A coin with two faces: understanding effects of ionizing radiation in space farming - the case of Solanum lycopersicum L. ‘Microtom’

Space exploration always has fascinated scientists, and in the last decades, the interest in this goal has increased exponentially. However, the realization of crewed space exploration missions or the permanence of human settlements on orbital stations or planetary habitats requires Life Support Systems (LSS) based on the interaction among abiotic and biotic elements, resembling terrestrial ecosystems. As on Earth, plants in Space would be involved in air regeneration through photosynthesis, water recovery, waste processing and food production, providing the astronauts with renewed resources and fresh food.

The extraterrestrial environment may be considered one the most extreme for organisms’ survival because of several Space factors constraining biological life (e.g., altered gravity, fluid-dynamic and microgravity interaction, modified pressure, temperature). Among them, ionizing radiation influences severely plant growth and metabolic processes.  Our study explores the response of tomato (Solanum lycopersicum L. ‘Microtom’), one of most widely cultivated crops, to different doses of sparsely ionizing radiation (IR), with two main goals: 1) to evaluate the morpho-functional mechanisms conferring radioresistance to guarantee their ecological role in closed, controlled environments; 2) to assess the possibility to use IR as a “biostimulant” to produce plant-derived functional food richer in functional compounds.

Different doses of X-rays (0.3, 1, 10, 20, and 30 Gy, 6 MV energy), were delivered on dry seeds (DS) and germinated seeds (GS) and compared to not-irradiated controls, to define a dose-response curve and check possible negative/positive outcomes on seedlings. After the irradiation treatments, seeds/seedlings were transferred to a climatic chamber and cultivated under controlled environmental conditions of light, temperature, relative humidity and photoperiod. For our study, we adopted a multidisciplinary approach that merges anatomical analyses with measurements of photosynthetic efficiency and biochemical traits, including polyphenols and other antioxidant compounds linked with the nutritional value of derived food. The growth and photosynthetic performance of DS and GS plants were followed during the whole plant life cycle, while anatomical traits and fruit antioxidant properties were determined at full maturity of the specific organs.

The results showed that the outcomes of radiation are dose-specific and dependent on the irradiated target stage, being SS plants more high-performing in photosynthetic activity and antioxidant content in fruits than SGs. Furthermore, some doses of X-rays act as a booster of bioactive compounds in fruits of both SS and SG plants. The outcomes of this research will be helpful to optimize crop production in Space and controlled environment agriculture systems. Moreover, the fine analysis of the relations between anatomical, eco-physiological and biochemical traits will furnish valuable insights to understand mechanisms of plant acclimation to stress, useful to manage cultivation factors to improve resource use optimization in controlled environments cultivations on Earth in line with sustainable development goals.