Separating the effects of hypobaria and hypoxia on lettuce: growth and gas exchange

The objectives of this research were to determine the influence of hypobaria (reduced atmospheric pressure) and reduced partial pressure of oxygen (pO2) [hypoxia] on carbon dioxide (CO2) assimilation (C(A)), dark-period respiration (DPR) and growth of lettuce (Lactuca sativa L. cv. Buttercrunch). Lettuce plants were grown under variable total gas pressures [25 and 101 kPa (ambient)] at 6, 12 or 21 kPa pO2)(approximately the partial pressure in air at normal pressure). Growth of lettuce was comparable between ambient and low total pressure but lower at 6 kPa pO2 (hypoxic) than at 12 or 21 kPa pO2. The specific leaf area of 6 kPa pO2 plants was lower, indicating thicker leaves associated with hypoxia. Roots were most sensitive to hypoxia, with a 50-70% growth reduction. Leaf chlorophyll levels were greater at low than at ambient pressure. Hypobaria and hypoxia did not affect plant water relations. While hypobaria did not adversely affect plant growth or C(A), hypoxia did. There was comparable C(A) and a lower DPR in low than in ambient total pressure plants under non-limiting CO2 levels (100 Pa pCO2, nearly three-fold that in normal air). The C(A)/DPR ratio was higher at low than at ambient total pressure, particularly at 6 kPa pO2- indicating a greater efficiency of C(A)/DPR in low-pressure plants. There was generally no significant interaction between hypoxia and hypobaria. We conclude that lettuce can be grown under subambient pressure ( congruent with25% of normal earth ambient total pressure) without adverse effects on plant growth or gas exchange. Furthermore, hypobaric plants were more resistant to hypoxic conditions that reduced gas exchange and plant growth.     

Hypobaria and hypoxia affects phytochemical production, gas exchange, and growth of lettuce

Hypobaria (low total atmospheric pressure) is essential in sustainable, energy-efficient plant production systems for long-term space exploration and human habitation on the Moon and Mars. There are also critical engineering, safety, and materials handling advantages of growing plants under hypobaria, including reduced atmospheric leakage from extraterrestrial base environments. The potential for producing crops under hypobaria and manipulating hypoxia (low oxygen stress) to increase health-promoting bioactive compounds is not well characterized. Here we showed that hypobaric-grown lettuce plants (25 kPa ≈ 25% of normal pressure) exposed to hypoxia (6 kPa pO₂ ≈ 29% of normal pO₂) during the final 3 d of the production cycle had enhanced antioxidant activity, increased synthesis of anthocyananins, phenolics, and carotenoids without reduction of photosynthesis or plant biomass. Net photosynthetic rate (P _N) was not affected by total pressure. However, 10 d of hypoxia reduced P _N, dark respiration rate (R _D), P _N/R _D ratio, and plant biomass. Growing plants under hypobaria and manipulating hypoxia during crop production to enhance health-promoting bioactive compounds is important for the health and well-being of astronauts exposed to space radiation and other stresses during long-term habitation.     

Effect of hypobaric conditions on ethylene evolution and growth of lettuce and wheat

Elevated levels of ethylene occur in enclosed crop production systems and in spaceflight environments, leading to adverse plant growth and sterility. There are engineering advantages in growing plants at hypobaric (reduced atmospheric pressure) conditions in biomass production for extraterrestrial base or spaceflight environments. Objectives of this research were to characterize the influence of hypobaria on growth and ethylene evolution of lettuce (Lactuca sativa) and wheat (Triticum aestivum). Plants were grown under variable total gas pressures [from 30 to 101 kPa (ambient)]. In one study, lettuce and wheat were direct seeded, germinated and grown in the same chambers for 28 d at 50 or 101 kPa. Hypobaria increased plant growth and did not alter germination rate. During a 10-day study, 28-day-old lettuce and 40-day-old wheat seedlings were transplanted together in the same low and ambient pressure chambers; ethylene accumulated in the chambers, but the rate of production by both lettuce and wheat was reduced more than 65% under 30 kPa compared with ambient pressure (101 kPa). Low O2 concentrations [partial pressure of O2 (pO2) = 6.2 kPa] inhibited ethylene production by lettuce under both low (30 kPa) and ambient pressure, whereas ethylene production by wheat was inhibited at low pressure but not low O2 concentration. There was a negative linear correlation between increasing ethylene concentration and decreasing chlorophyll content of lettuce and wheat. Lettuce had higher production of ethylene and showed greater sensitivity to ethylene than wheat. The hypobaric effect on reduced ethylene production was greater than that of just hypoxia (low oxygen).     

Ethylene reduces plant gas exchange and growth of lettuce grown from seed to harvest under hypobaric and ambient total pressure

Naturally occurring high levels of ethylene can be a problem in spaceflight and controlled environment agriculture (CEA) leading to sterility and irregular plant growth. There are engineering and safety advantages of growing plants under hypobaria (low pressure) for space habitation. The goals of this research were to successfully grow lettuce (Lactuca sativa cv. Buttercrunch) in a long-term study from seed to harvest under hypobaric conditions, and to investigate how endogenously produced ethylene affects gas exchange and plant growth from seed germination to harvest under hypobaric and ambient total pressure conditions. Lettuce was grown under two levels of total gas pressure [hypobaric or ambient (25 or 101 kPa)] in a long-term, 32-day study. Significant levels of endogenous ethylene occurred by day-15 causing reductions in photosynthesis, dark-period respiration, and a subsequent decrease in plant growth. Hypobaria did not mitigate the adverse ethylene effects on plant growth. Seed germination was not adversely affected by hypobaria, but was reduced by hypoxia (6 kPa pO₂). Under hypoxia, seed germination was higher under hypobaria than ambient total pressure. This research shows that lettuce can be grown from seed to harvest under hypobaria (≅25% of normal earth ambient total pressure).     

Ethylene reduces gas exchange and growth of lettuce plants under hypobaric and normal atmospheric conditions

Elevated levels of ethylene occur in controlled environment agriculture and in spaceflight environments, leading to adverse plant growth and sterility. The objectives of this research were to characterize the influence of ethylene on carbon dioxide (CO₂) assimilation (C_A), dark period respiration (DPR) and growth of lettuce ( Lactuca sativa L. cv. Buttercrunch) under ambient and low total pressure conditions. Lettuce plants were grown under variable total gas pressures of 25 kPa (hypobaric) and 101 kPa (ambient) pressure. Endogenously produced ethylene accumulated and reduced C_A, DPR and plant growth of ambient and hypobaric plants. There was a negative linear correlation between increasing ethylene concentrations [from 0 to around 1000 nmol mol⁻¹ (ppb)] on C_A, DPR and growth of ambient and hypobaric plants. Declines in C_A and DPR occurred with both exogenous and endogenous ethylene treatments. C_A was more sensitive to increasing ethylene concentration than DPR. There was a direct, negative effect of increasing ethylene concentration reducing gas exchange as well as an indirect ethylene effect on leaf epinasty, which reduced light capture and C_A. While the C_A was comparable, there was a lower DPR in hypobaric than ambient pressure plants – independent of ethylene and under non-limiting CO₂ levels (100 Pa pCO₂, nearly three-fold that in normal air). This research shows that lettuce can be grown under hypobaria (≅25% of normal earth ambient total pressure); however, hypobaria caused no significant reduction of endogenous ethylene production.     

Germination and growth of lettuce (Lactuca sativa) at low atmospheric pressure

The response of lettuce ( Lactuca sativa L. cv. Waldmann's Green) to low atmospheric pressure was examined during the initial 5 days of germination and emergence, and also during subsequent growth to vegetative maturity at 30 days. Growth took place inside a 66-l-volume low pressure chamber maintained at 70 kPa, and plant response was compared to that of plants in a second, matching chamber that was at ambient pressure (approximately 101 kPa) as a control. In other experiments, to determine short-term effects of low pressure transients, plants were grown at ambient pressure until maturity and then subjected to alternating periods of 24 h of low and ambient atmospheric pressures. In all treatments the partial pressure of O₂ was maintained at 21 kPa (approximately the partial pressure in air at normal pressure), and the partial pressure of CO₂ was in the range 66.5–73.5 Pa (about twice that in normal air) in both chambers, with the addition of CO₂ during the light phase. With continuous exposure to low pressure, shoot and root growth was at least as rapid as at ambient pressure, with an overall trend towards slightly greater performance at the lower pressure. Dark respiration rates were greater at low pressure. Transient periods at low pressure decreased transpiration and increased dark respiration but only during the period of exposure to low pressure. We conclude that long-term or short-term exposure to subambient pressure (70 kPa) was without detectable detriment to vegetative growth and development.     

Autonomous Biological System (ABS) experiments.

Three space flight experiments have been conducted to test and demonstrate the use of a passively controlled, materially closed, bioregenerative life support system in space. The Autonomous Biological System (ABS) provides an experimental environment for long term growth and breeding of aquatic plants and animals. The ABS is completely materially closed, isolated from human life support systems and cabin atmosphere contaminants, and requires little need for astronaut intervention. Testing of the ABS marked several firsts: the first aquatic angiosperms to be grown in space; the first higher organisms (aquatic invertebrate animals) to complete their life cycles in space; the first completely bioregenerative life support system in space; and, among the first gravitational ecology experiments. As an introduction this paper describes the ABS, its flight performance, advantages and disadvantages.     

Physiological,elemental, and stable isotope responses of the organs of mungbean to reduced atmospheric pressure

The effects of hypobaric conditions on stable isotope and mineral element concentrations during the germination of mungbean [Vigna radiata (Linn.) Wilczek] were evaluated. Mungbean seeds were cultured in lower atmospheric pressure (60 kPa) and normal air pressure (101 kPa) conditions, respectively. Oxygen and carbon dioxide partial pressures were maintained at 21 and 0.04 kPa, respectively. At 60 kPa, the fresh weight (FW) and dry weight (DW) of plants significantly increased by 5.41 and 9.62%, respectively, compared to those at 101 kPa after culturing for 7 d. Twelve mineral elements were compared among three organs (leaf, stem, and root) from seedlings grown under hypobaric and normal atmospheric conditions. This showed that lower air pressure generally improved element accumulation in the plant. A significantly lower value of δ ¹³C was observed at 60 kPa compared to that at 101 kPa. In addition, a significant increase in δ ¹⁵N value was detected in three different organs of plants grown under 60 kPa. Our survey provides a foundation for future field and laboratory studies on the influence of air pressure on plants, particularly in terms of stable isotope and mineral elements.     

Bios-3: Siberian experiments in bioregenerative life support

The Russian experience with the bioregenerative life support system Bios-3 at Krasnoyarsk, Siberia, is reviewed. A brief review of other bioregenerative systems examines Biosphere 2 in Oracle, Arizona, and the Bios-1 and Bios-2 systems that preceded Bios-3. Physical details of the Bios-3 facility are provided. The use of Chlorella and higher plants for gas exchange is examined. Long-term studies of human habitation are discussed. Other topics include microflora in Bios-3, the theory of closed systems, and problems for the future.     

Effects of hypobaric growth conditions on morphogenic potential and antioxidative enzyme activities in Saussurea involucrata

Effects of reduced atmospheric pressure on morphogenic potential and antioxidative enzyme activities in regenerated tissues of Saussurea involucrata were evaluated. Leaf explants were cultured at atmospheric pressure 30, 60 or 101 kPa on Murashige and Skoog (MS) medium with several plant growth regulators (PGRs). Oxygen and carbon dioxide partial pressures were maintained at 21 and 0.038 kPa, respectively. At 60 kPa, 12 shoots per explant were recorded, which was 1.5 and 2.1-folds higher than at 101 and 30 kPa, respectively. A shooting frequency of 80 % was observed at 60 and 101 kPa. Rooted plantlets were obtained on MS medium with indoleacetic acid. At 30, 60 and 101 kPa, rooting of shoots was 49, 72 and 85.6 %, respectively. The rooted plantlets were successfully acclimatized to soil. Activities of all of antioxidative enzymes determined in present study were affected by hypobaric conditions.     

Mechanism of soybean nodule adaptation to different oxygen pressures

Abstract. Soybean nodules showed the ability to adapt to oxygen pressures above and below ambient levels and this adaptation involved a decrease in cortical intercellular air-spaces with increasing oxygen pressure. Nodules were grown in oxygen pressures from 4.7 to 75 kPa and the decrease in number and size of cortical intercellular spaces with increasing oxygen pressure was the result of a change in cell structure and the deposition of an electron dense material within intercellular spaces. Exposure to a saturating pressure of acetylene caused a similar inhibition of respiration and nitrogenase activity in nodules developed in oxygen pressures from 4.7 to 47 kPa, suggesting that putative acetylene-induced changes in oxygen diffusion resistance occur by a different mechanism than that involved in long-term adaptation to oxygen. However, in nodules grown at 75 kPa oxygen, the initial specific activities were lower and did not show an acetylene induced decline. The results are discussed in terms of the current theories of regulation of nitrogenase activity by oxygen availability.     

Effect of atmospheric pressure on maize root growth and ethylene production

Maize (Zea mays) seedlings were exposed to elevated atmospheric pressures while growing in moist sand in open plastic envelopes to evaluate the effects of directly applied atmospheric pressure on ethylene production and root growth. Effects were evaluated after 24 h. The threshold pressures necessary to promote ethylene production and decrease root elongation were about 600 and 400 kPa, respectively. Direct atmospheric pressure, at levels up to 300 kPa, mimicked the control decrease in root diameter and increased diameter only slightly at 500 to 1200 kPa. In contrast, in previous work it was shown that physical impedance resulting from compression of the growth medium by external application of 100 kPa increased the ethylene production rate 4-fold and the root diameter 7-fold while reducing elongation 75% in 10 h. The relative insensitivity of roots to direct atmospheric pressure suggests that they perceive physical impedance, achieved experimentally by compressing the growth medium, via a surface mechanism rather than via a pressure-sensing mechanism.     

Short- and Long-term Stomatal Responses to Fluctuations in Environment in Southern European Greenhouses

Stomatal behaviour in cucumber (Cucumis sativus L.) was analysed and modelled as a function of different greenhouse environmental parameters, under variable summer conditions. Solar radiation was the main regulating factor. During the day, large atmospheric vapour pressure deficit increased transpiration which was followed by a reduction in stomatal aperture, suggesting the presence of a feedback response to water stress. However, stomatal behaviour was more sensitive to high atmospheric vapour pressure deficit when this was accompanied by a rapid decrease of solar radiation. The response to the difference between leaf and air temperature was also influenced by air vapour pressure deficit and duration of plant exposure to high evaporative demand. Calculation of the crop water stress index showed that the air vapour pressure deficit of 1 kPa used in the control treatment probably caused water stress and induced some hardening, a necessary condition for adaptation to summer climate in southern Europe. The importance of the interaction between climatic parameters and plant response in greenhouse environmental management is analysed. Classical models of stomatal resistance are also discussed.     

Role of barometric pressure in pulmonary fluid balance and oxygen transport

To examine the role of barometric pressure in high-altitude pulmonary edema, we randomly exposed five unanesthetized chronically instrumented sheep with lung lymph fistulas in a decompression chamber to each of three separate conditions: hypobaric hypoxia, normobaric hypoxia, and normoxic hypobaria. A combination of slow decompression and/or simultaneous adjustment of inspired PO2 provided three successive stages of simulated altitudes of 2,600, 4,600, and 6,600 m during which hemodynamics and lymph flow were monitored. Under both hypoxic conditions we noted significant and equivalent elevations in pulmonary arterial pressure (Ppa), cardiac output, and heart rate, with left atrial and systemic pressures remaining fairly constant. Normoxic hypobaria was also accompanied by a smaller but significant rise in Ppa. Lymph flow increased to a highly significant maximum of 73% above base line, accompanied by a slight but significant decrease in lung lymph-to-plasma protein ratio, only under conditions of combined hypobaric hypoxia but not under equivalent degrees of alveolar hypoxia or hypobaria alone. Arterial hypoxemia was noted under all three conditions, with arterial PO2 being uniformly lower under hypobaric conditions than when identical amounts of inspired PO2 were delivered at normal atmospheric pressure. We therefore hypothesize that alveolar pressure significantly alters the Starling forces governing transcapillary fluid flux in the lung and may affect the alveolar-arterial gradient for O2 as well.     

低压环境中植物的生长特性及适应机理研究进展

在低压受控生态生命保障系统中, 植物是关键的生物部件。在低压环境中, 植物会面临与常压不同的总大气压力(总压)、O₂分压和CO₂分压等大气环境条件。虽然植物在一定的低压环境中能完成完整的生长周期(从种子到种子), 但为了适应新的大气环境条件, 其生理生态特性均会发生改变。该文综述了低压对植物种子萌发、植株形态结构、生长特性、根系养分吸收、植株营养品质、叶片气体交换和乙烯(C₂H₄)释放的影响, 以及低压环境诱导的植物基因表达和相应的调控机理等, 从不同角度阐述了低压环境对植物生长的影响及植物对低压环境的适应机理, 并指出了将来需要进一步开展的试验研究方向。     

From the deep sea to the stars: human life support through minimal communities

Support of human life during long-distance exploratory space travel or in the creation of human habitats in extreme environments can be accomplished using the action of microbial consortia inhabiting interconnected bioreactors, designed for the purpose of reconversion of solid, liquid and gaseous wastes produced by the human crew or by one of the compartments of the bioregenerative loop, into nutritional biomass, oxygen and potable water. The microorganisms responsible for bioregenerative life support are part of Earth's own geomicrobial reconversion cycle. Depending on the resources and conditions available, minimal life support systems can be assembled using appropriately selected microorganisms that possess metabolic routes for each specific purpose in the transformation cycle. Under control of an engineered system, a reliable life-support system can hence be provided for.