Showing papers in "New Phytologist in 1989"

Pollen tube tip growth.

Abstract: This review considers pollen tube growth with regard to current information on pollen tube cytoplasm, wall structure and calcium ion interactions with pollen tubes. Pollen tubes have a marked cytoplasmic Polarity with a number of distinct zones along the tube, each with a characteristic complement of cytoplasmic and nuclear structures. The cytoplasmic structures are characteristic of secretory cells with extensive endoplasmic reticulum cisternae and numerous dictyosomes. The dictyosomes produce secretory vesicles that are mainly directed to the extending tip of the tube, where they provide new plasma membrane and wall components. The rates of secretory vesicle production and delivery have been estimated, allowing quantitative assessments of the rate of delivery of materials to the tip. Pollen tubes contain cytoskeletal components, with microtubules and microfilament strands lying axially in the main tube and diffuse microfilament strands at the tip. The tube wall consists of an outer fibrous layer containing pectins and an inner, more homogeneous layer containing callose and cellulose-like microfibrils, possessing both β-1,4 and β-1,3 linkages. Protein is also present in the wall. The tube tip lacks the inner callosic wall. This type of structure is considered to be different from that of elongating sporophyte tissue cells which are enclosed by a wall containing layers of cellulose microfibrils. Calcium ions are required for pollen tube growth and, in at least some species, act as a chemotropic agent. High concentrations of calcium ions in the external medium inhibit growth. Pollen tubes contain some calcium ions bound to the cell wall and larger amounts located intracellularly, which enter the tube at the tip. This intracellular calcium is present as ions that exist freely within the cytoplasmic Matrix and as ions bound to membrane systems. The highest concentrations in both of these pools are found at the tip and in both they decline towards the base. The structure of the tip and the activity involved in providing components for plasma membrane and Wall assembly provide a basis for considering possible mechanisms of tip growth. Two hypotheses to account for the regulation of tip extension are considered, cell wall control and cytoskeletal control. In the cell wall hypothesis, control depends on an interaction between internal turgor pressure and a plastic cell wall. The mechanical properties of the wall are assumed to be partly dependent on the availability of external calcium ions to crosslink acidic pectin chains. According to this hypothesis, high external calcium ion concentrations cause cessation of tip growth due to increased mechanical resistance of the tip wall. Various observations on plant cell-wall interactions with calcium ions and on experimentally-treated pollen tubes provide evidence that does not support this hypothesis. The cytoskeletal control hypothesis of tip growth depends on the internal tip cytoskeleton to contain the tube tip cytoplasm against the internal turgor pressure during cell wall assembly. The activities and mechanical properties of the cytoskeleton are assumed to depend on the availability of external calcium ions. High external concentrations are believed to cause a state of rigor in the cytoskeleton and hence a cessation of tip growth. Some experimental evidence is presented which suggests that the effects of excess calcium ions are on intracellular processes, and not extracellular ones. The mitochondrial zone behind the tip is believed to maintain the tip calcium ion concentration at an optimal level for growth. Some comparisons are made between tip growth in pollen tubes and that in other tip growing cells. CONTENTS Summary 323 I. Introduction 324 II. Cytoplasm 326 III. Wall structure 332 IV. Calcium 335 V. Tip growth 339 VI. Conclusions 350 References 351.

The development of endomycorrhizal root systems: VII. A detailed study of effects of soil phosphorus on colonization.

Abstract: Leek plants (Allium porrum L.) were grown in a controlled environment on a mixture (2: 1 w/w) γ-irradiated (1.0 Mrad) sandy loam and sand, at six concentrations of bicarbonate-soluble phosphorus (P) ranging from 22 to 344 mg P kg-1 (soil basis). Inoculum of the vesicular-arbuscular (VA) mycorrhizal fungus Glomus mosseae (Nicolson & Gerdemann) Gerdemann and Trappe was placed (M) or not (NM) in a layer 3 cm below the soil surface. At intervals of 10 d, lengths of main axes of roots and their lateral branches, and of the segments of infection within them, were measured. From these data we calculated the mean (harvest interval method} rates of linear extension of root tips and of infection fronts for each member of root, averaged over the whole root system. The mean delay, d, between a root encountering the layer of inoculum and the subsequent formation of internal infection, was also derived. Addition of P to soil did not affect rates of extension of roots, but increased the initiation of main axes and laterals. Infection segments extended twice as fast in laterals as in main axes. At low rates of addition, P did not affect fungal behaviour but increased the length of root available for colonization. When bicarbonate-soluble P exceeded 140 mg kg-1 , the rates of extension of infection fronts in both main axes and laterals were approximately halved, and d was considerably increased. The density (the area ratio of fungal to host tissue in a longitudinal squash) of the hyphae and arbuscules respectively, and the number of entry points per unit length of root, were greatly reduced by added P. However, the ratios of numbers of entry points/hyphal density and of arbuscule density/hyphal density were unaltered, and the morphology of the fungus was not noticeably affected. It is probable that formation of entry points was the rate-limiting step for colonization, and that this rate was reduced by added P.

Occurrence of nodulation in the Leguminosae

Abstract: summary Reports of nodulation in the Leguminosae are examined in the light of current views on the taxonomy of the family. In the subfamily Caesalpinioideae, nodulation is largely restricted to the tribe Caesalpinieae and the genus Chamaecrista from the Cassieae. All nodules studied have rhizobia retained within infection threads during the nitrogen fixing period. In the Mimosoideae, nodulation is general, except for 4 groups within the tribe Mimoseae, and a very few species of Acacia. The only tribe from the Papilionoideae which appears not to nodulate is the Dipterygeae, although the monogeneric Euchresteae has not been examined. A number of genera in the Swartzieae do not nodulate. Taking tile family as a whole, nodulation appears to be very uniform – certain sections nodulate, others do not.

Iron plaque on roots of Aster tripolium L.: interaction with zinc uptake

Abstract: summary The iron plaque on roots of Aster tripolium L. growing in waterlogged salt marsh soil adsorbed appreciable amounts of Zn and Cu, with maximum Zn/Fe and Cu/Fe ratios of 0.1 When concentrations of Zn or Cu adsorbed in the iron plaque are expressed as mg metal kg−1 FeOOH (assuming that iron plaque consists mainly of FeOOH), the Zn and Cu concentrations of the iron plaque was up to 680 and up to 2000 times higher than in the surrounding sediment, respectively. The Zn concentration in red roots (with iron plaque) was higher than in white roots (without iron plaque). Zn concentrations in field sampled roots were correlated with the amount of Zn on the roots and the Zn concentration in the soil, whereas Cu concentrations in the roots were only significantly correlated to the Cu concentration in the soil. In vitro experiments showed that red roots take up more Zn than white roots. Measurement of Zn uptake by excised roots showed that the uptake of Zn into the xylem fluid was significantly higher in roots with 500–2000 nmol Fe cm−2 on the root surface compared to roots with less than 500 or more than 2000 nmol Fe cm−2 on the root surface. The results indicate that iron plaque enhances uptake of Zn by the roots but may act as a barrier when large amounts of Fe are deposited on the root surface. The role of the iron plaque on roots of salt marsh plants growing in soil contaminated with heavy metals is discussed.

Soil disturbance reduces the infectivity of external hyphae of vesicular—arbuscular mycorrhizal fungi

Abstract: SUMMARY We conducted two glasshouse experiments to determine the effect of soil disturbance on the infectivity of external hyphae of vesicular–arbuscular (VA) mycorrhizal fungi. A nylon mesh was used to exclude plant roots while allowing fungal hyphae to grow into soil contained by the mesh. Hyphae of VA mycorrhizal fungi that had been separated from the original host plant root were still able to colonize bioassay plants rapidly and extensively. However, disturbance of the soil inside the mesh, by mixing for 1 min, almost eliminated subsequent VA mycorrhiza formation in that soil. Damage to the soil hyphal network by soil disturbance may contribute to the losses in mycorrhizal infectivity that have been observed after mining and cultivation.

Respiration and microalgal growth: a review of the quantitative relationship between dark respiration and growth

Abstract: SUMMARY Biomass-specific, dark respiration rates of microalgae vary by almost two orders of magnitude from 0.01 to 0.6 d−1. In general, dark respiration rates increase with growth rates in both intraspecitic and interspecific comparisons, but the precise relationship between respiration and growth rate varies. Under optimal conditions, respiration rates are about 20-30% of growth rates, but the ratio of respiration to growth increases under suboptimal conditions. The intercept of plots of respiration rate vs. growth rate varied from < 0.01 to 0.4 d1, and the slopes of these relationships varied from < 0.1 to 1.0. Minimum maintenance metabolic rates of about 0.01 d 1 have been estimated for some microalgae by extrapolation of the initial slope of the light curve for growth to a hypothetical negative growth rate in darkness. The dimensionless total energy cost of synthesis and maintenance for heterotrophie growth of microalgae using either endogenously or exogenously supplied carbohydrates is typically 0.4-1.2 for growth with ammonia or urea as a nitrogen source and 0.7-1.4 for growth with nitrate as the nitrogen source.

Appropriate controls for vesicular-arbuscular mycorrhiza research

Abstract: SUMMARY Sunflower (Helianthus animus L.) and mustard (Brassica hirta Moench.) plants were grown in autoclaved soil to which was added various components of soil inoculum. Use of inocula containing mycorrhizal propagules ultimately resulted in growth promotions which were related to improved phosphorus nutrition. Inoculation with soil containing mycorrhizal propagules caused an initial growth depression in comparison with plants receiving no inoculation, but inoculation with washed spores did not. For several reasons, the growth depression caused by inoculation with soil was not attributable to mycorrhizal infection. First, the growth depression was evident before mycorrhizal infections had become established. Second, the growth depression was also evident when either soil or soil sievings, both lacking mycorrhizal propagules, were used. Third, similar growth depressions were observed in mustard. The results suggest that careful selection of appropriate controls for mycorrhizal plants must be made, particularly if the emphasis is on early phases of plant growth. Use of non-sterile soil reduced the extent of mycorrhizal infection.

Interactions between infection by endophytic fungi and nutrient limitation in the grasses lolium perenne and festuca arundinacea

Abstract: SUMMARY Although grasses infected by fungal endophytes often exhibit increased growth relative to uninfected plants, the results of infection may be manifested as decreased growth in plants growing under stressful conditions. A preliminary experiment compared perennial ryegrass (Lolium perenne L.) infected by Acremonium lolii Latch, Christensen & Samuels with uninfected plants grown at three soil moistures and three nutrient concentrations. Total biomass, tiller number, and relative tillering rate were significantly affected by nutrient concentration and moisture. Although major differences between infected and uninfected plants were not apparent, relative growth rates of infected plants were significantly greater than uninfected plants in three of nine treatment combinations. Interactions between infection and nutrient limitation were examined in seedlings and adults of perennial ryegrass and tall fescue (Festuca arundinacea Schreb., infected by A. coenophialum Morgan-Jones and Gams). A significant nutrient x infection interaction for tall fescue seedlings indicated that, compared to uninfected seedlings, infected seedlings had greater biomass at the high nutrient level but a significantly lower biomass at the low nutrient level, suggesting a metabolic cost to the host due to competition with the fungus for nutrients or photosynthate. Infected perennial ryegrass seedlings had significantly more biomass at high and intermediate nutrient levels, but not at low nutrient levels. Although infection did not significantly affect total biomass in perennial ryegrass adults, in tall fescue adults the benefits of endophyte infection to host plants (recorded as greater biomass) became increasingly greater with increasing nutrient availability.

Interactions between ozone and plant cuticles

Abstract: SUMMARY The flux of ozone to plant surfaces results in its decomposition at the surface and its uptake through the cuticle. Ozone deposition velocity and ozone permeance were determined for a variety of isolated plant cuticles. Ozone deposition velocity was lower than determined with whole plants kept in darkness. It declined continuously during exposure to the gas and showed a‘recovery’effect after an interruption of the fumigation. It increased with the moisture content of the cuticles and decreased when the ozone concentration in the surrounding air was raised. The deposition velocity was much higher than cuticular ozone permeance at equivalent ozone concentrations. Due to the ozone decay in the cuticle, ozone permeance was much lower in thick than in thin cuticles. Even with the most permeable cuticles, ozone uptake under natural conditions is smaller than the flux through open stomata by a factor of at least 10000.

Quantification of frost damage in plant tissues by rates of electrolyte leakage.

Abstract: A method for assessing frost hardiness of plant tissues [using shoots of Picea rubens Sarg. syn P. rubra (Du Roi) Link] has been developed based upon the rate of electrolyte leakage from shoots immersed in distilled water after exposure to a range of freezing treatments. The relationship between conductivity (the electrolyte concentration in solution) and time has been shown to follow an asymptotic curve, which may be represented by a first-order equation: Ct - Co = (Cauto - Co) (1 - e-kt) where Ct is the conductivity at time t, Co is the initial conductivity, Cauto is the conductivity after autoclaving and k is the first-order rate constant (units time-1). The rate of electrolyte leakage (k) varies directly with the extent of tissue damage. In P. rubens a rate of 0.4% h-1 distinguished between shoots which eventually died, and shoots which remained alive. A minimum of 3 conductivity measurements (after 1 day, 5 days and after autoclaving) is required for a reliable estimate of k. This objective, quantitative method of assessing frost hardiness may therefore be used directly to estimate LT50 values within a population.

Responses of the non7hyphen;mycotrophic plant Salsola kali to invasion by vesicular–arbuscular mycorrhizal fungi

Abstract: SUMMARY Vesicular–arbuscular mycorrhizal fungi reduced the growth and survival of the non-mycotrophic weed, Salsola kali (Chenopodiaceae) in field and greenhouse experiments. To determine the mechanisms by which mycorrhizal fungi can affect a non-mycotrophic plant, we mapped the root responses of S. kali and the mycotrophic grass Agropyron dasystachyum to invasion by a mixture of Glomus spp. and by Gigaspora margarita. The fungi invaded the roots of both species within 10 days following seed germination. In A. dasystachyum normal mycorrhizal development occurred and no root browning or autofluorescence was observed, indicating a compatible reaction. In S. kali, the fungi initially invaded the roots and formed arbuscules and peletons. The roots reacted to invasion by autofluorescing bright yellow, suggesting lignification. With 1 or 2 days following root penetration by the fungus, the invaded root segment turned brown and, with the exception of some vesicles, the fungus disappeared from the root. The fungus sometimes then reinvaded new root segments farther down the root. These observations suggest that there is an incompatible reaction beween this non-mycotrophic annual and VA mycorrhizal fungi whereby the plant actively rejects infection. This response could form the basis for initiating studies on the genetics of mutualistic versus parasitic symbioses.

A comparative study of plant responsiveness to the duration of episodes of mineral nutrient enrichment

Abstract: summary Growth, dry matter partitioning and nitrogen capture were compared in two grasses of contrasted ecology, subjected to treatments involving pulses of nutrient enrichment of various durations supplied every 6 d over a period of 6 weeks. The potentially fast-growing Arrhenatherum elatius ssp. bulbosum (Willd.) Schubler and Martens was superior in rates of nitrogen capture and dry matter production when exposed to long (≥ 10 h) nutrient pulses whereas the slow-growing species Festuca ovina L. enjoyed an advantage in treatments providing nutrient pulses of between 0.1 and 10 h duration. These results are consistent with differences in mechanisms of resource foraging predicted by the C-S-R model of plant strategies and related models. It is concluded that the lower rates of turnover of tissues and the capacity of roots to remain viable under chronic nutrient stress are important components of the ability of Festuca ovina to exploit brief pulses of mineral nutrient enrichment. The range in duration of the pulses advantageous to F. ovina corresponds to that which might be expected to arise from the death and recovery of microbial populations in infertile soils.

Sequential response of whole plant water relations to prolonged soil drying and the involvement of xylem sap ABA in the regulation of stomatal behaviour of sunflower plants

Abstract: SUMMARY Sunflower plants (Helianihus animus cv. Tall Single Yellow} were grown in the greenhouse in drain pipes (100 mm inside diameter and 1 m long) rilled with John Innes No. 2 compost. When the fifth leaf had emerged, half of the plants were left unwatered for 6 days, rewatered for 2 days and then not watered for another 12 days. Measurements of water relations and abaxial stomatal conductance were made at each leaf position at regular intervals during the experimental period. Estimates were also made of soil water potentials along the soil profile and of ABA concentrations in xylem sap and leaves. Soil drying led to some reduction in stomatal conductance alter only 3 days but leaf turgors were not reduced until day 13 (6 days after rewatering). When the water relations of leaves did change, older leases became substantially dehydrated while high turgors were recorded in younger leaves. Leaf ABA content measured on the third youngest leaf hardly changed over the first 13 days of the experiment, despite substantial soil drying, while xylem ABA concentrations changed very significantly and dynamically as soil water status varied, even when there was no effect of soil drying on leaf water relations. We argue that the highest ABA concentrations in the xylem, found as a result of substantial soil drying, arise from synthesis in both the roots and the older leaves, and act to delay the development of water deficit in younger leases. In other experiments ABA solutions were watered on to the root systems of sunflower plants to increase ABA concentrations in xylem sap. The stomatal response to applied ABA was quantitatively very similar to that to ABA generated as a result of soil drying. There was a log-linear relationship between the reduction of leaf conductance and the increase of ABA concentration m xylem sap.

Hyphae of a vesicular—arbuscular mycorrhizal fungus maintain infectivity in dry soil, except when the soil is disturbed

Abstract: SUMMARY We examined the hypothesis that hyphae of a VA mycorrhizal fungus in dry soil remain infective, but that the infectivity decreases if the soil is disturbed. We used split-pots, and sealed cylinders of nylon mesh, to create volumes of dry soil which contained hyphae of Acaulospora laevis Gerd. and Trappe that were attached, outside the mesh, to roots of living plants of Acacia satigna. (Labill.) H. Wendl. After the soil was dried in situ, the hyphae were detached from the roots without soil disturbance, or the soil containing the hyphae was disturbed. The infectivity of the hyphae after these treatments was then compared with that of intact hyphae. External hyphae of A. laevis remained highly infective in soil in which the matric potential had reached −21 MPa. Hyphae detached from the original host plant roots were equally infective. However, disturbance of the dry soil containing the external hyphae severely reduced the infectivity of the hyphae. After fluorescent staining, fungal structures with fluorescing nuclei were readily visible both within, and attached to, roots of A. Saligna which had no fluorescing nuclei.

Effect of nitrogen source on growth response to salinity stress in maize and wheat

Abstract: summary The effect of ammonium and nitrate nutrition on maize and wheat grown hydroponically and salinity stressed was assessed from measurements of growth rate and gas exchange. In both maize and wheat the ammonium-grown plains were much more sensitive to salinity toxicity than nitrate-grown plants particularly when exposed to 60–80 mM salinity. Shoot growth was retarded to a far greater extent than root growth in salinity-stressed plants of both wheat and maize with either nitrogen source. There was no significant decrease of photosynthetic rate in salinity-stressed plants of either species fed nitrate or ammonium, except in severely wilted plants of both species fed nitrate or ammonium at the highest (80 mM) salinity concentration. The same was true for stomatal conductance, transpiration rate and transpiration ratio (water use efficiency). In nitrate-fed wheat, raising the calcium concentration from 2 to 12 mM in the presence of 60 mM salinity produced an 11% increase in growth. This effect is ascribed to improved nitrate uptake due to calcium protection of the nitrate transporter and was not evident in ammonium-grown wheat. Possible reasons for the differential effects of ammonium and nitrate nutrition are discussed.

Effects of acid mist on the frost hardiness of red spruce seedlings

Abstract: SUMMARY Seedlings of red spruce [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were exposed to mists containing equimolar (NH4, SO4 and HNO3 at pHs of 2.5, 2.7, 3.0, 35, 4.0 or 5.0. The mists were applied twice each week, amounting to 2 mm precipitation equivalent on each occasion, between July and December, to open-top chambers supplied with charcoal-filtered air. Frost hardiness of shoots excised from seedlings was determined on 6 occasions starting on 21 September, and was found to be strongly influenced by acid mist treatments, seedlings subject to the most acidic mists being the least frost hardy. On 21 September when the first sample was taken the lethal temperature for killing 50% of shoots (LT50) was – 11 °C for the least acidic (pH 5.0) mist and – 7 °C for the most acidic (pH 2.5). By 19 October, the LT50s of pH 5.0 and pH 2.5 mists were –27 and –15 °C respectively. All intermediate treatments ranked according to treatment concentration, with the smaller concentrations causing lower LT.50 values. The treatment at pH 30 provided S and N inputs to the seedlings similar to those experienced by red spruce at elevations of about 1000 m in the southern Appalachians. At pH 3.0, the frost hardiness LT10 during October was typically 8 °C higher than the pH 5.0 treatment, leading to a significant increase in the probability of frost damage at the LT10 level in an average October. The proximity of minimum night temperatures during September to December to the LT10 temperatures of red spruce shoots receiving large inputs of SO42-, NO3-, NH4+ and H+ suggests that decreases in frost hardiness caused by intercepted cloud water containing large concentrations of these ions may play a significant part in the observed decline at mountain-top locations.

The accumulation and the effect of heavy metals on the water fern Azolla filiculoides

Abstract: Azolla filiculoides Lamarck grown for 3-7 days in nutrient media containing 8-1 5 ppm of different heavy metals was found to contain about 10000 ppm cadmium, 1990 ppm chromium, 9000 ppm copper, 9000 ppm nickel and 6500 ppm zinc. The content of heavy metal in the roots was two- to five-fold higher than in the shoots, and 98 %h of the heavy metals were bound to the insoluble fraction of the Azolla. The content of heavy metals in dead dried Azollia was 3-7 times higher than in the living plants. Zinc and cadmium were more readily transferred from the roots to the shoots than nickel, copper and chromium. Growth in the presence of the heavy metals resulted in severe leakage of K', Mg2" and Na+ ions from the roots with less leakage of Ca2". The effect of heavy metals on the loss of K', Na+ and Mg2" from the shoots was more limited. Nitrogenase activity was almost completely inhibited by cadmium, nickel and zinc, while copper and chromium had less effect. Azolla may prove useful in the detoxification of industrial effluents.

Stimulation of ectomycorrhizal infection of Pinus radiata by some microorganisms associated with the mantle of ectomycorrhizas

Abstract: SUMMARY Microorganisms were isolated from inside the mantle of ectomycorrhizas formed by the fungus Rhizopogon luteolus Fr. and Nordh on the roots of Pinus radiata D. Don. growing in a sandy podzol. Different isolates of these microorganisms were then tested for their effect on three stages of mycorrhizal infection : growth of the fungus into the soil, growth along the root, and mycorrhiza formation. Experiments were performed in axenic and non-axenic conditions. A significant microbial population was present within the mantle of ectomycorrhizas (Gram negative bacteria being dominant) and a majority of these had a stimulating effect on mycelial growth of R. luteolus and/or mycorrhiza formation. These results suggest that some specific microbial communities have evolved to live in close association with ectomycorrhizal fungi in their symbiotic state, with mutual benefit. The potential use of such‘helper’microorganisms for improving the efficiency of ectomycorrhizal inoculation in forest nurseries is discussed.

Ectomycorrhizal mediation of competition between coniferous tree species

Abstract: SUMMARY To test the effect of ectomycorrhizal fungi (EMF) on interactions between host plants, Pseudotsuga menziesii (Mirb.) Franco and Pinus ponderosa Dougl. ex. Laws., seedlings were grown in replacement series in pasteurized soil with (a) no EMF added, (b) two EMF species added - Rhizopogon vinicolor Smith (specific to Douglas-fir) and R. ochraceorubens Smith (specific to pine), and (c) four EMF species added - the two Rhizopogon species plus two host generalists, Laccaria laccata (Scop. ex Fr.) Bk. & Br. and Hebeloma crustuliniforme (Bull.) Quel. A replacement series in unpasteurized forest soil also was included. Seedlings without added EMF were colonized by the greenhouse contaminant, Thelephora terrestris. Without added EM,IF (but with T. terrestris), the tree species mutually inhibited one another, producing Relative Yield Totals significantly < 1; with EMF added, mutual inhibition disappeared. With four EMF species added, Pseudotsuga menziesii seedlings were significantly larger in mixture than in monoculture, with no corresponding decrease in the size of Pinus ponderosa seedlings; this was due solely to seedlings with L. laccata, which apparently enhanced nitrogen (N) and phosphorus (P) uptake by Pseudotsuga menziesii at the expense of luxury consumption by Pinus ponderosa. Graphical analysis suggested that better growth of Pseudotosuga menziesii in mixture with EMF added was related to improved P nitrogen. Both N and P nutrition of Pinus ponderosa was better in mixture with two than no EMF species added; there was no clear nutrient effect with four EMF species added. Results indicate that EMF can reduce competition between plant species and perhaps increase overall community P uptake. However, patterns were specific to both EMF and tree species and were quite different in unpasteurized soils. Hence generalizations about the effects of EMF on plant-plant interactions must be made cautiously.

Glandular trichomes and essential oils in developing peppermint leaves: I. Variation of peltate trichome number and terpene distribution within leaves.

Abstract: The trichome number and the variability in mono and sequisterpene distribution of peltate trichomes within a leaf was investigated during development of peppermint (Mentha×piperita L.) leaves. A larger number of peltate trichomes was initiated on adaxial epidermises than abaxial epidermises, but during leaf development a greater gland number was produced on abaxial epidermises. Evidence showed that the trichome number was not fixed at the time of leaf emergence. On both epidermises, the trichome distribution was found to change with its position on the leaf. A striking monoterpene composition variability was present among individual trichomes and between different parts of the leaf. Different rates of peltate trichome development and distribution as well as significant differences in trichome number and density were found between the epidermises. A clear and opposite trend was found between menthone and menthol percentages during development. Menthone and menthol were also found to be present in larger amounts on adaxial epidermises, whereas neomenthol and isomenthol had highest percentages on abaxial epidermises. Of the total oil collected from peltate trichomes, 60%, was composed of menthone. isomenthol, menthol and neomenthol and of the remaining 40 %, more than half was represented by other mono and sesquiterpenes while the final percentage was made up of a few unidentified early eluting compounds of high Volatility.

New aspects on the acquisition of biotrophic status by a vesicular—arbuscular mycorrhizal fungus, Gigaspora margarita

Abstract: SUMMARY An in vitro system for culturing the vesicular arbuscular (VA) mycorrhizal fungus Gigaspora margarita Becker & Hall with Ri T-DNA transformed roots of carrot has been used to investigate the progressive dependency of a VA mycorrhizal fungus on its host during mycorrhizal establishment. The growth of hyphae from germinating spores of G. margarita was measured under different combinations of the following treatments: in absence or presence of root, with or without mycorrhizal colonization, with or without spore removal. Two distinct mechanisms of hyphal growth stimulation have been identified. The first required the presence of the root hut remained spore dependent. The second is exclusively root dependent. It has been demonstrated that the second mechanism is triggered when arbuscules are forming, indicating they are necessary for establishment of biotrophy. It is proposed that roots contribute to fungal growth by an inductive action, in addition to providing nutrients.

Ecophysiology of xerophytic and halophytic vegetation of a coastal alluvial plain in northern Venezuela

Abstract: summary Seasonal effects on leaf gas exchange and water relations were compared for Avicennia germinans, a true mangrove, and Conocarpus erectus, a mangrove associate, at coastal sites in northern Venezuela. On the Cienega el Ostional at Chichiriviche, A. germinans was most abundant around lagoons on the seaward side of the vegetation-free alluvial sand plain. C. erectus was the dominant shrub in inland communities, but the two species co-occurred on vegetation islands at the landward edge of the alluvial plain. On the vegetation islands of the Cienega el Ostional, gas exchange in A. germinans (a species with foliar salt glands) was less severely curtailed in the dry season compared with the rainy season than was gas exchange in C. erectus (a species lacking salt glands). Average rates of photosynthesis at near-saturating light intensities and total diurnal CO2 uptake were reduced in the dry season to 69 and 61%, respectively, of their values in the rainy for A. germinans, but to 48 and 30%, respectively, of their rainy-season values for C. erectus. Similarly, stomatal conductance and transpirational water loss were more reduced in the dry season for C. erectus than for A. germinans, with the result that C. erectus showed a 3.4-fold increase in water-use efficiency in the dry season compared with the rainy season. The importance of the soil environment in determining plant gas-exchange Patterns was evidenced by large seasonal shifts in dawn xylem tension for the two species (which increased from 1.34 MPa in the rainy season to 5.50 MPa in the dry season for A. germinans, and from 0.40 to 5.78 MPa for C. erectus). These values reflected changes in the soil environment caused by inundation of the upper soil layers by fresh water in the rainy season and a progressive increase in salt concentrations (to almost twice those in sea water) by evaporation from the soil in the dry season. Large changes in xylem tension were observed for both species during individual day–night cycles, reaching a maximum of 2.36 MPa for A. germinans. For C. erectus, the magnitude of these day-night changes was greatly reduced in the dry season, consistent with its very low transpiration rates at this time of year. Leaf-cell osmotic pressures also tended to be higher in A. germinans than C. erectus (attaining a maximum of 8.3 MPa for A. germinans in the dry season), and were related to the more seaward distribution of the true mangrove on the alluvial plain. Whereas leaves of A. germinans did not show any changes in succulence, leaf succulence in C. erectus increased with leafage and was slightly higher in the dry season than the rainy season. The more succulent leaves also had higher cell-sap osmotic pressures and NaCl concentrations. The most succulent leaves of C. erectus were observed for exposed shrubs growing on the shoreline. During the dry season, these shoreline plants showed high rates of gas exchange and low values for dawn xylem tension (0.89 MPa), indicating that they had access to relatively non-saline water from the shallow water table. On individual plants, exposed shoots had more succulent leaves and higher osmotic pressure and NaCl concentrations than sheltered leaves, demonstrating the importance of foliar absorption of salt borne in sea spray for the ionic relations of C. erectus. Thus, although the distribution of C. erectus is centred on brackish-water zones, this species can apparently extend from habitats with permanent access to a shallow water table through to areas where it is seasonally exposed to low soil water potentials and high salt concentrations in the substratum.

Uptake, translocation and assimilation of nitrogen from 15N‐labelled ammonium and nitrate sources by intact ectomycorrhizal systems of Fagus sylvatica infected with Paxillus involutus

Abstract: The uptake and assimilation of nitrogen from 15N-labelled ammonium and nitrate sources was followed in intact ectomycorrhizal systems containing Fagus sylvatica L. plants infected with the fungus Paxillus involutus (Mich. ex Pers.) Cohen & Couch. Plants were grown in flat perspex observation chambers containing non-sterile peat; the fungal mycelium growing from a host plant was allowed to cross a barrier and to colonize an area of peat from which roots had been excluded. Labelled ammonium chloride or sodium nitrate was fed to the mycelium, and the shoot, root and mycelial tissues analysed for total and 15N-labelled amino acid contents after a feeding period of 72 h. Both free and protein-incorporated amino acids were analysed. Labelled nitrogen was incorporated into a range of free amino acids, the principal sinks for assimilation being alanine, aspartate/asparagine and glutamate/glutamine. The spectrum of labelling in protein-incorporated amino acids was wider with significant incorporation into the above compounds but additional assimilation of nitrogen as glycine, valine, serine, leucine, isoleucine and arginine. In total 78% of the nitrogen assimilated was incorporated into proteinaceous material. Label was incorporated from both nitrogen sources but the levels of enrichment in most free and protein-bound amino acids were usually higher in systems supplied with ammonium than in those supplied with nitrate. The mean amount of nitrogen assimilated from nitrate was only 62% of that assimilated from ammonium. (Less)

Anther dehiscence in Lycopersicon esculentum Mill. I. Structural aspects

Abstract: SUMMARY The structure and chemistry of anther dehiscence in Lycopersicon esculentum (Solanaceae) has been examined using a range of microscopic techniques. No Single event or process has been identified as being solely responsible for the release of the pollen grains, instead, an integrated programme of development across a number of tissues appears to be involved. The earliest events in this process are changes in the intersporangial septa and growth of specific epidermal cells. Large numbers of calcium oxalate (druse) crystals accumulate in the septa and may play a part in the weakening and eventual enzymic dissolution of this cell layer. The differential growth of epidermal cells fulfils two roles; firstly it serves to define the point of rupture or stomium and, secondly the enlarged cells generate and transmit the force required to disrupt the unenlarged stomial cells. Adjacent loculi are unified within the anther first by the enzymic degradation of the crystal-filled septal cells, which is followed by similar degradation of the remaining connective tissue cells and the eventual rupture of the remnants of the tapetal walls. Once the epidermal cell system has been established, and adjacent loculi unified, the endothecium develops in the anther wall, but only in the region adjacent to the stomium in the distal third of the anther. Opening of the anther is achieved by a combination of the mechanical forces generated by tile enlarged epidermal, and possibly the endothecial, cells operating on the previously weakened stomial cells, which at the time of rupture have started to desiccate. Following rupture of the anther wall, the stomium is transformed from a slit to a wide-mouthed pore via the ordered desiccation and contraction of the endothecial and surrounding tissue. Although these final stages of dehiscence show some sensitivity to ambient vapour pressure differences, there is little doubt that the process of dehiscence is not purely a desiccatory one, indeed only specific localised areas of the anther degenerate and dehydrate, whilst the bulk of the anther tissue remains turgid and metabolically active. This multicomponential type of anther dehiscence is discussed in the perspective of other models proposed to explain anther opening in members of the Solanaceae, as well as systems suggested for other plants which clearly possess very different methods of dehiscence.

Effects of ectomycorrhiza on host growth and carbon balance in a semi‐hydroponic cultivation system

Abstract: SUMMARY Ectomycorrhizal and control Pinus sylvestris L. seedlings were grown under steady-state nutritional conditions in a semi-hydroponic system. Free access to balanced low concentrations of mineral nutrients did not inhibit mycorrhiza formation. The large variations in relative growth rate observed after inoculation with different fungi demonstrated that the mycobiont has a substantial influence over the proportion of host photosynthate allocated for maintaining the symbiosis. Analyses with 14C-tracer demonstrated that photosynthesis was considerably enhanced in mycorrhizal seedlings, even though N and Mg concentrations were similar in mycorrhizal and uninoculated seedlings. Although translocation and respiration were highly variable, they were enhanced in some seedlings. Nutrition had no demonstrable effects on shoot or root carbohydrate content, but auxin (IBA and IAA) treatments increased the carbohydrate concentrations. Several auxin treatments also accelerated translocation and respiration, but their effects on photosynthesis were variable. The results contradict some of the basic tenets in Bjorkman's (1942) carbohydrate theory, while providing circumstantial evidence in favour of Slankis's (1973) hormone hypothesis.

Interactions between fluorescent pseudomonads and VA mycorrhizal fungi

Abstract: SUMMARY Cucumber seeds were treated with rifampin-resistant derivatives of Pseudomonas putida (A12, NI or R-20) or R-20 or P. fluorescens (2–79 or 3871) and planted in soils with and without added inoculum of the VA mycorrhizal fungi Glomus intraradices Schenck & Smith or G. etunicatum Becker & Gerdemann. Populations of Pseudomonas spp. in the combined rhizosphere-rhizoplane soil were determined at 1–9 weeks after planting by dilution plating on a selective medium. At 1–3 weeks, the populations of all strains except R–20 were 1.5 to 7 times lower in the rhizosphere of cucumber roots colonized by G. intraradices, when compared to nonmycorrhizal plants. However, this effect was less consistent in 3- to 9-week-old plants. No significant difference was detected in populations of Pseudomonas strains between roots colonized by G. etunicatum and nonmycorrhizal roots. Strains 3871 and 2-79, which are antibiotic producers, delayed the germination of G. etunicatum spores in raw soil, but by seven days no significant differences in frequency of germination were detected. None of the fluorescent Pseudomonas strains affected the colonization of cucumber roots by G. etunicatum, as determined by measurements of mycorrhizal inoculum density root colonization relationships.

Which steps are essential for the formation of functional legume nodules

Abstract: Nodulation is reviewed in terms of the phenotypes proposed by Vincent (1980). Individual legumes may be infectible by one or more of the three bacterial genera (collectively known as rhizobia) Rhizobium, Bradyrhizobium, or Azorhizobium. The type of infection process by which rhizobia gain entry is largely governed by the host genotype. In addition to the widely studied root-hair pathway, infections may be associated with lateral root emergence or occur between root epidermal cells. The exact chemical and physical nature of the root hair/epidermal cell wall is likely to be a critical factor in determining whether infections can proceed. In addition to differing with species, wall composition may be influenced by soil chemical (e.g. Ca2+ ) and biotic factors (e.g. bacteria). Rhizobial features essential for infection include particular surface polysaccharides and the induction of nodulation genes by plant root exudates. Neither of these is likely to be a major barrier to the extension of nodulation to new hosts. Dissemination of rhizobia within developing nodules may be intercellular, via infection threads or by division of a small number of infected cells. All functional symbioses eventually have 'intracellular' bacteria, in the sense that rhizobia are geographically located within the boundary of the host cell walls. However, they remain extracellular in the sense that they are always confined by a membrane which is largely of host cell origin. In some genera they are also surrounded by infection thread walls, probably modified forms of 'invasive' infection thread walls, which allow differentiation of rhizobia into the nitrogen-fixing form. Thus, natural, functional, symbioses may (a) never involve a stage in which bacteria are confined within tubular infection threads or (b) never release bacteria from infection threads. These features are determined by host genotype. The one feature of legume nodules so far found never to vary is the stem-like character of a peripheral vascular system. This contrasts with the central vascular system of actinorhizas and the rhizobial-induced nodules on the Ulmaceous genus Parasponia. Although of great intrinsic interest, this character is unlikely to present an insurmountable barrier to the extension of nodulation to new species. Other features, such as the ability to produce haemoglobin are now known to the in the genetic makeup of many higher plants. The discovery of the wide range of nodule structures occurring in nature, together with work on mutant rhizobia which may bypass critical stages in the nodulation process, suggest various ways in which the extension of nodulation to non-nodulated legumes and to other (initially at least, dicotyledonous) plants may be engineered. CONTENTS Summary 129 I. Introduction 130 II. The symbionts 130 III. Stages in nodulation 132 IV. Stems and nodules 143 V. Prospects for finding/making new symbioses 144 VI. Conclusions 145 Acknowledgements 147 References 147.

Diurnal changes in radial oxygen loss and ethanol metabolism in roots of submerged and non-submerged rice seedlings

Abstract: Summary The effects of shoot submergence on root aeration were examined using floodwater Hushed with 1 or 2 kPa CO2 plus 10 or 21 KPa O2 or with air; the rooting media were stagnant. Oxygen regimes within the roots were evaluated from measurements of radial O2 loss to polarographic electrodes sleeving the extension zone of nodal roots, or, from dissolved O2 and ethanol in the rooting medium. With shoots submerged, root O2 and extension, as well as O2 and ethanol concentrations in the rooting medium underwent marked diurnal changes. In the dark, the radial O2 loss fell rapidly until root surface O2 became (0.001 mol m−3 (gas-phase equivalent < 0.1 kPa) and root extension slowed down or ceased. Oxygen, previously accumulated in the rooting medium, also declined markedly, whilst ethanol accumulated rapidly. When the lights come on, radial O2 loss resumed within 3 min and surged to a peak at c. 30 min before decreasing gradually to a lower level. Root extension also resumed, and a steady rise in dissolved O2 was accompanied by a decline in ethanol concentration. The expression of a diurnal cycling of O2, was attributed to high boundary-layer resistance to gas exchange between leaf and water, to C O2 supply and to a buffering effect by oxygen in the Hoodwater. Thus, during the day the escape of photosynthetic O2 was hindered, while at night, O2 flow from Hoodwater to leaf was restricted. Fluctuations in ethanol were attributed to the generation and subsequent consumption of ethanol by the roots in response to lower and higher internal O2 concentration. The O, surge at dawn was partly attributed to enhanced photosynthesis from accumulated internal CO2 and partly to a decline in O2 demand during the night as a result of substrate depletion in the plant. Stagnant floodwaters around the shoots led to much higher daytime rates of radial O2 loss from the roots, and to unexpected oscillations in radial O2 loss attributed to pressurization and de-pressurization during the expansion and subsequent release of O2-enriched bubbles from the leaves. With non-submerged plants, the diurnal cycles in O2 and ethanol concentrations were smaller and in the case of O2, different in pattern: although, initially at night, radial O2 loss declined, it often rose again later. This trend would be consistent with a reduction in respiration during the night following rapid substrate depletion at the high temperatures. Short roots grew day and night, but as roots became longer, apical O2 concentration declined and, because of this, growth ceased at c SO mm. The considerable influence of high temperatures on root aeration was confirmed by rapid increases in radial O2 loss when temperatures were lowered from 32 to 23 °C. The ecological significance of the findings is discussed and it is concluded that (a) a diurnal periodicity of root growth and of localized internal anoxia may be a normal feature of roots in submerged and even non-submerged rice, and (b) ethanol production may play an important role in root survival during the night.

Cadmium tolerance and co-tolerance in Silene vulgaris (Moench.) Garcke [=S. cucubalus (L.) Wib.]

Abstract: summary This paper is the first to report cadmium tolerance in a dicotyledonous species, Silene vulgaris (Moench.) Garcke. The response to cadmium of five populations originating from one uncontaminated and various heavy-metal contaminated sites was examined under standardized conditions for three weeks. The tolerance index (TI), based on the mean relative growth rate (R), was determined. Populations originating from cadmium-contaminated sites showed a distinct tolerance to cadmium. A population from a site enriched only with copper also exhibited a marked co-tolerance to cadmium. A clear difference in biomass production between the sensitive and tolerant populations was attained at 1 μM cadmium. An optimum biomass production in tolerant populations at a metal concentration higher than in the control, as demonstrated for zinc and copper, could not be established for cadmium. The pattern of cadmium uptake and translocation differed between tolerant and sensitive populations. All tolerant populations accumulated cadmium in the roots and showed some degree of restricted transport to the shoots. The effect of cadmium on the elemental distribution in roots and shoots was population-independent for some elements (copper, zinc, potassium) and population-specific for others (phosphorus, magnesium and sodium). The phenomenon of co-tolerance to cadmium is discussed in relation to possible tolerance mechanisms, especially with regard to metal-binding compounds (metallothioneins, phytochelatins).