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Effects of Common ISS Volatile Organic Compounds on Growth of Radish
Abstract
Radish (Raphanus sativus L.) is a salad type crop that is being evaluated for possible use on the International Space Station (ISS). The study will determine the growth and development of radish in the microgravity environment. A series of experiments were initiated to determine whether volatile organic compounds (VOC) that are commonly accumulated in closed systems of spacecraft atmosphere are biologically active. A survey of existing atmospheric samples from the space shuttle and ISS revealed over 260 compounds with potential biogenic activity of which a subset of 14 compounds have been selected for detailed evaluation. Initial screening is achieved by exposing radishes to VOC concentrations corresponding to 0.1 and 1.0 the Spacecraft Maximum Allowable Concentration (SMAC) of the contaminants. Biogenic effects of ethanol at 0.1 of the SMAC resulted in lower chlorophyll content, reduced growth rate, and lower yields. Chronic exposure to ethanol concentrations at 0.5 SMAC were lethal to radish. Radishes exposed to acetone did not show phytotoxic responses at concentrations up to the SMAC.
A review of past insights of space experiments with plants outlines basic space and gravity effects as well as gene expression. Efforts to grow plants in space gradually incorporated basic question on plant productivity, stress response and cultivation. The prospect of extended space missions as well as colonization of the Moon and Mars require better understanding and therefore research efforts on biomass productivity, substrate and water relations, atmospheric composition, pressure and temperature and substrate and volume (growth space) requirements. The essential combination of using plants not only for food production but also for regeneration of waste, and recycling of carbon and oxygen production requires integration of complex biological and engineering aspects. We combine a historical account of plant space research with considerations for future research on plant cultivation, selection, and productivity based on space-related environmental conditions.
Human space exploration missions require life support systems to sustain the crew. Plants help regulate the earth’s atmospheric and terrestrial environment; similarly, plants could be used in a closed system in space: cleaning carbon dioxide from the air, breaking down liquid and solid wastes, and producing fresh food. Advanced Life Support systems would use plants as part of a bioregenerative system. For space flight applications, the systems must be low in mass and energy consumption, and must be designed for reduced gravity operations. Four areas of research are reviewed in this article related to ALS system design, crop physiology and biology. The Water Offset Nutrient Delivery ExpeRiment (WONDER) will be conducted on the Space Shuttle to evaluate gravity-independent hydroponic systems and optimum water content targets for microgravity crop production. The Photosynthesis Experiment and System Testing and Operation (PESTO) project studied the photosynthesis rates and physiological responses of crops germinated and grown in microgravity. A project to study crop production systems in closed, controlled environments for multiple crops in the same root and aerial environment (International Space Station baseline conditions) is described. Finally, efficient light emitting diode (LED) lighting is being studied to determine plant photobiological responses to different light qualities.
A series of experiments were conducted to determine the sensitivity of radish to four light alcohols (ethanol, methanol, 2-propanol, and t-butanol) identified as atmospheric contaminants on manned spacecraft. Radish (Raphanus sativus L. 'Cherry Bomb' Hybrid II) seedlings were exposed for 5 days to concentrations of 0, 50, 100, 175, 250, and 500 ppm of each alcohol and the effect on seedling growth was used to establish preliminary threshold response values. Results show a general response-pattern for the four alcohol exposures at threshold responses of 10% (T10), 50% (T50) and 90% (T90) reduction in seedling length. There were differences in the response of seedlings to the four alcohols, with the T10 for t-butanol and ethanol (25 to 40 ppm) being 3 to 5x lower than for methanol or 2-propanol (110 to 120 ppm). Ethanol and t-butanol exhibited similar T 50 values (150 to 160 ppm). In contrast, T50 for methanol (285 ppm) and 2-propanol (260 ppm) were about 100 ppm higher than for ethanol or t-butanol. Chronic exposures to 400 ppm t-butanol, ethanol or 2-propanol were highly toxic to the plants. Radish was more tolerant of methanol, with T 90 of 465 ppm. Seeds did not germinate at the 500 ppm treatment of t-butanol, 2-propanol, or ethanol. There were significant differences in projected performance of plants in different environments, dependent upon the regulatory guidelines used. The use of exposure guidelines for humans is not applicable to plant systems.
A set of contained environment chambers have been
designed to study the effects of Volatile Organic
Compounds (VOCs) on plant growth and development.
The Volatile Organic Compound Analysis (VOCA)
system consists of six Lexan chambers, each with
independent VOC monitoring and control capacities. The
VOC exposure chambers are located within a larger
controlled environment chamber (CEC) which provides a
common air temperature, photoperiod, and light control.
Relative humidity, CO2 concentration, and VOC
concentration of the atmosphere are independently
controlled in each VOCA exposure chambers. CO2, air
temperature, relative humidity and PPF are continuously
monitored with software developed using IOControlTM
and IODisplayTM.
The concept of using higher plants to maintain a sustainable life support system for humans during long-duration space missions is dependent upon photosynthesis. The effects of extended exposure to microgravity on the development and functioning of photosynthesis at the leaf and stand levels were examined onboard the International Space Station (ISS). The PESTO (Photosynthesis Experiment Systems Testing and Operations) experiment was the first long-term replicated test to obtain direct measurements of canopy photosynthesis from space under well-controlled conditions. The PESTO experiment consisted of a series of 21-24 day growth cycles of Triticum aestivum L. cv. USU Apogee onboard ISS. Single leaf measurements showed no differences in photosynthetic activity at the moderate (up to 600 micromol m(-2) s(-1)) light levels, but reductions in whole chain electron transport, PSII, and PSI activities were measured under saturating light (>2,000 micromol m(-2) s(-1)) and CO(2) (4000 micromol mol(-1)) conditions in the microgravity-grown plants. Canopy level photosynthetic rates of plants developing in microgravity at approximately 280 micromol m(-2) s(-1) were not different from ground controls. The wheat canopy had apparently adapted to the microgravity environment since the CO(2) compensation (121 vs. 118 micromol mol(-1)) and PPF compensation (85 vs. 81 micromol m(-2) s(-1)) of the flight and ground treatments were similar. The reduction in whole chain electron transport (13%), PSII (13%), and PSI (16%) activities observed under saturating light conditions suggests that microgravity-induced responses at the canopy level may occur at higher PPF intensity.
Phytochemicals are naturally occurring compounds produced by plants. These compounds are essential for normal growth and development and can be specific for a given plant species or cultivar. Phytochemicals also are essential components of human nutrition and include carbohydrates, lipids, proteins, fiber, and vitamins. Secondary phytochemicals, such as polyphenols and flavanoids, provide human nutritional and health benefits. Phytochemicals associated with flavor and aroma play a significant, yet currently unquantified, psychological role in human mental health. In addition to the phytochemicals contained within plants, releasing of volatile and soluble phytochemicals into the environmentoccurs during growth and development. Examples of volatile phytochemicals include the fragrance of flowers and the aroma of freshly cut grass. Examples of soluble phytochemicals include allelopathic substances such as pyrethrins.
Super-Dwarf wheat plants were grown in the Russian/Bulgarian growth chamber called Svet (means light in Russian), in Space Station Mir, from August 12 to November 9, 1995 (90 days) and from August 5 to December 6, 1996 (123 days); a second 1996 crop grew from December 6, 1996 to January 14, 1997 (39 days). Environmental monitoring instrumentation was built at Utah State University and added to Svet for the experiments. That instrumentation functioned well in 1995, but four of six lamp sets (two lamps in each set) failed, as did the controller and a fan. Plants stayed alive but were mostly vegetative (contrary to ground controls under equivalent photon flux). New, higher intensity lamps and other equipment functioned well during 1996, and plants grew surprisingly well, producing about 280 heads and considerable biomass, but the heads were all sterile. A strong case can be made that the sterility was caused by high ethylene in the cabin atmosphere.
During the growing season of 1990, five staggered crops of radish (Raphanus sativus L.) were grown in the field, using the cultivars ‘Cherry Belle’, ‘Red Prince’, and ‘Red Devil B’. Half of the plants received a soil drench (100 ml plant−1; 100 mg litre−1 of ethylenediurea (EDU) once, early in plant development. Destructive harvests were carried out at 2-day intervals during vegetative development. Non-linear growth kinetics, derived from Richards' function, were fitted to the dry weight data of the total plant, main organs (shoot and hypocotyl) and to the dry weight ratio between below-ground and above-ground organs. Estimating the parameters of these non-linear functions and testing their differences between EDU-treated and untreated plants unveiled biologically meaningful information on the impact of different levels of ambient ozone (O3) during the growth periods. The modified function which was applied to the data of biomass partitioning between the major plant parts was more powerful in detecting transient alterations in assimilate allocation compared to the growth dynamics of individual plant organs. At low levels of O3, biomass partitioning towards the below-ground sink organs was slightly delayed and finally restricted in EDU-treated plants. When ambient O3 reached moderate levels, which did not cause visible foliar injury, assimilate partitioning between organs was only insignificantly altered during early growth when EDU-treatments were compared. As growth progressed, however, less assimilates were allocated towards the hypocotyl and roots in the plants not protected by EDU. This pattern was similar in all cultivars tested, but was smallest in ‘Cherry Belle’, which is known to be sensitive to O3 with respect to foliar injury.