The binding of zinc,but not cadmium,by phytic acid in roots of crop plants

Plant species adapted to soils enriched with heavy metals often accumulate these metals in their above or below ground organs. In this study, electron probe microanalysis of fractured, quench-frozen root specimens of common crop species shows that an appreciable quantity of Zn can be bound as Zn phytate (myo-inositolkis-hexaphosphate) within small vacuoles of cells in the root elongation zone of lucerne, soybean, lupins, tomato, rapeseed, cabbage, radish, maize and wheat exposed to high levels of Zn (80–300 M). Globular deposits of Zn phytate are most frequently observed in the endodermis of dicotyledonous species and in the pericycle of monocotyledonous species, but may also occur in the stele and inner cortex after prolonged exposure to toxic levels of Zn. The deposits could not be found in Zn-treated sunflower, field peas and Italian ryegrass. In three crop species, lucerne, soybean and maize, Zn-induced phytate globules were frequent, but exposure of roots to 30 M Cd did not induce the formation of Cd-containing globular deposits as observed inLemna minor (Van Steveninck et al., 1990a, 1992). Simultaneous Zn and Cd treatment induced the formation of Zn phytate globules as effectively as Zn alone, and Cd was not detected in the deposits.     

Nutrient content and photosynthesis of young yellowing Norway spruce trees (Picea abies L. Karst.) following calcium and magnesium fertilisation

Severely yellowed ten-year-old spruce trees growing in the Vosges Mountains on an acidic soil were fertilised with Magnesium lime during the spring of 1990. The effects of this treatment were assessed 18 months later. A very significant improvement of the mineral status of the trees was detected, with increasing Mg contents in the needles, and as a consequence, reduced yellowing and improved chlorophyll content. Only slight differences with control trees were observed for height increase. Effects of this improved nutrition on photosynthesis were tested measuring net CO₂ assimilation rates and chlorophyll a fluorescence. Light-saturated net assimilation rates of current-year needles were high, reaching 5.3 mol m^–2 s^–1 on a total needle area basis. The improvement in chlorophyll and Mg content had no significant effect on net assimilation rates or on any parameter describing photochemical functions of both current-and previous-year needles. Despite the strong inter-individual variability in needle chlorophyll and Mg contents (ranging from 0.2 to 0.8 mg g^–1 fresh weight, and 0.05 to 0.5 mg g^-1 dry weight respectively), photochemical efficiency of PS II under limiting irradiance only decreased significantly on older needles displaying Mg contents below 0.1 mg g^–1. It is concluded from these results that spruce trees exhibit a high degree of plasticity with regard to Mg deficiency on acidic soils, and that improved Mg nutrition and increased chlorophyll content do not necessarily improve photosynthesis and height growth.Abbreviations A light-saturated net CO₂ assimilation rate (mol m^–2 s^–1) - g_w light-saturated needle conductance to water vapour (mmol m^–2 s^–1) - _wp and _wm pre-dawn and mid-day needle water potential (MPa) - osmotic potential of sap expressed from needles (MPa) - PFD photosynthetic photon flux density (mol m^–2 s^–1) - F_v/F_m photochemical efficiency of PS II after 20 min dark adaptation - F/F_m ^' photochemical efficiency of PS II reaction centres after 10 min at a PFD of 220 mol m^–2 s^–1     

Effect of exogenous amino acids,glucose and citric acid on the patterns of short-term accumulation and loss of amino acids in the root-zone of sand-cultured forage rape (Brassica napus L.)

Amino acid loss from the roots of 25-day-old, sterile and non-sterile sand-grown forage rape plants, was determined over periods of up to 3.5 hours. Amino acid accumulation in the root-zone of sterile plants was concentration-dependent giving a convex accumulation profile. Amino acid levels in the root zone of non-sterile plants rapidly attained steady state values. Microbial assimilation of amino acids within the root zone appeared to lower amino acid concentrations, resulting in an underestimation of rates of amino acid loss from roots. The concentrations of most amino acids were higher after selected amino acids were supplied to the root zone. The response to exogenous acids was dependent on the concentration and composition of the acids added. Addition of a mixture containing ASN, GLN and GABA, each at 0.25 mM resulted in a greater increase in individual and total acid levels compared with a mixture containing ALA, SER, GLY and THR at the same concentration. Apparently, amino acids supplied exogenously competed with acids lost from the plant, by providing an alternative nutrient source for root zone micro-organisms. Addition of glucose and citric acid had a similar effect to addition of ALA, SER, GLY and THR, but were less effective than ASN, GLN and GABA at all concentrations tested. The nitrogen-rich amino acids ASN and GLN, and the -amino acid, GABA, appeared to compete more effectively with plant-derived acids than did ALA, SER, GLY and THR, the most abundent constituents of the plant-derived acids, which had the highest calculated rates of microbial consumption. Therefore, although bacterial consumption showed a dependence on amino acid concentration, a degree of selectivity for nitrogen-rich acids and gaba was also apparent.     

Patterns of short-term amino acid accumulation and loss in the root-zone of liquid-cultured forage rape (Brassica napus L.)

Amino acid release from roots of sterile and non-sterile, solution-grown, 7-, 21- and 60-days-old forage rape plants (Brassica napus L.), was measured over periods of up to 6 hours. With sterile plants, release of amino acids into a fixed volume of collection medium (6, 12, 70 mL) was concentration-limited, giving rise to similar convex accumulation profiles for individual acids. In contrast, amino acid accumulation in continuously circulating collection medium was not concentration limited, giving a linear accumulation pattern. The compositions of accumulating amino acids, which were similar to those measured in root extracts, did not change significantly. However, the proportions of ALA, GABA, GLU and ILE in both root extracts and root-derived amino acids increased as plants aged. Older plants released more amino acids per plant, while younger plants released more amino acids g^-1 root DW. Using non-sterile plants, the patterns of change in amino acid concentration and composition in the collection medium were completely different from those determined with sterile plants. In general, with 7-days-old plants, and 60-days-old plants that had recently become non-sterile, an initial rise in the concentration of all acids was followed by a fall to low levels. The loss of amino acids was apparently due to microbial consumption. Individual amino acids attained maximum concentration at different times during the collection process. This is attributed mainly to concentration-dependent differential assimilation of amino acids, since those with the highest initial concentrations, the major components of the mixtures released from roots, declined the earliest. When calculated rates of amino acid release from roots (R_r) and microbial consumption of amino acids (R_c) were compared (for 7-days-old plants), the highest ratios of R_c/R_r were found for ASN, ARG, GLU, GLN, and LYS. This suggests a degree of selectivity for glutamate and nitrogen-rich acids on the part of the consuming micro-organisms. With 21-days old plants and 60-days old plants grown entirely under non-sterile conditions, fluctuations in amino acid concentration were similar for all acids.     

Rapid identification ofRhizobium species based on cellular fatty acid analysis

As understanding of the evolutionary relationships between strains and species of root nodule bacteria increases the need for a rapid identification method that correlates well with phylogenetic relationships is clear. We have examined 123 strains ofRhizobium: R. fredii (19),R. galegae (20),R. leguminosarum (22),R. loti (17),R. meliloti (21), andR. tropici (18) and six unknowns. All strains were grown on modified tryptone yeast-extract (TY) agar, as log phase cultures, scraped from the agar, lysed, and the released fatty acids derivatized to their corresponding methyl esters. The methyl esters were analysed by gas-chromatography using the MIDI/Hewlett-Packard Microbial Identification System. All species studied contained 16:0, 17:0, 18:0 and 19cyclo_w9C fatty acids but onlyR loti andR tropici produced 12:0 3 OH,13:0 iso 3 OH,18:1_w9C and 15:0 iso 3 OH,17:0 iso 3 OH and 20:2_w6,9C fatty acids respectively. Principal component analysis was used to show that strains could be divided into clusters corresponding to the six species. Fatty acid profiles for each species were developed and these correctly identified at least 95% of the strains belonging to each species. A dendrogram is presented showing the relationships betweenRhizobium species based on fatty acid composition. The data base was used to identify unknown soil isolates as strains ofRhizobium lacking a symbiotic plasmid and a bacterium capable of expressing a symbiotic plasmid fromR. leguminosarum asSphingobacterium spiritovorum.     

Diurnal cycles of rhizosphere acidification byPinus contorta seedlings

The pH of the nutrient solution bathing the roots of four-month-oldPinus contorta var.latifolia Englm. seedlings was monitored continuously between additions of nutrients. Nitrogen was supplied in the form of NH₄NO₃, and was added three times per week in amounts relative to seedling fresh weight. No pH change was associated with the nutrient addition cycle; however, extinguishing of the lights at night resulted in a decrease in pH of almost half a pH unit in the first hour. The pH reverted to normal within a few hours. Re-illumination resulted in a pH increase of a smaller magnitude, but over a similar time span. Estimation of the proton extrusion rate gave values of about 17 µmol (g FW root)^–1 h^–1.     

Intraprotoplasmic feruloylation of arabinoxylans in Festuca arundinacea cell cultures

Graminaceous primary cell walls contain polysaccharides to which are esterified feruloyl residues. Ester biosynthesis is highly specific and the present experiments were performed to ascertain the likely site of feruloylation in living grass cell cultures. Cell cultures of tall fescue grass (Festuca arundinacea Schreber) incorporated exogenous l-[1-³H]arabinose into polymers at a linear rate after a short lag of approx. 1–3 min. Radiolabelled polymers did not start to accumulate in the culture medium until 20–35 min after [³H]arabinose was supplied. However, polymer-bound feruloyl-arabinose residues began to accumulate ³H after a lag of 1–3 min. Assuming that the onset of secretion of radiolabelled polymers into the medium indicates the time before which essentially all the radiolabel was internal to the plasma membrane, the results show that the polysaccharide-bound [³H]arabinose residues must have been feruloylated within the protoplast.Abbreviations AIR alcohol-insoluble residue - BAW butan1-ol/acetic acid/water (12:3:5 by volume) - BEW butan-1-ol/ ethanol/water (20:5:11 by volume) - EPW ethyl acetate/pyridine/ water (8:2:1 by volume) - R_Ara Chromatographic mobility relative to that of l-arabinose We are very grateful to Mr. Gundolf Wende for assistance with the characterisation of the feruloyl esters. K.E.M. is funded by a studentship from the Science and Engineering Research Council in collaboration with Zeneca Agrochemicals.