Humic acids mitigate salt and drought stress in soybean (Glycine max (L.) Merr.) in vitro cultures

The aim of this study was to evaluate the protective effect of humic acids (HA) with different molecular weight fractions on the soybean Progres cultivar under drought and salinity stress in in vitro. HA were isolated from peat samples according to the International Humic Substances Society procedure. Three HA treatments were tested: HA < 30 kDa, HA > 30 kDa, and unfractionated HA (Mix). Sterilized soybean seeds were cultured on nutrient media supplemented with 100 mmol·dm⁻³ NaCl or 150 mmol·dm⁻³ Mannitol to simulate salinity and drought stress, respectively. HA fractions were added at 0.005 g CHA·dm⁻³. No stress factors were used in the control samples. Biometric parameters (plant height, leaf number, root length, shoot and root biomass) and micromorphological traits (stomatal density and length) were measured. Micro- and macroelement contents in dry seedling matter were also analyzed. Soybean Progres exhibited greater sensitivity to salt than drought stress, shown by reduced biometric and micromorphological parameters and altered element contents. HA treatments demonstrated a protective role, which was dependent on the molecular fraction. The HA > 30 kDa fraction and HA Mix provided the greatest protective and, at times, stimulatory effects, notably increasing stomatal density and biometric values under stress. HA, in the presence of salt and drought stress, did not specifically affect the uptake of the analysed micro- and macroelements by soybeans. The fractions HA < 30 kDa and HA > 30 kDa caused a decrease in the uptake of most analyzed elements. Unfractionated HA predominantly mitigated the effects of applied stresses. For HA Mix, the levels of micro- and macroelements in soybean seedlings were generally comparable to those in control plants. This in vitro study demonstrates that humic acids, particularly those with a molecular weight greater than 30 kDa and unfractionated fractions, protect soybean Progres seedlings from drought and salinity stress by enhancing growth, stomatal traits, and nutrient balance.