Showing papers in "New Phytologist in 1991"

The hydraulic architecture of trees and other woody plants

Abstract: summary In this paper, we have reviewed how the hydraulic design of trees influences the movement of water from roots to leaves. The hydraulic architecture of trees can limit their water relations, gas exchange throughout the crown of trees, the distribution of trees over different habitats and, perhaps, even the maximum height that a particular species can achieve. Parameters of particular importance include: (1) the vulnerability of stems to drought-induced cavitation events because cavitation reduces the hydraulic conductance of stems, (2) the leaf specific conductivity-of stems because it determines the pressure gradients and most negative water potentials needed to sustain evaporation from leaves, (3) the water storage capacity of tissues because this might determine the ability of trees to survive long drought periods. All of these parameters are determined by the structure and function of anatomical components of trees. Some of the ecological and physiological trade-offs of specific structures are discussed.

Nutrient supply, nutrient demand and plant response to mycorrhizal infection.

Abstract: summary One of the most dramatic effects of infection by vesicular-arbuscular mycorrhizal fungi on the physiology of the host plant is an increase in phosphorus absorption. When phosphorus is limiting, the maximum extent to which mycorrhizal infection can improve plant performance is thus predicted to be a function of the phosphorus deficit of the plant, the difference between phosphorus demand and phosphorus supply. Phosphorus demand is defined as the rate of phosphorus absorption that would result in optimum performance of the plant as measured by growth rate, reproduction or fitness. The phosphorus supply is defined as the actual rate of phosphorus absorption under the prevailing conditions. Variation among plant taxa in morphological, physiological or phenological traits which affect either phosphorus demand or phosphorus supply (and thus phosphorus deficit) is predicted to lead to variation in potential response to mycorrhizal infection. The actual response to mycorrhizal infection is predicted to be a function of the increase in phosphorus uptake due to mycorrhizal infection and the phosphorus utilization efficiency of the plant. Demonstrated variability in responsiveness to mycorrhizal infection among plant taxa suggests that mycorrhizal fungi may play an important role in determining the structure of plant communities. Mycorrhizal infection may alter the phosphorus deficit or phosphorus utilization efficiency independently from its direct effect on phosphorus uptake, making the prediction of response to mycorrhizal infection based on the traits of non-mycorrhizal plants quite difficult. For example, infection may at times increase the rate of phosphorus accumulation beyond that which can be currently utilized in growth, reducing the current phosphorus utilization efficiency. Such momentary ‘luxury consumption’ of phosphorus may, however, serve a storage function and be utilized subsequently, allowing mycorrhizal plants ultimately to outperform non-mycorrhizal plants.

Architectural analysis of plant root systems 1. Architectural correlates of exploitation efficiency

Abstract: summary In order to determine whether variation in root system architecture can be said to be adaptive, it is necessary' to be able to define the costs and benefits of particular architectures. A simulation model of root growth is described that allows the development of root systems varying in several important architectural features, including topology, link lengths and radii, and branching angles. The program also estimates the exploitation efficiency of each root system, as the ratio of the volume of soil occupied by depletion zones (allowing for overlaps between competing roots) and the volume of tissue required to construct the system. Output from the model shows that high exploitation efficiency is associated with a herringbone topology (where branching is predominantly on the main axis) and with long interior and exterior links, although all these characteristics are associated with large tissue volumes and hence high construction cost, We predict, therefore, that such ‘expensive’ root systems will occur mainly in nutrient-poor soils and on plants characteristic of such soils.

On the relationship between specific root length and the rate of root proliferation: a field study using citrus rootstocks

Abstract: summary This study tested two hypotheses: (1) species with roots that have a high length to dry mass ratio or specific root length (SRL) also have the potential for high rates of root growth in small volumes of favourable soil and (2) variation in average root diameter fully accounts for variation in SRL. To minimize differences among shoots, the study used 13-year-old ‘Valencia’ sweet orange [Citrus sinensis (L.) Osbeck] trees budded to rootstocks representing a range of genotypes. Soil cores 7.4 cm in diameter and 14.2 cm deep were extracted from beneath the canopy, and the soil was sieved free of roots and replaced. Root length, diameter and dry weight of the roots in the disturbed soil and adjacent undisturbed soil were evaluated 5, 10, 19 and 40 weeks following soil replacement. The disturbed soil had a higher water content than the undisturbed soil for the first three sampling dates. Averaged across rootstocks, root length density increased in a linear fashion in the disturbed soil and was comparable to that in the undisturbed soil by 40 weeks. Mean root diameter of the fibrous roots (< 2 mm) declined with age. Rootstocks with the highest SRL had the most rapid rate of root proliferation (cm cm−3 wk−1) (r= 0.94) and the greatest rate of water extraction at 19 weeks (r= 0.79). Although variation in root diameter contributed to rootstock variation in SRL, the data also suggested that rootstocks of high SRL had roots with lower tissue density than those of low SRL (P < 0.05). The potential trade-offs of constructing root systems of high SRL are discussed.

Architectural analysis of plant root systems 2. Influence of nutrient supply on architecture in contrasting plant species

Abstract: summary Two components of root architecture (topology and link lengths) were measured in a group of 13 dicotyledon and eight grass species, under both high and low nutrient supply rates. Predictions were made, based on a simulation analysis, that root systems from plants grown in low nutrient conditions and those from plants characteristic of such conditions should have the more herringbone topology (branching principally on main axis) and longer links. Both these predictions were confirmed for the dicots, but data from grasses agreed with the predictions only in terms of geometry (link lengths) and not topology. The ecological character of the species was assessed on the assumption that species of low inherent relative growth rate were characteristic of infertile soils, Taxonomic patterns were also evident, indicating that some variation in root system architecture may be historical rather than adaptive.

The effect of soil disturbance on vesicular-arbuscular mycorrhizal fungi in soils from different vegetation types

Abstract: summary The infectivity of vesicular–arbuscular (VA) mycorrhizal fungi in three soils from different vegetation types was compared before and after disturbance. The relative quantities of infective propagules in the disturbed soils were estimated by an infectivity test after the soils were diluted. Spare numbers and mycorrhizal roots were also quantified in each soil. The mycorrhizal colonization of clover roots in the infectivity st was not decreased after soil from an annual pasture had been disturbed. In contrast, in both a forest soil and a heathland soil, the percentage root length colonized of test plants was almost halved if the soils had been disturhtd. In the pasture soil up to 10 times or 25 times more propaguies survived disturbance than in the forest soil or the heathland soil respectively. The large number of propaguies may allow the VA mycorrhizal fungi in the pasture soil to maintain maximum infectivity after soil disturbance.

Phosphorus depletion and pH decrease at the root–soil and hyphae–soil interfaces of VA mycorrhizal white clover fertilized with ammonium

Abstract: summary To study phosphorus (P) depletion and soil pH changes at the root–soil interface (rhizosphere) and at the hyphaesoil interface, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) plants were grown for 7 wk in two sterilized soils (Luvisol and Cambisol) in pots comprising five compartments: a central one for root growth, two adjacent compartments, separated from the central compartment by a nylon net of 30 μm mesh size, for growth of vesicular-arbuscular (VA) mycorrhizal [Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe] hyphae (hyphal compartments), and two outer compartments, separated from the hyphal compartments by a 0.45 μm membrane, which neither roots nor hyphae could penetrate (bulk soil compartments). Phosphorus was supplied as Ca(H2PO4)2 at a rate of 50 mg P kg−1 soil in the root compartment and 150 mg P kg−1 soil in the hyphal and bulk soil compartments. Nitrogen was supplied as (NH4)2SO4 at the rate of 300 mg N kg−1 soil uniformly to all compartments. In both soils, shoot dry weight and P uptake were much higher in mycorrhizal plants compared with non-mycorrhizal plants. Hyphae of VA mycorrhizal fungi contributed 70% (Cambisol) or 80% (Luvisol) to total P uptake of mycorrhizal plants. In the hyphal compartments, concentrations of both H2O-extractable soil P (Cambisol and Luvisol) and NaHCO3-extractable soil P (Luvisol) were decreased drastically. Soil P depletion profiles developed not only at the root-soil interface (rhizosphere), but also at the hyphae-soil interface and extended several millimetres from the hyphae surface into the soil. Likewise, the soil pH was decreased at the root-soil interface, in the hyphal compartment and also at the hyphae-soil interface. The results demonstrate that, similarly to roots, hyphae of VA mycorrhizal fungi have the ability to form a P depletion zone and a zone of altered pH in the adjacent soil. Thus, as well as at the root-soil interface, soil conditions at the hyphae–soil interface may also differ considerably from conditions in the bulk soil.

Mechanisms of action of abscisic acid at the cellular level

Abstract: summary Abscisic acid (ABA) has been implicated in the control of a diverse range of physiological processes in higher plants. In this review, we focus on the events which constitute the cellular responses to ABA. Current evidence suggests that it is possible to classify the responses to ABA on the basis of whether they are rapid, involving ion fluxes (typified by the stomatal response), or slower and requiring alterations to gene expression (for example the response of cereal embryos to ABA). In our consideration of ABA stimulus response coupling pathways, we have chosen to highlight the role of the calcium ion in the rapid responses, while we have concentrated on the contribution of as-acting elements and trans-acting factors in the regulation of ABA-responsive genes. We also draw attention to the possibility that interaction may exist between these pathways. Additionally, we discuss the controls of ABA concentrations during development and in response to environmental stimuli. Factors which contribute to the controls of ABA sensitivity are also reviewed. In our conclusions, we suggest that a general role for ABA may be to prepare tissue for entry into a new and different physiological state, perhaps by resetting the direction of cellular metabolism.

Stimulation of vesicular‐arbuscular mycorrhiza formation and growth of white clover by flavonoid compounds

Abstract: summary The effects of flavonoid compounds on VA mycorrhiza root colonization and growth of white clover (Trifolium repens L.) plants under growth chamber conditions are reported. The isoflavonoids, formononetin and biochanin A, previously identified from clover roots, stimulated colonization and growth of clover, while several other flavonoid compounds were inactive when tested at concentrations of 5 mg 1−1. The flavone, chrysin, when applied at concentrations higher than those tested for formononetin and biochanin A, also increased root colonization and plant growth. The stimulatory effects of the isoflavonoids on plant growth were mediated by VA mycorrhizal fungi and were dependent on concentration, period of growth and soil spore density. Maximum responses were found when 5 mg 1−1 solutions were applied to soil containing 2 to 4 VA mycorrhiza spores g−1 of soil. These results may provide insights on the molecular mechanisms of host-fungus interaction and for the development of technology to exploit the potential of the indigenous VA mycorrhizal fungi in field soil.

Achievable productivities of certain CAM plants: basis for high values compared with C3 and C4 plants.

Abstract: CAM species, which taxonomically are at least five times more numerous than C4 species, often grow-slowly, as is the case for various short-statured cacti and many epiphytes in several families, However, slow growth is not a necessary corollary of the CAM photosynthetic pathway, as can be appreciated by considering the energetics of CO2 fixation. For every CO2 fixed photosynthetically, C3 plants require 3 ATP and 2 NADPH, whereas the extra enzymatic reactions and compartmentation complexity for C4 plants require 4 or 5 ATP and 2 NADPH, and CAM plants require 5.5-6.5 ATP and 2 NADPH. Photorespiration in C8 plants can release some of the CO2 , fixed and also has an energetic-cost, whereas photorespiration is much less in C4 and CAM plants. Therefore, CAM plants can perform net CO2 fixation 15% more efficiently than C3 , plants, although 10% less efficiently than C4 plants. Using a simple model that assumes 8 photons per CO2 fixed and a processing time per excitation of 5 ms, a maximum instantaneous rate for net CO2 , uptake of 55 μmol m-2 s-1 is predicted. Measured maximal rates average 48μmol m-2 s-1 for leaves of six C3 species with the highest rates and 64 μmol m-2 s-1 for six such C4 species; CAM plants take up CO2 mainly at night, which is not directly related to the instantaneous rate of photon absorption. Net CO2 uptake integrated over 24 h, which is more pertinent to productivity than are instantaneous CO2 uptake rates, is similar for the three pathways, although the higher water-use efficiency of CAM plants can be an advantage during drought. Canopy architecture is crucial for the distribution of the photosynthetic photon flux density (PPFD) over the shoot, which determines net CO2 uptake per unit ground area and hence determines productivity. Maximal productivity for idealized canopies under optimal conditions is predicted to be about 100 Mg d. wt ha-1 yr-1 (1 Mg = 1 tonne), whereas actual values of environmental factors in the field approximately halve this prediction. The influence of environmental factors on net CO2 uptake can be quantified using an environmental productivity index (EPI), which predicts the fractional limitation on net CO2 uptake and is the product of a water index, a temperature index, and a PPFD index (nutrient effects can also be included). Using EPI with a ray-tracing technique to determine the PPFD index and taking into account respiration and carbon incorporated structurally, maximal productivity of CAM plants is predicted to occur at leaf or stem area indices of 4-5. In experiments designed using such shoot area indices, annual above-ground dry-weight productivities averaging 43 Mg ha-1 yr-1 have recently been observed for certain agaves and plutyopuntias. In comparison, the measured average annual productivity of the most productive plants is 49 Mg ha-1 yr-1 for six agronomic C4 species, 35 Mg ha-1 yr-1 for sis agronomic C3 species, and 39 Mg ha-1 yr-1 for six C3 tree species. Thus, CAM plants are capable of similar high productivities, which can become especially advantageous in regions of substantial water stress. Recognition of the high potential productivity of certain CAM species under optimal environmental conditions, exceeding that of most C3 species, may increase the cultivation of such CAM plants in various areas in the future. CONTENTS Summary 183 I. Introduction 184 II. Biochemistry of C3 , C4 , and CAM plants 185 III. CO2 uptake rates 188 IV. Canopy architecture and light absorption 193 V. Measured biomass productivity 198 VI. Conclusions 200 Acknowledgement 202 References 202.

Enzymatic studies on the metabolism of vesicular-arbuscular mycorrhizas. V, Is H+-ATPase a component of ATP-hydrolysing enzyme activities in plant-fungus interfaces ?

Abstract: Cytochemical methods were used to localize ATPase activity in different phases of development of the vesicular-arbuscular mycorrhizal association between Allium cepa L. and Glomus intraradices Schenck & Smith. In the absence of inhibitors the pattern of lead deposition (indicative of ATPase activity) at the arbuscular interface was similar to that previously published, whether material was prefixed in glutaraldehyde or formaldehyde (...)

Effect of a vesicular–arbuscular mycorrhizal fungus and rhizosphere micro-organisms on manganese reduction in the rhizosphere and manganese concentrations in maize (Zea mays L.)

Abstract: summary Maize (Zea mays L. ev. Tau) plants were grown in a calcareous soil for six weeks in pots having separate compartments for growth of roots and vesicular-arbuscular (VA) mycorrhizal fungal hyphae. Soil was sterilized and either left non-inoculated (sterile treatment), or was inoculated with rhizosphere micro-organisms only (MO–VA) or with rhizosphere micro-organisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol & Gerd.) Gerdemann & Trappe (MO +VA)]. Concentrations of Mn in roots and shoots, as well as exchangeable Mn in rhizosphere soil, decreased in the order MO–VA < MO + VA < sterile treatment. In all treatments, the concentration of exchangeable Mn was lower in the rhizosphere soil (0–5 mm distance from the root surface) than in the bulk soil (5–15 or 15–25 mm distance from the root surface). In the rhizosphere soil, the total microbial population was similar in mycorrhizal (MO + VA) and non-mycorrhizal (MO–VA) treatments, but the proportion of Fe- or Mn-reducers was 20- to 30-fold higher in the non-mycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The Mn+Iv-reducing potential (net balance between reduction and oxidation) in the rhizosphere soil was also distinctly lower in mycorrhizal treatment compared to non-mycorrhizal treatment. In the sterile treatment, low Mn+IV-reducing potential and correspondingly low concentration of exchangeable Mn in soil, compared to the other treatments, indicates the importance of micro-organisms in Mn reduction in soil and acquisition of Mn by plants. Therefore, the lower Mn concentrations in mycorrhizal plants are most probably caused by a shift in composition and activity of rhizosphere micro-organisms. As a side effect of the treatments, improved soil aggregation, as indicated by soil adhering to the nylon net (facing hyphal compartments) after plant harvest, occurred in non-mycorrhizal and sterile treatments but not in the mycorrhizal treatment.

Selective interactions between different species of mycorrhizal fungi and Rhizobium meliloti strains, and their effects on growth, N2-fixation (15N) and nutrition of Medicago sativa L.

Abstract: summary Three isolates of vesicular-arbuscular (VA) mycorrhizal fungi, belonging to the species Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, G. fasciculatum (Taxter sensu Gerd.) Gerd, and Trappe, and G. caledonium (Nicol. and Gerd.) Trappe and Gerd, were inoculated in dual combinations with six strains of Rhizobium meliloti with the aim of testing these combinations for functional compatibility with their common host plant, the legume Medicago sativa L. Symbiotic efficiency (promotion of plant growth and N and P nutrition) was found to be dependent on the particular combination of Rhizobium strain and Glomus species indicating selective and specific compatibilities between strains and isolates of the two types of microsymbiont, but also between them and the common host plant. Observed effects on plant growth were in general, though not always, related to the extent of VA mycorrhizal colonization. Although the different mycorrhizal and/or rhizobial treatments produced different effects on plant growth, the rate of nodule formation on M. sativa roots remained constant. Most mycorrhizal treatments increased the concentration and/or content of N in plant shoots but effectiveness was in the order: G. fasciculatum > G. mosseae > G. caledonium. In some cases, this increase in N-content may be a consequence of a P-mediated stimulation of N2-fixation by VA mycorrhiza, as ascertained using 15N. In other instances, however, the increase seems to reflect a VA mycorrhizal-mediated enhancement of N-uptake from soil. VA mycorrhizal inoculation decreased the concentration of Ca and Mg in plant shoots and a buffering effect of VA mycorrhiza in situations of nutrient excess in soil is proposed.

Leaf area compensation and nutrient interactions in CO2‐enriched seedlings of yellow‐poplar (Liriodendron tulipifera L.)

Abstract: summary The responses of yellow-poplar (Liriodendron tulipifera L.) seedlings to elevated levels of atmospheric CO2 were investigated to identify attributes governing growth and physiological responses to CO2. Based on the pattern of leaf initiation and nutrient requirements of the species, it was predicted that (1) CO2, enrichment would enhance growth of yellow-poplar seedlings both through accelerated leaf area production and through higher rates of carbon assimilation per unit leaf area; and (2) growth enhancement of yellow-poplar by CO2 enrichment would be reduced by nutrient limitations. The hypotheses were tested in an experiment in which yellow-poplar plants were grown from seed for 24 weeks in controlled-environment chambers. The experimental design comprised three atmospheric CO2 concentrations (371, 493, and 787 cm3m−3), two levels of mineral nutrients (unfertilized or weekly additions of complete nutrient solution), and three harvests (6, 12, and 24 weeks). Plant growth rate, water use, foliar gas exchange, component dry weights, and nutrient contents were measured. Both hypotheses were rejected. Whole-plant dry weight increased similarly with CO2, enrichment in plants provided with additional mineral nutrients and in unfertilized plants, although the fertilized plants grew 10-fold larger. The increase in dry weight resulting from elevated CO2 occurred only in root systems. Although leaves were produced continuously during the experiment, leaf area was slightly reduced in elevated CO2, and the whole-plant growth response was wholly attributable to an increase in carbon assimilation per unit leaf area. Although the compensation between photosynthesis and leaf area reduced the potential growth response to CO2, the reduction in leaf area ratio was associated with a significant increase in water-use efficiency. This unexpected result demonstrated the importance of feedbacks and interactions between resources in shaping the response of a plant to CO2.

The phytoplanktonic ways of life

Abstract: summary The adaptations of phytoplankton to life in suspension in water are considered with particular reference to hydromechanical factors ranging from molecular motion to ocean currents. The smallest phytoplankton, the picoplankton (0.2–2.0 μm), are the best adapted to the physico-chemical environment of the open waters of seas and lakes and, other things being equal, can out-compete the larger forms. The picophytoplankton are an autotrophic component in a microbial community, the ultraplankton, which also includes bacteria and flagellates up to about 20 μm in linear dimensions. This community is a highly dynamic and self-contained equilibrium system operating within a domain dominated by molecular diffusion. Within the photic zone it is limited, not by nutrient supply, but by its internal predator-prey relationships. It appears to be cosmopolitan, both in the sea and in freshwaters, to vary little either in time or space in species composition or in biomass concentration, and to contribute a minimum of organic carbon to higher trophic levels. There appears to be a fundamental divide in form and function between this and the microplankton, composed of organisms larger than about 20 μm. This community can only develop when nutrient levels are in excess of the concentrations required by the picoplankton. It is opportunistic, non-equilibrium in its dynamics, and highly variable in floristic composition and biomass concentration. Its life processes are dominated by turbulence. Nutrient supply is largely determined by turbulent eddy diffusion and movement of the organisms relative to the water mass. The microphytoplankton falls broadly into two types, one of which, exemplified by the diatoms, depends on turbulence to maintain it in the photic zone and the other, exemplified by the dinoflagellates and colony-forming cyanobacteria, relies on motility or buoyancy control to position it in a relatively stable water column so as to have best access to light and nutrients. The waxing and waning of microplankton populations is largely determined by hydrography and their floristic compositions by the interactions of the daily and seasonal rhythms of the organisms with the periodicities in the environment. In contrast to the ultraplankton microplankton species show distinct differences in biogeographical distribution. Throughout the discussion attention is drawn to the intimate relationships between the activities of phytoplankton and those of viruses, bacteria and zooplankton and the impossibility of getting a proper understanding of the physiology of the phytoplankton if they are considered in isolation.

Leaf demography and the seasonal internal cycling of nitrogen in sycamore (Acer pseudoplatanus L.) seedlings in relation to nitrogen supply

Abstract: summary The dynamics of the seasonal internal cycling of nitrogen (N) in Acer pseudoplatanus were studied in relation to N supply and leaf demography. Trees were grown in sand culture, and throughout 1988, supplied with either 1.0 mol N m −3(Low N) or 6.0 mol N m −3(High N), labelled with 15N to 5.0 atom %, in order to precondition their growth and capacity for N storage over the winter period. In 1989, the initial spring growth of leaves was independent of the current N supply at either 1.0 or 6.0 mol N m−3, and depended only upon the N supplied in 1988. A net loss of 15N from perennial tissues and recovery of 15N in the growing leaves confirmed that the current N supply had no effect on the remobilization of N from winter stores in the roots and stem. After 22 May 1989, remobilization of stored N for leaf growth had ceased and leaf demography was influenced by the current N supply. Plants preconditioned to 6.0 mol N m −3in 1988, but receiving only 1.0 mol N m−3in 1989, were unable to sustain previous leaf growth after 22 May and there was a decrease in leaf mass and area due to leaf abscission. In contrast, trees preconditioned with 1.0 mol N m−3in 1988, but receiving 60 mol N m−3in 1989 greatly increased their leaf growth after 22 May. This was a consequence of the stimulation by N uptake of growth of many small leaves on axillary shoots at the base of the canopy. Although leaf number increased, mean area per leaf decreased. The trees were inefficient at withdrawing N from senescing leaves in both years, allowing most of the N to fall from the tree with the leaves. The spring remobilization of N, therefore, was dependent largely upon the direct uptake of N into the stem and roots, in late summer and autumn, for storage over the winter period. The results are discussed in relation to leaf demography and the nitrogen use efficiency of deciduous trees.

Changes in germinability, ABA content and ABA embryonic sensitivity in developing seeds of Sorghum bicolor (L.) Moench. induced by water stress during grain filling

Abstract: summary The effect of intermittent water stress during grain filling on the germinability of developing seeds of S. bicolor was investigated. The drought treatment was imposed in cycles within the maturation period by withholding water for 5–6 days, rewatering at the end of each drought cycle and withholding water again. Changes in abscisic acid (ABA) content and embryonic sensitivity to ABA in the maturing seeds were also monitored in order to find out if there were any parallel changes with seed germinability resulting from drought conditions. Seeds developing in plants subjected to drought showed a high level of germinability earlier in the maturation period than did control seeds; consequently, they were less resistant to pre-harvest sprouting as shown when panicles were exposed to high humidity conditions. Very high levels of ABA accumulated in the early stages of development in seeds maturing on water-stressed mother plants; however. ABA content fell markedly when the seeds stopped growing, and remained significantly below those recorded in control seeds until the end of the maturation period. Development under drought conditions decreased the sensitivity of the isolated embryo to exogenous ABA by about 10-fold. The good agreement found between germinability, endogenous ABA concentrations and embryo sensitivity to ABA at different stages of development, suggests a key role for ABA as a major inhibitor of precocious germination and shows that changes in germinability caused by water stress during grain filling are likely to be related to changes in ABA pool size in the developing seed.

Evidence for the involvement of ethene in aerenchyma formation in adventitious roots of rice (Oryza sativa L.)

Abstract: summary Two varieties of rice (ev. Norin 36 and RB3) were either grown in stagnant or aerated 1/4-strength Hoagland's solution with or without exogenous ethene and a range of silver concentrations (an ethene antagonist), or were grown in flooded and drained soils. With cultivar Norin 36, AgNO3 was very effective in reducing porosity by inhibiting aerenchyma development. This effect was antagonized by gassing with increasing concentrations (1 or 2 μ1−l) of ethene. The results are consistent, therefore, with reports that cavity formation in roots is controlled by endogenous levels of ethene. Also, porosity varied with root length, and if a correction was made for this, the inhibitory effect of the silver ion on aerenchyma development appeared to be even greater. For the cultivar RB3, root porosities in solution culture appeared initially to be unaffected by ethene or AgNO2, but root lengths were reduced and root numbers increased by increasing ethene concentration. When these effects on root length were taken into account, it was concluded that ethene had increased root porosity, and AgNO3, had decreased it. Consistent with earlier studies, and with the ethene hypothesis, aerenchyma development in both varieties was also enhanced by soil waterlogging, with percentage porosities 12 units higher in flooded than in drained soil. The porosities of cv, RB3 were higher than those for cv. Norin 36, however, and the regression line of porosity against root length was steeper, indicating a greater predisposition to form aerenchyma in cv. RB3. This was confirmed in solution-grown roots, where AgNO3 had no significant effect on porosity in stagnant or in aerated roots of cv. RB3, and both 1 and 2 μ1−1 ethene increased the porosity in cv. RB3 roots to a similar level. Silver nitrate, however, did antagonize the effects of 1 and 2 μ1−1 ethene. In contrast, porosity in cv. Norin 36 was progressively reduced by AgNO3. It is concluded that, although there are intervarietal differences in sensitivity, an ethene promotion of gas-space formation can occur in rice, and ethene should not be ruled out as the endogenous promoter.

Vesicular-arbuscular mycorrhizas respond to corn and soybean cropping history

Abstract: summary Communities of vesicular-arbuscular (VA) mycorrhizal fungi were studied in a long-term crop rotation experiment at two locations (Waseca and Lamberton, Minnesota, USA). Spores of mycorrhizal fungi were counted and identified in experimental plots with a cropping history of either corn (Zea mays L.) or soybean [Glycine max (L.) Merrill]. Mycorrhizal fungal communities were affected by both location and cropping history. At Waseca, Glomus aggregation Schenck & Smith, G. leptotichum Schenck & Smith and G. occultum Walker spores were more abundant in soil with a corn history than a soybean history, while spores of G. microcarpum Tul. & Tul. exhibited the reciprocal pattern. Approximately 90% of the spores recovered at Lamberton were G. aggregation and did not vary with crop history. However, the spores of three other species: G. albidum Walker & Rhodes, G. mosseae Gerdemann & Trappe, and G. occultum, were more abundant in plots with a corn history than a soybean history. Densities of G. aggretatum spores were negatively correlated with soil pH at Waseca, but were unrelated to pH at Lamberton were the mean soil pH was lower. Our results indicate that mycorrhizal fungal species are individualistic in their responses to cropping history and edaphic factors.

The mechanisms of arsenate tolerance in Deschampsia cespitosa (L.) Beauv. and Agrostis capillaris L. Adaptation of the arsenate uptake system

Abstract: summary Arsenate influx isotherms in Deschampsia cespitosa (L.) Beauv. and Agrostis capillaris L. showed that tolerance to arsenate in these grasses is achieved at least in part by adaptation of the arsenate uptake system, which leads to reduced influx of arsenate in arsenate-tolerant plants. In D. cespitosa, this reduction in arsenate ioflux was by suppression of the Vmax of the high-affinity uptake system and by an increase in the Km of the low-affinity uptake system. In A. capillaris the reduction in arsenate uptake in arsenate-tolerant plants was by a decrease in the Vmax of both uptake systems and by an increase in the Km of the high-affinity uptake system. The differences in the rates of arsenate influx between tolerant and non-tolerant plants was much greater for D. cespitosa than for A. capillaris. The mechanism of arsenate tolerance in D. cespitosa and A. capillaris was the same mechanism of tolerance that had been demonstrated in arsenate-tolerant Holcus lanatus L. Reduced arsenate influx by adaptation of the arsenate uptake mechanism is a mechanism of tolerance that has evolved in three separate species.

Regulation of nutrient transfer between host and fungus in vesicular—arbuscular mycorrhizas

Abstract: summary Although the overwhelming majority of non-aquatic vascular plants form vesicular-arbuscular (VA) mycorrhizal associations, the extent of colonization of the host root by any given fungal symbiont varies considerably depending on host and environmental factors. Because VA mycorrhizal fungi are obligate biotrophs, transfer of photosynthate from host to fungus may be an important factor in regulating the extent of VA mycorrhizal formation. Host metabolites must cross the plasma membrane before becoming available to the fungus. Several studies on rates of root exudation under various environmental conditions show a strong correlation between rates of root exudation and percent of root length colonized by VA mycorrhizal fungi. However, passive leakage of simple metabolites from roots as the sole means of regulating fungal colonization seems improbable for an obligate biotroph which has not yet been successfully cultured on any artificial medium. So far there has been insufficient investigation of hormone interactions between symbionts, and of the interference by the fungus in host cell wall synthesis, to evaluate the possible role of these factors in controlling growth of VA mycorrhizal fungi. Cytochemical studies of the host-fungus interface suggest modification of host plasma membrane ATPase activity as arbuscules develop, but the function of this altered activity remains unresolved. The presence of a linked P1-photosynthate exchange mechanism on the host plasma membrane analogous to the P1-photosynthate translocator known to exist in the outer membrane of chloroplasts remains an uninvestigated possible mechanism for balancing photosynthate demand by the fungus with enhanced P uptake.

Physicochemical properties of plant growth regulators and plant tissues determine their distribution and redistribution: stomatal regulation by abscisic acid in leaves.

Abstract: summary Biophysical and biochemical information about plant growth regulators, biomembranes and cell compartments of stressed and unstressed leaves is presented. These data are incorporated into a physiological source-sink network, which allows the calculation of phytohormone concentrations at any time in each compartment on the basis of biophysical and biochemical laws. The following results and conclusions are deduced and discussed: (i) The summarized physicochemical properties (e.g. p Ka, partition coefficient octanol: water, membrane conductance of neutral and charged phytohormone species) differ between all known phytohormones. (ii) This information is sufficient to explain experimentally observed distribution and redistribution pattern of ABA. (iii) Only cytokinins and the ethylene precursor amino-cyclopropane-carboxylic acid are distributed evenly between cell compartments, if synthesis and degradation are absent. Only under these conditions does the bulk concentration of these growth regulators in plant tissue homogenates estimate concentrations in all compartments, (iv) For other growth regulators, there are uneven compartmental concentrations depending on pH, membrane potential and anion conductance of biomembranes, even if synthesis and degradation are absent, (v) Abscisic acid is the only phytohormone which distributes almost ideally according to the anion-trap mechanism for weak acids. Calculated expected values and measurements coincide, (vi) Under diurnal illumination regimes, the same redistribution pattern of ABA for C3 and CAM plants is expected. The influence of the extreme vacuolar pH change is small because of the low ABA percentage in CAM mesophyll vacuoles (maximum 2.7 % of the total ABA mass per unit leaf area), (vii) Under drought stress, complex compartmental pH-shifts in leaves induce a complicated redistribution of ABA amongst compartments, (viii) The final accumulation of ABA in guard cell walls is up to 16.1–fold over the initial value, (ix) A 2- to 3-fold ABA accumulation in guard cell walls is sufficient to induce closure of stomata. (x) The minimum time lag until stomata start to close is 1–5 min and it depends on the stress intensity and guard cell sensitivity to ABA. (xi) The primary target membrane of ‘stress’ is the plasmalemma, not thylakoids. (xii) The effective ‘stress sensor’, which induces the proposed signal chain finally leading to stomatal closure may be located in epidermis cells. Mesophyll cells support stomatal closure only synergistically. (xiii) The direct biophysical influence of drought stress (increase of transpiration until stomata close) on the ABA concentration in guard cell walls is considerable but slow, (xiv) A stress signal from the root system in the form of an increased ABA concentration is capable of regulating the stomatal conductance, if guard cell sensitivity to ABA remains constant. The total ABA content per unit leaf area declines only within about 1 or 2 wks (aftereffect), (xv) For other phytohormones, there is no, or only a moderate, redistribution after compartmental pH changes. For biophysical reasons, only ABA is principally capable of being a ‘stress messenger’ for stomata, and evolution appears to have selected it.

Absorption of atmospheric NO2 by spruce (Picea abies L. Karst.) trees

Abstract: summary When spruce (Picea abies L., Karst.) branches were exposed to 5.2–18.7 nl NO2 l−1the flux to the shoots increased linearly with increasing NO2 concentrations. At NO2 concentrations below 2.6 nl l−1uptake of NO2 by the shoots was not observed. The measured flux of NO2 to the shoots was found to be lower than the NO2 flux predicted from the shoot conductance to diffusion of H2O. These results are consistent with the existence of internal resistances for NO2 influx, e.g. production and emission of NO2 by the leaves. However, emission of NO2 was not observed. When the NO2 flux to the shoots was plotted versus transpiration rate, a linear increase was found with an y-axis intercept. The intercept may be interpreted as the NO2 flux to the cuticle and the bark; its value increased with increasing atmospheric NO2 concentrations. The flux of NO2 to the shoots showed diurnal variation with high levels in the light and low levels during darkness. NO2 flux to the branches was dependent on light intensity. This dependency can largely be explained by light dependent changes in shoot conductance. Daytime light intensity determined also the night-time shoot conductance and, hence, the NO2 flux to the branches during the night. The ratio of NO2 flux to transpiration rate was higher in the dark than in the light. Whether this observation can be explained by a light dependency of internal resistances remains to be elucidated. The absorption of NO2 by the shoots enhanced the in vitro NR activity of the needles, while NiR and GS activities were not increased significantly. One day after exposure to NO2 concentrations of 60 nl l−1, nitrate reductase (NR) activity was three times higher than that of untreated controls. However, after three days of NO2 exposure the NR activity declined to the level of untreated controls. Apparently, the increased in vitro NR activity upon NO2 fumigation is a transient phenomenon in spruce needles. The regulatory events that may modulate NR activity of the needles, when exposed to atmospheric NO2, are discussed.

The micromorphological distribution of bound calcium in needles of Norway spruce [Picea abies (L.) Karst.].

Abstract: summary The distribution of bound calcium in spruce needles is visualized by various microscopical techniques. Ca pectate occurs in the middle lamellae of cell walls and in a surprisingly great concentration in the thick cell walls of the sieve cells and transfusion parenchyma. The major fraction of the calcium, however, is bound in the form of insoluble Ca oxalate crystals, which occur extracellularly on the outside of the walls of mesophyll cells which face the intercellular spaces. Furthermore, numerous small crystals appear within the cell walls of the epidermal cells, especially in the cuticular layer. The development and distribution of these apoplastic crystals is described in detail. Some hypotheses are finally presented for possible interpretations of these unusual patterns of the crystallization of Ca oxalate outside the vacuole.

Uptake, accumulation and translocation of arsenate in arsenate-tolerant and non-tolerant Holcus lanatus L.

Abstract: summary Uptake, translocation and accumulation of arsenate was determined in arsenate-tolerant and non-tolerant genotypes of Holcus lanatus L. Over a 6 h period of growth in 005 mol m−3arsenate, non-tolerant genotypes accumulated arsenate to a much greater extent than tolerant plants. Tolerant plants transported a much greater proportion of As to their shoots compared with non-tolerant plants. Phosphate at a concentration of either 0.05 or 0.5 mol m−3decreased arsenate uptake in both tolerant and non-tolerant genotypes. When arsenate uptake was determined over 3 d at the same arsenate concentration, non-tolerant plants grown in the presence of 0 and 0.05 mol m−3phosphate died, while those growing in 0.5 mol m−3phosphate survived and continued to take up arsenate. At all three phosphate levels tolerant plants survived. Over the 3 d period transport of arsenate to the shoots decreased. With increasing phosphate levels in solution transport of arsenate to the shoots increased in tolerant plants. The results from these experiments are discussed in terms of strategies for metal tolerance in this species.

Effects of excess nitrogen on carbohydrate concentration and mycorrhizal development of Pinus sylvestris L. seedlings

Abstract: summary The interaction of nitrogen supply, carbohydrate availability and ectomycorrhiza has been the subject of several studies and has been interpreted as the regulatory mechanism of the symbiosis. Only a few studies have simultaneously examined the effect of external nitrogen supply on the internal concentration of N, the internal sugar pool and, in a quantitative way, the extent of mycorrhizal colonization. The latter has usually been estimated with low accuracy through root tip counting only. Using ergosterol as a quantitative descriptor of fungal biomass, several of the basic assumptions of the carbohydrate theory have been re examined in a semi-hydroponic cultivation system, where Scots pine seedlings mycorrhizal with Laccaria bicolor (Maire) Orton were grown at an approximately constant relative growth rate on a nutrition regime based on a balanced basic medium supplemented with excess ammonium or nitrate. Mycorrhizal biomass was strongly correlated with the shoot nitrogen. High nitrogen availability did not significantly affect internal sugar concentrations. The development of mycorrhiza significantly reduced sugar concentrations in both roots and shoots. The difference between ammonium and nitrate in reducing mycorrhizal development was due to differences in uptake alone. The findings tend to invalidate a basic assumption of the carbohydrate theory, that of high nitrogen availability leading to reduced sugar concentration, the latter limiting mycorrhizal development. The results also conflict with the view that the host's sugar concentration is elevated in the presence of mycorrhiza.

Comparative water uptake by roots of different ages in seedlings of loblolly pine (Pinus taeda L.)

Abstract: summary Magnetic resonance (MR) imaging was used to study water absorption from fine, moist sand by the taproot, first-order lateral roots, and tine roots of 9-month-old loblolly pine seedlings. Magnetic resonance imaging provides the opportunity for repeated non-destructive measurements of water uptake by roots growing in a solid medium such as sand. Root systems of container-grown seedlings were pruned to a taproot, one or two first-order laterals and attached fine roots, and were planted in small containers. Reference tubes filled with a mixture of CuSO4/D2O were placed in each container in the field of view. Roots of individual seedlings were repeatedly imaged at approximately 3 h intervals. Water uptake by individual roots was measured and uptake was calculated based on biomass, root length, and surface area. Based on weight and surface area, but not on root length, fine roots were more efficient than the lateral or taproots in water uptake. Measurement of water content in MR images of wet sand was confirmed by the imaging of Sand phantoms. These phantoms were tubes filled with fine sand at varied water contents (5–25%). Additional tubes of CuSO4 with D2O were also imaged. A linear relationship between signal intensity of moist sand (normalized against the CuSO4/D2O) and water conteni was demonstrated (R2= 0.97). Fitting the normalized signal intensity of the sand to the calculated linear regression allowed calculation of the water content of the sand.

Interactions between three alfalfa nodulation genotypes and two Glomus species

Abstract: summary Seedlings of a non-nodulating alfalfa (Medicago sativa L.) genotype (nod−fix−) and an ineffectively nodulating alfalfa genotype (nod+fix−) exhibited resistance to vesiscular-arbuscular mycorrhizal (VA mycorrhizal) colonization, when grown in pot cultures inoculated with either Glomus versiforme (Daniels and Trappe) Berth or Glomus intraradices Schenck and Smith. Appressoria, showing considerable variation in size and shape, developed on the root surface, but failed to form any internal structures such as arbuscules and vesicies. Wild-type (nod+fix−) alfalfa seedlings developed normal colonization. This phenomenon will be extremely useful in studying the processes of recognition and compatibility between plant species and VA mycorrhizal fungal species.

Fluxes of carbon and phosphorus between symbionts in willow ectomycorrhizas and their changes with time

Abstract: summary One way of viewing a mycorrhizal symbiosis is as a balance between the nutritional ‘benefits’ and carbon ‘costs’ to the phytobiont. Phosphorus acquisition efficiency (the amount of phosphorus taken up per unit of carbon allocated belowground) can be used as an indicator of this balance. In this study, phosphorus uptake and belowground carbon allocation were measured using ectomycorrhizal (M) (Thelephora terrestris (Ehrh.) Fr.) and non-mycorrhizal (NM) Salix viminalis L. cv. Bowles Hybrid. Following 50, 60, 85 or 98 d of growth in a gamma-irradiated soil/sand mixture containing 4 mg bicarbunate-extractable P kg−1, seven randomly-selected cuttings of each treatment were harvested and their P contents determined. Nine d prior to each harvest, the three median plants from the group of seven were pulse labelled with 14C to determine the relative allocation of C aboveground and belowground. Mycorrhizal colonization of willow caused a two-fold increase in growth owing to substantially higher P uptake. Phosphorus inflow rates were almost three times as high for M root systems as for NM root systems over the interval up to the first harvest (3.2 × 10 −12 and 1.2 × 1012 mol m−1 s–1, respectively). Over the interval from 50 to 98 d, inflows into M plants were 50% higher than into NM plants (1.4 × 1012 and 0.9 × 10−13 mol m−1 s−1 respectively). The M plants allocated about 25 times as much carbon belowground as the NM plants for both periods. The P acquisition efficiency was higher in M than in NM plants during the first interval (16% and 40% higher using two different calculation methods), whereas during the second interval it was higher in NM than in M plants (33% and 44% higher using the two different methods). Thus, ectomycorrhizas can be very effective in supplying P to their hosts even at an early stage of infection. Furthermore, it is suggested that a temporal separation exists in the maximal fluxes of P and C between the fungus and the host of the mycorrhizal association. The results are discussed in the context of the nutrient requirements and carbon economies of field-grown woody plants.

Root and soil respiration responses to ozone in Pinus taeda L. seedlings

Abstract: summary Respiration rates of roots of loblolly pine (Pinus taeda L.) seedlings decreased in response to elevated ozone (O3) concentrations. Estimated annual root respiration rates were 12% less in seedlings exposed to twice-ambient O3(7 h mean = 70.110 nl 1−1) than in seedlings exposed to sub-ambient O3 (7 h mean = 20.40 nl 1−1). Measurements taken during periods of relatively rapid and of negligible root growth suggest that the reduced root respiration may be due to both reduced maintenance respiration and reduced growth respiration. Respiration rates of the soil substrate of the O3-exposed seedlings were also below those of the substrate of seedlings exposed to sub-ambient O3. The study supports the theory that there is a reduced supply of photosynthates to the roots of plants exposed to elevated O3. It is hypothesized that a reduced supply of photosynthates to the roots may affect soil respiration rates by reducing root exudation rates and, consequently, reducing rhizosphere microbial populations.