Hyperspectral remote sensing of the impact of environmental stresses on nitrogen fixing soybean plants (Glycine max L.)

The influence of the environmental stress factors, salinity and enhanced UV-B radiation, on young nitrogen fixing soybean plants (Glycine max L.) was investigated by using hyperspectral reflectance data. Soybean is the leading oilseed crop produced and consumed worldwide. The soybean plants were grown in a growth clamber as water cultures on Helrigel nutrient solution. Three day's seedlings were inoculated with suspension of Bradyrhizobium japonicum strain 273. Salinity was performed at growth stage of 2^nd-4^th expanded leaves by adding of NaCl in the nutrient solution in concentrations 40 mM and 80 mM. Plants were divided into six groups. The first three groups consisted of untreated (control) and treated only with two NaCl concentrations plants. The other three groups (control and salinized) on the 14th day after the treatment were illuminated for four hours with UV-B radiation at intensity 64.4 μmol m^-2 s^-1. Spectral reflectance was registered by a portable fiber-optic spectrometer in the visible and near infrared (NIR) spectral ranges (450-850 nm). Data were subjected to statistical analysis through the Student's t-criterion in four spectral ranges: green, red, red-edge and NIR (520-580 nm; 640-680 nm; 690-720 nm; 720-780 nm). The results from spectral reflectance and biochemical analysis (evaluated stress markers) revealed that both treatments (salinity and salinity + UV-B radiation) bring the plants to stress and to decline of the biological nitrogen fixation. The UV-B treatment decreases the salinity action and partly restores the physiological state of the plants.