## 4 Discussion
At the same time, canola plants inoculated with the five bacterial strains showed an increase in \(\mathrm{F_{v}/F_{m}}\) suggesting, which indicated that maximal quantum efficiency of PSII photochemistry was enhanced by the mutualism with PGPB. \(\mathrm{F_{v}/F_{m}}\) values reported in this study ranged from 0.797 to 0.824 (Table 4), and such values are usually measured in unstressed plants [7]. Nevertheless, as reported in the literature, the \(\mathrm{F_{v}/F_{m}}\) ratio is considered less sensitive and responsive to the stresses in comparison with the quantum efficiency of PSII (\(\mathrm{\Phi PSII}\)) [63; 67]. Salt stress can cause a decrease in ETR and \(\mathrm{\Phi PSII}\) in the photochemical quenching process, with concomitant increases in \(\mathrm{qN}\), NPQ, and Y(\(\mathrm{NPQ}\)) as non-photochemical quenching mechanisms [68; 69]. These results are in line with previous indications, as canola plants exposed to salt stress exhibited a significant decrease in \(\mathrm{\Phi PSII}\) and ETR and a slight but not significant increase in NPQ values with respect to the saline control plants (Table 4). On the contrary, the inoculation with the bacterial strains allowed salt-stressed plants to maintain higher \(\mathrm{\Phi PSII}\) and ETR values than non-inoculated ones, except for plants inoculated with _Pseudomonas_ sp. Furthermore, the inoculation with bacterial strains reduced or did not affect the NPQ values in salt-stressed canola plants, compared with non-inoculated ones. The non-photochemical quenching (\(\mathrm{NPQ}\)) parameter provides information about the energy dissipation by heat in the photosynthetic apparatus exerting a photoprotective action [70]. Moreover, NPQ is also involved in the protection from oxidative stress [71]. In plants exposed to salt stress conditions, the NPQ value usually increases, indicating a smaller amount of energy available for photosynthesis. Nevertheless, in the current study, the greater quantum efficiency of PSII (\(\mathrm{\Phi PSII}\)) observed in the inoculated plants under salt stress, compared with saline control plants, indicated that the reaction centers in the thylakoid membrane were open, which led to less activity of the NPQ process (lower NPQ values) [72]. In fact, salt-stressed plants inoculated with _B. ambifaria_ or _Pseudomonas_ sp. showed a decrease in the NPQ values, compared with saline control plants, while the inoculation with the other strains did not affect this parameter. This response allowed salt-stressed plants inoculated with PGPB to maintain an adequate balance between photosynthetic electron transport and carbon metabolism. Therefore, data of the chlorophyll fluorescence parameters analyzed in this study (\(\mathrm{F_{v}/F_{m}}\), \(\mathrm{\Phi PSII}\), \(\mathrm{\Phi PSII}\), \(\mathrm{NPQ}\), and ETR) confirmed the beneficial effect of the PGPB inoculation in canola plants exposed to salt stress on the performance of the photosynthetic apparatus. Chlorophyll fluorescence images (Figure 6) showed that the parameters measured in dark-adapted leaves (\(\mathrm{F_{v}/F_{m}}\)), especially for inoculated or non-inoculated control plants, revealed a homogeneous pattern of distribution of chlorophyll fluorescence, whereas they showed an appreciable heterogeneous pattern of light utilization and photosynthetic activity in light-adapted leaves (\(\mathrm{\Phi PSII}\), and \(\mathrm{NPQ}\)), especially in plants inoculated with _A. brasilense_, _B. ambifaria_, _A. globiformis_ and _H. seropedicae_ both in control and stress conditions. Finally, the above-mentioned enhanced antioxidant activities detected in inoculated plants avoided the detrimental effects related to a damage of photosynthetic apparatus induced by ROS and described by [PERSON] et al. [5]. The inoculation of canola plants with _B. ambifaria_ enhanced photosynthetic activity in saline conditions even without variations in the antioxidant activity. A possible explanation for this result may be due to the production of the enzyme ACC deaminase by _B. ambifaria_[33].