The mechanism by which aluminum interferes with ion influx is not known. In this study, the effects of aluminum on the influx of the cations calcium, potassium, and ammonium and the anions nitrate and phosphate were measured in an aluminum-sensitive cultivar of barley (Hordeum vdgare 1.). Aluminum (100 PM) was found to inhibit the influx of the cations calcium (69%), ammonium (40%), and potassium (13%) and enhancing the influx of the anions nitrate (44%) and phosphate (17%). Aluminum interfered with the binding of the cations in the cell wall by the same order of magnitude as their respective influxes, whereas phosphate binding was strongly enhanced. The results are consistent with a mechanism whereby aluminum binds to the plasma membrane phospholipids, forming a positively charged layer that influences ion movement to the binding sites of the transport proteins. A positive charge layer would retard the movement of cations and increase the movement of anions to the plasma membrane in proportion to the charges carried by these ions. transport proteins, thereby impairing their function (Schroeder, 1988).Most of the studies on the effects of A1 on mineral nutrition have been long-term studies, lasting for a period of 1 or more d (Taylor, 1988). In the present study, to ensure that the inhibition of ion influx was the result of the impairment of membrane function rather than due to impairment of root function through general Al-induced toxicity, plants were grown in the absence of A1 and measurements of the effects of AI were camed out over a time period of 20 min or less. These conditions allowed us (a) to examine the effects of AI on short-term cation (Ca, K, ammonium) and anion (nitrate, phosphate) influx to reveal any patterns of discrimination between cation and anion transport, and (b) to critically evaluate the proposed basis of AI inhibition of ion uptake.

Aluminum-induced inhibition of root growth in the Al-sensitive cultivar Kearney of barley (Hordeum vulgare L.) is the result of disruption of both cell division in the meristematic region and cell expansion in the zone of elongation of the roots. In seedlings directly germinated in 50 μM Al, inhibition of root growth is detected 48 h after initiation of germination and it results primarily from the disruption of cell elongation. In seedlings germinated for 2 days under Al-free conditions, inhibition of root growth is apparent 8 h after transfer to 50 μM Al. In this instance, root growth inhibition is mainly the result of disruption of cell division in the meristematic region of the root. The calcium indicator dyes chlorotetracycline and Fluo-3 are used to study the distribution of intracellular calcium and its relationship to aluminum phototoxicity. Aluminum increases both chlorotetracycline and Fluo-3 fluorescence intensities. Fluorescence of the cytosolic calcium indicator dye Fluo-3 increases primarily in the zone of elongation of the roots of seedlings directly germinated in 50 μM aluminum. The increase in Fluo-3 fluorescence occurs concomitantly with major changes in both the length and width of the cells in the zone of elongation. The evidence suggests that changes in calcium homeostasis occurring in cells of the zone of elongation may be a major factor in the disruption of cell expansion and consequently root growth in seedlings directly germinated in 50 μM aluminum. Key words: aluminum, calcium, barley, chlorotetracycline, Fluo-3.

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A B S T R A C TFluorade was supplied as dissoliled N a F a t concentration? ranging froin 0 26 to 7.9 niLV (5-150 ppin) to three freshulater inicroalgae: Synechococcus leopoliensis (Raczb.) Koinarek (C)anoph)ta), Oscillatoria limnetica Lerninerrnann (C)anoph]ta) and Chlorella pyrenoidosa Chick (Chlorophqta) Groulth of C. pyrenoidosa xias unaffected bjjluorade, and uptake ofJuorzde b) thas organisin urns not detectable. Groultli of the c)aiioph)tes u a s temporaralj anhabated bj NaF. The durataon of thas groulth lag zncreased marked11 as the pH ictus lowered at constant external fluoride concentratzoii. In S. leopoliensis, fluoride uptake and znhibztion ofp1iotos)ntheJis b\ .YaF i ncreased an the savne waj as dzd the growth lag in response to PH. Growth-inhibitor? , \ ' aF treatnient5 decreased the .4TP leuel i n cells of S. leopoliensis 73% a n d also abolished phosphate uptake Cells of S. leopoliensis i n which Juoride-resistawe urns iiiduced b) p t i o r growth in noii-growth-anhibitor) leilels of AVaF accumulated much lessjuorzde than did normal ("sensitwe") cells, a n d also dad not respond to fluoride reduction of the A T P pool It is suggested (1) that fluoride enters \eiisitiile cells of S. leopoliensis principallj as uiidissociatecl HF, (2) that its major anhibitor) effect in these cells is the reduction in cellular ,4TP, (3) tlzat fluoride-resistniit cells accuiti ulate less fluoride dezleloping i n c r e a d per~neabilit~ to the fluoride anion. Ke) index ulords: acicl$catzori, hluegreeri algae, Clanophi ta, fluoride, phi topla nkton, tolera m e , tosicit)

Twenty-five pregnant women with suspected cervical incompetence were assessed by serial ultrasound. A dilating internal 0s was documented in one patient, incompetence was ruled out in two, and a 'slipping suture' was demonstrated in another; the remaining patients were subjected to cerclage on the basis of their history alone. Patients in whom the diagnosis of cervical incompetence is indefinite should have a diagnostic ultrasound scan to visualize the cervix for length, opening of the canal and integrity of the internal 0s. Selective ultrasonography may be beneficial in both the diagnosis and treatment of cervical incompetence.