Humic substances (HS) are the most important natural biostimulant of plants. However, the relationship between their structure and biological activity in plants is still not well recognized. The objective of this paper was to assess the influence of molecular fractions of humic acids (HA) (HA < 30 kDa and HA > 30 kDa) on reducing negative effects of drought stress in soybean (Glycine max (L.) Merr.) seedlings of Progres and Nawiko cultivars. Drought stress was induced in laboratory conditions by the addition of polyethylene glycol 6000 (PEG 6000) to make a water potential of −0.5 MPa. HA were extracted according to the International Humic Substances Society procedure, and then were separated into two molecular fractions by membrane filtration. The following physiological and biometric parameters were determined: chlorophyll content, photosynthesis activity, electrical conductivity, fresh and dry mass of overground and roots, and plant length. The enzyme activity and ion contents were also measured. Differences in response to drought stress with the addition of HA < 30 kDa and HA > 30 kDa or not to the Hoagland’s solution were observed among studied cultivars. Drought stress caused a decrease in the most physiological parameters and increase in peroxidase activity in the case of both studied cultivars. However, the results of biometric measurements showed that the Progres cultivar appears to have better tolerance to drought stress. The significant influence of water deficit on most macroelement content in dry matter leaves of both studied cultivars was not observed, while its effect on microelement uptake by soybean plants was concluded. In the case of the Progres cultivar, the results showed a significant decrease in microelement content in the dry matter of leaves, whereas in the leaves of Nawiko cultivar there was a significant increase. The influence of HA > 30 kDa and HA < 30 KDa fractions on physiological features of both studied cultivars was varied. HA > 30 kDa fraction better up-regulated the antioxidant defense system. Unfortunately, no effect of either HA fraction on the macro- and micronutrients uptake system of both studied cultivars was observed.

In the present study, the influence of humic acid (HA) molecular fractions (HA < 30 kDa and HA > 30 kDa) on enhancing the tolerance of seedlings of soybean [Glycine max (L.) Merr.] Progres and Nawiko cultivars to salt stress (50 mM NaCl) was investigated. HA were extracted from mountain fen soil and then were separated into two molecular fractions by membrane filtration and characterized by diffusion coefficient (Dapp), electrolytic conductivity (κ) and electrophoretic mobility (Ue). The following biometric parameters of tested plants were determined: total leaf area, height of plants, fresh and dry mass of the over ground part and roots as well as length of shoots cells and length of stomas. The chlorophyll content in ground tissue as well as the macro and microelements content in tested plants also were determined. The results showed that the κ, Ue and Dapp for HA > 30 kDa were lower than these for HA < 30 kDa. Adding NaCl caused increase κ and decrease Dapp and Ue. The salt stress caused a major decrease in biometric parameters in tested plants. HA > 30 kDa reduced the uptake of macro and microelements in the soybean Progres cultivar. In soybean Nawiko cultivar, it caused significant uptake of Fe and Zn. Soybean cultivars showed strong reaction to salt stress. HA molecular fractions reduced or eliminated the influence of the salt stress. However, HA > 30 kDa was more effective than HA < 30 kDa, due to its properties.

A b s t r a c t. This paper presents results of a study on the influence of 0-150 mmol NaCl dm -3 Hoagland solution on growth, chlorophyll content, photosynthesis and frost resistance of seedlings of three winter cereals: wheat -cv. Almari, rye -cv. Amilo, and triticale -cv. Tornado. Sodium chloride at 25 mmol dm -3 caused better growth of wheat shoots and roots, both of fresh and dry matter. Higher concentrations of NaCl in the medium decreased the biomass of the tested seedlings. The influence of NaCl on the chlorophyll content in the seedlings varied. The conductometry method showed that the resistance of the cell walls of wheat and rye to low temperature decreased in the presence of NaCl in the growth medium. Luminescence has shown that seedlings that grew in NaCl-containing medium indicated an impediment of electron flow at a lower temperature than the control plants.

Plants of soybean (Glycine max L.) cv. Augusta grew in the soil with addition of Ni in the concentration of 80 (C-80) or 120 mg•kg -1 dry mass (C-120) or without Ni (C-0), and they were subjected to UV-B (300 nm -315 nm) dose of 1.8 kJ m -2 d -1 for 28 days. The addition of nickel into soil in the concentration of 120 mg kg -1 (C-120) caused a decrease of the plant height by 35% compared to the control (C-0), and in the presence of UV-B (U-120) -by 43%. The shoot fresh and dry mass in the C-80 and C-120 plants were lower by 33% and 52% than in the C-0. In the presence of Ni, the intensity of net photosynthesis decreased by 55%. UV-B caused an increase of flavonoid content by 25% compared to the control (C-0), and Ni induced a reduction in the content of these compounds from 20% to 40%.

A laboratory research was carried out to examine the impact of humic acids (HA) on swelling and germination of ‘Nawiko’ and ‘Progres’ soybean seeds under salt stress (50 mM dm⁻³ NaCl) and water deficit stress (−0.5 MPa) induced by polyethylene glycol (PEG) 6000. HA in the form of a dry preparation was obtained from peat using the IHSS method. Tests on swelling and germination used non-fractionate preparation (NFHA) and two of its molecular fractions obtained using Millipore filters with a 30 kDa cut-off point. This enabled us to obtain two fractions: with a higher molecular weight, above 30 kDa (HMHA), and a lower molecular weight, below 30 kDa (LMHA). The carbon concentration in HA solutions, used in all tests, was 0.005 g C<span>_HA</span> dm⁻³. The results showed that HA mitigate the negative impact of salinity and water deficit on swelling and germination of soybean seeds.