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Long-term manned space-exploration missions and the permanence of human colonies on orbital stations or planetary habitats will require the regeneration of resources onboard or in-situ. Bioregenerative Life Support Systems (BLSSs) are artificial environments where different compartments, involving both living organisms and physical-chemical processes, are integrated to achieve a safe, self-regulating, and chemically balanced Earth-like environment to support human life. Higher plants are key elements of such systems and Space greenhouses represent the producers’ compartment. Growing plants in Space requires the knowledge of their growth responses not only to all environmental factors acting on Earth, but also to specific Space constraints such as altered gravity, ionizing radiations and confined volume. Moreover, cultivation techniques need to be adjusted considering such limitations. The type and intensity of environmental factors to be taken into account depend on the mission scenarios. Here, we summarize constraints and opportunities of cultivating higher plants in Space to regenerate resources and produce fresh food onboard. Both biological and agro-technological issues are considered briefly going through experiments both ground-based on Earth and in Space.

Summary.: Mediterranean ecosystems are characterized by seasonal and annual fluctuations in humidity and temperature which are considered limiting factors for plant growth and might have played a key role in the selection of species that compose the present vegetation. After anther release, pollen is generally exposed to various changes of temperature and humidity conditions, therefore its viability and consequently successful fruit set are strongly affected by these environmental parameters. The aim of this research was to study the effect of different combinations of humidity and temperature on pollen membrane integrity of Rosmarinus officinalis L. in order to investigate possible relations between pollen features and climatic conditions during flowering. This species is an evergreen perennial shrub, occurring among the sclerophyllous vegetation of Mediterranean maquis. In many areas of Southern Italy, it shows a flowering period starting from the beginning of winter and spreading throughout spring months. The analysis of data showed that duration of pollen viability of R. officinalis is enhanced by the co-occurrence of low temperature and high humidity. Moreover, these conditions are able to newly raise the viability of pollen kept at higher temperatures and lower humidity. These observations indicate that reactivation of the pollen membrane depends on low temperature apart from high humidity. Therefore “vernalization” of rosemary pollen enhances its viability, supporting that pollen behavior is adapted to winter condition and allows flowering in winter and early spring.

Summary.: In this study we evaluated the effect of simulated microgravity on pollen germination of both herbaceous and woody species in order to investigate the possibility of applying gametophytic selection for plant growth in the space environment. The behaviour of gametophytes exposed to the stress of clinostat rotation could be used to screen the degree of tolerance of the sporophyte to simulated microgravity. The use of male gametophyte selection overcomes the problems generally encountered by sporophytic selection in space especially for woody plants: the large size of plants and their long juvenile phase. In this experiment, pollen collected from just bloomed flowers of Prunus persica, P. avium, P. domestica, Pyrus communis, and Brassica rapa was subjected to tests assessing its viability by techniques such as fluorochromatic reaction. Once pollen viability was ascertained by fluorescence microscopy, pollen was placed on the growth medium in petri dishes both at 1 g and on the clinostat. After incubating for 1 day at room temperature, pollen was observed under a light microscope in order to detect parameters such as the percentage of germination and the growth direction. Then histochemical analyses were performed in order to verify the presence and distribution of nuclei, cytoplasm, and storage substances. Moreover, the presence, size, and morphology of callose plugs were observed. Results showed that the response of gametophytes to simulated microgravity is dependent on the species, some showing altered metabolism, others being unaffected.