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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.

Predictions of crop yield under future climate change are predicated on historical yield trends(1–3), hence it is important to identify the contributors to historical yield gains and their potential for continued increase. The large gains in maize yield in the US Corn Belt have been attributed to agricultural technologies(4), ignoring the potential contribution of solar brightening (decadal-scale increases in incident solar radiation) reported for much of the globe since the mid-1980s. In this study, using a novel biophysical/empirical approach, we show that solar brightening contributed approximately 27% of the US Corn Belt yield trend from 1984 to 2013. Accumulated solar brightening during the post-flowering phase of development of maize increased during the past 3 decades, causing the yield increase that previously had been attributed to agricultural technology. Several factors are believed to cause solar brightening, but their relative importance and future outlook are unknown(5–9), making prediction of continued solar brightening and its future contribution to yield gain uncertain. Consequently, results of this study call into question the implicit use of historical yield trends in predicting yields under future climate change scenarios.

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.

The physiological process underlying grain yield (GY) loss in maize as a result of weed competition is not understood clearly. We designed an experiment to test the hypotheses that early season stress caused by the presence of neighboring weeds will increase plant-to-plant variability (PPV) of individual plant dry matter (PDM) within the population. This increase in PPV will reduce GY through a reduction in harvest index (HI). Field experiments were conducted in 2008, 2009, and 2010. A glyphosate-resistant maize hybrid was cropped at a density of 7 plants m−2. As a model weed, winter wheat was seeded at the same time as maize and controlled with glyphosate at the 3rd or 10th to 12th leaf-tip stage of maize. Weed competition early in the development of maize decreased PDM and GY. This reduction in PDM, which occurred early in the development of maize, was attributed initially to a delay in rate of leaf appearance. Reductions in PDM were accompanied by an increase in PPV of PDM. This increase in PPV, however, did not reduce HI and did not contribute to the GY reductions created by weed competition, as hypothesized. As weed control was delayed, a reduction in fraction of photosynthetically active radiation (fIPAR) accounted for a further reduction in PDM and notably, a reduction in DMA from 17th leaf-tip stage through to maturity. The rapid loss of PDM and the subsequent inability to accumulate dry matter during maturation accounted for a rapid decline in kernel number (KN) and kernel weight (KW).

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.

The USA accounts for 38 and 35% of global maize and soybean production, producing a respective 320 and 84 Mt of these crops annually. More than 85% of those totals are produced in the north-central region known as the ‘Corn Belt’, where continuous maize and 2-year maize–soybean rotation are the dominant cropping systems. This chapter describes the climate, soil, and management practices of high-yield maize–soybean cropping systems in the Corn Belt. Major drivers for higher yields and resource-use efficiency are evaluated and opportunities for further improvement are discussed. Yield and input-use efficiency of maize and soybean in the US Corn Belt have increased steadily during the last 40 years as a result of (1) continuous genetic improvement, (2) intermittently phased periods of agronomic improvement, and (3) the synergistic interaction of improved genetics and agronomy. Future increases may be difficult to achieve as on-farm yields approach yield potential; however, some of the yield gap between on-farm yield and simulated yield potential can still be captured by fine-tuning crop management in a manner that increases yield, while simultaneously reducing the resource input amount or cost. Indeed, substantive opportunities exist for increased input-use efficiency by scheduling just-in-time irrigation events of the minimum amount needed, and by optimizing management of N fertilizer to be temporally and spatially effective.

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.

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.

Accurate prediction of phenological development in maize (Zea mays L.) is fundamental to determining crop adaptation and yield potential. A number of thermal functions are used in crop models, but their relative precision in predicting maize development has not been quantified. The objectives of this study were (i) to evaluate the precision of eight thermal functions, (ii) to assess the effects of source data on the ability to differentiate among thermal functions, and (iii) to attribute the precision of thermal functions to their response across various temperature ranges. Data sets used in this study represent >1000 distinct maize hybrids, >50 geographic locations, and multiple planting dates and years. Thermal functions and calendar days were evaluated and grouped based on their temperature response and derivation as empirical linear, empirical nonlinear, and process-based functions. Precision in predicting phase durations from planting to anthesis or silking and from silking to physiological maturity was evaluated. Large data sets enabled increased differentiation of thermal functions, even when smaller data sets contained orthogonal, multi-location and -year data. At the highest level of differentiation, precision of thermal functions was in the order calendar days < empirical linear < process based < empirical nonlinear. Precision was associated with relatively low temperature sensitivity across the 10 to 26°C range. In contrast to other thermal functions, process-based functions were derived using supra-optimal temperatures, and consequently, they may better represent the developmental response of maize to supra-optimal temperatures. Supra-optimal temperatures could be more prevalent under future climate-change scenarios, but data sets in this study contained few data in that range. EEA Balcarce Fil: Kumudini, S. The Climate Corp; Estados Unidos Fil: Andrade, Fernando Hector. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce-Unidad Integrada-Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina Fil: Boote, K.J. University of Florida. Department of Agronomy; Estados Unidos Fil: Brown, G.A. Breaking Ground; Estados Unidos Fil: Dzotsi, K.A. University of Florida. Department of Agricultural and Biological Engineering; Estados Unidos Fil: Edmeades, G.O. Hemmans; Nueva Zelanda Fil: Gocken, T. Monsanto; Estados Unidos Fil: Goodwin, M. Monsanto; Estados Unidos Fil: Halter, A.L. Dupont-Pioneer; Estados Unidos Fil: Hammer, G.L. University of Queensland; Australia Fil: Hatfield, J.L. USDA-ARS. National Laboratory for Agriculture and the Environment; Estados Unidos Fil: Jones, J.W. University of Florida. Department of Agricultural and Biological Engineering; Estados Unidos Fil: Kemanian, A.R. Pennsylvania State University. Department of Plant Science; Estados Unidos Fil: Kim, Sung Hyun. University of Washington. College of the Environment. School of Environmental and Forest Sciences; Estados Unidos Fil: Kiniry, J. United States Department of Agriculture. ARS; Estados Unidos Fil: Lizaso, J.I. Universidad Politécnica de Madrid. Departamento de Producción Vegetal; España Fil: Nendel, C. Leibniz Centre for Agricultural Landscape Research. Institute of Landscape Systems Analysis; Alemania Fil: Nielsen, R.L. Purdue University. Department of Agronomy; Estados Unidos Fil: Parent, B. INRA. Laboratory d’Ecophysiologie des Plantes sous Stress Environnementaux; Francia Fil: Stӧckle, C.O. Washington State University. Biological Systems Engineering; Estados Unidos Fil: Tardieu, F. INRA. Laboratory d’Ecophysiologie des Plantes sous Stress Environnementaux; Francia Fil: Thomison, P.R. Ohio State University. Department of Horticulture and Crop Science; Estados Unidos Fil: Timlin, D.J. USDA-ARS. Crop Systems and Global Change Lab; Estados Unidos Fil: Vyn, T.J. Purdue University. Department of Agronomy; Estados Unidos Fil: Wallach, D. INRA. Agrosystèmes et développement territorial; Francia Fil: Yang, H.S. Universidad de Nebraska - Lincoln. Department of Agronomy and Horticulture; Estados Unidos Fil: Tollenaar, M. The Climate Corp; Estados Unidos

The goal of a mechanistic model is to determine the outcome of weed–crop interference. An understanding of weed phenology is essential for construction of such models because phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to determine the influence of temperature and photoperiod on the phenological development of common lambsquarters. Common lambsquarters is a short-day species adapted to a temperature range of 6.5 to 44.5 C. Phenological development of common lambsquarters grown under a constant temperature of 20 C and an 8-h photoperiod was described in terms of biological days (Bd: chronological days at the optimum photoperiod and temperature). Three development phases of common lambsquarters were described as (1) a juvenile phase of 6.3 Bd, (2) a photoperiod-sensitive inductive phase of 8.2 Bd, and (3) a photoperiod-sensitive postinductive phase of 34.4 Bd. The photoperiod sensitivity of rate of development di...

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

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.

Mechanistic weed models focus on determining the outcome of weed and crop interference. An understanding of weed phenology is essential for simulation model development. Phenological development is a major factor determining the outcome of weed–crop competition. Growth cabinet studies were conducted to characterize the influence of photoperiod on the phenological development of redroot pigweed (Amaranthus retroflexus L.). Results indicated that redroot pigweed is a quantitative short-day species. Four development phases of redroot pigweed were described according to its response to photoperiod: (1) a juvenile phase of 1.1 d; (2) a photoperiod-sensitive inductive phase of 7.9 d; (3) a photoperiod-sensitive post-inductive phase of 39.9 d; and (4) a photoperiod-insensitive phase of 2.2 d. This information is useful for the development of mechanistic models and for comprehending the distribution and competitive ability of redroot pigweed with crops. The utilization of these results could help in predicting the phenological development of redroot pigweed. Key words: Phenology, mechanistic weed models, juvenile phase

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.

Understanding the environmental variables influencing the phenological development of weeds is essential for simulation model development. Temperature and photoperiod are important variables governing the phenological development of weeds. Growth cabinet studies were conducted to characterize the phenological development ofSetaria viridisin response to variations in temperature and photoperiod and to determine the duration of the juvenile phase and the effect of temperature and photoperiod on reproductive development.Setaria viridiswas adapted to a temperature range from 6.5 to 47 C. Phenological development ofS. viridiswas described accurately in terms of thermal days (cumulative day degrees above a base temperature) and biological days (Bd: chronological days at the optimum temperature and photoperiod). Four developmental phases ofS. viridiswere described: (1) a juvenile (photoperiod insensitive) phase of 2.6 Bd; (2) a photoperiod-sensitive inductive phase of 2.2 Bd; (3) a photoperiod-sensitive postinductive phase of 6.0 Bd; and (4) a photoperiod-insensitive inductive phase of 10.9 Bd. Photoperiod sensitivity ofS. viridisdid not differ with stage of development when expressed as a rate. Interpretation of constant sensitivity to photoperiod will simplify simulation of weed phenology in mechanistic models.

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.

Total number of initiated leaves and duration from sowing to silking increases when photoperiod is increased during the photoperiod-sensitive phase in maize ( Zea mays L.). Little is known, however, about possible other effects of photoperiod and incident photosynthetic photon flux density (PPFD) on rate of development and duration of life cycle. A study was undertaken to quantify effects of photoperiod and incident PPFD from sowing to the 15-leaf stage on rate of leaf appearance and duration of the grain-filling period. The short-season maize hybrid Pioneer 3902 was grown in growth cabinets from sowing to the 15-leaf stage with either (i) a 10 h photoperiod at high PPFD (650 μmol m −2 s −1 ), (ii) a 20 h photoperiod consisting of 10 h of high PPFD followed by 10 h of low PPFD (5–50 μmol m −2 s −1 ), or (iii) a 20 h photoperiod of high PPFD. From the 15-leaf stage to maturity the plants were placed under a 16 h photoperiod in a growth room. Increasing photoperiod from 10 to 20 h increased final number of initiated leaves and delayed silking but did not affect rate of leaf appearance. Doubling incident PPFD to a value similar to that under Ontario field conditions during the summer resulted in a 16% increase in rate of leaf appearance and in a significant increase in total number of initiated leaves. Differences in final number of initiated leaves and in rate of leaf appearance from sowing to the 15-leaf stage among treatments resulted in a 4-day difference in silking date between the 10 h photoperiod treatment and the two 20 h photoperiod treatments. Duration of the grain-filling period did not differ among the three treatments.

The main objective of the present work was to determine if chlorophyll fluorescence techniques could be used under field conditions to detect differences in leaf photosynthetic rates among commercial maize hybrids known to differ in agronomic performance. Chlorophyll fluorometry was used to estimate thylakoid electron transport rates (Je) in leaves of three commercial maize hybrids on 13 days between 48 days after sowing and first frost. The maximum quantum efficiency of thylakoid electron transport at low PPFD (φ), and the thylakoid electron transport rate when PPFD absorbed by leaves was equal to 1200 μmol m−2 s−1 (Je1200) were estimated using a curve-fitting approach. Canopy absorptance of incident PPFD (αc) and leaf absorptance of incident PPFD (αl) were measured at regular intervals. Significant fluctuations in both φ and Je1200 were observed over the season, and these appeared to be associated with different leaf temperatures on different measuring days. Measured values of Je1200 declined in a linear fashion over the season, but the decline was more rapid in an older hybrid than in two more modern hybrids. All three hybrids could be differentiated from one another on the basis of Je1200 measurements in the last part of the growing season. The two newer hybrids did not differ in their mean crop growth rates during this part of the season, however, despite the observed difference in Je1200. Regression analyses revealed a strong relationship between mean crop growth rates and αcJe1200, supporting the idea that chlorophyll fluorescence can be used to assess photosynthetic performance of maize under field conditions.

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.

Nitrogen (N) use efficiency (NUE) of fertilizer N can be accurately estimated by tracing the fate of soil applied labelled fertilizer, but the quantity of N remobilization from non‐kernel components into kernels in maize (Zea mays L.) plants is difficult to determine. A field experiment involving stem infusion with labelled 15N solution was conducted at Ottawa, Ontario (45°22'N, 75°43'W) for two years to determine whether stem infused 15N could be used to quantify N remobilization and the contribution of remobilized N to the grain. A current stay‐green commercial hybrid was grown at three fertilizer N rates and infused with 30 mL 15N solution [35.7 mmol N as 15NH4 15NO3 at 99.2 15N% atom enrichment (a.e.)] into the internode below the primary cob at anthesis. The control plants were infused with distilled water. Sampling occurred at 3 d, 2 wk and 5 wk after anthesis and at physiological maturity. Each plant was subdivided into ten components. Dry weight, total N concentration and 15N% a.e. were d...

The introduction of a more efficient means of measuring leaf photosynthetic rates under field conditions may help to clarify the relationship between single leaf photosynthesis and crop growth rates of commercial maize hybrids. A large body of evidence suggests that gross photosynthesis (AG) of maize leaves can be accurately estimated from measurements of thylakoid electron transport rates (ETR) using chlorophyll fluorescence techniques. However, before this technique can be adopted, it will first be necessary to determine how the relationship between chlorophyll fluorescence and CO2 assimilation is affected by the non-steady state PPFD conditions which predominate in the field. Also, it must be determined if the relationship is stable across different maize genotypes, and across phenological stages. In the present work, the relationship between ETR and AG was examined in leaves of three maize hybrids by making simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, both under controlled environment conditions and in the field. Under steady-state conditions, a linear relationship between ETR and AG was observed, although a slight deviation from linearity was apparent at low AG. This deviation may arise from an error in the assumption that respiration in illuminated leaves is equivalent to respiration in darkened leaves. The relationship between chlorophyll fluorescence and photosynthetic CO2 assimilation was not stable during fluctuations in incident PPFD. Since even minor (e.g. 20%) fluctuations in incident PPFD can produce sustained ( > 20 s) departures from the mean relationship between ETR and AG, chlorophyll fluorometry can only provide an accurate estimate of actual CO2 assimilation rates under relatively stable PPFD conditions. In the field, the mean value of ETR / AG during the early part of the season (4.70 ± 0.07) was very similar to that observed in indoor-grown plants in the vegetative stage (4.60 ± 0.09); however, ETR / AG increased significantly over the growing season, reaching 5.00 ± 0.09 by the late grain-filling stage. Differences in ETR / AG among the three genotypes examined were small (less than 1% of the mean) and not statistically significant, suggesting that chlorophyll fluorometry can be used as the basis of a fair comparison of leaf photosynthetic rates among different maize cultivars.