Effects of inoculation of root-associative Azospirillum and Agrobacterium strains on growth, yield and quality of pea (Pisum sativum L.) grown under different nitrogen and phosphorus regimes

•Pea was inoculated with Azospirillum and Agrobacterium to enhance N and P uptake.•Additionally, plants were supplied with less, moderate and optimum N and P levels.•PGPR inoculation increased the growth rate of pea plants compared to no inoculation.•PGPR were more effective at slightly less than optimum levels of N and P.•Root-associative PGPRs in legumes may increase nutrient (N and P) use efficiency. Nitrogen and phosphorus are essential for growth and development of crop plants and are required in large quantities for crops. However, nitrogen is less available to plants due to leaching, volatilization and denitrification and phosphorus due to insolubility that result in even higher fertilizer input. Plant growth promoting rhizobacteria (PGPR) have the potential to improve nutrient availability, reduce input cost and mitigate environmental issues incurred otherwise by substantial use of fertilizers in production of economically important horticultural crops such as pea. Therefore, this study was conducted with the objectives i) to determine whether the inoculation of pea plants with PGPR combined with the reduced mineral rates of nitrogen and phosphorus (NP) fertilizers will give similar growth, yield and quality as with full rates of NP fertilizer without inoculation and ii) what is the minimum percentage of NP that can be used for commercial pea cultivation. For this purpose, pea seeds were co-inoculated with a mixture of Azospirillum Er-20, a nitrogen fixing strain; and Agrobacterium tumefaciens Ca-18, a phosphorus solubilizing strain of rhizobacteria in combination with 60, 65, 70, 75, 80 and 100% (recommended) dose of NP fertilizer. The finding suggested that PGPR combined with NP fertilizer at a rate of 75% of the recommended dose led to increased plant height, fresh and dry weights of shoot and roots, number of leaves and branches, number and length of pods, number and seed weights and yield that were statistically equivalent to the full fertilizer rate (100% NP) without PGPR. Similar trend was observed for photosynthetic pigments (chlorophyll a and b, total chlorophyll concentration and total carotenoids), total phenolics and DPPH radical scavenging activity and antioxidant capacity. However, when PGPR were combined with NP doses less than 75% of the recommended amount, the results were not positively consistent. Without PGPR, NP rates lower than the recommended dose resulted in inconsistent effects and significantly reduced plant growth traits,