Abstract: Based on the measurement of photosynthetically active radiation (PAR) parameters in soybean and corn canopies as well as the calculation of reflectance and transmittance, the diurnal changes of the parameters and their affecting factors were studied in this paper. The results showed that the fraction of photosynthetically active radiation (FPAR) in the whole day was approached to a constant, especially at 8:00-11:00 and 14:00-16:00. The incidence curve of PAR in soybean canopy was smoother in sunny day than in cloudy day, but the reflectance and transmittance curves were not. Due to the absorbing and dispersing of cloud, the maximum PAR occurred one hour later in cloudy day than in sunny day. The spatial heterogeneity in vegetation canopy affected the parameters significantly, which had larger differences between different crops. Soybean canopy had a smaller spatial heterogeneity, and thus, the curves of its PAR parameters were much smoother. When the angle between linear quantum sensor and solar incidence projection was 30 degree, the bias extent of measured PAR parameters was the least, being 0.657%.

Key words: Poplar, Salinity stress, Growth, Enzyme activity