## 3 Results and Discussion
### The Leaf Greenness Index (SPAD) and the Photosynthetic Rate  
Weather conditions during the research had a significant effect on the SPAD index value only at the 1 node stage (BBCH\({}_{31}\)), with its highest value noted in 2019 (Figure 3). The microbial preparations had a significant effect on the SPAD index value at both test dates (BBCH\({}_{31}\) and BBCH\({}_{51}\)). Following the application of NPK and the preparation containing _P. azotofixans_ and NPK with all the biopreparations (_P. azotofixans + B. megaterium + B. subtilis_), significantly higher values of the SPAD index were noted at both growing stages. According to [PERSON] et al. [28], a higher SPAD index value results from the provision of a sufficient amount of N, supported by microbial preparations, which enables the improvement of the condition of plants. An increase in nitrogen fertilisation is accompanied by an increase in both the leaf greenness index value and the leaf area index (LAI) [77]. [PERSON] et al. [78] and [PERSON] et al. [79] also noted that nitrogen fertilisation significantly contributes to an increase in the flag leaf SPAD index value, which is positively correlated with the wheat grain yield. In the present study, no interaction was noted between the years of study and the preparations applied to the SPAD index values (data not shown). The photosynthetic process rate positively correlates with the mineral content in the soil [80; 81]. The photosynthetic intensity, however, can be impaired by almost any adverse environmental factor [81; 82]. One such factor is a soil water deficit, which results from the weather condition pattern. Abiotic stresses primarily reduce the photosynthetic performance of plants due to their adverse effect on chlorophyll biosynthesis, photosystem performance, electron transport mechanisms, gas exchange parameters and many more [57]. On both measurement dates, the years of the study unevenly affected the photosynthetic rate shaping (Figure 4), and on both observation dates, irrespective of the years, the highest photosynthetic rate was noted following the application of NPK + _P. azotofixans_ and NPK and all the preparations combined (_P. azotofixans + B. megaterium + B. subtilis_). The weather conditions throughout the years of the study differentiated the effect of biological preparations on the photosynthesis index. (Table 5). Such an example, as compared to the control plot (NPK), is obtaining lower photosynthetic rate values at the BBCH\({}_{31}\) stage following the application of NPK + _B. megaterium_ in 2018 and at the BBCH\({}_{51}\) stage in 2018 and 2019. This was confirmed by the strong positive correlation between the grain yield and the net photosynthetic intensity, as noted in a study by [PERSON] et al. [82], [PERSON] et al. [83], [PERSON] et al. [84] and [PERSON] et al. [85]. Such a relationship is indicative of the correct use of photosynthetic products in yield formation [86]. In  
Figure 3: The leaf greenness index (SPAD) 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—_Paenibacillus azotofixans;_ BM—_Bacillus megaterium;_ BS—_Bacillus subtilis._the present study, the regression equations describing the relationship of the photosynthetic rate at the wheat development stages of BBCH\({}_{31}\) and BBCH\({}_{51}\) are characterised by a determination coefficient of 0.40-0.99 (Table 6). At the wheat growth phase of BBCH31, when fertilising wheat with NPK alone, NPK + _P. azotofixans_, NPK + _B. megaterium_ and NPK + _B. subtilis_, an increase in the photosynthetic rate value was obtained with the maximum amount of rainfall and at the maximum temperature (Figure 5). As regards wheat fertilisation with NPK + _P.