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Dep. of Crop Sci., 1201 W. Gregory, Univ. of Illinois, Urbana, IL 61801
* Corresponding author (smoose{at}uiuc.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: IHP, Illinois High Protein • ILP, Illinois Low Protein • IRHP, Illinois Reverse High Protein • IRLP, Illinois Reverse Low Protein
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The majority of studies regarding genetic control of maize grain composition and the primary evidence for the negative yield–protein relationship in maize come from evaluations of the Illinois Long Term Protein Selection strains (Dudley et al., 1977; Boyat et al., 1980; Dudley and Lambert, 1992). The Illinois Protein Strains are the result of a long-term divergent recurrent selection program for grain protein concentration that has produced four strains that differ greatly in their kernel composition (Dudley and Lambert, 2004; also see Table 1). Illinois High Protein (IHP) and Illinois Low Protein (ILP) have been continuously selected for >100 cycles in the same direction, whereas the Illinois Reverse Low Protein (IRLP) and Illinois Reverse High Protein (IRHP) strains were obtained by reversing the direction of selection in ILP and IHP, respectively, beginning with generation 48. Physiological comparisons of IHP and ILP have demonstrated that differences in their relative concentrations of grain protein and starch depend on changes in C and N metabolism in source tissues, the translocation of photoassimilates to kernels during the grain-fill period, as well as utilization of assimilates by the kernel sink (reviewed in Below et al., 2004).
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The objectives of this study were (i) to confirm that the dramatic grain composition differences observed for the Illinois Protein Strains are also observed in testcrosses of derived inbreds to a current elite inbred; and (ii) to investigate the interactive effects of the resultant hybrids (genotype) and N supply on grain yield, yield components, and grain composition.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The Illinois Protein Strains have been maintained as populations and thus exhibit genetic variability for grain composition and other plant traits. To minimize the influence of genetic variability on this study and for future work with the Illinois Protein Strains, we first developed inbred lines from Cycle 90 for each of the four strains. Thirty S1 lines from Cycle 90 of each of the four strains (provided courtesy of Dr. John Dudley, University of Illinois) were self-pollinated to produce S4 lines. Each S4 line was evaluated during the summer of 2000 at Urbana, IL, for general plant vigor, maturity, synchronous anthesis and silking, and uniformity. On the basis of overall agronomic characteristics and grain composition representative of the original Cycle 90 population, one S4 line from each strain was further inbred to produce S6 lines. Grain composition was evaluated throughout the inbreeding process, with the four inbreds used in this study showing relative concentrations of protein, starch, and oil within the range of variation present for these traits in the Cycle 90 population (Table 1).
The S6 inbred line from each strain was increased by self pollination in the winter 2000–2001 nursery (Hawaiian Research, Molokai, HI), and was used as a male parent in crosses with the inbred line FR1064. These hybrids were made because we believed that evaluations of yield, grain quality, and N responses were most pertinent at the hybrid level. FR1064 was chosen because it has a grain composition typical of modern elite inbreds (90 g kg–1 protein; 775 g kg–1 starch; and 48 g kg–1 oil) and it has been used recently as a female parent in commercial hybrids. One of these commercial hybrids, FR1064 x LH185, was similarly evaluated for comparison. The protein-strain hybrid seed made in the Hawaiian winter nursery was used for the 2001 evaluation, with additional inbred and hybrid seed increases being made in the 2001 summer nursery for use in the 2002 evaluation. Seed of the commercial hybrid FR1064 x LH185 was obtained from Illinois Foundation Seeds, Inc. (Tolono, IL).
Field Experiments
Field experiments were conducted during the 2001 and 2002 growing seasons at the Department of Crop Sciences Research and Education Center in Champaign, IL, on plots that had previously been shown to be responsive to N fertilizer (Gentry et al., 2001). The soil type was a Drummer silty clay loam (fine-silty, mixed, superactive, mesic Typic Endoaquolls) with mean organic matter of 3.7% and a pH of 6.2. Soil analysis at the beginning of the 2001 growing season indicated a concentration of inorganic N (NO3––N and NH4+–N) of 13 µg g–1 in the top 20 cm of the soil profile. The field slope was <1% with a downward gradient from south to north. For at least the past 15 yr, the field had been divided into equal halves with similar soil characteristics, where each half was alternately cropped to maize or soybean [Glycine max (L.) Merr.]. Because of the rotation scheme used, the location of the trial in 2002 was adjacent to the 2001 experiment.
The four Protein Strain hybrids and the FR1064 x LH185 check hybrid were each grown under eight rates of fertilizer N (0 to 235 kg ha–1 in 34-kg increments) in a randomized complete-block experimental design with four replications for each genotype and N rate treatment combination. Each experimental plot was 5.3 m long by 3 m wide (four rows) in 2001 and 5.3 m long by 4.6 m wide (six rows) in 2002. Seeds were pretreated with fungicide (Maxim-Apron, Syngenta), overplanted with a machine cone planter, and thinned to achieve a stand density of 65000 plants ha–1. The planting date for 2001 was 26 April, but planting was delayed in 2002 until 22 May because of above-average precipitation during the months of April and May. Otherwise, the monthly temperature averages, seasonal cumulative heat units, and precipitation were similar between the two years and very close to the 30-yr averages (data not shown).
All major cultural practices were similar between the two years. Fertilizer treatments were hand applied in a diffuse band in the center of the row as ammonium sulfate when the crop was between the V2 and V3 growth stages (Ritchie et al., 1997). Immediately following fertilizer application it was incorporated with a field cultivator. Plots also received at planting an in-furrow application of cyfluthrin [cyano(4-fluoro-3-phenoxyphenyl)methyl 3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarboxylate] at a rate of 2.4 kg a.i. ha–1 to control western corn root worm (Diabrotica spp.) larvae and preemergence herbicide {methalachlor (N,N-dimethylbenzenamine), 2.3 kg a.i. ha–1, and atrazine [6-chloro-N-ethylamino)-1,3,5-triazine-2,4-diamine], 1.7 kg a.i. ha–1}. Plots were thinned and additional weed control was provided via hand cultivation. Supplemental water was supplied by a single overhead irrigation in early July in both 2001 (51 mm) and 2002 (76 mm) because of observable water stress (leaf rolling).
Grain Quality and Yield Measurements
Grain yield and composition were determined after the crop reached physiological maturity based on 50% of the plants in the plot exhibiting a visible black layer at the base of the kernels. This event took place on 3 and 6 September 2001 and 2002, respectively. All ears were harvested from one of the two center rows of each plot, and the fresh weight and moisture level of shelled grain used to estimate grain yield in megagrams per hectare. A representative 30-g subsample of the grain was ground, and the concentration of protein, oil, and starch in the ground sample was determined with a Dickey-John Instalab 600 near-infrared reflectance analyzer. Grain composition measurements were also made on the Protein Strain inbred lines and populations of approximately 100 plants from Cycle 90 of each of the four strains. These materials were grown in 2001 and/or 2002 with a supplemental N rate of 168 kg ha–1.
For yield components, individual kernel weight was obtained by drying 100 randomly selected kernels to constant weight in a forced-air oven at 75°C, and the total kernel number algebraically derived using values for grain yield and kernel weight. The percentage of the kernel constituted by the embryo was calculated by dissecting 30 mature kernels after imbibition for 24 h at 4°C in deionized H2O followed by separate weighing of the embryo and endosperm fractions.
Statistical Analysis
Excessive rainfall during the months of April and May delayed planting in 2002 to the extent that year effects are confounded with the impact of a late planting date (Cirilo and Andrade, 1994a; Bollero et al., 1996). In an initial step of our analysis, a model containing the effects of year and its interactions with N rate and hybrid was evaluated and shown to be significant (P < 0.05) for all variables. Moreover, a variance component analysis performed with PROC MIXED in SAS (SAS Institute, 2000) showed that the proportion of the total variance explained by the year effect was considerable, ranging from between 40 and 50%. Thus, data from the two years were analyzed separately.
For each parameter, the differential effects of fertilizer N rate for the five hybrids during both years were analyzed using PROC MIXED and the parameters of a quadratic regression were also fitted using this procedure in SAS. Main effect LSDs (P 
0.05) calculated with PROC GLM were included on the graphs in Fig. 1 and 2
to compare a hybrid across N rates, or to compare different hybrids at a given N rate.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The relative differences in grain protein concentration between the Protein Strain inbreds were manifested in the hybrids (Fig. 1A), with hybrid values generally intermediate between the Protein Strain and FR1064 inbred means. As expected, the IHP hybrid had the highest grain protein concentration, increasing linearly with N supply from 107 g kg–1 with no fertilizer N to 170 g kg–1 with the highest fertilizer rate. The IRLP hybrid exhibited only slightly lower amounts of grain protein compared with the IHP hybrid, which increased with N and reached a maximum concentration of 133 g kg–1 with 235 kg N ha–1. Both the IHP and IRLP hybrids had higher grain protein concentrations without any supplemental N than the FR1064 x LH185 hybrid showed at the highest N rate (89 g kg–1). Grain protein concentration of the IRHP hybrid showed only a modest response to fertilizer N that was maximized at 105 g kg–1. This response was intermediate between that observed for the IRLP and ILP hybrids but actually higher than what was observed for FR1064 x LH185 at each N rate (Fig. 1A). The grain protein concentration of the ILP hybrid was approximately 62 g kg–1, and unlike all the other hybrids evaluated, it was unaffected by increases in the N supply.
Genotypic differences in grain starch concentration and its response to fertilizer N were essentially inverse to that observed for protein concentration (Fig. 1B). The ILP hybrid had the highest starch concentration (approximately 760 g kg–1) that did not change with different rates of supplemental N. In contrast, the IHP hybrid exhibited the lowest kernel starch concentration that also decreased from 700 g kg–1 without N fertilizer to 630 g kg–1 with 235 kg N ha–1. Starch concentrations for both the IRLP and IRHP hybrids decreased with increasing N fertilizer, with values between those of the IHP and ILP hybrids. Similar to protein concentration, the IRHP hybrid had starch values and a grain starch response to N that was the most similar to the FR1064 x LH185 hybrid. Other evidence for an inverse starch and protein relationship in the Protein Strain hybrids comes from summing the grain starch and protein concentrations for the four hybrids grown at a typical field N rate of 164 kg N ha–1. For 2001, these combined starch and protein values ranged from 795 to 826 g kg–1, and none of these values was significantly different from the mean of 814 g kg–1 for the four hybrids. Similarly, starch plus protein concentrations for the Protein Strain hybrids grown in 2002 only ranged from 814 to 846 g kg–1, with a mean of 830 g kg–1.
For all hybrids, grain oil concentrations were not affected by either the N supply or the year (Fig. 1C). Hybrid differences (P 0.01) in grain oil were similar to those found for protein concentration, with IHP having the highest oil concentration (74 g kg–1), followed by IRLP (65 g kg–1). The ILP, IRHP, and FR1064 x LH185 hybrids all had similar and lower concentrations of grain oil (55 g kg–1). This quality component was primarily associated with differences in the proportion of the kernel constituted by the embryo, which was also unaffected by the N supply (Table 3). The IHP and IRLP exhibited the highest proportions of embryo in their kernels (average of 12.7% for 2001 and 10.7% for 2002), and had the highest oil concentrations (Fig. 1C). Conversely, ILP and IRHP had the lowest percentage of kernel as embryo (10.8% in 2001 and 9.0% in 2002) and the lowest concentrations of kernel oil.
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0.05) in grain yield, although their response to N supply was similar in both years (Fig. 2A, Table 2). Yield differences among the Protein Strain hybrids reflected their relative kernel starch concentrations, with the ILP hybrid showing the highest yields, the IHP hybrid the lowest, and the reverse strain hybrids in between the IHP and ILP hybrids. Each of the Protein Strain hybrids produced lower yields relative to FR1064 x LH185, particularly in 2001 where the maximum yields of the Protein Strain hybrids were similar to FR1064 x LH185 without any supplemental N. The impact of the late planting in 2002 was of a lesser magnitude in the Protein Strain hybrids than it was for FR1064 x LH185, affecting mainly the IHP and ILP hybrids (Fig. 2A). The yield component that best described differences in grain yield was kernel number, where the overall R2 value between yield and kernel number was 0.65. Changes in kernel number were also primarily responsible for the N-supply-induced increases in grain yield, as kernel number increased with higher levels of N supply for all hybrids (Fig. 2B, Table 2). For the most part, whenever there was a significant response in kernel number to N supply, the response was linear. Similar to grain yield, FR1064 x LH185 produced the greatest number of kernels per plant, reaching a maximum number of 740 kernels with 101 kg N ha–1 in 2001. Among the Protein Strain hybrids, ILP produced the highest number of kernels (682 plant–1 with 235 kg N ha–1), whereas IRHP produced the fewest number of kernels (mean of 445 plant–1). For all hybrids, kernel number was substantially lower in 2002 than 2001 (mean of 31%), which we attributed to the relatively late planting date. The negative effect of late planting on kernel number in 2002 was likely related to a lower crop growth rate around silking, which would cause a greater proportion of kernel ovaries to abort (Cirilo and Andrade, 1994b). This effect was most evident for the commercial hybrid, which may have overall higher yield potential due to the continued high crop growth rate around flowering.
Individual kernel weights were most strongly influenced by genotype, although small N responses were observed in all hybrids in both years. In most cases, the biggest increase in kernel weight was achieved with only 34 kg ha–1 of N fertilizer. In general, hybrids containing parents selected for low protein concentrations (ILP and IRHP) had the heaviest individual kernels, whereas kernels of hybrids that had a parent selected for high protein concentration were smaller (Fig. 2C). FR1064 x LH185 exhibited the greatest increase (19%) in kernel weight in response to N supply, especially in 2001. Compared with 2001, the late planting in 2002 decreased individual kernel weight of all the hybrids.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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As expected, the IHP hybrid had the highest concentrations of grain protein (and grain protein yields), whereas the ILP hybrid had the lowest concentration. The IRLP and IRHP hybrids generally showed intermediate phenotypes relative to the IHP and ILP hybrids that reflected the direction of selection for grain protein concentration. The reverse strains serve to confirm the strong genotypic control on grain composition traits; they also reveal significant genetic variability for these traits remained even after 48 cycles of forward selection.
Comparisons between the IHP and ILP hybrids also showed a genotype by N rate interaction for grain protein, with the IHP hybrid showing a near-linear positive increase in grain protein concentration in response to N, whereas protein in the ILP hybrid could not be increased by adding supplemental N (Fig. 1A). Grain protein concentration in the reverse strain hybrids responded positively to N in a similar fashion, indicating that reverse selection for grain protein concentration also reverses kernel responses to supplemental N.
The lack of N response in ILP hybrids is in agreement with both field evaluations and in vitro kernel culture studies of cycle 87 from the ILP strain (Wyss et al., 1991) where the grain N concentration could not be altered by increasing the N supply. We have confirmed this observation for our ILP inbred grown in the field with different rates of supplemental N (Moose and Below, 2002, unpublished data). Thus, ILP and its F1 hybrids appear to be sink-limited in their use of N to make seed storage proteins, and this limitation cannot be modulated by adding N. It is likely that changes in N accumulation by the vegetative source plant or N remobilization account for the dramatic differences in grain protein responses to N between IHP and ILP-derived hybrids, which is the focus of a separate study that will be reported elsewhere.
The positive association between protein and oil concentrations can be partially explained by differences in the structural components (i.e., the weight of the endosperm and the embryo) of the kernel (Table 3). Hybrids with small kernels would have a higher proportion of their kernels as embryo and endosperm aleurone layer, which contain almost all of the total grain oil. Similar differences in kernel characteristics have been reported for the IHP and ILP strains (generation 88), where compared with ILP, IHP had a smaller percentage of the kernel as endosperm, and a higher concentration of endosperm oil (Doehlert and Lambert, 1991). The lack of effect of N supply on grain oil concentration in the current study (Fig. 1C) can be explained by the failure of N to change the proportion of the kernel as embryo (data not shown). These findings are in agreement with Borrás et al. (2002), who reported that oil concentration of commercial hybrids remained constant under a range of post-flowering source-sink ratios, whereas the concentration of grain protein was affected by the imposed treatments.
Interestingly, while the inbred parents ranged in grain oil concentrations from 38 to 47 g kg–1 (Table 1), all the hybrids exceeded 50 g kg–1 of grain oil, with the IHP hybrid exceeding 70 g kg–1 (Fig. 1C). Similar results were obtained by Dudley and Lambert (2004) when they evaluated 100 plants from different cycles of the Illinois Protein Strains (spanning Cycles 67–99). Bulk testcrosses of the Protein Strains crossed to FR1064 showed higher grain oil levels compared with the Protein Strains per se, especially those involving IHP. These findings suggest that all of the Protein Strains combined with FR1064 show high-parent heterosis for grain oil. The explanation for this observation is unclear, but simultaneous increases in both grain protein and oil concentrations could be valuable from a grain quality or grain processing perspective.
The Inverse Yield–Protein Relationship
The Protein Strain hybrids show a strong negative association between grain protein and starch concentrations (Fig. 1A, 1B). Since both kernel weight and grain yield are highly correlated with starch concentration, these traits are also negatively impacted by protein concentration. This negative relationship has been reported previously for the Illinois Protein Strains materials (Dudley et al., 1977; Dudley and Lambert, 1992), as well as in other maize selection programs for high grain protein (Pollmer et al., 1978; Motto et al., 1980; Rossi et al., 2001). A negative relationship between grain yield and protein percentage has historically plagued cereal crops (Simmonds, 1995), and while the difference in metabolic cost of synthesizing protein vs. starch is often implicated (Bhatia and Rabson, 1976), a complete understanding of this interrelationship remains elusive.
The current study clearly demonstrates that the inverse relationship between kernel protein and starch concentration is impacted not only by genotype, but also by N availability. This finding implies that the N-mediated regulation of endosperm protein and/or starch synthesis plays a key role in the negative yield–protein relationship. Nitrogen appears to alter kernel composition by stimulating protein synthesis rather than inhibiting starch production, as evidenced by the positive yield increases in all hybrids with additional N and the fact that starch concentration did not decrease in the ILP hybrid grown at high N rates. Genetic selection for high grain protein in IHP has greatly enhanced the stimulation of seed protein synthesis in response to N; conversely, selection for low protein has eliminated this N response. These results agree with earlier reports of differences in how the IHP and ILP strains use N for yield and grain protein (Reggiani et al., 1985; Balconi et al., 1991; Wyss et al., 1991; and Rizzi et al., 1996), and provide a partial physiological explanation for the negative protein–yield association.
In addition to affecting seed metabolism and seed composition, N supply also impacts the reproductive sink capacity, as measured by increases in kernel number (Fig. 2B), and to a lesser extent increases in individual kernel weight (Fig. 2C). A positive response in kernel weight to increases in N supply is due to enhancement in kernel growth rate during the effective grain-filling period (Borrás and Otegui, 2001), while N-induced increases in kernel number result from a decrease in kernel abortion (Below, 2002). The lower grain yields of the Protein Strain hybrids compared with FR1064 x LH185 were primarily due to reductions in kernel number (Fig. 2). The highest-yielding Protein Strain hybrid (ILP) also produced the greatest number of kernels per plant (Fig. 2B), and like FR1064 x LH185, had both a high grain starch concentration (Fig. 1B) and relatively heavy individual kernel weights (Fig. 2C). A positive association between starch level and kernel number has been reported previously (Zinselmeier et al., 1999). Since most of the endosperm is starch (82–84%), it is easy to envision why high starch concentrations would lead to heavier individual kernels, which was observed in the current study. Conversely, the hybrids with the highest concentration of grain protein (IHP and IRLP) had the lowest concentrations of grain starch and the smallest individual kernel weights. Thus, one strategy to increase grain protein concentration while maintaining high yields may be to select genotypes that combine high protein concentration with a high number of kernels per ear.
Although the large differences in grain protein concentration and grain yield among the hybrids would be expected to impact protein yield expressed on a land-area basis, the protein yields of the hybrids generally exhibited the same relative rankings as the grain protein concentration of the male parent. When grown in 2001 with the highest N rate (234 kg N ha–1), protein yields were 1366, 1072, 757, and 618 kg ha–1 for the IHP, IRLP, IRHP, and ILP hybrids, respectively, compared with 1290 kg ha–1 for FR1064 x LH185. The 1366 kg ha–1 of grain protein produced by the IHP hybrid at a concentration of 170 g kg–1 is the highest combination of grain protein yield and grain protein concentration that we could identify in the literature (Pollmer et al., 1978; Boyat et al., 1980; Motto et al., 1980; Dudley and Lambert, 2004). An adequate N supply, however, was crucial to achieve this productivity, as the responses of grain yield and grain protein concentration to N supply were largely linear (Fig. 1 and 2; Table 2).
Utility of Protein Strain Inbreds and their Hybrids
This study has documented the extreme differences in kernel composition in inbreds derived from the Illinois Protein Strains, as well as hybrids that are relevant to current commercial maize production. Though the work reported here is based on only one inbred line derived from each Illinois Protein Strain, these inbred lines exhibit phenotypes representative of the strain (Table 1) and are in general agreement with data obtained from per se and testcross (also to FR1064) evaluations of the IHP, IRHP, and IRLP strains reported by Dudley and Lambert (2004). The ability to reproduce genetically defined materials that have been characterized for key compositional and agronomic parameters permits the design of replicated studies of the Protein Strains, which was not previously possible with the genetically heterogeneous strains themselves. The Protein Strain inbreds will also serve as reference genotypes for further genetic, physiological, and genomics studies to elucidate the molecular mechanisms responsible for the dramatic phenotypic differences that have resulted from a century of divergent selection for grain protein concentration.
The extreme relative protein and starch concentrations of the grain produced from the hybrids evaluated in this study may also be useful in pilot experiments to test the effects of elevated grain protein and starch concentrations on animal feed efficiency or the recovery of processing end products. We have only evaluated hybrids where the Protein Strain inbreds were crossed to a single tester, FR1064; and while FR1064 combines relatively well with these inbreds, other crosses may exhibit even greater grain yields or compositional differences. Further work with the unique genetic resources developed here should facilitate future efforts to improve grain quality in maize.
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ACKNOWLEDGMENTS |
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Received for publication January 10, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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