bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of ozone on foliar nitrogen metabolism ofPinus taedaL. and implications for carbohydrate metabolism
Authors: MANDERSCHEID R. , JÄGER H.‐J. , KRESS L. W.
Published: April 2006

summary

Dose‐response relationship of ozone effects on foliar nitrogen metabolism of two half‐sib families ofPinus taedaL. was studied. Trees were exposed to six ozone concentrations, ranging from 0.2 to 3 times the ambient, for two consecutive growing seasons (1988 and 1989) in open‐top chambers. Content of total chlorophyll, soluble protein and soluble amino acids, and activities of glutamine synthetase and glutamate dehydrogenase, were measured in second‐flush needles of 1989, harvested in November of 1989. Root collar diameter growth rate was also determined. Variation in depth to soil mottling, an indicator of changing redox conditions, significantly affected tree growth and nitrogen metabolism. Therefore depth to soil mottling was used as a covariate and dose‐response curves were provided with adjusted means. A decline of root collar diameter growth rate and foliar chlorophyll content was found at the highest ozone level. However, both these parameters were affected much more by soil conditions than by ozone. Glutamine synthetase, glutamate dehydrogenase and amino acids, especially glutamate and glutamine, increased by approx. 2‐fold and 3‐fold, respectively, at two and three times ambient ozone. However, soluble protein content was only affected at the highest ozone concentration, showing a 25‐fold increase. Nitrogen metabolism was more influenced by ozone than by the variation in edaphic conditions. No consistent ozone × family interaction on nitrogen metabolism could be found, but the two half‐sib families significantly differed in glutamate dehydrogenase activity and the contents of glutamate and glycine, indicating a regulatory function of this enzyme between nitrogen and carbon metabolism. The increase of amino‐N turnover and the following rise in soluble protein content are explained by the strategy of the plant to reallocate the nutrients from needles subjected to accelerated senescence by ozone. Possible implications of this process in carbohydrate metabolism and carbon partitioning are discussed.
Field studies on Norway spruce trees at high altitudes: II. Defence systems against oxidative stress in needles
Authors: POLLE ANDREA , RENNENBERG HEINZ
Published: April 2006

summary

Antioxidants were characterized in needles from mature spruce trees [Picea abies (L.) Karst.] grown in a forest on the Wank mountain in the Bavarian Calcareous Alps at three altitudes (870 m, 1270 m, and 1700 m). The effect of elevation, season and needle age on the activities of superoxide dismutase, glutathione reductase, ascorbate peroxidase, and the content of ascorbate and glutathione was analyzed. Ascorbate peroxidase and superoxide dismutase activity were little affected by stress from higher altitudes. Superoxide dismutase activity decreased with increasing needle age at all three altitudes. Maximum levels of glutathione and glutathione reductase were found in needles from the highest altitude, but the increase was not linear with increasing elevation. At the middle altitude, levels of glutatione reductase and glutatione were as low as, or even lower than at the lowest site. The mean ascorbate content increased with increasing altitude by about 2 μmol g−1 fresh weight per site. The seasonal change of the ascorbate content in the needles was twofold higher at the upper than at the lower sampling site. These findings suggest that superoxide dismutase and ascorbate peroxidase activities were sufficient to cope with a higher production of toxic oxygen species which probably occurs at high altitudes, and that adaptation was necessary for the antioxidant substrates and their regeneration system.
Secondary homostyly in Primula L.; evidence for the model of the ‘S’ supergene
Authors: WEDDERBURN F. M. , RICHARDS A. J.
Published: April 2006

summary

In Primula, in which 9% of the species are homostyle, it is estimated that 54% are polyploid, while only 4% of the cytologically examined heterostyle species are polyploid.

To investigate the supposed secondary and recombinational nature of homostyly in polyploid Primulas of section Aleuritia, crosses were made between three diploid heterostyle and five polyploid homostyle species. Patterns of pollen germination, pollen tube growth and seed set in interspecific illegitimate crosses were the same as those observed for intraspecific crosses. However, interspecific legitimate crosses showed a late‐acting incompatibility not seen in the intraspecific crosses. In all cases, homostyles acted as it they were of thrum mating type when male, but as if they were of pin mating type when female. We conclude that for all five homostyle species, homostyly has arisen secondarily by recombination within the ‘S’ complex linkage group. The relationship between secondary recombinational homostyly and polyploidy in Primula is discussed.

Although the small quantities of seed produced in crosses between four species of different ploidy levels were shown to be viable, substantial crossability barriers were demonstrated between all eight species. Thus, evidence of crossability supports the maintenance of these closely similar taxa as distinct species of disjunct distribution.
Association of esterase isozymes with morphology in F2 progenies of two growth variants in Poa annua L.
Authors: DARMENCY H. , BERTI A. , GASQUEZ J. , MATEJICEK A.
Published: April 2006

summary

The morphology, flowering behaviour and esterase genotype was examined for six F2 progenies of hybrids between the two main variants of Poa annua L. (namely, annual‐erect and perennial‐prostrate). The association of flowering time with leaf and tiller morphology and Est 1 allozyme genotype, previously described for the two variants in populations from Burgundy, was fully broken in the F2 generation, indicating that no linkage blocks were involved. Whether selection maintains the association of isozyme genotype and growth form is discussed.
Adventitious root formation in hypocotyl cuttings of Picea sitchensis (Bong.) Carr. — the influence of plant growth regulators
Authors: SELBY C. , KENNEDY S. J. , HARVEY B. M. R.
Published: April 2006

summary

A simple method for the study of adventitious root initiation on Sitka spruce [Picea sitchensis (Bong.) Carr.] hypocotyl cuttings is described and was used to examine the effects of plant growth regulators. Twenty‐one‐day‐old seedlings, grown at a low light intensity (15–20 μmol m−2 s−1) to stimulate hypocotyl elongation, provided easy to handle tissues which were responsive to root inducing treatments. Spontaneous rooting only occurred at a low frequency on untreated cuttings and reliable rooting depended upon auxin treatment. When continuous auxin treatment was used, indole‐3‐butyric acid (IBA) was more effective than either indole‐3‐acetic acid (IAA) or 1‐naphthaleneacetic acid (NAA), and was optimal between 10−6 and 10−5 M. Pulse treatments with 10−3 M IBA for as little as 10 min were sufficient to stimulate rooting. All other growth regulators examined were inhibitory to IBA induced rooting. 6‐benzylaminopurine (BA) was the most active being inhibitory down to a concentration of 3 × 10−7 M when it was applied in a mixture with IBA. Pre‐treatment of hypocotyl cuttings with BA prior to auxin treatment demonstrated strong cytokinin ‘carry‐over’ inhibitory effects on rooting. Gibberellic acid A3 (GA3), abscisic acid (ABA) and 2‐chloroethyl‐phosphoric acid (ethrel) were less potent inhibitors of rooting.
Sucrose‐metabolizing enzymes from leaves of barley infected with brown rust (Puccinia hordei Otth.)
Authors: TETLOW I. J. , FARRAR J. F.
Published: April 2006

summary

The capacity for sucrose synthesis in primary leaves of barley (Hordeum vulgare L.) infected with brown rust (Puccinia hordei Otth.) was assessed by measuring the activity of sucrose phosphate synthase (SPS). SPS activity gradually declined during rust infection to 32% of uninfected controls by the sporulation stage. Extractable SPS activity was constant in the light period in both uninfected and infected (sporulating) leaves; activity then declined in the first hour of darkness by 30% in uninfected and 60% in sporulating leaves until returning to the original activity at the start of the following light period. The activity of acid invertase (AI) increased more than four‐fold in rust‐infected (sporulating) primary leaves compared with controls. Extracellular AI activity in rusted tissue was two‐fold greater than in healthy. The consequences of the changes in enzyme activities in the diseased plant are discussed in relation to export from the diseased leaf.
Influence of intergeneric grafts between host and non‐host legumes on formation of vesicular–arbuscular mycorrhiza
Authors: GIANINAZZI‐PEARSON V. , GIANINAZZI S.
Published: April 2006

summary

Stem grafts were carried out between different varieties of vesicular‐arbuscular (VA) mycorrhizal host (pea, soybean) and non‐host (lupin) legumes. Only grafts on host rootstock were successful. Self‐grafts of three pea varieties (Amino, Solara, Frisson), and interspecific grafts between soybean (var. Amsoy 71) and pea (var. Frisson), did not generally affect development of, or arbuscule formation by, Glomus intraradices Schenck & Smith or G. mosseae Nicol. & Gerd. within the rootstock. In contrast, lupin (var. Alban and Lucky) scions greatly reduced colonization of the root cortex and completely inhibited arbuscule formation by both fungi in grafted pea rootstock. The possible role of mobile shoot factors in non‐host resistance to VA mycorrhizal fungi in lupins is discussed.
External hyphae of vesicular—arbuscular mycorrhizal fungi associated with Trifolium subterraneum L.
Authors: JAKOBSEN I. , ABBOTT L. K. , ROBSON A. D.
Published: April 2006

summary

Phosphorus transport by hyphae of the three VA mycorrhizal fungi, Acaulospora laevis Gerdemann & Trappe, Glomus sp. and Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders, associated with Trifolium subterraneum L. was investigated by means of radiotracer techniques. Plants with roots heavily colonized by each mycorrhizal fungus were transplanted to two‐compartment systems, where a hyphal compartment was separated from the main compartment by a fine mesh preventing root penetration. The hyphal compartment contained layers of 32P‐labelled soil, which were placed at 0, 1, 2.5, 4.5 or 7 cm from the root compartment.

A time‐course study over 37 d showed that Glomus sp. transported most 32P to shoots over soil‐root distances shorter than 1 cm. In contrast, A. laevis transported most 32P to shoots over soil–root distances longer than 1 cm. This ability of A. laevis to transport phosphorus over longer distances than Glomus sp. parallels previous observations that hyphae of A. laevis spread faster and further in soil than hyphae of the Glomus sp.

Scutellospora calospora transported much less 32P to plants, but accumulated more 32P in its hyphae, than the two other fungi. The higher specific radioactivity in the hyphae of S. calospora than of A. laevis and Glomus sp. indicated a retarded translocation of 32P in its hyphae or retarded transfer of 32P across its interface with the host. However, the poor phosphorus transport by S. calospora might also have resulted from its reaction to root trimming at transplanting; percentage root colonization by S. calospora decreased markedly after transplanting to the labelling system.
The ecology and functioning of vesicular—arbuscular mycorrhizas in co‐existing grassland species
Authors: SANDERS I. R. , FITTER A. H.
Published: April 2006

summary

Vesicular–arbuscular mycorrhizal infection and morphology was measured in six co‐existing plant species of a semi‐natural grassland during a two year period, in order (1) to measure within‐year variation in mycorrhizal infection, (2) to determine whether seasonal trends in mycorrhizal infection were the same in both years, and (3) to quantify and interspecific differences in this variation. Seasonal trends in total mycorrhizal infection in the six plant species grouped together were very different in the two years. When the plant species were considered individually distinct patterns emerged. Mycorrhizal infection in Plantago lanceolata L. and Rumex acetosa L. varied greatly both within and between years. In contrast, Festuca rubra L., Holcus lanatus L., Lathyrus pratensis L. and Trifolium pratense L. all exhibited more constant levels of infection within one year and between successive years. However, interspecific differences in infection occurred between these four species. Although there was temporal variation in hyphal density, arbuscule and vesicle density varied very little with time and thus could not be used to indicate whether there are predictable periods of host cost or benefit. Vesicle width varied significantly between the plant species; this might represent infection by different fungal species.
The ecology and functioning of vesicular–arbuscular mycorrhizas in co‐existing grassland species
Authors: SANDERS I. R. , FITTER A. H.
Published: April 2006

summary

Although vesicular–arbuscular mycorrhizas (VAM) are abundant in natural ecosystems, the evidence for the mutualistic effect of this association under field conditions is conflicting. VAM infection and inflow of phosphorus, copper, zinc and manganese was measured in six co‐existing plant species of a semi‐natural grassland, during a 2‐yr period, in order to establish whether patterns of nutrient uptake and plant growth in co‐existing species can be related to levels of mycorrhizal infection. There was no relationship between VAM infection and shoot concentrations of P, Cu, Zn or Mn, or plant biomass, and only in a few cases was nutrient inflow related to infection.

Although the results do not support the hypothesis that VAM infection is beneficial for plant nutrition, high levels of P inflow do occur at certain periods during the growth season, suggesting that mycorrhizas may promote plant nutrient uptake at these times. Owing to the abundance of VAM, the lack of speciation and high P inflows which can occur in the field, these mycorrhizal associations are likely to be mutualistic.
Effects of soil temperature on growth, biomass allocation and resource acquisition of Andropogon gerardii Vitman
Authors: DELUCIA EVAN H. , HECKATHORN SCOTT A. , DAY THOMAS A.
Published: April 2006

summary

The effects of root temperature on growth and resource acquisition (carbon, nitrogen and phosphorus) were examined in Andropogon gerardii, a dominant C4 grass of tallgrass prairies of the midcontinental USA. Soil temperature (5.9 to 35.5 °C) was manipulated independently from air temperature (c. 25/20 °C day/night) in a greenhouse. Total biomass at the end of the experiment and relative growth rate (RGR) were maximum at 25 °C soil temperature (Tsoil) and decreased at higher and lower temperatures. Tsoil had no effect on leaf area ratio (the ratio of leaf area to total biomass); thus the effect of Tsoil on RGR was primarily through its influence on net assimilation rate (NAR). Different mechanisms may have contributed to decreases in NAR at sub‐ and supra‐optimal Tsoil. Soil temperatures below 20 °C caused significant reductions in foliar nitrogen (N) and phosphorus (P) concentration. High concentrations of these nutrients in roots suggest that nutrient transport was more strongly inhibited than uptake, and low foliar N and P may have contributed to reduced photosynthetic rates observed at 5 and 10 °C Tsoil. Net photosynthesis was < 12 μmol m−2 s−1 at 5 and 10 °C Tsoil and > 20 °mol m−2 s−1 at 15–40 °C. Changes in net photosynthesis did not therefore contribute to the reduction in NAR at supra‐optimal Tsoil. The strong growth dependence of A. gerardii on Tsoil indicates that this abiotic factor may significantly limit productivity in tallgrass prairie, particularly early in the growing season.
Growth and biomass partitioning of populations of EuropeanPinus sylvestrisL. under simulated 50° and 60° N daylengths: evidence for photoperiodic ecotypes
Authors: OLEKSYN J. , TJOELKER M. G. , REICH P. B.
Published: April 2006

summary

Seedlings of 24 European populations of Scots pine (Pinus sylvestrisL.) were grown from seed for 4 months in controlled environments under photoperiod conditions of 50° or 60° N, but with similar temperatures and total daily quantum flux in both photoperiods. During the experiment the photoperiod was continually changed to simulate the photoperiod from 1 May to 1 September at each latitude. Secondary needle length, plant height and dry mass at harvest were greater at 60° than the 50° N photoperiod. All populations allocated significantly greater proportions of their biomass to roots at 50° than 60° N photoperiod, yet central populations allocated less to roots than northern ones in both photoperiods. Northern populations (origin > 55° N latitude) stopped height growth up to 50 d earlier than the other populations when grown under 50° N photoperiod conditions. Under the 60° N photoperiod, all populations, except the most southern one, developed terminal leader buds and ceased growth in height between 87 and 105 d after sowing. Using cluster analysis, populations with similar growth and photoperiod responses were assigned to two major groups: northern (61–56° N from Sweden and USSR), and central (55–40° N from the USSR, Poland, Germany, Belgium, France, Czechoslovakia, Yugoslavia, and Turkey). The most southern population, from Turkey, was classified independent from the central group based on growth characteristics alone. No single mechanism of height growth cessation can explain the response of all populations of 1‐yr‐old seedlings of Scots pine. Our findings support the hypothesis that northern plants respond to a combination of increasing night length and accumulated degree‐days. However, the evidence suggests that central and southern populations do not respond to increased night length as they all ceased growth earlier in the 60° than the 50° photoperiod. As hypotheses for further testing, we propose that central and southern populations are either photoperiodically insensitive or respond to short nights or to accumulated degree‐days at their native latitudes.
Tansley Review No. 36 Excited leaves
Authors: WALKER DAVID
Published: April 2006

Photosynthesis is largely to do with energy transduction; the conversion of light energy into electrical energy into chemical energy. Precisely how much light energy is needed to bring about the reduction of one molecule of carbon dioxide and the release of one molecule of oxygen (the quantum requirement) is a matter of fundamental importance and one which has attracted much past controversy. This article concludes that a minimum quantum requirement of eight, as demanded by the Z‐scheme, is obviously consistent with much contemporary work which puts the measured value for C3 leaves close to nine. Moreover, while values of less than eight (obtained in some circumstances with micro‐organisms), are a reminder that nothing is beyond challenge they are not, in the absence of confirmation and extension, sufficiently compelling to demand rejection of either the Z‐scheme or current measuring procedures. This article also shows why, even if the underlying minimum requirement was now accepted beyond all reasonable doubt, there would still be very good reasons for continuing, indefinitely, to measure actual photosynthetic efficiency in the natural environment. It discusses some of the implications of the fact that all plants, if not stressed, appear to photosynthesize at the same rate in low light. It explains the role of fluorescence in its relation to quantum yield, the possibility that the rate of photosynthesis might be determined from fluorescence measurements alone, and that a combination of fluorescence and gas exchange measurements could provide new information about the manner in which ‘dark respiration’ is affected by light. It indicates how contemporary interest in all of these matters has focused attention on the necessity for safe dissipation of excitation energy by leaves and on the manner by which this might be achieved.

CONTENTS

Summary
325

I.
Excitation
325

II.
Quantum requirement
326

III.
Learning from fluorescence
335

IV.
Safely dissipated
340

Acknowledgements
342

References
342
Root—stem junctions of a desert monocotyledon and a dicotyledon: hydraulic consequences under wet conditions and during drought
Authors: EWERS FRANK W. , NORTH GRETCHEN B. , NOBELF PARK S.
Published: April 2006

Junctions between shoot‐borne main roots and stems, which are a crucial part of the water‐flow pathway in plants, were investigated for the leaf succulent Agave deserti Engelm. and the sympatric stem succulent Ferocactus acanthodes (Lem.) Britton & Rose under wet conditions and during 21 d of drying in soil. During soil drying, the hydraulic conductance per unit pressure gradient (Kh) declined dramatically in the junctions and to a lesser extent in the roots, but not in stems. The decline in junction Kh was particularly important for A. deserti, which lacks vessels in its stem, because even under wet conditions its Kh was lower in stems and junctions than in roots. For both species, the decline in Kb was due to embolism in the connective tracheary elements at the junction. Such connective elements may be particularly vulnerable to embolism due to their large areas of unlignified primary cell wall. Because the embolism is reversible, the junctions act as rectifiers. Thus, high Kh under wet conditions allows for rapid water uptake following rainfall, and low Kh during drought helps limit water loss from the succulent shoots to a dry soil.
Long‐term exposure of Norway spruce,Picea abies(L.) Karst., to ozone in open‐top chambers
Authors: WALLIN G. , OTTOSSON S. , SELLDÉN G.
Published: April 2006

Rates of photosynthesis at different concentrations of CO2(0–1850 μmol mol−1) were measured using one clone of Norway spruce,Picea abies(L.) Karst. exposed to three different concentrations of ozone for four seasons (1985–8) in open‐top chambers. The treatments were non‐filtered air+ozone (NF+), non‐filtered air (NF) and charcoal‐filtered air (CF). The measurements were made on shoots of four different age classes. No significant effects were found on photosynthesis or on leaf conductance to CO2in current year shoots. In 1‐ to 3‐yr‐old shoots, leaf conductance to CO2and rates of net photosynthesis at both 330 μmol mol−1CO2(Pn330), and saturating concentrations of CO2(Pnmax), decreased with increasing shoot age and ozone concentration, although this was only significant forPn330 in 3‐yr‐old shoots. In current year shoots the intercellular concentration of CO2was significantly higher in NF+‐treated shoots compared with CF‐ and NF‐treated shoots, while in the 2‐ and 3‐yr‐old shoots it was significantly higher in NF‐ and NF +‐treated shoots compared with CF‐treated shoots. The carboxylation efficiency significantly decreased in 2‐ and 3‐yr‐old shoots from the NF and NF+ treatments compared with shoots from the CF treatment. The gas phase limitation of photosynthesis decreased with increasing shoot age and ozone concentration.
The influence of CO2 and O3, singly and in combination, on gas exchange, growth and nutrient status of radish (Raphanus sativus L.)
Authors: BARNES J. D. , PFIRRMANN T.
Published: April 2006

Five days after emergence radish (Raphanus sativus L. ev. Cherry Belle) plants were transferred to a phytotron at the GSF München, where they were exposed in four large controlled climate chambers to two atmospheric concentrations of CO2, (‘ambient’, daily means of ∼ 385 μmol−1; elevated, daily means of ∼ 765 μmol mol−1) and two O3 regimes (‘non‐polluted’ air, 24 h mean of 20 nmol mol−1; polluted air, 24 h mean of 73 nmol mol−1). Leaf gas‐exchange measurements were made at intervals, and visible O3 damage, effects on growth, dry matter partitioning and mineral composition were assessed at a final whole‐plant harvest after 27 d. In ‘non‐polluted air’ CO2 enrichment resulted in a progressive stimulation in Asat, whilst there was a decline in g which decreased E (i.e. improved WUEi). The extra carbon fixed in elevated CO2 stimulated growth of the root (+ hypocotyl) by 43 %, but there was no significant effect on shoot growth or leaf area. Moreover, a decline in SLA and LAR in CO2‐enriched plants suggested that less dry matter was invested in leaf area expansion. Tissue concentrations of N, S, P, Mg and Ca were lower (particularly in the root + hypocotyl) in elevated CO2, indicating that total uptake of these nutrients was not affected by CO2, and there was an increase in the C:N ratio in root (+ hypocotyl) tissue. In contrast, O3 depressed Asat, (∼ 26%) and induced slight stomatal closure, with the result that WUE, declined. All plants exposed to ‘polluted’ air developed typical visible symptoms of O3 injury, and effects on carbon assimilation were reflected in reduced growth, with shoot growth maintained at the expense of the root. In addition, O3 increased the P and K concentration in shoot and root (+ hypocotyl) tissue, indicating enhanced uptake of these nutrients from the growth medium. However, there was no affect of O3 on tissue concentrations of N, S, Mg and Ca. Interactions between the gases were complex, and often subtle. In general, elevated CO2 counteracted (at least in part) the detrimental effects of phytotoxic concentrations of O3, whilst conversely, O3reduced the impact of elevated CO2. Moreover, there were indications that cumulative changes in source: sink relations in O3‐exposed plants may limit plant response to CO2‐enrichment to an even greater extent in the long‐term. The future ecological significance of interactions between CO2 and O3 are discussed.
Pollination in species with dry stigmas: the nature of the early stigmatic response and the pathway taken by pollen tubes
Authors: ELLEMAN C. J. , FRANKLIN‐TONG V. , DICKINSON H. G.
Published: April 2006

Interaction between the pollen grain, pollen tube and the stigma surface has been studied in five species regarded as possessing dry stigma surfaces;Brassica oleraceaL.,Arabidopsis thaliana(L.), Heynh,Papaver rhoeasL.,Cosmos bipinnatusCav. andHelianthus annuusL.

InB. oleraceaandA. thaliana, stigmatic response to pollination includes events in the papillar cytoplasm and changes to the stigmatic surface beneath the grain. In particular a specialized outer element of the cell wall expands prior to pollen tube penetration. The pollen tube, which enters through a ‘foot’ of pollen coating, grows in a space generated between an inner and the outer element of the wall and extends to the base of the papilla where it enters the middle lamellae of the subjacent cell layer. However, inA. thalianatubes frequently were seen to penetrate all components of the stigmatic cell wall, an event only previously recorded in immature stigmas ofB. oleracea.In self pollinations of self‐incompatibleB. oleraceainvolving strong S (incompatibility) alleles no changes take place in the papillar cell wall.

InP. rhoeasthe stigmatic surface responds to self and cross pollination by the secretion of electron‐lucent material beneath the cuticle, causing it to become detached from the outer surface of the stigmatic cell wall. The pollen tubes then penetrate the cuticle and grow towards the base of the papilla in the space thus generated. The tubes continue to grow intercellularly in the transmitting tissue which lies horizontally in rays beneath the papillae.

In members of the Compositae,C. bipinnatusandH. annuus, both stigma and pollen respond to pollination by producing copious quantities of an electron‐opaque matrix, which frequently causes individual papillae to adhere together. Pollen tubes, which are formed following both compatible and incompatible intraspecific pollinations, grow into this matrix and toward the base of the papillae. There, in common with the other plants studied, they grow intercellularly and enter the transmitting tissue of the style.

These findings are discussed in the light of current views of the mechanisms operating during angiosperm pollination and of the significance of the stigmatic response to the functioning of self‐incompatibility mechanisms.
The influence of medium components on secondary embryogenesis of winter oilseed rape, Brassica napus L. ssp. oleifera (Metzg.) Sink.
Authors: LOH C. S. , LIM G. K.
Published: April 2006

The secondary embryogenic potential of Brassica napus L. ssp. oleifera (Metzg.) Sink. ev. Primor could be maintained for many years on complete Murashige and Skoog's medium without diminution. Experiments were conducted to investigate the effects of reducing or deleting certain components of the medium on secondary embryogenesis. Secondary embryogenesis was found to be significantly suppressed when strength of iron in the medium was reduced. Secondary embryogenesis, completely suppressed in ‘iron‐free’ medium, could be regained when iron was incorporated into the medium. Micronutrients and organic nutrients in the medium had no effect on secondary embryogenesis. A reduction in the strength of the medium or its macronutrients decreased secondary embryogenesis significantly.
Effects of dichlorobenzonitrile on the formation of cell wall appositions (plugs) in internodal cells of Chara corallina Klein ex. Willd, em. R.D.W. and Nitella flexilis (L.) Ag.
Authors: FOISSNER ILSE
Published: April 2006

The effect of dichlorobenzonitrile (DCB), an inhibitor of cellulose synthesis, on the formation of local cell wall appositions (plugs) was studied in internodal cells of the characeae Chara corallina Klein ex. Wild, em. R.D.W. and Nitella Flexilis (L.) Ag. DCB Significantly reduces size and number of fibrillar, membrane‐free plugs induced by 6 × 10−2 M Cacl2. Formation of these plugs leads to an increase in the cellulose content which is abolished in the presence of DCB. Plugs formed during treatment with DCB have a less regular structure, but membrane recycling appears not to be disturbed. Cells are able to recover from DCB treatment and normal plugs are then formed. DCB has only minor effects on size, number and structure of amorphous, membrane‐containing plugs induced by chlortetracycline (CTC) although their callose content is significantly enhanced. The data confirm that fibrillar plugs consist mainly of crystalline cellulose and that this polysaccharide does not play any role in the formation of amorphous plugs.
Microscopic evidence on how iron deficiency limits nodule initiation in Lupinus angustifolius L.
Authors: TANG CAIXIAN , ROBSON A. D. , DILWORTH M. J. , KUO J.
Published: April 2006

Lupins (Lupinus angustifolius L. ev. Yandee), grown in solution culture, have been used to study the sites and process of infection by Bradyrhizobium sp. (Lupinus) and the impairment of nodulation by iron deficiency.

Infection leading to nodulation occurred in an area of epidermal cells either lacking root hairs or with very young root hairs at the time of inoculation. Cells aged 13 h or over appeared not to be infected. Infection was initiated in the outer cortex. Rare, short infection threads were evident on day 4 after inoculation, 2 d after the initial division of cortical cells resulting from the bradyrhizobial inoculation. Bacteria had been released into the cytoplasm of cortical cells within 5 d after inoculation. Bacteroids multiplied in the cytoplasm, segregated passively and spread in the infection zones by repeated division of the invaded cells.

Under iron deficiency, initial cell division occurred in the outer cortex of host roots, as in iron‐sufficient plants after inoculation. Iron deficiency then limited further division of cortical cells. Only a few surviving infection sites developed nodules with normal structure but development was much slower than in iron‐sufficient plants.
Vesicular‐arbuscular mycorrhizal infection of grass seedlings in a degraded semi‐arid savanna in Botswana
Authors: VEENENDAAL E. M. , MONNAAPULA S. C. , GILIKA T. , MAGOLE I. L.
Published: April 2006

Infection of eight indigenous grasses with vesicular‐arbuscular mycorrhizal (VAM) fungi was studied in a savanna in South‐eastern Botswana. Infection of seedlings with VAM fungi was rapid just after germination at the onset of the rainfall season. Particularly high infection levels were found in the primary roots of seedlings but levels temporarily dropped with the development of the secondary roots. In 30 % of all cases germinating spores were found to be the source of infection, suggesting an important role for them in semi‐arid grasslands. The inoculum potential of soils from eroded and non‐eroded sites in the study area was compared. Subsoil was found to have a lower inoculum potential than topsoil. Topsoil from an eroded site without vegetation showed a lowered inoculum potential compared to sites with vegetation present. Topsoil from unvegetated sites where soil material was deposited however, did not always show a lower inoculum potential. This indicates that the transport and redistribution of water and soil material does not automatically result in lower infection levels.
Developmental biology of legume nodulation
Authors: HIRSCH ANN M.
Published: April 2006

Many legumes respond to Rhizobium inoculation by developing unique structures known as nodules on their roots. The development of a legume nodule in which rhizobia convert atmospheric N2 into ammonia is a finely tuned process. Gene expression from both partners of the symbiosis must be temporally and spatially coordinated. Exactly how this coordination takes place is an area of intense study. Nodule morphogenesis appears to be elicited by at least two distinct signals: one from Rhizobium, a product of the nod genes (Nod factor), and a second signal, which is generated within plant tissues after treatment with Nod factor. The identity of the second signal is unknown but changes in the balance of endogenous plant hormones or the sensitivity of plant tissues to these hormones are likely to be involved. These hormonal changes may be triggered by endogenous flavonoids produced by the root in response to inoculation with Rhizobium. There is some controversy as to whether the legume nodule is an organ sui generis or a highly derived lateral root. A resolution of this question may become more critical as attempts to induce nodules on non‐legume hosts, such as rice or maize, increase in number and scope.

CONTENTS

Summary
211

I.
Introduction
211

II.
Nodule development
213

III.
Nodule initiation
220

IV.
The second signal for nodule morphogenesis: role for the plant hormones ?
225

V.
Lateral root development
229

VI.
Are nodules modified lateral roots ?
229

VII.
Conclusions and future prospects
231

Acknowledgements and dedication
232

References
232
Leaf development inLolium temulentumL.: characterization of aslow‐to‐greenmutant
Authors: OUGHAM HELEN J. , THOMAS ANN M. , THOMAS BARRY J. , ROBERTS PETER C. , MUTINDA CHARLES , HAYWARD MICHAEL D. , DALTON SUSAN J.
Published: April 2006

A nuclear‐gene mutation of the C3 grassLolium temulentumL., which arose following cell suspension culture and plant regeneration, is manifested as delayed and incomplete greening, which occurs from the leaf tip downwards. Many plastids in the mutant exhibit abnormal morphology when examined by transmission electron microscopy; the plastid outer envelope lacks integrity and thylakoids, while still stacked, are spread over a wide area surrounded by diffuse stromal contents. These aberrant plastids can coexist with apparently normal chloroplasts in the same cell of mutant plants. Levels of chlorophyllaandb, and carotenoids, are all lower in the mutants than in normalLolium temulentum.Leaf length, absolute growth rate, and number of cells per unit length at the leaf base, are greatly reduced (20‐30% the normal values) inslow‐to‐greenplants, but relative growth rate, duration of leaf growth, length of cell division zone and proportion of cells dividing are little affected. This novel mutant is a potentially valuable resource for studying interrelationships between photosynthetic function and leaf extension growth in grasses.
Free radical processes and loss of seed viability during desiccation in the recalcitrant species Quercus robur L.
Authors: HENDRY GEORGE A. F. , FINCH‐SAVAGE WILLIAM E. , THORPE P. CHRISTOPHER , ATHERTON NEIL M. , BUCKLAND SARAH M. , NILSSON KATRINA A. , SEEL WENDY E.
Published: April 2006

Loss of moisture in mature seeds of Quercus robur L. was associated with loss of viability, a rise in lipid peroxidation and build‐up of free radicals. Radical‐initiated damage was largely confined to the embryonic axes as their moisture contents declined to below 47 %. The accumulation of a stable free radical in axial tissue, detected by electron para‐magnetic resonance (EPR), was indistinguishable from the EPR response previously shown in a moss on droughting and maize roots on desiccation. A minor higher‐field component appeared to represent an intermediate stage in the sequence of free radical reactions associated with loss of water. Using seeds from freshly abscised fruits dried to different moisture contents, protective mechanisms against activated forms of oxygen were monitored in cotyledons and in embryonic axes. The two tissues exhibit distinctly different molecular defences against oxidative attack; that in the cotyledons being predominantly enzymatic, with relatively high and increasing activities of superoxide dismutase and glutathione reductase; that in the axes being largely through the anti‐oxidants, ascorbic acid and alpha‐tocopherol. We conclude that a decrease in enzymic protection against oxidative attack in the axes, associated with diminishing concentrations of alpha‐tocopherol (and depletion of the precursor gamma‐tocopherol) as moisture was lost, was directly linked with lipid peroxidation and free radical formation in the axes and that these events taken together may contribute to loss of viability in these recalcitrant seeds.
Rapid identification of genetic variation of ectomycorrhizal fungi by amplification of ribosomal RNA genes
Authors: HENRION B. , TACON F. LE , MARTIN F.
Published: April 2006

There is a clear requirement to develop sensitive methods for detecting denned isolates of ectomycorrhizal fungi within the complex microbial communities of natural ecosystems and reforestation sites. We present a method that permits the rapid identification of an ectomycorrhizal isolate using enzymatic amplification (polymerase chain reaction) of DNA extracted either from pure cultures or ectomycorrhizas. A set of oligonucleotide primers capable of amplifying full‐length nuclear 17S and 25S ribosomal RNA genes, together with the ribosomal internal transcribed spacer and intergenic spacer, have been designed and could be used for amplifying target sequences from a wide range of ectomycorrhizal genera. Length polymorphism in the amplified rDNA and restriction endonuclease analysis of nearly 6‐0 kbp of amplified rDNA provided useful criteria for the rapid typing of isolates from different genera and species. Restriction endonuclease analysis of amplified DNA from 26 isolates representing four species of Laccaria (L. bicolor, L. laccata, L. proxima, L. tortilis) yielded up to 20 scored RFLPs and revealed interspecific and intraspecific polymorphism. Most of the polymorphisms were located within the regions corresponding to the internal transcribed spacer and intergenic spacer. The degree of variation observed was sufficient to discriminate several isolates from the same species. Genetic variation was correlated to some extent with geographical origin of the isolates. However, RFLPs of the rRNA genes cannot unambiguously discriminate all selected isolates within Laccaria species, requiring the development of additional DNA probes. Alone, or in combination with other DNA probes, the amplified rDNA genes may serve in the determination of pure fungal cultures and in the characterization of genetic variation of field ectomycorrhizal populations.
The distribution of some fungal and bacterial endophytes in maize (Zea mays L.)
Authors: FISHER P. J. , PETRINI O. , SCOTT H. M. LAPPIN
Published: April 2006

Endophytic bacteria and fungi were isolated from healthy maize plants collected in a field in Devon. The average bacterial counts in the stem core tissues showed that the plant parts closer to the soil were more heavily colonized by bacteria than those near the top of the plants, and that the lower and middle part of the stems hosted the most frequently isolated bacterial species. Of the fungal species isolated, 12 had a relative importance of more than 10% in the core, 15 in the epidermis, and only 5 in the leaves. In general the distribution patterns were different among the three tissue types studied, with core and epidermis of the stems showing almost equal colonization frequencies and the leaves being most heavily colonized. More fungal species were recovered from the core and epidermis of the stem than from the leaves. The fungi most frequently isolated showed some patterns of tissue specificity, with Alternaria alternata almost exclusively associated with the leaves and Aureobasidium pullulans var. melanigerum most often present in the epidermal tissues.

Assays showed low fungal colonization of seeds taken from freshly matured cobs and of seeds dried for 8 wk before testing, in contrast to higher colonization frequencies observed for the seeds used for planting. The mean values for fungal isolations in the stem pieces mostly in contact with the soil flora or close to the ground were lower than those of the more centrally placed sections. These areas of low fungal infection yielded the highest bacterial counts. The potential role of bacterial endophytes in biological control is briefly discussed.
The response of spring wheat (Triticum aestivum L.) to ozone at higher elevations. III. Responses of leaf and canopy gas exchange, and chlorophyll fluorescence to ozone flux
Authors: GRIMM A. GRANDJEAN , FUHRER J.
Published: April 2006

Spring wheat (Triticum aestivum L., cv. Albis) was grown in open‐top chambers and exposed to four different levels of ozone (O3) from the three‐leaf stage until maturity. The aim was to examine changes in leaf and canopy gas exchange, and in chlorophyll fluorescence, in response to O3 flux. Measurements were carried out periodically between full expansion and complete senescence of flag leaves. Fluxes to the canopy of CO2 (CERc.) (corrected for soilborne CO2), water vapour (Ec) and O3 were determined by using open‐top chambers as differential systems. Water use efficiency (WUEc.) was calculated from CERc, and Ec. Leaf CO2 (CERn) and H2O (En) exchange rates, stomatal conductance (g,(H2O)), and WUEn, were analyzed with a portable gas exchange analyzer. Effects of O3 flux on structural components of photosynthesis were examined by determining variable fluorescence (defined by the Fv/Fo ratio) in leaves after 60 minutes of dark‐adaptation or during the night.

The decline in CERc and CERn associated with senescence was accelerated by O3. Average CERn between flag leaf unfolding and late milk stage declined linearly with increasing O3 flux. The corresponding decline in average CERc was less pronounced. The quantitative effect of O3 flux on CERc corresponded well with the effect on grain yield. In young leaves, gs(H2O) was reduced in response to O3 but WUEn was unaffected. With progressing leaf age, WUEn declined. Thus, in the young leaves, O3 affected the stomata directly and, consequently, limitation of photosynthesis was primarily due to reduced CO2 diffusion. In contrast, in senescent leaves, the effect of O3 was mainly due to reduced carboxylation. Compared with WUEn WUEc responded differently to increasing O3 flux. During O2 fluxes at above‐ambient levels, WUEC tended to increase rather than to decrease. It is suggested that under O3 stress, factors controlling WUE at the canopy level differ from those operating at the level of single flag leaves. The decline in Fv/Fo measured after anthesis was stimulated by O2, but no effect of O3 was detected when Fv/Fo was measured during the night. This leads to the conclusion that the effect of O3 on photosynthetic structures is reversible and the reduction in photosynthesis in response to O3 flux is due to metabolic changes rather than to direct damage to structural components.
Mineral nutritional inter‐relations amongst stolons and tiller ramets in Agrostis stolonifera L.
Authors: MARSHALL C. , ANDERSON‐TAYLOR G.
Published: April 2006

The functional organization of the clonal plant Agrostis stolonifera L. was investigated in terms of the mineral nutrient inter‐relations between individual well‐established stolons and between adjacent tiller ramets on a single stolon. Tillers on developing stolons were rooted individually and treatments established in which different patterns of mineral nutrient supply were given to various parts of the clone. In the first experiment with plants consisting of 2 main stolons and where the ramets of each stolon were supplied with either nutrient solution or water, the stolons displayed distinctly independent nutrient economies irrespective of the pattern of nutrient supply. In a second experiment, where the effect of uniform or local nutrient supply to a single stolon of rooted ramets was followed, the growth of ramets was very closely related to their local nutrient supply. In a further experiment the performance of rooted and unrooted ramets was compared in contrasting nutrient regimes. When situated between ramets supplied with nutrients, the growth and development of an unrooted ramet closely matched one that was rooted and supplied with nutrient solution but greatly exceeded that of one that was rooted and given only water.

The independent mineral nutrient economies displayed by individual rooted stolons and by rooted tiller ramets is discussed in relation to the underlying physiological integration of the stolon axis that allows unrooted ramets and the unrooted extending apical region of a stolon to be provided with adequate mineral resources for their growth and development.
On the ecophysiology of the Clusiaceae in Trinidad: expression of CAM in Clusia minor L. during the transition from wet to dry season and characterization of three endemic species
Authors: BORLAND A. M. , GRIFFITHS H. , MAXWELL C. , BROADMEADOW M. S. J. , GRIFFITHS N. M. , BARNES J. D.
Published: April 2006

A study was made of photosynthesis and expression of crassulacean acid metabolism (CAM) in naturally exposed and shaded populations of Clusia minor L. during the transition from wet to dry season in Trinidad (mid‐February to mid‐April, 1990). At the start of the dry season, plants from exposed and shaded habitats showed a capacity for CAM either through the fixation of external or internal (respiratory) CO2. Exposed plants showed continuous uptake of CO2 over 24 h although dark fixation accounted for only a small proportion of CO2 fixed over the day. The expression of CAM was considerably enhanced as the dry season progressed with substantial increases in the overnight accumulation of titratable acidity, particularly in leaves of exposed plants. This was accompanied by a reduction in day‐time photosynthesis and an increase in dark fixation, with shaded plants showing only night‐time fixation of CO2. The magnitude of CAM in C. minor was substantial with a maximum ΔH+ of 1410 mol m‐3 measured in leaves from exposed branches. Both malic and citric acids were accumulated overnight. The highest citric:malic acid ratios were found in young leaves from exposed plants with 250 mol m‐3 malic and 125 mol m‐3 citric acid accumulated near the time of maximum CAM activity. Photosynthetic efficiency, measured as light responses of O2 evolution, also varied on a daily basis dependent on the incident photosynthetic photon flux density (PPFD). Apparent quantum yield and photosynthetic capacity showed marked reductions depending on the degree of exposure, suggesting that photoinhibitory responses are important under natural conditions. An analysis of three members of the Clusiaceae endemic to Trinidad showed that each had the capacity to induce CAM activity, despite being found in a narrow range of habitats which have higher rainfall than those of C. minor. However, despite the variable expression of CAM activity, carbon isotope composition suggested that when integrated throughout the year, carbon accumulation is predominantly mediated via the C3 pathway in all the species studied.