Ecophysiological characterization of carnivorous plant roots: Oxygen fluxes,respiration, and water exudation

Various ecophysiological investigations on carnivorous plants in wet soils are presented. Radial oxygen loss from roots of Droseraceae to an anoxic medium was relatively low 0.02 – 0.07 mol(O₂) m^{– 2} s^–1 in the apical zone, while values of about one order of magnitude greater were found in both Sarracenia rubra roots and Genlisea violacea traps. Aerobic respiration rates were in the range of 1.6 – 5.6 mol kg^–1 (f.m.) s^–1 for apical root segments of seven carnivorous plant species and 0.4 – 1.1 mol kg^–1 (f.m.) s^–1 for Genlisea traps. The rate of anaerobic fermentation in roots of two Drosera species was only 5 – 14 % of the aerobic respiration. Neither 0.2 mM NaN₃ nor 0.5 mM KCN influenced respiration rate of roots and traps. In all species, the proportion of cyanide-resistant respiration was high and amounted to 65 – 89 % of the total value. Mean rates of water exudation from excised roots of 12 species ranged between 0.4 – 336 mm 3 kg^–1 (f.m.) s^–1 with the highest values being found in the Droseraceae. Exudation from roots was insensitive to respiration inhibitors. No significant difference was found between exudation rates from roots growing in situ in anoxic soil and those kept in an aerated aquatic medium. Carnivorous plant roots appear to be physiologically very active and well adapted to endure permanent soil anoxia.     

Cultivation and carrageenan yield and quality ofKappaphycus alvarezii in the waters of Vietnam

The carrageenan-producing red algaKappaphycus alvarezii (Doty) Doty was brought to Vietnam from Japan in 1993. Branch fragments of this species were cultivated in a pond, lagoon, inlet and offshore in Vietnam for the first time. The best daily growth rate (DGR) of plants grown in the lagoon area attained 9–11 % day^–1 in May to June (cold season). The water temperature and salinity in this area ranged from 27.2–32.4 °C and 31.4–33.7 °C, respectively. DGR of plants grown in the inlet ranged from 7 to 9% day^–1 in June. Grazing by fish has been observed to occur in this area. The DGR of plants grown in the pond ranged from 5–6% in January–July, but decreased to less than 4% day^–1 in August (hot season). K. alvarezii in Vietnam showed a carrageenan yield of 18.8–24.6% and gel strength of 1566–1712 g cm^–2. These values are similar ones obtained fromK. alvarezii cultivated in the Philippines and Indonesia.     

Inhibition of isoproterenol-induced DNA and RNA synthesis in rat parotid and pancreas following removal of submandibular-sublingual glands

Summary [³H] thymidine incorporation into DNA of the parotid (PA) gland of adult and 20-day-old rats and into DNA of the pancreas (PANC) of 20-day-old rats was increased markedly following a 2-day regimen of isoproterenol (ISO) administration. However, when the submandibular-sublingual (SM-SL) glands had been removed just prior to initiation of the ISO injections, the [³H] thymidine incorporation into PA and PANC was inhibited, and cpm/mg protein of these organs was even lower than that of organs of untreated rats with SM-SL glands present. Removal of the PA glands just prior to initiation of the ISO regimen had no effect on the ISO-induced [³H] thymidine incorporation into DNA of PANC but partially inhibited that of the submandibular (SM) gland. It is suggested that the inhibitory effects on DNA and RNA synthesis that follow removal of SM-SL glands are attributable to the growth factors (epidermal growth factor and nerve growth factor) found in the rat SM gland. These factors appear to regulate normal DNA synthetic activity of exocrine glands as well as ₁-adrenoceptor mediated DNA synthesis. Cellular hypertrophy induced by the ISO was less markedly affected by absence of the SM glands, but a partial inhibition of [³H] uridine incorporation into RNA of PA of adult rats also occurred when SM-SL glands were removed prior to initiation of the ISO-regimen.     

Efficient transformation of beet (<Emphasis Type="Italic">Beta vulgaris</Emphasis>) and production of plants with improved salt-tolerance

Small bud tips of 1–3 mm in length were taken from multiple shoot clumps that derived from immature inflorescence cultures of beet as recipient for the Agrobacterium-mediated transformation and transgenic plants were obtained from eight genotypes. The optimal genetic transformation protocol was established as followed: the buds were immersed in Agrobacterium suspension of OD₆₀₀ =0.3–0.5 for 5–10 min, with vacuum infiltration (0.3–0.5 × 10⁵ Pa) or supplemented with 0.01% Silwet L-77, co-cultured for 2–4 days and followed by 10-day culture on medium containing 100 mg l⁻¹ cefotaxime, then the buds were selected on medium containing 10 mg l⁻¹ hygromycin B for three consecutive generations. The percentage of hygromycin-resistant buds after three selections varied from 13.3 to 30.6% with genotypes. The results of PCR and further Southern blotting of genomic DNA of hygromycin-resistant buds or plants showed that the exogenous hpt and AtNHX1 gene had been integrated into the genomes of some transformed buds or plants. The transgenic buds or plants with AtNHX1 gene encoding Na⁺/H⁺ antiport on the vacuole membrane of Arabidopsis showed improved salt-tolerance than the controls. AtNHX1gene inherited in some transgenic lines as Mendelian segregation. This result revealed that it was feasible to improve salt-tolerance of beets by the introduction of AtNHX1 gene into cultured buds.     

DNA,RNA and protein synthesis in ØLL55-infected Lactobacillus lactis

Lactobacillus lactis cells were infected with the bacteriophage ØLL55. The changes in DNA, RNA and protein synthesis were studied by following a long-term (over 3 h) incorporation of radioactive precursors into acid-insoluble material. Stimulation of DNA synthesis caused by phage occurred 30–35 min after infection and thymidine incorporation continued for about 70 min ceasing 10–20 min before the cells started to lyse. Cumulative (¹⁴C)-uracil incorporation into RNA continued at the level of uninfected cells for 30–40 min before starting to slow up. Protein synthesis in the infected cells followed that of a control culture for 40–50 min before the further incorporation of (¹⁴C)-leucine began to decrease.The additions of antibiotic inhibitors of RNA and protein synthesis (rifampicin and chloramphenicol, respectively) at various times before or during the prereplicative period showed that rifampicin, added up to 15 min after infection and chloramphenicol, added as late as 20–25 min after infection completely prevented the initiation of phage-genome replication. The later addition of these drugs did not prevent the out-burst of thymidine up-take, but promoted, however, a deduction in the initiations of new replication cycles. The results indicate that certain genes of ØLL55 genome must be expressed at the early stages of infection to confirm a proper onset and continuation of phage DNA replication.Abbreviations Rif rifampicin - CAL chloramphenicol - TCA trichloroacetic acid - cpm counts per minute     

Turnover and distribution of root exudates of Zea mays

Decomposition and distribution of root exudates of Zea mays L. were studied by means of ¹⁴CO₂ pulse labeling of shoots on a loamy Haplic Luvisol. Plants were grown in two-compartment pots, where the lower part was separated from the roots by monofilament gauze. Root hairs, but not roots, penetrated through the gauze into the lower part of the soil. The root-free soil in the lower compartment was either sterilized with cycloheximide and streptomycin or remained non-sterile. In order to investigate exudate distribution, 3 days after the ¹⁴C labeling, the lower soil part was frozen and sliced into 15, one-mm thick layers using a microtome. Cumulative ¹⁴CO₂ efflux from the soil during the first 3 days after ¹⁴C pulse labeling did not change during plant growth and amounted to about 13–20% of the total recovered ¹⁴C (41–55% of the carbon translocated below ground). Nighttime rate of total CO₂ efflux was 1.5 times lower than during daytime because of tight coupling of exudation with photosynthesis intensity. The average CO₂ efflux from the soil with Zea mays was about 74 g C g^–1 day^–1 (22 g C m^–2 day^–1), although, the contribution of plant roots to the total CO₂ efflux from the soil was about 78%, and only 22% was respired from the soil organic matter. Zea mays transferred about 4 g m^–2 of carbon under ground during 26 days of growth. Three zones of exudate concentrations were identified from the distribution of the ¹⁴C-activity in rhizosphere profiles after two labeling periods: (1) 1–2 (3) mm (maximal concentration of exudates) 2) 3–5 mm (presence of exudates is caused by their diffusion from the zone 1); (3) 6–10 mm (very insignificant amounts of exudates diffused from the previous zones). At the distance further than 10 mm no exudates were found. The calculated coefficient of exudate diffusion in the soil was 1.9 × 10^–7 cm² s^–1.     

Nitrogen retranslocation in plants of maize,lupin and cocklebur

Summary In a split root experiment translocation of N from shoot to root was studied using¹⁵NO ₃ ^– . The three plant species selected for this experiment differed significantly with respect to root NRA. For lupin, maize and cocklebur about 80, 50 and 6% of all absorbed NO ₃ ^– was assmilated in the roots, respectively.Although NO ₃ ^– was reduced in the roots of lupin and maize plants to a greater extent than required for the roots' demand for organic N, a significant phloem flow of N from shoot to roots was found in these plants. Unexpectedly, for cocklebur, the plant with the very low root NRA, the fraction of total N present in the root that has been imported from the shoot was only half that as found for lupin and maize.     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Protozoan Grazing and Bacterial Production in Stratified Lake Vechten Estimated with Fluorescently Labeled Bacteria and by Thymidine Incorporation

In stratified Lake Vechten, The Netherlands, protozoan grazing was estimated on the basis of uptake of fluorescently labeled bacteria and compared with bacterial production estimated on the basis of thymidine incorporation. By using a grazer-free mixed bacterial population from the lake in continuous culture, an empirical relationship between cell production and thymidine incorporation was established. Thymidine incorporation into total cold-trichloroacetic-acid-insoluble macromolecules yielded a relatively constant empirical conversion factor of ca. 10¹⁸ (range, 0.38 × 10¹⁸ to 1.42 × 10¹⁸) bacteria mol of thymidine⁻¹ at specific growth rates (μ) ranging from 0.007 to 0.116 h⁻¹. Although thymidine incorporation has been assumed to measure DNA synthesis thymidine incorporation appeared to underestimate the independently measured bacterial DNA synthesis by at least 1.5- to 13-fold, even if all incorporated label was assumed to be in DNA. However, incorporation into DNA was found to be insignificant as measured by conventional acid-base hydrolysis. Methodological problems of the thymidine technique are discussed. Like the cultures, Lake Vechten bacteria showed considerable thymidine incorporation into total macromolecules, but no significant incorporation into DNA was found by acid-base hydrolysis. This applied not only to the low-oxygen hypo- and metalimnion but also to the aerobic epilimnion. Thus, the established empirical conversion factor for thymidine incorporation into total macromolecules was used to estimate bacterial production. Maximum production rates (141 × 10⁶ bacteria liter⁻¹ h⁻¹; μ, 0.012 h⁻¹) were found in the metalimnion and were 1 order of magnitude higher than in the epi- and hypolimnion. In all three strata, the estimated bacterial production was roughly balanced by the estimated protozoan grazing. Heterotrophic nanoflagellates were the major consumers of the bacterial production and showed maximum numbers (up to 40 × 10⁶ heterotrophic nanoflagellates liter⁻¹) in the microaerobic metalimnion.     

Transfection of mouse cells with thymidine kinase gene of herpes simplex virus

A mouse cell line (LP1-1) was established from the murine L cells deficient in thymidine kinase (L-M(TK ^–)) by prolonged selective culture on the hypoxanthine-aminopterine-thymidine (HAT) medium following transfection with the thymidine kinase gene of herpes simplex virus type-I (HSVTK). Southern blot analysis has shown that the viral TK gene was integrated into one of the chromosomal loci by a single copy. From this established cell line, the 5-bromo-2-deoxyuridine (BrdU) resistant revertant was brought out at a frequency of 1×10^–6 and from these BrdU resistant revertants (LP1BU), one out of 1×10⁵ cells could return to the HAT-resistant phenotype. The established LP1-1 cell line showed a typical biphasic nature of DNA synthesis as determined by the ³H-thymidine incorporation test. The activity of thymidine kinase was shown to be equivalent to that of the DNA polymerase- when the whole nuclear fraction or the nuclear matrix were used for examination. These results indicate that the transfected viral TK gene can be expressed under the normal cell-cycle regulation and its gene product can act as a component of the multienzyme complex which is responsible for DNA replication.     

The fate of carbon and fertiliser nitrogen when dryland wheat is grown in monoliths of duplex soil

Triticum aestivum L. (cv. Gutha), a short-season wheat, was grown to maturity in large monoliths of duplex soil (sand over sandy-clay) in a daylight phytotron mimicking field conditions. Either ¹⁵N-labelled ammonium sulphate ((NH₄)₂SO₄) or urea was banded into the soil at a rate of 30 kg N ha^–1: even though roots were about 20% heavier when grown in the presence of (NH₄)₂SO₄ for 86 d (P<0.05), above-ground mass was not affected by the source of nitrogen. At four times through crop development up to grain-filling (50, 56, 70 and 86 d after sowing) shoots were labelled heavily with ¹⁴CO₂ with two purposes. First, to trace `instantaneous' assimilate movement over 24 h, revealing relative sink strengths throughout plants. This, in turn, allowed precise measurements of live root mass and the proportion of recent photoassimilates deposited in the rhizosphere. Although root systems were sparse, even in surface soil layers, they were strong sinks for photoassimilates early in development (0–50 d), supporting the conversion of inorganic applied nitrogen (N) to soil organic forms. In the presence of roots, up to 28% of ¹⁵N was immobilised, whereas only 12% of labelled ammonium sulphate was immobilised in unplanted plots in spite of a favourable moisture status in both treatments. The effect of plants on rates of ¹⁵N transformation is ascribed to recently imported photoassimilates sustaining rhizosphere metabolism. Not more than 15% of recently fixed carbon imported by roots was recovered from the rhizoplane, suggesting that a highly localised microbial biomass supported vigorous immobilisation of soil N. Thus, more than twice as much applied N was destined for soil organic fractions as for root material. By these processes, root- and soil-immobilised N become substantial stores of applied N and together with shoot N accounted for all the applied N under dryland conditions.     

Photosynthetic and leaf morphological characteristics in Leucaena leucocephala as affected by growth under different neutral shade levels

Morphological and physiological measurements on individual leaves of Leucaena leucocephala seedlings were used to study acclimation to neutral shading. The light-saturated photosynthetic rate (P_{n max}) ranged from 19.6 to 6.5 mol CO₂ m^–2 s^–1 as photosynthetic photon flux density (PPFD) during growth decreased from 27 to 1.6 mol m^–2 s^–1. Stomatal density varied from 144 mm^–2 in plants grown in high PPFD to 84 mm^–2 in plants grown in low PPFD. Average maximal stomatal conductance for H₂O was 1.1 in plants grown in high PPFD and 0.3 for plants grown in low PPFD. Plants grown in low PPFD had a greater total chlorophyll content than plants grown in high PPFD (7.2 vs 2.9 mg g^–1 on a unit fresh weight basis, and 4.3 vs 3.7 mg dm^–2 on a unit leaf area basis). Leaf area was largest when plants were grown under the intermediate PPFDs. Leaf density thickness was largest when plants were grown under the largest PPFDs. It is concluded that L. leucocephala shows extensive ability to acclimate to neutral shade, and could be considered a facultative shade plant.Abbreviations the initial slope of the photosynthesis vs PPFD curve - P_{n max} the light-saturated photosynthetic rate - PPFD photosynthetic photon flux density     

Promotion of root elongation by phosphorus deficiency

Decrease of culture solution pH and increase in cation/anion ratio in the plant were observed when horsegram (Macrotyloma uniflorum (Lam.) Verdc.) was grown in solution culture deficient in phosphorus. The effux of H⁺ from the roots of –P plants was observed in bromocresol purple agar. The length of root cells was considerably increased by –P treatment. Thus a close correlation between H⁺ excretion, length of the root cells and root elongation in response to P deficiency was established.     

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO ₃ ^– deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO ₃ ^– uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO ₃ ^– additions equivalent to 30 kg N ha^–1 year^–1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used ¹⁵N to determine the kinetic parameters of NO ₃ ^– uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO ₃ ^– –N m^–2 and determined the kinetic parameters of NO ₃ ^– and NH ₄ ⁺ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO ₃ ^– fertilization (range = non-detectable to 33 nmol NO ₂ ^– g^–1 h^–1). Maximal rates of NO ₃ ^– uptake in overstory trees also were low, ranging from 0.2 to 1.0 mol g^–1 h^–1. In seedlings, the mean V _max for NO ₃ ^– uptake in fine roots (1 mol g^–1 h^–1) was approximately 30 times lower than the V _max for NH ₄ ⁺ uptake (33 mol g^–1 h^–1). Our results suggest that A. saccharum satisfies its N demand through rapid NH ₄ ⁺ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO ₃ ^– .     

Impact of Agrobacterium tumefaciens-induced stem tumors on NO3− uptake in Ricinus communis

Developing tumors induced by Agrobacterium tumefaciens, strain C58, on stems of Ricinus communis L. var. gibsonii cv. Carmencita were shown to be strong metabolic sinks for sucrose and amino acids, thus causing higher nutrient demand in the host plant. However, NO₃ ^– uptake and, to a lesser extent, also H₂PO₄ ^– uptake were strongly inhibited. Correspondingly, NO₃ ^– concentration was lower in tumorised than in the control plants. NO₃ ^–reductase activity was the same in both plant types, but it was completely suppressed in the tumors. The electrical membrane potential difference of root cells was unaffected in tumorised plants when soil-grown, but significantly lowered when grown hydroponically. Consistent with the low NO₃ ^– uptake rate, NO₃ ^–-dependent membrane depolarisation at the onset of NO₃ ^–/2H⁺-cotransport was nearly zero. In the phloem sap, sucrose and amino acid concentrations were considerably lower in tumorised than in control plants, and lower below than above the tumor. The qualitative pattern of amino acids of the phloem sap of stems was almost the same in tumorised and control plants. It is concluded that neither the overall amino acid concentration nor special amino acids nor ammonium in the transport phloem suppress NO₃ ^– uptake in the roots. Aminocyclopropane-carboxylate, the precursor of ethylene, which is produced in the tumors in high amounts, was low in the stems and the same in both plant types. Thus, ACC and ethylene were ruled out as directly interfering with nutrient uptake in the roots. Root morphology was strongly affected during tumor development. Root fresh weight decreased to 50% of the controls and lateral root development was almost completely prevented. This suggests that the high tumor ethylene production, together with an increasing concentration of phenolic compounds, severely inhibits the basipetal auxin flow to the roots. Auxin accumulation and retention was confirmed by specifically enhanced expression of the auxin-responsive promoter of the soybean gene GH3:GUS in tumors induced in transgenic Trifolium repens L. Hence, root development is poorer and anion uptake inhibited in tumorised plants. This may be aggravated by abscisic acid accumulation in the tumor and its basipetal export into the roots. Moreover, sucrose depletion of the sieve tubes leads to energy shortage at the root level for maintaining energy-dependent anion uptake.     

In vitro induction of tetraploid in pomegranate (Punica granatum)

Tetraploid plants were obtained in pomegranate (Punica granatum L. var. `Nana') by colchicine treatment of shoots propagated in vitro. Shoots cultured on MS medium supplemented with 10 mg l^–1 colchicine, 1.0 mg l^–1 BA and 0.1 mg l^–1 NAA for 30 days produced tetraploids at a high frequency of 20%. No tetraploids were detected by treating the shoots in 5000 mg l^–1 colchicine for 114 h. Shoots treated by 5000 mg l^–1 colchicine for 96 h produced three morphological mutants with narrow leaves, which were later confirmed as mixoploids that separated into diploids and tetraploids after further subculture. In vitro tetraploid plants had shorter roots, wider and shorter leaves than the diploid ones. Tetraploid pomegranate plants grew and flowered normally in pots, but possessed flowers with increased diameter and decreased length compared to diploids. The number of pollen grains per anther was higher in tetraploids, but the viability of pollen decreased significantly.     

Production,standing biomass and natural abundance of <Superscript>15</Superscript>N and <Superscript>13</Superscript>C in ectomycorrhizal mycelia collected at different soil depths in two forest types

Nutrient uptake by forest trees is dependent on ectomycorrhizal (EM) mycelia that grow out into the soil from the mycorrhizal root tips. We estimated the production of EM mycelia in root free samples of pure spruce and mixed spruce-oak stands in southern Sweden as mycelia grown into sand-filled mesh bags placed at three different soil depths (0–10, 10–20 and 20–30 cm). The mesh bags were collected after 12 months and we found that 590±70 kg ha^–1 year^–1 of pure mycelia was produced in spruce stands and 420±160 kg ha^–1 year^–1 in mixed stands. The production of EM mycelia in the mesh bags decreased with soil depth in both stand types but tended to be more concentrated in the top soil in the mixed stands compared to the spruce stands. The fungal biomass was also determined in soil samples taken from different depths by using phospholipid fatty acids as markers for fungal biomass. Subsamples were incubated at 20°C for 5 months and the amount of fungal biomass that degraded during the incubation period was used as an estimate of EM fungal biomass. The EM biomass in the soil profile decreased with soil depth and did not differ significantly between the two stand types. The total EM biomass in the pure spruce stands was estimated to be 4.8±0.9×10³ kg ha^–1 and in the mixed stands 5.8±1.1×10³ kg ha^–1 down to 70 cm depth. The biomass and production estimates of EM mycelia suggest a very long turnover time or that necromass has been included in the biomass estimates. The amount of N present in EM mycelia was estimated to be 121 kg N ha^–1 in spruce stands and 187 kg N ha^–1 in mixed stands. The ¹³C value for mycelia in mesh bags was not influenced by soil depth, indicating that the fungi obtained all their carbon from the tree roots. The ¹³C values in mycelia collected from mixed stands were intermediate to values from pure spruce and pure oak stands suggesting that the EM mycelia received carbon from both spruce and oak trees in the mixed stands. The ¹⁵N value for the EM mycelia and the surrounding soil increased with soil depth suggesting that they obtained their entire N from the surrounding soil.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Nitrogen fixation associated with roots of Kallar grass (Leptochloa fusca L. Kunth)

Summary An ecological survey was made to measure the N₂-fixing activity in the rhizosphere of a salt-tolerant grass,Leptochloa fusca (L.) Kunth. Samples were obtained every month at two sites over a period of one year. Soil cores, unwashed, washed and surface-sterilized roots were subjected to acetylene reduction assay (ARA). ARA values up to 50 nmoles h^–1 for soil cores, 1095 nmoles g dry root^–1 h^–1 for unwashed roots, 4929 nmoles g dry root^–1 h^–1 for washed roots and 2494 nmoles g dry root^–1 h^–1 for surface-sterilized roots were observed but for most samples the range was 1–200 nmoles g dry root^–1 h^–1. A lag period of 5–7 h was observed before the onset of N₂-fixing activity by excised roots and O₂ levels had no effect on this lag. Values for roots incubated without preincubation were similar to those for unwashed preincubated roots. Activity was highest in September, October and November when the temperature is not very high and photosynthetic activity is reasonably good. N₂-fixing-bacteria were counted on the same samples by plate count and MPN methods, the latter being estimated on the basis of ARA and pellicle formation. Fairly high numbers of bacteria (10⁴–10⁷) were recorded in the histoplane fraction which indicates the presence of diazotrophs in the inner cells of grass roots.     

On the biogenesis of cytokinins in roots of Phaseolus vulgaris

Roots of intact bean plants were supplied with [¹⁴C]adenine by pulse-chase experiments. The rate of incorporation of radioactivity into tRNA and oligonucleotides of roots as well as the content of radioactive labeled cytokinin nucleotides in these RNA fractions were determined. On the average, 1/70 of the radioactivity incorporated into tRNA was localized in N⁶(²isopentenyl)adenosine. The half life of tRNA was estimated to be 65–70 h. Shortly after the pulse period, oligonucleotides contained zeatin riboside at a ratio of 1:800, on the basis of radioactivity. The half life of these oligonucleotides was determined to be about 8 h. The main free radioactive cytokinin of roots and leaves was zeatin. Comparing the rate of degradation of ¹⁴C-labeled tRNA and the oligonucleotides of roots and the rate of appearance of radioactive cytokinins in roots and leaves, we found strong indications for their dependency. The results contradict the hypothesis of de novo synthesis of cytokinins in roots of intact bean plants.Abbreviations AMP adenosine monophosphate - IPA N⁶(²isopentenyl)adenosine - IPAde N⁶(²isopentenyl)adenosine - Z zeatin - ZR zeatinriboside - TLC thin-layer chromatography - HPLC high performance liquid chromatography Part of the doctoral thesis, Bonn 1980