Plants in space face unique challenges, including chronic ionizing radiation and reduced gravity, which affect their growth and functionality. Understanding these impacts is essential to determine the cultivation conditions and protective shielding needs in future space greenhouses. While certain doses of ionizing radiation may enhance crop yield and quality, providing "functional food" rich in bioactive compounds, to support astronaut health, the combined effects of radiation and reduced gravity are still unclear, with potential additive, synergistic, or antagonistic interactions. This paper investigates the combined effect of chronic ionizing radiation and reduced gravity on seed germination and microgreens growth. Four cultivation scenarios were designed: standard Earth conditions, chronic irradiation alone, simulated reduced gravity alone, and a combination of irradiation and reduced gravity. An analysis of the harvested microgreens revealed that growth was moderately reduced under chronic irradiation combined with altered gravity, likely due to oxidative stress, primarily concentrated in the roots. Indeed, an accumulation of reactive oxygen species (ROS) was observed, as well as of polyphenols, likely to counteract oxidative damage and preserve the integrity of essential structures, such as the root stele. These findings represent an important step toward understanding plant acclimation in space to achieve sustainable food production on orbital and planetary platforms.