Mechanism of phosphorus-induced zinc deficiency in cotton. I. Zinc deficiency-enhanced uptake rate of phosphorus

The effect of withholding Zn on the uptake, translocation and accumulation of P was studied in cotton plants ( Gossypium hirsutum L. cv. Deltapine 15/21) grown in nutrient solutions under controlled environmental conditions. The influence of P on the uptake rate, translocation and distribution of ⁶⁵Zn in the plants was also examined. Increasing the P supply resulted in severe Zn deficiency symptoms (interveinal chlorosis) as well as P toxicity symptoms, which were characterized by leaf puckering and grayish-brown marginal necrosis. Zinc deficiency markedly increased the uptake and translocation rates of P over the whole concentration range tested (5x10^-5 to 1.25x10^-3 M ). Uptake and translocation rates of P increased with both level of P and severity of Zn deficiency. This often caused P toxicity symptoms on Zn-deficient leaves. In contrast to P, the concentrations of K and Mg in the leaves were not affected by Zn deficiency. Similar results were obtained for sunflower ( Helianthus annuus L.) and buckwheat ( Fagopyrum esculentum Moench) plants. Higher P concentrations in Zn-deficient leaves or shoots could not be attributed wholly to reduced shoot growth. This was also evident when Zn deficiency was compared with other micronutrient (Fe, Mn, and Cu) deficiencies. Only Zn-deficient plants showed enhanced uptake and translocation of P. In experiments with ⁶⁵Zn, a high P supply did not depress uptake and translocation of Zn. From the results obtained it is concluded that the P-induced Zn deficiency in cotton, as well as in other species, is primarily caused by enhanced P uptake and translocation and not by inhibition of Zn uptake.     

Uptake and distribution of 65Zn and 54Mn in wheat grown at sufficient and deficient levels of Zn and Mn II. During grain development

We investigated the uptake and distribution of Zn and Mn inwheat during grain development. Plants were grown in a chelate-bufferednutrient solution with one of the following treatments: control,low Zn or low Mn. Plants were dual-labelled with ⁶⁵Zn and ⁵⁴Mnat 2 and 8 wk post-anthesis for 5 and 24 h, respectively. Afterlabelling, the plants were separated into individual componentsfor analysis. In the plants harvested at 8 wk after anthesis,spikelets were separated into individual structures and analysedfor radioactivity. Little or no root-supplied ⁵⁴Mn was distributed to the leavesof both the controls and low-Mn plants during the grain developmentstages studied here. More ⁵⁴Mn was distributed to the head at8 wk than at 2 wk post-anthesis. In contrast, root-supplied⁶⁵Zn was transported to the leaves at 2 and 8 wk post-anthesis.More⁶⁵Zn was distributed to the leaves of the low-Zn plants thanthe controls during grain development.More esZn was detectedin the head towards grain maturity. Relatively larger amountsof ⁵⁴Mn than ⁶⁵Zn were found in different parts of the florets.Labelled Mn was found in relatively large quantities in thepalea, lemma and in the glumes, even though most ⁵⁴Mn was foundin the grain. A large percentage of the grain MMn was in theouter pericarp. During grain development leaves may still require Zn but notMn, which may be due to the requirement of Zn in maintainingmembrane structure and function. Distribution of Zn and Mn withinthe spikelets suggests that Zn may enter the grain via the phloemwhile Mnmay enter the grain via the xylem. Key words: Zinc, manganese, nutrient transport, grain development, wheat, Triticum aestivum     

Uptake and loss of dissolved zinc by the stickleback Gasterosteus aculeatus L.

Uptake and loss by sticklebacks of both stable zinc and ⁶⁵Zn in hard and soft water have been studied for periods of upto400 h. In calcium-free water, the ⁶⁵Zn uptake curve is approximately asymptotic over a period of 24 h, while in hard water internal ⁶⁵Zn levels are dropping at 24 h. Over 5 h, however, fish in hard tapwater absorb about 3.5 times more ⁶⁵Zn than those in calcium-free water. There is a positive linear relationship between log ⁶⁵Zn uptake and log wet weight of fish. Whole-body concentration factors (c.f.) at 16 h reach a maximum of 12.2 (mean=2.9), highest concentrations of ⁶⁵Zn being found in the gills (mean c.f.=5.l) and lowest concentrations in the gonads (mean c.f.=0.8). Over longer periods (400 h), internal stable zinc levels offish exposed to 1 and 4 p.p.m. Zn²⁺ remain little higher (max 28 %) than controls. ⁶⁵Zn efflux into zinc-free water falls to zero after 5 h, more zinc (78 %) being lost after uptake in tapwater than in calcium-free water (56 %).     

Transport of Zinc-65 at the Blood-Brain Barrier During Short Cerebrovascular Perfusion in the Rat: Its Enhancement by Histidine

Abstract: Zinc-65 transport into different regions of rat brain has been measured during short vascular perfusion of one cerebral hemisphere with an oxygenated HEPES-containing physiological saline at pH 7.40. The [Zn²⁺] was buffered with either bovine serum albumin or histidine. In each case uptake was linear with time up to 90 s. ⁶⁵Zn flux into brain in the presence of albumin followed Michaelis-Menten kinetics and for parietal cortex had a K _m of 16 n M and a V _max of 44 nmol/kg/min. Increasing concentrations of l -histidine enhanced ⁶⁵Zn flux into brain at [Zn²⁺] values between 1 and 1,000 n M . The combined effect of [histidine] and [Zn²⁺] was best accounted for by a function of [ZnHis⁺], i.e., flux = 64.4 · [ZnHis⁺]/(390 + [ZnHis⁺]) + 0.00378 · [ZnHis⁺], with concentrations being nanomolar. d -Histidine had an influence similar to that of l -histidine. ⁶⁵Zn flux in the presence of 100 µ M l -histidine was not affected by either 500 µ M l -arginine or 500 µ M l -phenylalanine. The results indicate specific transport of Zn²⁺ across the plasma membranes of brain endothelium. The enhancement due to histidine has been attributed to diffusion of ZnHis⁺ across unstirred layers "ferrying" zinc to and from transport sites.     

Transport interactions between cadmium and zinc in roots of bread and durum wheat seedlings

Field studies have shown that the addition of Zn to Cd-containing soils can help reduce accumulation of Cd in crop plants. To understand the mechanisms involved, this study used ¹⁰⁹Cd and ⁶⁵Zn to examine the transport interactions of Zn and Cd at the root cell plasma membrane of bread wheat ( Triticum aestivum L.) and durum wheat ( Triticum turgidum L. var. durum ). Results showed that Cd²⁺ uptake was inhibited by Zn²⁺ and Zn²⁺ uptake was inhibited by Cd²⁺. Concentration-dependent uptake of both Cd²⁺ and Zn²⁺ consisted of a combination of linear binding by cell walls and saturable, Michaelis-Menten influx across the plasma membrane. Saturable influx data from experiments with and without 10 µm concentrations of the corresponding inhibiting ion were converted to double reciprocal plots. The results revealed a competitive interaction between Cd²⁺ and Zn²⁺, confirming that Cd²⁺ and Zn²⁺ share a common transport system at the root cell plasma membrane in both bread and durum wheat. The study suggests that breeding or agronomic strategies that aim to decrease Cd uptake or increase Zn uptake must take into account the potential accompanying change in transport of the competing ion.     

Differential transport of Zn, Me and sucrose along the longitudinal axis of developing wheat grains

The hypothesis that Zn and Mn are transported within the grain in a similar manner to sucrose was investigated in the developing wheat grain. Detached ears were cultured in solution containing ⁶⁵Zn, ⁵⁴Mn and [¹⁴C]-sucrose for 10 to 120 min at 18–22 days post-anthesis. At different times the grain was cut transversely into 1-mm sections and the radioactivity in each section determined The embryo region was damaged in some grains to investigate the effect of reduced accumulation rate on the transport of ⁶⁵Za, ⁵⁴Mn and [¹⁴C]-sucrose to the embryo. The distribution of ⁶⁵Zn. ⁵⁴Mn and [¹⁴C]-sucrose between the endosperm cavity sap. endosperm, embryo and pericarp in grains labelled for 2.5 and 6 h at 18–22 days post-anthesis was also determined. [¹⁴C]-su-crose was initially high in the first, embryo-containing section of the grain but decreased progressively to the distal end of the grain. The amount of ⁶⁵Zn along the longitudinal axis of the grain was distributed evenly in each 1-mm section, whilst ⁵⁴Mn accumulated exponentially in the first proximal 1-mm section of the grain and was distributed evenly in the remaining sections. Damaging the embryo had no effect on ⁶⁵Zn and ⁵⁴Mn transport to the section containing the embryo. The pericarp contained almost all of the grain ⁶⁵Za and ⁵⁴Mn, with small amounts found in the embryo, endosperm and endosperm cavity sap. Increasing amounts of [¹⁴C]-sucrose were found in the endosperm as time progressed. The rate of accumulation of ⁶⁵Zn, ⁵⁴Mn and [¹⁴C]-sucrose was much higher in the embiyo than the endosperm: the difference between the embryo and endosperm was especially large for ⁶⁵Zn and ⁵⁴Mn. It is suggested that ⁶⁵Zn and ⁵⁴Mn are not transported within the grain in the same way as [¹⁴C]-sucrose. [¹⁴C]-sucrose moves laterally out of the vascular system of the crease into the endosperm cavity and is subsequently taken up and stored in the endosperm. In contrast, ⁶⁵Zn and ⁵⁴Mn appear to be retained within the vascular system of the crease and may be transported more slowly to grain parts such as the embryo and pericarp tissue.     

The trans-tissue pathway and chemical fate of 14C photoassimilate in carrot taproot

Axial and radial transport and the accumulation of photoassimilates in carrot taproot were studied using ¹⁴C labelling and autoradiography. Axial transport of the ¹⁴C labelled assimilates inside the taproot was rapid and occurred mainly in the young phloem found in rows radiating from the cambium. The radial transport of the assimilate inward (to cambium, xylem zone and pith) and outward (to phloem zone and periderm) from the conducting phloem was an order of magnitude slower than the longitudinal transport and was probably mainly diffusive. The cambial zone of the taproot presented a partial barrier in the inward path of the assimilate to the xylem zone. We suggest that this is due to the cambium comprising a strong sink for the assimilate on the basis that our previous work has shown that it contains very low concentrations of free sucrose. By contrast, a high accumulation of nonsoluble ¹⁴C was found in the cambium region in good agreement with the active growth of this zone. Autoradiography following the feeding of ¹⁴C labelled sugars to excised sections of taproot indicated that only a ring of cells at and/or just within the cambium take up sugars from the apoplast. This indicates that radial movement in the phloem and pith must be symplastic. An apoplastic step between phloem and xylem is possible. The rapid uptake of sugars from the apoplast at this point might represent a mechanism for keeping photoassimilates away from the transpiration stream and re-location back to the leaves.     

Distribution and remobilization of Zn and Mn during grain development in wheat

Wheat (Triticum aestivum cv. Aroona) was grown in siliceoussand with essential nutrients for unlimited growth except forthe following treatments: controls (sufficient Zn and Mn), lowMn (sufficient Zn) and low Zn (sufficient Mn) until anthesis.Replicate plants were harvested at anthesis; the remaining plantswere transferred to a chelate-buffered nutrient solution containingall essential nutrients except Zn and Mn to allow monitoringof the remobilization of existing Zn and Mn reserves withinthe plant. These plants were harvested 14 d post-anthesis andat grain maturity. At each harvest plants were separated intoindividual components. There were no growth differences between any of the treatmentsat the three harvests. Large amounts of Zn and Mn found in theroots and stems at anthesis were rapidly depleted during graindevelopment. The Zn content of the leaves increased from anthesisto 14 d post-anthesis, but then declined. The Mn content ofthe leaves increased throughout grain development in the controlswhilst remaining constant in the plants pre-grown at low Mn.The Zn and Mn content of the glumes, palea and lemma rose inthe controls from anthesis to 14 d post-anthesis; thereafterZn content declined but Mn content continued to increase. TheZn and Mn content of the grain rose sharply toward grain maturity.We conclude that Mn was not remobil-ized from the leaves ofwheat during grain development. Zinc was remobilized from theleaves, especially the flag leaf and from the leaves of thelow Zn plants. The post-anthesis accumulation of Zn and Mn withinthe glumes will be discussed in relation to the transport pathwaythat Zn and Mn use to enter the developing seed. Key words: Zinc, manganese, wheat, distribution, remobilization     

Actual escape area and lateral escape from the xylem of the alkali ions Na+, K+, Rb+ and Cs+ in tomato

Approximation of the total escape area of the xylem in an inbred line of tomato (Ly-copersicon escutentum Mill. cv. Tiny Tim) with help of the frequency distribution of xylem vessel radii provides the possibility to calculate realistic escape constant values from uptake experiments of several elements into tomato stem segments. Comparison of the lateral escape rates of ²⁴Na⁺, ⁴²K⁺, ⁸⁶Rb⁺ and ¹³⁴Cs⁺ indicate that Na⁺ escape is rate-limited by its uptake into a rather constant number of surrounding cells, regardless of changes in the total escape area of the xylem vessels. The escape of K⁺, Rb⁺ and Cs⁺ seems to be proportional to the surface area of the xylem vessels and their escape is apparently controlled by their transport across the cell walls of the transport channels. The calculated small values for the escape rate constants (apparent permeability of the xylem cell walls, ca 2–3 · 10−⁹ m s−⁷) are probably due to the presence of lignin in the xylem cell walls, the discrimination between ions as a result of differing affinities and selectivities and the presence of other solutes in the applied solution.     

Manipulation of xylem transport affects Zn and Mn transport into developing wheat grains of cultured ears

Zinc and manganese loading into developing wheat grain is little understood at present. The objective of this work was to investigate factors that may affect the rate of transport of Zn and Mn into developing wheat grain of cultured ears. Ears 18-22 days post-anthesis were cultured in solutions containing labelled Zn and Mn. The effect of additions of Cu, Fe, citrate, malate and EDTA to the culture solution was observed. The effect of humidity and awn removal as well as the sucrose status of the ears on Zn and Mn transport was also investigated. The effect of high concentration of Zn and Mn on [¹⁴C]-sucrose transport was determined. High humidity almost completely blocked transport of Zn and Mn into the grain. Awn removal reduced the transport of Zn and Mn to the lemma but not the grain. When the ears were depleted of sucrose (by maintaining them in the dark prior to labelling) transport of Zn and Mn to the grain was reduced compared to ears cultured with sucrose. The presence of Cu reduced the loading of Zn into the grain. There was little effect of Cu on Mn transport or Fe on either Zn or Mn transport. High concentrations of Zn and Mn in the culture solution did not affect [¹⁴C]-sucrose loading into the grain but loading of Zn and Mn was limited at high concentrations suggesting membrane saturation. This study demonstrates that sucrose status and humidity clearly influence the transport of Zn and Mn into the grain, and that other ions may influence Zn and Mn transport.     

Heavy metals in white lupin: uptake, root-to-shoot transfer and redistribution within the plant

The translocation of manganese (Mn), nickel (Ni), cobalt (Co), zinc (Zn) and cadmium (Cd) in white lupin (Lupinus albus cv. Amiga) was compared considering root-to-shoot transport, and redistribution in the root system and in the shoot, as well as the content at different stages of cluster roots and in other roots. To investigate the redistribution of these heavy metals, lupin plants were labelled via the root for 24 h with radionuclides and subsequently grown hydroponically for several weeks. 54Mn, 63Ni and 65Zn were transported via the xylem to the shoot. 63Ni and 65Zn were redistributed afterwards via the phloem from older to younger leaves, while 54Mn remained in the oldest leaves. A strong retention in the root was observed for 57Co and 109Cd. Cluster roots contained higher concentrations of all heavy metals than noncluster roots. Concentrations were generally higher at the beginning of cluster root development (juvenile and immature stages). Mature cluster roots also contained high levels of 54Mn and 57Co, but only reduced concentrations of 63Ni, 65Zn and 109Cd.     

Long distance transport of potassium in cereals during grain filling in detached ears

Ears of wheat plants ( Triticum aestivum L. cv. Kolibri), which were given different and uniform K⁺-nutrition in two experiments, were cut at 2, 4 and 6 weeks after anthesis at 15 cm below the ear. These detached ears were fed 30 m M (experiment 1) or 15, 30, 60 or 90 m M ⁸⁶Rb-K₂ malate (experiment 2) and 146 m M [¹⁴C]-sucrose. After a pulse period of 6 and 4 h, respectively, the ears were transferred to identical non-labeled solutions for additional 0, 4, 8 or 20 h.
About 50% of the K⁺ and sucrose supplied was absorbed by detached ears. This rate declined with plant age and decreasing transpiration. Within the 6 and 4 h uptake period less than 7% of the absorbed K⁺, but 20% of the sucrose taken up were incorporated into the grain. During the chase period labeled K⁺ in the grain increased to 15% and ¹⁴C even to 50% of total tracer uptake. Incorporation of labeled K⁺ into the grain was not affected by the previous K⁺ nutrition of the plant and was proportional to the K⁺ concentration in the uptake solution. Transition of K⁺ from xylem into phloem during its acropetal transport is assumed. No evidence was found that the grain itself could control its uptake of K⁺.     

K/Na selectivity at the plasmalemma of cortical root cells and preferential release of sodium to the xylem vessels in roots of Atriplex hortensis

Using excised roots of Atriplex hortensis L., cv. Gelbe Gartenmelde, the uptake, accumulation and xylem transport of K⁺ and Na⁺ have been measured. Influx as well as xylem transport proved to discriminate little between K⁺ and Na⁺, when considered in relation to the external solution. Both K⁺ and Na⁺ inhibited the uptake and xylem transport of each other to about the same degree. Measurements of intracel-lular Na⁺ fluxes by means of compartment analysis indicated that the low degree of K/Na discrimination during uptake was due to low influx selectivity. Moreover, K⁺/Na⁺ exchange at the plasmalemma was not very efficient in Atriplex roots. In order to establish the basis of the low K/Na discrimination in xylem transport, the rates of K⁺ and Na⁺ transport were related to the cytoplasmic K⁺ and Na⁺ concentrations to yield the selectivity ratio of transport, S(transport) = (φ_cx(K) × [Na⁺]_c)/(φ_cx(Na) × [K⁺]_c). Under all conditions this ratio was far below one indicating that Na⁺ was favoured during xylem release in excised roots of Atriplex at low external concentrations. The implications of this discrimination in favour of Na+ are discussed with respect to salt tolerance of A. hortensis .     

Selective transport of zinc, manganese, nickel, cobalt and cadmium in the root system and transfer to the leaves in young wheat plants

BACKGROUND AND AIMS: The uptake, translocation and redistribution of the heavy metals zinc, manganese, nickel, cobalt and cadmium are relevant for plant nutrition as well as for the quality of harvested plant products. The long-distance transport of these heavy metals within the root system and the release to the shoot in young wheat (Triticum aestivum 'Arina') plants were investigated. METHODS: After the application of 65Zn, 54Mn, 63Ni, 57Co and 109Cd for 24 h to one seminal root (the other seminal roots being excised) of 54-h-old wheat seedlings, the labelled plants were incubated for several days in hydroponic culture on a medium without radionuclides. KEY RESULTS: The content of 65Zn decreased quickly in the labelled part of the root. After the transfer of 65Zn from the roots to the shoot, a further redistribution in the phloem from older to younger leaves was observed. In contrast to 65Zn, 109Cd was released more slowly from the roots to the leaves and was subsequently redistributed in the phloem to the youngest leaves only at trace levels. The content of 63Ni decreased quickly in the labelled part of the root, moving to the newly formed parts of the root system and also accumulating transiently in the expanding leaves. The 54Mn content decreased quickly in the labelled part of the root and increased simultaneously in leaf 1. A strong retention in the labelled part of the root was observed after supplying 57Co. CONCLUSIONS: The dynamics of redistribution of 65Zn, 54Mn, 63Ni, 57Co and 109Cd differed considerably. The rapid redistribution of 63Ni from older to younger leaves throughout the experiment indicated a high mobility in the phloem, while 54Mn was mobile only in the xylem and 57Co was retained in the labelled root without being loaded into the xylem.     

Relationship between polar basipetal auxin transport and acropetal Ca2+ transport into tomato fruits

The dependence of acropetal Ca²⁺ transport on polar basipetal indoleacetic acid (IAA) transport was investigated in excised tomato fruits ( Lycopersicon esculentum L. Mill.) using an in vitro fruit system. Auxin transport inhibitors like triiodobenzoic acid (TIBA), chlorofluorenolmethyl ester (CME) and naphthylphthalamic acid (NPA) were used in order to investigate the effect of restricted polar basipetal auxin transport on the acropetal transport of ⁴⁵Ca²⁺, ⁸⁶Rb⁺ and ⁹⁸Sr²⁺ into the same fruits. TIBA and CME inhibited basipetal transport of IAA. particularly in 10- to 12-day-old tomato fruits, and simultaneously restricted the acropetal transport of ⁴⁵Ca²⁺. The auxin transport inhibitors failed to significantly reduce the upward transport of ⁸⁶Rb⁺ and the transport of ⁹⁶Sr²⁺ was less inhibited than that of ⁴⁵Ca²⁺. TIBA and CME did not significantly affect the acropetal transport of labelled water into the fruit, nor the cation-exchange capacity or K⁺ and Mg²⁺ concentrations in the tomato fruit. These results support the view that a part of the Ca²⁺-specific acropetal transport into tomato fruits is associated with the polar basipetal IAA transport. This Ca²⁺ transport is independent of the transpiration stream into the fruit and the cation exchange capacity of the fruit tissue.     

Single Doses of Acrylamide Reduce Retrograde Transport Velocity

Abstract: Single doses of acrylamide (0–1.3 mmol/kg) produced a dose-dependent decrease in the transport of ¹²⁵I-tetanus toxin to the perikarya of sensory neurons in dorsal root ganglia and motor neurons in ventral spinal cord. Acrylamide was a more potent inhibitor of retrograde transport in sensory axons than in motor axons. Substantially greater doses of N,N '-methylene-bis-acryl-amide, a reportedly non-neurotoxic analog of acrylamide, were required to alter the axonal transport of ¹²⁵I-tetanus toxin. Velocity of retrograde transport was assessed by determining the position of the leading edge of transported¹²⁵I-tetanus toxin at times following single doses of acrylamide. Acrylamide reduced the velocity of ¹²⁵I-tetanus toxin transport in a dose-dependent manner by up to 75%. No change in neuronal uptake of ¹²⁵I-tet-anus toxin was detected. It is concluded that single doses of acrylamide produce profound alterations in retrograde transport which precede the appearance of structural changes in affected nerve fibers.     

Rapid Brain Uptake of Manganese(II) Across the Blood-Brain Barrier

Abstract: ⁵⁴Mn²⁺ uptake into brain and choroid plexus from the circulation was studied using the in situ rat brain perfusion technique. Initial uptake from blood was linear with time (30 s to 6 min) and extrapolated to zero with an average transfer coefficient of ∼6 × 10^-5 ml/s/g for brain and ∼7 × 10^-3 ml/s/g for choroid plexus. Influx from physiologic saline was three- to fourfold more rapid and exceeded that predicted for passive diffusion by more than one order of magnitude. The lower uptake rate from blood could be explained by plasma protein binding as the free fraction of ⁵⁴Mn²⁺ in rat plasma was ≤30%. Purified albumin, transferrin, and α₂-macroglobulin were each found to bind ⁵⁴Mn²⁺ significantly and to restrict brain ⁵⁴Mn²⁺ influx. The results demonstrate that ⁵⁴Mn²⁺ is readily taken up into the CNS, most likely as the free ion, and that transport is critically affected by plasma protein binding. The results support the hypothesis that Mn²⁺ transport across the blood-brain barrier is facilitated by either an active or a passive mechanism.     

Major nitrogen compounds transported in xylem vessels from roots to top in Citrus trees

Four-year-old citrus trees ( Citrus unshiu Marcovitch) were fed via the roots with (¹⁵NH₄)2sO₄ or K¹⁵NO₃ as a nitrogen source. Nitrogenous compounds and their isotopic abundances in fine roots and xylem sap from trunks were assayed in order to obtain information on the species of nitrogen released by the root system into the ascending xyiem stream.
Arginine, asparagine, nitrate and proline in xylem sap accounted for 48, 21, 13 and 10%, respectively, of the total nitrogenous constituents tested in the sap. However, in the trees fed with labelled ammonium the main nitrogenous compound labelled with ¹⁵N in the xylem sap was asparagine and glutamine, which accounted for 79% and 18%, respectively, of total labelled nitrogen. In the xylem sap of trees fed with labelled nitrate, nitrate accounted for 94% of total labelled nitrogen. Nitrate and asparagine followed by glutamine showed the highest ratios of isotopic abundance in xylem sap as compared to fine roots. Proline and arginine had much lower ratios. These results indicate that nitrate, asparagine and glutamine are the main nitrogenous compounds released by the roots to the xylem stream, whereas arginine and proline are released into the xylern vessels by the trunk tissues. Furthermore, nitrate and asparagine are probably in steady movement upward in the trunk xylem, whereas glutamine is more easily taken up by the trunk tissues than nitrate and asparagine.     

The effects of water hardness upon the uptake, accumulation and excretion of zinc in the brown trout, Salmo trutta L.

The effects of water hardness (9 and 220 mgl⁻¹ as CaCO₃) upon zinc exchange in brown trout exposed to 0.77 μmol Zn 1⁻¹ have been investigated using artificial soft water (<49.9 μmol Ca ^l-1, <40.1 μmol Mg 1⁻¹) and mains hard water (1671.7 μmol Ca 1⁻¹, 493.6 μmol Mg 1⁻¹) of known composition. Both hard and soft water-adapted fish exhibited a bimodal pattern of net zinc influx. Net zinc influxes during both fast and slow uptake phases were significantly greater ( P <0.001) in soft (82.9 and 6.2 μmol Zn 100 g⁻¹ h⁻¹) than in hard water (46.3 and 2.4 μmol Zn 100 g h⁻¹). Zinc efflux (- 0.2 μmol Zn 100 g⁻¹ h⁻¹) was enhanced only in hard water during the slow net influx phase.
Brown trout exposed to zinc in hard water and placed in metal-free media exhibited a greater net efflux (- 25.6 μmol Zn 100 g⁻¹ h⁻¹) of the metal than did fish in soft water (-4.2 μmol Zn 100 g⁻¹ h⁻¹) treated in the same manner. Tissue ⁶⁵Zn activities reflected both the differences in uptake and excretion rates of the metal between hard and soft water fish. During zinc exposure (0.77 μmol Zn 1⁻¹) high water hardness reduced tissue burdens of the metal by reducing net branchial influx, and enhancing efflux of the metal in hard water fish.     

Calcium ion transport through tissue discs of the cortical flesh of apple fruit

Tissue discs cut from the cortical flesh of apple fruit (Malus domestica Borkh. ev. Granny Smith) were clamped between two chambers, and the transport of ⁴⁵Ca²⁺ from one chamber to the other was followed. After initial transport associated with partial infiltration of air spaces by the Ca²⁺ -containing solution, steady-state transport rates were achieved over several hours. Transprt was by diffusion through the apoplast, faciliated by exchange with binding sites on the cell walls. Cation competition was observed during Ca²⁺ loading, transport and unloading, suggesting that the presence of other cations and pH will be important in modifying Ca²⁺ transport through non-vascular tissue and in xylem unloading. Modification of the extracellular volume of solution by vacuum infiltration increased Ca²⁺ transport at high concentrations, suggesting that diffusion is the prime motive force when Ca²⁺ is abundant. When low concentrations were infiltrated, there was little effect on Ca²⁺ transport, and exchange had a strong influence. Transport was reduced at 1°C but this could be accounted for by physical effects of low temperature on diffusion and viscosity. The results are discussed in relation to the nature of the apoplast and the transport of Ca²⁺ in non-vascular plant tissue.