LIGHT INTERACTIONS WITH SOYBEAN CANOPIES (REMOTE SENSING, REFLECTANCE)

Research investigating interactions with solar radiation and its soybean growth and development has been limited. In light of this, the main thrust of these experiments was to quantify the effect of solar radiation intercepted by soybean plants. Solar radiation, particularly photosynthetically active radiation (PAR; 0.4 to 0.7 (mu)m) is the energy source used for plant growth. PAR is mainly intercepted by green leaves, the ratio of the area of green leaves to the area of soil is called the leaf area index (LAI). A three year study was conducted to (1) examine the effects of different cultural practices on the efficiency of converting PAR into dry phytomass, (2) develop a fast and accurate method of estimating LAI, and (3) develop models for estimating LAI and Intercepted PAR (IPAR) from spectral reflectance measurements. Soybeams were planted in May and mid June at 38 and 76 cm row widths in 1982 and at three row widths (18, 38, and 76 cm) in 1983 and 1984. Transmitted PAR (TPAR) was measured whenever clear sky conditions prevailed during both growing seasons. IPAR was calculated as 1.0 minus TPAR. The first study determined that energy intercepted from plant emergence to physiological maturity was significantly greater for soybeans planted in late May in narrow row widths than any other planting date/row width combination under investigation. The mean amount of intercepted energy at solar noon accumulated from emergence to physiological maturity, was highly correlated (r(\u272) = 0.86) with dry phytomass of soybeans. In the second study a fast and accurate method of estimating LAI from TPAR data was examined. Six models were investigated, the models developed using 57.5 (+OR-) 10(DEGREES) of sun zenith angle were superior. In the final study LAI could be reliably estimated ((+OR-)7%) using a ratio of the infrared to red spectral reflectance measurements. However, IPAR could be more accurately estimated ((+OR-)4%) using simulated Thematic Mapper reflectance data expressed through the greenness transformation