International audienc

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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