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In Canada, yield of short‐season soybean [Glycine max (L.) Merr.] cultivars has increased by approximately 0.5% per year since they were first cultivated in the early 1930s. Future yield gains may be dependent on an understanding of the changes made to soybean cultivars by breeding and selection. Our objective was to examine physiological differences associated with seed yield increase within a group of historical cultivars. At Ottawa, Ontario, we grew 14 cultivars representing seven decades of breeding and selection (1934–1992) in a randomized complete block design with four replications, across 4 years. Growth analysis provided data on leaf area and dry weight. Photosynthetic rate per leaf area was measured at several stages of development each year. Yield and harvest index were determined at maturity. The number of days to maturity and the total plant dry weight were not affected by the year of cultivar release. Seed yield, harvest index, and photosynthetic rate were found to have increased by 0.5% per year, while leaf area index decreased by 0.4% per year. The increase in seed yield with year of release was significantly correlated with an increase in harvest index, photosynthesis, and stomatal conductance and a decrease in leaf area index. Today's cultivars are more efficient at producing and allocating carbon resources to seeds than were their predecessors.

Remote-sensing techniques, in particular, multispectral visible and infrared (IR) reflectance, can provide Correlations between plant canopy reflectance and aboveground an instantaneous, nondestructive, and quantitative asbiomass can possibly be used for early prediction of crop yield. Field experiments were conducted in 1998 and 1999 on two soil types to sessment of the crop's ability to intercept radiation and assess whether measurements of canopy reflectance at given stages photosynthesize (Ma et al., 1996). The input of reflecof development could be used to discriminate high from low potential tance into yield production models has been shown to yields among genotypes with known differences in potential grain improve yield estimates (Clevers et al., 1994; Clevers, yield and whether a consistent relationship between yield and canopy 1997). Colwell (1956) was the first to use aerial IR phoreflectance could be used for screening and predicting soybean [Glytographs to monitor plant disease in the field. The cine max (L.) Merr.] yield in a variety trial. A 3-by-42 factorial experiamount of reflectance in the near IR (NIR) range ( ϭ ment, arranged in a randomized complete block design with three 700-1300 nm) is determined by the optical properties replications, was used on each soil type for both years. Three populaof the leaf tissues: their cellular structure and the air-cell tion densities (25, 50, and 75 seeds m Ϫ2 ) represented low, optimum, wall-protoplasm-chloroplast interfaces (Kumar and Silva, and high levels. Forty-two historical varieties represented nearly six decades (1934-1992) of soybean yield improvement in Canada. Can-1973). These anatomical characteristics are affected in opy reflectance was measured with a hand-held multispectral radiome-turn by environmental factors such as soil water and/or ter on three sampling dates (approximately R2, R4, and R5 stages) nutrient status (Gausman et al., 1969; Thomas et al., for each site. Grain yield at harvest was measured. Soybean grain yield 1971; Blackmer et al., 1994), soil salinity (Gausman and was highly positively correlated with canopy reflectance, expressed as Cardenas, 1968), and leaf age (Gausman et al., 1970).normalized difference vegetation index (NDVI), at all sampling dates. Reflectance in the visible red (R) range ( ϭ 550-675 Regression analyses showed a positive relationship between NDVI nm) has been used to estimate leaf chlorophyll and and grain yield, with R 2 up to 0.80 (P Ͻ 0.01) and progressive imcarotenoid (Benedict and Swidler, 1961; Thomas and provement from R2 to R5 stages. Population density did not affect Oerther, 1972;Filella et al., 1995) levels and, by extenthe yield-NDVI relationship at the development stages studied. Our sion, the photosynthetic capability of the crop. data suggest that canopy reflectance measured nondestructively between R4 and R5 stages adequately discriminates high-from low-The use of NIR or R spectral bands singly does not yielding genotypes and provides a reliable, fast, repeatable indicator account f...

Soybean [Glycine max (L.) Merr.] production in short‐season areas has increased greatly because of improvements in cultivars and production practices. Previous studies of genetic improvement in soybean have reported yield increases of 0.5 to 1% per year. To evaluate the genetic improvement of short‐season germplasm, 41 cultivars ranging from maturity group 000 to 0, released from 1934 to 1992, were grown for 8 station‐years in Ontario and Quebec. Yield, maturity, plant height, lodging score, 100‐seed weight, seed protein and oil levels, and yield stability were regressed on year of release to determine if improvements have been made. Yield has been improved about 0.5% per year during the period under study; however, since 1976 yield has been improved about 0.7% per year. There is evidence that the rate of genetic improvement of seed yield is accelerating. Significant lodging reduction was also observed. Seed protein levels have been reduced 4 g kg‐1 yr‐1 and seed oil levels have increased 4 g kg‐1 yr‐1. Although yield has been increased, yield stability has remained constant.

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breeding in short-season regions resulted in a yield improvement of 0.5% per year with an associated decrease Yield progress of short-season soybean [Glycine max (L.) Merr.] in protein concentration and some improvement in lodgcultivars in Canada has been approximately 0.5% per year since the early 1930s. Our objective was to identify changes in agronomic traits ing tolerance. In a related experiment, we grew 14 cultiassociated with yield increase within a selection of historical cultivars.vars selected from the original 41 to examine physiologi-Where applicable, we measured phenotypic stability of these traits.cal changes from 58 yr of genetic improvement (Mor-At Ottawa, ON, we grew 14 cultivars, representing seven decades of rison et al., 1999). We found that increased seed yield breeding and selection , in a randomized complete block was significantly correlated with decreased leaf area and design with four replicates, across 6 yr. Data were collected on seed increased photosynthetic and stomatal conductance yield, seed weight, plant height, plant population, lodging susceptibilrates per unit leaf area. ity, and foliar disease symptoms. Seed number per plant was calculatedOur objective in the current study was to examine from yield, seed weight, and plant population. Seed protein and oil the changes in agronomic traits associated with 58 yr of concentration were measured. The increase in seed yield with year soybean breeding in short-season cultivars. Specifically, of release was associated with a significant increase in the number of seeds produced per plant. There was no relationship between seed we examined plant height, lodging score, foliar damage yield and seed weight. A significant decrease in seed protein concen-score, plant stand, seed weight, seeds per plant, seed tration with year of release was offset by a significant increase in seed yield, and protein and oil concentration. We determined oil concentration. Newer cultivars were more phenotypically stable the phenotypic stability of these agronomic traits.for plant height than older cultivars. Modern cultivars were more efficient at establishing, supporting, and filling seeds on a per-plant MATERIALS AND METHODS basis than older cultivars.Abbreviations: CV, coefficient of variability; MG, maturity group.

Nitrogen fertilization is considered as an important source of atmospheric N 2 O emission. A seven site-year on-farm field experiment was conducted at Ottawa and Guelph, ON and Saint-Valentin, QC, Canada to characterize the affect of the amount and timing of N fertilizer on N 2 O emission in corn (Zea mays L.) production. Using the static chamber method, gas samples were collected for 28-days after preplant and 28-days after sidedress fertilization at the seven site-year, resulting in 14 monitoring periods. For both methods of fertilization, peak N 2 O flux and cumulative emission increased with the amount of N applied, with rates ranging from 30 to 900 lg N m À2 h À1 . Depending on N amount and time of application, cumulative emission varied from 0.05 to 2.42 kg N ha À1 , equivalent to 0.03% to 1.45% of the N fertilizer applied. Differences in N 2 O emission peaks among fertilizer treatments were clearly separated in 13 out of 14 monitoring periods. Total N 2 O emissions may have been underestimated compared with annual monitoring in 10 out of the 49 cases because the monitoring period ended before N 2 O efflux returned to the baseline level. The flux of N 2 O was negligible when soil mineral N in the 0-15 cm layer was o20 mg N kg À1 . While rainfall stimulated emission, soil temperature 415 1C was likely the driving force responsible for the higher levels of N 2 O found for sidedress than preplant application methods. However, caution must be taken when interpreting these later results as preplant fertilization may have continuously stimulated N 2 O emissions after the 28-days monitoring period, especially in situations where N 2 O effluxes have not fallen back to their baseline levels. Increasing fertilizer rates from 90 to 150 kg N ha À1 resulted in slight increases in yields, but doubled cumulative N 2 O emissions.

Assessment of crop N requirements is necessary to develop production systems with optimal N input. A field experiment with six maize (Zea mays L.) hybrids grown at three N fertilizer rates (0, 100, and 200 kg N ha−1) was conducted on a well‐drained sandy loam of the Grenville series (coarse‐loamy, mixed, mesic Typic Eutrochrepts) on the Central Experimental Farm at Ottawa, ON, in Canada (45°23' N, 75°43' W) for 3 yr (from 1991 to 1993) to evaluate whether canopy reflectance and greenness can measure changes in maize yield response to N fertility. Canopy reflectance, leaf area and greenness were measured on 11 dates from 4 wk before to 4 wk after anthesis. Grain yield at harvest was also measured. Direct radiometer readings at the 600‐ and 800‐nm wavelengths or a derived normalized difference vegetation index [NDVI = (800 nm − 600 nm)/(800 nm + 600 nm)] best differentiated N and hybrid treatments at most sampling dates. Canopy light reflectance was strongly correlated with field greenness at almost all growth stages (field greenness being a product of plant leaf area and leaf greenness measured with a chlorophyll meter, in this case a SPAD‐502). Both canopy light reflectance and field greenness measured preanthesis were correlated with yield at harvest. Light reflectance measured after anthesis differentiated hybrid differences in leaf senescence. Our data suggest that light reflectance measurements prior to anthesis may predict grain yield response and provide in‐season indications of N deficiency.