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We report here on results of experimental-theoretical investigation of high-order harmonic generation (HHG) in layers of CdSe semiconductor quantum dots of different sizes and a reference bulk CdSe thin film. We observe a strong decrease in the efficiency, up to complete suppression of HHG with energies of quanta above the bandgap for the smallest dots, whereas the intensity of below bandgap harmonics remains weakly affected by the dot size. In addition, it is observed that the ratio between suppression of above gap harmonics versus below gap harmonics increases with driving wavelength. We suggest that the reduction in the dot size below the classical electron oscillatory radius and the corresponding off the dots wall scattering limits the maximum acceleration by the laser field. Moreover, this scattering leads to a chaotization of motion, causing dephasing and a loss of coherence, therefore suppressing the efficiency of the emission of highest-order harmonics. Our results demonstrate a new regime of intense laser-nanoscale solid interaction, intermediate between the bulk and single molecule response., Comment: 32 pages, 19 Figures

The effect of acid irrigation and liming on fine root growth of Norway spruce (Picea abies [L.] Karst.) was studied in an approximately 80-year-old forest stand in southern Germany (Hoglwald). Root growth was measured mainly on root windows and in addition by soil core sampling. Root growth rate showed a typical pattern in the course of a year with a maximum in August. Acid irrigation depressed root growth rate, whereas liming, particularly in combination with acid irrigation, markedly increased root growth in the humic layer and the upper 0–5 cm of the mineral soil. The treatment effects on root growth in the mineral soil below 5 cm were small and not significant. Root growth rate was not correlated with the concentration of aluminium (Al) or the molar ratio of calcium (Ca) to Al in the soil solution. The results suggest that inhibition of root growth by acid irrigation is a direct effect of high proton concentrations in the irrigation water, and that enhancement of root growth by liming is caused by an improved supply of mineral nutrients and higher biological activity.

The longterm effect of acid irrigation and liming (dolomitic limestone) on the mineral element content of roots of Norway spruce (Picea abies [L.] Karst.) was investigated in an 80-year-old Norway spruce stand in South Germany (Bavaria). Soil cores of four soil depths (humic layer, 0–10, 10–20 and 20–30 cm) were taken over 2 years (August 1991 and August 1992) from six plots with different treatments (control, normal irrigation, acid irrigation solely or in combination with liming) and living short roots selected and analysed for calcium (Ca), magnesium (Mg), manganese (Mn) and aluminium (Al).

The siderophore rhizoferrin, produced by the fungus Rhizopus arrhizus, was previously found to be as an efficient Fe source as Fe-ethylenediamine-di(o-hydroxphenylacetic acid) to strategy I plants. The role of this microbial siderophore in Fe uptake by strategy II plants is the focus of this research. Fe-rhizoferrin was found to be an efficient Fe source for barley (Hordeum vulgare L.) and corn (Zea mays L.). The mechanisms by which these Gramineae utilize Fe from Fe-rhizoferrin and from other chelators were studied. Fe uptake from 59Fe-rhizoferrin, 59Fe-ferrioxamine B, 59Fe-ethylenediaminetetraacetic acid, and 59Fe-2[prime]-deoxymugineic acid by barley plants grown in nutrient solution at pH 6.0 was examined during periods of high (morning) and low (evening) phytosiderophore release. Uptake and translocation rates from Fe chelates paralleled the diurnal rhythm of phytosiderophore release. In corn, however, similar uptake and translocation rates were observed both in the morning and in the evening. A constant rate of the phytosiderophore's release during 14 h of light was found in the corn cv Alice. The results presented support the hypothesis that Fe from Fe-rhizoferrin is taken up by strategy II plants via an indirect mechanism that involves ligand exchange between the ferrated microbial siderophore and phytosiderophores, which are then taken up by the plant. This hypothesis was verified by in vitro ligand-exchange experiments.

The effects of suboptimal root zone temperatures (RZTs) on net translocation rates from the roots to the shoots and the concentrations of Fe, Mn, Zn, and Cu were examined in maize grown in nutrient solution or soil. Plants were grown at 12 °C, 18 °C and 24 °C RZT. At each RZT, the growth-related shoot demand for nutrients was varied by independently modifying the temperature of the shoot base (SBT) including the apical shoot meristem. The net translocation rates of Mn and Zn from the roots to the shoots were reduced at low RZTs, irrespective of the SBT and of the substrate (soil or nutrient solution). Obviously, the net translocation rates of Mn and Zn at low RZT were mainly regulated by temperature effects on the roots and not by the chemical nutrient availability in the rhizosphere or by shoot growth rate as controlled by SBTs. When both RZT and SBT were reduced, the decrease in net translocation rates of Mn and Zn was similar to the decline in the shoot growth rate and concentrations of Mn and Zn in the shoot fresh matter were not greatly affected or were even increased by low RZT. However, at high SBT and low RZT in nutrient solution, the depressed net translocation rates of Mn and Zn combined with the increased shoot growth resulted in significantly decreased concentrations of Mn and Zn in the shoot, indicating that Mn and Zn may become deficient even at high chemical availability. By contrast to Mn and Zn, the net translocation rates of Fe and Cu at all RZTs were markedly enhanced by increased SBTs. Accordingly, the concentrations of Fe and Cu in the shoot fresh matter were not greatly affected by RZTs, irrespective of the SBTs. These results indicate that the ability of roots to supply Fe and Cu to the shoot was internally regulated by the growth related shoot demand per unit of roots.

Two experiments were carried out in pots with three compartments, a central one for root and hyphal growth and two outer ones which were accessible only for hyphae of the arbuscular mycorrhizal fungus, Glomus mosseae ([Nicol. and Gerd.] Gerdemann and Trappe). In the first experiment, mycorrhizal and nonmycorrhizal bean (Phaseolus vulgaris L.) plants were grown in two soils with high geogenic cadmium (Cd) or nickel (Ni) contents. In the second experiment, mycorrhizal and nonmycorrhizal maize (Zea mays L.) or bean plants were grown in a non-contaminated soil in the central compartment, and either the Cd- or Ni-rich soil in the outer compartments. In additional pots, mycorrhizal plants were grown without hyphal access to the outer compartments. Root and shoot dry weight was not influenced by mycorrhizal inoculation, but plant uptake of metals was significantly different between mycorrhizal and nonmycorrhizal plants. In the first experiment, the contribution of mycorrhizal fungi to plant uptake accounted for up to 37% of the total Cd uptake by bean plants, for up to 33% of the total copper (Cu) uptake and up to 44% of the total zinc (Zn) uptake. In contrast, Ni uptake in shoots and roots was not increased by mycorrhizal inoculation. In the second experiment, up to 24% of the total Cd uptake and also up to 24% of the total Cu uptake by bean could be attributed to mycorrhizal colonisation and delivery by hyphae from the outer compartments. In maize, the mycorrhizal colonisation and delivery by hyphae accounted for up to 41% of the total Cd uptake and 19% of the total Cu uptake. Again, mycorrhizal colonisation did not contribute to Ni uptake by bean or maize. The results demonstrate that the arbuscular mycorrhizal fungus contributed substantially not only to Cu and Zn uptake, but also to uptake of Cd (but not Ni) by plants from soils rich in these metal cations.

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO3. In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.

Bicarbonate has been regarded as a major factor for inducing Zn deficiency in lowland rice, but the mechanisms responsible for this effect are not yet fully understood. The objective of the present study was to test whether early effects of bicarbonate (HCO3 −)are inhibition of root growth due to the accumulation of organic acids induced by HCO3 −. Solution culture experiments were conducted using two rice cultivars differing in susceptibility to Zn deficiency, and four bicarbonate concentrations (0, 5, 10, 20 mM). Bicarbonate (5–20 mM) strongly inhibited root growth of the Zn-inefficient cultivar within 4 days of treatments. In contrast, root growth of the Zn-efficient cultivar was slightly stimulated with bicarbonate at 5–10 mM and not affected at 20 mM. The inhibitory effect of bicarbonate on root growth in the Zn-inefficient cultivar was mainly that of impairment of new root initiation rather than suppression of elongation of individual roots. Bicarbonate (5–20 mM) increased the concentrations of malate, succinate and citrate in the roots of both cultivars, but to a greater extent for the Zn-inefficient than for the Zn-efficient cultivars. The results suggest that the impairment of root growth was likely to be the initial action of bicarbonate in inducing Zn deficiency in lowland rice, and the inhibitory effect of bicarbonate on root growth of the Zn-inefficient cultivar might result from high accumulation and an insufficient compartmentation of organic acids in the root cells.

It has been difficult to impose different degrees of Zn deficiency on Poaceae species in nutrient solution because most chelators which would control Zn to low activities also bind Fe3+ so strongly that Poaceae species cannot obtain adequate Fe. Recently, a method has been developed to provide buffered Fe2+ at levels adequate for rice using Ferrozine (FZ), and use of other chelators to buffer the other micronutrient cations. The use of Fe2+ buffered with FZ in nutrient solutions in which Zn is buffered with HEDTA or DTPA was evaluated for study of Zn deficiency in rice compared to a conventional nutrient solution technique. The results showed that growth of rice plants in FZ+HEDTA-buffered nutrient solution was similar to that in the conventional nutrient solution. Severe zinc deficiency symptoms were observed in 28-day-old rice seedlings cultured with HEDTA-buffered nutrient solution at Zn2+ activities < 10-10.6 M. With increasing free Zn2+ activities, concentrations of Zn, Fe, Cu, and Mn in shoots and roots were quite similar for the FZ+HEDTA-buffered nutrient solution and the conventional nutrient solution techniques. The percentages of water soluble Zn, Fe, Cu and Mn in shoots with HEDTA-buffered nutrient solution were also similar to those with the conventional solution. However, with DTPA-buffered nutrient solution, the rice seedlings suffered severe Fe deficiency; adding more FeFZ3 corrected the Fe-chlorosis but shifted microelement buffering. Further, much higher total Zn concentrations are required to provide adequate Zn2+ in DTPA-buffered solutions, and the contents of Mn and Cu in shoots and roots cultured with DTPA-buffered solutions were much higher than those with the conventional or HEDTA-buffered solutions. In conclusion, DTPA-buffered nutrient solutions are not suitable but the FZ/HEDTA-buffered nutrient solution technique can be used to evaluate genotypic differences in zinc efficiency in rice.

Field experiments were conducted during the rainy reasons of 1989, 1990 and 1991 on an acid sandy soil in Niger, West Africa, to assess the effect of millet straw application (+CR) on growth and N2 fixation of groundnut (Arachis hypogaea L.).

A pot experiment was carried out with pearl millet (Pennisetum americanum [L.] Leeke) growing in a sandy soil in which the upper (topsoil) and lower (subsoil) parts of the pots were separated by a perlite layer to prevent capillary water movement. Using microtensiometers a study was made to establish whether it was possible to measure hydraulic lift by which the upper part of the soil was rewetted when water was supplied exclusively to the lower part of the soil. Hydraulic lift occurred during the first seven days of the period of measurement, with a maximum water release to the soil of 2.7 Vol. % during one night (equivalent to 10.8 mL water in the top 10 cm of the soil profile). This magnitude was obtained at very high root length densities, so that water release from the roots would be expected to be much smaller under field conditions. Hydraulic lift ceased when the soil matric potential in the topsoil dropped below-10 kPa at the end of the light period and could not be re-established, neither by extending the dark period, nor after rewatering the topsoil. The disappearance of hydraulic lift could be explained in part through osmotic adaptation of plant roots and, thus prevention of water release from the roots in the topsoil. It is concluded that hydraulic lift may affect nutrient uptake from drying topsoil by extending the time period favourable for uptake from the topsoil.

The suitability of microtensiometers to measure the spatial variation of soil matric potential and its diurnal change was tested in a pot experiment with pearl millet (Pennisetum americanum [L.] Leeke) in a sandy soil as the soil dried out. The temporal and spatial resolution of this technique allowed precise measurement of soil matric potential and thus estimation of soil water extraction from different compartments as well as from the whole rooting zone. The technique also allowed the measurement of rehydration of plants at night and root water uptake rate per unit soil volume or per unit root length. The precision of determination of root water uptake depended greatly on the accuracy of the estimate of hydraulic conductivity, which was derived from a bare soil and might be different for a cropped soil owing to aggregation induced by the root system. A linear relationship between root length and water uptake was found (r2=0.82), irrespective of variation in soil water content between compartments and despite the variation in root age, xylem differentiation and suberin formation expected to exist between different compartments of the rooting zone. As the experiment was carried out in a range of soil matric potentials between −4 and −30 kPa, drought stress did not occur. Further information at lower soil matric potentials are required, to address questions such as the importance of soil resistance for water uptake, or which portion of the root system has to be stressed to induce hormonal signals to the shoot. The microtensiometer technique can be applied to soil matric potentials up to −80 kPa.

The effect of long-term (1983–1988) applications of crop residues (millet straw, 2–4 t ha-1 yr−1) and/or mineral fertilizer (30 kg N, 13 kg P and 25 kg K ha-1 yr-1) on uptake of phosphorus (P) and other nutrients, root growth and mycorrhizal colonization of pearl millet (Pennisetum glaucum L.) was examined for two seasons (1987 and 1988) on an acid sandy soil in Niger. Treatments of the long-term field experiment were: control (−CR−F), mineral fertilizer only (−CR+F), crop residues only (+CR−F), and crop residues plus mineral fertilizer (+CR+F).

In pot and field experiments conducted in 1990 and 1991 on an acid sandy, phosphorus (P) deficient soil in Niger, West Africa, the effect of seed coating on seedling emergence, early growth and grain yield of pearl millet (Pennisetum glaucum (L.) R. Br.) was studied. Seeds of pearl millet were coated with different rates (0; 0.5; 1.0; 2.0; 5.0; 10.0 mg P seed−1) and types of P fertilizers (single superphosphate, ammonium dihydrogen phosphate; monocalcium phosphate, sodium dihydrogen phosphate and sodium triphosphate). Seedling emergence was generally reduced at coating rates higher than 0.5 mg P seed−1 and prevented with single superphosphate and sodium triphosphate at rates higher than 5 mg P seed−1. No correlation was found between the pH and osmomolity of the coatings and final emergence of millet seedlings. The most favourable effect on plant growth and P content was achieved with ammonium dihydrogen phosphate (AHP) as seed coating. This was attributed to the enhancement effect of ammonium on P uptake. Compared to the untreated control dry matter production at 20 days after planting (DAP) was increased by 280%, P content per plant by 330%, total biomass at maturity by 30% and grain yield by 45%. Although seed coating with AHP may be harmful to seedlings emergence, it represents a suitable method to enhance early growth and increase yield of pearl millet.

The investigation of voltage/reactive power phenomena in large-scale power systems and the development of improved voltage control strategies require rather sophisticated simulation models, so that margins of power transmission and system limitations of real systems are correctly reproduced. For a large system this modeling has to be done as a compromise between the necesserary aggregation of parts of the complete system model and the model accuracy. An engineering-based methodology for such a modeling of the voltage/reactive power behaviour of large power systems is described. The resulting simulation model allows the realization of the usual voltage commands and allows detailed operating studies of voltage problems.

In a long-term field experiment millet straw application (+CR) increased soil pH and base saturation and strongly improved pearl millet (Pennisetum glaucum L.) growth on acid sandy soils. Aluminum (Al) toxicity may be responsible for poor millet growth in plots without crop residues (−CR). Laboratory experiments were conducted to verify this assumption. The concentrations of labile Al (8-hydroxyquinoline, 15 sec) in equilibrium soil solutions of top soil samples from field plots were 14.0 and 0.6 μM in unfertilized samples of −CR and +CR soil, respectively. The corresponding values for labile Al in fertilized (NPK) samples were 51.8 and 11.0 μM, respectively. A short-term (14 days) incubation of −CR soil with ground millet straw (0.1% w/w) increased soil solution pH and decreased total and labile Al in the soil solution by more than 44%. In a water-culture experiment with increasing concentrations of Al (0–60 μM) pearl millet proved to be very Al-tolerant compared to cowpea, peanut and soybean. A short-term (12 days) pot experiment with the incubated soil showed that root growth of pearl millet is not restricted by Al toxicity in the acid soils from Niger, but that after millet straw incubation root growth is considerably enhanced. Phosphorus (P) concentration in the soil solution was about three times higher in +CR (1.75 μM) than in −CR (0.52 μM) top soil. Since P is the most growth-limiting nutrient in those soils, the beneficial effect of crop residues on pearl millet is likely due to improvement of P nutrition by both increase in P mobility in the soil and enhancement of root growth.

To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-μm nylon net. The root compartment received a uniform level of P (50 mg kg−1 soil) in combination with low or high levels of P (50 or 150 mg kg−1 soil) in the hyphal compartments. Plants were inoculated withGlomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected.

White clover (Trifolium repens L.) plants were grown in a calcareous soil in pots with three compartments, a central one for root growth and two outer ones for growth of vesicular-arbuscular (VA) mycorrhizal (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe) hyphae (hyphal compartments). Phosphorus (P) was applied at three levels (0, 20 and 50 mg kg−1 soil) in the outer compartments in mycorrhizal treatments. Root and shoot dry weight were increased in mycorrhizal plants with hyphal access to outer compartments. Growth of the mycorrhizal hyphae in the outer compartments was not significantly affected by variation in P level in these compartments. However, both concentration and amount of P in roots and shoots sharply increased with increasing P supply in the outer (hyphal) compartments. With increasing P levels the calculated delivery of P by the hyphae from the outer compartments increased from 34% to 90% of total P uptake.

The spatial distribution of plant-available mineral nutrients in forest soils is often highly heterogeneous. To test the hypothesis that local nutrient enrichment of soil leads to increased root proliferation in the nutrient-rich soil zone, we studied the effects of nutrient enrichment on the growth and nutrient concentrations of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) roots. Three-year-old seedlings were grown for 9 months in split-root containers filled with nutrient-poor forest mineral soil, with one side supplemented with additional mineral nutrients. Root dry weight and root length in Scots pine and Norway spruce were increased in the nutrient-supplemented soil compared with the nonsupplemented side, whereas root growth in Douglas-fir was unaffected by nutrient enrichment. Of the three species examined, Norway spruce exhibited the highest root and shoot growth and the highest nutrient demand. Specific root length (m g −1 ) and the number of root tips per unit root length were not affected by local nutrient addition in any of the species. Despite increased root growth in Norway spruce and Scots pine in nutrient-supplemented soil, their root systems contained similar nutrient concentrations on both sides of the split-root container. Thus, coniferous trees may respond to local nutrient supply by increased root proliferation, but the response varies depending on the species, and may only occur when trees are nutrient deficient. As a response to local nutrient enrichment, increases in root dry matter or root length may be better indicators of pre-existing nutrient deficiencies in conifers than increases in root nutrient concentrations.

Iron uptake from ferrated (59Fe) pseudobactin (PSB), a Pseudomonas putida siderophore, by various plant species was studied in nutrient solution culture under short term (10 h) and long term (3 weeks) conditions. In the short term experiments, 59Fe uptake rate from 59FePSB by dicots (peanuts, cotton and sunflower) was relatively low when compared with 59Fe uptake rate from 59FeEDDHA. Iron uptake rate from 59FePSB was pH and concentration dependent, as was the Fe uptake rate from 59FeEDDHA. The rate was about 10 times lower than that of Fe uptake from the synthetic chelate. Results were similar for long term experiments.

In Fe nutrition of soil-grown plants, a series of possible rate-limiting steps are involved. These steps are either located in the bulk soil, at the soil-root interface (rhizosphere, rhizoplane), or within the plant (Fig. 1). The present stage of knowledge and important gaps of knowledge as highlighted in this symposium are summarized, and conclusions are drawn indicating areas where future research should be focussed. The main emphasis has been placed on aspects and conditions where Fe supply to the roots is in the suboptimal or deficiency range.

Etude sur six clones de pomme de terre en culture hydroponique aux temperatures optimale et supra-optimale. Observation de differences significatives entre les clones permettant de selectionner des genotypes thermoresistants et thermosensibles. Dans tous les cas, la temperature supra-optimale provoque une diminution de taille de l'appareil racinaire, ainsi qu'une courbure de sa zone apicale accompagnee d'une perte de granules d'amidon par la coiffe

The effect of application of the fungicide pentachloronitrobenzene (PCNB) at levels between 2 and 50 mg kg−1 soil on root growth, mycorrhizal infection and P uptake was studied in pot culture with oats (Avena sativa cv. Alfred) growing in a rendzina soil low in available P. The soil had been partially sterilized by X-ray, and half of the pots were inoculated with spores of the VAM-fungusGlomus mosseae (indigenous species).

The effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L. var. ‘Red Kidney’) grown in water culture was studied at different levels of manganese supply. Without silicon, growth depression and toxicity symptoms occurred already at 5 × 10−4 mM Mn in the nutrient solution. After addition of Aerosil (0.75 ppm Si), the plants tolerated 5 × 10−3 mM Mn and, at a higher silicon supply of 40 ppm, as much as 10−2 mM Mn in the nutrient solution without any growth depression. This increase in manganese tolerance was not caused by a depressing effect of silicon on uptake or translocation of manganese but rather by an increase in the manganese tolerance of the leaf tissue. In absence of silicon, 100 ppm Mn was already toxic for the leaf tissue, whereas with a supply of 40 ppm Si, this ‘critical level’ in the leaves was increased to more than 1000 ppm Mn. At lower manganese levels in the leaf tissue, a molar ratio Si/Mn of 6 within the tissue was sufficient to prevent manganese toxicity. Above 1000 ppm Mn, however, even a much wider Si/Mn ratio (> 20) could not prevent growth depression by manganese toxicity. With54Mn and autoradiographic studies, it could be demonstrated that, in absence of silicon, even at optimal manganese supply (10−4 mM), the distribution of manganese within the leaf blades was inhomogeneous and characterized by spot-like accumulations. In presence of silicon, however, the manganese distribution was homogeneous in the lower concentration range of manganese and still fairly homogeneous in the high concentration range. This effect of silicon on manganese distribution on the tissue level was also reflected on the cellular level. In the presence of silicon, a higher proportion of the leaf manganese could be found in the press sap,i.e., had been transported into the vacuoles, than in the absence of silicon. The increase in manganese tolerance of bean leaves by silicon therefore seems to be primarily caused by the prevention of local manganese accumulation within the leaf tissue which leads to local disorders of the metabolism and, correspondingly, growth depression.

Sunflower (Helianthus annuus L. var. Giganteus) and corn plants (Zea mays L. var. Velox) were grownin mono and mixed culture for 11 days in nutrient solution containing no iron, inorganic Fe3+ and FeEDTA (2 ppm Fe each) respectively to evaluate the interaction between both plant species on their ability for iron uptake. Both plant species developed chlorosis under iron stress (no iron, inorganic Fe3+); however, due to the ability of sunflower plants to lower the pH of the nutrient solution under iron stress conditions, in presence of inorganic Fe3+ the pH lowering was followed by regreening of sunflower in mono and mixed culture. In mixed culture but not in mono culture corn plants also regreened under these conditions. With respect to iron accumulation, therefore, the sunflower plants promoted the uptake and translocation of iron to the shoot of corn plants, i.e. an ‘Fe-efficient’ species can improve the iron nutrition of an ‘Fe-inefficient’ one. In contrast to this corn plants had little or no effect on the iron nutrition of sunflower plants. The corn variety used in this experiment developed chlorosis even in presence of 2 ppm Fe in form of EDTA. This high susceptibility to iron chlorosis was related to the P/Fe ratio in the shoot rather than to the capacity of the plants for absorption and translocation of iron, because sunflower plants with the same or even lower iron content but lower phosphorus content were green.

The recent proposal of Tipton and Thowsen (Plant Physiol 79: 432-435) that iron-deficient plants reduce ferric chelates in cell walls by a system dependent on the leakage of malate from root cells was tested. Results are presented showing that this mechanism could not be responsible for the high rates of ferric reduction shown by roots of iron-deficient bean (Phaseolus vulgaris L. var Prélude) plants. The role of O(2) in the reduction of ferric chelates by roots of iron-deficient bean plants was also tested. The rate of Fe(III) reduction was the same in the presence and in the absence of O(2). However, in the presence of O(2) the reaction was partially inhibited by superoxide dismutase (SOD), which indicates a role for the superoxide radical, O(2) ([unk]), as a facultative intermediate electron carrier. The inhibition by SOD increased with substrate pH and with decrease in concentration of the ferrous scavenger bathophenanthroline-disulfonate. The results are consistent with a mechanism for transmembrane electron transport in which a flavin or quinone is the final electron carrier in the plasma membrane. The results are discussed in relation to the ecological importance that O(2) ([unk]) may have in the acquisition of ferric iron by dicotyledonous plants.

One-year-old apple cuttings (Malus pumila var.domestica cv. M26) were grown for 6 months in pot culture with and without inoculum of the VA-mycorrhizal fungus (VAMF)Glomus macrocarpum in soil from a long-term fertilizer field experiment with different P availability (20, 210, and 280 mg CAL-extractable P kg−1). The indigenous VAMF propagule density was reduced by 0.5 Mrad X-irradiation. At harvest, non-inoculated and inoculated plants had similar proportions of root length bearing vesicles. Net dry weight of tree cuttings was significantly increased by inoculation only at 20 mg P kg−1 (+62%). Increasing P availability from 210 to 280 mg P kg−1 led to a 4-week depression of shoot elongation rate only in the inoculated plants. Uptake of P was significantly enhanced by inoculation at 20 and 210 mg P kg−1 (+64 and +12%, respectively). On average, inoculated plants had significantly higher concentrations of Zn in leaves and in roots (+16 and +14%, respectively) and of copper in stems and in roots (+13 and +126%, respectively). Proportion of vesicle bearing root length was significantly correlated with root caloric content. A lipid content of 0.9–4.5% in the root dry matter was attributed to the presence of vesicles corresponding to 1.6–8.2% of total root caloric content. As the control plants were also infected, the beneficial effect of VA-mycorrhiza on nutrient uptake and growth of apple cuttings was underestimated at all P levels. Furthermore, VAM-potential at the lowest P level was not fully exploited as onset of infection was most certainly delayed because of a decreased photosynthetic rate due to P deficiency. Energy drain by VAMF-infection was most probably underestimated considerably, due to, among others, loss of infected root cortex during root growth, sampling and staining. It is concluded that apple cuttings rely on VA-mycorrhizal P-uptake at least in low P soils. In high P soils, apple cuttings may profit predominantly from the uptake of Zn and Cu by the fungal symbionts.

The possible involvement of micro-organisms in the iron nutrition of sun-flower plants grown in nutrient solutions with different sources of iron (— Fe; inorganic Fe3+; Fe EDTA) was investigated. It is shown that micro-organisms play no significant role since the ability of plants under iron stress, to decrease the pH of the nutrient solution and to release reducing substances and riboflavin and to make Fe3+ available for uptake and chlorophyll formation was similar under both sterile and non-sterile conditions.

Ячмень, которому в стадии развития трех листъев давали32Р, в год обработки кустился сильнее и имел пониженную фертильность. Во втором поколении последействие выразилось только в ослаблении фертильности. Существенного влияния на развитие в этом поколении не наблюдалось. Повышение урожая сухого вещества при одновременном снижении урожая зерна тоже не было замечено.

Inorganic and organic phosphates (P) were measured in bulk soil, rhizosphere soil and mycorrhizal rhizoplane soil of Norway spruce. Various methods of P extraction and estimation were compared. In addition, acid phosphatase activity and mycelial hyphae length were determined. In soil solutions from various locations, about 50% (range 35%–65%) of the total P was present as organic P. Compared to the bulk soil, the concentrations of readily hydrolysable organic P were lower in the rhizosphere soil and in the rhizoplane soil; this difference was particularly marked in the humus layer. In contrast, the concentrations of inorganic P either remained unaffected or increased. A 2- to 2.5-fold increase was found in the activity of acid phosphatase in the rhizoplane soil in comparison to the bulk soil. There was a positive correlation (r = 0.83***) between phosphatase activity and the length of mycelial hyphae. The results stress the role of organic P and of acid phosphatase in the rhizosphere in the P uptake by mycorrhizal roots of spruce trees grown on acid soils.

Helianthus annuus L. responds to iron deficiency by forming a thickened cortex and abundant root hairs in a zone near the root apex that corresponds to the primary developmental stage. Cytological investigations revealed that within 24 to 48 h of iron deficiency most of the peripheral cells differentiate into transfer cells. The wall labyrinth is always situated on the peripheral walls that face the external medium. The cytoplasm of these cells is characterized by numerous mitochondria, extensive rough endoplasmic reticulum, and large leucoplasts containing protein bodies. These observations are discussed in relation to the fact that Helianthus, as an "iron efficient" plant, responds physiologically to iron deficiency by extrusion of H(+), production of reducing substances, and a steep increase in the uptake efficiency of Fe.

Water culture experiments are described which provide conclusive evidence that Mo uptake by tomato plants is markedly enhanced by P deficiency. In a longterm experiment, which ran for 11 days, in marked contrast to the uptake of other nutrients, a three fold higher Mo uptake rate was observed after only four days of withdrawal of P from the nutrient medium. In contrast to the gradual increase in pH of the nutrient medium of the plants supplied with P, the pH in the medium of the ‐P plants fell. Throughout the growth of these plants net H+ efflux could be accounted for by excess cation over anion uptake, indicating that organic acid extrusion plays no major role in the observed fall in pH. Further evidence that Mo uptake is enhanced in P deficient tomato plants is provided in short‐term nutrient solution experiments (1h and 4h) using radioactive molybdenum (99Mo). Compared with P sufficient plants, the uptake rates of 99Mo by P deficient plants were three to five times higher after 1h and nine to ...