Chlorophyll fluorescence of nitrogen fixing soybean plants (Glycine max L.) under stress conditions

Chlorophyll fluorescence technique was applied on young nitrogen fixing soybean plants (Glycine max L.) for detecting and assessment of the effects of two environmental stress factors - salinity and enhanced UV-B radiation. Biological nitrogen fixation by legume - Rhizobium symbiosis is important to agricultural productivity and is therefore of great economic interest. Soybean and soy foods attract worldwide attention for their potential health benefits and uses in food manufacturing. Soybean plants were grown under controlled conditions as water cultures on Helrigel nutrient solution. Three days seedlings were inoculated with suspension of Bradyrhizobium japonicum strain 273. Salinity was performed at the stage of 2^nd - 4^th trifoliate expanded leaves by adding of NaCl in the nutrient solution in concentrations 40 mM and 80 mM. On the 14^th day after the salinity treatment the half of the plants was treated with UV-B radiation at intensity of 64.4 μmol m^-2 s^-1 for four hours. Measurements were conducted on the same day. Hyperspectral fluorescence data were collected by a portable fiber-optic spectrometer in the visible and near infrared spectral ranges (600-900 nm). The spectral and statistical analysis was performed on fluorescence spectra normalized against the second maximum at five characteristic wavelengths. The statistical significance of the differences at p<;0.05 between data means of the control and treated plants were ascertained by the Student´s t-criterion. In the case of only salinity treatment the differences were statistically significant for both NaCl concentrations at all wavelengths with the exception of the rear slope. For the combined treatment (salinity + UV-B radiation) the differences in the spectra are non-significant at two of the wavelengths (in the middle of forefront and rear slope).