Horst Marschner

Different strategies in higher plants in mobilization and uptake of iron

Abstract Higher plants differ considerably in their capability for mobilization of iron in the rhizospere of soils with low iron availability. In the plant kingdom at least two different Strategies exist in the Fe deficiency‐induced root responses which lead to enhancement of both iron mobilization in the rhizosphere and uptake rate of iron. Strategy I is found in all dicots and in monocots, with the exception of the grasses (graminaceous species, e.g. barley, corn, sorghum). Strategy I is characterized in all instances, by an increase in the activity of a plasma membrane‐bound reductase leading to enhanced rates of Fe‐III reduction and corresponding splitting of Fe‐III‐chelates at the plasma membrane. Often, the net rate of H+ extrusion, i.e. acidification of the rhizosphere, is also increased. This acidification facilitates iron uptake by both enhancement of the reductase activity and solubilization of iron in the rhizosphere. An additional mobilization of sparingly soluble iron in the rhizosphere may o...

Mechanisms of adaptation of plants to acid soils

Major constraints for plant growth on acid mineral soils are toxic concentrations of mineral elements like Al, of H+, and/or low mineral nutrient availability either as a result of solubility (e.g. P and Mo), low reserves, and impaired uptake (e.g. Mg2+) at high H+ concentrations. Inhibition of root growth particularly by Al leads to more shallow root systems, which may affect the capacity for mineral nutrient acquisition and increase the risk of drought stress. Of the two principal strategies (tolerance and avoidance) of plants for adaptation to adverse soil conditions, the strategy of avoidance is more common for adaptation to acid mineral soils. Examples are (i) root-induced changes in the rhizosphere such as pH increase, (ii) release of chelators for Al, higher activity of ectoenzymes (acid phosphatases), and (iii) increase in root surface area via mycorrhizae. In order to have a better understanding of the principles of the mechanisms by which plants adapt to acid mineral soils more attention should thus be given to conditions at the root-soil interface.

Effect of mineral nutritional status on shoot-root partitioning of photoassimilates and cycling of mineral nutrients

Mineral nutrients taken up by the roots are, as a rule, transported in the xylem to the shoot, and photoassimilates transported in the phloem to the roots. According to the Thornley model of photosynthate partitioning, nutrient deficiencies should favour photosynthate partitioning to the roots. Examples are cited to show that this preferential partitioning is dependent on phloem mobility and hence on nutrient cycling from shoot to roots. Thus, root growth is enhanced under nitrogen and phosphorus deficiencies, but not under deficiencies of nutrients of low mobility in the phloem, such as calcium and boron. Enhanced root growth under nutrient deficiency relies on the import of both photosynthates and mineral nutrients. Cycling of mineral nutrients serves a number of other functions. These include the root supply of nutrients assimilated in the shoot (nitrate and sulphate reduction), maintenance of cation-anion balance in the shoot, providing an additional driving force for solute volume flow in the phloem and xylem, and acting as a shoot signal to convey nutrient demand to the root. Cycling of certain mineral nutrients through source leaves has a considerable impact on photosynthate export as demonstrated in impaired export under magnesium, potassium, or zinc deficiencies. Mineral nutrient deficiency can, therefore, affect photosynthate partitioning either directly via phloem loading and transport or indirectly by depressing sink demand.

Mobilization of iron and other micronutrient cations from a calcareous soil by plant-borne, microbial, and synthetic metal chelators

Mobilization of Fe, Zn, Cu, and Mn by various chelators from a calcareous soil was measured using a simple dialysis tube/complexing resin system. Root exudates from Fe-deficient barley increased the concentrations of all four metals in solution by, on average, a factor of 20, and the addition of complexing resin as a sink for heavy metal cations forced steady state solution concentrations to be reached sooner. Root exudates mobilized increasing amounts of the various micronutrients in the following order: Cu<Fe<Zn<Mn. Phytosiderophores isolated from root exudates of Fe-deficient barley mobilized similar amounts of Cu and Zn but somewhat more Fe and considerably more Mn than crude exudate. The synthetic chelators EDDHA and DTPA showed low specificity in micronutrient mobilization, but the microbial siderophore Desferal was relatively more specific, preferentially mobilizing Fe and Mn. The data indicates that phytosiderophores are capable of increasing the amount of complexed cations in solution. Despite their lack of specificity, phytosiderophores were just as effective as Desferal increasing the availability of Fe. Thus, phytosiderophores, as plant-borne chelators, are certainly of significance for the Fe nutrition of cereals grown in calcareous soils.

Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in a calcareous soil

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.Mycorrhizal inoculation doubled P concentration in shoot and root, and increased dry weight, especially of the shoot, irrespective of P levels. Mycorrhizal contribution accounted for 76% of total P uptake at the low P level and 79% at the high P level, and almost all of this P was delivered by the hyphae from the outer compartment. In the non-mycorrhizal plants, the depletion of NaHCO3-extractable P (Olsen-P) extended about 1 cm into the outer compartment, but in the mycorrhizal plants a uniform P depletion zone extended up to 11.7 cm (the length of the hyphal compartment) from the root surface. In the outer compartment, the mycorrhizal hyphae length density was high (2.5–7 m cm−3 soil) at the various distances (0–11.7 cm) from the root surface. Uptake rate of P by mycorrhizal hyphae was in the range of 3.3–4.3×10−15 mol s−1 cm−1.

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]

SummaryRelationships between root zone temperature, concentrations and uptake rates of NH4+and NO3− were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH4+and NO3− uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (Cmin) range of 100–150 μM uptake rates of NH4+were 3–4 times higher than those of NO3− The preference for NH4+uptake was also reflected in the minimum concentration (Cmin) values. Supplying NH4NO3, the rate of NO3− uptake was very low until the NH4+concentrations had fallen below about 100 μM. The shift from NH4+to NO3− uptake was correlated with a corresponding shift from net H+ production to net H+ consumption in the external solution. The uptake rates of NH4+were correlated with equimolar net production of H+. With NO3− nutrition net consumption of H+ was approximately twice as high as uptake rates of NO3− In the forest stand the NO3− concentration in the soil solution was more than 10 times higher than the NH4+concentration (<100 μM), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO3− uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

AbstractColonization of plant roots by arbuscular mycorrhizal fungi can greatly increase the plant uptake of phosphorus and nitrogen. The most prominent contribution of arbuscular mycorrhizal fungi to plant growth is due to uptake of nutrients by extraradical mycorrhizal hyphae. Quantification of hyphal nutrient uptake has become possible by the use of soil boxes with separated growing zones for roots and hyphae. Many (but not all) tested fungal isolates increased phosphorus and nitrogen uptake of the plant by absorbing phosphate, ammonium, and nitrate from soil. However, compared with the nutrient demand of the plant for growth, the contribution of arbuscular mycorrhizal fungi to plant phosphorus uptake is usually much larger than the contribution to plant nitrogen uptake. The utilization of soil nutrients may depend more on efficient uptake of phosphate, nitrate, and ammonium from the soil solution even at low supply concentrations than on mobilization processes in the hyphosphere. In contrast to ectomy...

Contribution of the VA mycorrhizal hyphae in acquisition of phosphorus and zinc by maize grown in a calcareous soil

An investigation was carried out to test whether the mechanism of increased zinc (Zn) uptake by mycorrhizal plants is similar to that of increased phosphorus (P) acquisition. Maize (Zea mays L.) was grown in pots containing sterilised calcareous soil either inoculated with a mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe or with a mixture of mycorrhizal fungi, or remaining non-inoculated as non-mycorrhizal control. The pots had three compartments, a central one for root growth and two outer ones for hyphal growth. The compartmentalization was done using a 30-μm nylon net. The root compartment received low or high levels of P (50 or 100 mg kg−1 soil) in combination with low or high levels of P and micronutrients (2 or 10 mg kg−1 Fe, Zn and Cu) in the hyphal compartments.Mycorrhizal fungus inoculation did not influence shoot dry weight, but reduced root dry weight when low P levels were supplied to the root compartment. Irrespective of the P levels in the root compartment, shoots and roots of mycorrhizal plants had on average 95 and 115% higher P concentrations, and 164 and 22% higher Zn concentrations, respectively, compared to non-mycorrhizal plants. These higher concentrations could be attributed to a substantial translocation of P and Zn from hyphal compartments to the plant via the mycorrhizal hyphae. Mycorrhizal inoculation also enhanced copper concentration in roots (135%) but not in shoots. In contrast, manganese (Mn) concentrations in shoots and roots of mycorrhizal plants were distinctly lower, especially in plants inoculated with the mixture of mycorrhizal fungi.The results demonstrate that VA mycorrhizal hyphae uptake and translocation to the host is an important component of increased acquisition of P and Zn by mycorrhizal plants. The minimal hyphae contribution (delivery by the hyphae from the outer compartments) to the total plant acquisition ranged from 13 to 20% for P and from 16 to 25% for Zn.

Acquisition of phosphorus and copper by VA-mycorrhizal hyphae and root-to-shoot transport in white clover

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.Hyphal access to the outer compartments also significantly increased both concentration and quantity of Cu in the plants. The calculated delivery of Cu by the hyphae from the outer compartments ranged from 53% to 62% of total Cu uptake, irrespective of the P levels and the amounts of P taken up and transported by the hyphae. However, the distribution of Cu over roots and shoots was largely dependent on P levels. With increase in P level in the outer compartments the calculated hyphal contribution to the total amount of Cu in the shoots increased from 12% to 58%, but decreased in the roots from 75% to 46%.In conclusion, uptake and transport by VA-mycorrhizal hyphae may contribute substantially not only to P nutrition, but also to Cu nutrition of the host.

Increase in membrane permeability and exudation in roots of zinc deficient plants

Summary The effect of Zn nutrition on the root membrane permeability and root exudation was studied in cotton ( Gossypium hirsutum L. cv. Deltapine 15/21), wheat ( Triticum aestivum L. cv. Cumhuriyet 75), tomato ( Lycopersicon esculentum L. cv. Super marmande), and apple ( Malus domestica cv. M26 rootstock) plants grown in nutrient solutions under controlled environmental conditions. Root exudation as an indicator for root plasma membrane permeability was measured by incubating roots of intact plants in aerated CaCl 2 (0.5 mM) solution. In all plant species studied, Zn deficiency increased root exudation (net efflux) of K + , amino acids, sugars and phenolics. Resupply of Zn to deficient plants for 8, 12 or 27 h increased the Zn concentration in the roots and simultaneously decreased the root exudation of all solutes studied. Omission of Ca from the incubation solution increased root exudation by a factor of about two in the Zn sufficient plants but only slightly in the Zn deficient plants. The levels of fatty acids and phospholipids were depressed in Zn deficient cotton roots. The depression was particularly evident in the case of unsaturated fatty acids. The results strongly suggest a distinct role of Zn in membrane integrity and thus also in root exudation. This role seems to be independent of the role of Ca and probably has ecological implications with respect to nutrient mobilization and microbial activity in the rhizosphere.