The influence of soil moisture and flooding on formation of VA-endo- and ectomycorrhizae in Populus and Salix

Native mixtures of extomycorrhizal fungi were found to infect Populus and Salix roots primarily in very moist but well drained soils in both the field and in controlled experiments (0 to –0.2 MPa), whereas native mixtures of VA-endomycorrhizal fungi infected roots over a much wider range of soil moisture (flooded to –3.4 MPa). Although a moisture gradient experiment showed endomycorrhizal formation was greater in moist soil than in very dry or flooded soils, this pattern was reversed in field transects along drainage gradients. Infection by VA-endomycorrhizal fungi in the field was the lowest where infection by ectomycorrhizal fungi was high, which suggests possible antagonism among the fungal symbionts. The narrow moisture range for ectomycorrhizal formation, and antagonism among endo- and ectomycorrhizal fungi, apparently combine to produce the mycorrhizal distributions found in nature.     

Primary photosynthetic processes in Heliobacterium chlorum at 15K

Absorbance changes induced by 25-ps laser flashes were measured in membranes of Heliobacterium chlorum at 15 K. Absorbance difference spectra, measured at various times after the flash showed negative bands in the Q_y region at 812, 793 and 665 nm. The first of these bands was attributed to the formation of excited singlet states of a long-wavelength form of antenna bacteriochlorophyll g (BChl g 808). Absorbance changes of shorter wavelength absorbing antenna BChls g were at least an order of magnitude smaller, indicating rapid excitation energy transfer (i.e. within the time resolution of the apparatus) from these BChls to BChl g 808. Excited BChl g 808 showed a bi-exponential decay with time constants of 50 and 200 ps. The bands at 793 and 665 nm may be attributed to the primary charge separation and reflect the photooxidation of the primary electron donor P-798 and photoreduction of a primary electron acceptor absorbing near 670 nm, presumably a BChl c or Chl a-like pigment. The bleaching of this pigment reversed with a time constant of 300 ps at 15 K and of 800 ps at 300 K. This indicates that electron transfer from the primary to the secondary electron acceptor is approximately 2.5 times faster at 15 K than at room temperature.Abbreviations BChl bacteriochlorophyll - FWHM full width at half maximum - P-798 primary electron donor - Tris tris(hydroxymethyl)amino methane     

Interdependence of growth, water relations and abscisic acid level in Phaseolus vulgaris during waterlogging

The interdependence between changes in growth and water relations after waterlogging was investigated by recording simultaneously growth, transpiration, water potential, turgor, leaf diffusion resistance and abscisic acid content in Phaseolus vulgaris L. cv. bruine Noord-Hollandse. Growth was inhibited immediately after flooding, whereas transpiration decreased gradually to a low level in about three days. The first two days after flooding a small increase in abscisic acid content in the leaves was observed which was accompanied by an increase in diffusion resistance. The increase in abscisic acid content could result from an inhibited export from the leaves. After the first two days a decrease in water potential and turgor was accompanied by a drastic increase in both abscisic acid content and diffusion resistance. This large increase in abscisic acid content occurred before the turgor had reached its minimum value. The change in diffusion resistance kept showing a lag of about one day with the change in abscisic acid content. The possibility is discussed that besides abscisic acid also its metabolite phaseic acid is involved in stomatal closure. After the formation of adventitious roots on the hypocotyl, abscisic acid level, diffusion resistance, water potential and turgor returned to the control values. Transpiration showed a slow recovery from the sixth day after flooding, whereas growth was inhibited for at least nine days. A remarkable similarity exists between our observations on the responses of bean plants to flooding and the well known responses to drought.     

Effects of anoxia and flooding on alcohol dehydrogenase in roots of Glyceria maxima and Pisum sativum

Flood tolerant Glyceria maxima and intolerant Pisum sativum were compared in respect of the effects of anoxia and flooding on the maximum catalytic activities of alcohol dehydrogenase in their roots. Small (<73%) increases in enzyme activity occurred when excised roots of both species were incubated in nitrogen for up to 2 days. Further incubation in nitrogen rapidly and permanently damaged the roots of both species. Enzyme activity in flooded roots of Glyceria was about double that in corresponding non-flooded roots. A marginally greater difference was found for roots of Pisum. It was concluded that the two species respond so similarly to the above treatments that variation in the extent of induction of alcohol dehydrogenase is unlikely to be a significant factor in determining their ability to tolerate flooding.     

A flash-photolysis study of the reactions of acaa 3-ttype cytochrome oxidase with dioxygen and carbon monoxide

The time course of absorbance changes following flash photolysis of the fully-reduced carboxycytochrome oxidase fromBacillus PS3 in the presence of O₂ has been followed at 445, 550, 605, and 830 nm, and the results have been compared with the corresponding changes in bovine cytochrome oxidase. The PS3 enzyme has a covalently bound cytochromec subunit and the fully-reduced species therefore accommodates five electrons instead of four as in the bovine enzyme. In the bovine enzyme, following CO dissociation, four phases were observed with time constants of about 10 s, 30 s, 100 s, and 1 ms at 445 nm. The initial, 10-s absorbance change at 445 nm is similar in the two enzymes. The subsequent phases involving hemea and Cu_A are not seen in the PS3 enzyme at 445 nm, because these redox centers are re-reduced by the covalently bound cytochromec, as indicated by absorbance changes at 550 nm. A reaction scheme consistent with the experimental observations is presented. In addition, internal electron-transfer reactions in the absence of O₂ were studied following flash-induced CO dissociation from the mixed-valence enzyme. Comparisons of the CO recombination rates in the mixed-valence and fully-reduced oxidases indicate that more electrons were transferred from hemea ₃ toa in PS3 oxidase compared to the bovine enzyme.     

Flooding-induced membrane damage, lipid oxidation and activated oxygen generation in corn leaves

Flooding effects on membrane permeability, lipid peroxidation and activated oxygen metabolism in corn (Zea mays L.) leaves were investigated to determine if activated oxygens are involved in corn flooding-injury. Potted corn plants were flooded at the 4-leaf stage in a controlled environment. A 7-day flooding treatment resulted in a significant increase in chlorophyll breakdown, lipid peroxidation (malondialdehye content), membrane permeability, and the production of superoxide (O ₂ ^- ) and hydrogen peroxide (H₂O₂) in corn leaves. The effects were much greater in older leaves than in younger ones. Spraying leaves with 8-hydroxyquinoline (an O ₂ ^- scavenger) and sodium benzoate (an .OH scavenger) reduced the oxidative damage and enhanced superoxide dismutase (SOD) activity. A short duration flooding treatment elevated the activities of SOD, catalase, ascorbate peroxidase (AP), and glutathione reductase (GR), while further flooding significantly reduced the enzyme activities but enhanced the concentrations of ascorbic acid and reduced form glutathione (GSH). It was noted that the decline in SOD activity was greater than that in H₂O₂ scavengers (AP and GR). The results suggested that O ₂ ^- induced lipid peroxidation and membrane damage, and that excessive accumulation of O ₂ ^- is due to the reduced activity of SOD under flooding stress.     

Leaf dynamics and insect herbivory in a Eucalyptus camaldulensis forest under moisture stress

Abstract The influence of soil moisture content on leaf dynamics and insect herbivory was examined between September 1991 and March 1992 in a river red gum (Eucalyptus camaldulensis) forest in southern central New South Wales. Long-term observations of leaves were made in trees standing either within intermittently flooded waterways or at an average of 37. 5m from the edge of the waterways. The mean soil moisture content was significantly (P≤0.05) greater in the waterways than in the non-flooded areas. Trees in the higher soil moisture regime produced significantly larger basal area increments and increased canopy leaf area. This increase in canopy leaf area was achieved, in part, through a significant increase in leaf longevity and mean leaf size. Although a greater number of leaves was initiated and abscissed per shoot from the non-flooded trees, more leaves were collected from litter traps beneath the denser canopies of the flooded trees. Consumption of foliage by insects on the trees subjected to flooding compared to the non-flooded trees was not significantly different. However, the relative impact of insect herbivory was significantly greater on the non-flooded trees. Leaf chewing was the most common form of damage by insects, particularly Chryso-melidae and Curculionidae. No species was present in outbreak during this study. Leaf survival decreased as the per cent area eaten per leaf increased. In addition, irrespective of the level of herbivory, leaf abscission tended to be higher in E. camaldulensis under moisture deficit. The influence of soil moisture content on the balance between river red gum growth and insect herbivory is discussed.     

Impact of grazing on vegetation dynamics in former ricefields

Abstract. In the Rhône delta, Juncus gerardi and Scirpus maritimus are often the dominant species in abandoned rice fields which are artificially flooded in early spring to improve forage production. Under these conditions they occur either in mixed communities, or form monospecific stands. Monitoring the vegetation dynamics in quadrats located in six abandoned rice fields artificially flooded from November to April confirmed the important role of grazing. In ungrazed plots, communities dominated by Scirpus maritimus mixed with Juncus gerardi developed fast. After 42 months of management Scirpus maritimus had established in nearly all quadrats and continued to expand, whereas Juncus gerardi had started to decline. In grazed plots Juncus gerardi alone dominated and continued to increase in cover up to the 42nd month. Scirpus maritimus established at low densities mainly in quadrats where Juncus gerardi was initially absent. Introduction of seeds of Scirpus maritimus in communities of Juncus gerardi under controlled conditions demonstrated the existence of the phenomenon of preemption. The increase in cover of Juncus gerardi suggests that the preemption of Juncus gerardi over Scirpus maritimus plays a more pronounced role in the field in the presence of grazing.     

Reconstitution of iron-sulfur center B of Photosystem I damaged by mercuric chloride

Incubation of thylakoid membranes from spinach with low concentrations of mercuric chloride induces the loss of one of the iron-sulfur centers, F_B, in Photosystem I (PS I) and inhibits the electron transfer from PS I to the soluble electron carrier, ferredoxin. Reconstitution of this damaged iron-sulfur center has been carried out by incubating treated thylakoid membranes with exogenous FeCl₃ and Na₂S in the presence of-mercaptoethanol under anaerobic conditions. Low temperature EPR measurements indicate that center F_B is largely restored. Kinetic experiments show that the restored F_B can be photoreduced from P700. However, these reconstituted thylakoid membranes are still incompetent in the photoreduction of ferredoxin and NADP⁺, even though ferredoxin binding to the modified membranes was not impaired, indicating additional changes in the structure of the PS I complex must have occurred.     

Enhanced ethylene production by primary roots of Zea mays L. in response to sub-ambient partial pressures of oxygen

Ethylene production by primary roots of 72–h-old intact seedlings of Zea mays L. cv. LG11 was studied under ambient and sub-ambient oxygen partial pressures (pO₂) using a gas flow-through system linked to a photoacoustic laser detector. Despite precautions to minimize physical perturbation to seedlings while setting-up, ethylene production in air was faster during the first 6h than later, in association with a small temporary swelling of the roots. When roots were switched from air (20–8kPa O₂) to 3 or 5kPa O₂ after 6h, ethylene production increased within 2—3 h. When, the roots were returned to air 16 h later, ethylene production decreased within 2—3 h. The presence of 10kPa CO₂ did not interfere with the effect of 3kPa O₂. Transferring roots from air to 12–5kPa did not change ethylene production, while a reduction to 1 kPa O₂ induced a small increase. The extra ethylene formed in 3 and 5 kPa O₂ was associated with plagiotropism, swelling, root hair production, and after 72 h, increased amounts of intercellular space (aerenchyma) in the root cortex. Root extension was also slowed down, but the pattern of response to oxygen shortage did not always match that of ethylene production. On return to air, subsequent growth patterns became normal within a few hours. In the complete absence of oxygen, no ethylene production was detected, even when anaerobic roots were returned to air after 16 h.