Tree fruits have often been deliberated on as potential crops in bioregenerative life support systems for use on long-duration missions (Wheeler, 2003). A continuous supply of fresh fruit could provide unique nutritive contributions to the crew’s diet and offer enhanced menu diversity, an important consideration on long-duration missions (Bourland et al., 2000). Regardless of their nutritional and menu diversification benefits, tree fruits have been precluded as candidate crops based on architectural, juvenility, and phenological constraints associated with their normal growth and development. Tree fruits are large, take a long time to mature, and in the case of temperate species, require a cold dormancy period between fruiting cycles. The dormancy requirement also means food production from these crops is periodic, which presents a further barrier to use in bioregenerative life support systems. There is also concern with tree crops regarding their harvest index (i.e., the ratio of edible biomass to total biomass), as trees tend to dedicate significant resources to the development of structural tissue (wood) relative to reproductive (edible) tissue.
The FasTrack crop breeding system presented in Srinivasan et al. (2012; 2014) and briefly summarized herein, takes advantage of the Flowering Locus T1 (
In addition to early flowering, the
The most obvious obstacle for growing trees in a spaceflight environment is their large size. Typical mature plum orchard trees can range in height from 3-4 m (Day et al., 2013), making them impractical for use in any foreseeable spaceflight or planetary exploration plant growth system. In order to be considered as a candidate crop, the entire architecture of the tree needs to be reduced to the point that the tree could be grown in the same systems used for such candidate crops as tomato and pepper. The over-expression of
Perhaps less obvious than the architectural barriers, but equally limiting in terms of spaceflight applications, is the prolonged period of exclusively vegetative growth that occurs in the juvenile phase leading up to the first flowering and fruit set. For
Although the FT-plum development phase is still a somewhat protracted timeframe for experiments on the International Space Station (ISS), it should be noted the majority of this development time could elapse on the ground as a lead up to a spaceflight experiment. The already accelerated flower development associated with the
The use of bioregenerative systems as a crew food supply – in whole or part – during extended duration missions would require a constant production of foodstuffs (Wheeler, 2000). This can be accomplished with staggered plantings, or by using indeterminate crop species capable of continual food production. Although fruit trees are perennial and capable of multiple crops, most are not indeterminate for the purposes of bioregenerative life support. Most tree fruit production is phenologically regulated – particularly those species that evolved in temperate climates (Childers et al., 1995) – implying that sometime during a mission the trees would need to enter a cold dormancy phase, during which time they would neither produce food nor contribute to other life support functions (i.e., air revitalization and water purification). Under the influence of
Harvest index is a plant productivity metric used to describe the relative distribution of biomass between the edible and inedible components of a crop (Hay, 1995). It was originally developed primarily for cereal crops but has since been used for a wide range of crops. Tree crops – such as plum – are generally considered to have a low harvest index, at least in the short term, as the plant directs its resources to the development of vegetative and structural elements (i.e., wood). In the long term (i.e., 20-40 years), it can be argued the harvest index is actually quite high given the multiple harvests, and field studies with ultra-dwarf fruit trees have shown biomass partitioning to fruit can be quite high once the trees develop beyond their juvenility phase (Palmer, 1988). The overexpression of
Bone loss and its impact on the health of crewmembers has been identified by NASA scientists as one of the greatest challenges to interplanetary space exploration and long-duration stays on the ISS. For crew members of the Russian MIR and ISS, the decrease in bone mineral density can range from of 1.0-1.6% per month in the hip and lumbar spine (LeBlanc et al., 2000). In the space environment, microgravity, radiation exposure, and immunological changes can all contribute to bone loss, but the most pronounced effects result from abnormal loading of the skeleton.
A variety of interventions, including exercise or loading regimens (Baldwin et al., 1996; Yang et al., 2009), drug therapies (Bikle et al., 1994; Turner et al., 1998), and dietary modifications (Globus et al., 2009; Smith et al., 2005; Zwart et al., 2004), have been considered as countermeasures. The appeal of dietary interventions is they could provide a practical and safe component to an osteoprotective regimen through the incorporation of foods rich in specific nutrients or non-nutrient bioactive components (e.g., polyphenolic compounds) or dietary supplements. The ideal dietary intervention would have the capacity to suppress the catabolic activity of the osteoclast cells (i.e., resorption), while maintaining or up-regulating the anabolic activity of osteoblast cells (i.e., formation). Additionally,
Accumulated scientific evidence has demonstrated the beneficial effects of dried plums (
In terms of their bioactive components, plums are considered a nutrient dense fruit serving as a good dietary source of potassium and vitamin K, as well as a rich source of phenolic compounds (Donovan et al., 1998; Kayano et al., 2004; Stacewicz-Sapuntzakis et al., 2001). Dried plum is of particular interest because it has received the highest oxygen radical absorbance capacity (ORAC) ranking among the most commonly consumed fruits and vegetables (McBride, 1999). In one of our recent studies, we showed an extract of plum phenolic compounds accounted for > 90% of the effects of plum on bone in an aging osteopenic animal model (B.J. Smith, unpublished results). While we recognize other components in plum (e.g., oligosaccharide) likely contribute to the benefits of plum on bone due to their ability to promote calcium uptake by cells (Weaver, 2005; Weaver et al., 2011), our findings indicate the specific phenolic compounds – seemingly unique in plum – are in large part responsible for the beneficial effects on bone.
The FT-plum phenotypes address all the major obstacles that have prevented inclusion of tree fruits for bioregenerative life support applications; however, FT-plums are still at a low technical readiness level (TRL). Many questions need to be answered before FT-plums can be accepted as a candidate space food crop. Early questions are centered on basic horticultural management and production under spaceflight conditions, including performance under super elevated CO2 levels typical in crew cabin environments. Further considerations include, but are not limited to, growth and performance under a 24 h photoperiod, propagation and growth in available spaceflight hardware, pollination considerations in space, and the safety issues regarding the consumption of the genetically modified material. Research aimed at increasing the TRL of these FT-plums by addressing these issues is currently underway.
The resultant phenotypes associated with the ectopic expression of