New Phytologist

ROLFE, STEPHEN A.; SCHOLES, JULIE D.

doi: 10.1111/j.1469-8137.1995.tb03056.xpmid: 33863168

A computer‐controlled video system is described where images of chlorophyll fluorescence from oat (Arena saliva L.) leaves are captured throughout an induction curve. The fluorescence parameters φ11 and ΔFm/F'm which represent photosystem II photochemical efficiency and non‐photochemical quenching respectively are calculated from images taken under actinic and saturating illumination. They are compared with the equivalent parameters determined from simultaneous measurements made with a non‐imaging modulated fluorimeter. The leaves are imaged in the chamber of an infra‐red gas analyser, allowing the relationship between the fluorescence parameters and CO2 assimilation to be determined. Under a series of illumination conditions and partial pressures of CO2, the values of φ11 determined by image analysis are in close agreement with those obtained by modulated fiuorimetry and are linearly related to CO2 assimilation. The values of ΔFm/Fm determined by image analysis are linearly related to those obtained by modulated fluorimetry but the absolute values differ between the two systems. The video system described can be used to examine quantitative changes in fluorescence parameters in a heterogeneous system.

DOLAN, LIAM; ROBERTS, KEITH

doi: 10.1111/j.1469-8137.1995.tb03061.xpmid: 33863160

The root of Arabidopsis thaliana (L.) Heynh. undergoes dramatic morphological changes during secondary thickening. We have used monoclonal antibodies that recognize two cell surface arabinogalactan protein epitopes (AGPs) and a pectic polysaccharide to document cell surface changes during the process of secondary thickening. These antibodies recognize cells in various stages of differentiation. An AGP epitope recognized by JIM 14 is expressed at the plasma membrane of most tells in the root, but is most highly expressed in mature sieve tube elements, JIM 13 identifies an epitope that is expressed in a discrete ring of cells in the periderm and during xylem vessel element differentiation, disappearing as lignificntion of the vessel elements proceeds, These antibodies reveal a biochemical differentiation at the surface of cells which mirrors obvious morphological differentiation events. Since AGPs have been shown previously to have a dramatic effect on the developmental capabilities of cells it is possible that the epitopes recognized by these antibodies might have an important developmental role during differentiation.

NORBY, RICHARD J.; WULLSCHLEGER, STAN D.; GUNDERSON, CARLA A.; NIETCH, CHRISTOPHER T.

doi: 10.1111/j.1469-8137.1995.tb03058.xpmid: 33863171

Forests have a prominent role in the global carbon cycle, but their response to a changing atmosphere cannot be measured directly. Experimental observations of small trees in CO2‐enriched atmospheres must be interpreted carefully if they are to be relevant to the potential responses of forest trees. We grew1 white oak (Quercus alba L.) saplings for four complete growing seasons in open‐top chambers with different partial pressures of atmospheric CO2 White oak saplings produced 58% more dry mass in 50 Pa CO2 and 135% more in 65 Pa, compared with plants in ambient (35 Pa) CO2 Although this result might suggest a substantial potential for increased carbon storage in forests, the large difference in growth rate could be attributed to a stimulation of growth very early in the experiment. There was not a sustained effect of C2 on relative growth rate after the first year, and the increased absolute growth rate could persist only so long as leaf area could increase, a condition that would not occur indefinitely in a forest. Nevertheless, annual stem wood production per unit area (growth efficiency) was 37 %, greater in elevated CO2. This increase in growth efficiency, a response that is consistent across diverse studies, implies a potential increase in carbon sequestration by forests, subject to critical assumptions about forest canopy development in a CO2‐enriched atmosphere

STAL, LUCAS J.

doi: 10.1111/j.1469-8137.1995.tb03051.xpmid: 33863161

In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro‐scale ecosystems are characterized by steep and fluctuating physico‐chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat‐forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo‐ and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate‐reducing bacteria are important in the end‐oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide‐dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat‐forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non‐heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non‐heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen‐sensitive nitrogenase from inaction.

YOUNG, NICOLA; ASHFORD, ANNE E.

doi: 10.1111/j.1469-8137.1995.tb03052.xpmid: 33863162

Intact mature sclerotia of Sclerotium rolfsii sacc, and Sclerotium cepivorum Berk, produced in culture are impermeable to the apoplastic tracer sulphorhodamine G. Both of these species produce sclerotia with rinds. Some movement of sulphorhodamine into sclerotia of Rhizoctonia solani Kühn, which have no rind, occurred but the fluorochrome was arrested after permeation of at most the outer five layers of cells. In all cases, low permeability depended on an intact outer layer, and when sclerotia of each species were bisected to provide direct access of sulphorhodamine to all tissue layers, the fluorochrome permeated the cell walls and extracellular matrix (where present) of many cells within the sclerotium. A marked reduction in permeability of intact sclerotia occurs at maturity in a number of species and might he important in long‐term Survival.

SEGSCHNEIDER, HANS‐JOSEF; AND, JÜRGEN WILDT; FÖRSTEL, HILMAR

doi: 10.1111/j.1469-8137.1995.tb03060.xpmid: 33863169

Exposures of sunflowers (Helianthus annuus L. var. Giganteus) to 15NO2, at concentrations between 5.0 and 72.9 ppb resulted in its uptake by the plants during illumination as well as during darkness. The NO2, flux increased linearly with increasing NO2, concentrations in both treatments. Evolution of NO2 by sunflowers was not observed. During illumination, measured and potential N Of uptake were equal, which indicates that the uptake of NO2 was only limited by stomatal resistance, whereas in darkness, an additional internal resistance to NO. cannot be excluded. After 15NO2, exposures, the highest proportions of the 15N tracer were fixed as protein nitrogen or as heterocyclic nitrogen compounds and glucosamine. Under both treatments there were remarkably high 15N enrichments in the fraction ‘soluble amino acid nitrogen’, indicating a rapid assimilation of the nitrogen derived from 15NO2, A linear relation was found between the δ15N values of nitrogen pools investigated and the 15NO2 concentrations after exposures in light as well as after night‐time fumigation. Apparently, all the enzyme systems involved in the assimilation of the NO2 nitrogen must be available in sufficient amounts in plant tissues during exposures. Even after exposures in the dark, about 95% of the absorbed 15NO2, nitrogen was fixed in reduced organic nitrogen compounds, which indicates that the assimilation of atmospheric NO2, might not depend on the production of reduction equivalents by photosynthesis. The mechanism of NO, assimilation via nitrate/nitrite reductase within the exposed sunflowers is discussed.

KIRK, G. J. D.; BAJITA, J.B.

doi: 10.1111/j.1469-8137.1995.tb03062.xpmid: 33863159

Rice plants (Oryza sathiva L., cv. IR34) were grown with their roots sandwiched between cylinders of an anaerobic low‐Zn Mollisol. After periods of root‐soil contact of up to 12 d (total plant age c. 28 d) the profiles of different Zn fractions, reduced and oxidized Fe, and pH in the soil near the root plane' were determined. The concentration of easily plant‐extractable Zn in the soil (measured by extraction in I M KCl) was negligible, and so it was necessary for the plants to induce changes in the soil to solubilize Zn. After 6 d, there was a substantial accumulation of Zn associated with organic matter and amorphous ferric hydroxide within 4–5 mm of the root plane. Over the next 6 d, the accumulation continued but there was a substantial depletion of the accumulated fractions within 2 mm of the root plane. The zones of accumulation and depletion coincided with zones of Fe(III) accumulation and soil acidification in which the pH decreased from the bulk soil value of 7.3 by over 0.2 pH units; i.e. a two‐fold increase in H+ concentration, The acidification was the result of H+ released from the roots to balance excess intake of cations over anions, and H+ generated in the oxidation of Fe(H) by root‐released O2. At the high pH and CO., pressure of the experimental soil (7.3 and c. 0.9 kPa. respectively), soil acidity diffusion is fast and consequently the pH drop at the root surface was small. The rate of Fe oxidation peaked before 6 d, but the acidification and Zn accumulation continued beyond 6 d unabated. It is concluded that Fe oxidation released Zn from highly insoluble fractions, and that this Zn was re‐adsorbed on Fe(OH)3 and on organic matter in forms that were acid‐soluble and therefore accessible to the plants.

MARKKOLA, A. M.; OHTONEN, R.; TARVAINEN, O.; AHONEN‐JONNARTH, U.

doi: 10.1111/j.1469-8137.1995.tb03063.xpmid: 33863163

Biomasses of ectomycorrhizal and saprotrophic fungal communities partitioned into sporophores and non‐reproductive structures were estimated in mature Scots pine (Pinus sylvestris L.) stands along an urban nitrogen and sulphur pollution gradient in northern Finland. The average total biomass of fungi varied in the four pollution zones from 14.6 to 20.2 g d. Wt kg−1 soil d. wt and from 73.3 to 108.0 g d. Wt m−2, the mycelia of both mycorrhizai and saprotrophic fungi in the soil comprising 72–80% of the total. The annual carbon allocation to the fungal communities was calculated to vary between 9 and 26% of the estimated annual carbon assimilation at the Scots pine sites. The size of the mean fungal biomass fractions decreased in the following sequence: mycelia in the soil > fungal biomass in fine roots estimated in terms of chitin > sclerotia > fungal biomass in fine roots estimated in terms of ergosterol > sporopbores of mycorrhizal fungi > sporophores of saprotrophie fungi. A positive correlation was obtained between the number of Scots pine mycorrhiza and the average sporophore yield of mvcorrhiaal fungi for three successive years. Tbe sporophore biomass of the mycorrhizal fungi was smaller at the most polluted than at the least polluted sites. The total fungal biomass allocation was not affected by urban pollution.

DITOMMASO, ANTONIO; WATSON, ALAN K.

doi: 10.1111/j.1469-8137.1995.tb03054.xpmid: 33863165

The combined effects of planting density and of different inoculation frequencies ofColletotrichum coccodes (Wallr.) Hughes on growth and competitive performance of Abutilon theophrasti Medik. were studied using a soybean‐A. theophrasti target‐neighbour design in a controlled environment. In both trials of the experiment, A. theophrasti inoculated at the highest planting density (four plants per ppt) suffered significantly greater reductions in height (41%) than did A. theophrasti at the lowest density (one plant per pot) (7%). Above‐ground biomass and leaf area reductions, however were significantly greater at the highest density for only one of the trials. Soybean plants grown with inoculated A. theophrasti at the two highest planting densities had a significantly greater above‐ground biomass (61%) and leaf area (68%) than did plants grown with uninoculated A. theophrasti at the same densities. By contrast, at the two lower densities, soybean above‐ground biomass and leaf area were not increased significantly fallowing inoculation. Either one or two C. coccodes inoculations caused the greatest reductions in A. theophrasti growth compared with uninoculated plants. Conversely, three applications of the fungus generally resulted in less severe disease symptoms and resulted in the smallest decreases in A. theophrasti growth. Induced systemic resistance following two inoculations might have played an important role in limiting disease. However, the significantly greater biomass and height of A. theophrasti plants subjected to the triple C. caicudes treatment, compared with plants receiving either one or two inoculations in one of the trials, provides some evidence of a possible compensatory response in .4. theophrasti. The relevance of these findings for biological weed control is examined.

GARDNER, SIMON D. L.; TAYLOR, GAIL; BOSAC, CREANA

doi: 10.1111/j.1469-8137.1995.tb03057.xpmid: 33863167

Leaf extension was stimulated following exposure of three interamerican hybrid poplar clones (Populus trichocarpa P. deltoides); ‘Unal’, ‘Boelare’, and ‘Beaupre’ and a euramerican clone ‘Primo’ (Populus nigra×P. deltoides) to elevated CO2, in controlled environment chambers. For all three interamerican clones the evidence suggests that this was the result of increased leaf cell expansion associated with enhanced cell wall extensibility (WEx), measured as tensiomerric increases in cell wall plasticity. For the interameriean clone ‘Boelare’, there was also a significant increase in cell wall elasticity following exposure to elevated CO2 (P⩽ 0.001). The effect of elevated CO2 in stimulating cell wall extensibility was confirmed in a detailed spatial analysis of extensibility made across the lamina of expanding leaves of the clone ‘Boelare’. For two of the interamerican hybrids, ‘Unal’ and ‘Beaupre’, both leaf cell water potential Ψ and turgor pressure (P) were lower in elevated than in ambient CO2. By contrast, no significant effects on the cell wall properties or leaf water relations for the euramerican hybrid ‘Primo’ were observed following exposure to elevated CO2. suggesting that the mechanism for increased leaf extension in elevated CO2, differed, depending on clone. The cumulative total length of leaves of ‘Boelare’ grown in elevated CO2, was significantly increased (P≤ 0.05) and since leaf number was not significantly increased in any inter‐american clone it is hypothesized that final leaf size was stimulated in elevated CO2 for these clones. By contrast, there was no significant effect of CO2 on cumulative total leaf length for the euramerican clone ‘Primo’, but leaf number was significantly increased by elevated CO2. The measurements suggest that total tree leaf area was stimulated for a range of poplar hybrids exposed to elevated CO2. Given the short rotation of a coppiced crop, it is likely that increased leaf areas will result in enhanced stemwood production when hybrid poplars are grown in the CO, concentrations predicted for the next century.