## 4 Discussion
The observed increase in both non-enzymatic and enzymatic antioxidant responses, induced by PGPB inoculation, confirms the literature data on the mutualism between plants and some bacterial species [20; 57]. The presence of bacteria mitigates plant oxidative stress through various mechanisms consistent with the differences observed between the five PGPB strains that were tested. Plants inoculated with _A. brasilense, A. globiformis_, and _Pseudomonas_ sp. showed an enhanced production and accumulation of phenolic compounds, compared with non-inoculated ones (Table 2). The antioxidant enzymatic activity involved SOD in plants inoculated with _A. brasilense, A. globiformis_, and _Pseudomonas_ sp. (Figure 4) and APX activity in plants inoculated with _H. seropedicae_ (Figure 5). No variations were detected in the antioxidant activity of canola inoculated with _B. ambifaria_. This bacterium may elicit other antioxidant enzymes, such as catalase, glutathione peroxidase, or guaiacol peroxidase. Therefore, further studies are needed to confirm this hypothesis. Soil salinity can dramatically affect the development of leaves, causing chlorosis and necrosis and then early senescence, thus impacting photosynthesis as well, either directly (i.e., biosynthesis of chlorophylls, regulation of photosynthetic enzymes) or indirectly (regulation of antioxidant enzyme system) [58]. Furthermore, the induced closure of stomata reduces the rate of CO\({}_{2}\) assimilation [59]. [PERSON] and [PERSON] (2005) [55] reported that the amounts of photosynthetic pigments generally decrease in salt-stressed plants, but there are also cases in which the concentration of these molecules do not vary significantly or even increase in saline conditions, as reported by [PERSON] et al. [10] in canola plants. In the present study, salinity stress induced a significant increase both in contents of chlorophyll and carotenoids in non-inoculated canola plants, compared with the control, confirming findings reported by [PERSON] et al. [10]. At the same time, plants inoculated with _A. brasilense_ and _Pseudomonas_ sp. and exposed to salt showed, compared with the controls in saline conditions, an increase in both pigments. On the contrary, plants under salt stress inoculated with _B. ambifaria_ and _H. seropedicae_ showed a reduction in chlorophyll content but an enhancement in carotenoids (Table 3). Our results are in accordance with previous studies that showed an increase, compared with saline control, in the amount of chlorophyll and carotenoids in plants of _Brassica napus_[52] and _Triticum durum_[50] exposed to salt stress and inoculated with PGPB strains. The increase in photosynthetic pigments in PGPB-inoculated plants suggests the capacity of bacterial inoculation to reduce the detrimental effects of salt, by ameliorating the activities of electron transporters associated with photosynthesis [60], as well as the biosynthesis of proteins and enzymes related to pigment stabilization [61]. Furthermore, [PERSON] et al. (2006) [62] reported that inoculation of wheat with _Azospirillum_ under salt stress enhanced the production of auxiliary photoprotective pigments such as carotenoids, composed of carotenxanthophyls, which may protect chlorophylls from oxidation during exposure to salt stress. As reported in the literature, the inoculation of plants with PGPB can mitigate, on a certain level, some of the deleterious effects of salinity, enhancing plant growth, biomass accumulation, yield, and photosynthetic performance [63; 64; 65; 66]. To estimate the beneficial effects of PGPB inoculation on photosynthetic performance and to study the spatial heterogeneity of photosynthesis, chlorophyll fluorescence imaging was used. In this present study, the maximal quantum efficiency of PSII photochemistry (\(\mathrm{F_{v}/F_{m}}\)) was not affected in salt-stressed plants, as it indicated no damage to the PSII reaction center.