Influence of a low xylitol-dose on mutans streptococci colonisation and caries development in preschool children.

AIM: To evaluate the effect of xylitol-containing tablets on mutans streptococci colonisation and caries development in preschool children. STUDY DESIGN: Randomised single-blind prospective design. METHODS: The material consisted of 132 healthy 2-year-old children, 71 boys and 61 girls and they were assigned to a xylitol tablet (test) group or a non-intervention control group. The mean age was 2 years + 1 month in both groups. The drop-out rate was 10.6% during the 2-year trial. The test group was given 1-2 xylitol tablets (0.5-1g) per day during 1.5 years. Mutans streptococci (MS) enumeration was performed at baseline and semi-annually in the children and at baseline or shortly after in the mothers with a chair-side technique. Caries prevalence was scored at baseline and the age of 4 years. RESULTS: No statistically significant differences in MS colonisation were disclosed between the test and control groups at baseline or any of the designated follow-ups. A statistically significant positive relationship was found between the maternal salivary MS levels and the colonisation of the children in the control group at 2.5 years, 3 years and 3.5 years (r=0.39, r=0.35; r=0.30; p<0.01, p<0.01 and p<0.05) but not in the xylitol tablet group (p<0.05). The mean caries prevalence was lower in the test group compared with the control group at 4 years of age (dmfs 0.38 +/-1.05 vs. 0.80 +/-2.60) but the difference was not statistically significant. CONCLUSION: The findings do not support a low-dose xylitol tablet program for caries prevention in preschool children.

The strategy of the wheat plant in acclimating growth and grain production to nitrogen availability.

Two cultivars of spring wheat (Triticum aestivum L.) were grown to maturity in hydroponic cultures. Nitrogen accumulation was controlled by daily growth-limiting additions of nitrate together with all other nutrients in excess. Six different curves of N accumulation were used, with the same relative changes from day to day, but with different amplitudes. These curves were obtained by using the same mathematic formula of the N accumulation curves but varying the value of initial N content. The total amount of nitrogen added varied from 20 mg plant(-1) to 65 mg plant(-1). Plant bioproductivity showed a linear response to accumulated N. The number of grains per plant increased linearly with increased N availability whereas grain weights were essentially unaffected. Grain N concentrations and N content varied slightly, with highest values generally at the lower N availability levels. The quantitatively most important response to increased N availability was an increased number of earbearing tillers per plant. This varied from 0.1 tiller plant(-1) at maturity when given 20 mg N plant(-1), up to about 2 tillers plant(-1) when given 65 mg N plant(-1). Not all tillers that were initiated developed ears. The reduction of tillers seems to be one important mechanism in adapting plant productivity to N availability. Other individual characters influenced by N availability were straw height and the number of spikelets per spike. The two cultivars behaved in a qualitatively similar manner over the range of N availability even though they quantitatively differed in grain size, N concentrations and yield.

Nitrogen Utilization in Lemna: II. Studies of Nitrate Uptake Using NO(3).

(13)N-labeled nitrate was used to trace short-term nitrate influx into Lemna gibba L. G3 in experiments where disappearance of both radioactivity and total nitrate from the incubation medium was measured continuously and simultaneously. In plants performing net nitrate uptake from an initial nitrate concentration of 40 to 60 micromolar, there was no discrepancy between net uptake and influx, irrespective of the N status of the plants, indicating that concomitant nitrate efflux was low or nil. Plants treated with tungstate to inactivate nitrate reductase were able to take up nitrate following induction of the uptake system by exposure to a low amount of nitrate. Also, in this case, net uptake was equivalent to influx. In tungstate-treated plants preloaded with nitrate, both net uptake and influx were nil. In contrast to these observations, a clear discrepancy between net uptake and influx was observed when the plants were incubated at an initial nitrate concentration of approximately 5 micromolar, where net uptake is low and eventually ceases. It is concluded that plasmalemma nitrate transport is essentially unidirectional in plants performing net uptake at a concentration of 40 to 60 micromolar, and that transport is nil when internal nitrate sinks (vacuole, metabolism) are eliminated. The efflux component becomes increasingly important when the external concentration approaches the threshold value for net nitrate uptake (the nitrate compensation point) where considerable exchange between internal and external nitrate occurs.

Nitrogen Utilization in Lemna: III. Short-Term Effects of Ammonium on Nitrate Uptake and Nitrate Reduction.

The effects of ammonium application on nitrate utilization were studied in N-limited cultures of Lemna gibba L. G3. Addition of ammonium instantaneously inhibited net nitrate uptake by at least 60%, followed by a slight recovery. The inhibition was equally clear after near-complete inactivation of glutamine synthetase by application of l-methionine-d,l-sulfoximine. Experiments where (13)N-labeled nitrate was used as an influx tracer revealed that ammonium specifically inhibited influx, but did not promote nitrate efflux. Nitrate accumulation was relatively more inhibited than nitrate reduction and net uptake. Nitrate reductase, extracted and assayed in vitro in the presence of the thiol proteinase inhibitor leupeptin, was unaffected by short-term treatment of the plants with either nitrate, ammonium, or ammonium nitrate. Nitrate reductase activity recovered in the absence of leupeptin was considerably lower; however, it was enhanced by all the nitrogen sources, with ammonium as the most potent. It is argued that the effect of ammonium on nitrate utilization in Lemna is due to inhibition of nitrate influx, and that the effect should be attributed to ammonium itself, not to a newly formed nitrogen derivative. The decreased nitrate flux caused a decrease in nitrate reduction, whereas the activity of nitrate reductase per se rather is stabilized by presence of ammonium.

Short-term studies of NO 3 (-) uptake in Pisum using (13)NO 3 (-).

Influx, efflux and net uptake of NO 3 (-) was studied in Pisum sativum L. cv. Marma in short-term experiments where (13)NO 3 (-) was used to trace influx. The influx rate in N-limited plants was similar both during net uptake at external concentrations of around 50 µM, and at low external NO 3 (-) concentrations (4-6 µM) when net uptake was practically zero. Efflux could be inferred from discrepancies between influx and net uptake but was never very high in the N-limited plants during net uptake. Close to the threshold concentration for not NO 3 (-) uptake, efflux was high and equalled influx. Thus, the threshold concentration can be regarded as a NO 3 (-) compensation point. The inclusion of NH 4 (+) in the outer medium decreased influx by about 40% but did not significantly affect efflux. The roles of NO 3 (-) fluxes and nitrate-reductase activity in regulating/limiting NO 3 (-) utilization are discussed.