## 1 Introduction  
The continued increase in land affected by salinization [1] makes it necessary to select crops that are salt tolerant, to avoid the yield loss that would otherwise be associated with high soil salinity. Salt-stressed plants undergo osmotic and oxidative stresses that negatively influence their growth. Photosynthetic activity can be affected by salinity as well [2], e.g., studies by [PERSON] et al. [3] highlighted that _Eruca sativa_ (arugula) had lower photosynthetic activity when watered with 100 mM NaCl solution than when water was employed. Generally, in salt-sensitive plants exposed to salinity, the amount of chlorophyll decreases, and the performance of the photosystems and electron transport mechanism are negatively impacted [4]. This can also involve damage of PSII by reactive oxygen species (ROS) causing a decrease in photosynthetic activity [5]. Photosynthesis is one of the physiological processes that are extremely susceptible to environmental stress, and for this reason, photosynthetic efficiency can be a good biomarker of environmental pressure in stressed plants [6]. In this context, chlorophyll fluorescence imaging can be analyzed to evaluate plant stresses, allowing in vivo analysis that it is non-destructive [7]. Moreover, it allows the evaluation of the heterogeneity in photosynthetic functions throughout a leaf, owing to image analysis of the quantum efficiency of photosystem II (PSII) in plants affected by several stresses, including heavy metals [8; 9], salinity [10], pharmaceuticals [11], and emerging contaminants [12]. _Brassica napus_ L. is a very important crop commonly named canola. Its agronomical relevance derives from food use (cultivars containing low quantities of erucic acid and glucosinolate) and from the high amounts of oils, useful for biodiesel production [13]. This species is a glycopyte, which is quite sensitive to the presence of salt in the soil, even though its genotypes can range from sensitive to tolerant [10]. Different approaches can be applied to help canola plants to overcome salt stress, including plant acclimation [10] and seed priming [14]. In addition, the establishment of a mutualistic relationship between plants and plant growth-promoting bacteria (PGPB) [15; 16; 17; 18; 19; 20] can be a good strategy to help plants to overcome salt stress. PGPB [21] are soil bacteria able to improve plant growth directly (by producing beneficial compounds) or indirectly (by inhibiting plant pathogens). Species belonging to the genera _Azospirillum_, _Burkholderia_, _Pseudomonas_, and _Enterobacter_ have been reported to be PGPB [16; 22]. Several beneficial activities have been attributed to them, including the synthesis of osmoprotectants, siderophores, and phytohormones (auxin, cytokins, gibberellins, and abscisic acid). Some strains can also fix atmospheric nitrogen, solubilize organic and inorganic phosphate, and they can reduce the synthesis of stress ethylene by the plants as a consequence of the activity of the enzyme 1-aminocyclopropane-1-carboxylate (ACC deaminase) [16; 23; 24; 25; 26]. Despite the growing interest in the use of PGPB to alleviate plant stress to salinity, in the literature, there are few reports concerning the effect of PGPB on physiological and biochemical response in plants exposed to stressful levels of salt [27; 28]. The bacterial species, selected for this study, _Azospirillum brasilense_ strain CD, _Arthobacter globiformis_ strain CD, _Burkholderia ambiria_ strain PHP7, _Herbospirillum seropedicae_ strain Z67, and _Pseudomonas_ sp. strain UW4 share the above-mentioned PGPB characteristics. All these strains produce auxins and polysaccharides [29; 30; 31; 32; 33]. Moreover, _B. ambiria_, _H. seropedicae_, and _Pseudomonas_ sp. have ACC deaminase activity [15; 33; 34; 35; 36], while _A. brasilense_ and _A. globiformis_ do not possess this activity [20; 33], even though they belong to genera in which the ACC deaminase has been detected in other strains [37; 38; 39].