New Phytologist

COLE, L.; DEWEY, F. M.; HAWES, C. R.

doi: 10.1046/j.1469-8137.1998.00237.xpmid: N/A

Extracellular matrices associated with conidia and germ tubes of Botrytis fabae (Sard.) sporelings grown on Vicia faba L. leaves were clearly visualized by epi‐fluorescence microscopy following immunolabelling with the monoclonal antibodies, BC‐KH4 and BC‐FD7‐G9. These antibodies were raised against surface washings of B. cinerea, are directed against B. cinerea and B. fabae, and are known to recognize carbohydrate epitopes on a glycoprotein. Both BC‐KH4 and BC‐FD7‐G9 also labelled matrix material located at the surface of penetration and infection hyphae inside the leaf tissue by epi‐fluorescence microscopy. Such matrix material was not visible by DIC microscopy.

COLE, L.; DEWEY, F. M.; HAWES, C. R.

doi: 10.1046/j.1469-8137.1998.00230.xpmid: N/A

Immunoelectron microscopy using the anti‐pectin monoclonal antibody JIM 7 confirmed earlier observations that pectin degradation is a primary event in the process of host cell wall breakdown during the development of chocolate spot disease (causal agent: Botrytis fabae (Sard.)) of broad bean. Close examination of infected and non‐infected Vicia faba L. leaves indicated a loss of JIM 7‐labelling, and therefore, methyl‐esterified pectin, from swollen walls of infected and contiguous epidermal cells. Modified mesophyll walls also possessed less methyl‐esterified pectin than healthy walls. Enzymes which attack methyl‐esterified pectin appeared to be most active in regions of host tissue close to sites of fungal infection.

CHAFFEY, N. J.; BARLOW, P. W.; BARNETT, J. R.

doi: 10.1046/j.1469-8137.1998.00241.xpmid: N/A

Aspects of the structure and ultrastructure of the fusiform cambial cells of the taproot of Aesculus hippocastanum L. (horse chestnut) are described in relation to the seasonal cycle of cambial activity and dormancy. Particular attention is directed at cell walls and the microtubule and microfilament components of the cytoskeleton, using a range of cytochemical and immunolocalization techniques at the optical and electron‐microscopical levels. During the dormant phase, cambial cell walls are thick and multi‐layered, the cells possess a helical array of cortical microtubules, and microfilament bundles are oriented axially. In the early stages of reactivation, vesicle‐like profiles are associated with the cell walls, whereas arrangement of the cytoskeletal elements remains unchanged. In the succeeding active phase, the cell walls are thin, and cortical microtubules form a random array, although microfilament bundles maintain a near‐axial orientation. The observations are discussed in relation to the seasonal cycle of wall structure and cortical microtubule rearrangement within the vascular cambium of hardwood trees. It is suggested that the cell‐wall thickening at the onset of cambial dormancy, which is associated with the presence of a helical cortical microtubule array, should be considered to be secondary wall thickening, and that selective lysis of this secondary wall layer during cambial reactivation restores the thinner, primary wall found around active cambial cells.

BURKE, R. M.; CAIRNEY, J. W. G.

doi: 10.1046/j.1469-8137.1998.00235.xpmid: N/A

Isolates of the ericoid mycorrhizal fungus Hymenoscyphus ericae (Read) Korf et Kernan, and the ectomycorrhizal fungi Suillus variegatus (Swartz ex Fr.) and Pisolithus tinctorius (Pers.) Coker & Couch, along with a Cortinarius sp. and the white rot Phanerochaete chrysosporium Burdsall were examined for the ability to oxidize carbohydrates to their corresponding lactones and to excrete the H2O2 produced thereby. All except Phanerochaete chrysosporium were found to express cellobiose oxidase (cellobiose dehydrogenase, EC 1.1.19.88) and glucose oxidase (β‐d‐glucose∶oxygen 1‐oxidoreductase, EC 1.1.3.4) when grown on cellobiose and glucose respectively. Production of extracellular H2O2 was visualized during growth on both substrates using ABTS as the chromogen. According to the Fenton reaction, H2O2 will react with hydrated or chelated Fe(II) in the environment to produce hydroxyl (Fenton) radicals, HO·. Mycelial extracts from each of the mycorrhizal fungi produced HO· in the presence of cellobiose and Fe(II), presumably mediated by H2O2 produced by cellobiose oxidase activity in the extracts. Conditions favourable to HO· production were shown to exist in Modified Melin–Norkrans medium, and the data discussed in relation to previously observed lignin degradation by mycorrhizal fungi.

NIELSEN, KAI L.; BOUMA, TJEERD J.; LYNCH, JONATHAN P.; EISSENSTAT, DAVID M.

doi: 10.1046/j.1469-8137.1998.00242.xpmid: N/A

Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean (Phaseolus vulgaris L.). Plants were grown in solid‐phase‐buffered silica sand providing a constant supply of low (1 μm) or moderate (10 μm) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low‐P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non‐mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below‐ground respiration at days 21, 28 and 35 after planting (29–40%) compared with plants grown with moderate P supply (18–25%). Relatively greater below‐ground respiration in low P plants was mainly a result of their increased root∶shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40%. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low‐P plants. Our results support the hypothesis that root C costs are a primary limitation to plant growth in low‐P soils.

KLEIER, CATHERINE; FARNSWORTH, BLAKE; WINNER, WILLIAM

doi: 10.1046/j.1469-8137.1998.00229.xpmid: N/A

Brassica rapa L. (rapid‐cycling Brassica), was grown in environmentally controlled chambers to determine the interactive effects of ozone (O3) and increased root temperature (RT) on biomass, reproductive output, and photosynthesis. Plants were grown with or without an average treatment of 63 ppb O3. RT treatments were 13°C (LRT) and 18°C (HRT). Air temperatures were 25°C/15°C day/night for all RT treatments.

STAL, LUCAS J.; WALSBY, ANTHONY E.

doi: 10.1046/j.1469-8137.1998.00228.xpmid: N/A

Measurements were made of the rates of nitrogenase activity (acetylene reduction) by cyanobacteria collected from the Baltic Sea at 2‐h intervals, over a period of 24 h, and incubated under natural light. By relating the chlorophyll‐specific rate of N2 fixation (PN) to the mean photon irradiance (I) at different periods, a PN/I curve was constructed. A mathematical description of this relationship was used in the calculation of rates of N2 fixation at different depths and times of day from continuous measurements of surface irradiance and light attenuation. By relating these calculations to the vertical distribution of chlorophyll due to Aphanizomenon flos‐aquae, an estimate of the daily integral of N2 fixation by the population of this cyanobacterium was obtained: it varied from 0·39 to 0·71 mmol m−2 with a mean value of 0·53 mmol−2 over 9 d. Comparisons with similar calculations of the daily integral of photosynthesis over the same period indicated the atomic ratio of N/C fixed is about 0·22, not very different from the N/C ratio expected for the elemental composition of these cyanobacteria. It is demonstrated that when buoyant gas‐vacuolate colonies float up during calm periods the increased irradiance that they experience supports an increased rate of N2 fixation. This increase is less than that seen for carbon fixation because photosynthesis is negated by respiration at low irradiances and is less inhibited at high irradiances.

FLORES‐MOYA, ANTONIO; GÓMEZ, IVÁN; VIÑEGLA, BENJAMÍN; ALTAMIRANO, MARÍA; PÉREZ‐RODRÍGUEZ, EDUARDO; MAESTRE, CRISTINA; CABALLERO, ROSARIO M.; FIGUEROA, FÉLIX L.

doi: 10.1046/j.1469-8137.1998.00233.xpmid: N/A

The effects of full‐spectrum solar radiation and of solar radiation deprived of total u.v. radiation (λ<395 nm) on the endemic Mediterranean red alga Rissoella verruculosa (Bertoloni) J. Agardh were studied in situ in early summer, from sunrise to sunset. Photosynthetic performance, pigment content and the activities of enzymes related to nutrient uptake, were monitored under both radiation conditions throughout a daylight period.

CLAYDEN, STEPHEN R.

doi: 10.1046/j.1469-8137.1998.00243.xpmid: N/A

Ascospore germination, thallus initiation, and areole and prothallus development in the lichen Rhizocarpon lecanorinum Anders were examined using light, fluorescence and scanning electron microscopy. The ascospore germ hyphae remain very short and do not form a prothallus‐like mycelium. Instead, a compact soredium‐like granule develops directly from sporeling contact with a compatible species of Trebouxia. Diffuse initial stages involving non‐trebouxioid algae are lacking. The onset of thallus differentiation is marked by the deposition of rhizocarpic acid in an incipient cortical layer within the apical part of the granule. As pigmentation and cortex‐formation transform this structure into a typical areole, radiating prothallus hyphae are simultaneously initiated from its basal margin. Most areoles formed subsequently in the marginal prothallus lack subtending melanized hyphae and apparently stem from overgrowth by the prothallus of photobiont cells on, or in, the substratum. Apothecia reach maturity in thalli as small as 2 mm in diameter. It is proposed that the lack of diffuse hyphal growth in sporelings and telescoped morphogenesis of R. lecanorinum are part of a life history strategy geared to precocious, heavy investment in ascospore production. The R. lecanorinum–Trebouxia symbiosis has a number of features which make it well‐suited for further studies of the life history and development of prothallus‐forming crustose lichens with sexually reproducing mycobionts.

CRITTENDEN, P. D.

doi: 10.1046/j.1469-8137.1998.00236.xpmid: N/A

Concentrations of NH4+, NO3−, PO43−, K+, Ca2+ and Mg2+ in snow meltwater resulting from summer snow showers were monitored before and after its passage through monospecific stands of the Antarctic macrolichen Usnea sphacelata R. Br. The sampling was conducted under field conditions near Casey Station in East Antarctica between January and March. Total snow deposition during the 61‐d period was 44±1 mm (rainfall equivalent depth) delivering 362±10, 87±2 and 9±1 μmol m−2 of NH4+, NO3− and PO43−, respectively. Meltwater that had percolated through U. sphacelata was depleted in NH4+ and NO3− equating with a retention by the lichen of 87 and 92%, respectively, of the total wet deposition of these ions. Lichen‐modified meltwater was slightly enriched in PO43−, but because the volume of the lichen percolate was smaller than that of the original snow deposition, the lichen achieved a net gain of 9% of the total P deposited. Lichen percolate was also enriched in metal cations. Potassium loss associated with the melting of the heaviest snowfall (18 mm) was equivalent to only 0·05% of the total K in the lichen suggesting that ion loss did not signal significant cellular damage. There was also a progressive increase in NH4+ concentration in unmodified meltwater from 3 to 21 nmol ml−1 over a 3‐d period whereas levels in the lichen‐modified meltwater remained unchanged at 4 nmol ml−1. This enrichment in NH4+ might have resulted from dry deposition onto the melting snow pack of NH3 emitted from nearby penguin rookeries. During the study, tagged thalli of U. sphacelata made a 2% loss in dry mass although they appeared healthy and, at the end of the study, showed an effective quantum yield (ΔF/Fm′) comparable with field material. The results are discussed in relation to the time of year that is likely to be most suitable for lichen growth in this continental Antarctic environment and the potential growth‐led demand for N in U. sphacelata.