Specificity of interplant cycling of phosphorus: The role of mycorrhizas

The cycling of nutrients from dying roots of one plant (the donor) to other intact plants (the receivers) was examined in a series of pot experiments. In each pot receiver plants formed either the same or a different type of mycorrhiza as the donor plant and was therefore respectively either capable or incapable of forming mycorrhizal hyphal links to the donor. There was a preferential transfer of ³²P from the dying roots of the vesicular-arbuscular mycorrhizal (VAM) Lolium perenne to VAM-infected trees Acer pseudoplatanus and Fraxinus excelsior compared to the ectomycorrhizal (ECM) Larix eurolepis, this despite an apparently greater competitive ability of L. eurolepis to obtain ³²P from the soil. Following the death of L. perenne roots there was also an increase in total P in the VAM tree receiver. These findings could not be explained by similarities in rooting distribution of the VAM-infected plants.In a similar study of the transfer of ³²P between heathland plants there was a preferential cycling of ³²P from one ericoid mycorrhizal Calluna vulgaris to another rather than to the VAM Molinia caerulea. In contrast, when ³²P was supplied directly to the soil, M. caerulea obtained significantly more ³²P than C. vulgaris. These results are discussed in relation to the potential role of interplant mycorrhizal links in the cycling of nutrients within partially closed cycles and the implications that this might have for species balance in plant communities.     

Nonhydraulic signalling of soil drying in mycorrhizal maize

Our objectives were to (1) verify that nonhydraulic signalling of soil drying can reduce leaf growth of maize, (2) determine if a mycorrhizal influence on such signalling can occur independently of a mycorrhizal effect on leaf phosphorus concentration, plant size or soil drying rate, and (3) determine if leaf phosphorus concentration can affect response to the signalling process. Maize (Zea mays L. Pioneer 3147) seedlings were grown in a glasshouse with root systems split between two pots. The 2 x 3 x 2 experimental design included two levels of mycorrhizal colonization (presence or absence of Glomus intraradices Schenck & Smith), three levels of phosphorus fertilization within each mycorrhizal treatment and two levels of water (both pots watered or one pot watered, one pot allowed to dry). Fully watered mycorrhizal and nonmycorrhizal control plants had similar total leaf lengths throughout the experiment, and similar final shoot dry weights, root dry weights and leaf length/root dry weight ratios. Leaf growth of mycorrhizal plants was not affected by partial soil drying, but final plant leaf length and shoot dry weight were reduced in half-dried nonmycorrhizal plants. At low P fertilization, effects of nonhydraulic signalling were not evident. At medium and high P fertilization, final total plant leaf length of nonmycorrhizal plants was reduced by 9% and 10%, respectively. These growth reductions preceded restriction of stomatal conductance by 7 d. This and the fact that leaf water potentials were unaffected by partial soil drying suggested that leaf growth reductions were nonhydraulically induced. Stomatal conductance of plants given low phosphorus was less influenced by nonhydraulic signalling of soil drying than plants given higher phosphorus. Soil drying was not affected by mycorrhizal colonization, and reductions in leaf growth were not related to soil drying rate (characterized by time required for soil matric potential to drop below control levels and by time roots were exposed to soil matric potential below typical leaf water potential). We conclude that mycorrhizal symbiosis acted independently of phosphorus nutrition, plant size or soil drying rate in eliminating leaf growth response to nonhydraulic root-to-shoot communication of soil drying.Abbreviations and Symbols ANOVA analysis of variance - Cs stomatal conductance(s) - med medium - P probability - matric potential(s) - water potential(s) This work was supported by the U.S. Department of Agriculture grant No. 91-37100-6723 and a University of Tennessee Professional Development Research Award to R.M.A. We thank Angela Berry for the graphics.     

Interactions between plants: the role of mycorrhizae

Summary We present and discuss evidence, mostly from our own research, on some possible roles of mycorrhizae in interactions between plants. Experiments investigating whether seedlings become more rapidly infected with mycorrhiza if they are near large, already infected plants have shown that contact between the seedling's roots and established mycelium sometimes speeds up infection but on other occasions it does not. The reason for the discrepancies is not clear. Mycorrhiza can substantially alter the balance between competing plant species in a way that would not be predicted from their response when growing separately. An experiment involving large and small plants of the same species growing together showed little effect of mycorrhiza on the balance between plants of different sizes. The rate of transfer of ³²P between plants of Lolium perenne or Plantago lanceolata was so slow, even when they were mycorrhizal, that phosphorus transfer between living plants seems unlikely to be of major ecological importance. However, nitrogen was found to be transferred much more freely than phosphorus between P. lanceolata plants. Situations are discussed in which there could be a source-sink relationship between plants causing net flow of carbon or mineral nutrients from one to the other. If nutrients pass from dying roots to living plants via mycorrhizal links, this could result in preferential nutrient cycling between species that share the same type of mycorrhiza. Some evidence is presented that this does happen.     

A comparison of phosphorus and nitrogen transfer between plants of different phosphorus status

Summary This study has two objections: (1) to compare transfers of phosphorus (³²P) with nitrogen (¹⁵N) from undefoliated and defoliated mycorrhizal P-rich plants to an adjacent mycorrhizal plant and (2) to determine whether the improved nutrient status of a plant growing with a nutrient-rich plant is due primarily to movement of nutrients from roots of its nutrient-rich neighbor (= nutrient transfer) or to reduced nutrient uptake by its nutrient-rich neighbor (=shift in competition). Two plants of Plantago lanceolata were grown in a three-pot unit in which each of their root systems were split, with part in the central shared pot and part by themselves in an outside pot. There were three treatments: (1) no added P; (2) P added in the outer pot to only plant, termed the donor plant, since it might provide P to the companion plant, acting as a receiver; and (3) as in the previous treatment but the P-fertilized donor plant was also clipped. To encourage the formation of hyphal links between roots of the different plants, transfers were determined when root length densities were high (90 to 130 cm cm^-3 soil) and when 56 to 85% of the root length was infected with vesicular-arbuscular mycorrhizae. Phosphorus fertilization enhanced P but not N movement within donor plants. Regardless of treatment, N transfer from donor to receiver plants was an order of magnitude greater than P transfer and in amounts that could potentially affect plant nutrition in very infertile soils. Phosphorus transfer was very small in any of the treatments. Although P fertilization and clipping improved P status of receiver plants, P transfer was not indicated as the main reason for the improved nutrition. A shift in competition between donor and receiver plants was likely the major factor in the shift in nutrition of the receiver plants.     

Excretion of total phosphorus,dissolved reactive phosphorus,ammonia, and urea by Lake MichiganMysis relicta

Rates of nutrient release byMysis relicta in Lake Michigan were measured on five nights at a 45-m station near Milwaukee, Wisconsin, U.S.A., in the summer of 1977. Nocturnal vertical migrations of the mysids were monitored with both echosounder tracings and vertical net tows. Estimates of the total areal dry mass of the mysids ranged from 600 to 1 820 mg m–2. Rates of release of dissolved reactive phosphorus, total phosphorus, ammonia, and urea were measured in dark incubations on shipboard. Excretion experiments were initiated immediately after mysids were collected from each of several vertical net hauls. The depths of maximum mysid densities corresponded approximately with a deep phytoplankton peak located in the vicinity of the thermocline. Semiquantitative demands for N and P by phytoplankton within this peak were obtained from¹⁴C estimates of primary production from a previous study, assuming a constant C:N:P ratio for the algae. These algal nutrient demands were compared to potential N and P release by the mysids to obtain a first approximation of the relative rates of nutrient supply and demand for the field phytoplankton populations. Our analysis indicates that mysids may directly supply about 1–10% of the daily N and P demands of the phytoplankton in the deep peak. However, indirect interactions betweenMysis relicta and other organisms, such as small zooplankton and fishes, could be major factors in nutrient recycling within the metalimnion and subthermocline region of Lake Michigan.Contribution No. 238, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Wisconsin.Contribution No. 238, Center for Great Lakes Studies, University of Wisconsin-Milwaukee, Wisconsin.     

Nutrient transport between ryegrass plants differeing in nutrient status

Summary It is known that nitrogen and phosphorus can be transferred from one living plant to another, but it is not known whether the amounts transferred are large enough to influence the growth of the species in the field. Two Lolium perenne plants were grown per pot of unfertilized soil. During 25 weeks one plant (the donor) in each pot was fed nutrients through leaves; donors in control pots received only water. The receiver plants which shared a pot with the nutrient-fed donors increased significantly, compared with the controls, in leaf number and concentration of phosphorus, though not in concentration of nitrogen, potassium, calcium or magnesium. The rate of phosphorus transfer agreed well with previous results using ³²P. The results are compared with rates of phosphorus uptake in the field. It is concluded that nutrient transfer could have an influence on the balance between coexisting species in the field, but probably the influence will be small.     

Boundaries between soil compartments formed by microporous hydrophobic membranes (GORE-TEXR) can be crossed by vesicular-arbuscular mycorrizal fungi but not by ions in the soil solution

Various container systems have been described in which soil regions available to hyphae only are separated from the mycorrhizal root region by 30–60 m mesh screens to study nutrient exchange between plants and fungi in the mycorrhizal symbiosis. The screens designed up to now prevent penetration by roots but allow easy passage of fungal hyphae as well as diffusion or mass flow of water and nutrient solutions. We tested hydrophobic microporous polytetrafluorethylene (PTFE) membranes (GORE-TEX^R) with 5 to 15 m diameter pores in an attempt to obtain a better seal between compartments and to prevent uncontrolled nutrient transport by diffusion or mass flow. We found that these membranes completely prevented diffusion or mass flow of ions between two soil compartments but could be penetrated easily by the vesicular-arbuscular mycorrhizal fungus Glomus mosseae, as demonstrated by the rapid colonization of soybean roots (Glycine max L.) from an inoculum across the membranes.     

Carbon discrimination,water-use efficiency,nitrogen and phosphorus nutrition of the host / mistletoe pair Eucalyptus behriana F. Muell and Amyema miquelii (Lehm. ex Miq.) Tiegh. at permanently low plant water status in the field

Summary Under conditions where both plants had permanently low water status, the mistletoe, Amyema miquelii (Lehm. ex Miq.) Tiegh., had lower nitrogen contents in leaf tissue than its host, Eucalyptus behriana F. Muell. The parasite transpired less than its host which is consistent with the hypothesis that mistletoe transpiration acts as a nitrogen gathering mechanism. Nitrogen and phosphorus contents were generally low in both plants; they were positively correlated, and mistletoes reduced nutrient contents of infested hosts. The carbon discrimination ratio, ¹³C (a measure of water-use efficiency) of each plant was within the range reported for other mistletoes and their hosts. Although it did not differ significantly between host and parasite it indicated lower water-use efficiency in the mistletoe. For the nitrogen content of host leaves the gradient within the pair, (¹³C), is much lower compared to the correlation given by Ehleringer et al. (1985). It is concluded that at permanently low water status on nitrogen and phosphorus deficient soils a water-saving strategy accompanied with slow growth is more appropriate for both mistletoe and host.     

Moisture retention properties of a mycorrhizal soil

The water relations of arbuscular mycorrhizal plants have been compared often, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affects soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by (1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (_m) began to decline, changes in _m per unit change in soil water content were smaller in mycorrhizal than in the two nonmycorrhizal soils. Within the range of about –1 to –5 MPa, the mycorrhizal soil had to dry more than the nonmycorrhizal soils to reach the same _m. Soil characteristic curves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly more water stable aggregates and substantially higher extraradical hyphal densities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root length, root surface area and root volume densities were similar in mycorrhizal and nonmycorrhizal soils.     

Impacts of invading N2-fixing Acacia species on patterns of nutrient cycling in two Cape ecosystems: evidence from soil incubation studies and 15N natural abundance values

This study examines the impacts of woody, N₂-fixing invasive Acacia spp. on the patterns of nutrient cycling in two invaded ecosystems of differing nutrient status in the Cape floristic region. Patterns of soil nutrient mineralization were measured by a field incubation method while the significance of the fixation process in altering nutrient cycling was assessed by the ¹⁵N natural abundance technique. The results confirm earlier reports that invasion by woody shrubs results in organic matter and nutrient enrichment of surface soils of both ecosystems. However, patterns of nutrient availability (phosphorus and nitrogen) were not necessarily enhanced. In the more fertile strandveld both phosphorus and nitrogen (significant at P<0.10) showed trends towards enhanced annual mineralization rates upon invasion, while in the low nutrient fynbos system only phosphorus followed this trend. It is unclear whether this differential response is a consequence of plant- or soil-derived feedbacks on the decomposition processes in each system. The ¹⁵N values of the soils from the invaded sites of both ecosystems indicated a strong influence of the alien species on the soil nitrogen component. However, as with other studies of natural ecosystems, the contribution of nitrogen from fixation could not be readily quantified with the ¹⁵N natural abundance method because of problems in selecting suitable non-N₂-fixing reference plants. A technique of disrupting nodule structure and function, by fumigation with O₂, to obtain the ¹⁵N value of a non-N₂-fixing speciment of the study species was tried and found to overcome some of the problems associated with the lack of suitable reference plants. With this technique it was possible to detect the almost total dependence of A. saligna on N₂-fixation in the fynbos soils with their low nitrogen mineralization rates. In the strandveld ecosystem with much higher soil nitrogen release rates A. cyclops was only partly dependent on fixation (about half) for its nitrogen. The nutrient enrichment of both ecosystems and trends towards enhanced rates of nutrient mineralization could have profound implications on the long-term success of alien invader clearing operations and the restoration of the indigenous flora at these sites.     

Evaluation of structural and physiological plant characteristics in relation to the distribution of cadmium in maize inbred lines

To establish the structural and physiological characteristics related to the genotypic variation in Cd distribution between maize inbred lines (shoot Cd excluders and non-shoot Cd excluders), shoot and root morphological parameters were studied on plants grown in nutrient solution. Furthermore, the xylem sap composition and the desorbability of Cd from roots of these inbreds have been compared. No relationship between the morphological characteristics of either shoot (specific leaf area and leaf area ratio) or root (specific root length, specific surface area, and average diameter) and Cd distribution could be assessed. Cadmium concentrations in the xylem exudates from non-shoot Cd excluders were higher than those from shoot Cd excluders, but not related to citrate and malate concentrations. The absolute and relative amounts of Cd desorbed from roots of shoot Cd excluders were about twice as high compared to those of the non-shoot Cd excluders, especially at the lowest Cd concentration in solution. The absence of a relationship between shoot or root morphological parameters and Cd partitioning and the differences between both groups in the amounts of Cd desorbed, even at similar root Cd concentrations, indicate that the differential Cd distribution between shoot Cd excluders and non-shoot Cd excluders may be related to the observed differences in root Cd concentration, desorption characteristics and binding capacity of Cd inside and/or outside the root and its distribution within the roots.     

Mutagenic and membranal effect of a phytotoxic molecule isolated from olive leaves parasitized by the fungusCycloconium oleaginum Cast

A phytotoxic substance (C₂₃H₄₄O₃) which is named Substance A, was purified from olive leaves infected withCycloconium oleaginum Cast. The mutagenic effect of this substance was detected using TA 100 and TA 102 strains ofSalmonella in the Ames test usingBacillus subtilis strains M45 rec^–, H17 rec⁺ in the rec assay. Another substance manifesting the mutagenic effect was found in the extract from theCycloconium oleaginum culture. This substance was not detected in the extract from contaminated olive leaves. Substance A increased electrolytes leakage from tissue of olive leaves, thus manifesting its phytotoxicity.     

Analysis of the cell cycle in an arbuscular mycorrhizal fungus by flow cytometry and bromodeoxyuridine labelling

Summary The cell cycle of an arbuscular mycorrhizal fungus,Glomus versiforme, was determined by flow cytometric analysis of nuclei isolated from spores and mycorrhizal roots of leek, and by immunogold staining after bromodeoxyuridine (BrdU) uptake by DNA. The aims of our work were to establish: (i) whether there are changes in ploidy during fungal growth and morphogenesis, (ii) when and where the cell cycle is activated. Our results demonstrate that nuclei isolated from quiescent spores ofG. versiforme are arrested in the GO/G1 phase (99.2%), whereas fungal nuclei from mycorrhizal roots are in the synthetic (S) (10.1%) and G2/M phase (3.9%). Nuclei undergoing DNA synthesis were detected in situ after BrdU uptake. Labelled nuclei were observed in intercellular hyphae and in large arbuscular trunks. This paper demonstrates that colonization of an arbuscular mycorrhizal fungus is linked to activation of its cell cycle.Abbreviations AM fungi arbuscular mycorrhizal fungi - BrdU 5-bromo-2-deoxyuridine - PI propidium iodide - DAPI 4,6-diamidino-2-phenylindole     

Sensitive 1H-31P correlations with 5′ methylene protons of DNA via homonuclear double-quantum coherence

A novel pulse sequence is presented for the correlation of 5 and 5 protons in DNA with phosphorus. Double-quantum coherence between the methylene protons is used to generate ¹H5-³¹P and ¹H5-³¹P cross peaks in an HMQC-type experiment. The resolution for these cross peaks is significantly improved over that of conventional HSQC experiments, as cross peaks between ¹H4 and ³¹P are largely suppressed and a 3D version of the experiment can be performed with little penalty in sensitivity. In addition, sensitivity is favoured by slower relaxation of the double-quantum coherence and a more favourable multiplet fine structure in the acquisition dimension.     

The role of mycorrhizal infection in the resistance of Vaccinium macrocarpon to manganese

The role of mycorrhizal infection in the resistance of Vaccinium macrocarpon to manganese was investigated in perlite culture containing nutrient solution amended with Mn at 0, 250, 500 or 1000 g/ml. Shoot and root dry weights of the mycorrhizal plants were higher than nonmycorrhizal plants. The mycorrhizal plants produced significantly longer main roots than the nonmycorrhizal plants. Differences between shoot and root Mn concentrations of mycorrhizal and nonmycorrhizal plants arose by reduction of Mn in the leaves of mycorrhizal plants and a corresponding increase in root tissues.     

Contact with ballotini (glass spheres) stimulates exudation of iron reducing and iron chelating substances from barley roots

Contact between roots and Fe-containing solid substrate is known to facilitate acquisition of iron by plants, but the actual mechanism of this contact effect is not yet clear. This study was undertaken to evaluate the effect of root contact with ballotini (glass spheres) on exudation of substances capable of reducing or chelating insoluble Fe(III) compounds by the roots of barley (Hordeum vulgare L. cv. Europa) seedlings. Seedlings with roots encountering mechanical impedance (i.e., in contact with ballotini) produced more lateral roots than the seedlings with unimpeded (i.e., freely suspended) roots in the nutrient solution. Nutrient solution bathing roots in contact with ballotini showed higher concentrations of Fe(III)-chelating (83% on day 7) and Fe(III)-reducing (107% on day 12) substances than solutions bathing unimpeded roots. The pH of all solutions rose continuously during the course of the experiment but was always lower (by a nonsignificant degree) in the solutions with roots in contact with ballotini than in those with unimpeded roots. The data indicate that under natural soil conditions the amount of Fe-chelating and Fe-reducing root exudates may be higher than is usually measured from roots of terrestrial plants artificially suspended in nutrient solution.     

Direct assessment of symbiotically fixed nitrogen in the rhizosphere of alfalfa

Rhizodeposition has been proposed as one mechanism for the accumulation of significant amounts of N in soil during legume growth. The objective of this experiment was to directly quantify losses of symbiotically fixed N from living alfalfa (Medicago sativa L.) roots to the rhizosphere. We used ¹⁵N-labeled N₂ gas to tag recently fixed N in three alfalfa lines [cv. Saranac, Ineffective Saranac (an ineffectively nodulated line), and an unnamed line in early stages of selection for apparent N excretion] growing in 1-m long polyvinylchloride drainage lysimeters in loamy sand soil in a greenhouse. Plants were in the late vegetative to flowering growth stage during the 2-day labelling period. We determined the fate of this fixed N in various plant organs and soil after a short equilibration period (2 to 4 days) and after one regrowth period (35 to 37 days). Extrapolated N₂ fixation rates (46 to 77g plant^–1 h^–1) were similar to rates others have measured in the field. Although there was significant accretion of total N in rhizosphere compared to bulk soil, less than 1% was derived from newly fixed N and there were no differences between the excreting line and Saranac. Loss of N in percolate water was small. These results provide the first direct evidence that little net loss of symbiotically-fixed N occurs from living alfalfa roots into surrounding soil. In addition, these results confirm our earlier findings, which depended on indirect ¹⁵N labelling techniques. Net N accumulation in soil during alfalfa growth is likely due to other processes, such as decomposition of roots, nodules, and above ground litter, rather than to N excretion from living roots and nodules.     

Effects of nutrient addition on growth and rhizosphere pH of Leucadendron ‘Safari Sunset’

A greenhouse experiment was conducted to establish the optimal fertilization management for Leucadendron Safari Sunset (Leucadendron salignum × L. laureolum). Addition of NPK and micronutrient fertilizer or raising phosphorus concentration by itself increased total fresh weight and improved plant growth. Elevating the total nutrients or P concentration increased N and P concentrations. Clusters of proteoid roots were present along the root system of plants irrigated with tap-water, whereas few proteoid roots developed on plants irrigated with complete nutrient solution when only P was omitted, and none developed in any of the other treatments with P present.The pH in leachate of plants irrigated with tap water or with high nutrient levels (100, 20 and 100 mg L^-1 of N, P and K, respectively) was consistently lower compared with leachate without plants. Increasing concentration resulted in a significant change in the pH values (pH in leachate with plant – pH in leachate without plant) from negative to positive. This phenomenon is probably related to rhizosphere effects due to plant and microbial activity.     

A comparison of mineral uptake and translocation by above-ground and below-ground root systems ofSalix syringiana

Nutrient uptake and translocation by above-ground adventitious roots and below-ground roots of woodySalix syringiana saplings were studied with gamma spectrometry. Each of four radionuclides (⁷⁵Se,¹³⁸Cs,⁵⁴Mn, and⁶⁵Zn) administered to adventitious and belowground roots were detected in stems and leaves within one month. Nuclides tended to be immobilized in the leaves and branches closest to the adventitious roots that absorbed them, while nuclides absorbed from below-ground sources were distributed more evenly throughout the plant. The capacity of adventitious roots to acquire nutrients from above-ground sources suggests they function as a potential auxiliary pathway of nutrient uptake and might enhance plant nutrient status where below-ground root uptake it hindered by adverse soil conditions.     

Mannosyl-β(1-4)-N-acetylglucosaminyl-β(1-N)-urea,a compound isolated from the urine of patients with β-mannosidosis

A previously unknown substance, mannosyl-(1–4)-N-acetylglucosaminyl-(1-N)-urea, has been isolated from the urine of patients with -mannosidosis in addition to the main metabolite mannosyl-(1–4)-N-acetylglucosamine. Structural investigation was carried out by fast atom bombardment mass spectrometry and high-resolution¹H-nuclear magnetic resonance spectroscopy at 500 MHz. It was postulated that the occurrence of this carbohydrate-urea conjugate in urine results mainly from urine handling.