Seaweed-derived biostimulant (Kelpak®) and phosphate-solubilising endophytic species enhanced the phosphate status of cucumber seedlings under phosphorus deprivation
Phosphorus (P) is an indispensable nutrient and a limiting factor affecting plant growth and development. To address P deficiency in plants, synthetic phosphate fertilisers are often used despite concerns over their ecological impacts. The use of microbial and non-microbial biostimulants is a plausible, eco-friendly alternative to enhance plant P acquisition. In this study, the in vitro quantitative P-solubilising potential of four endophytic species ( Aspergillus niger , Fusarium oxysporum , Serratia marcescens , and Paenibacillus polymyxa ) was evaluated. The P-acquisition-enhancing potential of endophyte-inoculated cucumber seedlings grown on P-starved hydroponic medium and supplemented with commercial seaweed biostimulant (Kelpak ® ) was also investigated. The four endophytic strains demonstrated the ability to solubilise the phosphates from substrates containing aluminium, calcium and iron. Aspergillus niger had the highest P-solubilising capacity from the three substrates. Under P-starvation, the four endophytic species had the capacity to colonise the internal tissues of the seedlings, and P-limitation led to a reduction in the root biomass, photosynthetic pigment concentration and root-to-shoot ratio in cucumber seedlings. However, endophytic inoculation and Kelpak ® supplementation of the growth medium enhanced leaf area and foliar P content in cucumber seedlings. Endophytic inoculations with or without Kelpak ® supplementation decreased phosphatase activity in root exudates of the seedlings. The results revealed that the four endophytic strains have the capacity to solubilise P and colonise cucumber root tissues. The endophytic species, in conjunction with the seaweed extract, enhanced the morpho-physio-biochemical adaptive responses of cucumber seedlings to P-limitation. Thus, inoculating cucumber seedlings with beneficial microbes and adding Kelpak ® to the growth medium is a promising, sustainable strategy to enhance cucumber production under P limitation.