Spectral estimates of absorbed radiation and phytomass production in corn and soybean canopies

Numerous studies have reported a linear relation between phytomass production and absorbed photosynthetically active radiation (APAR) for a wide range of plant species. Related work has shown that APAR may be estimated from multispectral (visible and near infrared) remotely sensed observations. The combination of these two concepts may provide the basis for development of physically-based agronomic monitoring systems. These concepts are subjected to examination for two primary, mid-latitude crops; corn (Zea mays L.) and soybeans (Glycine max Merr.). Phytomass, green leaf area index (LAI), absorbed PAR, and multispectral reflectance factors of corn and soybean were measured periodically from planting to mid-grain fill in two growing seasons. As green LAI increased, the fractional APAR asymptotically approached a maximum value between 0.95 and 0.97. Fractional APAR displayed a linear relation to the normalized difference vegetation index (NDVI), which is relatively independent of species, throughout the growing seasons. However, deviations in the relation were observed between pre- and post-onset of senescence. For both corn and soybean, a linear relation between cumulative APAR and cumulative aboveground phytomass production was found. However, the rate of accumulation per unit APAR was more than twice as great for corn as for soybean. This agrees with previous reports comparing C4 grasses and C3 legumes. These results indicate that remotely sensed measurements contribute valuable information concerning energy / mass accumulation in plant canopies. However, implementation of this approach in crop monitoring will clearly require a capability to discriminate between, at least, corn and soybean. The influence of stress events, such as drought, nutrient limitations, and disease, will also require further consideration.