H. Marschner

Effect of silicon on manganese tolerance of bean plants (Phaseolus vulgaris L.)

SummaryThe 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.

Mucilage Protects Root Meristems from Aluminium Injury

Summary Depression of root elongation, the most obvious toxic effect of Al on cowpea (Vigna unguiculata) growth, is mainly a result of inhibition of cell division in the root tip meristems. Since root tips are the sites of most intensive mucilage secretion, uptake of Al into the meristems could be affected by this mucilage secretion. In solution culture a full size mucilage represents 17 % of the dry matter of the apical 5 mm of the primary root tip to which it adheres. Mucilage is continuously produced at a rate of 1.6% of the dry matter increment due to root elongation. When the mucilage is periodically removed from the root tips with a brush, root elongation is considerably more inhibited by Al, indicating a protective function of the mucilage against Al injury. Higher Al sensitivity of roots without mucilage is related to higher Al contents in the tissue of the root tips. Mucilage has a very high and specific binding capacity for Al, thus reducing Al uptake into the root tip meristems. The results indicate an important ecological role for mucilage excretion by root tips in the Al tolerance of plants growing in acid mineral soils.

Effect of Aluminium on Root Growth, Cell-division Rate and Mineral Element Contents in Roots of Vigna unguiculata Genotypes

Summary Inhibition of root elongation, the most sensitive parameter for the toxic effect of Al on the growth of cowpea (Vigna unguicalata), was mainly the result of inhibited cell division in the root-tip meristems. In secondary roots, which are more sensitive to Al than primary roots, 5 h after the beginning of the Al treatment the cell-division rate was drastically reduced and after 10 h completely inhibited. But after 18 h cell division had started again, although Al was continuously supplied to the roots indicating a recovery from the «primary Al-shock». Inhibition of cell division after short-term Al treatment (6 h) was due to an accumulation of Al in the roots, especially in the apical 1 cm root tip. 50 % of the Al content of the root tip was bound to the adhering mucilage. The nutrient concentrations in the root tip did not give any indication of an involvement of induced nutrient deficiency in short-term growth depression by Al. Considerable differences in Al tolerance existed between the cowpea genotypes studied. Less inhibition of root elongation in Al tolerant genotypes was related to lower Al uptake, especially into the 1 cm root tip, rather than to less inhibition of Ca and P uptake. The results show that short-term adaptation to high Al concentrations is an important aspect of Al tolerance and has to be taken into consideration in short-term screening for genotypical Al tolerance. Studies on the physiological reasons for the toxic effect of Al and genotypical differences in Al tolerance have to be focussed on the meristematic zones of the root, the sites of the depressing effect of Al on root elongation.

Accumulation of zinc and Organic Acids in Roots of Zinc Tolerant and Non-tolerant Ecotypes of Deschampsia caespitosa

A much higher zinc level was necessary to inhibit root elongation in the zinc tolerant ecotype as compared to the non-tolerant ecotype of Deschampsia caespitosa. In the presence of a range of high levels of zinc, zinc accumulated to a much higher concentration in the roots of the tolerant ecotype, especially in the root sap. Accumulation of citrate in the root sap was highly correlated to the accumulation of zinc. Gel filtration chromatography of the root sap showed zinc to be mainly present as zinc-citrate. This was the only zinc complex found. The malate concentration of the root sap was much lower than the concentration of citrate. However the malate content of aqueous root homogenates was comparable or even greater than the content of citrate, suggesting that malate and citrate are located in different compartments within the cell. The results are consistent with a model of zinc tolerance in which zinc is complexed with citrate in the vacuole.

Effect of K+ and Na+ on growth of leaf discs of sugar beet and spinach

Summary Leaf discs of sugar beet and spinach were grown on sterile substrates in which either the concentration of K + in the absence of Na + or the ratio of K + /Na + were varied. With increasing supply of K + in the absence of Na + fresh and dry weight, cell size and chloroplast number/cell and the amount of chlorophyll/disc increased but chlorophyll/mg fresh weight remained constant or decreased. In spinach and especially sugar beet at all levels of K + addition of Na + increased fresh and dry weight, cell size and chloroplast number/cell. The amount of chlorophyll/disc, however, was either not increased (spinach) or decreased (sugar beet). With the exception of the absence of both K + and Na + in all other treatments there was a general correlation between cell size and number of chloroplasts/cell. The size of the chloroplasts was not much influenced by the different treatments. The results suggest that K + cannot be replaced by Na + in chlorophyll formation but to a large extent K + can be replaced by Na + in cell expansion and chloroplast multiplication. The greater cell expansion in the presence of Na + compared to K + in sugar beet could be caused by different effects of both ions on carbohydrate metabolism.

Effect of Excessive Manganese Supply on Uptake and Translocation of Calcium in Bean Plants (Phaseolus vulgaris L.)

Summary In short- and long-term water culture experiments an excessive supply of manganese inhibited the calcium translocation into the leaves of bean plants ( Phaseolus vulgaris L.). This inhibition was not based on a reduction of the transpiration rate, nor could it be explained by a simple cation competition for binding sites. Particularly in fast growing leaf tissue, the inhibition of calcium translocation was much more pronounced than that of other cations, for example potassium. Inhibited calcium translocation resulted in typical symptoms of calcium deficiency (e.g., «crinkle leaf») in the shoot apices and youngest leaves at toxic levels of manganese. Excessive manganese supply caused the cation exchange capacity in the leaf tissue to be distinctly reduced, and the calcium uptake into the AFS of isolated leaf segments from leaves showing manganese toxicity was lower. It is concluded that under conditions of manganese toxicity the IAA-oxidase-activity is increased with a concomitant decrease in auxin level. The lower auxin level causes inhibition of cell wall expansion and, in turn, a reduction in formation of new negative binding sites and thus decreased calcium translocation into this tissue.

Root Growth and Zn Uptake by Two Ecotypes of Deschampsia caespitosa as Affected by High Zn Concentrations

Summary Inhibition of root elongation by high Zn concentrations was quite different in two ecotypes of Deschampsia caespitosa. A 5 to 10 times higher Zn supply was necessary to induce the same inhibition in the Zn tolerant as compared to the non-tolerant ecotype. After 10 h exposure to 100 μM Zn, which severely depressed root growth in the non-tolerant but not in the tolerant ecotype, the Zn concentrations in the roots were similar in both ecotypes. In short term uptake (10 h) and desorption studies (30 min) using 65Zn, up to 50% of the Zn(65Zn) taken up could be desorbed by unlabelled Zn within 30 min at 4°C. Zn bound in the cell walls was estimated to account for up to 30% of the total Zn content of the roots. Binding of Zn in the cell walls could not explain the differences in Zn tolerance of the two ecotypes. At the same degree of inhibition of root-elongation rate in both ecotypes, in the tolerant ecotype the 1 mm apical root sections had approximately a 3 times greater intracellular Zn content than the same sections of the non-tolerant ecotype. This suggests that the responsible factor for Zn tolerance is not exclusion from the cells but rather a higher internal tolerance of the root tip meristems. Since large vacuoles could not be observed in the 1 mm apical root sections, compartmentation of Zn in the vacuoles could only be of minor importance. Accordingly we suggest that specific binding of Zn in the cytoplasm is a more likely mechanism for the detoxification of Zn in the apical meristems.

Calcium-Umlagerung in Bohnenfrüchten während des Samenwachstums: Redistribution of Calcium in Bean Fruits during Seed Development

Summary Bean plants ( Phaseolus vulgaris L. var. «Saxa») were grown in water culture under controlled climatic conditions. The water transport in the xylem into the fruit and within the dorsal and ventral vascular strands of the fruit was traced with the dye Pyranin. Simultaneously the influx of 45 Ca into the fruit, its movement in the vascular strands of the fruit and its transfer from the pod into the developing seed was studied. With onset of seed growth the influx of water in the xylem and of Ca into the fruit sharply declines. This decline is more pronounced in the dorsal vascular strand, which is in direct contact with the seeds. Within both vascular strands of the fruit, however, distinct alternating fluxes of water in the xylem and of Ca in the basal and apical directions occur. In the dorsal vascular strand translocation in the basal direction, i.e. towards the fruit stem, dominates. During seed development the amount of Ca in the seeds increases continuously, whereas the amount of Ca in the pod decreases at a similar rate. The high mobility of Ca within the pod and the preferential uptake of Ca by the seed from the ventral side of the pod as compared to that from the dorsal side are further indications for a direct transfer of Ca from the pod tissue into the seed via the seedcoat, i.e. an apoplastic Ca transport. Within the bean fruit the seedcoat has both the highest Ca content and the highest cation exchange capacity. The oxalate content of the bean fruit tissue is very low. According to these results obtained from bean fruits, the Ca supply to the developing seed is effectively regulated by the demand of the seed. Redistribution of Ca from the pod through the seedcoat into the cotyledons is the dominating mechanism of Ca supply. In later stages of seed growth, in particular, there appears to be no long-distance transport of Ca in the vascular system into the seed.

Wirkung von 2,3,5-Trijodbenzoesaure (TIBA) auf den Calcium-transportund die Kationenaustauschkapazität in Sonnenblumen

Summary In water culture experiments with sunflower, a single spraying of the shoot with TIBA(100 ppm) induced growth depression and calcium deficiency symptoms within 8-10 days, and led finally to necrosis of the shoot apex. Simultaneous spraying with CaSO 4 twice a day prevented this induced calcium deficiency and nearly overcame the TIBA effect on growth. TIBA only inhibited translocation of calcium into the shoot apex, but not that of potassium,magnesium or phosphate. Particularly calcium translocation from the petioles into the blades of the youngest leaves was inhibited by TIBA. TIBA lowered the transpiration rate of the fully developed leaves only but not of theyounger ones and caused insufficient formation of sclerenchymatic tissue in the lower part of the stem. The differentiation of the vascular strands in the upper stem and in the youngest leaves, however, was hardly influenced by TIBA. TIBA decreased significantly the cation exchange capacity (CEC) in the upper part ofthe stem, especially in the apical bud and the youngest leaves. Much less calcium but more potassium was translocated into those parts with lowered CEC. The results indicate a connection between the TIBA-induced inhibition of calcium translocationinto the shoot apex and the inhibitory effect of TIBA in IES transport. This inhibition of IES transport could lead to a lower activity of the IES-stimulated proton efflux pump in the elongation zones of the shoot apex. This, in turn, could cause a decrease in formation of new exchange sites (i.e. CEC) in these regions and therefore a lower attraction of the shoot apex for calcium.

Umtausch von kalium in verschiedenen Wurzelzonen von Maiskeimpflanzen

Summary 1. At various distances from the root tip 42 K was supplied to a 3 cm long zone of the primary root of intact corn seedlings. At the same time the remainder of the root system was placed into an identical solution, but without labelled potassium. The duration of the experiments was 24 hours. A remarkable exchange of K + ( 42 K) between roots and the external solution was detectable. Approximately 20 % of K + ( 42 K) taken up by a 3 cm long root zone were exchanged in other root zones by K + from the external solution and 42 K was released there (K + -efflux). This exchange and efflux of K + mainly occurred in the root tip zones; here the efflux rate was more than 40 %. 2. The high rate of K + -turnover within an intact corn root was shown by supply of 42 K to different root zones. Despite of the relative high concentration of K + in the external solution (1 meq/l) supplied to the whole root system, there was an additional pronounced translocation of K + from other root parts to the tip zones of the primary root and the side roots. Therefore, the root tip zones acted as a «sink« for K + taken up in other root regions. In fully differentiated zones (ca. 6–9 cm behind the root tip), however, this «sink« action for K + was scarcely visible. 3. Due to this translocation from other root zones and the exchange processes with K + from the external solution (K + -efflux) within 24 hours more than 70 % of K + were exchanged in those tip zones. 4. The results support the view that within the cytoplasm K + is easily exchangeable and that even in the presence of Ca ++ the plasmalemma of corn roots is highly permeable for K + in both directions, at least in the concentration range used in this experiment.