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 (D_app), electrolytic conductivity (κ) and electrophoretic mobility (U_e). 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, U_e and D_app for HA > 30 kDa were lower than these for HA < 30 kDa. Adding NaCl caused increase κ and decrease D_app and U_e. 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.

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%.

Environmental factors, such as light and drought, affect plant growth and functioning both independently and through their mutual interactions. The aim of this study was to evaluate the effects of three levels of photosynthetic photon flux density (PPFD: 400, 800, and 1200 μmol·m⁻²·s⁻¹) on the growth and physiological responses of spring barley (Hordeum vulgare L.) seedlings of the Ella and Kucyk cultivars grown under control conditions or PEG 6000-induced osmotic stress (−0.5 MPa), applied to simulate water-deficit conditions. After 28 days of the experiment, the fresh and dry mass of shoots and roots, SPAD index, electrolyte leakage (EL), chlorophyll fluorescence parameters, and delayed chlorophyll luminescence parameters were determined. PEG-induced osmotic stress significantly reduced shoot and root fresh mass in both cultivars, whereas its effect on dry mass accumulation was less pronounced. Greater shoot and root dry mass was observed at higher PPFD levels under both the control and stress conditions. Most measured traits varied among PPFD treatments, whereas osmotic stress primarily affected biomass-related parameters. Significant PPFD × drought interactions were detected for root fresh mass and Ft/F₀ in cv. Ella, and for root fresh mass, root dry mass, SPAD, EL, Y(II), and Ft/F₀ in cv. Kucyk. SPAD values were generally higher under stress conditions in cv. Kucyk, while remaining relatively stable in cv. Ella. Most chlorophyll fluorescence parameters, including Fv/Fm, Y(II), and Rfd, remained relatively stable across treatments. Ft/F₀ and the NDS parameter of delayed chlorophyll luminescence showed the greatest variation among the applied PPFD treatments. Overall, treatments characterized by higher PPFD were associated with greater biomass accumulation, while physiological performance remained largely stable under PEG-induced osmotic stress. However, the magnitude and nature of the responses depended on both cultivar and trait.