D. N. Munns

Adsorption of Al3+ to phosphatidylcholine vesicles.

Aluminum toxicity to soil and aquatic organisms is widespread, but the mechanisms of toxicity are unknown. To understand these mechanisms, it is important to know how aluminum reacts with cell surfaces. In this report, we studied adsorption of Al3+ to liposomes composed of phosphatidylcholine, the most abundant phospholipid in plasma membranes of eukaryotic cells. Our equilibrium dialysis and electrophoresis experiments both showed that Al3+ has a 560-fold higher affinity for the phosphatidylcholine surface than Ca2+. Unlike previous reports for adsorption of divalent metals, adsorption of Al3+ to phosphatidylcholine was predicted only approximately by the Stern model. Adsorption of AlF2+ and AlF2+ to the surface was not detectable at the activities we used. From our data, we calculate that Al3+ at 5 x 10(-6) activity could neutralize the surface charge on plant cell plasma membranes and cause a surface potential shift from -30 to +11 mV. This is consistent with non-specific Al3+ inhibition of cation uptake by root cells. Al3+ adsorption to phosphatidylcholine may also play a role in aluminum uptake into cytoplasm by endocytosis.

Sensitivity of the common bean (Phaseolus vulgaris L.) symbiosis to high soil temperature

SummaryExperiments were done to test whether N fixation is more sensitive to high soil temperatures in common bean than in cowpea or soybean. Greenhouse experiments compared nodulation, nitrogenase activity, growth and nitrogen accumulation of several host/strain combinations of common bean with the other grain legumes and with N-fertilization, at various root temperatures. Field experiments compared relative N-accumulation (in symbiotic relative to N-fertilized plants) of common bean with cowpea under different soil thermal regimes. N-fertilized beans were unaffected by the higher temperatures, but nitrogen accumulation by symbiotic beans was always more sensitive to high root temperatures (33°C, 33/28°C, 34/28°C compared with 28°C) than were cowpea and soybean symbiosis. Healthy bean nodules that had developed at low temperatures functioned normally in acetylene reduction tests done at 35°C. High temperatures caused little or no suppression of nodule number. However, bean nodules produced at high temperatures were small and had low specific activity. ForP. vulgaris some tolerance to high temperature was observed among rhizobium strains (e.g., CIAT 899 was tolerant) but not among host cultivars. Heat tolerance ofP. acutifolius andP. lunatus symbioses was similar to that of cowpea and soybean. In the field, high surface soil temperatures did not reduce N accumulation in symbiotic beans more than in cowpea, probably because of compensatory nodulation in the deeper and cooler parts of the soil.

Nitrogen fixation potential of beans (Phaseolus vulgaris L.) compared with other grain legumes under controlled conditions

SummaryGreenhouse experiments were done under favorable conditions to compare effective bean symbioses with cowpea and soybean symbioses and with N-fertilized controls. Growth, N-accumulation, nodulation, acetylene reduction, hydrogen evolution and the effect of native rhizobia on symbiotic performance were evaluated. Relative N accumulation (in symbiotic plants relative to N-fertilized plants) was higher for soybean (43%) than for the other symbioses (25–39%) at four weeks, but from four to six weeks both soybean (96%) and cowpea (92%) accumulated relatively more N than did beans (56–78%). Inferior performance of beans could not be atributed to differences in acetylene reduction or nodule weight, but bean nodules were smaller and more numerous and evolved more hydrogen (Relative energetic efficiency was 0.5 to 0.7 in bean, 0.95 in cowpea and soybean). Relative N accumulation was influenced by N accumulation characteristics of the fertilized plants as well as the symbiotic plants. The vegetative N-fixation period of early maturing beans was shorter than for cowpeas of similar maturation date; the beans flowered earlier and had a longer pod-filling period. There was no evidence that the common bean symbiosis was more sensitive than the others to competition from native rhizobia. With mixed populations of effective rhizobia, hydrogen evolution in otherPhaseolus species (P. acutifolius, P. coccineus, P. filiformis, P. lunatus) was similar to that inP. vulgaris and higher than in cowpea or soybean. Although failure to establish effective nodulation is often considered the reason for poor N-fixation by common bean in the field, the species may be genetically predisposed to poor fixation because of symbiotic inefficiency and the short vegetative fixation period.

Lipid bilayer permeation by neutral aluminum citrate and by three α-hydroxy carboxylic acids

Abstract Several groups have proposed that aluminum (Al) may permeate biologival membranes as a neutral complex with citrate. We tested this hypothesis by measuring aluminum citrate flux across unilamellar phospholipid vesicles (liposomes). Results from two independent procedures show that lipid bilayer permeation by the neutral aluminum-citrate complex is slow (P∼=1·10−11 cm·s−1). We then compared aluminum-citrate permeation with permeation by a series of α-hydroxy carboxylic acids and by trimethylcitrate. This comparison showed that the aluminum-citrate flux is limited by diffusion across the water/lipid interface. This is due to hydrogen bonding between water and the citrate carboxyl groups, and by hydration of the bound metal in the aqueous phase. By analogy with citric acid, steric hindrance of diffusion within the bilayer does not affect the permeation rate of aluminum citrate. Elevated tissue levels of Al in subjects fed a diet supplemented with citric acid and Al(OH)3 cannot be explained by lipid bilayer permeation of the neutral complex.

Effect of CaSO4 and NaCl on mineral content ofLeucaena leucocephala

The effects of varying CaSO4 and NaCl levels on the nutrient content ofLeucaena leucocephala were established by examining the concentrations of Na, Ca, Cl, K and Mg in leucaena roots, stems and leaves. Leucaena was grown in nutrient solution at four levels of CaSO4 (0.5, 1.0, 2.5 and 5.0 mM) and NaCl (1, 25, 50 and 100 mM), in randomized blocks with five replications. Leucaena excluded sodium from stems and leaves when NaCl concentration was 50 mM or less. Sodium uptake decreased as CaSO4 concentration increased. Calcium uptake was affected by NaCl concentration when substrate CaSO4 concentration was 0.5 mM. At this level, 100 mM NaCl caused a marked decrease in leaf calcium and a marked increase in leaf Cl. In all other treatments, Cl uptake was not affected by CaSO4 concentration. Potassium uptake was strongly depressed as NaCl concentration increased at low Ca concentration, but this effect was offset at high Ca. Magnesium uptake decreased as CaSO4 levels increased.

Effects of CaSO4 and NaCl on growth and nitrogen fixation ofLeucaena leucocephala

Effects and interactions of varying CaSO4 and NaCl levels on growth and nitrogen fixation ofLeucaena leucocephala K8 were examined. Leucaena was grown in nutrient solution at four levels of CaSO4 (0.5, 1.0, 2.5 and 5.0 mM) and NaCl (1, 25, 50 and 100 mM) in randomized blocks with five replications. While NaCl significantly reduced plant growth, additions of CaSO4 increased plant height, leaf number, and biomass of salt treated plants. For the nonsaline treatments, high CaSO4 levels slightly depressed growth, which contradicts suggestions that Leucaena has a high calcium requirement. A significant calcium/sodium interaction was not seen for nodule number or weight. Nodule number was significantly depressed by 100 mM NaCl and nodule weight of the salt stressed plants significantly increased as CaSO4 concentration increased from 0.5 to 2.5 mM. Effects of NaCl and CaSO4 on nitrogen content of plant parts were inconclusive. The promotion of Leucaena salinity tolerance by addition of CaSO4 may be attributed to the effect of calcium in maintaing the selective permeability of membranes.

Nodulation and growth ofPhaseolus vulgaris in solution culture

SummaryConditions and techniques for achieving good nodulation ofPhaseolus vulgaris L. in continuously aerated solution were developed from greenhouse experiments.If nodules had been established, their growth and activity and the growth of the plant were at least as good in solution culture as in gravel culture. Nodule formation was observed within 10 days of inoculation in small volumes of solution culture (1 liter). In large volumes (19 liters), similarly prompt nodulation occurred only if the plants were inoculated before or immediately after the seedlings were transferred to the solution from gravel or vermiculite; and the nodules were restricted to the roots that had been present at the time of transfer. Delayed inoculation, 2 days after transfer to large volume solutions, led to sparse nodulation observed only after 3 weeks. Delay or inhibition of nodulation in large volumes of solution could not be explained by failute of bacteria to colonize roots or by sparsity of root hairs.Nodule initiation in solution culture was severely inhibited at pH below 5.4. An additional problem in growing N2-dependent bean in solution culture was the buildup of Cl− to toxic levels in the plant in nitrate-free media, even at solution concentrations as low as 0.4 mM Cl−. Daily addition of 0.5 to 1.0 mg N per plant delayed nodule growth and activity slightly, but increased plant growth and alleviated the severe N-deficiency that otherwise developed before the onset of N2-fixation.

Depression of legume growth by liming

SummaryEarly growth or nodulation of certain species was depressed when a Hawaiian Oxisol was limed at rates above 6 tons/ha (pH 6). In 8 legumes, the depression later gave way to positive response. This was evident in plant weights of Desmodium intortum and Glycine wightii var. Cooper, and in pod weights of Phaseolus vulgaris. A transient depression was observed visually in Desmodium canum, Dolichos axillaris, Glycine wightii var. Tinaroo, and Trifolium subterraneum.In Stylosanthes guyanensis and S. fruticosa, the depression persisted throughout the experiment (6 months).Growth was not depressed in Arachis hypogea, Coronilla varia, Glycine max, Leucaena leucocephala, Medicago sativa, Trifolium repens, or Vigna sinensis.

Uptake of aluminum into root cytoplasm: Predicted rates for important solution complexes

Abstract In this report we predict the rate of aluminum absorption into root cytoplasm as solutes that are common in acid soil solutions. Our predictions of passive influx rely upon permeability coefficients estimated from known values for similar compounds. We also consider aluminum absorption by endocytosis and absorption by competition with iron for iron‐specific transport mechanisms. Our calculations indicate that passive flux of Al3+ or of common charged Al‐complexes is unlikely to contribute measurably to total aluminum absorption , but endocytosis of Al3+ bound to the plasma membrane surface may dominate uptake under some conditions. Among neutral Al‐complexes, AlF3 o absorption should be detectable; by comparison, passive influx by Al‐citrateo should not be detectable.

Enzymatic breakdown of pectin and acid-inhibition of the infection of Medicago roots by rhizobium

SummaryProduction of a pectinase precedes infection. Acidity in the pH-range 4.5 to 5.5, which inhibits production of root-nodules by inhibiting the infection of the root-hairs, also inhibits the action of the pectinase on pectin. The enzymes pH-requirements may determine those of nodulation.