Plant reproduction during spaceflight: importance of the gaseous environment

Plant reproduction is a complex developmental process likely to be disrupted by the unusual environmental conditions in orbital spacecraft. Previous results, reviewed herein, indicated difficulties in obtaining successful seen production in orbit, often relating to delayed plant development during the long-term growth necessary for a complete plant life cycle. Using short-duration exposure to spaceflight, we studied plant reproduction in Arabidopsis thaliana (L.) Heynh, during three flight experiments: CHROMEX-03 on STS-54 (6 d), CHROMEX-04 on STS-51 (10 d), and CHROMEX-05 on STS-68 (11 d). Plants were 13 - 14 d old (rosettes) at time of launch and initiated flowering shoots while in orbit. Plants were retrieved from the orbiters 2 - 3 h after landing and reproductive material was immediately processed for in-vivo observations of pollen viability, pollen tube growth, and esterase activity in the stigma, or fixed for later microscopy. Plants produced equal numbers of flowers to those controls growing on the ground but required special environmental conditions to permit fertilization and early seed development during spaceflight. In CHROMEX-03, plants were grown in closed plant growth chambers (PGCs), and male and female gametophyte development aborted at an early stage in the flight material. In CHROMEX-04, carbon dioxide enrichment was provided to the closed PGCs and reproductive development proceeded normally until the pollination stage, when there was an obstacle to pollen transfer in the spaceflight material. In CHROMEX-05, an air-exchange system was used to provide a slow purging of the PGCs with filtered cabin air. Under these conditions, the spaceflight plants apparently had reproductive development comparable to the ground controls, and immature seeds were produced. In every aspect examined, these seeds are similar to those produced by the ground control plants. The results suggest that if the physical environment around the plant under spaceflight conditions meets the physiological demands of the plant, then reproductive development can proceed normally on orbit.