Canopy photosynthesis (CO2 fixation expressed on a ground area basis) (Wells, 1991) and crop growth rate (an indirect estimate of canopy photosynthesis) (Shibles and Weber, 1965; Buttery, 1969; Board and Harville, 1994) are directly related to LAI and radiation interception during the initial stages of vegetative growth. Canopy photosynthesis, therefore, increases rapidly during early vegetative growth (Larson et al., 1981; Christy and Porter, 1982; Acock et al., 1985) and reaches its maximum level when radiation interception is complete. There is no evidence for an optimum
LAI in soybean (Shibles and Weber, 1965); consequently, there is no change in canopy photosynthesis (or crop growth rate) as LAI increases beyond the level required for maximum radiation interception. Canopy photosynthesis, crop growth rate and ultimately yield will be reduced if the canopy does not reach maximum interception by the beginning of reproductive growth (Lee et al., 2008). Reaching maximum interception before flowering does not contribute directly to yield, but it may aid weed control (Buhler and Hartzler, 2004). Jiang and Egli (1995) found that stress that reduced vegetative growth before growth stage R1 had no effect on yield if there was enough LAI to ensure maximum radiation interception by the beginning of flowering.
The rate of evapotranspiration follows the increase in LAI and radiation interception, especially when a dry soil surface limits soil evaporation (Heatherly and Elmore, 2004). Consequently, the rapid early development of leaf area associated with narrow rows and high populations may increase evapotranspiration (Heatherly and Elmore, 2004).
Radiation-use efficiency (RUE; dry matter produced per unit of intercepted solar radiation) is often used to evaluate the productivity of crop communities since it represents an estimate of the efficiency with which the community converts solar radiation into dry matter (Sinclair and Muchow, 1999). It seems to provide a simple characterization of productivity, but estimates are often quite variable since they require determination of dry matter accumulation and radiation interception (Sinclair and Muchow, 1999), making it difficult to detect small differences between treatments or cultivars.
RUE is sensitive to any variation in photosynthesis, including those caused by environmental conditions (e.g. temperature, water stress, nutrient availability) and plant species. Since soybean is a C3 legume that produces leaves with high protein levels and seeds with a high energy content, the maximum RUE (i.e. measured under non-stress conditions) is less than in many other crops. Sinclair and Muchow (1999) concluded after an extensive review that the average RUE for soybean is 1.02 g MJ-1 (based on total solar radiation), which is lower than estimates for maize (1.6-1.7 g MJ-1) and wheat (Triticum species) (1.4-1.7 g MJ-1).
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