Physiological regulation of plant-atmosphere ammonia exchange

Plants have a compensation point for NH₃ which ranges from 0.1 to 20 nmol mol^-1, and may be several-fold higher or lower than naturally occurring atmospheric NH₃ concentrations. This implies that NH₃ fluxes over vegetated surfaces are bi-directional and that ammonia exchange with the atmosphere in many cases contributes significantly to the nitrogen economy of vegetation. Physiological regulation of plant–atmosphere NH₃ fluxes is mediated via processes involved in nitrogen uptake, transport and metabolism. A rapid turnover of NH₃ ⁺ in plant leaves leads to the establishment of a finite NH₃ ⁺ concentration in the leaf apoplastic solution. This concentration determines, together with that of H⁺, the size of the NH₃ compensation point. Barley and oilseed rape plants with access to NH₃ ⁺ in the root medium have higher apoplastic NH₃ ⁺ concentrations than plants absorbing NO₃ ^-. Furthermore, the apoplastic NH₃ ⁺ concentration increases with the external NH₃ ⁺ concentration. Inhibition of GS leads to a rapid and substantial increase in apoplastic NH₃ ⁺ and barley mutants with reduced GS activity have higher apoplastic NH₃ ⁺ than wild-type plants. Increasing rates of photorespiration do not affect the steady-state NH₃ ⁺ or H⁺ concentration in tissue or apoplast of oilseed rape, indicating that the NH₃ ⁺ produced is assimilated efficiently. Nevertheless, NH₃ emission increases due to a temperature-mediated displacement of the chemical equilibrium between gaseous and aqueous NH₃ in the apoplast. Sugarbeet plants grown with NO₃ ^- seem to be temporarily C-limited in the light due to a repression of respiration. As a consequence, the activity of chloroplastic GS declines during the day causing a major part of NH₃ ⁺ liberated in photorespiration to be assimilated during darkness when 2-oxoglutarate is supplied in high rates by respiration. This revised version was published online in June 2006 with corrections to the Cover Date.     

New taxa and combinations in Old World Urticaceae

In anticipation of the publication of full revisions, a diagnosis of a new species of Droguetia is presented together with two new combinations for subspecies of Droguetia iners , a new combination for a species and a variety on Didymodoxa and a new combination for a subspecies of Australina pusilla . The name Elatostema trinerve Hochst. (1845) is shown to be conspecific with and antedate Urera cameroonensis Wedd. (1869). The new combination Urera trinervis is therefore made.     

Pathways for bicarbonate transfer across the serosal membrane of turtle urinary bladder: Studies with a disulfonic stilbene

Summary Bicarbonate is transferred across the serosal (S) membrane of the epithelial cells of the turtle bladder in two directions. Cellular HCO ₃ ^– generated behind the H⁺ pump moves across this membrane into the serosal solution. This efflux of HCO ₃ ^– is inhibited by SITS (4-isothiocyano-4-acetamido-2,2-disulfonic stilbene). When HCO ₃ ^– is added to the serosal solution it is transported across the epithelium in exchange for absorbed Cl^–. This secretory HCO ₃ ^– flow traverses the serosal cell membrane in the opposite direction. In this study the effects of serosal addition of 5×10^–4 m SITS on HCO ₃ ^– secretion and Cl^– absorption were examined. The rate of H⁺ secretion was brought to zero by an opposing pH gradient, and 20mm HCO ₃ ^– was added toS. HCO ₃ ^– secretion, measured by pH stat titration, was equivalent to the increase inMS Cl^– flux after HCO ₃ ^– addition. Neither theSM flux of HCO ₃ ^– nor theMS flux of Cl^– were affected by SITS. In the absence of electrochemical gradients, net Cl^– absorption was observed only in the presence of HCO ₃ ^– in the media; under such conditions, unidirectional and net fluxes of Cl^– were not altered by serosal or mucosal SITS. H⁺ secretion, however, measured simultaneously as the short-circuit current in ouabain-treated bladders decreased markedly after serosal SITS. The inhibition of the efflux of HCO ₃ ^– in series with the H⁺ pump and the failure of SITS to affect HCO ₃ ^– secretion and Cl^– absorption suggest that the epithelium contains at least two types of transport systems for bicarbonate in the serosal membrane.     

荔枝胚胎败育与酚类抑制物质的关系

在荔枝 (LitchichinensisSonn .)胚胎败育发生期 ,以系统溶剂法从正常或败育胚珠中初步提取酚类抑制物质 ,通过TLC分离与纯化 ,用GC MS联用仪进一步分离鉴定 ,并以标准品核对。试验首次从荔枝胚珠中分离鉴定出酚类抑制物质对羟基苯甲酸 (p_HBA)。生物活性测定表明 ,p_HBA是一种很强的生长抑制物质。在败育胚珠中其含量及IAA氧化酶活性均显著高于正常胚珠 ,IAA水平则明显低于正常胚珠 (P <0 .0 1)。因此认为 ,p_HBA参与了荔枝胚胎发育的调节 ,高含量的p_HBA是通过促进IAA侧链的氧化并影响促进和抑制生长的物质之间的平衡而导致荔枝胚胎的败育     

HeLa细胞端粒酶最适反应条件及活性重建

端粒是真核细胞染色体末端的重复ＤＮＡ序列 ,其生物学功能是防止染色体ＤＮＡ降解、末端融合、非正常重组和染色体的缺失[1] .由于存在“末端复制问题” ,随着老化人体细胞端粒重复序列长度不断缩短 ,但在生殖细胞中由于端粒酶的存在 ,端粒序列并不缩短 .端粒酶是由蛋白质和ＲＮＡ构成的核蛋白 ,是依赖ＲＮＡ的ＤＮＡ聚合酶 ,在ＤＮＡ3’端合成端粒重复序列[2 ] .研究表明 ,在 85 %～ 95 %的人肿瘤细胞中可以检测到端粒酶的活性[3 ,4 ] ,而在正常体细胞中除生殖细胞和造血干细胞等极少数细胞中存在端粒酶活性外 ,均检测不到端粒酶活性 ,这…     

Differential capacity for high-affinity manganese uptake contributes to differences between barley genotypes in tolerance to low manganese availability

There is considerable variability among barley (Hordeum vulgare) genotypes in their ability to grow in soils containing a low level of plant available manganese (Mn). The physiological basis for the tolerance to low Mn availability is unknown. In this work, Mn2+ influx and compartmentation in roots of the Mn-efficient genotype Vanessa and the Mn-inefficient genotype Antonia were investigated. Two separate Mn transport systems, mediating high-affinity Mn2+ influx at concentrations up to 130 nm and low-affinity Mn2+ influx at higher concentrations, were identified in both genotypes. The two genotypes differed only in high-affinity kinetics with the Mn-efficient genotype Vanessa having almost 4 times higher V(max) than the inefficient Antonia, but similar K(m) values. Online inductively coupled plasma-mass spectrometry measurements verified that the observed differences in high-affinity influx resulted in a higher Mn net uptake of Vanessa compared to Antonia. Further evidence for the importance of the differences in high-affinity uptake kinetics for Mn acquisition was obtained in a hydroponic system with mixed cultivation of the two genotypes at a continuously low Mn concentration (10-50 nm) similar to that occurring in soil solution. Under these conditions, Vanessa had a competitive advantage and contained 55% to 75% more Mn in the shoots than did Antonia. Subcellular compartmentation analysis of roots based on 54Mn2+ efflux established that up to 93% and 83% of all Mn was present in the vacuole in Vanessa and Antonia, respectively. It is concluded that differential capacity for high-affinity Mn influx contributes to differences between barley genotypes in Mn efficiency.