Light quality alters the response to ionizing radiation in seedlings of legume species in terms of development and nutritional traits

For manned Space exploration, the possibility to integrate astronauts’ diet with fresh food directly produced onboard of Space platforms is essential. Apart from the design of space greenhouses where crops can be cultivated seed-to-seed, the production of easy ‘vegetal systems’, such as seedlings, can be an opportunity since they can be produced in a few days, in small volume, with low energy. However, ionizing radiation is still one of the main constraints for organisms’ growth in extra-terrestrial environments and, although seeds are considered much more resistant than animals to ionizing radiation, seedlings in the first developmental stages are very sensitive organisms. The aim of this study was to analyze the combined effect of low-LET (Linear Energy Transfer) ionizing radiation and light quality on the development of soybean (Glycine max (L.) Merr.) and Azuki bean (Vigna radiata (L.) R. Wilczek) seedlings. Irradiation with X-rays was performed on germinated seeds when the radicle was 1 cm long. Indeed, germinated seeds are considered vulnerable to ionizing radiation because of the occurrence of actively-proliferating meristems. Seedlings were irradiated with three doses of X-rays, (0.3, 10 and 20 Gy) and then incubated into four conditions of light: white light (W), red light (R), red and blue light (RB), and dark (D). Seedlings were then subjected to the preparation for microscopy and biochemical analyses to detect possible light-dependent radiation-induced modifications to the structure and nutritional value. Light and epi-fluorescence microscopy observations, coupled with digital image analysis, allowed to quantify anatomical traits such as tissue and cell size, and the amount of phenolic compounds in different organs of the seedlings. Nutritional analyses were focused on anti-oxidant compounds. The overall analysis showed that the seedling-response to increasing doses of X-rays is dependent on light quality. Results also showed that the response is species-specific. In conclusion, the possible occurrence of radiation-induced increase in antioxidant compounds in bean seedlings can be severely influenced by light quality already at very early stages of development. Therefore, the interaction between ionizing radiation and other environmental conditions should be taken into account in the shielding design of plant-based modules of bioregenerative systems.