Sand Culture System Research Articles

Feasibility of Irrigating Pickleweed (Salicornia bigeloviiTorr) with Hyper‐saline Drainage Water

Reuse of drainage water (DW) for irrigation reduces the volume of DW requiring treatment or disposal. We conducted a greenhouse study to evaluate the performance of the halophyte Salicornia bigelovii Torr. when irrigated with hyper-saline DW and seawater (SW) treatments, ranging from 1/3 strength to full strength (18-49 dS m(-1)), in a sand-culture system. Results indicate that Salicornia grows well over the entire range of iso-osmotic SW and DW salinity treatments. Moreover, when boron (B) was added to SW treatments to concentrations equivalent to that of corresponding 1/3- and 2/3-strength DW treatments (i.e., 9 and 17 mg L(-1)), growth was not affected, and tissue B concentrations were <150 mg kg(-1) dry wt. However, when plants were irrigated with synthetic DW where B was reduced to solution culture levels (0.5-1.0 mg L(-1)), plants generally performed worse than when irrigated with actual DW high in B at the same salinity level. Evapotranspiration (ET) rates exceeded that lost from an evaporation pan from 1.5 to 2.5 times. Using a method accounting for changes in the isotopic signature of water in the reservoir due to evaporation, we estimated that high ET rates were due primarily to high transpiration rates (>78% of ET). The salt content in the tissue was very high (ash content 43-52%), but ionic composition in the shoot tissue reflected that of the treatment water used to irrigate the plants. These data indicate that hyper-saline DW, characteristic of California's San Joaquin Valley, can be used to irrigate Salicornia and substantially reduce drainage volumes.

Evidence from near-isogenic lines that root penetration increases with root diameter and bending stiffness in rice.

Deep rooting can be inhibited by strong layers, although there is evidence for species and cultivar (cv.) differences in their penetration ability. Here, the availability of near-isogenic lines (NILs) in rice (Oryza sativa L.) was exploited to test the hypothesis that increased root diameter is associated with greater root bending stiffness, which leads to greater root penetration of strong layers. Wax/petrolatum discs (80% strong wax) were used as the strong layer, so that strength can be manipulated independently of water status. It was found that good root penetration was consistently associated with greater root diameter and bending stiffness, whether comparisons were made between cvs or between NILs. With NILs, this effect was seen with 'research' lines bred from recombinant inbred lines of a cross between cvs Bala and Azucena and also in improved lines developed from cv. Kalinga III by introgression of parts of the genome from Azucena. Much of the bending behaviour of roots could be explained by treating them as a simple cylinder of material. In both wax disc and sand culture systems, roots that had encountered a strong layer had lower bending stiffness than roots that had not encountered a strong layer which is a novel result and not previously reported.

Growth Response of Various Perennial Grasses to Increasing Salinity

ABSTRACT Although the effect of salinity on plant growth has been the focus of a substantive research effort, much of this research has failed adequately to separate the various growth-limiting aspects of salinity; thus, the results are confounded by multiple factors. Eight perennial grass species were grown in a sand-culture system dominated by sodium chloride (NaCl) [electrical conductivities (ECs) between 1.4 and 38 dS m−1], with sufficient calcium (Ca) added to each treatment to ensure that Na-induced Ca deficiency did not reduce growth. Of the eight perennial grass species examined, Chloris gayana cv. ‘Pioneer’ (Rhodes grass) was the most salt-tolerant species, while Chrysopogon zizanioides cv. ‘Monto’ (vetiver) was of only moderate tolerance. However, observed salinity tolerances tended to be lower than those expected from published values based on the threshold-salinity (bent-stick) model. This discrepancy may be due in part to differences in the evapotranspirational demand between studies; i.e., an increase in demand accelerates the accumulation of sodium (Na) in the shoots and hence decreases apparent salinity tolerance. It was also observed that the use of a non-saline growth period (to allow seed germination and establishment) results in the overestimation of vegetative salinity tolerance if not taken into consideration. This situation is particularly true for species of low salt tolerance, due to their comparatively rapid growth in the non-saline medium compared with growth at full salinity.

Nitrogen Requirement of Prosopis velutina Seedlings

The objective of this study was to determine whether mesquite (Prosopis velutina) seedlings have a preference for the ammonia or nitrate form of nitrogen (N), and to determine the optimum rate of N to maximize growth and minimize N leaching when seedlings are grown in different substrates. Mesquite seedlings were fertigated with different ratios of NH4+: NO3- to determine effects on shoot and root growth and N-uptake efficiency. Nutrient solution containing 67% NH4+ : 33% NO3- resulted in greatest biomass after 120 days of fertigation. N leachate remained stable until 12 weeks after the onset of treatment, but increased significantly by week 16. Subsequently, mesquite seedlings were grown in sand or soilless media and were fertigated with a solution of 67 % NH4+: 33% NO3- at a rate of 25, 50, 100, or 200 mg·L-1 of N. After 60 days, plants in media produced 41% more leaves and total biomass compared to those in sand. Leaf number was greatest for plants grown at 200 mg·L-1 of N in both substrates. Root biomass of plants in media showed no response to increasing N concentrations while root biomass of seedlings in sand were similar for the three lower N concentrations and nearly doubled for the highest one. Shoot biomass of seedlings receiving 25, 50, or 100 mg·L-1 of N was similar, but more than doubled for plants fertigated with 200 mg·L-1 of N. N leachate losses were highest from seedlings growing in sand and receiving the two higher N fertigations, those in media had greatest N leachate loss when fertigated at 200 mg·L-1 of N. For balanced mesquite seedling growth and minimum N leaching losses, concentrations between 50 to 100 mg·L-1 of N are recommended. Implications of using a sand culture system vs. soilless growing substrate for nutrition studies will be discussed.

Critical Tissues for Nutrient Diagnostics and Optimal Nutrients for Enhancing Yield of Processing Carrots

Carrot (Daucus carota L. var sativus) production in Nova Scotia is challenging as carrots are grown under cool temperatures, rainfed conditions, and in mineral soils usually of low fertility. Growers must rely on fertilizer inputs to optimize yields. Excess application can result in high costs and may lead to soil and environmental problems. There is no up-to-date solidly-based, fertilizer recommendation available for carrot production in Nova Scotia. A greenhouse trial was conducted to identify the critical tissue(s) at various growth stages and optimal tissue nutrient concentrations for yield and quality. This will provide a diagnostic tool for assessing plant nutrient health and the opportunity to correct nutrient deficiencies to prevent yield losses, as well as provide an up-to-date fertilizer recommendation. Dicer carrot seeds, variety Red Core Chatenay were grown in sand culture system that used a gravity-fed drip irrigation system. Nine fertility treatments consisting of a complete 20-20-20 plus micronutrients fertilizer was used to deliver at 0, 50, 100, 150, 200, 250, 300, 350, and 400 ppm equivalent of N, P, and K. Soil and plant tissue samples were taken at 4 and 9 weeks and at final harvest at 13 weeks. Critical tissues varied for each element studied at each of the growth stages. Results suggest 0 and 50 ppm treatments did not provide enough fertilizer to obtain maximum growth while plants receiving above 300 ppm were found to be more susceptible to disease. The treatment with 100 ppm N, P, and K was optimal, being significantly higher in yield and quality than all treatments except 150 ppm.

Fertility and pH effects on polyphenol and condensed tannin concentrations in foliage and roots

Plant secondary compounds such as polyphenols and tannins are purported to influence nutrient cycling by affecting organic matter degradation, mineralization rates, N availability and humus formation. In turn, site conditions have been shown to affect plant phenolic concentrations. In a greenhouse sand culture system we investigated the influence of fertility and pH on foliar and fine root polyphenol and tannin concentrations of two pines and three ericaceous species that grow in the pygmy forest of coastal California. Higher fertility levels increased plant growth up to 10-fold while N concentrations increased 1.2–2.4 fold in foliage and roots. Concentrations of foliar total phenols (TP) and condensed tannins (CT) ranged between 130–300 mg g−1 and 100–320 mg g−1 on a dry weight basis, respectively. Root TP and CT concentrations were lower than corresponding foliar levels, ranging between 60–150 mg g−1 for TP and 80–190 mg g−1 for total CT. As fertilizer levels decreased, concentrations of foliar TP and CT increased 1.2–2.0 fold. Effects of fertility on root TP and CT concentrations were not always significant and not as great as those seen in the foliage (0–33% increase under low versus high fertility). Effects of pH level on plant growth and chemical composition were less consistent, and often confounded by an interaction with fertility. Foliar C:N, TP:N and CT:N ratios, which may be predictors of litter quality, were significantly higher under low fertility than high fertility. These ratios were generally higher at pH 6.5 than pH 3.5. Similar trends were obtained from a greenhouse soil study which included lime and fertilizer treatments. Our results indicate that changes in soil nutrient supply have a greater influence on secondary compound production than do changes in solution pH.

Involvement of cytokinin in the stimulation of nodulation by low concentrations of ammonium inPisum sativum

Nodulation in pea (Pisum sativumL.) grown in hydroponic and sand culture systems is stimulated by low concentrations (&lt;1.0 mM) of ammonium, but the physiological mechanisms underlying this stimulation are unknown. The current study involves a series of experiments, which investigate if the ammonium‐induced stimulation of nodulation involves changes in endogenous hormone (auxin and cytokinin) levels.P. sativumL. cv. Express was grown in growth pouches for 1 week with mineral N (0.5 and 2.0 mMNH4+or NO3–) or for 3 weeks exposed to exogenous indole‐3‐acetic acid (IAA) or 6‐benzylaminopurine (BAP) at a range of concentrations (10‐9−10‐5 M). Ammonium enhanced nodulation on the basis of both early whole plant (nodules plant−1) and specific nodulation (nodules g−1root DW), especially in 0.5 mMtreatment in which nodulation was approximately 4‐fold of the mineral‐N‐free control 1 week after inoculation. Correspondingly, the roots treated with ammonium contained much higher levels oft‐zeatin (Z) and lowert‐zeatin riboside (ZR) than that the control or nitrate‐treated plants. There was no significant difference in IAA levels between the control and ammonium treatments. Exogenous application of BAP for 3 weeks at concentrations of 10‐7−10‐5 Mstrongly inhibited nodulation. However, 10−9 MBAP, but not IAA, significantly enhanced nodulation. These data support the theory that a relatively high ratio of cytokinin:auxin in roots is favourable for nodule initiation, but that an excessively high level of cytokinin inhibits nodulation. Based on these results we propose that stimulation of nodulation by low concentrations of ammonium may be mediated through increasing Z level in roots, which alters the balance of cytokinin and auxin, which in turn induces cortical cell divisions leading to nodule initiation.

Functional analysis of beet necrotic yellow vein virus (BNYVV) RNA4 in fungal transmission

Full-length RNA4 of beet necrotic yellow vein virus (BNYVV) and its mutants, including a frame-shift and deletions within the coding region, were inserted into a transcription plasmid. Transcripts derived from these plasmids and RNAs extracted from a BNYVV isolate containing only RNAs 1, 2 and 3 were coinoculated to Tetragona expansa. Then these recombinant isolates were inoculated to sugar beet plants grown in a sand culture system containing Polymyxa betae that had been freed of virus particles to test efficiency of BNYVV transmission. The result showed that the efficiency of BNYVV transmission among the plants by Polymyxa betae was highly decreased by different deletions in the coding region of RNA4, but not affected by the frame-shift mutant with four nucleotides insertion. The observations suggest that at least some of the RNA4 sequence encompassing the deleted region of 580 nucleotide acids (525 –1105 nt) are necessary for high efficient transmission of the virus by the fungus.

Buckwheat ( Fagopyrum esculentum Moench) has high capacity to take up phosphorus (P) from a calcium (Ca)-bound Source

Two experiments were carried out in a growth chamber to investigate the phosphorus (P)-uptake efficiency of Fagopyrum esculentum Moench (buckwheat) and Triticum aestivum (spring wheat) from a Ca-bound form. The first experiment was based on a sand-culture system with either rock phosphate (RP) or CaHPO4 (CaHP) as the P source and nitrate or ammonium nitrate as nitrogen source. A highly calcareous soil was used in the second experiment. Buckwheat was shown to be highly efficient in taking up Ca-bound P compared to spring wheat. When plants were supplied with nitrate, the total P uptake by buckwheat from RP was nearly 10-fold higher than that of spring wheat (20.1 compared with 2.1 mg P pot−1). Changing nitrogen source from nitrate only to ammonium nitrate increased P uptake by spring wheat substantially, but not buckwheat. High P-uptake efficiency of buckwheat was also demonstrated using the field soil, but to a lesser extent, which may be related to the difference in Zn supply between sand culture and field soil. It is suggested that buckwheat may be included in intercropping or crop rotation systems to activate P sources in calcareous soils. The principal mechanism of P uptake efficiency of buckwheat may be its ability to acidify the rhizosphere; however, further study is needed to unravel the regulation of root excretion of H+ and its molecular basis in order to exploit buckwheat's genetic capability to utilise sparingly soluble P from soil.

An in vitro approach to investigate medicinal chemical synthesis by three herbal plants

Ever since regulatory changes introduced herbals into mainstream supermarkets and pharmacies, there has been an explosion of demand for herbal plants and extracts which can be used to improve human health and well being. Science still lacks a basic mechanistic understanding of how environmental triggers regulate phytochemical accumulation, but this gap can be bridged using in vitro models to examine herbal species responses. For St. John's wort (Hypericum perforatum), uniform in vitro shoot cultures were set up as a parallel to a previously established sand culture system for investigation of physical and chemical environmental factors that control hypericin accumulation. Cytokinin supplementation of shoot culture medium resulted in a proliferation of abundant leaf glands with enhanced levels of hypericin, as compared to controls. Cell cultures of echinacea (Echinacea angustifolia) were established, and hydrophilic pharmacological components (caffeic acid derivatives) were detected. A protocol of rigorous explant pretreatment, and use of newly emerging vegetative shoots permitted establishment of axenic kava (Piper methysticum) callus, which was used to regenerate roots (organogenesis). Kavapyrone synthesis was achieved in both undifferentiated cell cultures and in cultured roots, although at lower levels than found in in vivo root systems. The predominance of kavain and methysticin in both forms of the in vitro cultures was parallel to the relative proportions from kava roots in vivo. The cell and organ cultures of all three herbal medicinals provide advantageous, easily-manipulated models to decipher environmental controls of phytochemical biosynthesis.

Differential effects of light and nitrogen on production of hypericins and leaf glands in Hypericum perforatum

The effect of light intensity and root nitrogen supply on the levels of leaf hypericins was examined for St. John’s wort ( Hypericum perforatum L.) grown in a sand culture system with artificial lighting. Increasing the light intensity illuminating St. John’s wort plants from 106 to 402 μmol·m –2·s –1 resulted in a continuous increase in the level of leaf hypericins. Using a leaf dissection approach, the association of hypericins with the dark glands on the leaves was shown, and it was found that increasing light intensity resulted in a parallel increase in the number of dark glands. In this respect, a linear relationship was observed between leaf gland number and the level of leaf hypericins ( R = 0.901). While a decrease in nitrogen supply to St. John’s wort plants also yielded an increase in the level of leaf hypericins, this response occurred in a discontinuous manner over the range of nitrogen levels tested and no significant effect upon the number of dark leaf glands was observed. Overall, these effects of increased light intensity and decreased nitrogen supply on leaf hypericins appear to be independent and additive, and may reflect differences in the sites and processes where these environmental parameters impact production of these phytochemicals.

Arbuscular mycorrhizal fungi alter phosphorus relations of broomsedge (Andropogon virginicus L.) plants

Broomsedge (Andropogon virginicus L.) is a dominant grass revegetating many abandoned coal-mined lands in West Virginia, USA. Residual soils on such sites are often characterized by low pH, low nutrients, and high aluminium. Experiments were conducted to assess the resistance of broomsedge to limited phosphorus (Pi) availability and to investigate the role that arbuscular mycorrhizal (AM) fungi play in aiding plant growth under low Pi conditions. Pregerminated mycorrhizal and non-mycorrhizal seedlings were grown in a sand-culture system with nutrient solutions containing Pi concentrations ranging from 10 to 100 microM for 8 weeks. Non-mycorrhizal plants exhibited severe inhibition of growth under Pi limitation (<60 microM). Colonization by AM fungi (combined Glomus clarum Nicolson & Schenck and Gigaspora gigantea (Nicol. & Gerd.) Gerd. & Trappe) greatly enhanced host plant growth at low Pi concentrations, but did not benefit growth when Pi was readily available (100 microM). In comparison to non-mycorrhizal plants, mycorrhizal plants had higher phosphorus use efficiency at low Pi concentrations and maintained nearly constant tissue nutrient concentrations across the gradient of Pi concentrations investigated. Manganese (Mn) and sodium (Na) accumulated in shoots of non-mycorrhizal plants under Pi limitation. Mycorrhizal plants exhibited lower instantaneous Pi uptake rates and significantly lower C(min) values compared to non-mycorrhizal plants. These patterns suggest that the symbiotic association between broomsedge roots and AM fungi effectively maintains nutrient homeostasis through changes in physiological properties, including nutrient uptake, allocation and use. The mycorrhizal association is thus a major adaptation that allows broomsedge to become established on infertile mined lands.

Interdependence of phosphorus, nitrogen, potassium and magnesium translocation by the ectomycorrhizal fungus Paxillus involutus.

• Translocation is shown of phosphorus, nitrogen, potassium and magnesium to a P-deficient host from ectomycorrhizal fungal hyphae. • Mycorrhizal (with Paxillus involutus) and nonmycorrhizal P-deficient spruce (P. abies) seedlings were grown in a two-compartment sand-culture system. Hyphal translocation of nutrients from the inner compartment (penetrated only by hyphae) to the host was measured using mass balance (for N, P and K) or stable isotope (15 N and 25 Mg) methods. • Addition of P to the hyphal compartment strongly stimulated hyphal growth, and this also increased both seedling P status and growth. Hyphae translocated nonlimiting elements in addition to P, contributing 52, 17, 5 and 3-4%, respectively, to total P, N, K or Mg plant uptake. The potential role of the ectomycorrhizal mycelium in K acquisition was demonstrated. Translocation to mycorrhizal seedings of N, K and Mg was strongly reduced when hyphal P-fluxes ceased; this translocation of nonlimiting nutrients depended on simultaneous translocation of P. • The ectomycorrhizal mycelium has an active role in P acquisition from sources not available to roots. Nutrient fluxes within fungal hyphae are interdependent and strong coupling of N, K and Mg fluxes with long-distance P translocation in the mycorrhizal mycelium occurs.

Nitrogen limitation in mycorrhizal Norway spruce (Picea abies) seedlings induced mycelial foraging for ammonium: implications for Ca and Mg uptake

Although many studies support the importance of the external mycelium for nutrient acquisition of ectomycorrhizal plants, direct evidence for a significant contribution to host nitrogen nutrition is still scarce. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus (Batsch) Fr. in a sand culture system with two compartments separated by a 45-μm Nylon mesh. Hyphae, but not roots, can penetrate this net. Nutrient solutions were designed to limit seedling growth by nitrogen. Hyphal density in the hyphal compartment, host N status and shoot growth of mycorrhizal seedlings significantly increased in response to NH4 + addition to the hyphal compartment. Labeling the compartment only accessible to hyphae with 15NH4 + showed that the increase in N uptake in the mycorrhizal seedlings was a result of hyphal N acquisition from the hyphal compartment. These results indicate that hyphae of P. involutus may actively forage into N-rich patches and improve host N status and growth. In the mycorrhizal seedlings, which received additional NH4 + via their external mycelium, the increase in NH4 + supply less negatively affected Ca and Mg uptake than in nonmycorrhizal seedlings, where the additional NH4 + was directly supplied to the roots. This was most likely due to the close link of NH4 + uptake and H+ extrusion, which, in the nonmycorrhizal seedlings, lead to a strong acidification in the root compartment, and subsequently reduced Ca and Mg uptake, whereas in the mycorrhizal seedlings the site of intensive NH4 + uptake and acidification was in the hyphal and not in the root compartment. Our data support the idea that the ectomycorrhizal mycelium connected to an N-deficient host may actively forage for N. The mycelium may also be important as a biological buffer system ameliorating negative influence of high NH4 + supply on cation uptake.

Significance of root exudates in acquisition of heavy metals from a contaminated calcareous soil by graminaceous species

Graminaceous species release phytosiderophores for mobilization of iron (Fe) under Fe deficiency stress. In this study the possible risk that these root exudates might also enhance the uptake of toxic heavy metals from contaminated soils, was investigated. For this purpose wheat (Triticum aestivum L. cv. Piko) and sorghum (Sorghum bicolor (L.) Moench cv. E‐610), differing in release of phytosiderophores were precultured with roots in nylon bags in a sand culture system for low and high Fe nutritional status (+/‐Fe supply) for two weeks. After this preculture the nylon bags were brought in contact with a sewage sludge or heavy metal salt‐contaminated calcareous soil in a climate chamber for an additional 8 days growth period. At harvest the concentration of Fe, Zn, Ni and Cd in roots and shoots were analysed. The results clearly show that both plant species have mobilized Fe and heavy metals from the contaminated soil. Plants precultured for low Fe nutritional status had a higher uptake of Fe, Zn, Ni and Cd (up to 200%) than control plants with adequate Fe nutritional status. This enhanced acquisition of heavy metals was particularly expressed in wheat as an Fe efficient graminaceous species with corresponding higher release of phytosiderophores under Fe deficiency stress than sorghum.

Fate of 2,4,6‐trinitrotoluene in axenic sand culture systems containing smooth bromegrass

AbstractPlants have the potential to metabolize the munition 2,4,6‐trinitrotoluene (TNT) in contaminated soils, sediments, and natural waters. However, microbial interference must be eliminated to demonstrate an intrinsic capacity for the metabolism of TNT by plants. An axenic sand culture system for smooth bromegrass (Bromus inermis Leyss) was established to investigate the influence of TNT on smooth bromegrass growth and metabolism. Shoot length growth was inhibited by 40% after 14 d of exposure to TNT at 36 mg/L in the sand solution, whereas photosynthetic and respiration rates were similar to controls. Addition of [14C]‐labeled TNT to the system resulted in the roots containing 21.3% and the shoots containing 3.8% of the radioactivity. The [14C]TNT in media that contained plants was reduced about 50% compared to media without plants. Trace amounts (0.03%) of the initial [14C]TNT was converted to 14CO2 during a 5‐d incubation period. The TNT and its metabolites were observed in root and shoot extracts by radiochromatographic analysis. The major TNT metabolites identified were 2‐amino‐4,6‐dinitrotoluene and 4‐amino‐2,6‐dinitrotoluene. Previous exposure of the plants to TNT did not increase TNT metabolism or prevent reduction of shoot length growth. From these results we concluded that smooth bromegrass is capable of taking up and metabolizing TNT.

The mycorrhizal fungus Paxillus involutus transports magnesium to Norway spruce seedlings. Evidence from stable isotope labeling

Although it is well established that ectomycorrhizas improve the mineral nutrition of forest trees, there has been little evidence that they mediate uptake of divalent cations such as Mg. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus Batsch in a sand culture system with two compartments separated by a 45-μm Nylon mesh. Hyphae, but not roots, can penetrate this net. Labeling the compartment only accessible to hyphae with 25Mg showed that hyphae of the ectomycorrhizal fungus Paxillus involutus transported Mg to their host plant. No label was found in nonmycorrhizal control plants. Our data support the idea that ectomycorrhizas are important for the Mg nutrition of forest trees.

Growth and nutrient composition of Ca2+ use efficient and Ca2+ use inefficient genotypes of tomato

The response of two tomato lines (Lycopersicon esculentum Mill. Ca2+ use efficient line 113 and Ca2+ use inefficient line 67) to a range of constant low Ca2+ concentrations was investigated in a sand culture system. Four Ca2+ concentrations were established and maintained throughout the experiment: 0.038, 0.75, 1.51 and 3.75 mM CaCl2 on a constant background of 1.1 mM NaCl. Response to Ca2+ was determined by analysis of growth parameters and of shoot Ca2+, Na+, K+ and Cl− concentrations. Differences in Ca2+ and K+ use efficiencies were expressed as the calcium utilization efficiency ratio, or CaER, and potassium utilization efficiency ratio, or KER, (mg of dry weight produced·mg−1 of Ca2+ or K+ in plant). Dry weight production of line 113 was significantly higher than line 67, and was associated with a higher CaER and KER. The Ca2+ treatments differentially affected shoot Ca2+, Na+, Cl− and K+ concentrations. As expected, shoot Ca2+ and Cl− concentrations increased whereas Na+ concentration decreased with Ca2+ treatments. Line 113 had more than twice the amount of Na+ in shoot tissue than line 67. The K+ to Na+ ratio was twice as high in line 67 than in line 113. No evidence for higher soluble Ca2+ contributing to higher Ca2+ utilization was observed. The relationship between Ca2+ use efficiency and growth was not correlated with higher percentages of soluble Ca2+ in leaf tissue or with differences in root morphology. Differences in Ca2+ use efficiency alone could not explain the higher growth rate in line 113. This study demonstrated that the physiological factors involved in the genetic control of Ca2+ use efficiency should be assessed under a range of constant low Ca2+ concentrations in order to observe the physiological changes taking place. Thus, the use of Ca2+ deficient conditions are to be avoided as it may interfere with the expression of the physiological factors involved in Ca2+ use efficiency.

Ectomycorrhizas and cadmium toxicity in Norway spruce seedlings.

We studied the effects of ectomycorrhizal colonization by Laccaria bicolor (Maire) Orton S238 and Paxillus involutus (Batsch) Fr. 533 on cadmium (Cd) toxicity in Norway spruce seedlings (Picea abies (L.) Karst.). Both mycorrhizal and nonmycorrhizal seedlings were exposed to 0 (control), 0.5 or 5 &mgr;M CdSO(4) for 9 weeks in a sand culture system with frequent addition of nutrient solutions. In pure culture, P. involutus and L. bicolor showed similar Cd tolerance. However, in symbiosis, the Cd treatments decreased colonization by L. bicolor but not by P. involutus. Paxillus involutus ameliorated the negative effects of 0.5 &mgr;M Cd on shoot and root growth and chlorophyll content of old needles, whereas L. bicolor did not. Mycorrhizal colonization did not affect Cd concentrations of old needles and roots of seedlings. Despite differences between the ectomycorrhizal fungi in colonization and ability to alleviate Cd toxicity of seedlings, both species reduced Cd concentrations of young needles to a similar degree compared with nonmycorrhizal seedlings. However, in the 0.5 &mgr;M Cd treatment, the Cd content of needles of seedlings colonized by P. involutus was increased, whereas the Cd content of needles of seedlings colonized by L. bicolor was similar to that of needles of nonmycorrhizal seedings. When the amount of Cd translocated to needles was expressed on a root length basis to account for differences in the size of the root systems, the amount of Cd translocated to the needles was similar in seedlings mycorrhizal with P. involutus and in nonmycorrhizal seedlings. All mycorrhizal seedlings were similarly affected by 5 &mgr;M Cd, indicating that the amelioration efficiency of ectomycorrhizal fungi is dependent on the metal concentration to which the roots are exposed. Concentrations of P, K, Ca, Mg and Mn were decreased by 5 &mgr;M Cd to a similar extent in both nonmycorrhizal and mycorrhizal seedlings. In contrast to L. bicolor, P. involutus increased P uptake and altered patterns of root branching. We conclude that mycorrhizas alleviate Cd-induced reductions in growth of Picea abies seedlings. Although the two mycorrhizal fungi examined differed in their ability to alleviate Cd toxicity, these differences were not related to differences in Cd uptake or translocation to the shoot of the mycorrhizal seedlings. We suggest that amelioration of Cd toxicity by P. involutus may be a result of improved P nutrition or changes in root morphology, or both.

Rhizodeposition and C-partitioning of Lolium perenne in axenic culture affected by nitrogen supply and defoliation

This study investigated the effects of N-supply and partial defoliation on C-partitioning, root morphology and soluble rhizodeposition, for Lolium perenne grown in axenic sand culture systems percolated with nutrient solution. Plants were grown for 36 d in nutrient solutions with differing N concentrations (4 mM or 0.02 mM NH4 +NO3 -), and effects of repeated defoliation to 4 cm were determined. The ‘low N’ supply reduced (P < 0.05) dry matter accumulation, with proportionately increased partitioning to the root systems. Root morphology was also altered at ‘low N’, with development of a finer root system, manifest as increased (P < 0.05) specific root length. Concurrent with these effects on growth of L. perenne, ‘low N’ increased (P < 0.05) exudation of C-compounds from roots on a per g root basis. Defoliation was found to increase exudation (P < 0.05) of soluble compounds for periods of 3-5 d following each cut, at both N-supply rates. The effects of N-supply and defoliation are of importance in understanding the coupling of plant productivity to nutrient cycling in soils with differing N availabilities and for grassland systems which are subject to grazing.