Effect of nitrogen stress and abscisic acid on nitrate absorption and transport in barley and tomato

The potential of barley (Hordeum vulgare L.) and tomato (Lycopersicon esculentum Mill.) roots for net NO ₃ ^- absorption increased two-to five fold within 2 d of being deprived of NO ₃ ^- supply. Nitrogen-starved barley roots continued to maintain a high potential for NO ₃ ^- absorption, whereas NO ₃ ^- absorption by tomato roots declined below control levels after 10 d of N starvation. When placed in a 0.2 mM NO ₃ ^- solution, roots of both species transported more NO ₃ ^- and total solutes to the xylem after 2 d of N starvation than did N-sufficient controls. However, replenishment of root NO ₃ ^- stores took precedence over NO ₃ ^- transport to the xylem. Consequently, as N stress became more severe, transport of NO ₃ ^- and total solutes to the xylem declined, relative to controls. Nitrogen stress caused an increase in hydraulic conductance (L _p) and exudate volume (J _v) in barley but decrased these parameters in tomato. Nitrogen stress had no significant effect upon abscisic acid (ABA) levels in roots of barley or flacca (a low-ABA mutant) tomato, but prevented an agerelated decline in ABA in wild-type tomato roots. Applied ABA had the same effect upon barley and upon the wild type and flacca tomatoes: L _p and J _v were increased, but NO ₃ ^- absorption and NO ₃ ^- flux to the xylem were either unaffected or sometimes inhibited. We conclude that ABA is not directly involved in the normal changes in NO ₃ ^- absorption and transport that occur with N stress in barley and tomato, because (1) the root ABA level was either unaffected by N stress (barley and flacca tomato) or changed, after the greatest changes in NO ₃ ^- absorption and transport and L _p had been observed (wild-type tomato); (2) changes in NO ₃ ^- absorption/transport characteristics either did not respond to applied ABA, or, if they did, they changed in the direction opposite to that predicted from changes in root ABA with N stress; and (3) the flacca tomato (which produces very little ABA in response to N stress) responded to N stress with very similar changes in NO ₃ ^- transport to those observed in the wild type.Abbreviation and symbols ABA abscisic acid - J_v exudate volume - L_p root hydraulic conductance     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Exogenous Tryptophan Increases Synthesis, Storage, and Intraneuronal Metabolism of 5-Hydroxytryptamine in the Rat Hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.     

A new species of coccinellid feeding on scale insects in West Pakistan

Résumé Les auteurs décriventPseudoscymnus simmondsi, espèce nouvelle deCoccinellidae prédatrice dePinnaspis strachani (Cooley) surFicus palmata au Pakistan occidental. Les autres espèces du genre sont connues seulement de l'Extrême-Orient.

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This research has been financed in part by a grant made by the United States Department of Agriculture, Agricultural Research Service.     

Environmental responses of plants and ecosystems as predictors of the impact of global change

An understanding of plant responses to fluctuations in environment is critical to predictions of plant and ecosystem responses to climate change. In the northern hemisphere, the northern limits of distribution of major biomes are probably determined by the tolerance of their dominant physiognomic types (e.g., deciduous hardwood trees) to minimum winter temperatures and can thus be predicted from long-term patterns of temperature fluctuations. At a more detailed level, the responses of functional groups of plants to altered climate can be predicted from their known responses to fluctuations in soil resources (nutrients and water) and the expected effect of climatic change on these soil resources. Laboratory and field experiments demonstrate the feasibility of this approach.     

Differential localization of syntaxin isoforms in polarized Madin-Darby canine kidney cells.

Syntaxins, integral membrane proteins that are part of the ubiquitous membrane fusion machinery, are thought to act as target membrane receptors during the process of vesicle docking and fusion. Several isoforms of the syntaxin family have been previously identified in mammalian cells, some of which are localized to the plasma membrane. We investigated the subcellular localization of these putative plasma membrane syntaxins in polarized epithelial cells, which are characterized by the presence of distinct apical and basolateral plasma membrane domains. Syntaxins 2, 3, and 4 were found to be endogenously present in Madin-Darby canine kidney cells. The localization of syntaxins 1A, 1B, 2, 3, and 4 in stably transfected Madin-Darby canine kidney cell lines was studied with confocal immunofluorescence microscopy. Each syntaxin isoform was found to have a unique pattern of localization. Syntaxins 1A and 1B were present only in intracellular structures, with little or no apparent plasma membrane staining. In contrast, syntaxin 2 was found on both the apical and basolateral surface, whereas the plasma membrane localization of syntaxins 3 and 4 were restricted to the apical or basolateral domains, respectively. Syntaxins are therefore the first known components of the plasma membrane fusion machinery that are differentially localized in polarized cells, suggesting that they may play a central role in targeting specificity.     

Apical targeting in polarized epithelial cells: There's more afloat than rafts

Most metazoan cells are 'polarized'. A crucial aspect of this polarization is that the plasma membrane is divided into two or more domains with different protein and lipid compositions or example, the apical and basolateral domains of epithelial cells or the axonal and somatodendritic domains of neurons. This polarity is established and maintained by highly specific vesicular membrane transport in the biosynthetic, endocytic and transcytotic pathways. Two important concepts, the 'SNARE' and the 'raft' hypotheses, have been developed that together promise at least a partial understanding of the underlying general mechanisms that ensure the necessary specificity of these pathways.     

Long–term responses to factorial, NPK fertilizer treatment by Alaskan wet and moist tundra sedge species

At thirteen sites in northern and central Alaska, full factonal and partial factonal NPK fertilizer experiments were completed in wet and moist tundras The effects of the fertilizers on N and P concentrations in leaves, leaf mass per tiller, tilling, and flowering of the dominant sedge species ( Eriophorum vaginatum. E angustifolium, and Carex aquatilis ) were documented for 3–10 yr at each site Almost all sites showed at least some response to the fertilizer treatment, but the specific treatments and elements that caused the responses varied greatly among sites There also was no consistent difference in responses between wet and moist sites There was, however, a consistent temporal pattern to the responses observed, starting with increases in N and P concentration in leaves in the year that fertilizer was added, followed by growth and tilling responses in the second year, and increased flowering in the third year The increases in N concentration generally only appeared in the first year, while increased P concentrations were sustained for 3–4 yr Effects of fertilizer on flowering were still significant after 6 years in some cases Results are discussed in terms of the role of "nutrient limitation" arctic plant communities and detemuning their productivity following disturbance or fertilization     

Basolateral plasma membrane localization of ouabain-sensitive sodium transport sites in the secretory epithelium of the avian salt gland

The distribution of Na+ pump sites (Na+-K+-ATPase) in the secretory epithelium of the avian salt gland was demonstrated by freeze-dry autoradiographic analysis of [(3)H] ouabain binding sites. Kinetic studies indicated that near saturation of tissue binding sites occurred when slices of salt glands from salt-stressed ducks were exposed to 2.2 μM ouabain (containing 5 μCi/ml [(3)H]ouabain) for 90 min. Washing with label-free Ringer's solution for 90 min extracted only 10% of the inhibitor, an amount which corresponded to ouabain present in the tissue spaces labeled by [(14)C]insulin. Increasing the KCl concentration of the incubation medium reduced the rate of ouabain binding but not the maximal amount bound. In contrast to the low level of ouabain binding to salt glands of ducks maintained on a freshwater regimen, exposure to a salt water diet led to a more than threefold increase in binding within 9-11 days. This increase paralleled the similar increment in Na+-K+-ATPase activity described previously. [(3)H]ouabain binding sites were localized autoradiographically to the folded basolateral plasma membrane of the principal secretory cells. The luminal surfaces of these cells were unlabeled. Mitotically active peripheral cells were also unlabeled. The cell-specific pattern of [(3)H]ouabain binding to principal secretory cells and the membrane-specific localization of binding sites to the nonluminal surfaces of these cells were identical to the distribution of Na+-K+-ATPase as reflected by the cytochemical localization of ouabain-sensitive and K+-dependent nitrophenyl phosphatase activity. The relationship between the nonluminal localization of Na+-K+-ATPase and the possible role of the enzyme n NaCl secretion is considered in the light of physiological data on electrolyte transport in salt glands and other secretory epithelia.