Low O2 Treatment Research Articles

Decreased calcification to photosynthesis ratio in coccolithophores under reduced O2 and elevated CO2 environment

We examined the physiological performance in the most cosmopolitan coccolithophorid, Emiliania huxleyi and Gephyrocapsa oceanica, which were treated with 8.3 (ambient O2 (AO)), 4.6 (medium O2 (MO)), and 2.5 (low O2 (LO)) mg L–1 O2 under 400 ppm (ambient CO2 (AC)) and 1000 (high CO2 (HC)) ppm CO2 conditions. Elevated CO2 decreased the specific growth rate of cells cultured under AO and LO conditions in both species, but it increased the rate in the MO-grown E. huxleyi. Regardless of the CO2 levels, diminished O2 concentration inhibited the growth rate in E. huxleyi while accelerating the rate in G. oceanica. LO reduced the particulate organic carbon (POC) production rate compared to the AO treatment in both species. Additionally, the decrease was higher in the HC cultures than in the AC ones. LO also inhibited the production rate of particulate inorganic carbon (PIC) compared to the AO/AC treatment. Due to a higher reduction in the production rate of PIC than POC, the PIC/POC ratio was decreased in the LO treatment compared to the AO/AC treatment. The current study reveals that low O2 can, individually or in combination with high CO2, considerably affect the physiology of marine photoautotrophic organisms.

Combined effects of ocean deoxygenation, acidification, and phosphorus limitation on green tide macroalga, Ulva prolifera

Ocean deoxygenation, acidification, and decreased phosphorus availability are predicted to increase in coastal ecosystems under future climate change. However, little is known regarding the combined effects of such environmental variables on the green tide macroalga Ulva prolifera. Here, we provide quantitative and mechanistic understanding of the acclimation mechanisms of U. prolifera to ocean deoxygenation, acidification, and phosphorus limitation under both laboratory and semi-natural (mesocosms) conditions. We found that there were significant interactions between these global environmental conditions on algal physiological performance. Although algal growth rate and photosynthesis reduced when the nitrogen-to‑phosphorus (N/P) ratio increased from 16:1 to 35:1 under ambient CO2 and O2 condition, they remained constant with further increasing N/P ratios of 105:1, 350:1, and 1050:1. However, the increasing alkaline phosphatase activities at high N/P ratios suggests that U. prolifera could use organic P to support its growth under phosphorus limitation. Deoxygenation had no effect on specific growth rate (SGR) but decreased photosynthesis under low N/P ratios of 16:1, 35:1, and 105:1, with reduced activities of several enzymes involved in N assimilation pathway being observed. Elevated CO2 promoted algal growth and alleviated the negative effect of deoxygenation on algal photosynthesis. The patterns of responses to high CO2 and low O2 treatments in in situ experiments were generally consistent with those observed in laboratory experiments. Our results generally found that the strong physiological acclimation capacity to elevated CO2, low O2, and high N/P could contribute to its large-scale blooming in coastal ecosystem.

Comprehensive insight into the chlorophyll degradation mechanism of postharvest broccoli heads under elevated O2 controlled atmosphere

Postharvest broccoli heads undergo yellowing quickly in unsuitable conditions, which diminishes their quality and commercial value. We investigated the pattern and molecular regulatory mechanism of chlorophyll (Chl) degradation in broccoli under elevated O2 controlled atmosphere. As results, the 40% O2 + 5% CO2 treatment accelerated the Chl degradation and the change in color of postharvest broccoli heads, while the 5% O2 + 5% CO2 treatment delayed the yellowing. The results indicated that Chl degradation pattern of postharvest broccoli heads concurrently involved the Pheide a oxygenase (PAO)/phyllobilin and oxidative degradation pathways, and the latter might be the dominant pathway of Chl degradation. The regulatory mechanism of elevated O2 on Chl degradation may be related to its ability to accelerate cell damage and death, destroy the light-harvesting complex II, and promote up-regulation of genes in the PAO/phyllobilin and oxidative degradation pathways. In contrast, low O2 treatment may down-regulate the expression of Chl degradation-related genes by increasing H2O2 content, thus delaying the yellowing of broccoli heads.

Effects of Sulfur Dioxide Exposure on Leaf Mass per Area of Selected Gymnosperms and Implications for Interpreting the Plant Fossil Record

Premise of research. The role of sulfur dioxide (SO2) in plant responses to large igneous province volcanism is difficult to determine but important in relation to understanding ecosystem responses during mass extinction events. Leaf mass per area (LMA) is an important leaf measure for tracking plant responses to environmental pressures in the fossil record, but how it responds to SO2 exposure is unclear, particularly for nonangiosperm taxa.Methodology. Five gymnosperm species were grown in simulated paleoatmospheric conditions that included SO2 to determine how the plants responded.Pivotal results. All plants’ growth in high SO2 showed a significant decrease in LMA compared with the control (with no SO2). When grown in a high CO2, low O2, and high SO2 treatment, most plants continued to show a decrease in LMA compared with the control but an increase compared with the high SO2 treatment with ambient CO2 and O2.Conclusions. These findings suggest that exposure to SO2 significantly decreases LMA, potentially reducing preservation potential in the fossil record. Exposure to combined SO2 and high CO2 reduces this effect, but this may also obscure the effects of SO2, leading to interpretations that ecosystem instability is primarily driven by increasing CO2, which may not be the case.

Short-term Low Oxygen Treatment Immediately After Planting Alters Asparagus officinalis var. UC157’s Harvest Timing

To control asparagus harvest timing, we investigated the effects of short-term low (5%) oxygen (O2) treatment in the cultivation area on asparagus growth and yield using a closed cultivation system. During 120 days of cultivation, low O2 treatments were initiated at 0 to 4, 20 to 24, and 40 to 44 days after planting (DAP). The sprouting spears and control crown yield gradually decreased with increasing DAP. However, low O2 treatment at 0 to 4 DAP significantly delayed the decrease until 80 DAP, although the total yield did not change during cultivation. In contrast, low O2 treatments at 20 to 24 and 40 to 44 DAP did not affect yield performance. Taken together, short-term low O2 treatment immediately after planting can change the harvest timing of white asparagus and can be used for effective asparagus culturing in a closed system, such as a plant factory.

Root-zone hypoxia reduces growth of the tropical forage grass Urochloa humidicola in high-nutrient but not low-nutrient conditions.

The perennial C4 grass Urochloa humidicola is widely planted on infertile acidic and waterlogging-prone soils of tropical America. Waterlogging results in soil anoxia, and O2 deficiency can reduce nutrient uptake by roots. Interestingly, both nutrient deficiencies and soil waterlogging can enhance root cortical cell senescence, and the increased gas-filled porosity facilitates internal aeration of roots. We tested the influence of nutrient supply and root-zone O2 on root traits, leaf nutrient concentrations and growth of U. humidicola. Plants were grown in pots in a completely randomized design under aerated or stagnant deoxygenated hydroponic conditions and six nutrient regimes, with low to high concentrations of all essential elements, for 28 d in a controlled-temperature greenhouse. The standard acid solution (SAS) used was previously designed based on infertile acidic soils of the tropical America savannas, and step increases in the concentration of SAS were used in aerated or deoxygenated 0.1 % agar solution, which mimics changes in gas composition in waterlogged soils. Measurements included shoot and root growth, root porosity, root anatomy, radial O2 loss, and leaf tissue nutrient concentrations. Shoot dry mass was reduced for plants in stagnant compared with aerated conditions at high, but not at low, levels of mineral nutrition. In low-nutrition stagnant solution, roots were shorter, of greater porosity and had smaller radial thickness of the stele. Suberized lamellae and lignified sclerenchyma, as well as a strong barrier to radial O2 loss, were documented for roots from all treatments. Leaf nutrient concentrations of K, Mg and Ca (but not N, P and S) were higher in aerated than in stagnant conditions. Under low-nutrient conditions, plant growth in stagnant solution was equal to that in aerated solution, whereas under higher-nutrient regimes growth increased but dry mass in stagnant solution was less than in aerated solution. Slow growth in low-nutrient conditions limited any further response to the low O2 treatment, and greater porosity and smaller stele size in roots would enhance internal O2 movement within roots in the nutrient-limited stagnant conditions. A constitutive barrier to radial O2 loss and aerenchyma facilitates O2 movement to the tips of roots, which presumably contributes to maintaining nutrient uptake and the tolerance of U. humidicola to low O2 in the root-zone.

Effect of a low oxygen storage condition on betacyanin and vitamin C retention in red amaranth leaves

We investigated the effects of different O2 concentrations (air or 2 kPa O2) and temperatures (5 °C or 20 °C) on the respiration rate, betacyanin content and VC content to provide basic information on the most optimal conditions for practical storage and transportation of red amaranth. The respiration rates were obviously reduced by low temperatures and low O2 treatments. However, an increase in the CO2 production was detected in red amaranth stored at 20 °C-2 kPa O2 after 30 h. The total betacyanin content changed slightly after storage for 3 days, and there was no significant difference among storage treatments. The VC contents of all the treatments were greatly reduced (more than 50%) after storage. The VC content of red amaranth stored under low temperature either with or without low O2 was not different from the control. Nevertheless, low O2 (2 kPa) at 20 °C significantly slowed the loss of VC. Our results mainly suggested that the O2 level is one of the key factors affecting the quality of red amaranth during storage.

Hypoxia Controls Hypocotyl and Radicle Length in Germinated Impatiens Seeds

Sowing germinated seeds for bedding plant production can decrease the production time and reduce profit losses from sporadically or poorly germinating seeds. Low concentrations of O2 have been used to control radicle length in Impatiens wallerana Hook. F., but only a brief exposure period could be used (12 to 24 h). The effects of prolonged exposures are unknown. Our first objective was to determine if impatiens seedlings could be acclimated to an extreme hypoxic environment by a preliminary exposure to a less severe hypoxic exposure. Our second objective was to determine the effects of longer-duration (greater than 24 h) treatments at low-O2 concentrations on hypocotyl and radicle length, abnormal seedling development, and subsequent plant growth and development. Our third objective was to provide a commercial recommendation of a low-oxygen treatment or treatments that could be used for temporary storage of unused germinated seeds. Germinated seeds were placed in various combinations of 0.5%, 1.0%, and 1.5% O2 for either 4 days (Expt. 1) or 3 days (Expt. 2) followed by 24 h in air to simulate shipping. Germinated seeds were less tolerant of 0.5% O2 than greater O2 concentrations, especially during the first 2 days of treatment, and more abnormal seedlings developed at 0.5% O2. Germinated seeds were more tolerant of 0.5% O2 during Days 3 and 4 of the treatment period or when days at 0.5% O2 were interspersed between days of 1.0% or 1.5% O2. This indicates that germinated impatiens seeds can tolerate extremely low-O2 for longer durations (greater than 24 h) when first acclimated to lower O2 concentrations. Treatment of 1.5% O2 for Days 1 and 2 followed by 0.5% O2 for Day 3 is recommended for commercial use. Hypocotyl and radicle length was controlled to 1 mm without development of abnormal seedlings. Fully grown plants from seedlings that received low-O2 treatments were not different from control plants with regards to hypocotyl and radicle length, percentage abnormal seedlings, and plant growth and quality, ensuring there were no long-term detrimental effects.

Effects of Low Temperature O2 Treatment on the Electrical Characteristics of Amorphous LaAlO3 Films by Atomic Layer Deposition

Amorphous LaAlO3 films were deposited on hydrogen-terminated silicon substrates by atomic layer deposition (ALD) at 300 oC. The precursors were lanthanum tris(N,N'-diisopropylformamidinate), trimethylaluminum (TMA) and water. Capacitance-voltage measurements made on ALD MoN/LaAlO3/Si stacks showed humps especially at low frequencies. They were effectively removed by O2 treatment at 300 o C without affecting the dielectric constant (κ~15). The O2 treatment can be carried out either after deposition of a LaAlO3 film, or after each ALD cycle. The O2 treatment also lowered the leakage current from 80 mA cm-2 to 1 mA cm-2 for EOT = 1.3 nm. This indicates that oxygen vacancies are the main defects in as-deposited LaAlO3. Oxygen treated LaAlO3 is one of the best candidates for future high-κ dielectric material due to its low leakage, low defect density and abrupt interface with silicon.

Effects of Low Temperature O2 Treatment on the Electrical Characteristics of Amorphous LaAlO3 Films by Atomic Layer Deposition

not Available.

Effects of Short-term Exposure to Low Oxygen Atmospheres on Phsyiological Responses of Sweetpotato Roots

Sweetpotato roots were stored under a continuous flow of 0% or 1% O2 (balance N2) or air for 7 days at 20°C to study the effects of short-term exposure to low O2 on their physiological responses and quality. During the course of the experiment, no visible signs of injury or decay were observed. However, low O2 treatments increased the soluble solid content and weak off-odors were detected by olfactory evaluation in roots stored at 0% O2. The intensity of off-odors increased as the concentrations of acetaldehyde and ethanol increased in roots during storage. Ethanol concentrations were higher than those of acetaldehyde, which remained low during storage in 1% O2 and air, but increased greatly in roots stored at 0% O2. Pyruvate decarboxylase (PDC) activities in roots exposed to 0% or 1% O2 increased by 3.1- and 2-fold respectively over levels in roots stored in air by day 7. Alcohol dehydrogenase (ADH) activities in roots exposed to 0% or 1% O2 increased by 1.6- and 1.7-fold respectively over levels in roots stored in air by day 7. ADH-specific activity was about 10-times that of PDC. The pH of root homogenate exposed to air remained constant, whereas the pH increased and decreased, respectively, in roots stored at 0% or 1% O2. PDC showed stability over the pH range 5.5–7.0, whereas ADH exhibited stability over the pH range 6.0–7.5. The Km of PDC in sweetpotato was 0.56 mM for pyruvate, whereas the Km of ADH was 0.19 mM for acetaldehyde. From these results, there may be some potential for the short-term exposure of sweetpotato roots to low O2 in place of low temperature treatment to prolong shelf-life, although ethanol fermentation may be accelerated under low O2 atmospheres.

Physiological and quality responses of Chinese chive leaves to low oxygen atmosphere

Abstract Harvested leaves of Chinese chive were stored in 0, 1 or 3% O2 (balance N2), or air for 7 days at 20 °C to determine the effects of low O2 atmospheres on their physiology and quality. Leaf yellowing was visible at day 5 in air, whereas low O2 treatment delayed yellowing and retarded chlorophyll and protein degradation that accompanied leaf senescence. The respiration rates of leaves stored at low O2 atmospheres were substantially lower than those of the control during storage. However, at 0% O2, undesirable off-odors were induced and visible anaerobic injury appeared in stored leaves, presumably due to a high accumulation of acetaldehyde and ethanol in the tissue. The contents of acetaldehyde and ethanol were very low during storage at 1% O2, 3% O2, or air. At 0% O2, ethanol and to a lesser extent acetaldehyde, rapidly accumulated in the leaf tissue. Pyruvate decarboxylase (PDC) activity greatly increased in leaves exposed to 0 or 1% O2, while its activity in leaves exposed to 3% O2 was only slightly higher than that of the control. Alcohol dehydrogenase (ADH) activity greatly increased in leaves exposed to 1 or 3% O2, while its activity in leaves exposed to 0% O2 was only slightly higher than that of the control. The activity of ADH was about 250 times that of PDC during storage. Changes in ADH isozymes correlated well with changes in ADH activity. The potential for using low O2 atmospheres to help in maintaining the quality of Chinese chive leaves is discussed.

EFFECT OF SOME POSTHARVEST TREATMENTS AND CONTROLLED ATMOSPHERE (CA) STORAGE ON BASIC QUALITY CRITERIA OF PEACHES AND NECTARINES

The peach (Flavorcrest and Red Top) and nectarine (Fantasia and Fairlane) cultivars used in the study were stored in normal and controlled-atmosphere conditions. Peaches and nectarines were subjected to some treatments (fungicide solution, low O2 and intermittent warming treatment) either before or during storage. Physical and chemical changes (weight loss, respiration rate, fruit flesh firmness, titratable acidity, total sugar, overall appearance) were recorded at 15-day intervals during 45 days storage and after 10 days shelf life at the end of the storage. Results are given.

Quality of Lettuce Stored in a Low Oxygen Closed System.

For the low cost vegetable transportation, the applicability of a low O2 closed system was examined under ambient temperature. Heads of leaf lettuce grown under the controlled climate of a commercially operated plant factory individually placed in airtight containers filled with normal air or with low O2 mixed gas (5% 02+95% N2) were stored for 11 d and 13 d at 15°C and for 7 d at 20°C. Populations of mesophilic aerobic bacteria from levels of 107 to 109 CFU/g were detected at the end of storage on lettuce heads in control treatment (normal air). Contrary to this, final populations on lettuce heads in low O2 treatment were 102- to 105-fold less than those in control treatment. As the final populations were less than 1 x 107 CFU/g, leaves of lettuce heads in low O2 treatment were not spoiled for all storage experiments. Supplementary examinations indicated that low O2 treatment was not effective to suppress bacterial populations on lettuce heads having large populations as about the level of 105 CFU/g or more at the beginning of storage. Hence, the low O2 closed system can be applied exclusively to the leaf lettuce grown in plant factory that can normally assure small populations of bacteria. Adding to this bacterial point of view, cumulative quantity of CO2 showed a reduction in respiration of lettuce head in low O2 treatment. Moreover, large amount of ascorbic acid was lost in control treatment as compared to that in low O2 treatment. However, no significant difference was observed in weight loss regardless of treatment.

Physiological responses and quality attributes of Japanese pear ‘Kosui’ fruit kept in low oxygen atmospheres

SummaryJapanese pear ‘Kosui’ fruits were stored under a continuous flow of 0%, 1%, 3%, 5% and 10% O2 (balance N2) or air for 7 d at 20°C to study the effects of low O2 on their physiological responses and quality attributes. Low O2 treatments did not significantly influence changes in skin colour and soluble solids content. However, weak off-flavours were detected in the fruits stored at 0% O2 on day 3, and the intensity of these off-flavours increased as storage progressed. The concentrations of acetaldehyde in fruit increased throughout the storage period. The ethanol concentration was greatly increased in fruits stored at 0% O2. Moreover, ethanol concentrations were much higher than those of acetaldehyde and remained very low during storage in air, but their concentration were just slightly increased in fruits exposed to 1%, 3%, 5% and 10% O2. Pyruvate decarboxylase activity was greatly increased in fruits exposed to 1% and 3% O2, while its activity in fruits exposed to 5% and 10% O2 were only slightly higher than that of the control and at 0% O2 at the same level as the control. Alcohol dehydrogenase (ADH) activity greatly increased in fruit exposed to 0%, 1%, 3% and 5% O2, while at 10% O2, ADH was only slightly higher than the control. Changes in ADH isozymes correlated well with changes in ADH activity. The homogenate pH of fruits exposed to 1%, 3%, 5% and 10% O2 and air remained constant, while in fruit stored at 0% O2 their pH increased. The potential for using low O2 atmospheres to help in maintaining the quality of Japanese pear ‘Kosui’ is discussed.

Hypoxic acclimation prevents avocado mesocarp injury caused by subsequent exposure to extreme low oxygen atmospheres

Avocado fruit (Persea americana Mill., cv. Hass), preclimacteric and ripening-initiated, were either hypoxically pretreated (HPT, 3% O2 for 24 h) or exposed directly to 1 and 0.25% O2 for 1–3 days (NHPT) at 20 °C. Low O2 treatments resulted in fruit maintaining higher flesh firmness. Hypoxic acclimation of preclimacteric and ripening-initiated avocado fruit increased their tolerance to subsequent 0.25 and 1% O2 levels, as assessed by visual quality attributes when the fruit returned to air. Hypoxic pre-treatment also produced a stronger expression of the anaerobically induced ADH isoenzymes than in NHPT avocado fruit that were subjected directly to 1 and 0.25% O2 for 1–3 days at 20 °C. Thus, acclimation of avocado fruit to hypoxia resulted in a beneficial increase in tolerance to subsequent ultra-low O2 treatments. © 2001 Elsevier Science B.V. All rights reserved.

Low-oxygen treatment for inhibition of decay and ripening in organic bananas

SummaryApplication of a low-O2 atmosphere for several days prior to storing organic banana clusters was effective in preventing decay development on the crown cuts, and in delaying ripening. Preclimacteric banana clusters (Musa spp. AAA group cultivar Ziv) were treated with a low-oxygen atmosphere (2%) at 208C for 24, 48 and 72.h immediately after harvest. After removal from low-O2 stress, the bananas were stored at 128C for 21.d, and were then treated with ethylene at 188C for 24.h and transferred to shelf life at 208C for an additional 4.d. The low-O2 treatments were compared with the commercial treatment of dipping the crown cuts in 0.2% thiabendazole (TBZ). The low-O2 stress for 48 or 72.h was effective in preventing decay after shelf life, but less so than the TBZ treatment. Low-O2 for 24.h was not effective enough, and the 72.h treatment markedly impaired colour development after ripening with ethylene. The 48.h low-O2 treatment which resulted in the best decay prevention also retarded ripening processes (colour, ®rmness, respiration and ethylene production) and reduced chilling injury symptoms, without impairing the taste. The level of reducing sugars in the 48.h-treated fruit was similar to that in the control fruit after ripening and shelf life. The endogenous levels of acetaldehyde (AA) and ethanol produced by the treated fruit were lower than those in the control fruit during shelf life. It seems that there is a high potential for this physical treatment of low-O2 stress to replace chemical treatments, as means of maintaining the quality of organic bananas.

Mango Tolerance to Reduced Oxygen Levels in Controlled Atmosphere Storage

`Haden' and `Tommy Atkins' mangoes (Mangifera indica L.) were stored in air, 2, 3, 4 or 5 kPa O2 plus N2, or 25 kPa CO2 plus air for 14 days at 15 °C or 21 days at 12 °C, respectively, then in air for 5 days at 20 °C to determine their tolerance to reduced O2 levels for storage times encountered in typical marine shipments. All low O2 treatments reduced mature green mango respiration (CO2 production), however, elevated ethanol production occurred in 2 and 3 kPa O2 storage, with the levels two to three times higher in `Tommy Atkins' than `Haden'. In contrast, `Haden' fruit at the onset of the climacteric also accumulated ethanol in 4 kPa O2 and produced 10 to 20-fold more ethanol in 2 and 3 kPa O2 than preclimacteric fruit. While there were no visible injury symptoms, off flavor developed in mature green fruit at 2 kPa O2 and in ripening initiated fruit at 2 and 3 kPa O2. Ethanol production was not affected by storage in 25 kPa CO2. Ethylene production was reduced slightly by low O2, however, `Haden' fruit also showed a residual inhibitory effect on ethylene production after 2 or 3 kPa O2 storage, while `Tommy Atkins' fruit stored in 2 kPa O2 produced a burst of ethylene upon transfer to air at 20 °C. Fruit firmness, total sugars, and starch levels did not differ among the treatments, but 2, 3 or 4 kPa O2 and 25 kPa CO2 maintained significantly higher acidity than 5 kPa O2 or air. The epidermal ground color responded differently to low O2 and high CO2 in the two mango cultivars. Only 2 kPa O2 maintained `Haden' color better than air, while all low O2 levels maintained `Tommy Atkins' color equally well and better than air. High CO2 was more effective than low O2 in maintaining `Haden' color, but had about the same effect as low O2 on `Tommy Atkins'. Results indicate that preclimacteric `Haden' and `Tommy Atkins' mango fruit are able to tolerate 3 kPa O2 for 2 or 3 weeks at 12 to 15 °C and that tolerance to low O2 decreases as mangoes ripen. Results also show that low O2 and high CO2 affect mango ripening differentially.

Transgenic cotton (Gossypium hirsutum) over-expressing alcohol dehydrogenase shows increased ethanol fermentation but no increase in tolerance to oxygen deficiency

Cotton (Gossypium hirsutumL.) was transformed with constructs for the over-expression of two enzymes involved in ethanol fermentation, alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC), with the goal of increasing waterlogging tolerance. Four independent transgenic lines transformed with the cotton Adh2 cDNA driven by the CaMV 35S promoter showed ectopic expression of this isozyme in leaves and up to 20-fold greater in vitro ADH activity in roots. Under conditions of O2 deficiency, excised roots from these transgenic plants showed up to 80% increase in ethanol evolution compared to untransformed roots. Conversely, one line transformed with a construct containing the Adh2 coding region in the antisense orientation showed a 65% decrease in ADH activity and a 25% decrease in ethanol production from anaerobic roots relative to untransformed cotton. Lines transformed with a rice Pdc1 cDNA driven by the CaMV 35S promoter showed high levels of expression of the transgene-encoded protein in leaves, but only very low levels of protein and no in vitro enzyme activity detectable in the roots of these plants. Roots from plants transformed with the 35S-Pdc construct did not produce more ethanol than roots from untransformed controls. We tested the ability of cotton roots to withstand low O2 treatments under hydroponic conditions. Neither the ‘ADH’ nor the ‘PDC’ transgenics showed more tolerance than the wild-type on the basis of root growth under a mild stress (5% O2), a strong stress (0% O2 with or without a 5% O2 pretreatment), or in recovery growth following these treatments. Our results show that over-expression of ADH can lead to ethanol over-production (even though the activity of this enzyme by far exceeds that of PDC, its precursor in the pathway), but this is not sufficient to increase waterlogging tolerance in cotton.

Interactions between the Timing of the Application of Hypoxia and Rate of Sugar Accumulation and Activities of Invertase, Sucrose Synthase, and Sucrose Phosphate Synthase

Previous research has shown that subjecting bananas to low O2 treatment during the climacteric rise decreases the rate of sugar accumulation but the fruits eventually ripen. In the present study we applied low O2 in fruits whose ripening had been initiated by exogenous C2H4 and in preclimacteric ones. In preclimacteric fruits low O2 suppressed the climacteric rise during the duration of the experiment (20 days). It completely inhibited the increase in sugars, invertase and sucrose phosphate synthase (SPS) activities while there was a sharp increase in sucrose synthase (SS). In control fruits the increase in sugar content coincides with a sharp increase in invertase, and SPS and a decline in SS. Hypoxia inhibited the increase in invertase and SPS while it induced an increase in SS. Nevertheless, the activities of invertase and SPS in the climacteric hypoxic fruits was higher than in hypoxic preclimacteric ones. The results, thus, indicate that the imposition of low O2 at the preclimacteric stage is much more efficient in delaying banana ripening than when it is applied after the initiation of ripening.