The effect of lime on phosphorus adsorption and barley growth in three acid soils

For three acid soils from Santa Catarina, Brazil, lime application and time of incubation with lime had little effect on the adsorption of added phosphorus. In two soils with high contents of exchangeable aluminium, solution P and isotopically exchangeable P were decreased by incubating with lime for 1 month: phosphorus was probably adsorbing on freshly precipitated aluminium hydrous oxides. In one soil with less exchangeable aluminium, P in solution was increased by liming. After 23 months lime increased solution and exchangeable P possibly due to crystallization of aluminium hydrous oxides reducing the number of sites for P adsorption. All these changes were however small. In a pot experiment, lime and phosphorus markedly increased barley shoot and root dry matter and P uptake. Although liming reduced P availability measured by solution P, isotopically exchangeable P and resin extractable P, it increased phosphorus uptake by reducing aluminium toxicity and promoting better root growth. The soil aluminium saturation was reduced by liming, but the concentration of aluminium in roots changed only slightly. The roots accumulated aluminium without apparently being damaged.     

Fine-root biomass and soil properties in a semideciduous and a lower montane rain forest in Panama

The distribution of root biomass and physical and chemical properties of the soils were studied in a semideciduous and in a lower montane rain forest in Panama. Roots and soil samples were taken by means of soil cores (25 cm deep) and divided into five, 5-cm deep sections. Soils were wet-sieved to retrieve the roots that were classified in four diameter classes: very fine roots (<1 mm), fine roots (1–2 mm), medium roots (2–5 mm) and coarse roots (5–50 mm). Soil samples were analyzed for organic carbon, total nitrogen, available phosphorus, exchangeable bases, cation exchange capacity, pH, aluminium and exchangeable acidity. Total root biomass measured with the soil corer (roots <50 mm in diameter) was not different between the forests (9.45 t ha^-1), while biomass of very fine roots was larger in the mountains (2.00 t ha^-1) than in the lowlands (1.44 t ha^-1). The soils in the semideciduous forest were low in available phosphorus, while in the mountains, soils had low pH, high exchangeable aluminium and exchangeable acidity, and low concentration of exchangeable bases. Phosphorus was in high concentration only in the first 5 cm of the soil. In both forests, there was an exponential reduction of root biomass with increasing depth, and most of the variation in the vertical distribution of roots less than 2 mm in diameter was explained by the concentration of nitrogen in the soils. The results of this study support the hypothesis that a large root biomass in montane forests is related to nutrients in low concentration and diluted in organic soils with high CEC and low bulk density, and that fine root biomass in tropical forests in inversely related to calcium availability but not a phosphorus as has been suggested for other forests.     

The Effect of Aluminium and some other Trivalent Metal Cations on Cell Division in the Root Apices of Allium cepa

The morphological abnormalities of root systems treated withaluminium salts are such that they may be explained by an inhibitoryeffect of aluminium on either cell division or cell extension.Elongation of onion roots was completely inhibited after 6–8hours' treatment with 10^–3and 10^–4M aluminium sulphatesolutions. Examination of aceto-carmine squashes of root apicesshowed that the cessation of root elongation was closely correlatedwith the disappearance of mitotic figures. The time taken forcomplete inhibition of cell division and root elongation wasdependent on the ambient temperature. Abnormalities of the mitoticapparatus were not seen. Treatment of onion roots with othertrivalent metals, gallium, indium, and lanthanum, produced similarresults. It is concluded that some mechanism associated with cell divisionis highly sensitive to aluminium and is permanently damagedby short exposures. The results are not satisfactorily explainedby the well-known effect of aluminium on phosphorus uptake.     

Root-zone temperature and salinity: Interacting effects on tillering,growth and element concentration in barley

The effects of root-zone salinity (0, 30, and 60 mmol L^–1 of NaCl) and root-zone temperature (10, 15, 20, and 25°C) and their interactions on the number of tillers, total dry matter production, and the concentration of nutrients in the roots and tops of barley (Hordeum vulgare L.) were studied. Experiments were conducted in growth chambers (day/night photoperiod of 16/8 h and constant air temperature of 20°C) and under water-culture conditions. Salinity and root temperature affected all the parameters tested. Interactions between salinity and temperature were significant (p<0.05) for the number of tillers, growth of tops and roots, and the concentration of Na, K, P in the tops and the concentration of P in the roots. Maximum number of tillers and the highest dry matter were produced when the root temperature was at the intermediate levels of 15 to 20°C. Effect of salinity on most parameters tested strongly depended on the prevailing root temperature. For example, at root temperature of 10°C addition of 30 mmol L^–1 NaCl to the nutrient solution stimulated the growth of barley roots; at root temperature of 25°C, however, the same NaCl concentration inhibited the root growth. At 60 mmol L^–1, root and shoot growth were maximum when root temperature was kept at the intermediate level of 15°C; most inhibition of salinity occurred at both low (10°C) and high (25°C) root temperatures. As the root temperature was raised from 10 to 25°C, the concentration of Na generally decreased in the tops and increased in the roots. At a given Na concentration in the tops or in the roots, respective growth of tops or roots was much less inhibited if the roots were grown at 15–20°C. It is concluded that the tolerance of barley plant to NaCl salinity of the rooting media appears to be altered by the root temperature and is highest if the root temperature is kept at 15 to 20°C.     

Differences between maize and wheat in growth-related nutrient demand and uptake of potassium and phosphorus at suboptimal root zone temperatures

The effects of low root zone temperatures (RZT) on nutrient demand for growth and the capacity for nutrient acquisition were compared in maize and wheat growing in nutrient solution. To differentiate between direct temperature effects on nutrient uptake and indirect effects via an altered ratio of shoot to root growth, the plants were grown with their shoot base including apical shoot meristem either within the root zone (low SB), i.e. at RZT (12°, 16°, or 20°C) or, above the root zone (high SB), i.e. at uniformly high air temperature (20°/16° day/night).At low SB, suboptimal RZT reduced shoot growth more than root growth in wheat, whereas the opposite was true in maize. However, in both species the shoot growth rate per unit weight of roots, which was taken as parameter for the shoot demand for mineral nutrients per unit of roots, decreased at low RZT. Accordingly, the concentrations of potassium (K) and phosphorus (P) remained constant or even increased at low RZT despite reduced uptake rates.At high SB, shoot growth at low RZT in both species was higher than at low SB, whereas root growth was not increased. At high SB, the shoot demand per unit of roots was similar for all RZT in wheat, but increased with decreasing RZT in maize. Uptake rates of K at high SB and low RZT adapted to shoot demand within four days, and were even higher in maize than in wheat. Uptake rates of P adapted more slowly to shoot demand in both species, resulting in reduced concentrations of P in the shoot, particularly in maize.In conclusion, the two species did not markedly differ in their physiological capacity for uptake of K and P at low RZT. However, maize had a lower ability than wheat to adapt morphologically to suboptimal RZT by increasing biomass allocation towards the roots. This may cause a greater susceptibility of maize to nutrient deficiency, particularly if the temperatures around the shoot base are high and uptake is limited by nutrient transport processes in the soil towards the roots.     

The influence of root temperature on 14C assimilate profiles in wheat roots

Summary The rate of translocation of ¹⁴C assimilates from leaves to seminal roots in wheat seedlings was considerably reduced by lowering root temperature from 20° to 10° or 5° although the total translocation of ¹⁴C to the roots after 24 h was little affected by temperature. The lowered root temperatures (particularly 5°) resulted in a more uniform distribution of assimilate along the roots than did a temperature of 20°, the ratios of radioactivity/cm in the apical cm, elongating zone, and basal parts of the root after 24 h being 14.0:9.6:1 in 20° roots by contrast with 2.8:1:1 in 5° roots. Temperature effects on assimilate distribution may help explain the observations that for roots grown below 15° ion uptake is sustained in older parts and that roots grown at a low temperature are thicker than roots grown at a higher one.     

Mechanisms of reaction of hematoxylin with aluminium-treated wheat roots

Hematoxylin stain is used for localization of aluminium in plant root tissue and is the basis of a rapid assay of relative At tolerance among wheat cultivars. In the present study, mechanisms by which hematoxylin might selectively stain Al-sensitive wheat roots have been examined. The results are consistent with the idea that in Al-sensitive cultivars, hematoxylin forms complexes with Al that precipitate with phosphate as AlPO₄ in intercellular spaces: (1) Al and P are co-localized in the cell wall region of the outer cortex of Al-stressed roots by using x-ray microanalysis: (2) the molybdenum blue histochemical stain for extracellular phosphate reveals areas of stain that parallel those observed with hematoxylin; (3) in vitro, the presence of phosphate in an Al-hematoxylin reaction mixture causes formation of precipitate when the P/Al ratio exceeds 1. 0. I suggest that selective hematoxylin staining of Al-sensitive wheat cultivars is the result of direct damage by Al to root cells, leading to leakage of phosphorus into the cell wall region. Cultivars whose roots are damaged by Al in this way are likely to be judged Al-sensitive when other criteria such as growth or crop yield are used.     

Architectural Tradeoffs between Adventitious and Basal Roots for Phosphorus Acquisition

Adventitious rooting contributes to efficient phosphorus acquisition by enhancing topsoil foraging. However, metabolic investment in adventitious roots may retard the development of other root classes such as basal roots, which are also important for phosphorus acquisition. In this study we quantitatively assessed the potential effects of adventitious rooting on basal root growth and whole plant phosphorus acquisition in young bean plants. The geometric simulation model SimRoot was used to dynamically model root systems with varying architecture and C availability growing for 21 days at 3 planting depths in 3 soil types with contrasting nutrient mobility. Simulated root architectures, tradeoffs between adventitious and basal root growth, and phosphorus acquisition were validated with empirical measurements. Phosphorus acquisition and phosphorus acquisition efficiency (defined as mol phosphorus acquired per mol C allocated to roots) were estimated for plants growing in soil in which phosphorus availability was uniform with depth or was greatest in the topsoil, as occurs in most natural soils. Phosphorus acquisition and acquisition efficiency increased with increasing allocation to adventitious roots in stratified soil, due to increased phosphorus depletion of surface soil. In uniform soil, increased adventitious rooting decreased phosphorus acquisition by reducing the growth of lateral roots arising from the tap root and basal roots. The benefit of adventitious roots for phosphorus acquisition was dependent on the specific respiration rate of adventitious roots as well as on whether overall C allocation to root growth was increased, as occurs in plants under phosphorus stress, or was lower, as observed in unstressed plants. In stratified soil, adventitious rooting reduced the growth of tap and basal lateral roots, yet phosphorus acquisition increased by up to 10% when total C allocation to roots was high and adventitious root respiration was similar to that in basal roots. With C allocation to roots decreased by 38%, adventitious roots still increased phosphorus acquisition by 5%. Allocation to adventitious roots enhanced phosphorus acquisition and efficiency as long as the specific respiration of adventitious roots was similar to that of basal roots and less than twice that of tap roots. When adventitious roots were assigned greater specific respiration rates, increased adventitious rooting reduced phosphorus acquisition and efficiency by diverting carbohydrate from other root types. Varying the phosphorus diffusion coefficient to reflect varying mobilities in different soil types had little effect on the value of adventitious rooting for phosphorus acquisition. Adventitious roots benefited plants regardless of basal root growth angle. Seed planting depth only affected phosphorus uptake and efficiency when seed was planted below the high phosphorus surface stratum. Our results confirm the importance of root respiration in nutrient foraging strategies, and demonstrate functional tradeoffs among distinct components of the root system. These results will be useful in developing ideotypes for more nutrient efficient crops.     

Effect of adsorbed cations on phosphorus uptake by excised roots

Pretreatment of excised roots of Hordeum vulgare, Zea mays, and Glycine max with various salt solutions affected their subsequent rate of phosphorus absorption from 2 × 10⁻⁵m KH₂PO₄. The rate of absorption was greatest for roots pretreated with trivalent cations, intermediate with divalent cations and lowest with monovalent cations. It appeared that the pretreatment involved a rapid exchange reaction at the root surface which was reversible. A 1 min pretreatment was effective for more than 20 min. The acceleration of phosphorus uptake by roots produced by polyvalent cations may be due partly or entirely to a greater reduction in the electrical potential at the root surface or within the pores of the negatively charged cell wall by polyvalent cations than by monovalent cations.     

The temperature-dependence of the maximum axial growth pressure of roots of pea (Pisum sativum L.)

Although it has been suggested that the maximum axial growth pressure of roots is temperature-dependent, this has not previously been tested experimentally. In this paper we report the temperature-dependence of the maximum axial growth pressure of completely mechanically-impeded roots of pea (Pisum sativum L. cv Meteor). Maximum growth pressures were somewhat lower at 15 and 20°C than at 10, 25 or 30°C, but there was no overall trend for maximum growth pressure to increase or decrease with temperature. Turgor pressure in unimpeded roots varied little with temperature and we suggest that cell wall tension in completely impeded roots also varies little with temperature.     

Cell wall peroxidase against ferulic acid, lignin, and NaCl-reduced root growth of rice seedlings

The changes in cell wall peroxidase activity against ferulic acid (FPOD) and lignin level in roots of NaCl-stressed rice seedlings and their correlation with root growth were investigated. Increasing concentrations of NaCl from 50 to 150 mmol L⁻¹ progressively decreases root growth. The reduction of root growth by NaCl is closely correlated with the increase in FPOD activity extracted from the cell wall. In contrast, lignin level was reduced by NaCl. Since proline and ammonium accumulations are associated with root growth inhibition caused by NaCl, we determined the effect of proline or NH₄Cl on root growth and FPOD in roots. Exogenous application of NH₄Cl or proline markedly inhibited root growth and increased FPOD activity in rice seedlings in the absence of NaCl. An increase in FPOD activity in roots preceded inhibition of root growth caused either by NaCl, NH₄Cl, or proline. Our results suggest that cell-wall stiffening catalyzed by FPOD may participate in the regulation of root growth reduction of rice seedlings caused by NaCl.     

The role of cell wall components from groundnut roots in solubilizing sparingly soluble phosphorus in low fertility soils

Groundnuts have a superior ability to take up P from soils with low P fertility compared to sorghum and soybean. Previous experiments showed that this ability was neither attributable to better root development nor to root exudates capable of solubilizing Fe- and Al-bound P, the sparingly soluble P forms in soils. Direct "contact reactions" between cell wall components from these 3 plant species (groundnut, soybean and sorghum) and P-fixing Fe and Al minerals were examined. Cell wall preparations from groundnut roots showed a superior P solubilizing ability than those of soybean and sorghum. Cell wall activity of groundnut roots may thus at least partly explain the superior growth of this crop under P-deficient conditions. To characterize the active site responsible for P solubilization, effects of pH, heat, addition of cations, and digestion with enzymes (pectinase and cellulase) or HCl on P solubilization were investigated. Conclusion are 1) Solubilizing ability is not related to root CEC because soybean with higher root CEC showed an inferior solubilizing ability compared to groundnut. 2) The reaction site of cell-walls of groundnut roots is stable against heating and digestion with cellulase and pectinase. 3) Solubilizing ability was severely reduced by digestion with HCl. 4) Pre-treating cell walls with either Al3+, Fe3+, or Ga3+ decreased solubilizing ability but cations with lower valency such as Na+, K+, Ca2+ or Mg2+ had no effect. Soaking roots of groundnuts grown in solution culture in 0.5 M NaOH for 30 seconds prior to cell wall preparation led to a 30% reduction in solubilization of P from FePO4 without permanently damaging plants. This suggests that 5) the active component of the cell walls was located on the root epidermal cell surfaces. Based on these results a phosphorus solubilizing mechanism is proposed.     

Changes in percentage organic carbon content during ontogeny

Changes in percentage organic carbon content were assessed during the first five weeks of growth of Uniculm barley (Hordeum vulgare) and Brussels sprouts (Brassica oleracea) plants grown in controlled-environment conditions at two constant temperatures, 16° and 22°C. Foliage (leaf laminae), stem, and root material was assayed in both species, together with leaf sheaths of barley and cotyledon laminae of Brussels sprouts. In barley, there was a decline in percentage organic carbon content with increasing foliage age in plants grown at 22°C, but in sheath material there was no significant change at either temperature. Root material showed a decline in percentage carbon content at both growth temperatures, whereas stems showed the opposite trend. Similar results were found in Brussels sprouts, with an overall decline in percentage carbon content in foliage at 22°C and a rise in stem material at both growth temperatures. However, roots showed no significant change in percentage carbon content over the experimental period. The results demonstrate that percentage organic carbon content may change during plant growth.     

The response to auxin of rapeseed (Brassica napus L.) roots displaying reduced gravitropism due to transformation by Agrobacterium rhizogenes

It has recently been documented that, compared to untransformed controls, the roots of oilseed rape (Brassica napus L. CV CrGC5) seedlings transformed by Agrobacterium rhizogenes A4 show a reduced gravitropic reaction (Legué et al. 1994, Physiol Plant 91: 559–566). After stimulation at 90°C or 135°, the transformed root tips curve, but never reach a vertical orientation. In the present study, we investigated the causes of reduced gravitropic bending observed in stimulated transformed root tips. First, we localized the gravitropic curvature in normal and in transformed roots after 1.5 h of stimulation. The cells involved in root curvature (target cells) corresponded at the cellular level to the apical part of the zone of increasing cell length. In transformed roots grown in the vertical position, these cells showed a reduction in cell length compared to controls. Because auxin is considered to be the gravitropic mediator, the response of normal and transformed roots to exogenous auxin was studied. Indole-3-acetic acid (IAA) was applied along the first 3 mm using resin beads loaded with the hormone. In comparison to normal roots, transformed roots showed reduced bending toward the bead at all points of bead application. Moreover, the cells which responded to IAA corresponded to the target cells involved in the gravitropic reaction. The level of endogenous IAA was lower in transformed roots. Thus, it was concluded that the modified behavior of transformed roots during gravitropic stimulation could be due to differences either in IAA levels or in reactivity of the target cells to the message from the cap.Abbreviations DEZ distal elongation zone - ELISA enzymelinked immunosorbent assay - T-DNA DNA transferred from Agrobacterium rhizogenes to the plant genome This work was supported by the Centre National d'Etudes Spatiales.     

Depletion of extracellular calcium increases cadmium toxicity in barley root tip via enhanced Cd uptake-mediated superoxide generation and cell death

Whereas severe Cd stress (150 µM Cd) causes root growth arrest as a consequence of marked superoxide generation leading to extensive cell death in the root tips, mild Cd stress (15 µM Cd) evokes morphogenic responses, such as reduced root elongation and radial root expansion, resulting in shorter and thicker roots. Similar to the low Cd concentration-caused mild stress, treatment of roots with either Ba to remove exchangeable or EDTA to remove both exchangeable and tightly bound cations, including Ca and Mg, from the apoplast, induced root growth inhibition and swelling. However, pre-treatment of roots with Ba had a synergistic effect on the development of these mild Cd stress-induced morphogenic responses, but without the development of any other symptoms in the root tips. In turn, EDTA pre-treatment markedly increased the toxicity of Cd in barley root tips via enhanced Cd uptake-mediated superoxide generation, which evoked extensive cell death in the transition zone of root tips identically to the high Cd concentration-induced severe stress. While the mild stress-induced responses were alleviated by the inhibition of auxin signalling pathway, the severe stress-induced symptoms were prevented by Ca, but not Mg, supplementation or by the inhibition of Cd uptake into the root symplasm. Therefore, the appropriate concentration of Ca in the apoplast is crucial to prevent the rapid accumulation of Cd in the symplasm, which above a certain threshold level leads to the huge superoxide generation and cell death.     

Boron-induced amelioration of aluminium toxicity in a monocot and a dicot species

Previous research has reported inconsistent results from experiments on the influence of boron (B) on plant sensitivity to potentially toxic aluminium (Al) concentrations. Differences in B requirement and cell wall properties among species, especially between Poaceae and dicots, may account for this. This investigation reports amelioration by B of Al-induced inhibition of root elongation in Al-sensitive cucumber (Cucumis sativus), but not in Al-sensitive maize (Zea mays). Vital staining, however, also revealed a positive influence of B supply on Al tolerance in maize. In both species, adequate B supply decreased Al-induced damage of cell integrity. In cucumber, increasing B supply enhanced Al concentrations and haematoxylin staining in root tips. In maize, no differences for root Al among B treatments were observed. These results indicate that the positive effect of B on Al resistance was not due to less Al accumulation in root tips. Enhanced concentrations of reduced glutathione were found in roots of Al-stressed maize plants growing with adequate B. It is concluded that adequate B supply is essential for prevention of Al toxicity in both the dicot and the monocot species. In dicot cucumber, the B-induced amelioration of root elongation, despite higher Al accumulation in root tips, indicates B-induced change in either or both Al speciation and compartmentation in the tips. The protection by an adequate B supply of roots against Al-induced cell death suggests a role for B in the defence against oxidative stress. This is supported by the observation that Al induced enhanced levels of GSH in roots of maize plants growing with adequate B supply but not in those growing with either deficient or excess B concentrations.     

Temperature effects on steady-state growth,phosphorus uptake,and the chemical composition of a marine phytoplankter

The marine chrysophyteMonochrysis lutheri was grown in phosphorus-limited continuous cultures at temperatures of 15°, 18.8° and 23°C. The effect of temperature on the maximum growth rate was well-defined by the Arrhenius equation, but the Q₁₀ for this alga (1.7) was somewhat lower than has been determined previously for many other phytoplankton species (2.0–2.2). The minimum phosphorus cell quota was relatively unaffected by temperature at 18.8°C and 23°C, but doubled in magnitude at 15°C. As a result, the internal nutrient equation of Droop described the relationship between specific growth rate and phosphorus cell quota well at 18.8° and 23°C, but was less successful at 15°C. The major limitation in using the Droop equation is that the ratio between the minimum and maximum cell quotas must be known, thus necessitating the need to establish the true maximum growth rate by the cell washout technique. In addition, the phosphorus uptake rate on a cell basis at a given steady state growth rate (=specific uptake rate) increased dramatically at 15°C, whereas the turnover rate of total available phosphorus was unaffected by temperature. Both the nitrogen and carbon cell quotas were relatively unaffected by growth rate at a given temperature, but the average values increased slightly with decreasing temperature. The overall conclusion is that phytoplankton growth and limiting-nutrient uptake rates are only synchronous at or near the optimum temperature. Because these types of responses are species specific, much additional data on temperature effects will be required before the importance of including such effects in phytoplankton-nutrient models can be determined.     

Effects of liming on rhizosphere chemistry and growth of fine roots and of shoots of sessile oak (Quercus petraea)

Soil acidification can be detrimental to root growth and nutrient uptake, and liming may alleviate such acidification. In the following study, seedlings of sessile oak (Quercus petraea Liebl. M.) were grown in rhizotrons and subjected to liming (L) or gypsum (G) treatments and compared with the control (C). In order to study and interpret the impact of these calcium rich treatments on fine root development and tree growth, the following parameters were assessed: fine root biomass, fine root length, seedling development (height, diameter, leaves), seedling biomass, nutrient content of roots and seedlings, bulk soil and soil solution chemistry and rhizosphere soil chemistry. The results show that liming increased bulk soil pH, exchangeable Mg, Ca and the Ca/Al molar ratio, and decreased exchangeable Al, mainly in the A-horizon. Gypsum had a similar but smaller impact on exchangeable Al, Ca, H⁺ and the Ca/Al molar ratio in the A-horizon, but reacted with depth, so that exchangeable Mn, Mg and Ca were increased in the B-horizon. In the rhizosphere, the general pattern was determined by the treatment effects of the bulk soil. Most elements were more concentrated in the rhizosphere than in bulk soil, except for Ca which was less concentrated after liming or gypsum application. In the B-horizon rhizosphere pH was increased by the treatments (L > G,C) close to the root tips. Furthermore, the length of the zone with a positive root-induced pH increase was greater for the limed roots as compared with both the other treatments. Fine root growth was stimulated by liming (L > G,C) both in terms of biomass and length, whereas specific root length was not obviously affected apart from the indication of some stimulation after liming at the beginning. The live:dead ratio of fine roots was significantly higher in the limed rhizotrons as compared to the control (G not assessed), indicating lower mortality (higher longevity). Shoot growth showed greater lime-induced stimulation (L > G,C) as compared to root growth. As a result the shoot:root ratio was higher in the limed rhizotrons than in the control (L > G,C). Liming induced a higher allocation of P, S, Mg, Ca and K to the leaves, stem and twigs. Gypsum showed similar effects, but was only significant for S. Liming increased the foliar Ca/Al ratio by both increasing foliar Ca and decreasing foliar Al, whereas gypsum did not clearly improve foliar nutrition. This study suggests that a moderate application of lime can be successful in stimulating seedling growth, but that gypsum had no effect on seedling growth. It can be concluded that this lime-induced growth stimulation is directly related to the improved soil fertility status, and the alleviation of Al toxicity and acid stress, resulting in better foliar nutrition. The impact of liming on fine roots, as a consequence, was not limited to a stimulation of the total amount of fine roots, but also improved the root uptake performance. This revised version was published online in June 2006 with corrections to the Cover Date.     

The infection process ofFusarium oxysporum in cotton root tips

Summary Conidia ofFusarium oxysporum f. sp.vasinfectum started to germinate on the roots of cotton (Gossypium barbadense L.) 6 h after inoculation and formed a compact mycelium covering the root surface. 18 h later, penetration hyphae branched off and infected the root. The number of penetration hyphae increased with the number of conidia used for inoculation. The optimal temperature for penetration was between 28 and 30 °C. The highest numbers of penetration hyphae were found in the meristematic zone, 40 percent less in the elongation and root hair zones, and none in the lateral root zone. The fine structure of the infection process was studied in protodermal cells of the meristematic zone and in rhizodermal cells of the elongation zone. The penetration hyphae were well preserved after freeze substitution and showed a Golgi equivalent consisting of three populations of smooth cisternae. Plant reactions were found already during fungal growth on the root surface. In the meristematic zone, a thickening of the plant cell wall due to an apposition of dark and lightly staining material below the hyphae occurred. This wall apposition increased in size around the hypha invading the plant cell and led to the formation of a prominent wall apposition with finger-like projections into the host cytoplasm. In the elongation zone, the deposits around the penetration hypha appeared less thick and the dark inclusions were less pronounced. High pressure freezing of infected cells revealed, thatF. oxysporum penetrates and grows within the host cells without inducing damages such as plasmolysis, cell degeneration or even host necrosis. We suggest thatF. oxysporum has an endophytic or biotrophic phase during colonization of the root tips.Abbreviation Ph penetration hyphae     

A preliminary evaluation of the use of microbial culture filtrates for the control of contaminants in plant tissue culture systems

The partitioning of carbon between reserve polysaccharide and alkaloid secondary products was investigated in batch cultures of transformed roots of Datura stramonium grown in media in which the carbon substrate concentration was held constant and the level of mineral nutrients was varied. The growth and accumulation of starch and hyoscyamine was examined in roots grown at temperatures of 20°C, 25°C or 30°C in media containing 5% sucrose and levels of mineral nutrients varying from 1/4 to twice the standard level of Gamborg's B5 salts. The dry matter content was highest (up to 15% w/w) in roots grown at either 20°C or 25°C in medium of the lowest ionic strenth (1/4 B5 salts) and decreased as the ionic strength was raised (down to 7% w/w with 2 B5 salts). Up to half of this decrease could be accounted for by loss of starch from the roots. At 20°C and 25°C, the starch content of the roots grown in medium of the lowest ionic strength (1/4 B5) was 40 mg g^-1 and 22 mg g^-1 fresh weight respectively but decreased to less than 1 mg g^-1 weight at either temperature when the ionic strength of the medium was raised to 2 B5. At 30°C, starch accumulation was severely inhibited in all media. In contrast, varying either the temperature or the ionic strength of the medium had only a small effect on hyoscyamine accumulation which remained at between 0.4–0.6 mg g^-1 fresh weight. Although increases in the level of mineral salts had little effect on the hyoscyamine content of the roots, total yields however, increased due to stimulation of growth. Time course experiments showed that cultures grown at either 20°C or 25°C continued to accumulate both starch and hyoscyamine into late stationary phase.