## 3 Results and Discussion  
### Grain Yield and Protein Content  
The lowest yield obtained in 2018 results from the unfavourable rainfall distribution during this growing season (Figure 1). Excessive rainfall during the sowing and emergence of wheat in 2017 (September-October) might have contributed to the poor rooting of the plants, which could have resulted in lower yields in 2018. Furthermore, low rainfall duringwinter (February-March 2018), as well as water shortages during the intensive growth (May-June 2018), adversely affected the growth and development of the plants. During this period, the unfavourable water balance was exacerbated by the occurrence of higher temperatures. A reduced yield is a typical response to water stress, as both the photosynthetic intensity and the plant growth processes are reduced [53; 54; 55; 56; 57]. High temperatures combined with low rainfall increase the loss of nutrients (N, P, K), particularly in poor soils [58]. Adequate weather conditions occurred in the growing seasons of 2016/2017 and 2018/2019, which resulted in higher yields (by 2.64 t and 1.72 t h\({}^{-1}\), respectively) as compared to the season of 2017/2018. A higher yield can primarily result from a relatively normal temperature and the appropriate distribution of the optimum rainfall amount during the May-June period, which is critical for cereal growth in Poland [59]. Irrespective of the years, as compared to the control plot (NPK), the highest grain yield was noted following the application of the three combined microbial preparations (NPK + _P. azotofixans_ + _B. megaterium_ + _B. subtilis_), as well as NPK + _P. azotofixans_ by 19.6% and 18.4%, respectively. In the years of the study, the effect of the application of the microbial preparations on the grain yield was not uniform (Table 3). In a study by [PERSON] et al. [60], the introduction of _Bacillus subtilis_, _Bacillus megaterium_ and _Azospirillum brasilense_ into the soil increased the grain yield by 24%, 19% and 19%, respectively, while their application in combination resulted in an increase in the grain yield by 33% as compared to the control (with no microbial preparations). In the authors' studies in 2017 and 2019, the highest grain yields were obtained following the application of NPK + _P. azotofixans_ and NPK and all the preparations combined (_P. azotofixans_ + _B. megaterium_ + _B. subtilis_), while in 2018, optimal yields were observed only following the application of all the three preparations in combination. The preparation with _P. azotofixans_ was applied under optimal conditions for wheat growth and development, and all the preparations applied in combination performed well under extreme weather conditions thanks to their ability to supplement each other. In general, the effects of microbial preparations are more noticeable under unfavourable weather conditions [61] and less noticeable under optimal conditions prevailing during the crop-growing season [62]. Bacterial endophytes found in plants employ direct or indirect mechanisms to improve the growth and development of plants and increase their tolerance to biotic and abiotic stresses [63; 64; 65]. According to [PERSON] et al. [66], [PERSON] and [PERSON] [67], [PERSON] and [PERSON] [68], plants inoculated with _B. subtilis_ can improve their growth under water stress conditions, mainly due to increased water use efficiency. Figure 1: The winter wheat grain yield after application of microbial preparations (mean value and standard deviation), (**a**) average for the years, (**b**) average for treatment. Values followed by the same letters do not differ significantly in [PERSON]’s (HSD) test (_p_ < 0.05). PA—_Paniobacillus azotofixans_; BM—_Bacillus megaterium_; BS—_Bacillus subtilis_. The quality characteristics of wheat grains are determined by the genetic conditions and modified by the cultivation technology type and the weather conditions [1; 2; 3].