Sweetpotato is being studied at Tuskegee University as a potential crop for use in the U.S. National Aeronautics and Space Administration's Advanced Life Support program to provide food for long-term space exploration missions. Stem cuttings are commonly used in the propagation of sweetpotato. Although seeds of several crops have been grown in microgravity and their growth compared with ground-based controls (Cowles et al., 1994; Levine and Krikorian, 1991), plants that have been propagated vegetatively have not been studied under these conditions. Sweetpotato stem cuttings offer distinct advantages for space flight studies, especially those of short duration. Cuttings develop roots easier and quicker than seeds and the genetic makeup can be maintained from one initial planting.
Levine and Krikorian (1991) initiated roots of the monocot Hemerocallis (Baker.) M. Hotta and three populations of the dicot Haplopappus gracilis (Nutt.) Gray during a 5-d shuttle flight and reported greater overall root production compared with ground controls. Abrahamson et al. (1991) exposed eight sprouted seedlings [six alfalfa (Medicago sativa L.) and two white clover (Trifolium repens L.)] to microgravity for 6 d on a shuttle flight and found that root length:shoot length and root length:total length were greater compared with ground controls.
Plant regeneration from seeds during space flight studies has shown a decrease in the level of amyloplasts and a disorientation of root growth resulting from the absence of a strongly dominant gravity vector (Smith and Luttges, 1994). In addition, decreases in plant tissue starch from space flight have been one of the most consistent responses to microgravity (Brown et al., 1996). Musgrave et al. (2005) evaluated seed storage reserves and glucosinolates in Brassica rapa L. grown on the International Space Station and reported that deposition of storage reserves was more advanced in ground controls, whereas glucosinolate accumulation was enhanced by microgravity. They concluded that the spaceflight environment adversely influenced the overall flavor and nutritional quality of this crop by its direct impact on metabolite production. In contrast, Stutte et al. (2006) reported that there were no differences in the content of starch and soluble sugars in the leaves of flight and ground-based wheat (Triticum aestivum L.) plants grown for 21 d. They also found very little difference in cell development except that chloroplasts in the leaves of the plants grown in microgravity were more ovoid and the thylakoid membranes trended toward greater packing density. The researchers concluded that the space flight environment exerted minimal impact on wheat metabolism.
There is evidence that indicates plant responses when grown in space may be influenced by the gaseous environment. For example, Musgrave et al. (1998, 2000) obtained smaller seeds and variable weight in space experiments and have hypothesized that the composition of the gaseous environment changed as a result of the lack of buoyancy-driven convection in microgravity. Ground-based studies by Blasiak et al. (2006) reported that accretionary seed growth in pepper (Capsicum annum L.) was limited by the availability of oxygen and suggested that the variation in seed quality could be attributed to localized limitations in oxygen supply. Shoots of B. rapa grown in microgravity had greater sucrose and total soluble carbohydrates compared with ground control shoots, and it was suggested that this response was the result of root zone hypoxia caused by microgravity-induced changes in fluid and gas distribution (Stout et al., 2001).
Successful root growth is the all-important first step in the establishment of a sweetpotato crop in a closed environment. Of particular importance to this crop is the rapid growth of adventitious roots because these will influence the eventual development of storage roots. If sweetpotato is to be used successfully in future bioregenerative systems that recycle wastes into food, water, and oxygen, reliable plant propagation and growth in microgravity must be demonstrated necessitating a comprehensive understanding of the effects of gravity on both the plant's physiology and environment (Stout et al., 2001).
The primary objective of this experiment was to demonstrate the feasibility of the use of stem cuttings for plant propagation in microgravity. The root growth, distribution of amyloplasts in the root cells, and carbohydrates in the stems were examined and compared with their ground-based counterparts.
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