C2H4 Evolution Research Articles

(Poster Award Winner 1) Investigating the Impact of Ethylene Carbonate Reaction Pathways on the Solid Electrolyte Interphase in Li-Ion Batteries

A high-performing Li-ion battery relies heavily on the solid electrolyte interphase (SEI), which is composed of various decomposition products, primarily from reduced additives or electrolyte species such as ethylene carbonate (EC). The composition of the SEI evolves and continues to form during the operation of a Li-ion battery, making it challenging to fully understand through post-mortem analysis alone. To gain a complete understanding of the EC reaction pathways and their respective contributions to the SEI, it is necessary to employ analytical techniques that monitor the battery during operation. Online Electrochemical Mass Spectrometry (OEMS) is one such technique that can sample the gas composition of a battery, providing an overview of voltage-dependent gas evolution processes.1 This presentation focuses on two EC reaction pathways that occur on a model carbon electrode.2 The first pathway, the "EC ring-opening" reaction pathway, is initiated by hydroxides/alkoxides at potentials above 1.5 V vs. Li+/Li and is associated with CO2 evolution.3 , 4 The second pathway, the "EC reduction" pathway, occurs at potentials below 0.9 V vs. Li+/Li and is identified by C2H4 evolution.5 Both reaction pathways contribute to the formation of the SEI, and they compete with each other. If EC ring-opening is favoured, it can suppress EC reduction. Figure 1 illustrates the competing relationship between the two different EC reaction pathways, where the addition of EC ring-opening initiators (H2O) increases CO2 and reduces C2H4. It is essential to consider all possible EC reaction pathways, not just the desired one, when forming the SEI. The presentation concludes with a discussion of the potential implications that EC ring-opening may have on a Li-ion cell. Figure 1 . Comparison between the total CO2 and C2H4 evolution during the initial cyclic voltammetry cathodic sweep in a Glassy carbon | 1 M LiPF6 EC:DEC | LiFePO4 cell. References R. Lundström and E. J. Berg, J. Power Sources, 485, 229347 (2021).R. Lundström, N. Gogoi, X. Hou, and Berg Erik J., Submitted (2023).M. Metzger, B. Strehle, S. Solchenbach, and H. A. Gasteiger, J. Electrochem. Soc., 163, 1219–1225 (2016).L. Gireaud, S. Grugeon, S. Laruelle, S. Pilard, and J.-M. Tarascon, J. Electrochem. Soc., 152, A850 (2005).R. Imhof and P. Novák, J. Electrochem. Soc., 145, 1081–1087 (1998). Figure 1

Designing Heteroatom-Codoped Iron Metal-Organic Framework for Promotional Photoreduction of Carbon Dioxide to Ethylene.

Rational engineering active sites and vantage defects of catalysts are promising but grand challenging task to enhance photoreduction CO2 to high value-added C2 products. In this study, we designed an N,S-codoped Fe-based MIL-88B catalyst with well-defined bipyramidal hexagonal prism morphology via a facile and effective process, which was synthesized by addition of appropriate 1,2-benzisothiazolin-3-one (BIT) and acetic acid to the reaction solution. Under simulated solar irradiation, the designed catalyst exhibits high C2 H4 evolution yield of 17.7 μmol g-1 ⋅h, which has been rarely achieved in photocatalytic CO2 reduction process. The synergistic effect of Fe-N coordinated sites and reasonable defects in the N,S-codoped photocatalyst can accelerate the migration of photogenerated carriers, resulting in high electron density, and this in turn helps to facilitate the formation and dimerization of C-C coupling intermediates for C2 H4 effectively.

Designing Heteroatom‐Codoped Iron Metal–Organic Framework for Promotional Photoreduction of Carbon Dioxide to Ethylene

AbstractRational engineering active sites and vantage defects of catalysts are promising but grand challenging task to enhance photoreduction CO2 to high value‐added C2 products. In this study, we designed an N,S‐codoped Fe‐based MIL‐88B catalyst with well‐defined bipyramidal hexagonal prism morphology via a facile and effective process, which was synthesized by addition of appropriate 1,2‐benzisothiazolin‐3‐one (BIT) and acetic acid to the reaction solution. Under simulated solar irradiation, the designed catalyst exhibits high C2H4 evolution yield of 17.7 μmol g−1⋅h, which has been rarely achieved in photocatalytic CO2 reduction process. The synergistic effect of Fe‐N coordinated sites and reasonable defects in the N,S‐codoped photocatalyst can accelerate the migration of photogenerated carriers, resulting in high electron density, and this in turn helps to facilitate the formation and dimerization of C−C coupling intermediates for C2H4 effectively.

Oxygen vacancy–rich Cu2O@Cu with a hydrophobic microenvironment for highly selective C[sbnd]C coupling to generate C2H4

Cu2O is promising to catalytically convert CO2 into C2 products by overcoming the instability and slow multi-electron transfer. Regulating catalytic construction and interface microenvironment is challenging to stabilize Cu2O and selectively convert CO2 to C2+ hydrocarbons. A photocatalyst (Cu2O@Cu-CN) of oxygen vacancy–rich Cu2O@Cu symbiont embedded in N-doped carbon skeleton is achieved by ligand competition strategy. The Schottky junction and the hydrophobic microenvironment in the catalyst together stabilize the active site against the Cu2O redox reaction. The hydrophobic interface can make the catalyst favorable for trapping CO2. The asymmetric interface with oxygen vacancy can enhance the adsorption and activation of CO2 and *CO, thus reducing the energy barrier for the formation of *OCCO intermediates and promoting the coupling conversion of *OCCO to C2H4. The C2H4 evolution rate reaches 46.27 μmolC2H4 gcat-1h−1 with a high selectivity of 40.3 % using triethanolamine (TEOA) as a sacrificial agent, and the apparent quantum yield (AQY) is as high as 14.41 % (λ = 420 nm). The C2H4 evolution rate is 2.10 μmolC2H4 gcat-1h−1 in water without TEOA. Moreover, the productive rate of C2H4 still maintains 95.37 % after lasting 20 h, showing a robust long − term stability.

Operando Analysis of Gas Evolution in TiNb2O7 (TNO)-Based Anodes for Advanced High-Energy Lithium-Ion Batteries under Fast Charging.

TiNb2O7 (TNO) is regarded as one of the promising next-generation anode materials for lithium-ion batteries (LIBs) due to its high rate capabilities, higher theoretical capacity, and higher lithiation voltage. This enables the cycling of TNO-based anodes under extreme fast charging (XFC) conditions with a minimal risk of lithium plating compared to that of graphite anodes. Here, the gas evolution in real time with TNO-based pouch cells is first reported via operando mass spectrometry. The main gases are identified to be CO2, C2H4, and O2. A solid-electrolyte interphase is detected on TNO, which continues evolving, forming, and dissolving with the lithiation and delithiation of TNO. The gas evolution can be significantly reduced when a protective coating is applied on the TNO particles, reducing the CO2 and C2H4 evolution by ∼2 and 5 times, respectively, at 0.1C in a half-cell configuration. The reduction on gas generation in full cells is even more pronounced. The surface coating also enables 20% improvement in capacity under XFC conditions.

Individual and interactive effects of temperature and light intensity on canola growth, physiological characteristics and methane emissions

Earlier studies have shown that plants produce methane (CH4) under aerobic conditions, and that this emission is not microbial in nature. However, the precursors of aerobic CH4 remain under debate, and the combined effects of environmental factors on plant-derived CH4 requires further attention. The objective of this study was to determine the interactive effects of temperature and light intensity on CH4 and other relevant plant parameters in canola (Brassica napus L.). Plants were grown under two temperature regimes (22/18 °C and 28/24 °C, 16 h light/8 h dark) and two light intensities (300 and 600 μmol photons m−2 s−1) for 21 days after one week of growth under 22/18 °C (16 h light/8 h dark). In this study, higher temperature had little effects on CH4 emissions from plants, indicating the mitigating effects of higher light intensity. Higher light intensity, however, significantly decreased CH4, which was inversely related to plant dry mass. Higher light intensity decreased stem height, leaf area ratio, chlorophyll, nitrogen balance index, leaf moisture, methionine (Met) and ethylene (C2H4), but increased specific leaf mass, photochemical quenching, flavonoids, epicuticular wax, lysine and tyrosine. The results revealed that increased CH4 emissions from plants could be related to changes in plant physiological activities, which portrayed themselves in increased C2H4 evolution, and methylated amino acids, such as Met. We conclude that higher light intensity reduces Met and, in turn, CH4 and C2H4 emissions, but lower light intensity enhances CH4 formation through cleavage of methyl group of amino acids by reactive oxygen species, as previously suggested.

Solid Electrolyte Interphase (SEI) Formation on the Graphite Anode in Electrolytes Containing the Anion Receptor Tris(hexafluoroisopropyl)borate (THFIPB)

Addition of small amounts of tris(hexafluoroisopropyl)borate (THFIPB) has previously been shown to improve the capacity of graphite anodes in a wide temperature window during long-term cycling in electrolytes based on LiPF6 in ethylene carbonate (EC) and diethylene carbonate (DMC) solvents. Here, we demonstrate that the addition of THFIPB accelerates the LiPF6 hydrolysis, and consumes residual water. The SEI formation and composition was studied by operando electrochemical mass spectrometry (OEMS), X-ray photoelectron spectroscopy (XPS), and ex situ diffuse reflectance spectroscopy (DRIFT). Differences in the surface products are observed for the THFIPB containing electrolyte already upon exposure of graphite anodes to the electrolyte. Both the mechanism and the kinetics of the EC reduction reaction are affected, as evidenced by the shift in the C2H4 evolution peak to higher potentials as compared to the reference electrolyte. Addition of THFIPB leads to formation of a SEI layer enriched in inorganic components; salt reduction products at high potentials, and inorganic carbonate at lower potentials. The SEI formed is more conductive but slightly less passivating. The SEI formed in the reference electrolyte is dominated by organic compounds, also at high potentials, facilitated by the trace amounts of water inevitably found in the electrolyte.

Electrolyte and SEI Decomposition Reactions of Transition Metal Ions Investigated by On-Line Electrochemical Mass Spectrometry

We use on-line electrochemical mass spectrometry (OEMS) to elucidate and quantify the electrolyte reduction on graphite caused by transition metal ions. To have a controlled system, we use ethylene carbonate (EC) with 1.5 M LiPF6 and representative amounts of Ni(TFSI)2 or Mn(TFSI)2 as model electrolytes, combined with a 2-compartment cell in which anolyte and catholyte are separated by an impermeable solid lithium ion conductor. Focusing on C2H4 evolution as a marker for EC reduction, we find that both Ni2+ and Mn2+ lead to enhanced gas evolution on pristine graphite electrodes once the potential is decreased to below the TM2+/TM0 redox potential, demonstrating that the reduced transition metals are active toward electrolyte reduction. If the electrodes are preformed in a TM-free electrolyte and subsequently cycled in an electrolyte containing either Mn2+ or Ni2+, the activity of nickel toward electrolyte decomposition is greatly reduced, whereas the electrolyte with manganese still shows a strong ongoing C2H4 generation. The use of vinylene carbonate during formation partially suppresses the gas evolution from manganese. Using OEMS and post-mortem ATR-FTIR, we finally show that reduced manganese can decompose organic SEI components into Li2CO3, thereby compromising the integrity of the SEI and enabling the additional reduction of electrolyte.

Gas Evolution at Graphite Anodes Depending on Electrolyte Water Content and SEI Quality Studied by On-Line Electrochemical Mass Spectrometry

The gas evolution during the formation of graphite electrodes is quantified by On-line Electrochemical Mass Spectrometry (OEMS) for dry electrolyte (< 20 ppm H2O) and 4000 ppm H2O containing electrolyte to mimic the effect of trace water during the formation process. While the formation in dry electrolyte mainly shows ethylene (C2H4) from the reduction of ethylene carbonate (EC) and small amounts of hydrogen (H2), the formation in water-containing electrolyte yields large amounts of H2 and considerable amounts of CO2 in addition to the expected C2H4 evolution. We could show that a protective solid-electrolyte interphase (SEI) layer formed by pre-cycling the graphite electrode in 2% vinylene carbonate (VC) containing electrolyte can reduce the H2 evolution in water-containing electrolyte by a factor of 7.5 compared to a pristine graphite electrode. Consequently, the ability of graphite electrodes to form an SEI prevents excessive gassing from trace water, which, e.g., is observed for non-SEI forming lithium titanate (LTO) electrodes.

Effect of storage conditions on physico-chemical attributes and physiological responses of 'milk' (Musa spp., AAB group) banana during fruit ripening

Knowledge of fruit physico-chemical properties and physiological responses is important for the design and development of appropriate postharvest handling and storage systems to reduce losses and maintain quality. The objective of this study was to investigate the changes in physico-chemical attributes and physiology of ‘milk’ banana cultivar at three ripening stages (unripe, fully ripe and overripe) under three storage conditions [refrigerated storage, 11°C–12°C and 95.5% RH; warm (room) storage, 20°C–22°C and 82%–85% RH; and 12-h cycle of 28°C/70% RH and 18°C/50% RH, simulating cyclic day/night conditions]. Fruit physico-chemical characteristics (weight loss, firmness, colour, pH, total soluble solids, titratable acidity, sugar:acid ratio and vitamin C) and physiological processes (C2H4 and CO2 evolution) varied significantly among storage conditions for each of the three ripening stages tested. The least reduction in fruit weight, highest firmness and peel colour change were found using refrigerated storage compared to the other storage conditions. Refrigerated storage at 11°C–12°C and 95.5% RH offered significant benefits in postharvest handling of ‘milk’ banana by reducing fruit weight-loss, extending storage life by two weeks, and enhancing fruit vitamin C content during ripening.

Responses of ethylene and methane consumption to temperature and pH in temperate volcanic forest soils

SummaryThere is limited knowledge about the consumption and interaction of methane (CH4) and ethylene (C2H4) in forest soils under disturbances of temperature and acidification. Temperate volcanic forest topsoils (0‐5 cm) sampled under different tree species (e.g. Pinus sylvestris, Cryptomeria japonica and Quercus serrata) were used to study the capacities for CH4 and C2H4 consumption and their sensitivity to temperature and pH. We also studied the responses of soil nitrogen (N) transformations to temperature and relationships to consumption of both CH4 and C2H4. The C2H4 consumption rates increased with temperature up to 35oC, whereas the optimum temperature for CH4 consumption rates was approximately 25oC. Both Q10 values and activation energies for CH4 consumption rates over the range 5 to 25oC were larger than corresponding values for C2H4 consumption rates. The rates of nitrous oxide (N2O) and nitric oxide (NO) evolution and net N mineralization in the soils increased exponentially with temperature up to 35oC, with relatively large Q10 values and activation energies for NO evolution. In these forest topsoils, rates of CH4 and C2H4 consumption at pH &lt; 4.0 were negligible, and the pH optimum for both consumptions varied from 5.5 to 6.2. Most of the tested forest soils had an optimum pH for CH4 and C2H4 consumption that was above natural pH values, which indicated that soil acidification would inhibit CH4 and C2H4 consumption in situ. There was a high rate of net C2H4 evolution from forest soils acidified experimentally to pH &lt; 4.0, particularly from Cryptomeria japonica forest soil, and 67% of the variation in C2H4 evolution rates could be accounted for by the increase in soil water‐soluble organic carbon concentrations. Previous studies have shown that addition of C2H4 in headspace gases can inhibit atmospheric CH4 consumption in such forest soils. Hence, the evolution of C2H4 from temperate volcanic forest soils at decreasing pH can exacerbate inhibition of the soil atmospheric CH4 consumption in situ.

1-Methylcyclopropene Inhibits Degreening But Stimulates Respiration and Ethylene Biosynthesis in Grapefruit

We determined the effects of 1-methylcyclopropene (1-MCP) and ethylene on color change and CO2 and ethylene production in grapefruit. Treatment with 1-MCP at concentrations equal to or greater than 75 nL·L−1 inhibited ethylene-induced degreening, but increasing 1-MCP concentrations greater than 150 nL·L−1 did not cause additional inhibition of degreening. Although ethylene-induced degreening was inhibited by 1-MCP, the effect was transient. Treating grapefruit with 15 to 75 nL·L−1 1-MCP resulted in a slight suppression of CO2 production, whereas treatment with 150 or 300 nL·L−1 1-MCP resulted in rates of CO2 production significantly higher than nontreated fruit. 1-MCP treatment also caused a very pronounced increase in the rate of C2H4 production that was both dose- and time-dependent. The effects of 1-MCP on respiration and ethylene evolution were reduced if fruit was subsequently exposed to ethylene. Fruit treated with 1-MCP alone had the highest rates of CO2 production, fruit treated with ethylene after 1-MCP or ethylene alone had intermediate rates of CO2 production, and control fruit had the lowest rate of CO2 production. Rates of C2H4 evolution were ≈200 nL·kg−1·h−1 from control and C2H4-treated fruit compared with ≈10,000 nL·kg−1·h−1 from 1-MCP-treated fruit; fruit treated with ethylene after 1-MCP had ethylene production rates of ≈400 nL·kg−1·h−1. Our results lend further support for a regulatory role for ethylene in degreening of citrus and suggest that endogenous levels of ethylene regulate ethylene production.

Apple Scald Development and Regulation

We have studied scald development by comparing changes in gene expression, C2H4 evolution, and α-farnesene and conjugated trienol contents in scald-resistant cultivars, i.e., `Gala' and `Braeburn', and scald-sensitive cultivars, i.e., `Red Delicious' and `Granny Smith'. We also carried out similar comparisons between controls and treatments that diminished scald symptoms in sensitive cultivars. The data show that scald development is critically dependent on the initiation of the climacteric rise in C2H4 evolution, since treatments that suppress the latter inhibit scald development at low temperatures coincident with a suppression of α-farnesene and conjugated trienols. However, in scald-resistant cultivars, there is an increase in α-farnesene and conjugated trienols, although to a lower degree than in the sensitive cultivars. This indicates that factors other than the auto-oxidation of α-farnesene are also involved in scald development. Analytical data show that malonyl dialdehyde (MDA) increases only in scalded areas, which, in turn, suggests that oxidative reactions are involved in scald development. Storage of `Granny Smith' at temperatures above 7 °C prevents the development of scald without affecting the accumulation of α-farnesene and conjugated trienols. This in turn suggests that chilling temperatures induce as yet unknown enzymes that contribute to scald development. In short, the data show that in addition to cultivars, low temperature stress and the induction of the C2H4 climacteric play a crucial role in scald development. Preliminary data show that treatment of `Granny Smith' apples with olive oil emulsions suppress scald development symptoms.

Synergistic Effects of the Combined Application of MCP and Low O2 on Apple Fruit Ripening

We have studied the effects of MCP and low O2, applied singly and in combination, on apple fruit ripening at 1, 7, and 18 °C. The single application of 2 ppm MCP is more effective in delaying the onset of the C2H4 climacteric than is 1% O2. However, the combined application has a much larger effect than the single applications of either MCP or 1% O2. For instance, at 7 °C, the onset of the C2H4 climacteric occurs at 15, 50, and 90–95 days for the controls, 1% O2 and 2 ppm MCP, respectively, whereas the combined application of 2 ppm MCP and 1% O2 suppressed the initiation of the C2H4 climacteric for 200 days, the duration of the experiment. The retardation of the climacteric onset by the treatments is associated with the suppression of ACC-synthase (ACS1) and the putative receptor ERS1. The accumulation of their transcripts is critically dependent on the rate of C2H4 evolution. As expected, the combined application of MCP and 1% O2 completely suppressed the expression of both genes. Yet when the fruits were transferred to 18 °C in air, they ripened normally. A similar pattern of inhibition in response to the above treatments was also observed with a C2H4-dependent MAPK. The expression of ETR1, ETR2 and ACC-oxidase was not affected by the treatments. The nature of this strong effect of the combined application of MCP and low O2 is not clear. It should be pointed out that MCP does not inhibit the induction of hypoxic proteins such as ADH.

Temporary Relationship Between C2 H4 Evolution and C2 H4 Recptor Genes

`Granny Smith' apples were stored at 1 °C, 5 °C and 20 °C, then treated at the preclimacteric stage with 2 ppm MCP, various O2 concentrations, and MCP + low O2. All the treatments greatly retarded the onset of the C2 H4 climacteric, and hence ripening. MCP + low O2 was much more effective than were the applications of MCP and low O2 singly. Even at 20 °C, 4.04 kPa O2 inhibited the rise in C2 H4 evolution for 145 d. Neither low O2 nor MCP inhibited the System 1 C2 H4 evolution. The suppression of the climacteric rise in C2 H4 evolution was accompanied by a strong inhibition of the accumulation of ERS1 C2 H4 receptor and ACS transcripts. On the other hand, ETR1 receptor was constitutively expressed. When climacteric fruits were treated with MCP, and with low O2 + MCP the rate of C2 H4 evolution decreased sharply. This occurred simultaneously with a decrease in ERS1 mRNA. Moreover, the decrease in ERS1 mRNA paralleled the decrease in C2 H4 evolution. The data thus indicate that the initiation and sustainment of the C2 H4 climacteric requires the presence of functional C2 H4 receptors. The expression of ETR2 and ERS2 is also under investigation.

REDUCTION OF CHILLING INJURY IN STORED BANANA FRUITS BY JASMONIC ACID DERIVATIVE AND ABSCISIC ACID TREATMENT

Applications of n-propyl dihydrojasmonate (PDJ) and abscisic acid (ABA) were carried out aiming to reduce chilling injury in banana fruits at chilling temperature. In the first experiment, banana fruits were treated with PDJ by either spraying or by infiltration at 82.7 kPa for 3 min, with 0.5 or 1.0 mM concentration, prior to storage at 5 °C for 0-4 weeks. When sprayed with 1.0 mM, the highest rate of reduction in chilling injury was found after cold storage at 5 °C for 1 week and subsequent transfer to 20 °C for ripening. Compared with other treatments, these treated fruits increased C2H4 evolution and CO2 production, while membrane permeability and chilling injury index were lowered. Endogenous ABA in the peel of 1.0 mM sprayed fruits was higher than other treatments and approximately two times that of untreated fruits. In a second experiment, ABA spraying resulted in a decrease of chilling injury in comparison with the other applications, with 0.5 and 1.0 mM after storage at 5 °C for 1 week and subsequent transfer to 20 °C for ripening, as in the first experiment.

Extension of the shelf life of banana fruit by 1-methylcyclopropene in combination with polyethylene bags

The effect of the new anti-ethylene compound 1-methylcyclopropene (1-MCP) in combination with polyethylene bags on the ripening of harvested banana fruit was investigated. 1-MCP treatment delayed peel colour change and fruit softening, and extended shelf life in association with suppression of respiration and C2H4 evolution. Banana fruit ripening was delayed when exposed to 0.01–1.0 μl 1-MCP/l for 24 h, and increasing concentrations of 1-MCP were generally more effective for longer periods of time. Similar results were obtained with fruit sealed in polyethylene bags (0.03 mm thick) containing 1-MCP at various concentrations, but longer delays in ripening were achieved. The greatest longevity of about 58 days was realised by packing fruit in sealed polyethylene bags with 1-MCP at either of 0.5 or 1.0 μl/l. Analyses of C2H4 and CO2 concentrations within polyethylene bags confirmed that 1-MCP suppressed both C2H4 evolution and respiration. Thus, application of 1-MCP in combination with the use of polyethylene bags can greatly extend the postharvest life of banana fruit.

C2H2 and Ar induce rapid declines in nitrogenase activity and CO2 evolution in nodules of Datisca glomerata

Preliminary studies have indicated that after addition of C2H2 there is a rapid decline in nitrogenase activity in the nodules of Datisca glomerata. The present work was undertaken to determine whether (1) there is also a decline in respiration and (2) the decline is associated with the cessation of ammonia production. The rates of C2H4 and CO2 evolution by nodulated root systems of Datisca were measured as a function of time after exposure to C2H2. The peak rate of C2H4 evolution occurred at 30 s after C2H2 exposure, while the rate of CO2 evolution started to decline at 60 s after exposure to C2H2. Incubation of nodules in a gas mixture containing Ar also caused a decline in CO2 evolution. Further, pretreatment with Ar eliminated most of the C2H2‐induced decline in nitrogenase activity and CO2 evolution. These C2H2‐ and Ar‐induced declines in Datisca nodules are more rapid than those reported in any other nodules. They are evidence that continued ammonia formation is essential for maintenance of normal nitrogenase activity in Datisca nodules.

Utilization of the acetylene reduction assay to screen for tolerance of symbiotic N2 fixation to limiting P nutrition in common bean

The limitation of symbiotic nitrogen fixation due to P deficiency restricts the development of a sustainable agriculture, particularly in Mediterrancan and tropical soils. Common bean genotypes, APN18, BAT271, PVA846, POT51, G2633 and G12168, were grown in an aerated N‐free nutrient solution at low (72 μmol plant‐1 week‐1) and control P supplies (360 μmol plant‐1 week‐1). Nitrogenase activity was estimated by in situ measurements of acetylene reduction activity (ARA) in a flow‐through system. During the assays, maximum values of ARA (peak ARA) were reached between 20 and 30 min after exposure to C2H2, depending on P treatment and growth stage. Thereafter, a decline in C2H4 evolution was observed. This decline was most pronounced in low‐P plants and there was a significant genotypic effect. ARA per plant was decreased by P deficiency, mostly because nodulation was delayed and the number and mass of nodules were reduced. The ARA decrease during pod filling was also activated by P deficiency. ARA per g dry weight nodule was increased by P deficiency in G2633 and G12168, unchanged in APN18, BAT271 and POT51 and decreased in PVA846. Except for the climbing type IV G2633, total N at harvest for all P treatments was correlated with the cumulative value of peak ARA and with peak ARA at early pod‐filling which was the highest peak ARA throughout the growth cycle of type III bushy genotypes. We conclude that if phenology and growth habit are carefully considered, peak ARA is a reliable screen of genotypes for N2 fixation tolerance to P deficiency. Selection of lines with early nodulation under P deficiency is also advisable, and the effect of P deficiency on the nodule functioning needs to be considered.

Effects of Low O2 on the Timing of the Onset of the C2H4 Climacteric and Subsequent Ripening of `Gala' Apples at 1°C

We have investigated the effects of different O2 concentrations on the onset of the climacteric rise in C2H4 evolution and subsequent ripening of `Gala' apples. The data show that the effect of low O2 on the timing of the onset of the C2H4 climacteric is saturable in that for low O2 to retard the onset of the rise in C2H4 evolution its concentration must be decreased below 8%. Further, the results suggest that the retarding effects of low O2 on the onset of ripening cannot be ascribed to the inhibitory effects of low O2 on the action of the low levels of C2H4 evolution by preclimacteric apples. The results concerning the rate of C2H4 evolution and accumulation of the ACC-oxidase protein show that application of low O2 after the initiation of the climacteric rise in C2H4 is not as effective in delaying senescence as when it is imposed early, i.e., at the preclimacteric stage.