## 3 Results  
Morphological Identification of Cultivable Endophytic Bacteria in Pea Embryos with Putative Nitrogen Fixation Activity  
Twenty pea accessions were divided into six cultivar groups, including DGPS (dry grain pea samples S18P01-04), FGPS (fresh grain pea samples S18P05-08), SPS (snow pea samples S18P09-12), SCPS (sweet crispy pea samples S18P13-16), SEPS (pea tip [seedling of pea] samples S18P17-19), and SHPS (pea sprout [shoot of pea] sample S18P20) which was used as a control. Twenty-seven putative NFB isolates were identified from pea embryos based on their sizes, shapes, texture, and colors (Table S1; Figure 1a). These strains were somewhat evenly distributed in DGPS (S18P01-1, S18P01-2, S18P02-1, S18P02-2, S18P02-3, S18P03-1, S18P03-2, S18P04-1, S18P04-2, and S18P04-3), FGPS (S18P05-1, S18P05-2, S18P06-1, S18P06-2, S18P06-3, S18P07-1, S18P07-2, S18P07-3, S18P08-1, S18P08-2, and S18P08-3), and SPS (S18P09, S18P10, S18P11-1, S18P11-2, S18P12-1, and S18P12-2), with no putative NFB isolates being identified from SCPS, SEPS, or SHPS. Nutrient Solubilization and Utilization Activities of Nitrogen-Fixing Bacteria (NFB) Isolated from Pea Embryos  
Nitrogen activities of the 27 putative NFB cultures ranged from 10.67 to 89.45 and averaged at 71.95 nmol C\({}_{2}\)H\({}_{4}\)/mL/h (Table 1). They were classified into four nitrogenase activity categories of less than 40 (S18P02-1 [10.67]), 40-60 (S18P11-1 [54.37]), 60-80 (21 isolates), and above 80 (S18P05-1 [85.02], S18P08-2 [89.45], and S18P12-1 [85.86]). The three  
Figure 1: Isolation of nitrogen-fixing bacteria (NFB) from pea embryos and their nutrient solubilization activities. (**a**) Colonial morphology of two (S18P11-1 and S18P09) representative endophytic bacterial strains. Twenty-seven culturable endophyticytic bacterial strains isolated in this research may possess putative nitrogen fixation capability. (**b**) Two NFB isolates S18P05-2 and S18P08-3 are also phosphorus-s solubilizing bacteria (PSB) capable of solubilizing both inorganic and organic phosphorus. Inorganic phosphorus screen plates (**top**) and egg yolk plates (**bottom**) plates were used to test bacterial abilities to solubilize inorganic and organic phosphorus, respectively. The emergence of clear (**top**) and turbid (**bottom**) halos indicates bacterial solubilization of inorganic and organic phosphorus, respectively. (**c**) Among 13 isolated NFB that are also potassium-s solubilizing bacteria (KSB), S18P02-2 (**left**) and S18P12-1 (**right**) have the highest potassium solubilization capacities. Bacterial potassium solubilization was measured by the emergence of clear halos on the silicate plates. Both S18P02-2 and S18P12-1 generated clear halos with diameters as large as 2.2 cm. (**d**) Quantification assays using the Salkowski reagent show that all isolated NFB secrete IAA, with the highest and the lowest levels of IAA production observed in S18P05-2 and S18P11-1, respectively. strongest NFB were isolated from FGPS (S18P05-1 and S18P08-02) and SPS (S18P12-1). Overall, 26 out of 27 isolates, excluding S18P02-1, were verified as NFB with considerable nitrogen fixation capability. PSB and KSB solubilize soil phosphorus and potassium, respectively, for their easier absorption by plants [60; 61]. We screened our NFB collection for PSB and KSB by measuring the halos they generated on certain nutrient plates, which has been demonstrated to be a convenient and reliable approach [62]. Eleven out of twenty-six NFB isolates generated clear halos on inorganic phosphorus screen plates (Table 1), demonstrating their ability to solubilize inorganic phosphorus. On the other hand, only 3 out of 26 isolates were organic phosphorus solubilizers, as revealed by their ability to generate turbid halos on egg yolk plates (Table 1). Of particular interest, S18P05-2 and S18P08-3 are the only two NFB that are also dual-role PSB capable of solubilizing both inorganic and organic phosphorus (Figure 1b). KSB catabolize potash feldspar powder in the silicate medium.