Abstract In den Jahren 1985 und 1986 wurden Stickstoffdüngungsversuche zu Winterweizen durchgeführt, um den Nitrat‐Schnelltest in Pflanzen (Halmbasis) mit einem neu entwickelten Schnelltest auf Amino‐Stickstoff (Amino‐N‐Schnelltest) in Blättern in seiner Aussagekraft für den Stickstoffversorgungsgrad der Pflanzen zu vergleichen. Die Versuche sollten außerdem klären, welchen Einfluß die Witterungsbedingungen bei Düngung mit Kalkammonsalpeter (KAS) und Ammonnitratharnstofflösung (AHL) auf die Eignung der beiden Schnelltests zur besseren Bemessung der Spätdüngung haben. Die Ergebnisse haben gezeigt, daß sich der Nitrat‐Schnelltest gut zur Bestimmung der N‐Versorgung von Winterweizenpflanzen eignet, wenn der Stickstoff hauptsächlich in Form von Nitrat über die Wurzel aufgenommen wird. Dies ist in der Regel bei der N‐Düngung in Salzform (z. B. KAS), aber auch bei Flüssigdüngung mit AHL der Fall, wenn der Dünger – wie bei der Spätdüngung – mit Schleppschläuchen über den Boden ausgebracht wird. Bei der AHL‐Düngung über die Blätter zum Schossen und zum Ährenschieben ist in Verbindung mit trockener Witterung – Versuchsjahr 1986 – jedoch mit einer erhöhten Aufnahme über die Blätter zu rechnen, die bei Anwendung des Nitrat‐Schnelltests eine Unterbewertung des N‐Versorgungsgrades und somit eine Überdosierung der N‐Düngung (Größenordnung: 20 kg N/ha) zur Folge haben kann. Unter diesen Bedingungen ist der Amino‐N‐Schnelltest in den Blättern (Preßsaft) besser zur Ermittlung der N‐Versorgung geeignet, weil er mit Glutamin und Aminosäuren die wichtigsten Speicherformen von reduziertem N in Weizenpflanzen erfaßt. Wenn allerdings nach einer AHL‐Düngung über die Blätter der größte Teil durch Regen abgewaschen wird – Versuchsjahr 1985 –, ist die Aussagekraft beider Testverfahren vergleichbar gut. Eine Nitratspeicherung in der Halmbasis z.B. infolge geringer Einstrahlungsintensität im Frühjahr (verminderte Nitratreduktion) kann mit dem Amino‐N‐Schnelltest allerdings nicht erfaßt werden und führt entsprechend zu einer Unterbewertung des N‐Versorgungsgrades.
Abstract The effect of varied zinc (Zn) and iron (Fe) supply on the release of Zn and Fe mobilizing phytosiderophores from roots was studied in Zn‐efficient Aroona and Zn‐inefficient Durati wheat genotypes (Triticum aestivum cv. Aroona; T. durum cv. Durati) grown under controlled environmental conditions in nutrient solution for 25 days. Phytosiderophore release was determined by the measurement of Zn and Fe mobilizing capacity of root exudates from a Zn‐loaded resin and from freshly precipitated FeIII hydroxide as well as identification by HPLC analysis. Visual Zn‐deficiency symptoms, such as necrotic patches on leaves and reduction in shoot length, appeared first and more severely in Zn‐inefficient Durati, although the concentrations of total Zn in shoot and root tissues were the same in both genotypes. Zinc‐efficient Aroona responded to Zn deficiency by increasing phytosiderophore release usually after 10 days growth in nutrient solution, whereas the phytosiderophore release in Durati remained at a very low level during 25 days growth. In contrast, under Fe deficiency and also under both Fe and Zn deficiency, Aroona and Durati released similarly high amounts of phytosidero‐ phores. HPLC analysis of root exudates revealed that the same phytosiderophores were released under Zn as under Fe deficiency, 2'‐deoxymugineic acid (DMA) being the dominant phytosiderophore. Besides DMA, 3‐hydroxymugineic acid (HMA) could also be identified mainly, however, in root exudates of Zn and of Fe‐deficient Aroona. In Zn‐deficient Durati, both DMA and HMA were released in much lower amounts. The results demonstrate that phytosiderophore release can occur under Zn deficiency as well as under Fe deficiency, and enhanced release of phytosiderophores under Zn‐deficiency stress may be causally involved in Zn efficiency in genotypes of graminaceous species.
To test the ability of vesicular–arbuscular mycorrhizal (VAM) hyphae to take up water, phosphorus, nitrogen, and potassium, mycorrhizal and nonmycorrhizal couchgrass (Agropyron repens) or white clover (Trifolium repens) plants were grown in pots with separated compartments for roots and hyphae growth. Soil solution transfer between compartments was blocked by a 2-mm air gap. Total shoot contents of phosphate and nitrogen, but not of potassium, were higher in mycorrhizal plants with access to the hyphal compartment. Hyphal uptake from the outer compartment accounted for 49% of the total phosphate and 35% of the total nitrogen taken up by mycorrhizal plants. This was associated with depletion of extractable phosphate, [Formula: see text]-nitrogen, and also [Formula: see text]-nitrogen in the soil of the hyphal compartments. In contrast, no difference in water loss from the hyphal compartments was measured by tensiometers under well-watered and water-stress conditions whether hyphae were present or not. Severance of the external hyphae did not affect water loss from the outer compartments. The results show the ability of VAM hyphae to transport considerable quantities of phosphate and nitrogen to the plant from soil zones several centimetres from the root. However, there was no evidence for a significant direct water transport by VAM hyphae to plants. Key words: Agropyron repens (couchgrass), Glomus mosseae, nitrogen, phosphorus, vesicular–arbuscular mycorrhiza, water.
The production of superoxide radical (O2−) was studied in plasma membrane vesicles isolated by aqueous polymer two-phase partitioning from roots of zinc-sufficient and zinc-deficient bean (Phaseolus vulgaris L. cv. Prélude) plants. The two populations of vesicles were highly enriched in plasma membrane and had similar composition as evidenced by the specific membrane marker enzymes. Vesicles from zinc-deficient roots showed higher rates of NAD(P)H oxidation compared to vesicles from zinc-sufficient plants. The NAD(P)H-dependent formation of O2− in plasma membrane vesicles was also highly increased by zinc deficiency. For both activities, a higher response to zinc deficiency was observed when NADPH was used as electron source. Re-supply of zinc to deficient plants for 24 h substantially decreased the rates of NAD(P)H oxidation and 02− production in isolated vesicles. The NADPH-dependent O2− generation was strongly stimulated by FAD and showed a high pH optimum; it was scarcely affected by Triton X-100 or even inhibited in the presence of FAD and was almost insensitive to Antimycin A. The results suggest the presence at the plasma membrane of bean roots of an O2− generating activity, preferentially utilizing NADPH, which is affected by the zinc nutritional status of the plant. This finding, together with previous observations on cytosolic and microsomal fractions prepared from zinc-deficient roots of different plants, is consistent with a role of zinc in membrane stabilization by controlling the level of oxidizing O2 species.
Abstract Spatial variation in the growth of pearl millet [ Pennisetum glaucum (L.) R. Br.] over short distances is a problem in field experiments in the Sahel, but the causes are still poorly understood. Data from a 3‐yr experiment with millet were used to compare four data types for their usefulness for reducing variation not related to treatment: (i) soil chemical data, (ii) residuals of the first year's yield data, (iii) a traditional fertility classification system, and (iv) plant vigor scores. The completely randomized experiment consisted of four factors combined to 48 treatments, replicated twice. There were three levels of millet crop residues (CR), two levels of broadcast P, and four genotypes; the fourth factor had two levels and varied over years. Whereas chemical analyses of the topsoil did not explain overall variation, residuals of plant scores used as covariates led to a reduction in residual variation of 32% for straw and 51% for grain yield in 1991. Most satisfactory, however, was the use of residuals of plant scores to classify plots into two strata of relatively low and high inherent soil productivity (a retrospective procedure called post stratification ). In low‐productivity plots, a CR application of 2000 kg ha −1 (compared with 500 kg ha −1 ) increased millet straw yield by an average of 42% and grain yield by 48% for the first 2 yr. In contrast, under high productivity, yields were barely influenced by treatments. The application of P, however, was equally effective in both productivity strata. The results show that vigor scores can be useful to clarify treatment effects on millet growth. The different responses of crop residues and P in the two productivity strata also indicate that nonchemical parameters such as soil mechanical resistance may contribute to soil microvariability in the Sahel.
Abstract Although per growing season nutrient uptake of adequately growing forest trees is less than the nutrient uptake of annual crop species, nutrient uptake per unit root length in trees is considerable. Because of high heterogeneity of soil conditions and root growth in forest soils, modelling of uptake processes is even more difficult for forest than for crop stands. Detailed studies show that white lips of growing tree roots have a high nutrient uptake capacity. However, most root tips are usually colonised by mycorrhizal fungi. These fungi can participate substantially in tree nutrient uptake, in particular in the utilisation of organically‐bound phosphorus and nitrogen in soils. Mycorrhizal hyphae, root tips, and older root zones can all absorb water, but their actual contribution is difficult to assess. In this review, experimental results from our laboratory and literature data are used to describe the potential activity of tree roots and mycorrhizas in nutrient and water uptake. Methodology for in situ measurements must be developed to quantify at different forest sites the actual contribution of mycorrhizas and different root parts.
In order to manipulate the shoot demand for mineral nutrients per unit root weight, maize ( Zea mays L.) seedlings were grown in nutrient solution with different temperatures in the root zone and at the shoot base. The aerial temperature was kept uniform at 24/20°C day/night. At a root zone temperature (RZT) of 24°C, shoot growth was reduced by decreasing the shoot base temperature (SBT) to 12°C; at a RZT of 12°C, shoot growth was increased by raising the SBT to 24°C. At both RZT root growth was not affected by the SBT. Thus, the shoot demand for nutrients per unit root was either increased by raising, or decreased by lowering the SBT. The net uptake rate of potassium (K), as determined from accumulation rates between sequential harvests, was not affected within the first 3 days after lowering the SBT, whereas net translocation rates of K into the shoot and translocation rates in the xylem exudate of decapitated plants were markedly reduced. Obviously, translocation of K into the shoot seems to be regulated independently from K uptake into the root cells. Translocation rates of K in the xylem exudate of decapitated plants were markedly reduced when the nutrient solution was replaced by CaCl 2 solution during exudation. But, depending on the SBT before decapitation, significant differences remained in the translocation rates of K even when K uptake from the nutrient solution was prevented. From the results it is suggested that xylem loading of K is regulated separately from K uptake from the external solution and that the adaptation of K translocation to shoot demand is coupled with an altered capacity of the root for xylem loading.
Abstract In order to study the effect of different growth rates of the shoot apex, i.e. shoot demand, on the remobilization of iron (Fe) from mature (primary) leaves, bean (Phaseolus vulgaris L.) plants were precultured with 8x10‐5 M FeEDTA for four days. Thereafter, plants were grown for another six days at various levels of Fe (0.0, 1.0, and 10.0μM FeEDTA), and simultaneously treated with or without shading of one primary leaf. Dry weight increment of the shoot apex decreased with decreased Fe in the nutrient solution. Shading of one primary leaf decreased total dry weight of plants irrespective of Fe supply, but increased the dry weight of the shoot apex of plants supplied without Fe or with only 1.0μM Fe. In these plants, the concentration of chlorophyll and Fe in the shoot apex corresponded with the treatment effects on dry weight of the shoot apex. Shading induced senescence of the shaded leaf, decreased the content of "active Fe"; (extractable in dilute acid), and also enhanced the remobilization of Fe and copper (Cu) from the shaded leaf. The remobilization of Fe from primary leaves was not related to the severity of chlorosis in the shoot apex (the Fe demand of sink tissue), indicating that only a certain fraction of the total Fe in mature leaves can be remobilized.
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