Growth requirement for N as a criterion to assess the effects of physical manipulation on nitrate uptake fluxes in spinach

The effects of physical manipulation of hydroponically grown plants of spinach (Spinacia oleracea L., cvs Subito and Glares) on nitrate uptake fluxes were studied in a long-term experiment (3 days), and in short-term label experiments (2 h) with ¹³N-nitrate and ¹⁵N-nitrate. In the long-term experiment, net nitrate uptake rate (NNUR) was measured by following the nitrate depletion in the uptake solution, which was replaced at regular intervals. In the short-term experiments, NNUR and nitrate influx were measured by simultaneous application of ¹³N-nitrate and ¹⁵N-nitrate. Plants were gently transferred into the labelled uptake solution, as is usually done in nutrient uptake studies. In addition, a more severe physical manipulation was carried out, including blotting of the roots, to mimic pretreatments which involve more handling of the plants prior to uptake measurements. Nitrate influx was measured immediately after physical manipulation and after 2 h of recovery. To assess the impact of the physical manipulation the experimentally determined nitrate uptake fluxes were compared with the N demand for growth, defined as relative growth rate (RGR) times plant nitrogen concentration (PNC) of parallel plants, which were left undisturbed. Nitrate influx and efflux were both subject to changes after physical manipulation of the plants. Physical handling, however, did not always result in an alteration of NNUR, which complicates the determination of the length of the recovery period. The impact of the handling and the time course of the recovery depended on the severity of the disturbance and were independent of the light conditions during the experiments. Even after a gentle transfer of the plants, recovery, in most cases, was not complete within 2 h. The data emphasise the need for minimal disturbance of plants during the last hours prior to nutrient uptake measurements.     

Slow Sodium: An Oral Slowly Released Sodium Chloride Preparation

The use of a slowly released oral preparation of sodium chloride is described. It was given to patients and athletes to treat or prevent acute and chronic sodium chloride deficiency. Gastrointestinal side effects were not encountered after the ingestion of up to 500 mEq in one day or 200 mEq in 10 minutes.     

Composition and synthesis of fatty acids in atherosclerotic aortas of the pigeon

The composition, synthesis, and esterification of fatty acids were studied in aortas of White Carneau and Show Racer pigeons after perfusion of the aortas with a medium containing acetate-1-(14)C. For both breeds of pigeons the principal change in aortic fatty acids, in response to an atherogenic diet, was a marked increase in the percentage of oleic acid in the cholesteryl ester fraction. In atherosclerotic aortas incorporation of acetate-1-(14)C into the phospholipid and glyceride fractions increased 2-fold, while a much greater increase (up to 10-fold) was seen in incorporation into cholesteryl esters. In those birds receiving the atherogenic diet, palmitic acid accounted for approximately 50% of the fatty acid radioactivity, compared with approximately 25% from control aortas. Calculation of fatty acid synthesis showed the major newly synthesized fatty acids to be stearic acid in the phospholipid fraction; stearic, palmitic, and oleic acids in the glycerides; and oleic acid in the cholesteryl esters. The pattern of fatty acid synthesis was closely similar to the actual fatty acid composition of the aorta. In atherosclerotic aortas an increased synthesis of all fatty acids was seen, but the greatest increase was seen in the synthesis of oleic acid and its esterification to cholesterol.     

Reiterated transfer RNA genes of Xenopus laevis

The proportion of the Xenopus laevis genome complementary to “7 S” RNA, unfractionated transfer RNA and some selected aminoacyl-tRNAs, and the sequence complexity of these RNA species, have been determined by following the kinetics of RNA-DNA hybridization on filters under conditions of RNA excess at optimum rate temperature. For hybridization of aminoacyl-labelled tRNAs, conditions for optimum aminoacylation were first determined and, where necessary, aminoacyl-tRNAs were treated with nitrous acid to prevent discharge during annealing. Neither the extent nor rate of hybridization was affected by this treatment.“7 S” RNA, coded for by 580 genes per haploid complement of chromosomes, reacts like a single family of nucleotide sequences, whereas about 43 basic tRNA sequences are coded for by at least 7800 genes. If hybrids are not treated with RNase A, the apparent tDNA redundancy is some 23% greater but no more nucleotide sequences are detectable. Taken together, the results suggest that each tRNA sequence is, on average, 200-fold reiterated.The reiteration varies, however, for different aminoacyl-tRNAs. Thus, hybridization resolves only one valyl-tRNA which is coded for by 240 genes, but at least four leucyl-tRNA sequences can be distinguished by hybridization, each of which is on average 90-fold reiterated. Reiteration also varies for the two methionyl-tRNAs detectable both by hybridization and by reversed phase chromatography: tRNA₁^Met and tRNA₂^Met are 310- and 170-fold reiterated, respectively, and each is kinetically homogeneous. These saturation values are almost exactly additive and are not influenced by the presence of other tRNA species. Thus the results suggest that Xenopus tRNAs are no more heterogeneous than would be predicted by the genetic code, despite the high but variable multiplicity of tRNA cistrons.     

Growth factor modulation of mitogenic responses and proteoglycan synthesis by human periodontal fibroblasts

In order to understand the relationship between specific growth factors and matrix synthesis by periodontal cells, we have investigated the effects of platelet-derived growth factor BB (PDGF-BB), insulin-like growth factor-I (IGF-1), and growth hormone on DNA and proteoglycan synthesis by cultured human gingival and periodontal ligament fibroblasts in vitro. PDGF-BB and IGF-1, but not growth hormone, were mitogenic for both periodontal ligament fibroblasts and gingival fibroblasts, although the periodontal ligament cells responded more strongly. The mitogenic response was accompanied by alterations in expression of matrix proteoglycan mRNA. For both the gingival and periodontal ligament cells, there was a decrease in mRNA for decorin and an increase in mRNA for versican following exposure to IGF-1 and PDGF-BB. Although no change was seen in response to PDGF, biglycan mRNA level was increased by IGF-1 in periodontal ligament fibroblasts. With the gingival fibroblats, biglycan mRNA levels were unaffected by IGF-1, PDGF-BB, or growth hormone. These findings suggest variable responses of fibroblasts to growth factors depending upon anatomical site within the periodontium. Moreover, there appears to be a correlation between cell proliferation and the types of proteoglycan synthesised with decorin expression being suppressed, and versican being increased during fibroblast proliferation. J. Cell. Physiol. 174:353–361, 1998. © 1998 Wiley-Liss, Inc.     

Muscle function in men and women during maximal eccentric exercise

This study assessed muscle fatigue patterns of the elbow flexors in untrained men and women to determine if sex differences exist during acute maximal eccentric exercise. High-intensity eccentric exercise is often used by athletes to elicit gains in muscle strength and size gains. Development of fatigue during this type of exercise can increase risk of injury; therefore, it is important to understand fatigue patterns during eccentric exercise to minimize injury risk exposure while still promoting training effects. While many isometric exercise studies have demonstrated that women show less fatigue, the patterns of fatigue during purely eccentric exercise have not been assessed in men and women. Based on the lack of sex differences in overall strength loss immediately post-eccentric exercise, it was hypothesized that women and men would have similar relative fatigue pattern responses (i.e., change from baseline) during a single bout of maximal eccentric exercise. Forty-six subjects (24 women and 22 men) completed 5 sets of 10 maximal eccentric contractions on an isokinetic dynamometer. Maximal voluntary isometric contraction strength was assessed at baseline and immediately following each exercise set. Maximal eccentric torque and contractile properties (i.e., contraction time, work, half relaxation time, and maximal rate of torque development) were calculated for each contraction. Men and women demonstrated similar relative isometric (32% for men and 39% for women) and eccentric (32% for men and 39% for women) fatigue as well as similar deficits in work done and rates of torque development and relaxation during exercise (p > 0.05). Untrained men and women displayed similar relative responses in all measures of muscle function during a single bout of maximal eccentric exercise of the elbow flexors. Thus, there is no reason to suspect that women may be more vulnerable to fatigue-related injury.     

Colonization dynamics and facilitative impacts of a nitrogen‐fixing shrub in primary succession

Abstract. Whether nitrogen‐fixing plants facilitate or inhibit species change in primary succession is best resolved by examining their impacts throughout the plant's entire life cycle from arrival to senescence. We experimentally examined two aspects of the successional impacts of a nitrogen‐fixing shrub, Coriaria arborea, on Mt. Tarawera, a volcano in New Zealand: factors limiting Coriaria colonization and impacts of Coriaria‐induced soil changes on a later successional tree, Griselinia littoralis. Coriaria germination was promoted by artificial wind protection and by the presence of heath shrubs. Transplanted Coriaria seedlings survived only if nodulated with Frankia, and the addition of Coriaria‐enriched soils slowed Coriaria seedling growth and did not improve seedling survival. This explained why Coriaria seedlings were found mostly in protected habitats away from adult Coriaria, and suggested that Coriaria thickets are not self‐replacing. Coriaria increased soil fertility by developing a 4 cm thick organic soil horizon that was richer in nitrogen (tenfold) and phosphorus (threefold) than pre‐Coriaria stages. These soil changes resulted in three‐ to sixfold increases in growth of Griselinia when it was grown in Coriaria‐enriched soils in a glasshouse. Coriaria's net effect on primary succession is facilitative, but the establishment of Coriaria is itself facilitated by the amelioration of the physical habitat by earlier colonists, suggesting facilitation is important throughout the life cycle of Coriaria. Sequential facilitative events determine the order of species replacements in this study but inhibition, linked to the developmental stages of Coriaria, may determine the rate of species change.