Wetting of the upper leaf surface of Juglans regia L. and of model surfaces colonized by epiphytic micro-organisms was investigated by measuring contact angles of aqueous solutions buffered at different pH values. During June to October 1995, contact angles of aqueous solutions on the leaf surface of J. regia decreased by angles ranging from 12° (low pH values) to 25° at high pH values. At the end of this vegetation period, wetting was strongly dependent on pH showing significantly lower contact angles with alkaline solutions (pH 9·0) than with acidic solutions (pH 3·0). Contact angle titration measured angles on the leaf surface as a function of the pH of buffered aqueous solutions, covering a pH range from 3·0 to 11·0. Titration curves revealed inflection points around 7·5, indicating the existence of ionizable carboxylic groups at the interface of the phylloplane. Altered leaf-surface wetting properties observed on the intact leaf surface could be simulated in model experiments by measuring contact angles on artificial surfaces colonized by Pseudomonas fluorescens and by epiphytic micro-organisms isolated from the phylloplane of J. regia. Strong evidence is provided that interfacial carboxylic groups derive from epiphytic micro-organisms present on the phylloplane. Results suggest that the age-dependent increase in, and pH dependence of, wetting as leaves mature are related to the presence of epiphytic micro-organisms on the phylloplane. Ecological consequences of increased leaf-surface wetting, concerning the structure of the leaf surface as a microhabitat for epiphytic micro-organisms, are discussed.

CONTENTS

Summary 389

I. INTRODUCTION 389

II. 1. The SNARE hypothesis 393

2. SNAREs and associates 395

(a) NSF 395

(b) SNAP 395

(c) SNARE 395

(d) Rab and Rab-associated proteins 397

(e) Other interacting proteins 398

(f) Clostridial neurotoxins 399

3. New perspectives on SNARE function 400

(a) Revisiting the SNARE hypothesis 400

(b) Roles for SNARE proteins 401

III. VESICLE TRAFFICKING IN PLANTS 402

1. Identifying secretory proteins in plants 402

2. Placing syntaxins in order 403

3. Functional analysis of plant SNAREs in vesicle trafficking 404

4. Small GTPases 405

5. Searching for a vesicle–fusion complex 405

IV. REGULATION OF VESICLE TRAFFICKING IN PLANT CELLS 406

1. Hormonal control of secretion 406

(a) Auxin-evoked secretion and cell growth 406

(b) Gibberellic acid and secretion by barley aleurone 407

(c) Abscisic acid control of stomatal aperture 407

2. Coupling secretion to cytosolic-free Ca+ concentration 408

3. Stress-evoked secretion 409

(a) Mechanical stress 409

(b) Wounding and pathogen attack 409

V. CONCLUSION 410

Acknowledgements 411

References 411

Eukaryotic cells share a set of secretory pathways for the flux of membrane and protein material. In 1993, ideas about the functioning of three major proteins of the neurosecretory complex were consolidated in the SNARE hypothesis, which proposed that the interaction of these proteins provides both the specificity for vesicle targeting and the molecular machinery for fusion between vesicle and target membranes. Subsequetly, the organization, molecular mechanics and control of vesicle trafficking have become topics of intense research, and the hypothesis has evolved to accommodate new discoveries from the analysis of secretion in yeast and mammals. It is likely to be challenged again as more information comes to light about secretory processes in plants. New tools for measuring and manipulating vesicle traffic and secretion are now being used, drawing on in vivo fluorescence and capacitance recording as well as genetic engineering. These new technologies have already begun to yield details wholly unexpected from past studies. Here we focus on recent findings relating to the mechanisms of vesicle trafficking and the background to these developments, highlighting both current understanding of the molecular events of secretion and the gaps therein, as well as discussing emerging themes from work with plants.

Since the beginning of the Bronze Age the acquisition of metals and the mastering of metal processing has been a driving force in human civilisation. It is difficult to imagine, however, how a plant could possibly profit from accumulating excessive amounts of a metal for which there is no metabolic need. Leaving behind an environmentally degraded wasteland, human mining and smeltering activities contributed to providing the niches for such plants, sometimes even accomplishing seed dispersal – through seeds attached to the boots of migrating miners. A stunning example of this is found in the healthy populations of Thlaspi caerulescens in the Cevennes (southern France), the leaves of which have been found to contain the toxic and non-essential metal cadmium (Cd) at concentrations between 1000 and 3000 mg kg−1 dry biomass (R. D. Reeves, unpublished). In this issue, experiments under controlled experimental and glasshouse conditions are reported, confirming an extraordinary level of Cd accumulation and tolerance in these populations (Lombi et al., pp. 11–20).

Birch cuttings (Betula pendula Roth) were grown in a sand-culture system with two concentrations (0·05, HF and 0·005%, LF) of fertilizer containing macronutrients and micronutrients, and were exposed to 90/40 nl l−1 O3 (day/night) and <3 nl l−1 O3 (control) for one growing season in the field fumigation chambers at Birmensdorf (Switzerland). Leaves of different ages were analysed for gas exchange, contents of chlorophyll, protein, and for metabolites as well as enzyme activities of carbohydrate metabolism.

Ozone reduced net photosynthesis and chlorophyll contents in mature leaves of both fertilization treatments, whereas that of protein was only reduced in high-fertilized plants (HF). However, net photosynthesis, chlorophyll, and protein increased in young leaves of low-fertilized plants (LF). The effects of ozone on enzyme activities of carbohydrate metabolism were most pronounced in leaves of LF plants. Specific activities of the sucrose-cleaving enzymes, sucrose synthase and alkaline invertase, were induced, whereas acid invertase was unchanged. Extractable activity of sucrose phosphate synthase, which is a key enzyme of sucrose synthesis, was reduced. Levels of fructose 2,6-bisphosphate, an inhibitor of sucrose synthesis, were increased in leaves of O3/LF plants, but reduced in O3/HF plants. In addition, activities of enzymes involved in starch metabolism, ADP-glucose pyrophosphorylase and starch phosphorylase, were lowered in ozone-treated samples and the ratio of ATP[ratio ]ADP was increased.

It is concluded that chronic ozone exposure leads to an inhibition of sucrose synthesis and favours sucrose degradation. This effect is modulated by the nutrient status of the plants, indicating higher O3 tolerance in HF plants. Furthermore, as the metabolic responses in the ozone-treated samples resemble very closely those observed under end-product inhibition of photosynthesis, we assume that the O3 effect is mainly due to reduced photosynthate export.

Young spruce (Picea abies [L.] Karst; 2-yr) and beech trees (Fagus sylvatica L.; current-year and 1-yr-old seedlings) were grown under field conditions at a rural site near the city of Basel (Schönenbuch; 400 m above sea level) and 2-yr-old spruce trees at an elevated montainous site (Wengernalp; 1890 m above sea level). The plants were exposed in open-top-chambers (OTC) either to charcoal-filtered air or ambient air with ozone being the predominant air pollutant at both sites. The exposures lasted up to 1 yr. At the end of the growth period the plants were harvested and the pool sizes of the major non-structural carbohydrates were determined. Exposure to ambient air compared with filtered air controls caused an increase of glucose, fructose, pinitol, starch and, in some cases, sucrose in needles whereas carbohydrate contents in roots and twigs of spruce were reduced, in particular at Wengernalp. In beech saplings at Schönenbuch, a significant rise of sucrose and, in some cases, of raffinose, together with a significant reduction of glucose and fructose was observed in buds, leaves, twigs, and roots in ambient air compared with filtered air. In phloem exudates of current year beech seedlings, a 54–160% higher content of carbohydrates was found in ambient air than in filtered air. In roots of the beech saplings, the amount of soluble carbohydrates increased and starch contents mostly decreased. In general, starch storage in roots and stems was more abundant in beech trees. Spruce, especially at the mountainous site, accumulated high amounts of soluble carbohydrates in needles, in particular glucose.

Two tetraploid subspecies of Dactylis glomerata L., subsp. reichenbachii (Hausm.) Stebbins et Zohary and subsp. glomerata, occur in the French Alps. The former is confined to dolomitic, south-facing, alpine lawns above 2000 m, whereas the latter occurs in non-dolomitic habitats in subalpine meadows mainly below 1900 m. Previous studies of allozyme variation have shown that genetic introgression between the two subspecies occurs over large areas. By contrast, morphologically intermediate individuals only occur in an extremely narrow area, suggesting that the morphological and physiological differences between the two subspecies is of adaptive significance. A reciprocal clone transplant experiment was set up to examine (1) any genetic differences between subspecies indicative of ecotypic differentiation in relation to habitat characteristics and (2) the level of phenotypic plasticity in the two subspecies. Genetic differentiation was confirmed by a statistically significant taxon × site interaction effect in anova for all traits studied. The glomerata populations produced more tillers, longer leaves and higher culms in all sites, especially in their home environment. However, reichenbachii populations produced more seeds than the glomerata populations in the original reichenbachii environment, suggesting ecotypic differentiation between the two subspecies. This result might also explain why the glomerata subspecies is unable to colonize dolomitic habitats occupied by the reichenbachii subspecies. The reichenbachii populations showed less plasticity than the glomerata populations for leaf length and floriferous tiller number, a result which is discussed in the context of the response of plants from productive and non-productive habitats to environmental variation.

Two arbuscular mycorrhizal fungi, Scutellospora calospora and Glomus sp. ‘City Beach’, were grown in soil conditions suitable for colonization of Allium porrum. Effects on plant growth and phosphate uptake were examined. Isolates of S. calospora (including the one used here) have been shown by others to vary in their stimulation of plant growth, and appear to be inefficient in the transfer of P from the fungus to the host. Our hypothesis was that such isolates of S. calospora may be more aggressive colonizers than other fungi, thus preventing growth increases or producing strong growth depressions in the host plant. In fact, inoculation with either fungus increased growth of plants in their respective soils both low in P (P0) and with added P (P1) by 42 d. The effect on growth due to mycorrhizal symbiosis (i.e. mycorrhizal growth response, % MGR) at 42 d was higher in plants grown in P0 soil. Plants colonized by Glomus sp. ‘City Beach’ had a greater % MGR than plants colonized by S. calospora. Both fungi colonized plants to high levels. The percentage of root length colonized was higher in P0 soil than in P1 soil at 21 d. The internal development of S. calospora appeared less affected by addition of P than Glomus sp. ‘City Beach’ at the early harvests. Formation of arbuscules followed the same trends as total colonization. Shoot P concentration was significantly higher in mycorrhizal plants than in non-mycorrhizal plants, by 21 d in P1 soil and by 28 d in P0 soil. Different percentage responses to added P based on total plant dry weight (% PGR) were observed at 28 and 42 d between the plants colonized by the two fungi. The increased P content due to mycorrhizal colonization (% MPR) differed with soil P. For both fungi grown in their respective soils, the response was greater in plants grown in P0 soil. Although the isolate of S. calospora used is an aggressive arbuscular mycorrhizal fungus with some hosts, it promotes a strong positive plant growth response in A. porrum after a mild initial growth depression.

The representation of root activity in models is here confined to considerations of applications assessing the impacts of changes in climate or atmospheric [CO2]. Approaches to modelling roots can be classified into four major types: models in which roots are not considered, models in which there is an interplay between only selected above-ground and below-ground processes, models in which growth allocation to all parts of the plants depends on the availability and matching of the capture of external resources, and models with explicit treatments of root growth, architecture and resource capture. All models seem effective in describing the major root activities of water and nutrient uptake, because these processes are highly correlated, particularly at large scales and with slow or equilibrium dynamics. Allocation models can be effective in providing a deeper, perhaps contrary, understanding of the dynamic underpinning to observations made only above ground. The complex and explicit treatment of roots can be achieved only in small-scale highly studied systems because of the requirements for many initialized variables to run the models.

Endogenous polyamine content of the ectomycorrhizal fungus Paxillus involutus, as well as the activity of its biosynthetic enzymes in relation to mycelia ageing were investigated in this work. Polyamines in free, PCA-soluble and insoluble conjugated forms, are present in Paxillus involutus mycelia in relatively high amounts and the ratio of putrescine to spermidine is age-dependent. Both arginine- and ornithine-decarboxylases are present, but putrescine biosynthesis proceeds mostly via ornithine decarboxylase and decreases with the age of mycelia. There was a large release of free polyamines from mycelia which showed age-dependent features. Clear polyamine uptake was observed in 2-wk-old mycelia and no competition between putrescine and cadaverine was detected. Putrescine uptake seems to reduce ornithine decarboxylase activity, but does not affect arginine decarboxylase.

To test the hypothesis that fructan structure can be used as a taxonomic character for the Poaceae we examined the accumulation of the linear, β-2,6-linked fructan series with a terminal glucose residue within the tribes Aveneae and Poeae. Only Dactylis glomerata L. (Poeae) has been shown to contain this series, making it unique among fructan structures and a potential taxonomic marker. To this end, 13 members of the tribes Aveneae and Poeae were surveyed for the presence of this fructan series by comparing extracts of water-soluble carbohydrate by TLC with an extract of D. glomerata. The fructans of four species were similar to those of D. glomerata, as determined by TLC. Further comparison by anion-exchange chromatography and linkage analysis demonstrated that Phalaris aquatica L., Puccinellia stricta (Hook. f.) C. H. Blom and Holcus lanatus L. contained a linear, β-2,6-linked series of fructan with a terminal glucose residue but that Lagurus ovatus L., although containing predominantly β-2,6-linked fructan was composed of more than one series of fructan, one with a terminal glucose residue and one with an internal glucose residue. A linear β-2,6-linked series of fructans with a terminal glucose residue was the dominant fructan accumulated by some species of genera belonging to both the Aveneae (Phalaris and Holcus) and the Poeae (Dactylis and Puccinellia). As both of these tribes also contain genera that accumulate fructans with an internal glucose residue, e.g. Avena and Lagurus (Aveneae) and Lolium (Poeae), structure of fructan cannot be used to distinguish these tribes. Review of the literature, however, showed that it is possible to separate the supertribes Triticodae and Poodae on the basis of the structure of the fructan that they accumulate.

To clarify the role of seasonal change, competitive response and nutrient availability in the competitive asymmetry of grassland species a competition experiment was conducted on Holcus lanatus, Anthoxanthum odoratum and Festuca ovina, which represent a successional sequence of decreasing nutrient availability. Seven harvests were taken over two growing seasons. At each harvest the dry weight of plant parts, dead leaves, leaf area and plant height were measured. Three key traits that determine the successional status of the species were studied: specific leaf area, specific shoot height, and dead leaf fraction.

The response of these traits to competition appeared to be limited and insufficient to change the competitive relations in the experiment. However, all three traits showed marked seasonal changes which resulted in superior growth and survival in winter of the species adapted to nutrient-poor environments. The findings support the theory that competitive asymmetry increases at higher nutrient levels. It is postulated that the directionality of light makes it possible for the dominant species to monopolize this resource more easily than nutrients.

Ammonia was necessary for the origin of life. However, NH3 would not have been a significant component of the neutral or mildly reducing (N2, CO2, H2O) atmosphere which characterized the Hadean earth (>3·8 Gyr ago), especially in view of the u.v. lability of NH3, the greater u.v. output of the young sun than pertains today, and the absence of an atmospheric u.v. screen. Heterogeneous phase reactions, following atmospheric chemistry, have been proposed as a prebiotic NH3 source. Lightning, or bolides (meteorites and comets), generated NOx., and Fe2+ in the sea could have reduced NO2− resulting from the NOx. to NH4+; this disequilibrium NH4+ level could have supported the production of the nitrogenous organic building blocks of life. NOx. and NHy thus could both have had important roles in the origin and evolution of life. Burgeoning biota could soon have depleted abiotically generated NH4+, and biological N2 fixation could have evolved in its present Fe-demanding, O2-sensitive forms in the Archaean O2-free, Fe2+-rich environment. Organic matter could have driven biological denitrification reactions based on NO2− and NO3− generated abiologically using some redox components which evolved in earlier chemolithotrophs and photolithotrophs, regenerating atmospheric NOx. and producing N2O. Any atmospheric NH3 leaking from oceanic biology would have been subject to u.v. breakdown and rain-out. Although O2-evolving photosynthesis probably began in the Archean some 3·5 Gyr ago, any O2 accumulation was local until 2·0 Gyr ago (Proterozoic) due to consumption by oxidation of Fe2+ and S2−. However, localized O2 accumulation before 2·0 Gyr ago could account for the observed early evolution of cytochrome oxidase and the possibility of O2-consuming chemolithotrophic and chemoorganotrophic nitrification, with further possibilities of NOx. production. Oxygen accumulation globally from 2·0 Gyr onwards coincided approximately with the evolution of eukaryotes, which contributed phagotrophy to the reactions of the N cycle as well as the nutrification-like aerobic production of NO. by nitric oxide synthetase, while lightning and bolides could now generate NO. from N2 and O2.

Evidence for terrestrial ecosystems is found from 1·0 Gyr onwards; NOx. and NH3 generated by terrestrial biota stands a greater chance of escaping to the atmosphere than do these compounds generated in the sea where recycling within the water body is likely. As CO2 levels fell and O2 levels rose, NH3 cycling in the photorespiratory carbon oxidation cycle might have been evident as early as 1 Gyr ago, although this does not seem to be a major contributor to atmospheric NH3 today. Embryophyte evolution on land 450 Myr ago, together with symbionts and biophages, increased primary productivity and N cycling on land, with greater quantitative possibilities for NOx. and NH3 escape to the atmosphere. The evolution of lignin (and related phenylpropanoids) at least 400 Myr ago, with associated NH3 recycling in vascular land plant, does not seem (on present evidence) to increase NH3 loss to the atmosphere significantly. Biomass burning occurred at least 350 Myr ago with lightning as the likely ignition source; such burning yielded NO., NH3, N2O and N2 from organic N. As in earlier times, the existence of terrestrial embryophyte vegetation has been punctuated by major bolide impacts, with the Cretaceous-Tertiary boundary impact generating perhaps 104 as much NO. as a year's thunderstorms do today, although the toxicity of NOx.per se to vegetation might not have been the major effect of the NOx. on biota. Terrestrial plants suffered fewer extinctions at this time than did many other major taxa. Despite such very significant generators of atmospheric combined N as major bolide impacts, the mean levels of NHy, NOx. and N2O (and O3) in the atmosphere during the 450 Myr existence of embryophyte vegetation was lower than the current, globally averaged, anthropogenically influenced values. Current globally averaged levels of atmospheric combined N are thus not without precedent in the history of life, globally for NOx., and locally for NHy, so vegetation has had evolutionary experience of high atmospheric combined N. However, this should not make us complacent about the impact of current wide-spread and continuing anthropogenic inputs of combined N, in view of the rate and extent of the inputs, and their combination with other aspects of local anthropogenic influence (e.g. SO2 from burning of high-S coal), stratospheric O3 depletion, and global environmental change (increasing CO2, temperature and sea-level).

The relative ozone resistance of 20 European and two American populations of Plantago major was examined, and relationships with climatic factors at the source of the plant material were explored using data provided by participants in the ICP-Crops initiative (International Co-operative Programme to Investigate the Effects of Air Pollutants and Other Stresses on Agricultural and Semi-Natural Vegetation). Plants grown from seed were exposed to either charcoal/Purafil® filtered air (CF<5 nmol mol−1 O3) or CF+ozone (70 nmol mol−1 O37 h d−1) over a 2-wk period in controlled environment chambers, and effects on mean plant relative growth rate (R) and allometric root/shoot growth (K) determined. Ozone resistance (R%) was calculated from (R03/RCF)×100.

Populations exhibited contrasting sensitivities to ozone, without the development of typical visible symptoms of injury. A positive relationship was found between relative ozone resistance and descriptors of the ozone-climate at the site of seed collection for the year of, and the 2 yr before, seed collection. The best predictors of inherent ozone resistance were shown to be cumulative ozone exposure indices calculated according to current United Nations Economic Commission for Europe (UN-ECE) critical level guidelines for the pollutant (i.e. the accumulated hourly average ozone exposure over a threshold level of 40 nmol mol−1 (AOT40) or 30 nmol mol−1 (AOT30) calculated during daylight hours for the consecutive 3-month period of the year experiencing the highest ozone concentrations). No relationships were found between ozone resistance and climatic factors (temperature, precipitation, sunshine hours, humidity) or the concentrations of other air pollutants (SO2, NO2, NO).

These findings support the view that current ambient levels of ozone in many regions of Europe are high enough to promote evolution of resistance to the pollutant in native plant populations. The significance of these findings to the debate over the establishment of separate critical levels for the protection of natural and semi-natural vegetation is discussed.

Production of enzymes which degrade plant cell wall macromolecules has been studied in relatively few ericoid fungal isolates, although these polymers are a major component of the organic litter and an important source of nutrients for these fungi. Our aims were to investigate whether the ability to degrade the wall pectic component, only reported for one isolate, is a general feature of ericoid fungi. Of about 35 isolates from different geographic regions, all were capable of growing on pectin as the sole carbon source. Polygalacturonase (PG) activity was detected to a different degree in the culture filtrates and independently of the fungal growth rate. Solid and liquid isoelectric focusing allowed separation and identification of several polygalacturonase isoforms. Among the fungal isolates investigated, those from the northern hemisphere produced mostly acidic isoforms, whereas isolates from South Africa secreted more abundantly basic isoforms. However, purification and biochemical characterization of several PG isoforms from the different isolates revealed an optimal activity in the acidic pH range for all the PG enzymes tested. Polygalacturonase enzymes seem to be an important component of the enzymatic arsenal secreted by ericoid fungi during their saprotrophic life. In addition, they could also play a role during root colonization, since penetration across the plant cell wall is a prerequisite for the establishment of endomycorrhizal symbiosis.

The hydraulic architecture of the secondary hemiepiphyte Monstera acuminata was examined in native plants from Los Tuxtlas, Veracruz, Mexico, to determine how it compared to better-known growth forms such as trees, shrubs, lianas and primary hemiepiphytes. Monstera acuminata starts its life cycle as a prostrate herb. As it ascends a tree or other vertical support, the stem becomes thicker, produces larger leaves, and may die back from the base upwards until only aerial feeding roots serve to connect the stem to the soil. Unlike the pattern of vessel-size distribution along the stems of woody dicotyledons, M. acuminata has its wider vessels at the top of the stem, decreasing in diameter towards the base. Also peculiar is the fact that Huber values (axis area/distal leaf area) tend to increase exponentially at higher positions within the plant. Based on the hydraulic conductivity (kh) and leaf-specific conductivity (LSC, kh/distal leaf area), the base of the stem potentially acts as a severe hydraulic constriction. This constriction is apparently not limiting, as aerial roots are produced further up the stem. The plants have remarkably strong root pressures, up to 225 kPa, which may contribute to the maintenance of functional vessels by refilling them at night or during periods of very high atmospheric humidity, as in foggy weather and rain. In common with dicotyledonous plants, vessel length, vessel diameter, kh, specific conductivity (ks, kh/axis area) and LSCs were all positively correlated with axis diameter. The features of the hydraulic architecture of M. acuminata may be an evolutionary consequence of an anatomical constraint (lack of vascular cambium and therefore of secondary growth) and the special requirements of the hemiepiphytic growth form.

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.

‘Tiny Tim’ tomato plants were exposed to five CO2 treatments (375 (ambient), 450, 525, 600 or 675 μmol mol−1) in combination with O3 (0 or 80 nmol mol−1). Biomass was evaluated following 3, 5, 7 and 13 wk exposure. Biomass following 13 wk exposure also included weekly harvests of mature tomato fruit beginning week 8. Carbon dioxide enrichment significantly enhanced total vegetative plant d. wt at each harvest, as well as cumulative yield of mature fruit, whereas O3 significantly suppressed total vegetative plant d. wt at each harvest and reduced total cumulative fruit yield. The magnitude of these changes varied with the development of tomato from early growth to mature fruit yield. Carbon dioxide enrichment reduced the detrimental effects of O3 on total vegetative plant d. wt of tomato following 3, 5, 7 and 13 wk exposure. Final mature fruit yield was 24% higher under enriched CO2 treatments than in ambient CO2. Ozone suppressed final yield by 31% following exposure to 80 nmol mol−1 O3 when compared with exposure to charcoal-filtered (CF) air. The impacts of both CO2 and O3 on yield were, however, dependent upon the presence or absence of the other gas. In the absence of O3, yields were very similar for the ambient and elevated CO2 treatments, but in the presence of O3, yields under ambient CO2 were greatly suppressed whereas yields under elevated CO2 were similar to those in the absence of O3. Thus, enriched CO2 ameliorated most of the suppressive effect of O3 on yield of mature fruit.

Most recent reviews of plant salinity response have included the concept of a nutritional disturbance as one likely mechanism by which shoot growth might be inhibited. None the less, few studies of dicotyledonous plants have presented data on nutrient transport into the most intensively growing shoot tissues. In this paper net nutrient deposition was followed for 3 d in 8 sequential, growing leaves of Lactuca sativa, which were grown either in conditions of moderate salinization, or in a growth-stimulating concentration of NaCl. The nutrient deposition was studied from 0.7 to 3.7 d following completion of stepwise salinization. This deposition was followed in immature leaves, which had attained only 1–2% of ultimate leaf mass by the completion of the study. In such young leaves development is still dominated by cell division. The transport of Ca2+ specifically to the youngest leaves was reduced by more than twice as much as was K+ transport. Transport of the other major divalent cationic nutrient, Mg2+, was not decreased for these leaves. The factors of increase for Na+ and Cl− after 3.7 d after completion of salinization averaged 152 and 62% over control levels for the three youngest leaves (for Na+ and Cl−, respectively). Though significant, these increases were only 27 and 14% as great as increases in three leaf sets of more developed growing leaves. Decreases in net K+ deposition and leaf K+ concentration were not greater for the youngest than they were for the oldest leaves. Net S deposition was reduced 44% more in younger than older growing leaves, but for most leaves not beyond the level expected due to reduced sink strength. The reduction in net P deposition also seemed more related to reduced sink strength, but was reduced to approx. 50% in both younger and more developed growing leaves. While Fe concentration was not reduced by salinization at any developmental stage, Zn2+ net transport and Zn concentration were both reduced in the two youngest leaves (57 and 70%, respectively). Given the moderate treatment imposed (Na:Ca ratio of 22) the results suggest that Ca2+ transport to the youngest leaves is probably highly sensitive to salinization of the root medium and is perhaps a key physiological response in the inhibition of leaf growth.

In Plantago lanceolata L. the effect of Cu(II) additions to the nutrient solution on root-associated Fe(III) reductase was studied in a factorial design with different Cu(II) and Fe(III) concentrations. Iron starvation resulted in approx. an eightfold increase in root Fe reduction at the level of intact plants and twofold enhancement in the specific activity of both NADH-linked FeEDTA reductase and H+-ATPase in isolated root plasma membrane vesicles. In plants exposed to low (0·3–0·7 μM) Cu and suboptimal Fe levels, reduction activity at the root surface was further increased and associated with more severe interveinal chlorosis than plants grown in Cu-free medium. In Fe-sufficient plants, withholding Cu over a prolonged period slightly enhanced the reduction activity.

Addition of high (5 μM) Cu concentrations to Fe-free medium inhibited the induction of the physiological responses by Fe-deficiency stress. In plants without Fe supply but with adequate Cu supply, short-term application of 5 μM CuSO4 completely inhibited the reduction activity. Neither incubation of the plasma membrane vesicles before measurement nor incubation of intact plants with Cu before isolation caused a significant decrease in reductase activity. The results are interpreted as indicating different mechanisms underlying Cu-induced alterations in iron nutrition.

Arbuscular mycorrhizal (AM) fungi extensively colonize the root cortex under low-soil-phosphate (P) conditions, whereas infection is limited under high-P conditions. Fungal growth under both P conditions might be influenced by plant defence-related gene expression. In this study, we used in situ hybridization methods to compare the cellular localization of three defence-related mRNAs in non-infected bean roots and in relation to fungal infection units. In non-infected and infected roots, mRNAs encoding acidic and basic endochitinases were generally most abundant in the vascular cylinder. High-P-grown mycorrhizal roots showed localized accumulation of the acidic endochitinase mRNA in cortical cells containing arbuscules and in their immediate vicinity (one to five cell layers). The pattern of accumulation of the basic endochitinase mRNA was not affected by P or AM fungal infection. At the low P concentration, the β-1,3-glucanase mRNA accumulated predominantly in the vascular cylinder of non-infected roots. Suppression of β-1,3-glucanase mRNA accumulation in these tissues was observed in non-infected roots at the high-P and in mycorrhizal roots at both P concentrations. The observed suppression extends at least several mm from fungal infection units, characterizing a systemic effect. Beta-1,3-glucanase mRNA accumulated also around a number of cortical cells containing arbuscules only at the low P concentration. The localized accumulations of the endochitinase and β-1,3-glucanase mRNAs suggest that the encoded proteins might be involved in the control of intraradical fungal growth.