Combined effect of ionizing radiation and light on morpho-anatomical and nutritional traits of soybean seedlings intended as integrator for astronauts’ diet

For Space exploration, the realization of long-term manned missions requires the possibility to grow plants in extra-terrestrial environments. Indeed, life support in Space is based on the in situ regeneration of resources (e.g. air, water and food) needed by the crew that can be achieved in plant-based Closed Ecological Life Support Systems (CELSS). In such artificial ecosystems, higher plants, as every photosynthetic organism, utilize CO2 produced by humans and provide oxygen, can utilize nutrients derived from human wastes and provide not only food, but also non-nutritive benefits as they resemble effective countermeasures against conditions of isolation which elicit behavioural changes in crewmembers (1). The integration of astronauts’ diet with fresh food directly produced onboard of Space platforms can also involve the production of sprouts as easy ‘vegetal systems’ that can be grown in a few days, in small volume, with low energy (2). However, one of the main constraints for the establishment of extra-terrestrial outposts is the presence of high levels of ionizing radiation which affect organisms’ growth. Therefore, one of the main challenge is ensuring the productivity of edible plants in CELSS even in such harsh conditions. However, plants are much more resistant than animals to ionizing radiation: doses that are detrimental for animals can be ineffective or induce positive responses in plants (3). The effects of ionizing radiation can be the result of genetic aberrations (4) and their severity depends on radio-resistance that is variable among the species and cultivar as well as among target tissues/organs. Moreover, the possible interaction between ionizing radiation and other environmental factors has been poorly investigated. The aim of this study was to analyze the combined effect of low-LET (Linear Energy Transfer) ionizing radiation and light on the development of soybean (Glycine max (L.) Merr.) seedlings. Irradiation with Xrays was not performed on dry seeds, which are considered one of the most resistant stage, but on germinated seeds when the radicle was at least 1 cm long. Indeed, germinated seeds are considered delicate systems, vulnerable to ionizing radiation because of the occurrence of actively-proliferating meristems. Seedlings were irradiated with three doses of X-rays, namely 0.3, 10 and 20 Gy, and then incubated into two conditions of light, namely absence of light (dark) and presence of white light (w) for a week. Seedlings were then collected and subjected to the preparation for microscopy and biochemical analyses and to detect possible radiation-induced modifications to the structure and nutritional value of fruits under the two light conditions. 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 influenced by light. Results suggested that the possible occurrence of radiation-induced positive outcomes in plants can be severely influenced by other environmental conditions: such a phenomenon should be taken into account in the shielding design of plant based modules of bioregenerative systems.