Cultivating watermelon (Citrullus lanatus) in Martian regolith simulant after seed inoculation with plant growth-promoting bacteria

The human colonisation of space is no longer confined to the realm of science fiction. Among various celestial bodies, Mars has emerged as a primary target for long-term human settlement in the future. One of the major challenges in such missions is ensuring a sustainable food supply. Transporting food across such vast distances is both costly and impractical, prompting growing interest in space farming and the use of in situ resources, such as Martian regolith, for plant cultivation. In this study, we explored the potential of growing watermelon (Citrullus lanatus) under simulated Martian conditions using a substrate which mimics the composition of Martian regolith. To mitigate nutrient deficiency, the seeds were treated with plant growth-promoting bacteria (PGPB) isolated from the rhizosphere of Miscanthus × giganteus grown in heavy metal-contaminated soil, yielding bacterial strains tolerant to the metal-rich and nutrient-poor conditions analogous to those of the Martian regolith simulant. Statistically significant differences in the plant growth parameters - including height, root length, fresh weight, and leaf area - were observed between the plants grown on soil and regolith substrates, while the chlorophyll content showed no significant variation, suggesting a preserved photosynthetic function despite abiotic stress. Additionally, seed inoculation with a bacterial consortium consisting of Pseudomonas chlororaphis, Bacillus safensis, and B. cereus/thuringiensis improved the germination rates compared to the untreated control. This research represents the first attempt to cultivate watermelon in a Martian regolith simulant and highlights the potential of PGPB as a promising strategy to enhance plant performance under extreme conditions. Further studies are needed to optimise microbial consortia and regolith amendments for future space agriculture applications.