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CROP PHYSIOLOGY & METABOLISM

Kernel Number Determination in Argentinean Maize Hybrids Released between 1965 and 1993

^a INTA Balcarce-Universidad Nacional de Mar del Plata, CC 276, 7620 Balcarce, Argentina
^b Facultad de Agronomía, UBA, Av. San Martín 4453
^c Dep. of Plant Agriculture, Crop Science Building, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1

^* Corresponding author (lecharte{at}mdp.edu.ar).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Grain yield and the stability of harvest index are greater in newer than in older Argentinean maize (Zea mays L.) hybrids. The objective of this study was to elucidate mechanisms underlying the superior yield and harvest index stability of newer Argentinean maize hybrids using the relationship between kernel number per plant (KN_P) and plant growth rate during the period bracketing silking (PGR_s). Three experiments were performed at Balcarce, Argentina, during two growing seasons (1998–2000). Maize was grown under a wide range of plant densities (from 2 up to 30 plants m^–2) to generate contrasting availability of resources per plant. Growth of individual plants during the period bracketing silking was estimated through a nondestructive method on the basis of relationships between actual shoot dry matter and morphometric variables, including stem and ear diameters and ear length. Detasseling and silk pollination synchronization treatments were imposed in one experiment to also modify available resources per kernel and kernel sink strength. Newer hybrids set more kernels per unit PGR_s than older hybrids as is indicated by (i) the lower threshold PGR_s for kernel set and (ii) greater potential kernel number at high availability of resources per plant, for newer than for older hybrids. At low and intermediate PGR_s, the greater kernel set per unit PGR_s in newer vs. older hybrids was attributable to greater partitioning of dry matter to the topmost ear during the period bracketing silking, whereas number of kernels set per unit of ear growth rate did not differ. In contrast, kernel set per unit of ear growth rate was greater in newer than in older hybrids when PGR_s was high. Results of this study indicate that genetic yield improvement in maize is attributable, in part, to increased partitioning of dry matter to the ear during the critical period bracketing silking.

Abbreviations: EGR_s, growth rate of the topmost ear during the critical period bracketing silking • KN_p, kernel number per plant • KN₁, kernel number of the topmost ear • PGR_s, plant growth rate during the critical period bracketing silking

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
YIELD OF NEWER ARGENTINEAN MAIZE hybrids is greater than that of older ones (Echarte et al., 2000). Genetic yield improvement in North American maize hybrids has been associated with increased dry matter accumulation and not with the proportion of above-ground dry matter that is partitioned to the grain (i.e., harvest index), which has remained relatively stable (Tollenaar and Lee, 2003). Contrarily, harvest index has increased from older to newer Argentinean hybrids (Echarte and Andrade, 2003). More recently released hybrids are more tolerant to high plant-density stress than older hybrids (Russell, 1984; Castleberry et al., 1984; Tollenaar et al., 1992; Duvick, 1997; Tollenaar and Wu, 1999; Echarte et al., 2000; Tollenaar and Lee, 2002). This is associated with higher stability in harvest index of newer hybrids (Echarte and Andrade, 2003), which may result from their ability to maintain a high kernel number per plant (KN_P) as resource availability per plant decreases (Echarte and Andrade, 2003).

Grain yield improvement is highly associated with kernel number (e.g., Andrade et al., 1996; Echarte et al., 2000; Tollenaar et al., 2000). Kernel number per plant is associated with plant growth rate during the critical period bracketing silking (PGR_s) (Aluko and Fischer, 1988; Tollenaar et al., 1992; Andrade et al., 1999; Vega et al., 2001a). In maize, the KN_P–PGR_s relationship has been described by two successive curves to account for the first and second ear in prolific, or a single curve in nonprolific plants (Tollenaar et al., 1992; Andrade et al., 1999; Vega et al., 2001b). A particular feature of the KN_P–PGR_s relationship is the significant PGR_s threshold for kernel set which probably reflects the abrupt decreases in dry matter partitioning to the ear when resources per plant are low (Edmeades and Daynard, 1979; Tollenaar et al., 1992; Andrade et al., 1999; Vega et al., 2001a). The lower threshold of biomass per plant measured at physiological maturity for yield observed in newer hybrids (Echarte and Andrade, 2003) could be associated with a lower PGR_s threshold for grain set in comparison with older hybrids. On the other hand, the greater KN_P response to increases in resource availability per plant of newer hybrids (Echarte and Andrade, 2003) could be the result of a greater potential kernel number in the topmost ear. Greater kernel number per plant in newer hybrids was related to a greater kernel set in the topmost ear and not to a greater prolificacy (Echarte and Andrade, 2003).

Differences among hybrids in the number of kernels set per unit of PGR_s (i.e., KN_P/PGR_s) may be attributable to either or both dry matter partitioning to the ear and number of kernels set per unit of ear growth rate during the period bracketing silking (Andrade et al., 1999; Vega et al., 2001b). First, increased partitioning to the ear could increase KN_P/PGR_s. Genetic reduction in tassel size or tassel removal have generally favored dry matter partitioning to the ear and increased kernel set (Fischer and Palmer, 1984; Bolaños and Edmeades, 1993; Edmeades et al., 1993). Tassel size has declined linearly in U.S. hybrids from the 1930s to the 1990s (Tollenaar et al., 2000). A lower response of kernel number to tassel removal would be anticipated in newer than in older hybrids if the reduction in tassel size has been associated with a concomitant reduction in tassel dominance over the ear. Second, an increase in the number of kernels set per unit of ear growth rate during the critical period bracketing silking (EGR_s) could increase KN_P/PGR_s. Differences in the assimilate requirement per kernel (Edmeades and Daynard, 1979; Edmeades et al., 1993; Vega et al., 2001a) may account for differences in number of kernels set per unit of EGR_s among hybrids. In addition, differences in kernel number per unit EGR_s may be associated with differences in synchronization of fertilization of florets within the ear (Cárcova et al., 2000; Cárcova and Otegui, 2001). Therefore, if kernel number set per unit of ear growth is a mechanism underlying a greater KN_P/PGR_s of newer hybrids, (i) a lower minimum assimilate requirement per kernel and/or (ii) a lower kernel set response to improved synchronism in silk pollination would be expected in newer than in older hybrids.

In this work, we examine the mechanisms that underlie the differences in kernel number per unit of PGR_s between newer and older Argentinean maize hybrids. The PGR_s threshold for kernel set and the response of KN_P to PGR_s increments could be involved in the high kernel number and HI stability of new Argentinean maize hybrids. We tested the hypothesis that the PGR_s threshold for grain set is lower and the kernel response to PGR_s increments is greater in newer than in older hybrids by examining the response of KN_P to resource availability of individual plants rather than that of plot means. In addition, we examined whether dry matter partitioning to the ear and kernel number set per unit of ear growth rate during the critical period bracketing silking is associated with the greater kernel number per unit of PGR_s of a newer versus an older maize hybrid.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site and Crop Management
Maize was grown at Balcarce, Argentina (37°45'S, 58°18'W; elevation 130 m), during the 1998–1999 (Exp. 1) and the 1999–2000 growing seasons (Exp. 2 and 3). Crops were fertilized with 35 kg P ha^–1 before sowing, and with 150 kg N ha^–1 at V6 (Ritchie and Hanway, 1982). Soil water to 1-m depth was kept over 50% of maximum available water by sprinkler irrigation. Weeds and insects were effectively controlled.

Plant Material and Experimental Design
The maize hybrids Morgan 400, DeKalb 4F36, DeKalb 664, and DeKalb 752 (Exp. 1) and DeKalb F880 and DeKalb 752 (Exp. 2 and 3) were sown on 6 Oct. 1998 (Exp. 1) and 8 Oct. 1999 (Exp. 2 and 3). Each of these hybrids was among the three topmost cultivated hybrids in the Argentinean Pampas for at least 5 yr after their release (Table 1). In all three experiments, plant density was used as the source of experimental variation for KN_P and PGR_s. Plots were oversown and thinned to the desired plant densities at V3. The experimental design was a split-plot randomized complete-block design with three replications, with plant densities as main plots and hybrids as subplot. Plant densities at harvest were 2, 4, 8, 16, and 30 plants m^–2 in Exp. 1 and 2, and 8 and 16 plants m^–2 in Exp. 3. Subplots comprised five 7-m-long rows at low plant densities and six to seven 7-m rows at intermediate (8 plants m^–2) and higher plant densities (16–30 plants m^–2). The distance between rows was 0.7 m in all cases. In Exp. 3, treatments of detasseling and artificial synchronous pollination were applied at random within each experimental unit. Detasseling was performed by hand (n 80 plants per hybrid) when the tassel was still surrounded by four leaves (i.e., 6.5 ± 0.14 d before silking, average for both hybrids and plant densities). In plants chosen for artificial synchronous pollination (n 80 plants per hybrid), both the uppermost and the second ears were bagged before silk emergence. Both ears were pollinated 5 d after first silks emerged from the husks of the uppermost ear. In border rows, additional plants were sown 2 wk after 8 Oct. 2000 to assure adequate pollen availability during the whole period.

Destructive Sampling
Morphometric variables, i.e., basal stem diameter and diameter and length of the topmost ear, were measured on a density-dependent number of plants (three plants per replicate at low plant density and six to eight plants per replicate at high plant densities). Diameter of the stem and the ear were measured on the widest section. Immediately after measurements, plants were harvested, leaving borders of at least 1 m between successive harvests. Plants were separated into leaf blade, stem plus sheath, and ears and oven dried at 65°C until constant weight. Allometric relationships were established between morphometric variables and dry weights of shoot and female reproductive structures. Reproductive structures included kernels and rachis of the topmost ear. Models fitted to shoot dry weight are summarized in Table 2 and models fitted to dry weight of the topmost ear are summarized in Table 3.

Data Analysis
Growth rate during the critical period for kernel set was estimated as the ratio between accumulated biomass in shoots or topmost ear and the duration of the period. We assumed a linear relationship between biomass accumulation per plant and days during the period bracketing silking, and female reproductive biomass to be negligible at 10 d before silking (beginning of the critical period bracketing silking).

The relationship between KN₁ and PGR_s was investigated using a nonlinear model (Model 1; Jandel Scientific, 1991). This model was chosen because it includes parameters with biologically meaning parameters (Echarte and Andrade, 2003).

Parameter a₁ quantifies the potential number of kernels set in the topmost ear, and b₁ is a measure of the curvilinearity of the KN₁–PGR_s relationship. A large b₁ value indicates that the curve approaches a straight line. The parameter x₀ (g plant^–1 d^–1) represents the PGR_s threshold for kernel set in the uppermost ear. Free iteration of parameters yielded large errors in the estimation of the PGR_s threshold for kernel set (parameter x₀). Particularly, the model did not adequately estimate KN₁ for plants with KN₁ < 200 kernels. Most of the residuals of the model were negative for these plants, especially for older hybrids. This was mainly explained by a large variability in KN₁ close to the threshold and to the high initial slope of the KN₁–PGR_s relationship. Therefore, parameter x₀ was set in the model as an input based on PGR_s data from nonbarren plants bearing nubbins. The threshold of PGR_s for kernel set was estimated as the average PGR_S of plants that set 1 to 100 kernels. Selection of a narrow range of KN₁, i.e., 1 > KN₁ > 50 did not provide enough number of plants to perform valid statistic comparisons among hybrids. Barren plants were not included in the calculation of the PGR_s threshold because they underestimate its value.

Additionally, we fitted the relationship between KN₁ and the ear growth rate at the period bracketing silking (EGR_s) using Model 2 (Vega et al., 2001a); which presented a greater R² than Model 1.

Parameters a₂ and b₂ represent the initial slope and the curvilinearity of the KN₁–EGR_s relationship, respectively. Low b₂ indicates that the curve approaches to a straight line. Parameter x_t quantifies the EGR_s threshold to set kernels.

The minimum assimilate requirement per kernel (mg kernel^–1 d^–1) was estimated as the mean EGR_s KN^–1₁ for the interval of EGR_s at maximum kernel set per unit of EGR_s ± 10%. The maximum kernel set per unit of EGR_s was obtained from the hyperbolic KN₁–EGR_s relationship (Vega et al., 2001a).

Data were processed by t test of parameters and t tests were used to assess differences between hybrids in KN_P and EGR_s for intervals of EGR_s or PGR_s.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Relationship between KN_P and PGR_s
The relationship between KN_P and PGR_s was curvilinear (Fig. 1) with a PGR_s threshold for kernel set (x₀). A trend toward a plateau for kernel number of the topmost ear (KN₁) at high values of PGR_s indicates morphogenetic limitations in reproductive plasticity. There was not a clear trend in mean PGR_s at each plant density with year of hybrid release (data not shown). At fixed ranges of PGR_S, KN₁ was greater in newer than in older hybrids (P < 0.05). For example, the newer hybrid DK752 set 26 and 51% more kernels than the older hybrid M400 at low (i.e., 0.5 < PGR_s < 1.5 g d^–1) and at high (i.e., 5 < PGR_s < 6 g d^–1) resource availability per plant, respectively. This supports contentions of a previous analysis based on mean plot data which concluded that greater KN_P set per unit of PGR_s would primarily underlie the greater KN_P of newer hybrids (Echarte et al., 2000). The threshold PGR_s (x₀) was higher for older hybrids (0.82 g plant^–1 d^–1 > x₀ > 1.18 g plant^–1 d^–1) than for newer hybrids (0.52 g plant^–1 d^–1 > x₀ > 0.65 g plant^–1 d^–1) (Table 4). In accordance to these values of PGR_s thresholds for kernel set, more than half of the plants of the old hybrids and only 16% of the plants of the newer hybrids were sterile at PGR_s from 0.5 to 1 g d^–1 (Fig. 2a) . Those proportions decreased to 17 and 0% for older and newer hybrids at PGR_s from 1 to 1.5 g d^–1 (Fig. 2b). In addition, supporting a lower threshold PGR_s for kernel set in newer than in older hybrids, the PGR_s values below which half of the plants were sterile were lower in newer (0.82 and 0.91 g d^–1 for DK664 and DK752, respectively) than in older hybrids (1.58 and 1.43 g d^–1 for M400 and DK4F36, respectively). The degree of curvilinearity of the KN₁–PGR_s relationship was lower in newer than in older hybrids (parameter b₁, Table 4—a large b₁ value indicates that the curve approaches a straight line). Consequently, KN₁ continues to increase to greater PGR_s in newer than in older hybrids (Fig. 1). A comparison of the newer hybrid DK752 with the older hybrid DKF880 showed that both number of kernel rows (20 vs. 14) and number of kernels per row (37 vs. 28) were greater in the newer hybrid at intermediate plant density (i.e., 8 plants m^–2).

View larger version (39K): [in this window] [in a new window]   Fig. 1. Relationship between kernel number per uppermost ear (KN1) or per plant (KNP) and plant growth rate during a period bracketing silking (PGRs) in five maize hybrids released in Argentina in different decades. Curves are the fitted Model 1. Triangles represent kernel number of prolific plants (kernel number of the topmost and second ear). Other symbols represent KN1 at low (2–4 plants m–2; solid circles), intermediate (8 plants m–2; squares), and high plant densities (16–30 plants m–2; white circles).

View this table: [in this window] [in a new window]   Table 4. Threshold of PGRs for kernel set (x0) estimated as the mean PGRs of plants that set 1 to 100 kernels, potential kernel number (a1), degree of curvilinearity (b1), and R2 of Model 1 (KN1 = a1 x {1 – exp[–(PGRs – x0)/b1]} if PGRs x0 and KN1 = 0 if PGRs < x0) fitted to the KN1–PGRs relationship of five Argentinean maize hybrids.

View larger version (27K): [in this window] [in a new window]   Fig. 2. Frequency distributions (%) of kernel number per plant for plants growing at (a) 0.5 > PGRs > 1 g d–1 and (b) at 1 > PGRs > 1.5 g d–1 in four maize hybrids released in Argentina at different decades (data from Exp. 1).

The lower x₀ of newer hybrids, which is associated with an improved performance under stress or high plant density, could result from indirect selection under progressively higher plant densities in breeding programs and from hybrid evaluation across a wide range of environments, including low-yield environments (Troyer, 1996; Reeder, 1997; Tollenaar and Lee, 2002; Fasoula and Fasoula, 2002). Similarly, genotype selection for tolerance to mid-season drought stress may lead to changes that also contribute to increased tolerance to nitrogen and high plant density stress (Bänziger et al., 2002). Andrade et al. (2002) showed that the KN_P– PGR_s relationship is not influenced by the nature of the cause for variation in PGR_s (e.g., plant density, nitrogen, water). Therefore, it is likely that kernel set in older and newer hybrids will follow the specific KN_P–PGR_s relationships irrespective of the nature of the environmental stress. Greater dry matter partitioning to the ear at the period bracketing silking, and/or greater kernel number set per unit of ear growth rate (Andrade et al., 2000; Vega et al., 2001a), could underlie the decrease in x₀ and the increase in kernel set per unit PGR_s with year of release.

Partitioning of Dry Matter to the Topmost Ear and Kernel Set per Unit EGR_s
Growth rate of the topmost ear during the period bracketing silking was greater in the newer hybrid DK752 than in the older hybrid DKF880, across the whole range of PGR_s (P < 0.05, Fig. 3a) . In addition, mean PGR_s at each plant density was greater in the newer hybrid DK752 than in the older hybrid (data not shown). As a consequence, the proportion of plants with very low EGR_s (i.e., EGR_s < 0.2 g d^–1) was greater in the older hybrid (55%) than in the newer hybrid (23%) at high plant densities (16–30 plants m^–2). When expressed at equal EGR_s, however, KN₁ did not differ between the two hybrids at low and intermediate EGR_s (i.e., EGR_s from 0 to 0.5 g d^–1, Fig. 3b). Therefore, these results support the contention that the greater number of kernels set per unit of PGR_s of the newer hybrid was not influenced by a greater kernel number set per unit of ear growth rate at relatively low resource availability per plant. In contrast, the number of kernels set per unit of ear growth rate was greater in the newer hybrid (P < 0.05, Fig. 3b) at low plant densities (i.e., EGR_s 0.6 g d^–1) because of a larger reproductive plasticity, i.e., potential kernel number per ear (Table 4). In addition, at high resource availability per plant, Vega et al. (2001a) showed that EGR_s of the topmost ear is lower in prolific plants compared with nonprolific plants of the same hybrid. This suggests that a low mean EGR_s of the topmost ear at high resource availability per plant could be associated, in part, with a high prolificacy (i.e., large proportion of prolific plants). However, since prolificacy did not present a clear trend with the year of hybrid release (Echarte et al., 2000; Echarte and Andrade, 2003) and since at intermediate and low plant densities the oldest and the newer hybrids presented the same proportion of prolific plants (27.5 and 28.3% for DKF880 and DK752, respectively), a lower growth rate of the topmost ear in the older hybrid is not associated with a greater prolificacy.

View larger version (29K): [in this window] [in a new window]   Fig. 3. (a) Mean growth rate of the topmost ear (EGRs) vs. 1.5-g d–1 intervals of plant growth rate during a period bracketing silking (PGRs) and (b) relationship between kernel number of the topmost ear and ear growth rate during a period bracketing silking of an older (DKF880) and a newer hybrid (DK752) in Exp. 2. In Fig. 3a values at the x axis indicate the upper limit of the interval. Bars indicate standard error. Number of individuals ranged from 10 to 100 depending on the range of PGRs. **indicates significant differences between hybrids within the range of PGRs at P < 0.05. In Fig. 3b, curves are the fitted Model 2 for DKF880 (gray line) and DK752 (black line). Fitted regression were: DKF880, KN1 = 1807 ± 189.9 x (EGRs – 0.028 ± 0.01)/[1 + 2.77 ± 0.464 x (EGRs – 0.028 ± 0.01)] if EGRs xt and KN1 = 0 if EGRs < xt, R2 = 0.78; DK752, KN1 = 1191 ± 89.5 x (EGRs – 0.007 ± 0.019)/[1 + 0.76 ± 0.12 x (EGRs – 0.007 ± 0.019)] if EGRs xt and KN1 = 0 if EGRs < xt, R2 = 0.89. The regressions were significant at P < 0.05.

View larger version (26K): [in this window] [in a new window]   Fig. 4. (a) Mean kernel number per uppermost ear (KN1) vs. 1.5-g d–1 intervals of plant growth rates during a period bracketing silking (PGRs), (b) mean growth rate of the topmost ear (EGRs) vs. 1.5-g d–1 intervals of PGRs in an older (DKF880) and newer hybrid (DK752) in Exp. 3. Values at the x axis indicate the upper limit of the interval unless indicated otherwise. Bars indicate the standard error. Number of individuals in each treatment ranged from 10 to 40 depending on the range of PGRs. **indicates significantly different from the control at P < 0.05.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Results reported in the current study elucidate the mechanisms involved in differences in kernel set between older and newer hybrids. Newer Argentinean hybrids set more kernels per unit PGR_s than older Argentinean hybrids as indicated by (i) the lower threshold PGR_s for kernel set and (ii) the greater potential kernel number at high availability of resources per plant, for newer than for older hybrids. At low and intermediate PGR_s, the greater kernel set per unit of PGR_s in newer vs. older hybrids was attributable to a greater partitioning of dry matter to the topmost ear during the period bracketing silking, whereas the number of kernels set per unit of ear growth did not differ. The KN₁ response to detasseling in the older hybrid, but not in the newer one, and the lack of KN₁ response to synchronous pollination in both hybrids supported this contention. In contrast, kernel set per unit of ear growth rate was greater in newer than in older hybrids when PGR_s was high. The greater partitioning of dry matter to the ear in newer hybrids will contribute to greater yield stability and harvest index stability relative to those of older hybrids when resources per plant decreases. In addition, the greater partitioning of dry matter to the ear and the greater number of kernel set per unit of ear growth rate at high PGR_s for newer than for older hybrids will contribute to greater yield in newer than in older hybrids grown at low plant density or in case of an irregular plant stand (i.e., gaps in the row). The failure of others (cf., Tollenaar et al., 1992; Maddonni et al., 2000; Luque, 2000) to show a significant difference between older and newer hybrids in the threshold PGR_s for kernel set may be attributable to the use of analyses based on plot means, which do not allow precise estimations of the KN_P–PGR_S relationship at low PGR_s. Results of this study indicate that genetic yield improvement in maize is attributable, in part, to increased partitioning of dry matter to the ear during the critical period bracketing silking.

	ACKNOWLEDGMENTS

 
This work was supported by the Instituto Nacional de Tecnología Agropecuaria (INTA), the Research Council of Argentina (CONICET), the Universidad Nacional de Mar del Plata, the Secretaría de Ciencia y Técnica (SECYT), Monsanto Argentina S.A., and Mycogen. L. Echarte held a scholarship from CONICET. This work is part of a thesis submitted by L. Echarte in partial fulfillment for the requirements for the doctoral degree, Universidad Nacional de Mar del Plata.

Received for publication November 1, 2003.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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