The growth of higher plants in space greenhouses: cultivation strategies to minimise plant sensitivity to space stressors and maximise resource use efficiency

The sustainability of long duration manned missions in space relies on plant-based Bioregenerative Life Support Systems (BLSSs). The growth of higher plants in space is constrained by multiple stressors, some of them distinctive of extra-terrestrial environments (especially reduced gravity levels and ionising radiations), the others related to the technical complexity of regulating the environmental conditions (e.g. light, atmosphere composition, temperature, water and nutrients availability) in closed ecological systems. Whenever acting at suboptimal levels, such factors may operate synergistically to low-gravity and radiation, thus enhancing the effects of space factors. The current research efforts in agrotechnology aim to realise space greenhouses where environmental factors can be precisely modulated and optimised in order to maximise plant growth efficiency and reduce plant sensitivity to space stressors. In this paper, we summarise the experience of our research group which, for long time, has been involved in studies aiming to evaluate the possibility of cultivating higher plants in space to regenerate resources and produce fresh food onboard. Both biological and agrotechnological issues have been pursued. In particular, the response of plants to altered gravity and ionising radiations has been investigated with both experiments on Earth and in space. These studies allowed to evidence the most critical growth phases where stress-induced aberrations are more likely to occurr. Hence, wide attention has been dedicated to agro-technologies applied to BLSSs in order to identify cultivation protocols to maximise plant growth efficiency and to provide edible products with suitable nutritional quality to fulfil the astronauts needs. Current experiments within the space greenhouse research are focused on the hydroponic cultivation of soybean [Glycine max (L.) Merr.] within the research program MELiSSA (Micro-Ecological Life Support System Alternative) of the European Space Agency (ESA). Soybean is a candidate species for BLSSs because seeds are characterised by high nutritional value, can produce fresh sprouts or can be transformed in several edible products (soymilk and soy pulp). Being aware that the maximization of plant growth efficiency requires the proper choice of the biological material, we designed an objective criterion for the preliminary theoretical selection of the cultivars most adaptable to BLSSs also in the sight of Space constraints. Then, the performance of candidate cultivars was evaluated in bench tests in hydroponics; cultivation trials have been or are currently focused to analyse the response of plants to different hydroponic systems, to different sources of nitrogen and to the presence of specific microbes in the rhyzosphere. Plants’ response is evaluated in terms of morphological traits (e.g. plant size and seed set), physiological traits (e.g. gas exchange, nutrient uptake), chemical composition of seeds and their products, and technical parameters such as resource use efficiency and non-edible biomass production (waste). In the sight of future transfer of ground-based BLSSs research towards space outposts, more and more strict relations between biological and agrotechnological objectives is desirable to optimise plant growth protocols and environmental control to reduce the sensitivity of plants to space-related stressors to reach the final goal of maximizing plant growth and resource use efficiency.