**3.1 Plant material**

The experiments were carried out on young soybean plants (*Glycine max L., cultivar Pavlikeni 101)*. They were grown as water cultures in a growth chamber under controlled conditions (16/8 photoperiod day to night, photon flux density 90 mol m-2 sec-1, humidity 60-70%, and temperature 25 1 C). Soybean seeds were surface sterilized with 70% ethanol and washed afterwards several times in distilled water. Then, they were let to germinate on a damp filter paper at 24 °С for three days in dark. On the 4th day seeds were transferred in plastic vessels with half strength well aerated Helrigel nutrient solution (Helrigel, 1898) and were put to growth clamber.
