Plants

Like bacteria, plants were exposed to spaceflight very early in the space program. Seeds of five species were first sent up on Sputnik-4 in 1960. Since then there has been a bias to send a variety of plants into space rather than picking one or two species and studying them in detail over the decades. In part this is because different scientists have "specialty" systems that they work on, or they pick certain plants as best for particular tests. In part, it is because of practical concerns (e.g., a need for plants with short life spans to match short spaceflights) or a desire to see whether a variety of possible foodstuffs would do well. A few of the plants types sent so far have included algae, carrots, anise, pepper, wheat, pine, oat, mung beans, cress, lentils, corn, soybeans, lettuce, cucumbers, maize, sunflowers, peas, cotton, onion, nutmeg, barley, spindle trees, flax, orchids, gladiolas, daylilies, and tobacco.

Shoots of cell culture-derived daylily (.Hemerocallis cv. Autumn Blaze) and haplopappus (Haplopappus gracilis) have been selected because they represent both major groups of the plant kingdom. The daylily represented herbaceous monocotyledonous plants and the haplopappus represented annual dicotyledonous plants. Haplopappus is valuable for studies of chromosome behavior because it has only four chromosomes in its diploid state (Figure 2-09). Daylily was chosen for the study because it has special lcaryotypic features (features related to the number, size, and configuration of chromosomes seen in the metaphase portion of mitosis) and it is a species for which a great deal of culture technology has been developed.

Arabidopsis plant has been chosen because it has many genes in common with humans. This flowering herb is a member of the mustard (Brassicaceae) family that is widely used as a model organism in plant biology. This species is a flowering herb that is widely used for research in plant genetics. With a small, completely sequenced genome, rapid life cycle (about 6 weeks from germination to mature seed), prolific seed production and easy cultivation in restricted space, it offers important advantages for basic research in genetics, development, and molecular biology in the space environment. In addition, this plant uses a chemical, glutamate, much as it is used in the human brain, that is, as a chemical messenger. However, as mentioned earlier, the reproducibility of these very short life cycle plant appears to be questionable. Mouse-ear cress was also chosen because its small size allowed it to fit easily into the experiment hardware (Musgrave et al. 1995).

Woody plants, such as conifer, can also be used to evaluate the effect of microgravity on the ability of plants to form a reinforcement tissue known as reaction wood. On Earth, woody plants produce this distinctive reinforcement tissue when their stems are bent contrary to their normal orientation. The reaction wood formation restores the stems to its upright position, which contributes to the plant's survival, but it has an adverse effect on wood quality and texture. Conifer seedlings placed in a plant growth facility (see Chapter 3, Section 3.3) can be used for this research.

Plants can be harvested and preserved chemically to stop their growth and development at predetermined intervals (Figure 2-10), and then frozen for postflight analysis. Electron and light microscopic study of the samples can define the time and place or reaction wood formation, while biochemical analysis enables the scientists to study the regulatory enzymes and genes involved.

Figure 2-10. On the Space Shuttle Columbia's middeck, an astronaut works with the Brassica Rapa plants being grown for comparing changes in ultrastructure, biochemical composition and function induced by the spaceflight environment on the photosynthetic apparatus of its seedlings at different stages of vegetative development. Photo courtesy of NASA.

Figure 2-10. On the Space Shuttle Columbia's middeck, an astronaut works with the Brassica Rapa plants being grown for comparing changes in ultrastructure, biochemical composition and function induced by the spaceflight environment on the photosynthetic apparatus of its seedlings at different stages of vegetative development. Photo courtesy of NASA.

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