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Different approaches can be taken to obtain an insight into the physiological basis of genetic improvement of corn. The first approach has been taken in our review of the physiological basis of genetic improvement in corn. Genetic improvement of corn hybrids in the United States has been associated with an increase of the plant density at which maximum grain is attained. Both practical (i.e., economic yield) and physiological considerations favor an analysis of genetic improvement in corn at the optimum plant density. Interpretation of the frequently complex hybrid X environment X plant density interactions among old and new corn hybrids can be facilitated by a basic understanding of the physiological basis of the plant-density response. As reported earlier, several experiments were carried out in Ontario to identify the physiological basis of genetic improvement of corn hybrids over the last three decades.

Characterization of the pre- and post-silking period differences in dry matter (DM) accumulation and nitrogen (N) uptake and partitioning between older and newer maize ( Zea mays L.) hybrids is useful in the context of providing possible mechanisms of yield and N efficiency gains over the decades of genetic improvement. However, there is substantial uncertainty about the mechanisms by which DM and N partitioning into distinct plant organs at silking (R1) affect their respective post-silking dynamics in modern versus older maize hybrids. Clarity is also lacking about management impacts on how source (leaf and stem) strength and sink (grain) strength drive post-silking DM (PostDM) and post-silking N (PostN) dynamics in genotypes of different eras. In this two-year and two-location study, we compared two newer hybrids (commercialized in 2005) to one older hybrid (commercialized in 1975) in 2012 and to two older hybrids (the same 1975 hybrid, and one commercialized in 1967) in 2013. All hybrids were compared under two N fertilizer rates (55 kg N ha −1 , 220 kg N ha −1 ) and three densities (54,000 pl ha −1 , 79,000 pl ha −1 , 104,000 pl ha −1 ). Although both moderate and high plant densities increased leaf N contents at silking and remobilized N from leaves during grain fill, density × hybrid interactions were not significant for these or almost all parameters measured. Older hybrids consistently partitioned more of their total DM at silking to stem than leaf relative to both newer hybrids. Both newer and older hybrids increased PostDM (an average increase from 8.3 to 10.1 Mg ha −1 ) and PostN (an average increase from 36.3 to 63.6 kg N ha −1 ) in response to the higher N rate over the 2-year period. Newer hybrids accumulated 2.1–2.3 Mg ha −1 more grain DM than the single older hybrid in 2012, and newer hybrids accumulated 1.3 and 3.1 Mg ha −1 more grain DM than the 1975 and 1967 hybrids in 2013 when overall PostDM gains were much higher than in 2012. In 2013, more of the Grain N content (GrainN) was derived from post-silking N uptake in newer hybrids versus older hybrids. Plant component DM and N changes between silking and maturity stages in 2013 suggested 33% of final grain N originated from leaves (with no net DM depletion), and 22% of grain N originated from stems (accompanied by a net 20% DM depletion), during grain filling in a rather consistent manner for all four hybrids. However, newer hybrids maintained a higher leaf DM and leaf N content at maturity (despite a lower leaf N concentration and higher grain N harvest index) compared to older hybrids. These results indicated that retaining leaf function by enhancing leaf biomass and N content and, consequently, PostN accumulation during the grain filling, benefited from a higher DM partitioning to leaves at silking in newer hybrids.

Nitrogen use efficiency is higher in newer than in older maize (Zea mays L.) hybrids, but the physiological mechanisms underlying differences in N-use efficiency are unknown. The objective of this study was to quantify differences between an older and a newer maize hybrid in their response to N availability throughout the life cycle at both the leaf and the whole-plant level. An older and a newer maize hybrid were grown in a field hydroponic system located near Guelph, ON, in 2005 at a high and a low N level. Leaf carbon exchange rate (CER), chlorophyll index, and the thylakoid electron transport rate (ETR) were measured weekly from 2 wk presilking to 8 wk postsilking. Plant-component dry matter and N content were determined from 1 wk presilking to maturity. At the leaf level, leaf CER declined during the grain-filling period, and the decline was greater under low than high N availability. The decline in leaf CER during the grain-filling period was less in the newer than in the older hybrid under both high and low N availability, and differences in leaf CER were associated most strongly with a reduction in leaf CER per unit absorbed photosynthetic photon flux density. At the whole-plant level, reduction in grain yield in low vs. high N was greater in the older than in the newer hybrid. The hybrid x N interaction for grain yield was attributable predominantly to a greater decline in the proportion of dry matter allocated to the grain in the older hybrid.

Accurate estimates of total leaf area and the vertical leaf area profile are important in process-based crop growth models. The ben-shaped function that quantifies the area-per-leaf profile of a maize (Zea mays L.) plant can be used to estimate the area-per-leaf profile. The objectives of this study were to quantify the effects of maize hybrid, soil N, plant density, and row spacing on the coefficients of the bell-shaped function. The coefficients of the bell-shaped function that quantify (i) the breadth of the area-per-leaf profile, (ii) the skewness of the area-per-leaf profile, and (iii) the position of the largest leaf were estimated using nonlinear regression in four datasets. Datasets consisted of the fully expanded leaf areas of all leaves on maize plants grown in studies performed in Ontario, Canada, between 1997 and 2001 that included combinations of maize hybrids, plant densities, N levels, and row spacing. Observations fitted well to the bell-shaped function (r 2 > 0.95). The breadth of the area-per-leaf profile decreased under high soil N level and high plant density, and was lower for a newer than an older hybrid, whereas the opposite occurred with the position of the largest leaf. In contrast, the degree of skewness was not significantly altered by any of the factors examined in this study. Because of the relatively small impact of the examined agronomic factors on the coefficients of the bell-shaped function, a general model using mean coefficient values was validated with independent datasets. Results showed that this general bell-shaped function is a robust predictor of the area-per-leaf profile in maize.

Few studies have examined the inheritance and interrelationships of both grain yield and the underlying physiological processes in maize (Zea mays L.). The objective of this study was to establish genetic relationships between the physiological components of grain yield and to examine the inheritance of grain yield and its component processes (i.e., additive and the nonadditive genetic effects). Twelve F 1 hybrids, obtained by mating three male and four female inbred lines using a North Carolina Design II, were evaluated in trials conducted in Ontario from 2000 to 2002. Dry matter accumulation (DMA) at four stages of development, harvest index, leaf area index (LAI), stay green, and grain yield were measured. Variation among the 12 hybrids was significant for all traits evaluated, and the range in mean grain yield was 28% of the mean. Using the genetic effects partitioned by a Design II analysis, we dissected the physiological mechanisms that influenced favorable or unfavorable contributions to grain yield. Using the highest- and lowest-yielding hybrids in the study (i.e., maximum genetic variation), we attempted to dissect the physiological reasons for the difference in grain yield. This analysis, however, was unsuccessful in dissecting grain yield in terms of physiological mechanisms using a quantitative genetic model. Reasons for this failure may be, in part, (i) the relatively low contribution of statistically significant genetic effects to the differences between the hybrids; and (ii) partitioning of the difference between hybrids in four general combining ability (GCA) estimates and two specific combining ability (SCA) estimates results in small estimates relative to the grand mean.

Although heterosis in maize (Zea mays L.) has been studied since the early 1900s, very little is known about how heterosis affects the physiological components of grain yield. The objective of this study was to quantify the physiological basis of heterosis for grain yield in maize by examining maize hybrids and their parental inbred lines in terms of grain yield and its component processes, dry matter accumulation (DMA) at maturity, and the partitioning of DMA to the grain (i.e., harvest index), as well as in terms of the physiological processes underlying those two components. The genetic material consisted of 12 maize hybrids and seven parental inbred lines. Experiments were conducted from 2000 to 2002 at the Elora Research Station, ON, Canada. Data were recorded on grain yield, DMA at four stages of development, harvest index, leaf area index (LAI), final leaf number, leaf width and length, rate of leaf appearance, stay green, ear number, kernel number and weight, and number of days to silking and physiological maturity. Mean heterosis across the 3 yr was 167% for grain yield and 85 and 53% for its two component processes, DMA at maturity and harvest index, respectively. Results show that heterosis for grain yield in maize can be attributed to (i) heterosis for DMA before silking, which results mainly from greater light interception due to increased leaf size; (ii) heterosis for DMA during the grain-filling period, which results from greater light interception due to greater maximum LAI and increased stay green, and (iii) heterosis for harvest index.

Heterosis for grain yield in maize (Zea mays L.) manifests itself through its effects on the components of grain yield, dry matter accumulation at maturity, harvest index, and its effects on physiological processes underlying these components, such as leaf CO 2 exchange rate (CER). The objectives of this study were (i) to quantify the pattern of leaf CER throughout the grain-filling period in maize hybrids and their parental inbred lines, and (ii) to determine the mode of inheritance of leaf CER during the grain-filling period. Studies were performed with 12 F 1 hybrids and their seven inbred parents grown hydroponically in the field at the Cambridge Research Station, ON, Canada, in 2002. Data were recorded on leaf CER from silking to maturity, and grain yield, aboveground dry matter, and root dry matter at maturity. Mean leaf CER of hybrids was not different from that of their parental inbred lines at silking. However, significant differences became apparent 2 wk after silking and became increasingly larger as plants advanced toward maturity. In general, leaf CER differed among inbred lines but did not differ among hybrids. Combining ability analysis showed that predominantly additive genetic effects influence the expression of leaf CER late in the season. Finally, the maintenance of leaf CER throughout a plant's life cycle, rather than potential leaf CER, is positively associated with dry matter accumulation during the grain-filling period and grain yield.

Grain yield improvement of maize (Zea mays L.) hybrids has been associated with delayed leaf senescence. The objective of this study was to quantify the vertical profile of leaf senescence during the grain-filling period in an older hybrid ('Pride 5') and two more recent maize hybrids ('Pioneer 3902' and 'Pioneer 3893'). Leaf senescence was rated visually from silking to maturity on each individual leaf across the vertical leaf-area profile along the stem of maize plants growing in the field at 1, 3.5, and 12 plants m -2 near Elora, ON, Canada, during the 1999 to 2001 growing seasons. Maximum leaf area index (LAI) at silking was greater in newer hybrids than in the older hybrid. Rate of leaf senescence across hybrids and plant population densities progressed at a linear rate of 0.44% d -1 during the first half of the grain-filling period, whereas the rate was 1.87% d -1 during the second half of the grain-filling period. Rates of leaf senescence were 3.4 and 2.1 times greater in the older hybrid than in the newer hybrids during the first and second half of the grain-filling period, respectively. During the first half of the grain-filling period, leaf senescence increased from the medium to the highest plant population density, whereas rates of senescence during the second half of the grain-filling period declined with an increase in plant population density for the older hybrid and rates were lowest at the medium plant population density for the newer hybrids. A top-bottom profile of leaf senescence was observed during the second half of the grain-filling period, with leaves in the central section of the canopy being the last leaves to senesce, and this phenomenon was more marked in the newer hybrids.

Average commercial maize yield in the US has increased from about 1 Mg/ha in the 1930s to about 7 Mg/ha in the 1990s. Although the increase has been the result of both genetic and agronomic-management improvements, we contend that most of this improvement is the result of the genotype×management interaction. The genetic improvement in maize yield is associated neither with yield potential per se, nor with heterosis per se, but it is associated with increased stress tolerance, which is consistent with the improvement in the genotype×management interaction. The potential for future yield improvement through increased stress tolerance of maize in the US is large, as yield potential is approximately three times greater than current commercial maize yields. The mechanism by which maize breeders have improved stress tolerance is not known, but we speculate that increased stress tolerance may have resulted from the selection for yield stability. Stability analyses were performed on a number of high-yielding maize hybrids, including three hybrids that have been involved in some of the highest maize yields recorded in producers’ fields, to examine the relationship between yield and yield stability. Results of the stability analyses showed that high-yielding maize hybrids can differ in yield stability, but results do not support the contention that yield stability and high grain yield are mutually exclusive.

The response of dry matter accumulation and leaf photosynthesis in maize (Zea mays L.) to low temperature has been documented during early phases of development, but little is known about the low-temperature response of maize during later phases of development. Studies were conducted in 1999 at the Cambridge Research Station, Ontario, Canada, to quantify the effect of low night temperature during grain filling on leaf photosynthesis of short-season maize hybrids. Plants were grown in a hydroponic system in the field with plants in the low-night temperature treatments exposed to 4°C from late afternoon (17:00 h) to the next morning (9:00 h). Plants of three maize hybrids (i.e., an older hybrid, Pride 5, and two more recent hybrids, Pioneer 3902 and Cargill 1877) were exposed to one night or three consecutive nights of 4°C at weekly intervals from tasseling to 6 weeks after silking. Carbon exchange rate (CER) was measured at 10:00, 12:00, 14:00, and 16:00 h on the second leaf above that subtending the topmost ear. Dark-adapted chlorophyll fluorescence (Fv/Fm) was measured at 9:00 h at 6 weeks after silking. Leaf CER of control plants declined almost linearly from about 50 μmol m−2 s−1 at tasseling to about 20 μmol m−2 s−1 at 6 weeks after silking with the rate of decline in the older hybrid approximately two times greater than that in the two newer hybrids. No trend in the reduction in cold-stressed leaf CER relative to the field-grown control was apparent from tasseling to 6 weeks after silking. Reduction in CER was greater during the morning than during the afternoon after exposure to 4°C and the reduction in leaf CER increased from 19.4% after one night, to 25.8% after two nights, and 30.2% after three nights. Mean reduction in leaf CER after one night at 4°C differed significantly among the three hybrids and was 29.7% for Pride 5, 15.4% for Pioneer 3902, and 13.5% for Cargill 1877. The reduction in leaf CER due to low night temperature was associated with a reduction in leaf chlorophyll fluorescence. In conclusion, maize hybrids differ significantly in leaf CER response to cold night temperature during the grain-filling period.

Leaf senescence is typically associated with loss of chlorophyll and decline in photosynthetic capacity. The objectives of this study were: (i) to quantify the relationship between chlorophyll (SPAD) and soluble-carbohydrate concentrations in maize ( Zea mays L.) leaves during leaf senescence and (ii) to examine whether this relationship differed between an old and a recent maize hybrid. Field experiments were conducted in 1995 at two locations. A range of leaf soluble-carbohydrate concentrations and SPAD values was obtained through various source-manipulation of source activity by defoliation and thinning. Defoliation treatments were imposed at 3, 4, and 5 weeks after silking. The thinning treatment was imposed at 3 weeks after silking. Two maize hybrids were composed: Pride 5 (old) and Pioneer 3902 (recent). Leaves at three leaf positions, near the topmost ear and the ear internode were sampled at weekly intervals, from 3 weeks after silking until visual completion of leaf senescence. Leaf and internode soluble-carbohydrate concentration declined following defoliation, although the response in leaves was delayed compared to that in the stem. SPAD readings and soluble-carbohydrate concentrations were positively correlated below a plateau value of 55 mg glucose equivalents g −1 leaf. The correlation between SPAD and soluble-carbohydrate concentration was similar for the old and recent maize hybrid.

Leaf senescence in a recent maize (Zea mays L.) hybrid is delayed relative to that in an older maize hybrid and the trait is associated with an improvement of the ratio of assimilate supply (i.e., source) and demand (i.e., sink) during grain filling. This study examined whether effects of source : sink ratio of leaf longevity in an old and more recent hybrid are associated with changes in leaf nitrogen (N) concentration and N uptake during grain filling. A 3-year field study was conducted with maize hybrids Pride 5 (old) and Pioneer 3902 (recent) grown at two soil-N levels: 150 kg−1 N ha−1 was broadcast in the high N treatment while none was added to the low N treatment. Four imposed source : sink treatments ranged from partial defoliation to no grain. Leaf N of the control treatments did not differ between the two hybrids, but the decline in leaf N from the control to the no-sink treatment was larger for Pioneer 3902 than for Pride 5. Total N uptake in above-ground portions was 10 and 18% greater in the new than in the old hybrid under low and high soil-N conditions, respectively. The difference in the total N uptake between the two hybrids could be attributed to post-silking N uptake. The proportion of N in the grain derived from post-silking N uptake was 60% for Pioneer 3902 and 40% for Pride 5 and this proportion was positively associated with the source : sink ratio. Higher rates of N uptake in Pioneer 3902 vs. Pride 5 appear to be, in part, the result of higher rates of dry matter accumulation of the newer hybrid during grain filling.

Total seasonal dry matter accumulation of recently released commercial maize ( Zea mays L.) hybrids is greater than that of older hybrids. Part of this difference may be attributable to lower rates of respiration among newer hybrids. The objective of this study was to determine if hybrids released between 1959 and 1995 differ in their rates of respiration of mature leaves. Early morning rates of dark respiration were compared among six hybrids, grown at two different soil nitrogen (N) levels in replicated field trials at two locations. Measurements were made over 9 or 11 weeks, beginning ≈50 days after sowing, and continuing until the onset of leaf senescence. Leaf respiration was measured as the rate of CO 2 efflux from leaf disks in a simple open-flow gas-exchange system, and was calculated on both a leaf area ( R LA ) and a leaf dry weight ( R DW ) basis. This measurement system revealed significant effects of crop age, leaf position, time of day, and N fertilization treatments on rates of dark respiration of leaves in the field. Significant differences in mean seasonal rates of R LA and R DW were detected among the six hybrids studied. The ranking of the six hybrids with respect to R DW was the same at both locations in the absence of nutrient stress due to low soil N. A strong negative correlation between R DW and total seasonal dry matter accumulation among these six hybrids indicates that a reduction in rates of leaf respiration may have contributed to improvement in agronomic performance of commercial maize hybrids grown in Ontario between 1959 and the present.

Grain yield improvement of maize (Zea mays L.) hybrids in Ontario has been associated with increased stress tolerance of newer hybrids. This study was conducted to assess the effect of low soil N, weed interference, and soil water deficit on dry matter accumulation and grain yield of an older and a more recent hybrid. Studies were carried out at the Elora Research Station in 1992, 1993, and 1994 on a London loam (Aquic Hapludalf) that had been tile drained. The experiment was arranged in a split-split-split-plot design with four replications. Treatments consisted of two soil-N levels (70 and 225 kg ha -1 ), weed-free all season and weedy from the 3- to 4-leaf stage of maize, irrigation and no irrigation, and the hybrids 'pioneer 3902' (new) and 'Pride 5', (old). Maize phenology, dry matter accumulation, grain yield, stomatal conductance, leaf chlorophyll fluorescence, and leaf chlorophyll content were measured. Irrigation generally did not influence the measured parameters. Silking date and stomatal conductance during the grain-filling period were affected less by low soil N and weed interference in Pioneer 3902 than in Pride 5. Aboveground dry matter at silking and maturity was reduced about 20% by single stresses of low soil N and weed interference and about 55% by the combination of both stresses. Hybrid x stress treatment interactions for dry matter were not significant. The reduction in grain yield due to weed interference was 21% greater in Pride 5 than in pioneer 3902. The hybrid x N interaction for grain yield was not significant, because a smaller reduction in harvest index due to N stress in the new than in the old hybrid was offset by a larger reduction in dry matter accumulation during the grain-filling period. The differential response of the two hybrids to stress was greatest during the silking period, as indicated by the greater reduction in harvest index and kernel number in the old than in the new hybrid due to stress.

Improvement of nitrogen use efficiency in maize production requires quantifying genotypic variability in response to soil N. The objective of this field study was to quantify the relationship between leaf N concentration and leaf photosynthetic rate from pre-anthesis to grain fill for six maize (Zea mays L.) hybrids. Maize hybrids were grown at the Central Experimental Farm at Ottawa in Ontario (45°23'N, 75°43'W) in 1991, 1992, and 1993 at three N fertilizer rates (0, 100, and 200 kg N ha -1 ). A method of analysis was developed to relate leaf photosynthesis measurements at comparable irradiance levels to leaf N concentration measurements. Photosynthetic response to leaf N was characterized by two fitted coefficients, N 0 , threshold leaf N concentration for photosynthesis, and P 3 , photosynthetic rate at high (30 g N kg -1 dry weight) leaf N concentration. There were no hybrid differences in N 0 (P > 0.05). There were differences in P 3 , notably a significantly lower P 3 value at anthesis for an old hybrid compared with recent commercial hybrids. In 1992, a recent commercial hybrid, Pioneer 3902, had the highest P 3 value, the second highest crop growth rate (CGR) and the highest grain yield, but hybrid differences in P 3 were not generally correlated with CGR or grain yield (P > 0.05). Growth stage had a significant effect on both N 0 and P 3 ; both declined at later growth stages (P ≤ 0.05). The decline in P 3 was greatest in Pride 5 in 1993. Photosynthate production during grain fill was potentially limited in all hybrids by a decline in P 3 as the season progressed, although the degree of decline varied with year and hybrid. This analysis quantified the relationships between leaf N concentration and leaf photosynthetic rate for different growth stages and hybrids for evaluation of hybrid N use efficiency.

In a mixture of crop and weeds, competition exists between plants for incident solar radiation, soil nutrients, and soil moisture. Integrated weed management manipulates cropping-system factors such that the competitive relationship favors growth of the crop at the expense of the weed; however, relatively few studies have been reported on the influence of cropping-system factors of crop-weed competition. The objectives of this study were to evaluate the impact of weed interference on the performance of four maize (Zea mays L.) hybrids and to examine the influence of soil N level on the competitiveness of the hybrids. Experiments were conducted during 1990, 1991, and 1992 at the Elora Research Station, Elora, ON, on a London loam (Aquic Hapludalf) soil that had been tile drained [...]

Increased prolificacy and reduced barrenness have been identified as physiological traits in maize (Zea mays L.) hybrids that are tolerant of environmental stresses induced by high plant density. The objective of this study was to investigate ear and kernel formation under a range of plant densities in old and new hybrids. Experiments were carried out during 1987 and 1988 at two locations in Ontario with nine maize hybrids representing three decades of yield improvement in Ontario, grown at 2, 4, 8, 10, and 13 plants m -2 (.)

Studies were conducted to investigate whether genetic improvement in dry matter accumulation of maize (Zea mays L.) hybrids recommended in Ontario from the late 1950s to the late 1980s is associated with cold-temperature tolerance during early phases of development. The maize hybrids Pride 5 (released in 1959) and Pioneer 3902 (released in 1988) were compared at 16/7, 23/14, and 33/24 °C under a 16-h photoperiod with a photosynthetic photon flux density of 650 μmol m−2 s−1 in long-term and short-term temperature experiments conducted in controlled-environment cabinets. In the long-term temperature experiment, plants were grown at the three temperature regimes from the 4- to the 12-leaf stage. Total and plant component dry matter was determined at the 8-, 10-, and 12-leaf stage, and leaf photosynthesis and chlorophyll fluorescence were measured at the 10-leaf stage. In the short-term temperature experiment, plants were exposed to the three temperature regimes during a 3-d period after the 9-leaf stage, followed by 2 d at 23/14 °C. Dry matter accumulation during the 5-d period was measured and leaf photosynthesis and chlorophyll fluorescence were measured during each of the last 3 d of the 5-d period. Results showed a highly significant temperature effect on all measured parameters. Dry matter of Pride 5 at the 10-leaf stage was higher than that of Pioneer 3902, but rates of dry matter accumulation and leaf photosynthesis did not differ among hybrids, and hybrid × temperature interactions were not significant for these parameters. Hybrid × temperature interactions were significant for dry matter partitioning and the fluorescence parameter Fv/Fm, suggesting better low-temperature tolerance for Pride 5. Results of the short-term temperature study showed a significant hybrid × temperature interaction for dry matter accumulation, with Pride 5 higher than Pioneer 3902 at the low temperature regime and Pride 5 lower than Pioneer 3902 at the high temperature regime. Results of these experiments suggest that improvement over the past 30 yr of Ontario maize hybrids is not associated with improved low-temperature tolerance during early development. Key words: Maize, low-temperature tolerance, dry matter accumulation, dry matter partitioning, photosynthesis, chlorophyll fluorescence

Understanding of the physiological basis for increased maize (Zea mays L.) yields over the last three decades may contribute to future genetic improvement. Recent maize production systems have tended to increase plant densities to maximize grain yield. The objective of this field study was to determine if there were changes in the response of leaf photosynthetic rates to increasing plant densities in four hybrids grown in Ontario from 1959 to 1989. The four hybrids, numbered from the oldest to most recent hybrid ((1) Pride 5, (2) United 106, (3) Pioneer 3978 and (4) Pioneer 3902) were grown at 20 000, 80 000 and 130 000 plants ha−1. Leaf photosynthetic response to irradiance (PRI) and crop growth rate (CGR) were measured near silking and during late grainfilling, leaf area index (LAI) was measured near silking and total grain yield was measured after maturity. The LAIs of recent hybrids tended to be larger than for old hybrids at comparable plant densities. Leaf photosynthetic rates declined in all hybrids at increasing densities, but the decline occurred at lower LAIs in the older hybrids. As a result, despite the higher LAIs of recent hybrids, they showed an equal or higher PRI at all plant densities. The higher PRI of recent hybrids was correlated with higher CGRs and grain yields. These results suggest that increases in optimum plant density for grain and increases in yield may be attributable, in part, to higher PRI at elevated LAIs in recent hybrids. Key words: Leaf area index, plant density, leaf photosynthesis, Zea mays L.

The optimum plant density for total grain yield increased from old to more recent hybrids, but the increase in optimum plant density did not continue for hybrids from the 1970s era to hybrids of the 1980s era. Approximately one-third of the genetic gain in machine-harvestable grain yield can be attributed to reduced stem lodging. A comparison of the oldest and most recent hybrids indicates that approximately 15% of the genetic gain in total grain yield can be attributed to increased harvest index. Consequently, increase in total dry matter accumulation attributed 85% to the genetic gain in total grain yield

Research attention to improving source and sink strength in maize production is requisite for enhancing yield. Improvement in source strength has been achieved with higher post-silking dry matter accumulation, whereas historical improvement in sink strength has been mostly attributed to increasing kernel number (KN) per unit area, in part because KN is known to be more vulnerable to abiotic stresses compared to kernel weight (KW). However, KW can also vary widely as it is dependent on both genotype and dry matter accumulation during the post-silking period. In order to illustrate the consequences of breeding efforts over a 4-decade period for enhancing source and sink strength at varying nitrogen rates and plant densities, a 2-year and 2-location study was conducted in 2013 and 2014. Eight commercial hybrids from DeKalb released from 1967 to 2005 were compared at 2 nitrogen rates (55 and 220 kg N ha −1 ) and 3 plant densities (54,000 (D1), 79,000 (D2) and 104,000 (D3) plants ha −1 ). Breeding progress increased grain yield per hectare (GY) by an average of 66 kg ha −1 year −1 , and grain yield per plant (GYP) by 0.91 g plant −1 year −1 across all treatments and environments. This yield increase with hybrid improvement was attributed more to an increase in KW (1.29 mg kernel −1 year −1 across all treatments and both locations), than to any increase in KN. The overall source-sink ratio (SSR − ratio of post-silking dry matter accumulation to kernel number per plant) also increased by an average of 1.25 mg kernel −1 year −1 across all treatment and locations. The hybrid improvement in SSR was more pronounced at the high N rate or low plant density. Post-silking dry matter accumulation (PostDM) increased by an average of 54 kg ha −1 year −1 across all treatments and locations. KW was highly correlated with ear growth rate (EGR) during grain fill. New hybrids had much higher KW gain per unit of EGR. Newer hybrids also had a longer active grain filling period, but the correlation of post-silking dry matter accumulation to the duration of active grain filling period was weak. This study showed that the breeding progress for yield gain in these DeKalb hybrids was achieved by (i) longer duration of the grain filling period plus longer leaf stay green that accompanied a higher PostDM of newer hybrids, (ii) enhanced source to sink strength during grain filling by a higher SSR in newer hybrids, (iii) improved efficiency for transferring source from cob and husk to grain by increasing KW gain per unit of EGR, and (iv) enhanced stress tolerance in newer hybrids to maintain grain yield even under high density.

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 (KNP) and plant growth rate during the period bracketing silking (PGRs). 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 PGRs than older hybrids as is indicated by (i) the lower threshold PGRs 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 PGRs, the greater kernel set per unit PGRs 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 PGRs 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. Fil: Echarte, Maria Mercedes. Instituto Nacional de Tecnología Agropecuaria; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Andrade, Fernando Héctor. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Vega, Claudia Rosa Cecilia. Universidad de Buenos Aires. Facultad de Agronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Tollenaar, M.. University of Guelph; Canadá