Spectral quality may be used to alter plant disease development in CELSS.

Plants were grown under light emitting diode (LED) arrays with different spectral qualities to determine the effects of light on the development of tomato mosaic virus (ToMV) in peppers and powdery mildew on cucumbers. One LED array supplied 100% of the photosynthetic photon flux (PPF) at 660 nm, a second array supplied 90% of the PPF at 660 nm and 10% at 735 nm, and a third array supplied 98% of the PPF at 660 nm with 2% in the blue region (380-500 nm) supplied by blue fluorescent lamps. Control plants were grown under metal halide (MH) lamps. Pepper plants inoculated with ToMV and grown under 660 and 660/735 LED arrays showed marked increases in both the rate and the severity of symptoms as compared to inoculated plants grown under the MH lamp or 660/blue array. Pepper plants grown under the 660/blue array did not develop symptoms as rapidly as inoculated plants grown under the 660 or 660/735 arrays, but they did develop symptoms faster than inoculated plants grown under the MH lamp. The numbers of colonies of powdery mildew per leaf and the size of each colony were greatest on inoculated cucumber plants grown under the MH lamp.     

Integrating biological treatment of crop residue into a hydroponic sweetpotato culture.

Residual biomass from hydroponic culture of sweetpotato [Ipomoea batatas (L.) Lam.] was degraded using natural bacterial soil isolates. Sweetpotato was grown for 120 days in hydroponic culture with a nutrient solution comprised of a ratio of 80% modified half Hoagland solution to 20% filtered effluent from an aerobic starch hydrolysis bioreactor. The phytotoxicity of the effluent was assayed with Waldmann's Green' lettuce (Lactuca sativa L.) and the ratio selected after a 60-day bioassay using sweetpotato plants propagated vegetatively from cuttings. Controlled environment chamber experiments were conducted to investigate the impact of filtrate from biological treatment of crop residue on growth and storage root production with plants grown in a modified half Hoagland solution. Incorporation of bioreactor effluent, reduced storage root yield of 'Georgia Jet' sweetpotato but the decrease was not statistically significant when compared with yield for plants cultured in a modified half Hoagland solution without filtrate. However, yield of 'TU-82-155' sweetpotato was significantly reduced when grown in a modified half Hoagland solution into which filtered effluent had been incorporated. Total biomass was significantly reduced for both sweetpotato cultivars when grown in bioreactor effluent. The leaf area and dry matter accumulation were significantly (P < 0.05) reduced for both cultivars when grown in solution culture containing 20% filtered effluent.     

Carbon balance and productivity of Lemna gibba, a candidate plant for CELSS.

The photosynthesis and productivity of Lemna gibba were studied with a view to its use in Controlled Ecological Life Support Systems (CELSS). Photosynthesis of L. gibba floating on the nutrient solution could be driven by light coming from either above or below. Light from below was about 75% as effective as from above when the stand was sparse, but much less so with dense stands. High rates of photosynthesis (ca. 800 nanomoles CO2 g dry weight (DW)-1 s-1) were measured at 750 micromoles m-2 s-1 PPF and 1500 micromoles mol-1 CO2. This was attained at densities up to 660 g fresh weight (FW) m-2 with young cultures. After a few days growth under these conditions, and at higher densities, the rate of photosynthesis dropped to less than 25% of the initial value. This drop was only partly alleviated by thinning the stand or by introducing a short dark period at high temperature (26 degrees C). Despite the drop in the rate of photosynthesis, maximum yields were obtained in batch cultures grown under continuous light, constant temperature and high [CO2]. Plant protein content was less than reported for field grown Lemna. When the plants were harvested daily, maintaining a stand density of 600 g FW m-2, yields of 18 g DW m-2 d-1 were obtained. The total dry weight of L. gibba included 40% soluble material (sugars and amino acids), 15% protein, 5% starch, 5% ash and 35% cellulose and other polymers. We conclude that a CELSS system could be designed around stacked, alternate layers of transparent Lemna trays and lamps. This would allow for 7 tiers per meter height. Based on present data from single layers, the yield of such a system is calculated to be 135 g DW m-3 d-1 of a 100% edible, protein-rich food.     

Effect of CO2 levels on nutrient content of lettuce and radish.

Atmospheric carbon-dioxide enrichment is known to affect the yield of lettuce and radish grown in controlled environments, but little is known about CO2 enrichment effects on the chemical composition of lettuce and radish. These crops are useful model systems for a Controlled Ecological Life-Support System (CELSS), largely because of their relatively short production cycles. Lettuce (Lactuca sativa L.) cultivar 'Waldmann's Green' and radish (Raphanus sativus L.) cultivar 'Giant White Globe' were grown both in the field and in controlled environments, where hydroponic nutrient solution, light, and temperature were regulated, and where CO2 levels were controlled at 400, 1000, 5000, or 10,000 ppm. Plants were harvested at maturity, dried, and analyzed for proximate composition (protein, fat, ash, and carbohydrate), total nitrogen (N), nitrate N, free sugars, starch, total dietary fiber, and minerals. Total N, protein N, nonprotein N (NPN), and nitrate N generally increased for radish roots and lettuce leaves when grown under growth chamber conditions compared to field conditions. The nitrate-N level of lettuce leaves, as a percentage of total NPN, decreased with increasing levels of CO2 enrichment. The ash content of radish roots and of radish and lettuce leaves decreased with increasing levels of CO2 enrichment. The levels of certain minerals differed between field- and chamber-grown materials, including changes in the calcium (Ca) and phosphorus (P) contents of radish and lettuce leaves, resulting in reduced Ca/P ratio for chamber-grown materials. The free-sugar contents were similar between the field and chamber-grown lettuce leaves, but total dietary fiber content was much higher in the field-grown plant material. The starch content of growth-chamber lettuce increased with CO2 level.     

Growth of soybean and potato at high CO2 partial pressures.

Soybean and potato plants were grown in controlled environments at carbon dioxide (CO2) partial pressures ranging from 0.05 to 1.00 kPa. The highest yields of edible biomass occurred at 0.10 kPa for both species, with higher CO2 levels being supraoptimal, but not injurious to the plants. Stomatal conductance rates of upper canopy leaves were lowest at 0.10 kPa CO2, while conductance rates at 0.50 and 1.00 kPa were significantly greater than 0.10 kPa. Total water use by the plants was greatest at the highest CO2 pressures (i.e. 0.50 and 1.00 kPa); consequently, water use efficiencies (biomass produced/water used) were low at the highest CO2 pressures. Based on previous CO2 studies in the literature, the increased conductance and water use at the highest CO2 pressures were surprising and pose interesting challenges for managing plants in a CELSS, where CO2 pressures may exceed optimal levels.     

Super-optimal CO2 reduces wheat yield in growth chamber and greenhouse environments.

Seven growth chamber trials (six replicate trials using 0.035, 0.12, and 0.25% CO2 in air and one trial using 0.12, 0.80, and 2.0% CO2 in air) and three replicate greenhouse trials (0.035, 0.10, 0.18, 0.26, 0.50, and 1.0% CO2 in air) compare the effects of super-optimal CO2 on the seed yield, harvest index, and vegetative growth rate of wheat (Triticum aestivum L. cvs. USU-Apogee and Veery-10). Plants in the growth chamber trials were grown hydroponically under fluorescent lamps, while the greenhouse trials were grown under sunlight and high pressure sodium lamps and in soilless media. Plants in the greenhouse trials responded similarly to those in the growth chamber trials; maximum yields occurred near 0.10 and 0.12% CO2 and decreased significantly thereafter. This research indicates that the toxic effects of elevated CO2 are not specific to only one environment and has important implications for the design of bio-regenerative life support systems in space, and for the future of terrestrial agriculture.     

Regulating plant/insect interactions using CO2 enrichment in model ecosystems.

The greenhouse environment is a challenging artificial ecosystem in which it is possible to study selected plant/insect interaction in a controlled environment. Due to a combination of "direct" and "indirect" effects of CO2 enrichment on plant photosynthesis and plant development, canopy productivity is generally increased. In this paper, we discuss the effects of daytime and nighttime CO2 enrichment protocols on gas exchange of pepper plants (Capsicum annuum L, cv Cubico) grown in controlled environments. In addition, we present the effects of thrips, a common Insect pest, on the photosynthetic and respiratory activity of these plant canopies. Carbon dioxide has diverse effects on the physiology and mortality of insects. However, our data indicate that thrips and whiteflies, at least, are not killed "directly" by CO2 levels used to enhance photosynthesis and plant growth. Together the data suggest that the insect population is affected "indirectly" by CO2 and that the primary effect of CO2 is via its effects on plant metabolism.     

Potato growth in a porous tube water and nutrient delivery system.

Potato (Solanum tuberosum L.) cv. 'Norland', vegetative growth and tuber productivity grown in the porous water and nutrient delivery system (PTNDS) developed by the Wisconsin Center for Space Automation and Robotics were compared with the vegetative growth and tuber productivity of plants grown in a peat:vermiculite potting mixture (PT/VR). The plants were grown at 12, 16, and 24-h light periods, 18 degrees C constant temperature, 70% relative humidity, and 300 micromol m-2 s-1 photosynthetic photon flux. Canopy height of plants grown in the PT/VR system was taller than that of plants grown in the PTNDS system. Canopy height differences were greatest when the plants were grown under a 24-h photoperiod. Leaf and stem dry masses were similar for plants grown in the two systems under the 12-h photoperiod. Under the 24-h photoperiod, leaf and stem dry masses of plants grown in the PT/VR system were more than 3 times those of plants grown in the PTNDS system. Tuber dry masses were similar for plants grown in the two systems under the 12-h photoperiod. Under the 24 h-photoperiod, tuber dry weights of plants grown in the PT/VR system were more than twice those of plants grown in the PTNDS system. A slightly higher harvest index (ratio of tuber weight to leaf plus stem weight) was noted for the plants grown in the PTNDS than for the plants grown in the PT/VR system. Plants grown in the PTNDS system at the 24-h photoperiod matured earlier than plants grown at this photoperiod in the PT/VR system. Vegetative growth and tuber productivity of plants grown under the 16-h photoperiod generally were intermediate to those noted for plants grown under the 12 and 24-h photoperiods. These results indicate that potato plants grown in a PTNDS system may require less plant growing volume, mature in a shorter time, and likely produce more tubers per unit area compared with plants grown in the PT/VR system. These plant characteristics are a distinct advantage for a plant growing unit of a CELSS.     

Proximate nutritional composition of CELSS crops grown at different CO2 partial pressures.

Two CELSS candidate crops, soybean (Glycine max) and potato (Solanum tuberosum), were grown hydroponically in controlled environments maintained at carbon dioxide (CO2) partial pressures ranging from 0.05 to 1.00 kPa (500 to 10,000 ppm at 101 kPa atmospheric pressure). Plants were harvested at maturity (90 days for soybean and 105 days for potato) and all tissues analyzed for proximate nutritional composition (i.e. protein, fat, carbohydrate, crude fiber, and ash content). Soybean seed ash and crude fiber were higher and carbohydrate was lower than values reported for field-grown seed. Potato tubers showed little difference from field-grown tubers. With the exception of increased crude fiber of soybean seed with increased CO2, no trends were apparent with regard to CO2 effects on proximate composition of soybean seed and potato tubers. Crude fiber of soybean stems and leaves increased with increased CO2, as did soybean leaf protein (total nitrogen). Potato leaf and stem (combined) protein levels also increased with increased CO2, while leaf and stem carbohydrates decreased. Values for leaf and stem protein and ash were higher than values generally reported for field-grown plants for both species. Results suggest that CO2 partial pressure should have little influence on proximate composition of potato tubers or soybean seed, but that high ash and protein levels might be expected from leaves and stems of crops grown in controlled environments of a CELSS.     

CO2 crop growth enhancement and toxicity in wheat and rice.

The effects of elevated CO2 on plant growth are reviewed and the implications for crop yields in regenerative systems are discussed. There is considerable theoretical and experimental evidence indicating that the beneficial effects of CO2 are saturated at about 0.12% CO2 in air. However, CO2 can easily rise above 1% of the total gas in a closed system, and we have thus studied continuous exposure to CO2 levels as high as 2%. Elevating CO2 from 340 to 1200 micromoles mol-1 can increase the seed yield of wheat and rice by 30 to 40%; unfortunately, further CO2 elevation to 2500 micromoles mol-1 (0.25%) has consistently reduced yield by 25% compared to plants grown at 1200 micromoles mol-1; fortunately, there was only an additional 10% decrease in yield as the CO2 level was further elevated to 2% (20,000 micromoles mol-1). Yield increases in both rice and wheat were primarily the result of increased number of heads per m2, with minor effects on seed number per head and seed size. Yield increases were greatest in the highest photosynthetic photon flux. We used photosynthetic gas exchange to analyze CO2 effects on radiation interception, canopy quantum yield, and canopy carbon use efficiency. We were surprised to find that radiation interception during early growth was not improved by elevated CO2. As expected, CO2 increased quantum yield, but there was also a small increase in carbon use efficiency. Super-optimal CO2 levels did not reduce vegetative growth, but decreased seed set and thus yield. The reduced seed set is not visually apparent until final yield is measured. The physiological mechanism underlying CO2 toxicity is not yet known, but elevated CO2 levels (0.1 to 1% CO2) increase ethylene synthesis in some plants and ethylene is a potent inhibitor of seed set in wheat.     

Biochemical,photosynthetic and productive parameters of Chinese cabbage grown under blue–red LED assembly designed for space agriculture

Currently light emitting diodes (LEDs) are considered to be most preferable source for space plant growth facilities. We performed a complex study of growth and photosynthesis in Chinese cabbage plants (Brassica chinensis L.) grown with continuous LED lighting based on red (650 nm) and blue (470 nm) LEDs with a red to blue photon ratio of 7:1. Plants grown with high-pressure sodium (HPS) lamps were used as a control. PPF levels used were about 100 μmol/(m² s) (PPF 100) and nearly 400 μmol/(m² s) (PPF 400). One group of plants was grown with PPF 100 and transferred to PPF 400 at the age of 12 days. Plants were studied at the age of 15 and 28 days (harvest age); some plants were left to naturally end their life cycle. We studied a number of parameters reflecting different stages of photosynthesis: photosynthetic pigment content; chlorophyll fluorescence parameters (photosystem II quantum yield, photochemical and non-photochemical chlorophyll fluorescence quenching); electron transport rate, proton gradient on thylakoid membranes (ΔpH), and photophosphorylation rate in isolated chloroplasts. We also tested parameters reflecting plant growth and productivity: shoot and root fresh and dry weight, sugar content and ascorbic acid content in shoots. Our results had shown that at PPF 100, plants grown with LEDs did not differ from control plants in shoot fresh weight, but showed substantial differences in photophosphorylation rate and sugar content. Differences observed in plants grown with PPF 100 become more pronounced in plants grown with PPF 400. Most parameters characterizing the plant photosynthetic performance, such as photosynthetic pigment content, electron transport rate, and ΔpH did not react strongly to light spectrum. Photophosphorylation rate differed strongly in plants grown with different spectrum and PPF level, but did not always reflect final plant yield. Results of the present work suggest that narrow-band LED lighting caused changes in Chinese cabbage plants on levels of the photosynthetic apparatus and the whole plant, concerning its development and adaptation to a varying PPF level.     

Comparison of aerobically-treated and untreated crop residue as a source of recycled nutrients in a recirculating hydroponic system.

This study compared the growth of potato plants on nutrients recycled from inedible potato biomass. Plants were grown for 105 days in recirculating, thin-film hydroponic systems containing four separate nutrient solution treatments: (1) modified half-strength Hoagland's (control), 2) liquid effluent from a bioreactor containing inedible potato biomass, 3) filtered (0.2 micrometer) effluent, and 4) the water soluble fraction of inedible potato biomass (leachate). Approximately 50% of the total nutrient requirement in treatments 2-4 were provided (recycled) from the potato biomass. Leachate had an inhibitory effect on leaf conductance, photosynthetic rate, and growth (50% reduction in plant height and 60% reduction in tuber yield). Plants grown on bioreactor effluent (filtered or unfiltered) were similar to the control plants. These results indicated that rapidly degraded, water soluble organic material contained in the inedible biomass, i.e., material in leachate, brought about phytotoxicity in the hydroponic culture of potato. Recalcitrant, water soluble organic material accumulated in all nutrient recycling treatments (650% increase after 105 days), but no increase in rhizosphere microbial numbers was observed.     

Utilization of white potatoes in CELSS.

Potatoes (Solanum tuberosum) have a strong potential as a useful crop species in a functioning CELSS. The cultivar Denali has produced 37.5 g m-2 d-1 when grown for 132 days with the first 40 days under a 12-h photoperiod and a light:dark temperature cycle of 20 degrees C:16 degrees C, and then 92 days under continuous irradiance and a temperature of 16 degrees C. Irradiance was at 725 micromoles m-2 s-1 PPF and carbon dioxide at 1000 micromoles mol-1. The dried tubers had 82% carbohydrates, 9% protein and 0.6% fat. Other studies have shown that carbon dioxide supplementation (1000 micromoles mol-1) is of significant benefit under 12-h irradiance but less benefit under 24 h irradiance. Irradiance cycles of 60 minutes light and 30 minutes dark caused a reduction of more than 50% in tuber weight compared to cycles of 16 h light and 8 h dark. A diurnal temperature change of 22 degrees C for the 12-h light period to 14 degrees C during the 12-h dark period gave increased yields of 30% and 10% for two separate cultivars, compared with plants grown under a constant 18 degrees C temperature. Cultivar screening under continuous irradiance and elevated temperatures (28 degrees C) for 8 weeks of growth indicated that the cvs Haig, Denali, Atlantic, Desiree and Rutt had the best potential for tolerance to these conditions. Harvesting of tubers from plants at weekly intervals, beginning at 8 weeks after planting, did not increase yield over a single final harvest. Spacing of plants on 0.055 centers produced greater yield per m2 than spacing at 0.11 or 0.22 m2. Plants maintained 0.33 meters apart (0.111 m2 per plant) in beds produced the same yields when separated by dividers in the root matrix as when no separation was made.     

Yield comparisons and unique characteristics of the dwarf wheat cultivar 'USU-Apogee'.

Extremely short, high yielding cultivars of all crop plants are needed to optimize the food production of bioregenerative life support systems in space. In the early 1980's, we examined over a thousand wheat genotypes from the world germplasm collection in search of genotypes with appropriate characteristics for food production in space. Here we report the results of 12 years of hybridization and selection for the perfect wheat cultivar. 'USU-Apogee' is a full-dwarf hard red spring wheat (Triticum aestivum L.) cultivar developed for high yields in controlled environments. USU-Apogee was developed by the Utah Agricultural Experiment Station in cooperation with the National Aeronautics and Space Administration and released in April 1996. USU-Apogee is a shorter, higher yielding alternative to 'Yecora Rojo' and 'Veery-10', the short field genotypes previously selected for use in controlled environments. The yield advantage of USU-Apogee is 10 to 30% over these other cultivars, depending on environmental conditions. USU-Apogee (45-50 cm tall, depending on temperature) is 10 to 15 cm shorter than Yecora Rojo and 1 to 4 cm shorter than Veery-10. USU-Apogee was also selected for resistance to the calcium-induced leaf tip chlorosis that occurs in controlled-environments. Breeder seed of USU-Apogee will be maintained by the Crop Physiology Laboratory and seed is available for testing on request.     

Effects of lighting and air movement on temperatures in reproductive organs of plants in a closed plant growth facility

Temperature increases in plant reproductive organs such as anthers and stigmas could cause fertility impediments and thus produce sterile seeds under artificial lighting conditions without adequately controlled environments in closed plant growth facilities. There is a possibility such a situation could occur in Bioregenerative Life Support Systems under microgravity conditions in space because there will be little natural convective or thermal mixing. This study was conducted to determine the temperature of the plant reproductive organs as affected by illumination and air movement under normal gravitational forces on the earth and to make an estimation of the temperature increase in reproductive organs in closed plant growth facilities under microgravity in space. Thermal images of reproductive organs of rice and strawberry were captured using infrared thermography at air temperatures of 10–11 °C. Compared to the air temperature, temperatures of petals, stigmas and anthers of strawberry increased by 24, 22 and 14 °C, respectively, after 5 min of lighting at an irradiance of 160 W m⁻² from incandescent lamps. Temperatures of reproductive organs and leaves of strawberry were significantly higher than those of rice. The temperatures of petals, stigmas, anthers and leaves of strawberry decreased by 13, 12, 13 and 14 °C, respectively, when the air velocity was increased from 0.1 to 1.0 ms⁻¹. These results show that air movement is necessary to reduce the temperatures of plant reproductive organs in plant growth facilities.     

Excess nutrients in hydroponic solutions alter nutrient content of rice, wheat, and potato.

Environment has significant effects on the nutrient content of field-grown crop plants. Little is known, however, about compositional changes caused by controlled environments in which plants receive only artificial radiation and soilless, hydroponic culture. This knowledge is essential for developing a safe, nutritious diet in a Controlled Ecological Life-Support System (CELSS). Three crops that are candidates for inclusion in a CELSS (rice, wheat, and white potato) were grown both in the field and in controlled environments where the hydroponic nutrient solution, photosynthetic photon flux (PPF), and CO2 level were manipulated to achieve rapid growth rates. Plants were harvested at maturity, separated into discrete parts, and dried prior to analysis. Plant materials were analyzed for proximate composition (protein, fat, ash, and carbohydrate), total nitrogen (N), nitrate, minerals, and amino-acid composition. The effect of environment on nutrient content varied by crop and plant part. Total N and nonprotein N (NPN) contents of plant biomass generally increased under controlled-environment conditions compared to field conditions, especially for leafy plant parts and roots. Nitrate levels were increased in hydroponically-grown vegetative tissues, but nitrate was excluded from grains and tubers. Mineral content changes in plant tissue included increased phosphorus and decreased levels of certain micronutrient elements under controlled-environment conditions. These findings suggest that cultivar selection, genetic manipulation, and environmental control could be important to obtain highly nutritious biomass in a CELSS.     

Growing root, tuber and nut crops hydroponically for CELSS.

Among the crops selected by the National Aeronautics and Space Administration for growth in controlled ecological life support systems are four that have subsurface edible parts -- potatoes, sweet potatoes, sugar beets and peanuts. These crops have been produced in open and closed (recirculating), solid media and liquid, hydroponic systems. Fluorescent , fluorescent plus incandescent and high pressure sodium plus metal halide lamps have proven to be effective light sources. Continuous light with 16 degrees C and 28/22 degrees C (day/night) temperatures have produced highest yields for potato and sweet potato, respectively. Dry weight yields of up to 4685, 2541, 1151 and 207 g m-2 for for potatoes, sweet potatoes, sugar beets and peanuts, respectively, have been produced in controlled environment hydroponic systems.     

Controlled environments alter nutrient content of soybeans.

Information about compositional changes in plants grown in controlled environments is essential for developing a safe, nutritious diet for a Controlled Ecomological Life-Support System (CELSS). Information now is available for some CELSS candidate crops, but detailed information has been lacking for soybeans. To determine the effect of environment on macronutrient and mineral composition of soybeans, plants were grown both in the field and in a controlled environment where the hydroponic nutrient solution, photosynthetic flux (PPF), and CO2 level were manipulated to achieve rapid growth rates. Plants were harvested at seed maturity, separated into discrete parts, and oven dried prior to chemical analysis. Plant material was analyzed for proximate composition (moisture, protein, lipid, ash, and carbohydrate), total nitrogen (N), nonprotein N (NPN), nitrate, minerals, amino acid composition, and total dietary fiber. The effect of environment on composition varied by cultivar and plant part. Chamber-grown plants generally exhibited the following characteristics compared with field-grown plants: 1) increased total N and protein N for all plant parts, 2) increased nitrate in leaves and stems but not in seeds, 3) increased lipids in seeds, and 4) decreased Ca:P ratio for stems, pods, and leaves. These trends are consistent with data for other CELSS crops. Total N, protein N, and amino acid contents for 350 ppm CO2 and 1000 ppm CO2 were similar for seeds, but protein N and amino acid contents for leaves were higher at 350 ppm CO2 than at 1000 ppm CO2. Total dietary fiber content of soybean leaves was higher with 350 ppm CO2 than with 1000 ppm CO2. Such data will help in selecting of crop species, cultivars, and growing conditions to ensure safe, nutritious diets for CELSS.     

Growth and development in higher plants under simulated microgravity conditions on a 3-dimensional clinostat.

Growth and development of etiolated pea (Pisum sativum L. cv. Alaska) and maize (Zea mays L. cv. Golden Cross Bantam) seedlings grown under simulated microgravity conditions were intensively studied using a 3-dimensional clinostat as a simulator of weightlessness. Epicotyls of etiolated pea seedlings grown on the clinostat were the most oriented toward the direction far from cotyledons. Mesocotyls of etiolated maize seedlings grew at random and coleoptiles curved slightly during clinostat rotation. Clinostat rotation promoted the emergence of the 3rd internodes in etiolated pea seedlings, while it significantly inhibited the growth of the 1st internodes. In maize seedlings, the growth of coleoptiles was little affected by clinostat rotation, but that of mesocotyls was suppressed, and therefore, the emergence of the leaf out of coleoptile was promoted. Clinostat rotation reduced the osmotic concentration in the 1st internodes of pea seedlings, although it has little effect on the 2nd and the 3rd internodes. Clinostat rotation also reduced the osmotic concentrations in both coleoptiles and mesocotyls of maize seedlings. Cell-wall extensibilities of the 1st and the 3rd internodes of pea seedlings grown on the clinostat were significantly lower and higher as compared with those on 1 g conditions, respectively. Cell-wall extensibility of mesocotyls in seedlings grown on the clinostat also decreased. Changes in cell wall properties seem to be well correlated to the growth of each organ in pea and maize seedlings. These results suggest that the growth and development of plants is controlled under gravity on earth, and that the growth responses of higher plants to microgravity conditions are regulated by both cell-wall mechanical properties and osmotic properties of stem cells.     

Effects of long-term low atmospheric pressure on gas exchange and growth of lettuce

The objectives of this research were to determine photosynthesis, evapotranspiration and growth of lettuce at long-term low atmospheric pressure. Lettuce (Lactuca sativa L. cv. Youmaicai) plants were grown at 40 kPa total pressure (8.4 kPa _pO₂$p_{{\text{O}}_2}$) or 101 kPa total pressure (20.9 kPa _pO₂$p_{{\text{O}}_2}$) from seed to harvest for 35 days. Germination rate of lettuce seeds decreased by 7.6% at low pressure, although this was not significant. There was no significant difference in crop photosynthetic rate between hypobaria and ambient pressure during the 35-day study. The crop evapotranspiration rate was significantly lower at low pressure than that at ambient pressure from 20 to 30 days after planting (DAP), but it had no significant difference before 20 DAP or after 30 DAP. The growth cycle of lettuce plants at low pressure was delayed. At low pressure, lettuce leaves were curly at the seedling stage and this disappeared gradually as the plants grew. Ambient lettuce plants were yellow and had an epinastic growth at harvest. The shoot height, leaf number, leaf length and shoot/root ratio were lower at low pressure than those at ambient pressure, while leaf area and root growth increased. Total biomass of lettuce plants grown at two pressures had no significant difference. Ethylene production at low pressure decreased significantly by 38.8% compared with ambient pressure. There was no significant difference in microelements, nutritional phytochemicals and nitrate concentrations at the two treatments. This research shows that lettuce can be grown at long-term low pressure (40 kPa) without significant adverse effects on seed germination, gas exchange and plant growth. Furthermore, ethylene release was reduced in hypobaria.