## 4 Discussion  
The implementation of PGPR in plant production systems can be achieved through various methods, including seed coating, root dipping, leaf spraying, and soil drenching; the choice of method depends on the crop species, the specific SynCom composition, and the environmental conditions [45]. The main limiting factors affecting the effectiveness of PGPR inoculants in enhancing plant nutrition or acting as biocontrol agents are related to (1) the competition or predation of the PGPR strains with the already established microbiome [46; 47; 48], (2) the survival of the PGPR strains used to inoculate plants [49; 50] and/or (3) the loss of the efficacy of the promoting traits under stress conditions [51; 52]. In this study, we isolated five free-living N-fixing (diazotrophic) _Bacillusaceae_ and evaluated their ability to promote the growth of a model plant (_L. sativa_ L.) as SynCom. Although the high selectivity of host plants limits successful colonization of the N-fixing bacteria [30; 53], biofertilization through leaf spraying of diazotrophs has been shown to increase crop yield [9; 53; 54; 55; 56] and plant resistance to pathogens [57]. Evaluation of the efficacy and cost-effectiveness of diazotrophic biofertilizers requires a comparative analysis of soil and foliar application methods. This approach mirrors the evaluation strategies used for traditional fertilization methods, where optimization of nutrient delivery and economic considerations are paramount. For example, [PERSON] et al. [58], who studied the effects of N fertilization methods on wheat, showed that the foliar application of organic (urea) and inorganic (ammonium nitrate) N allowed an increase in grain protein content comparable to that obtained with soil fertilization but with a lower dose of fertilizer. This finding highlighted the potential for foliar application of N fertilizers to maximize nutrient use efficiency and minimize environmental impact. This concept can be applied to the inoculation of PGPR diazotrophs, considering that, among their traits, they can enhance the plant's access to N.  
In the present study, we used a soilless culture system to reduce the potential for competition or predation by indigenous rhizosphere microbial communities and three different inoculation methods to evaluate better the efficacy of the PGPR: root, foliar, or root and foliar application. Microorganisms applied to the leaf surface, similarly to the phyllosphere microbiome, encounter and must survive fluctuating environmental conditions, including variations in temperature, humidity, wind speed, and radiation, which cause significant physiological stress [27; 28]. The leaf surface is an oligotrophic environment with limited nutrient availability to support the growth of the epiphytic microbial community [25; 26]. The spatial distribution of carbon (C), N, and micronutrients follows the distribution of water availability, as water droplets are effective sinks for the outward diffusion of photosyntates through the cuticle [59]. The leaf cuticle, a hydrophobic layer composed of cutin \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & \multicolumn{2}{c}{NL} & \multicolumn{2}{c}{**ALA**} & \multicolumn{2}{c}{**TLA**} & \multicolumn{2}{c}{**LMA**} \\  & \multicolumn{2}{c}{**No. Leaves Per Plant**} & \multicolumn{2}{c}{**cm\({}^{2}\)**} & \multicolumn{2}{c}{**cm\({}^{2}\)**} & \multicolumn{2}{c}{**g m\({}^{-2}\)**} \\ \hline C & 9.60 & B & 18.43 & A & 177.38 & A & 21.98 & B \\ R & 10.00 & AB & 15.79 & AB & 157.36 & AB & 23.08 & B \\ L & 9.60 & B & 13.38 & B & 128.79 & B & 24.95 & B \\ RL & 10.80 & A & 16.36 & AB & 176.20 & A & 29.46 & A \\ s.e. & & & & & & & \\ C & 0.24 & & 0.84 & & 10.93 & & 0.54 & \\ R & 0.32 & & 0.74 & & 5.95 & & 0.82 & \\ L & 0.24 & & 1.10 & & 12.28 & & 1.72 & \\ RL & 0.37 & & 0.72 & & 7.02 & & 0.90 & \\ \hline \end{tabular}
\end{table}