Response of Winter Wheat (<i>Triticum aestivum</i> L.) to Selected Biostimulants under Drought Conditions

To prevent the staggering degradation of the environment, restrictions in the use of plant protection products and fertilizers are being strengthened every year. Therefore, methods for improving plant tolerance to unfavorable environmental conditions are sought to positively affect both plants and the natural environment. Here, we evaluated and compared the efficacy of four commercial biostimulants on the tolerance of winter wheat to drought stress. The effects of the following biological agents: <i>Bacillus</i> sp., soil bacterial strains, free amino acids, and humic substances on winter wheat were assessed in a pot experiment under full hydration soil moisture and under drought. Among the studied biostimulants, the two based on bacterial strains had the strongest beneficial effects on improving the tolerance of wheat plants to drought. In plants treated with either of these two, the highest level of CO₂ assimilation was recorded under drought. Moreover, in the same plants, the decrease in transpiration value due to drought was the smallest. The highest stomatal conductance under drought was also noted in these same plants. The results of chlorophyll fluorescence also indicate the smallest damage to the photosynthetic apparatus in the plants on which these bacterial biostimulants were used. Under drought, the lowest initial fluorescence values were noted for these bacterial preparations, as were the highest values of maximum fluorescence. On the other hand, a parameter indicating stress was reduced due to drought in all plants, except for those treated with one of these preparations. Another parameter showing the efficiency of the use of light photons in the photosynthesis process increased only in plants treated with one of these preparations, whereas for other plants it decreased due to drought, with the smallest decrease observed in plants treated with the other preparations. The most effective work of the photosynthetic apparatus in such treated plants was observed by the fastest transport of electrons through photosystems under drought. Additionally, under drought, the highest grain yield was obtained in plants treated with one of these bacterial preparations. The drought stress resistance index indicated that among all tested formulations, plants treated with either of these bacterial preparations scored the best. The use of these two biostimulants is recommended for comparative efficacy studies in the field, to help combat the drought-related yield losses of wheat.