Synergistic Effects of Microbial Biostimulants and Calcium in Alleviating Drought Stress in Oilseed Rape

The study aimed to examine the changes in winter oilseed rape (Brassica napus L.) under simulated prolonged drought and to assess the effects of a microbial biostimulant ProbioHumus and calcium, individually and in combination, in order to improve the plant's drought resistance and to identify the biochemical processes occurring in the plant tissues. The oilseed rape cv. 'Visby' was grown under controlled laboratory conditions. CaCO₃ (hereafter, Ca) (3.71 g) was added to the soil of one pot at 70 g m^-2. Seedlings at the 3-4 leaf stage were sprayed with ProbioHumus 2 mL 100 mL^-1 and exposed to drought for 8 days to achieve a high water deficit. Irrigation was then resumed, and recovery was assessed after 4 days. The data showed that the microbial biostimulant alleviated the physiological and biochemical response of oilseed rape to drought stress. ProbioHumus + Ca reduced plant wilting by increasing leaf relative water content (RWC) by 87% and induced drought tolerance by increasing endogenous proline content 4-fold, increasing photosynthetic pigment content in leaves by 10-28%, reducing H₂O₂ by 53% and malondialdehyde (MDA) by 45%, and stimulating stomata opening (by 2-fold on the upper and 1.4-fold in the lower leaf surface), vs. drought control. The most effective measure to increase plant survival and/or resume growth after drought was the application of a microbial biostimulant with additional calcium to the soil. The practical implications of this research point to the potential benefits of applying these ecological measures under field conditions.