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In field-grown maize (Zea mays L.) at different growth stages xylem exudate was collected and analyzed for nitrogen compounds and sugars. The plants were grown at two levels of nitrogen (N) fertilization (low N, high N). In an additional experiment in some plants the cobs were removed after anthesis. The concentrations of nitrate- and reduced-N in the exudate were greater at high than at low N fertilization. At both levels of N fertilization, during plant development the concentrations of nitrate-N declined but not those of reduced-N, leading to an increase in the percentage of reduced-N up to 80-90 per cent of the total N in the exudate. This pattern may reflect either an increase in the proportion of nitrate reduction in the roots, or of N cycling between the shoot and the roots, or both. Increased internal cycling of N was considered to be the main cause of the significant increase in the concentrations of reduced N in the exudate after cob removal. The concentrations of reducing sugars and sucrose in the exudate, at both levels of N fertilization, increased after anthesis to more than 20 mM, and after ear removal to more than 50 mM. Cycling of sugars between shoot and roots and mobilization of sugars previously stored in the roots are discussed as possible causes for the high sugar concentrations in the exudate, which might significantly contribute to the osmotic driving force for the volume flow in the xylem.

Graminaceous species can enhance acquisition of iron (Fe) by release of phytosiderophores mainly from apical root zones. However, phytosiderophores are readily degradable by microorganisms. To study the effect of rhizosphere microorganisms in Fe acquisition, maize plants were grown axenically or inoculated with a mixture of microorganisms in a limestone substrate supplemented with a small amount of FeIII-oxide. Axenic plants grew well without Fe deficiency symptoms and released considerable amounts of phytosiderophores. In contrast, inoculated plants showed severe symptoms of Fe deficiency chlorosis and much less phytosiderophores were detectable in the substrate. The severity of Fe deficiency chlorosis was strongly influenced by the mode of water supply either continuously by glass fibre wicks or by periodic short-term flooding. Inoculated plants became more chlorotic, when watered by flooding than plants watered by glass fibre wicks. This was suggested to be due to greater microbial degradation of phytosiderophores as a consequence of higher microbial population density in apical root zones, the sites of phytosiderophore release. To prove this hypothesis maize plants were grown in a silty loam soil. Short-term periodic flooding resulted in a uniform distribution pattern of rhizosphere microorganisms along the root axis, whereas in non-flooded plants the number of rhizosphere microorganisms was lower in apical root zones and increased sharply from 0–20 to 20–40 mm distance from the root apex. It is concluded, that in solid substrates a low population density of rhizosphere microorganisms in the apical root zone is of particular importance for efficient Fe acquisition by phytosiderophores in graminaceous species like maize.

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.

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.

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.

Summary In order to measure the pH at the soil-root interface (rhizosphere) of growing roots, 4 different plant species (maize, wheat, chickpea and white clover) were grown in plexiglass boxes in a soil of pH 6.0. Nitrogen was supplied at different rates and in different forms (nitrate, ammonium and N2 fixation, resp.). After a growth period between 4 and 10 days the soil and the roots were infiltrated with agar (35–38°C) containing bromocresol purple as pH indicator. The pH in the rhizosphere was also measured by microelectrodes inserted into the agar. Compared to the bulk soil (pH 6.0) in the rhizosphere of maize plants the pH increased to about 7.5 with NO3--N and dropped to about 4.0 with NH4+-N as the N supply. However, with NO3--N striking pH differences were seen both along the roots and between the various root zones of the maize plants. In contrast to maize, the pH of the rhizosphere of chickpea seedlings decreased to about 4.5 even when supplied with NO3--N. In white clover the rhizosphere pH depended on whether nitrogen was supplied as NO3--N or via N2 fixation (i.e. rhizobial infection): with N3--N the rhizosphere pH increased to about 6.5 and with N2 fixation decreased to about 4.5. In mixed culture of maize and chickpea there were striking differences in pH values in the rhizosphere varying from 4.5 to 7.0. The results demonstrate the extent to which the pH in the rhizosphere can differ from that further from the roots, depending on plant species and nitrogen source. The method described in this paper offers a simple way to obtain an overall view of this pH pattern in the rhizosphere of intact roots.

Summary Roots of intact plants (Helianthus annuus; Calathea warscewizii; Zea mays) were embedded in an agar medium (0.75%) containing nutrient solution and either bromocresol purple (pH indicator) or BPDS (chelator for FeII). Around the apical root zones of Fe deficient sunflower plants, enhanced reduction of FeIII (formation of red coloured FeII(BPDS)3) and increased proton efflux (i.e. acidification of the medium) could be observed within a few hours. Under Fe deficiency, enhanced reduction of manganese (MnFeIV) could also be demonstrated in the same root zones when the roots were embedded in agar with MnCO2. In the agar media with bromocresol purple the effect of the source of nitrogen (NC3- and NH4+ respectively) on pH changes along the roots can be demonstrated within a few hours. Along corn roots with NCO3- supply the pH rises steeply only around certain zones of the seminal root. The pH changes along roots (either induced by the source of nitrogen or the Fe nutritional status) can also be directly measured by inserting microelectrodes into the agar media. With this agar method more detailed information can be obtained on the response of the various root zones of intact plants to either the nutritional status of the plant or to the supply of different nutrients.

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.

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.

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.

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.

Summary K+, Na+, and Ca2+ were determined in stele and root cortex of 9 days old maize (Zea mays L., Velox) and bean plants (Phaseolus vulgaris L., Saxa) after application of these ions either during the pregrowing period to the whole root system, or — labelled with 42K, 22Na or 45Ca — for 24 hours only to a certain 3 cm long root zone of intact plants. It is shown that K+ was distributed equally in the root system between cortex and stele. Na+ was accumulated more in the stele than in the cortex all over the root system, whilst Ca2+ was accumulated more in the root cortex than in the stele in the zone of application. In contrast to K+ and Na+, there was no translocation of Ca2+ in the stele in direction to the root tip (i.e. no phloem transport). The results show that active ion uptake mechanisms seem to be located in the stele as well as in the root cortex.

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.

Summary In sugar beet ( Beta vulgaris L.) growing under controlled climatic conditions, the partial replacement of K + by Na + in the nutrient solution (Na-treatment) significantly increased the dry weight, area and succulence of the leaves, and the number of stomata per leaf area. However, net photosynthesis per leaf area was unaffected. At high water availability (substrate water potential = -0.2 bar), plants of the Na-treatment had higher rates of water consumption than plants supplied with K + only; however, at low water availability (-4.0 bar) the opposite was observed. Under transient «drought-stress» (-7.5 bar, imposed by addition of mannitol for 3 h to the nutrient solution), in plants of the Natreatment stomatal resistance the water vapour diffusion increased more rapidly with onset of stress and decreased more slowly after stress release than in plants supplied with K + only. In agreement with this the leaves of the Na-treatment maintained a higher relative water content (RWC) during «drought stress», and after stress release regained more rapidly both the original RWC and the original rate of net photosynthesis. The abscisic acid (ABA) content of the leaves during transient «drought-stress» and stress release showed a similar pattern in plants supplied with either Na + or K + . No correlation existed between stomatal resistance and ABA content of the leaves. The results demonstrate that Na + increases the «tolerance» of sugar beet leaves to low water availability in the substrate. The beneficial effect of the application of sodium fertilizers in increasing the dry matter production of sugar beet under low rainfall conditions is consistant with these results.

Carrier-free 85Sr was applied to the foliage of three genera of woody forage plants (Ceanothus, Adenostoma and Quercus) and subsequent translocation into untreated portions of the plants was measured. Experiments were conducted in the field and in the greenhouse. In all cases the bulk of the retained dose remained with the directly treated material. However, substantial concentrations of 85Sr were found in the new growth arising on previously treated branches of Ceanothus and Adenostoma. The amounts translocated under field conditions under which plants were subject to periodic wetting and drying were greater than those translocated by plants in the greenhouse.

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.