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Maize (Zea mays L.) breeders through selec- tion have had profound impacts on the maize genome. In this study we examine one aspect of this intense selection pressure, the extent and nature of genetic variation present in an elite maize breeding cross. Specifi cally genetic variation is examined with regards to genotype × environment interactions (G × E), magnitude of the genetic variance (Vg) estimates, and the underlying grain yield quantitative trait loci (QTL). Using two elite Iodent sister-lines that are 64% identical by descent, 128 recombinant inbred lines (RILs) were generated and testcrossed to a Stiff Stalk inbred line (CG102). Hybrid RILs were grown in 24 trials encompassing 4 yr, three locations, and three planting densities. Addi- tive main effects and multiplicative interaction analysis resolved the trials into eight unique patterns of G × E. Smaller Vg estimates were associated with the more frequently observed patterns of G × E. Nine single-effect QTL and four epistatic interactions were detected across seven of the G × E patterns; however, the single- effect QTL and epistatic interactions were, in general, specifi c to a G × E pattern. In summary, we found extensive linkage disequilibrium (LD), reduced Vg in the more commonly occurring G × E patterns, and genetic variation due to larger effect epistatic interactions and smaller single effect QTL specifi c to the G × E pattern. Conse- quences of the genetic variation are discussed in relation to modern maize breeding programs.

In production agriculture, it is not uncommon for a crop to experience both intra- and interspecific competition during the normal course of development. Although the competition between crop plants (i.e., intraspecifc) is often considered independently of crop-weed competition (i.e., interspecific), the mechanisms through which yields are reduced may be common to both. The objective of this study was to use the experimental structure of a critical time for weed removal study to examine the timing and effect of intraspecific competition on maize (Zea mays L.) biomass accumulation and phenological development. A field trial was conducted in which maize stands were thinned from a higher to a lower density at six stages of development. Results indicated that intraspecific competition at densities of 8 and 16 plants m -2 did not affect maize biomass accumulation until the 14th and 12th leaf tip stages, respectively. Before these stages, maize seedling growth at 8 or 16 plants m -2 was not resource limited. Increases in leaf area index and specific leaf area at the onset of intraspecific competition, and the recovery of plants following the removal of competitors, suggest that reductions in the rate of crop growth and development may have been linked to competition for light quantity.

Summary The effect of early weed emergence on crop seedling development has not been analysed within the context of a critical period study. Experiments were conducted to quantify the influence of a low light quality environment (i.e., low ratio of reflected red to far-red light (R ⁄ FR)) on maize seedling growth and development under non-limiting resource conditions. Weed-addition and -removal series were constructed, such that the effects of R ⁄ FR on seedling growth and development were isolated from those of direct competition. Maize seedlings responded to the presence of weeds within 24 h of addition by increasing plant height, which was followed by a subsequent reduction in the rate of leaf appearance. Seedling biomass and leaf area decreased linearly in the weed removal series with increasing duration of weed presence. Conversely, seedlings in the weed-addition series were unaffected. These results demonstrate that early exposure to weeds reduced the rate of seedling growth and development and that this effect was most pronounced if it was initiated upon emergence. This suggests that the existence of a period of developmental sensitivity to R ⁄ FR precedes the defined critical period for weed control in maize. These early physiological changes triggered by the R ⁄ FR ratio may contribute to the onset of the critical time of weed removal.

Mounting concerns over the cost and environmental impact of N fertilizer combined with progressively higher plant densities in maize (Zea mays L.) production systems make progress in maize N use efficiency (NUE) and N stress tolerance essential. The primary objectives of this 3-yr field study were to (i) evaluate the N responsiveness, NUE, and N stress tolerance of multiple modern maize genotypes using suboptimal, optimal, and supraoptimal plant densities (54,000,79,000, and 104,000 plants ha -1 , respectively) with three levels of side-dress N (0, 165, and 330 kg N ha -1 ), (ii) identify key morphophysiological responses to the simultaneous stresses of intense crowding and low N availability, and (iii) consider our results with extensive reference to literature on maize morphophysiological responses to plant crowding and N availability. At optimal and supraoptimal plant densities, maize receiving 165 kg ha -1 of side-dress N displayed strong N responsiveness, high NUE, pronounced crowding tolerance, and plant density independence. However, crowding tolerance was contingent on N application. Relative to less crowded, N-fertilized environments, the 104,000 plants ha -1 , 0 kg N ha-1 treatment combination exhibited (i) reduced pre- and postanthesis plant height (PHT), stem diameter (SD), and total biomass; (ii) greater preflowering leaf senescence and lower R1 leaf areas at individual-leaf, per-plant, and canopy levels; (iii) enhanced floral protandry; (iv) lower pre- and postanthesis leaf-chlorophyll content; (v) lower per-plant kernel number (KN P ), individual kernel weight (KW), grain yield per plant (GYp), andharvest index per plant (HI P ); and (vi) enhanced per-plant grain yield variability (GY CV ). Genetic efforts to improve high plant density tolerance should, therefore, simultaneously focus on enhancing NUE and N stress tolerance.

Heterosis in maize (Zea mays L.) confers stress tolerance. To better understand the physiological mechanisms underlying the differential response of a maize hybrid (CG60 × MBS1236) and its parental inbred lines to shading stress, studies were conducted in a field hydroponic system in Ontario, Canada, from 2004 to 2006. Shading stress consisted of a 55% reduction in incident solar radiation and was implemented either for a 30- to 33-d period before silking starting at the 7-leaf tip stage, a 21-d period during silking, or a 21-d period after silking. Mean reduction in total dry matter at maturity (TDM) due to the shading treatments was 18%, and this reduction was similar for the three shading periods. Heterosis for grain yield was greater when plants were exposed to shading during the presilking and silking periods compared to the unshaded control. This increase was attributable to increased heterosis for both harvest index and TDM. In contrast, shading during the grain-filling period did not increase heterosis for grain yield. Heterosis for grain yield was highly associated with heterosis for kernel number. Heterosis for kernel set was attributable, in part, to the relationship between kernel number and plant growth rate (PGR) during the period bracketing silking and the inherent lower PGR of the inbred lines as compared to the hybrid. Kernel set was also affected by shading during the presilking period, in particular, in one of the two inbred lines.

Genetic yield improvement in maize (Zea mays L.) has been associated with heterosis and increased tolerance to high plant densities, but the physiological processes underlying heterosis for tolerance to plant density stress have not been identified. The objective of this study was to quantify the response of heterosis for grain yield to increasing plant density, and to examine the processes underlying this response. Field experiments were conducted in Ontario, Canada, during the 2005 and 2006 growing seasons in which the hybrid CG60 x MBS1236 and its parental inbred lines were grown at a low plant density (4 plants m- 2 ), a high plant density (12 plants m -2 ), and a plant density resulting in an approximately equal leaf area index (LAI) for all three genotypes. Increasing plant density from 4 to 12 plants m- 2 resulted in an increase in heterosis for grain yield and harvest index (HI), but did not affect heterosis for dry matter at maturity. Heterosis for dry matter accumulation did not differ between the low and the high plant density treatment, increased from the presilking to the postsilking period, and was only to a small extent attributable to a higher maximum LAI and light interception by the hybrid as compared to its parental inbred lines. Increased heterosis for Hl was associated with a greater plant-to-plant variability and a higher threshold plant dry matter for HI at maturity in the inbred lines as compared to the hybrid.

Soybean rust (SBR; caused by Phakopsora pachyrhizi Syd. and P. Syd.) leads to premature leaf loss and yield reduction. The objectives of this study were to assess effects of SBR infection on soybean (Glycine max (L.) Merrill) yield and to identify causes for the yield reduction. Experi- ments were conducted in the 2005-2006 and 2006-2007 growing seasons at Londrina, Brazil. The fi ve treatments were SBR infection begin- ning at either (i) the R2 or (ii) R5 growth stages; nondiseased defoliation treatments to mimic the leaf loss when SBR started at either (iii) the R2 or (iv) R5 growth stages; and (v) a disease-free, nondefoliated control. The control and defolia- tion treatments were protected against SBR by fungicide applications. Disease severity, lesion area, and leaf area were monitored from R2 to R7. Biomass and seed yield were measured at maturity. Mean SBR-induced yield reductions were 67% when infection started at R2 and 37% when infection started at R5. Leaf loss alone reduced yield signifi cantly in only one year and only when defoliation treatments were begun at R2 (31% in 2005-2006). Soybean rust-induced yield loss was attributable to (i) premature leaf loss, (ii) reduction in canopy green leaf area due to SBR lesions, (iii) reduction in dry mat- ter accumulation per unit absorbed radiation by the nonlesion green leaf area, and (iv) reduction in harvest index. The response of harvest index was attributable to reduced seed set and seed mass resulting likely from SBR-induced reduc- tions in rate of dry matter accumulation.

Yield loss in soybean [Glycine max (L.) Merr.] due to soybean rust (SBR) (caused by Phakospora pachyrhizi Syd. & P. Syd.) has been associated with reduced radiation interception and reduced radiation use efficiency by green leaf area. The objective of this study was to determine the mechanisms involved in the SBR-induced reductions in radiation use efficiency by quantifying the effect of SBR on (i) absorption of photosynthetic photon flux density (PPFD) and (ii) carbon exchange rate (CER) per unit absorbed PPFD of soybean leaves. A controlled-environment study was conducted using a split-plot design with three replications over time, with two disease levels as main plots and two soybean cultivars as subplots. The two disease levels were a disease-free control and inoculation with P. pachyrhizi. Measurements included leaf CER, PPFD absorptance, and chlorophyll fluorescence. As disease severity increased, there was a small linear decline in leaf absorptance and a negative exponential decline in leaf CER. At 50% disease severity, absorptance was reduced by 10% and CER was reduced by 68% compared with the disease-free control. The relative decline in CER was similar at low and saturating PPFD levels. Soybean rust-induced reductions in CER were mainly associated with lower efficiency of photosystem II (PSII) photochemistry and damage to PSII reaction centers. Therefore, visual indicators such as light absorptance and disease severity do not reflect the degree of impact of SBR on leaf photosynthesis.

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.

Conflicting results have been reported on the effects of spacing and emergence variability on grain yield in maize (Zea mays L.). Effects of spacing and emergence variability on maize grain yield are the net result of the responses of all plants within the stand. The objective of this study was to quantify effects of spacing and emergence variability on crop yield in terms of increased or decreased crowding stress on resource capture (i.e., dry matter accumulation) and resource utilization (i.e., dry matter partitioning) of the individual plants within the crop canopy. Results of previously reported studies were analyzed in terms of plant dry matter accumulation, leaf area, plant growth rate during the critical period for kernel set bracketed by silking (PGR s ), grain yield, and harvest index, that is, the proportion of dry matter partitioned to the grain at maturity. Results show that a moderate increase in plant-spacing variability does not influence maize grain yield at the canopy level because reductions in grain yield of plants that experience enhanced crowding stress is compensated, in part, by increased yield of plants that experience reduced crowding stress; crowding stress affected dry matter accumulation but did not affect harvest index. In contrast, plant-emergence variability reduced grain yield at the canopy level because the reduction in grain yield was attributable, in part, to a reduction in harvest index of plants with PGR s less than the threshold for kernel set. Hence, plants can compensate for factors that influence resource capture, but cannot compensate for a reduction in factors that influence resource utilization.

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.

Planter type, maintenance, and operation play an important role in uniform stand establishment in corn (Zea mays L.). Research was conducted to determine if planter type affects corn yield by altering plant spacing and emergence variability and to determine if planting speed and tillage influence these effects. This experiment was performed at two locations in south-central Ontario during a 2-yr period. Treatments were established with conventional tillage (CT) and no-tillage (NT) as main plots, three planter types (vacuum meter, finger-pickup, and air seeder) with differing mechanisms including varied seed-singulating mechanisms as subplots, and two planting speeds of 7.2 and 11.3 km per hour (kph) as sub-subplots. Planter type affected stand uniformity with mean standard deviation (SD) of within-row plant spacing of 7.9,10.3, and 19.9 cm for vacuum meter, finger pickup, and air seeder, respectively. A higher SD was observed in NT for finger pickup and air seeder but remained the same for vacuum meter. For all planters, SD increased at faster planting speeds. The number of days required to achieve 50% emergence was similar between the vacuum meter and finger pickup but was greater for the air seeder, especially when planting speed was increased and NT was used. Final plant population was unaffected by planter and planting speed treatments. Overall, grain yields decreased 35.9 kg ha -1 for each centimeter increase in within-row plant spacing SD and 292.8 kg ha -1 per day of delay in emergence. Results suggest that grower's attention to corn planter mechanisms and maintenance is more critical under a NT system or when operating speeds are increased.

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.

Nonuniform plant spacing within the row in corn (Zea mays L.) may reduce grain yield. To investigate the response of corn to plant spacing variability, experiments were conducted at two locations in south-central Ontario during 2000 and 2001. Six plant spacing treatments, 6.7 to 16.2 cm in standard deviations (SD), were established by planting Roundup Ready corn with increasing proportions of conventional corn seeds and then removing the conventional corn using glyphosate before three-leaf stage. Using SD as well as short gap, long gap, double, and cluster as an index of plant spacing variability, effects of plant spacing variability on corn growth and grain yield were investigated. Averaged across locations and years, grain yield was not significantly affected by plant spacing variability. Plant spacing variability also had no significant effect on leaf number, plant height, leaf area index, and harvest index. There were no correlations between plant spacing variability and stalk lodging and barren or double ears. The lack of strong correlations among plant growth, grain yield, and plant spacing variability indicates that spacing uniformity within the range used in this study is not a significant factor in determining grain yield under commercial conditions and common plant densities used in Ontario.

Earlier spring planting to maximize the duration of the growing season has increased the importance of early-season cold tolerance in maize (Zea mays L.). The objectives of this study were to assess the response of several physiological parameters associated with cold tolerance in maize during early autotrophic development and to quantify variability in the response to cold stress among 49 maize inbred lines. At the 7-leaf tip stage, maize inbred lines were subjected to two day/night temperature regime treatments, 25/15°C (control) and 15/3°C (cold). Carbon exchange rate (CER), leaf chlorophyll content, quantum efficiency of Photosystem II, leaf conductance, dry weight, rootlshoot ratio, and rate of development were measured at the 8-leaf tip stage for genotypes under both treatments. The cold treatment effects were significant for all parameters except root/shoot ratio. Genetic diversity for most traits investigated was observed for the response of the inbred lines to cold stress. Results of this study show that leaf CER and rate of development are good discriminators of cold tolerance during early phases of development.

Light quantity (photosynthetic photon flux density, PPFD) and quality (red:far-red ratio, R:FR) may affect phenological development of weed species growing under a crop canopy. An indoor study was conducted to quantify the effects of incident PPFD and R:FR on development and dry matter accumulation of redroot pigweed (Amaranthus retroflexus L.). Pigweed was grown in growth cabinets from the one-leaf stage to the initiation of seed set under three different PPFD/R:FR treatments: (i) high PPFD (550 μmol m -2 s -1 ) and high R:FR (1.4) (HH), (ii) low PPFD (180 μmol m -2 s -1 ) and high R:FR (1.4) (LH), and (iii) low PPFD (180 μmol m -2 s -1 ) and low R:FR (0.8) (LL). The experiment was undertaken at 12- and 16-h daylengths with three replications. Rate of leaf appearance (RLA) was accelerated with an increase in PPFD (HH vs. LH) at both daylengths. The FR enrichment (LL) negated the effect of low PPFD on RLA under the 12-h but not under the 16-h daylength. Low PPFD delayed the occurrence of floral primordia, flowering and initiation of seed set. Plant height was a result of the complementary effects of PPFD and R:FR. Total dry matter accumulation and partitioning, with the exception of dry matter accumulation to the stem, were influenced by PPFD only. Results of this study show that both light quality and quantity influence the phenology of pigweed.

Maize (Zea mays L.) leaf photosynthesis during the grain-filling period is affected by low (4°C) night temperatures. Three factors that are potentially involved in this phenomenon were examined: (i) duration of cold exposure, (ii) acclimation prior to exposure to light, and (iii) level of incident photosynthetic photon flux density (PPFD) following cold exposure. Studies were carried out with plants grown hydroponically under both field and controlled-environment conditions. Three hybrids ('Pride 5', 'Pioneer 3902', and 'Cargill 1877') were used in the field experiments and Pioneer 3902 was used in controlled-environment experiments. Plants were exposed to 4°C in the dark for either 2 or 16 h and, subsequently, acclimated for either 0 or 1 h in the dark before exposure to high PPFD. Four incident PPFD levels (400, 650, 1200, and 2000 μmol m -2 s -1 ) after cold exposure were examined. Both duration of cold exposure and acclimation after cold exposure affected the reduction in leaf carbon exchange rate (CER). Leaf CER was reduced by 18.0% after a 2-h exposure and by 30.4% after a 16-h exposure to 4°C, and leaf CER was reduced by 20.4 and 28.0% for 1- and 0-h acclimation, respectively. Dark-adapted F v /F m (F v = variable fluorescence; F m = maximum chlorophyll fluorescence), that is, maximum quantum efficiency of Photosystem II, was 0.71 after cold exposure during the night compared with 0.81 for the control. The F v /F m was affected by duration of cold exposure (0.74 and 0.68 for 2- and 16-h exposure, respectively) but not by acclimation. Reduction in leaf photosynthesis after cold exposure was linearly related to the incident PPFD level. Results support the contention that the reduction in CER due to low night temperature is not associated with photoinhibition.

repeatedly demonstrated that knowledge of phenological development is critical in understanding crop growth, An understanding of the environmental variables influencing the yield potential, and for prediction of phenology (Grant, phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables 1989; Miller et al., 1993). Similarly, it is essential to governing the phenological development of weeds. Growth cabinet understand weed phenology in order to develop prestudies were conducted to characterize the phenological development dictive models. of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] in response to Barnyardgrass is a common and troublesome C4 weed variations in temperature and photoperiod and to determine the dura- capable of infesting a variety of crops such as rice (Oryza tion of the juvenile phase and the effect of temperature and photoperiod sativa L.), soybean [Glycine max (L.) Merr.], cotton on reproductive development. Barnyardgrass was adapted to a tempera- (Gossypium hirsutum L.), corn (Zea mays L.), wheat ture range of 6.5 to 528C. Phenological development of barnyardgrass (Triticum aestivum L.), vegetables, beans, root crops, was described in terms of thermal days (cumulative day degrees above forage crops, orchards, and pastures in most of the aga base temperature for leaf appearance, tiller appearance, and shoot

A retrospective analysis of the physiological basis of genetic yield improvement may provide an understanding of yield potential and may indicate avenues for future yield improvement. Rate of yield improvement of maize (Zea mays L.) hybrids in Ontario, Canada has been ≈1.5% yr⁻¹ during the last five decades. Comparison of short-season hybrids representing yield improvement from the late 1950s to the late 1980s showed that genetic yield improvement was 2.5% per year and that most of the genetic yield improvement could be attributed to increased stress tolerance. Differences in stress tolerance between older and more recent hybrids have been shown for high plant population density, weed interference, low night temperatures during the grain-filling period, low soil moisture, low soil N, and a number of herbicides. Yield improvement is the result of more efficient capture and use of resources, and the improved efficiency in resource capture and use of newer hybrids is frequently only evident under stress. Improved resource capture has resulted from increased interception of seasonal incident radiation and greater uptake of nutrients and water. The improved resource capture is associated with increased leaf longevity, a more active root system, and a higher ratio of assimilate supply by the leaf canopy (source) and assimilate demand by the grain (sink) during the grain-filling period. Improvements of resource use under optimum conditions have been small, as leaf photosynthesis, leaf-angle distribution of the canopy, grain chemical composition, and the proportion of dry matter allocated to the grain at maturity (i.e., harvest index) have remained virtually constant. Genetic improvement of maize has been accompanied by a decrease in plant-to-plant variability. Results of our studies indicate that increased stress tolerance is associated with lower plant-to-plant variability and that increased plant-to-plant variability results in lower stress tolerance.

Recent evidence suggests that chlorophyll fluorescence techniques can be used to determine photosynthetic rates of maize (Zea mays L.) leaves much more rapidly than is possible with conventional gas exchange methods. However, the accuracy of such measurements depends directly on the accuracy with which leaf absorptance of incident photons in the PAR (photosynthetically active radiation) region can be estimated. Our objectives were (i) to monitor changes in leaf absorptance in a typical maize crop over the growing season, and (ii) to determine if absorptance of maize leaves could be accurately estimated with a SPAD 502 hand-held chlorophyll meter (Minolta Corporation, Ramsey, NJ). Leaves from six leaf positions were harvested on eleven dates between 55 and 134 d after planting. An integrating sphere and field-portable spectroradiometer were used to measure reflectance and transmittance of leaf tissue samples at 5-nm intervals between 400 and 700 nm, and a SPAD value was also determined for each sample. Greening of young leaves was associated with decreases in both reflectance and transmittance in the middle wavelengths of the PAR region, while leaf senescence was associated with increases in both reflectance and transmittance, primarily in the longer wavelengths of the PAR region. Leaf absorptance of incident photons in the PAR region by healthy, fully expanded leaves ranged from approximately 0.88 to 0.91; much lower values were observed for young, chlorotic, or senescing leaves. Regression analysis revealed a strong relationship (r 2 = 0.98) between leaf absorptance and SPAD value, suggesting that the SPAD meter could be used to provide a rapid estimate of leaf absorptance in the field.

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.

Grain yield improvement of maize (Zea mays L.) hybrids in Ontario has been associated with increased stress tolerance. Effects of shortterm water-deficit stress on canopy gas exchange of an old and a new maize hybrid were studied. Plants of Pride 5 (old) and Pioneer 3902 (new) were grown under controlled-environtrtent conditions until tassel emergence and, subsequently, plants were transferred to enclosures, each containing the aboveground parts of four plants. Canopy photosynthesis, respiration, transpiration, and stem water potential were measured continuously for 7 d. Plants were either well watered or exposed to a water-deficit stress by withholding water supply until net canopy photosynthesis declined to zero and, subsequently, plants were rehydrated. Under well-watered conditions, the two hybrids did not differ significantly in terms of canopy photosynthesis, transpiration, and stem water potential. In contrast, hybrids responded differently to a water-deficit stress. Cumulative canopy photosynthesis and transpiration during the drying cycle were reduced by 21 and 31%, respectively, in Pioneer 3902 and by 34 and 31%, respectively, in Pride 5. The threshold stem water potential related stomatal closure, and thus, photosynthesis and transpiration, was lower for Pioneer 3902 than for Pride 5. During the recovery day, canopy photosynthesis was 53% higher and canopy transpiration was 31% higher in Pioneer 3902 than in Ride 5. Respiration per unit CO 2 fixed was lower in Pioneer 3902 than in Pride 5 in all conditions.

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 [...]

Development of an integrated weed management system requires detailed information on crop-weed interactions, including the impact of the relative competitive ability of the crop during serious phases of development on weed growth. The objective of this study was to quantify effects of maize (Zea mays L.) plant density on weed interference in maize throughout the growing season. Experiments were carried out during 1990, 1991, and 1992 at the Elora Research Station, Elora, ON, on a London loam (Aquic Hapludalf) soil that had been tile drained. Maize was grown at three plant densities (4, 7, and 10 plants m −2 ) under three weed pressures. Weed pressures were established by varying the weedfree period after maize planting: all season (weed free), planting to 5to 7-leaf stage of maize (medium weed pressure), and planting to 3- to 4-leaf stage of maize (high weed pressure) [...]

Interception of solar irradiation by leaf canopies is influenced by the canopy architecture of crops, which is a function of shape, distribution, and orientation of the leaves that constitute the canopy. The objective of this study was to test the hypothesis that leaf azimuthal distribution in a maize (Zea mays L.) canopy is not influenced by plant density and row width. Experiments were conducted at Elora, Ontario, with maize grown at 4, 7, and 10 plants m -2 at a 0.76-m row width in 1991 and 1992 and with maize grown at 7 and 10 plants m -2 at a 0.5-m row width in 1992. Leaf orientation was recorded in 16 azimuthal classes for leaves from the bottom to the topmost leaf position

Rye (Secale cereale L.) cover crops often delay development and reduce yield of corn (Zea mays L.). A 3-yr study (1982-1984) was conducted to investigate the influence of flour rye cultivars and one wheat (Triticum aestivum L.) cultivar, rate of N application in the spring, and removal of above-ground cereal phytomass on growth and development of corn. Experiments were carried out at the Elora Research Station, Ontario, in a split-plot design, with two N levels (145 and 220 kg ha −1 ) and five cereal cultivars as main plots, and cereal phytomass removal or retention as sub-plots. Cereal cultivars were planted in September after corn harvest and corn was planted in the spring [...]

Summary Crop–weed competition is comprised of both resource dependent and resource independent processes. While many studies have focused on the role that resource dependant competition plays in reducing crop yields, few have investigated whether resource independent effects may contribute to these losses. In this study, we identify the red-to-far-red ratio as a variable that contributing to resource independent competition and tested the hypothesis that the expression of shade avoidance in response to weeds reduces maize fitness (i.e., kernel number) in the absence of resource dependent competition. Seedlings were grown in a field fertigation system under two light quality environments: an ambient and a low red-to-far-red ratio environment, which were designed to simulate weed-free and weedy conditions respectively. Plants that expressed classic shade avoidance characteristics set fewer kernels per plant and partitioned less biomass to the developing ear. Shade avoidance also doubled the plant-to-plant variability in these yield parameters (i.e., kernel number and harvest index) without affecting the mean or frequency distribution of shoot biomass at maturity. We propose that shade avoidance should be viewed as an integral component of the process of competition. This resource independent response precedes and conditions the crop seedling for the onset of resource dependent competition.

A rye (Secale cereale L.)-corn (Zea mays L.) double-cropping system has the potential to maximize annual phytomass production in Ontario. Field experiments were conducted at Elora and Woodstock, ON, from 1981 to 1984 to evaluate the impact of rye harvest and corn planting date combinations during the spring as well as tillage methods on annual above-ground phytomass production. Winter rye was planted in late September or early October after corn harvest and either chemically killed in early May or harvested as silage before corn planting. The rye/tillage treatments consisted of no-rye/rototill, rye/rototill, and rye/no-till. Corn phenology was monitored during the growing season and corn above-ground dry matter was harvested in late September []

Differences in dry matter accumulation among crop cultivars can be attributed to differences in either the absorptance of incident photosynthetically active radiation (PAR) and/or the conversion of absorbed PAR into dry matter. This study was conducted to quantity the contribution of radiation use efficiency (RUE, g dry matter MJ −1 ) to the difference in dry matter accumulation between an old and a new maize (Zea mays L.) hybrid. An old (Pride 5) and a new (Pioneer 3902) maize hybrid were grown during 1989 and 1990 at the Elora Research Station, Ontario, 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 in Ontario have shown that corn (Zea mays L.) yields are reduced when corn is seeded immediately after rye (Secale cereale L.) harvest or chemical kill of winter rye. A study was conducted in 1983 and 1984 on a Maryhill (Typic Hapludalf) loam soil to determine the effect of spring tillage systems and timing of rye chemical kill on the subsequent corn crop. The rye was seeded in early October after corn silage harvest (...)

The use of a winter rye (Secale cereale L.) corn (Zea mays L.) doable cropping sequence in combination with appropriate tillage practices could increase biomass production and reduce soil erosion potential in southern Ontario. A 3-yr study (1982–1984) was conducted at two locations to determine the potential of this sequence for double cropping, and to evaluate spring tillage systems and management of the rye residue on subsequent productivity of corn. Winter rye was planted in early October after corn silage harvest, and either chemically killed or harvested as silage in the spring before corn planting. Rye treatments consisted of no rye, rye harvested in the spring and rye residue left on the plots. Spring cultivation treatments were no-till, tandem discing, and mold board plowing followed by secondary tillage. The use of a winter rye cover crop delayed corn development and reduced corn biomass yield by 11% at the Elora location and by 17% at the Woodstock location. The adverse effect of the rye crop was more pronounced under no-till than where the soil was tilled. Removal or retention of the rye residue had no consistent effect on the subsequent corn crop. An allelopathic effect resulting from the rye crop may be one plausible explanation for the reduction in corn yield. Total biomass yield (rye + corn) was increased relative to corn alone, if the soil was cultivated. Therefore, a winter rye-corn sequence may still be of interest, despite a reduction in corn yield, especially if advantages such as total biomass production and the potential for decreased soil erosion during fall and winter are considered. Research supported by Agriculture Canada (ERDAF).

During the maize (Zea mays L.) hybrid era (1939 to present), commercial grain yields have improved neady sixfold and the genetic component of the improvement has been estimated as approximately 60%. In this paper, we examine physiological factors and successful breeding strategies that underlie the yield improvement. Grain yield is the product of accumulating dry matter and allocating a portion of the total dry matter to the grain. The processes influencing dry matter accumulation are commonly referred to as the "source" components, while the processes influencing allocation of dry matter to the grain are referred to as the "sink" components. On the source side, changes in leaf canopy size and architecture account for only a minor portion of the improvement. The majority of the improvement in source capacity is due to visual and functional "stay-green." On the sink side, the improvement is through changes in the relationship between kernel number per plant and plant growth rate during a period bracketing silking. In a breeding context, these improvements have been made (i) in a "closed" gemplasm pool stratified into heterotic groups; (ii) through use of a pedigree method of breeding structured to mimic reciprocal recurrent selection and thereby improving both additive and nonadditive genetic effects; and (iii) by a gradual increase in plant population densities during the hybrid era as the constant source of stress during both inbred line development and hybrid commercialization. Functional stay-green and the sink establishment dynamics still represent opportunities for yield improvements. It is essential that source and sink are kept in balance, and that improvement in one accompanies a simultaneous improvement in the other. One strategy for exploiting these opportunities is to incorporate high plant population density trials into inbred line development programs.

Potential yield benefits from improving within-row plant spacing variability and plant emergence variability in corn (Zea mays L.) production are often questioned by growers. Research was conducted at two locations in south-central Ontario during a 2-yr period to quantify the effects and interactions of plant spacing variability and plant emergence variability on growth and grain yield of corn. Nine treatments were established by hand planting corn rows with repeating six-plant sequences consisting of uniform and nonuniform spacing, even and uneven emergence, and their combinations. Spacing treatments consisted of (i) uniform within-row plant spacing of 20 cm; (ii) one 40-cm gap associated with a double; and (iii) one 60-cm gap associated with a triple in each six-plant sequence. Emergence treatments included uniform early emergence, a two-leaf stage delay, and a four-leaf stage delay for one plant in each six-plant sequence. Only plant emergence variability significantly affected plant height, leaf area index (LAI), dry matter accumulation, and grain yield. Compared with the uniformly early emerged plants, one out of six plants with a two-leaf stage delay in emergence reduced yield by 4%, and one out of six plants with a four-leaf stage delay reduced yield by 8%. Whereas corn plants next to a gap demonstrated compensatory growth, plants adjacent to a late emerging corn plant did not exhibit compensatory growth. These results indicate that corn is more responsive to plant emergence variability than plant spacing variability. Variation in plant emergence reduced yield, whereas variation in within-row spacing did not affect yield, and interactions between the two factors were not significant.

Crop dry matter production, in the absence of other limiting fac-tors, is determined by the absorption of photosynthetically activeradiation (PAR) by a crop canopy and the efficiency with whichabsorbed PAR is converted into dry matter. This study was con-ducted to investigate the effects of plant density and phase of de-velopment on PAR absorption and conversion efficiency of two maize (Zea mays L.) hybrids. The maize hybrids Pioneer 3851 and Pioneer3925 were grown in 1984 and 1985 at plant densities of 3.9, 6, and10 plants m 2 in irrigated field experiments near Elora, Ontario, ona London loam (Aquic Hapludalf) soil that had been systematicallytile drained. Conversion efficiency was estimated from rates ofaboveground dry matter accumulation and PAR absorption from ap-proximately 2 wk before to 6 wk after silking. PAR absorption wasestimated from continuous measurements of incident PAR, PAR re-flectance and PAR transmittance. Crop growth rates declined duringthe phase of development under study due to a decline in incidentPAR and a decline in conversion efficiency from 5.8 to 3.5%; PARabsorptance was approximately 89% and remained fairly stable. Leafarea index among density-hybrid treatments ranged from 2.0 to 4.8,and PAR absorption of the latter was 9% higher than that of theformer. Although crop growth rates of the plant densities were notdifferent during the grain-filling period, differences in PAR absorp-tion were too small to show a significant effect of plant density onconversion efficiency. Photosynthetic conversion efficiency appearsto be influenced by phase of development, but the effects of plantdensity on efficiency appear to be small.

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