Effects of Temperature and Photoperiod on Development Rates of Nine Soybean Varieties in the Mississippi Valley

Time to flowering in soybeans Glycine max (L.) Merr. is affected by environmental conditions, temperature and photoperiod being the leading environmental factors. Most of available experimental data in the Mississippi Valley indicate midand late spring plantings. Planting dates in soybean crops vary significantly, and late plantings are not uncommon, especially in years with extreme spring weather events. The development rates of soybean are cultivar specific but also affected by temperature and photoperiod. The objective of this study was to quantify differences in development rates in field soybean crops encountering different daylength patterns. Nine soybean varieties were planted on three different dates in two soil types. Each treatment had five replications. Phenological observations and their quantitative analysis confirmed the earlier proposed hypothesis by Acock et al. (1997) that the daily increment in reproductive stage could be simulated as a linear function of photoperiod with slopes of these linear functions different before and after solstice. INTRODUCTION Soybean yields are highly sensitive to the length and timing of pod-filling period (Egli et al., 1984). Since Garner and Allard (1920) discovered photoperiodism, agricultural researchers made many attempts to account for photoperiodism when predicting the stages of reproductive development. To study the effects of management practices, soil properties and weather on the developmental rates, we need a reliable understanding of how environmental factors affect soybean ontogeny. Also, there is a need to predict growth stages for use in some irrigation-scheduling programs (Foroud et al., 1993; Specht et al., 1989). To quantify plant ontogeny means to define stages of development and to describe progress between the stages as a function of environmental factors (Acock et al., 1997). For soybean crops, the progress between stages is most often described as a continuous progression (Hodges and French, 1985; Summerfield et al., 1993; Sinclair et al., 1991). Summerfield et al. (1993) concluded that temperature and photoperiod are the leading factors in determining the rate of progress towards flowering. Photoperiod affects significantly on floral induction (Hicks, 1978). Several authors have also observed an effect of photoperiod on seed-filling rates (Grimm et al., 1994; Seddigh et al., 1989; Thomas and Raper, 1976), but some cultivars did not demonstrate this effect (Johnson et al., 1960). It has been also noticed that the rates of development depend on whether the daylength is increasing or decreasing (Constable and Rose, 1988; Lawn and Byth, 1972). The responses of soybean developmental rates to environmental factors are cultivar-specific (Constable and Rose, 1988; Grimm et al., 1994; Summerfield et al., 1993). The equations expressing these responses can be the same for several cultivars, but with different coefficients for each cultivar. These coefficients can be viewed as cultivarspecific parameters (Reddy et al., 1995). The southern USA is an important soybean growing region. During the period between 1991 and 1995, data on soybean growth and development were collected in the Mississippi Valley in seven southern states to validate the soybean crop simulator GLYCIM (Acock and Trent, 1991). These data could be used to quantify relationships between environmental variables and developmental rates for several soybean cultivars. The objective of this study was to quantify differences in Proc. 4 IS on Cropmodels Eds. J.H. Lieth & L.R. Oki Acta Hort. 593, ISHS 2002 202 developmental rates in field soybean crops encountering different daylength patterns. MATERIAL AND METHODS