were cut at ground level and divided into sample and subsample portions. The subsample consisted of five randomly selected plants in the harvest area. After recording the fresh weight of both portions, the subsample was separated into leaf and stem and the sample portion was discarded. Moisture content of each subsample was determined by measuring its weight before and after drying at 80°C until weight did not change for two consecutive weighing dates. Total dry weight of the sample area was estimated by multiplying total sample fresh weight and percentage dry matter of the subsample. Total aboveground DMA at physiological maturity was estimated by cutting 10 subsample plants at ground level and separating the subsample in ear and nonear portions. Only the ears of the remaining plants in the sample area were harvested. Dry weight of each subsample fraction and of the remaining ears in the sample area was determined after drying at 80°C until weight did not change for two consecutive weighing dates. The DMA at maturity was computed from dry weights of the ear and nonear fractions of the subsample and the remaining ears in the sample area.
Grain Yield and Harvest Index
All ears from the sample area at maturity were shelled and grain weight of the sample was determined. Grain yield was expressed at 0% moisture. Harvest index was estimated as the proportion of grain weight to total aboveground dry weight in the 10 subsample plants in each plot at maturity.
Kernel Number, Kernel Weight, and Ear Number
The number of kernels per unit area was calculated from complete grain sample at maturity using a seed counter. The 1000-kernel weight was also determined from the same sample. Number of ears per plant was recorded before harvest at maturity; only ears containing 10 kernels or more were included in the count.
Leaf Area Index and Stay Green
The LAI of each entry at the 14-leaf stage and at silking was recorded by a LICOR leaf-area meter model 3100 (LI-COR, Lincoln, NE). For partly senesced leaves, the senesced portion was cut away from the leaf before measurement so that only green leaf area was determined. The LAI was estimated for each entry by dividing the leaf area of five randomly selected plants by the corresponding ground surface. In the first 2 yr of the study, stay green was determined by visually assessing the degree of green tissue at 6 wk postsilking. To measure this trait more accurately, in the third year of the study, five plants were randomly selected and tagged within each plot, and leaf area was estimated by measuring leaf length and maximum leaf width of all leaves on each selected plant at silking; leaf area was estimated as leaf length x leaf width x 0.75 (Montgomery, 1911) and summing for all leaves measured on each plant. Every leaf was again examined at 6 wk postsilking and the stay green of each entry was determined as the proportion of green leaf area at 6 wk postsilking relative to green leaf area at silking by visually rating the proportion of leaves remaining green (cf., Valentinuz and Tollenaar, 2004).
Light Interception
Light interception was not measured in this study, but effects of LAI on light interception were estimated using the following equation: IA = 1– e–k x LAI, where IA is the fraction of incident solar radiation absorbed by the crop canopy, and k is the extinction coefficient of the crop canopy (Tollenaar and Dwyer, 1999). The extinction coefficient (k) was assumed to be 0.65 in the estimates of light interception.
Leaf Number and Rate of Leaf Appearance
Rate of leaf appearance was determined by counting number of visible leaf tips per plant at approximately the six-leaf stage and the 14-leaf stage in 2000 and 2001. Total leaf number along the stem of the plants was determined by marking Leaf Positions 6 and 10 on five randomly selected plants in each plot before the first leaf senesced and taking final count immediately after anthesis.
Silking and Maturity Dates
Number of days from planting to silking and maturity were determined for each entry. Silking date was the first day silks were visible on the topmost ear of at least 50% of plants in a plot. Date of maturity was defined as the first day when grain of at least 50% of plants in a plot attained black layer.
Statistical Analysis
Analyses of variance were performed for each trait in both hybrid and inbred line experiments in each year using PROC GLM of SAS v. 8.2 (SAS Institute, 1999). In addition, a variance analysis was performed for the combination of two or more years using the following linear model:
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j is the effect of year, 
i(j) is the block effect nested within a year, 
k is the effect of entry, ()jk is the interaction effect of year and entry, and 
ijk is the random effect of the subplot units. Midparent heterosis, the measure of the average superiority of a hybrid over its parental inbred lines, was calculated for each trait as [(F1 – MP)/MP] x 100, where F1 is the cross mean and MP is midparental value. Mean heterosis was estimated from the heterosis values of the 12 hybrid and midparental means for each of the 2 or 3 yr. Phenotypic correlations (r) were performed between heterosis for grain yield and either grain yield, DMA at maturity, harvest index, heterosis for kernel number and heterosis for kernel weight, and between heterosis for kernel number and heterosis for DMA from the 14-leaf stage to silking using PROC CORR of SAS v. 8.2. Genetic correlations (rG) using four replications across years between grain yield and DMA at maturity and grain yield and harvest index were calculated for both hybrids and inbred lines as described by Bernardo (2002). |
RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Heterosis for Physiological Processes Underlying the Grain-Yield Component Processes
Rates of Dry Matter Accumulation Throughout Life Cycle
Heterosis for rates of DMA varied among environments and throughout the life cycle, indicating that heterosis should be considered as a dynamic attribute. The magnitude of the mean heterotic response for grain yield, DMA at maturity, and harvest index varied with environment: 385, 161, and 99%, respectively, in 2000; 163, 88, and 44%, respectively, in 2001; and 117, 56, and 40%, respectively, in 2002. Although heterotic responses varied greatly among environments, the relative magnitude of heterosis was consistent for grain yield and its component processes. Heterosis for DMA also varied throughout the life cycle: 90% at the six-leaf stage, to 163% at the 14-leaf stage, 57% at silking, and 85% at maturity. Heterosis for rates of DMA was 90% during the interval from planting to the six-leaf stage, 153% between the six- and the 14-leaf stage, 58% between the 14-leaf stage and silking, and 134% during the grain-filling period. This developmental aspect of heterosis allows us to assess its contribution to the total DMA of F1 hybrids at maturity (Fig. 1)
. Among the four phases of development, the period from the six- to 14-leaf stage displayed the highest heterosis, but only 19% of total DMA at maturity was accumulated during this phase. In contrast, the period from the 14-leaf stage to silking contributed 36% and the period from silking to maturity contributed 45% (Fig. 1). The grain-filling period is the phase that displays the second highest heterosis for DMA during the life cycle and it is also the phase during which hybrids accumulate most dry matter. Interestingly, the greatest differences for DMA among the hybrids occurred during the grain-filling period (Ahmadzadeh, 2003).
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Leaf Area Index
Heterosis for LAI declined from 140% at the 14-leaf stage to 88% at silking (Table 1). Mean LAI at the 14-leaf stage for hybrids was 1.30 and for inbred lines was 0.55. Heterosis for LAI at this stage was attributable, in part, to a greater number of emerged leaves due to a greater rate of leaf appearance for hybrids than for inbred lines from the six- to 14-leaf stage (Table 1). Mean LAI at silking for hybrids increased to 2.95 and for inbred lines increased to 1.55. Total number of the leaves per plant at silking was only slightly greater in hybrids than in their parental lines. Mean final leaf number of the hybrids was 16.9 and that of inbred lines was 16.0, resulting in a small, but statistically significant heterosis of 5.5% (Table 1). Consequently, heterosis for maximum LAI in our study was because of leaf size rather than leaf number, similar to other studies (Torigoe et al., 1987). The relative contribution of leaf length vs. maximum leaf width to heterosis for LAI and the effect of leaf position on heterosis for LAI were assessed in 2002 when leaf length and maximum leaf width were measured for each leaf on the plant. Heterosis for leaf length was approximately double of that for maximum leaf width and heterosis for leaf area increased with leaf position from the bottom to the top of the plant (Ahmadzadeh, 2003). Heterosis for leaf size has been attributed to a greater cell number rather than to a greater cell size (Uchimiya and Takahashi, 1973; Pavlikova and Rood, 1987). Heterosis for maximum LAI was fairly consistent across years and hybrids, ranging from 70% in 2002 to 103% in 2000, and from 70% for CG58 x LH61 to 110% for CG57 x CGG33. Heterosis for green LAI increased after silking, as the heterosis for stay green at 6 wk postsilking was 91% (Table 1).
Dry Matter Accumulation from Planting to Silking
Heterosis for DMA between planting and silking (Table 1) was associated with heterosis for light interception that was estimated from LAI during this phase of development. At the 14-leaf stage, mean estimated light interception was 57% for the hybrids (LAI = 1.30) and 30% for inbred lines (LAI = 0.55). In contrast, mean estimated light interception at silking was 85% for hybrids (LAI = 2.95) and 63% for inbred lines (LAI = 1.55). Although our estimates of heterosis for light interception may be biased by differences in extinction coefficient between and among hybrids and inbred lines, estimates show that the decline in heterosis for rate of DMA from the six- to 14-leaf stage (153%) to that from the 14-leaf stage to silking (58%) was strongly associated with a decline in heterosis for light interception from the 14-leaf stage (90%) to silking (35%).
Heterosis for DMA from planting to silking was probably not associated with differences in leaf photosynthesis between hybrids and inbred lines. We have found that maximum leaf photosynthesis did not differ between this set of hybrids and their parental lines at silking (Ahmadzadeh et al., 2004). In a study with a different set of hybrids and their parental inbred lines, Armstrong (2004) reported no differences between hybrids and inbred lines in maximum leaf photosynthesis from early stages of development until silking in one growing season and small but significantly greater photosynthesis in the hybrids in another growing season.
Dry Matter Accumulation during Grain-Filling Period
Mean heterosis for DMA during the grain-filling period was 134% (Table 1), which resulted from heterosis for both estimated light interception and leaf photosynthesis and a slightly longer duration of this period for hybrids than for inbred lines. Heterosis for estimated light interception at silking was 35%, but heterosis for light interception probably increased as stay green, a visual assessment of the proportion of LAI that remained green, was greater in hybrids than in inbred lines (Table 1). Stay green varied both among hybrids and among inbred lines, with very low values for the inbred line CG33 and its hybrids (Tables 4 and 5). Mean heterosis for stay green was 91%. The magnitude of the heterotic effect for stay green in this study, however, is underestimated because stay green was recorded at the same calendar date for hybrids and inbred lines. Mean silking date of inbred lines was 5.5 d later than that of hybrids and, consequently, hybrids were at a more advanced stage of development. The LAI of inbred lines at silking (Table 5) was well below the level needed for 95% light interception and, consequently, heterosis for stay green implies that heterosis for light interception increased when the grain-filling period advanced from silking to maturity. There was no heterosis for maximum leaf photosynthetic rates at silking for this set of hybrids and inbred lines grown hydroponically in the field (Ahmadzadeh et al., 2004). However, leaf photosynthetic rate per unit green leaf area declined as the grain-filling period progressed and the decline was more rapid in the inbred lines than in the hybrids, resulting in significant and increasing heterosis for leaf photosynthesis as the grain-filling period progressed (Ahmadzadeh et al., 2004). Hence, differences in duration of the grain-filling period, light interception, and the conversion of absorbed solar irradiance into dry matter contributed to heterosis for DMA during the grain-filling period. In addition, it has been shown that maize leaf photosynthesis during the grain-filling period is reduced by abiotic stresses and that this reduction is greater in stress-susceptible than in stress-tolerant hybrids (Tollenaar and Lee, 2002; Ying et al., 2002). Heterosis for leaf photosynthetic rate during the grain-filling period will be even greater if inbred lines are less tolerant to abiotic stresses than hybrids.
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Heterosis for kernel number was not associated with rate of DMA during the period bracketed by silking. Influences of biotic and abiotic factors such as plant population density on kernel number in maize are conferred, in part, through rate of DMA during the period bracketed by silking (Tollenaar et al., 1992, 2000; Edmeades et al., 2000). Mean rates of DMA from the 14-leaf stage to silking in 2001 and 2002 were 249 kg ha–1 d–1 for hybrids and 157 kg ha–1 d–1 for inbred lines, resulting in a mean heterosis for rate of DMA during this period of 58%. Hence, a mean heterosis for kernel number of 109% (Table 1) may have resulted, in part, from heterosis for rate of DMA. This contention is supported by results reported in the literature showing that kernel number of hybrids and their parents was similar when both groups were compared at equal LAI around silking (Johnson and Tanner, 1972; Djisbar and Gardner, 1989). However, heterosis for kernel number in hybrids was not associated with heterosis for rate of DMA from the 14-leaf stage to silking (r = 0.31; P > 0.05). The lack of association between kernel number and DMA from the 14-leaf stage to silking among and between hybrids and inbred lines can be demonstrated by various examples: (i) The hybrid CG59 x CG33 was among hybrids with the highest kernel number, and the hybrid CG69 x CG33 was among hybrids with the lowest kernel number (Table 6), but mean DMA from the 14-leaf stage to silking in 2001 and 2002 was similar for both hybrids (Tables 4 and 6); and (ii) mean kernel number was three times greater for the hybrid CG57 x LH145 than for the inbred line CG69, but mean DMA between the 14-leaf stage and silking in 2001 and 2002 was similar for the hybrid and the inbred line (Tables 4–7).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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(i) Heterosis for LAI. Heterosis for LAI was attributable predominantly to leaf size, although greater rates of leaf appearance in hybrids contributed to the difference in LAI between hybrids and inbred lines at the 14-leaf stage. The DMA is the integration across time of light interception by the crop canopy, which is a function of LAI, and the conversion of absorbed irradiance into dry matter, that is, leaf photosynthesis. Heterosis for DMA before silking was attributable mainly to heterosis for light interception, whereas differences in leaf photosynthesis were probably small (Ahmadzadeh et al., 2004; Armstrong, 2004).
(ii) Heterosis for stay green. Heterosis for DMA during the grain-filling period is attributable, in part, to heterosis for light interception due to heterosis for maximum LAI and stay green. The grain-filling period was the most important phase for heterosis of DMA at maturity.
(iii) Heterosis for sustaining rate of photosynthesis of green leaf area during the grain-filling period. Heterosis for rate of DMA during the grain-filling period can be attributed also to the increasing difference in photosynthesis per unit green leaf area between hybrids and their parental inbred lines as the grain-filling period advanced from silking to maturity (Ahmadzadeh et al., 2004).
(iv) Heterosis for partitioning of aboveground dry matter to the grain (i.e., harvest index). Heterosis for harvest index was strongly associated with heterosis for kernel number, but heterosis for kernel number was not associated with rate of DMA during the sensitive period for kernel establishment. Heterosis for kernel number could be associated with dry matter partitioning to the kernels during the sensitive period for kernel establishment, as recently reported studies have shown that differences in kernel number among hybrids, and among a hybrid and its parental inbred lines, are associated with dry matter partitioning to the kernels during this period (Echarte and Tollenaar, 2004; Echarte et al., 2004).
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication October 15, 2003.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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