The effect of humidity and light on cellular water relations and diffusion conductance of leaves ofTradescantia virginiana L.

Turgor (_p) and osmotic potential (_s) in epidermal and mesophyll cells, in-situ xylem water potential (-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (_p-epi, _p-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.g/_p-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.Symbols E transpiration - g leaf conductance - w leaf/air vapour concentration difference - -epi water potential of epidermal cells - -mes water potential of mesophyll cells - -xyl water potential of xylem - _p-epi turgor pressure of epidermal cells - _p-mes turgor pressure of mesophyll cells - _s-epi osmotic potential of epidermal cells - _s-mes osmotic potential of mesophyll cells     

Endogenous abscisic acid and 2-trans-abscisic acid in alternate bearing ‘Wilking’ mandarin trees

The relationship of abscisic acid (ABA) and 2-trans-abscisic acid (t-ABA) to alternate bearing has been examined in Wilking mandarin (Citrus reticulata Blanco) trees. Leaves, stems and buds of trees loaded with fruit (on trees) had 4.3, 6.0 and 2.2 fold higher ABA levels than the corresponding organs from off trees. Leaves had higher ABA levels than stems and buds in both on and off trees. t-ABA was non-detectable in Wilking leaf, stem and bud tissue. Amounts of t-ABA not exceeding 40% of the ABA content, were found in Shamouti and Valencia orange buds and in Wilking fruit peel.The elevated levels of ABA in on tree organs may reflect a stress imposed by the fruit overload.     

Sodium ions are necessary for growth and energy transduction in the marine cyanobacterium Oscillatoria brevis

The maximal growth rate of the marine cyanobacterium Oscillatoria brevis was reached at 200–400 mM NaCl and pH 9.0–9.6. NaCl was found (i) to stimulate the rate of the light-supported generation across the cytoplasmic membrane of the cells and (ii) to decrease the sensitivity of level and motility of the O. brevis trichomes to protonophorous uncouplers. The Na⁺/H⁺ antiporter, monensin, increased both and the uncoupler sensitivity of the cells. The data obtained agree with the assumption that O. brevis possesses a primary Na⁺ pump in its cytoplasmic membrane.Abbreviations ATP adenosine-5-triphosphate - TTFB tetrachlortrifluoromethylimidazol - CCCP carbonyl cyanide m-chlorophenylhydrazone - Na⁺ transmembrane electrochemical potential differences of Na⁺ - transmembrane electric potential difference - pNa transmembrane pNa difference     

Influence of Phosphorus Application on Water Relations, Biochemical Parameters and Gum Content in Cluster Bean Under Water Deficit

Relative water content (RWC), leaf water potential (_w) and osmotic potential (_s), contents of chlorophyll (Chl) a, Chl b, soluble sugars, and seed quality (gum content) were used to evaluate the role of phosphorus in alleviation of the deleterious effect of water deficit in clusterbean (Cyamopsis tetragonoloba L. Taub). Under water stress, _w, _s, and Chl and gum contents decreased and soluble sugar contents increased. Phosphorus application increased Chl and sugar contents in control plants and ameliorated negative effects of water stress.     

Sex identification by male-specific growth hormone pseudogene (GH-Ψ) in Oncorhynchus masou complex and a related hybrid

It is often difficult to identify sexes of many fish species by conventional cytological method because of the lack of heteromorphic sex chromosomes. Isolation of sex-specific molecular markers is thus important for sexing and for understanding sex chromosome evolution in these species. We have identified genetic sexes by PCR-based male-specificity of a growth hormone pseudogene (GH-) in masu and Biwa salmon, two subspecies of the Oncorhynchus masou complex, and their hybrid Honmasu. PCRs with primers designed from sequences of chinook salmon GH genes amplified GH-I and GH-II fragments in both sexes, but a third GH- fragment was detected in predominant proportion of males and very few phenotypic females. The consistency of phenotypic sex with genetic sex identified by GH- for masu salmon, Biwa salmon and Honmasu was 93.1, 96.7 and 94%, respectively. The remaining individuals showed inconsistency or deviation from sex-specificity: a few phenotypic males lacked the GH-, whereas a few phenotypic females possessed the GH-. Sequence of the putative GH- fragment from such females was identical to that from genetic males, and shared about 95% homology with the corresponding GH- fragment from chinook salmon. This result confirmed that these females were really GH--bearing individuals. PCR analyses with primers designed from masu salmon GH- gave identical results, indicating that the absence of GH- in a few males was not resulted from primer mismatching. These GH--bearing females and GH--absent males were more likely to originate from spontaneous sex reversion than from crossing-over between GH- and the sex determination gene/region.     

An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure

Summary Solute osmotic potentials (_x) in the vessels of hydroponically grown maize roots were measured to assess the osmotic-xylem-sap mechanism for generating root pressure (indicated by guttation). Solutes in vessels were measured in situ by X-ray microanalysis of plants frozen intact while guttating. Osmotic potentials outside the roots (_o) were changed by adding polyethylene glycol to the nutrient solution. Guttation rate fell when _o was decreased, but recovered towards the control value during 3–5 days when _o was greater than or equal to –0.3 MPa, but not when _o was equal to –0.4 MPa. In roots stressed to _o = –0.3 MPa, _x, was always more positive than _o, and _x changed only slightly (ca. 0.05 MPa). Thus the adjustment in the roots which increased root pressure cannot be ascribed to _x, contradicting the osmotic-xylem-sap mechanism. An alternative driving force was sought in the osmotic potentials of the vacuoles of the living cells (_v), which were analysed by microanalysis and estimated by plasmolysis. _v showed larger responses to osmotic stress (0.1 MPa). Some plants were pretreated with abundant KNO₃ in the nutrient solution. These plants showed very large adjustments in _v (0.4 MPa) but little change in _x (0.08 MPa). They guttated by 4 h after _o was lowered to –0.4 MPa. It is argued that turgor pressure of the living cells is a likely alternative source of root pressure. Published evidence for high solute concentrations in the xylem sap is critically assessed.Abbreviations _o external water potential - _x osmotic potential of xylem sap - _v osmotic potential of vacuolar sap - EDX energy dispersive X-ray microanalysis - CSEM cryo-scanning electron microscope - LN₂ liquid nitrogen - PEG polyethylene glycol     

The ability of acidic pH,growth inhibitors,and glucose to increase the proton motive force and energy spilling of amino acid-fermenting <Emphasis Type="Italic">Clostridium sporogenes</Emphasis> MD1 cultures

Clostridium sporogenes MD1 grew rapidly with peptides and amino acids as an energy source at pH 6.7. However, the proton motive force (p) was only –25 mV, and protonophores did not inhibit growth. When extracellular pH was decreased with HCl, the chemical gradient of protons (ZpH) and the electrical membrane potential () increased. The p was –125 mV at pH 4.7, even though growth was not observed. At pH 6.7, glucose addition did not cause an increase in growth rate, but increased to –70 mV. Protein synthesis inhibitors also significantly increased . Non-growing, arginine-energized cells had a of –80 mV at pH 6.7 or pH 4.7, but was not detected if the F₁F₀ ATPase was inhibited. Arginine-energized cells initiated growth if other amino acids were added at pH 6.7, and and ATP declined. At pH 4.7, ATP production remained high. However, growth could not be initiated, and neither nor the intracellular ATP concentration declined. Based on these results, it appears that C. sporogenes MD1 does not need a large p to grow, and p appears to serve as a mechanism of ATP dissipation or energy spilling.Mandatory disclaimer: Proprietary or brand names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product, and exclusion of others that may be suitable.     

Seasonal Changes in Photosynthesis and Stomatal Conductance of Five Plant Species from a Semiarid Ecosystem

In order to determine whether stomatal closure alone regulates photosynthesis during drought under natural conditions, seasonal changes in leaf gas exchange were studied in plants of five species differing in life form and carbon fixation pathway growing in a thorn scrub in Venezuela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 deciduous shrubs), Alternanthera crucis (C4 deciduous herb), and Prosopis juliflora and Capparis odoratissima (evergreen phreatophytic trees). Xylem water potential () of all species followed very roughly the precipitation pattern, being more closely governed by soil water content in I. carnea and A. crucis. Maximum rate of photosynthesis, Pmax, decreased with in I. carnea, J. gossypifolia, and A. crucis. In I. carnea and J. gossypifolia stomatal closure was responsible for a 90 % decline in net photosynthetic rate (PN) as decreased from -0.3 to -2.0 MPa, since stomatal conductance (gs) was sensitive to water stress, and stomatal limitation on PN increased with drought. In A. crucis, PN decreased by 90 % at a much lower (-9.3 MPa), and gs was relatively less sensitive to . In P. juliflora and C. odoratissima, Pmax, gs, and intercellular CO2 concentration (Ci) were independent of soil water content. In the C3 shrubs stomatal closure was apparently the main constraint on photosynthesis during drought, Ci declining with in I. carnea. In the C4 herb, Ci was constant along the range of values, which suggested a coordinated decrease in both gs and mesophyll capacity. In P. juliflora Ci showed a slow decrease with which may have been due to seasonal leaf developmental changes, rather than to soil water availability.     

Stereospecificity and electrogenicity of amino acid transport in Riccia fluitans

In the aquatic liverwort Riccia fluitans, membrane depolarization (_m), change in membrane conductance (g_m), and current-voltage (I-V) characteristics in the presence of different amino acids as well as the uptake of ¹⁴C-labeled amino acids were measured. L-isomers of the tested amino acids generate larger electrical effects (_m, g_m) than D-isomers, and the I-V characteristics show that the positive electrical inward-current of 20 mA m^-2 generated by 0.5 mM D-serine is only about 50% of the current generated by adding 0.5 mM L-serine. Whereas - and -amino acids rapidly depolarize the membrane to the same extend, with -aminobutyric acid (-AB) and dipeptides no significant electrical effects have been measured. The uptake kinetics of ¹⁴C-labeled amino acids display three components: (I) A saturable high-affinity component with K_s-values of 48 M D-alanine, 12 M -aminoisobutyric acid (AIB), 9 M L-alanine, 8 M L-proline, and 6 M L-serine, respectively; (2) an apparently linear low-affinity component, and (3) an also linear but unspecific component at concentrations >20 times the given K_s-value. Uptake of ¹⁴C-labeled AIB can be inhibited competitively by all tested neutral amino acids, the L-isomers being more effective than the D-isomers, as well as by ammonium or methylamine. Vice versa, AIB competitively inhibits uptake of L-serine and L-alanine. It is concluded that an uncharged stereospecific carrier for the investigated amino acids exists in the plasmalemma of Riccia fluitans. Accumulation ratios of about 50 suggest secondary active transport driven by a transmembrane electro-chemical gradient (mainly _m) which is generated by the electrogenic proton pump. It is suggested that this carrier binds to the amino group forming either a charged binary complex with positively charged amines (Felle 1980), or an uncharged complex with -AB or dipeptides, whereas electrogenic transport of - and -amino acids is mediated by a ternary carrier complex, probably charged by a proton.Symbols and Abbreviations _m membrane potential (mV) - E_co equilibrium potential (mV) of the transport system - g_m membrane (slope) conductance (Sm^-2) - g_m change in g_m - I-V curve current-voltage curve - AIB -aminoisobutytric acid - -AB -aminobutyric acid     

Tetramethyl Rhodamine Methyl Ester (TMRM) is Suitable for Cytofluorometric Measurements of Mitochondrial Membrane Potential in Cells Treated with Digitonin

A new method for cytofluorometric analysis of mitochondrial membrane potential has been developed by using TMRM as a cationic, mitochondrial selective probe. The method is based on limited treatment of cultured cells with digitonin which permeabilises the plasma membrane and leaves mitochondria intact. The resulting signal of TMRM-stained cells thus represents only the probe accumulated in mitochondria. Fibroblasts and cybrids were used as a model cell systems and optimal conditions for digitonin treatment and staining by TMRM were described. The TMRM signal collapsed by valinomycin, KCN and antimycin A and FCCP titration was used to gradually lower and characterise the stability of . The method is suitable for sensitive measurement of in different types of cultured cells.     

Abscisic acid, temperature and salinity interactions on growth and some mineral elements in Carthamus plants

Growth and contents of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), chloride (Cl), phosphorus (P) and sulphur (S) in shoot and root tissues of Carthamus tinctorius plants were measured at combinations of four nutrient solution osmotic potentials (_s=0, -0.3, -0.6 and -0.9 MPa) induced by NaCl and CaCl treatments, three constant temperatures (T) ranging from 15 to 35°C and four abscisic acid (ABA) concentrations (0,10,50 and 100 mg L^–1). Unstressed and stressed plants grown in optimal temperature conditions (25°C) maintained higher growth rates (dry mass production) than plants grown under low and high temperatures (15 and 35°C respectively). Shoot and root growth (dry mass production) were largely inhibited by salinity but the magnitude of growth inhibition was temperature dependent. Safflower plants respond to salinity stress by increases in Ca, Cl and to a lesser extent Na in their shoots and roots and by a decrease in the ratio of fresh to dry weight. The ratio of K/Na was decreased progressively on salinization. With stressed plants, ABA application reduced the toxicity of salt treatment, improved K uptake under salinity, effectively increased K/Na ratio and helped the plants to avoid Na toxicity and sometimes enhanced growth. The effect of ABA on the growth was more pronounced at optimum temperature (25°C). The association between the internal mineral element concentrations was largely affected by ABA application and temperature change but a wide fluctuation in response was noticed. The effects of single factors (_s, T and ABA) on the growth and mineral contents were statistically significant. Also, bifactorial (_s× T, _s × ABA and T × ABA) and three factorial (_s × T × ABA) interactions significantly affected the parameters. Further statistical treatment of the data (coefficient of determination ²) led to four important findings: (1) Salinity (_s) was dominant in affecting Ca and Cl contents in both shoot and root as well as root Na content. (2) Temperature (T) had a dominant effect on growth, shoot K, Mg, P, S and root P, and S contents (3) The share of _s × T × ABA interaction was dominant for root Na and Mg contents. (4) The single factors and their interactions had a dual role in their subsidiary effects.Abbreviations ABA abscisic acid - _s osmotic potential - ² coefficient of determination - F.wt fresh weight - d.m. dry matter - T temperature - MPa mega pascal - SAR sodium adsorption ratio - P phosphorus - S sulphur     

Dormancy termination of western white pine (<Emphasis Type="Italic">Pinus monticola</Emphasis> Dougl. Ex D. Don) seeds is associated with changes in abscisic acid metabolism

Western white pine (Pinus monticola) seeds exhibit deep dormancy at maturity and seed populations require several months of moist chilling to reach their uppermost germination capacities. Abscisic acid (ABA) and its metabolites, phaseic acid (PA), dihydrophaseic acid (DPA), 7-hydroxy ABA (7OH ABA) and ABA-glucose ester (ABA-GE), were quantified in western white pine seeds during dormancy breakage (moist chilling) and germination using an HPLC–tandem mass spectrometry method with multiple reaction monitoring and internal standards incorporating deuterium-labeled analogs. In the seed coat, ABA and metabolite levels were high in dry seeds, but declined precipitously during the pre-moist-chilling water soak to relatively low levels thereafter. In the embryo and megagametophyte, ABA levels decreased significantly during moist chilling, coincident with an increase in the germination capacity of seeds. ABA catabolism occurred via several routes, depending on the stage and the seed tissue. Moist chilling of seeds led to increases in PA and DPA levels in both the embryo and megagametophyte. Within the embryo, 7OH ABA and ABA-GE also accumulated during moist chilling; however, 7OH ABA peaked early in germination. Changes in ABA flux, i.e. shifts in the ratio between biosynthesis and catabolism, occurred at three distinct stages during the transition from dormant seed to seedling. During moist chilling, the relative rate of ABA catabolism exceeded ABA biosynthesis. This trend became even more pronounced during germination, and germination was also accompanied by a decrease in the ABA catabolites DPA and PA, presumably as a result of their further metabolism and/or leaching/transport. The transition from germination to post-germinative growth was accompanied by a shift toward ABA biosynthesis. Dormant imbibed seeds, kept in warm moist conditions for 30 days (after an initial 13 days of soaking), maintained high ABA levels, while the amounts of PA, 7OH ABA, and DPA decreased or remained at steady-state levels. Thus, in the absence of conditions required to break dormancy there were no net changes in ABA biosynthesis and catabolism.Abbreviations ABA abscisic acid - ABA-GE abscisic acid glucose ester - DPA dihydrophaseic acid - 7OH ABA 7-hydroxy abscisic acid - 8OH ABA 8-hydroxy abscisic acid - MRM multiple reaction monitoring - PA phaseic acid     

Abscisic acid metabolism —vacuolar/extravacuolar distribution of metabolites

The biotransformation of abscisic acid (ABA) was studied in cell suspension cultures of Lycopersicon esculentum. The ABA was converted by the cells to phaseic acid, nigellic acid, dihydrophaseic acid, abscisic acid--D-glucopyranosyl ester (ABA-Glc) and other ABA and phaseic acid conjugates. Investigation of their cellular distribution showed that the conjugated forms were located only in the vacuoles whereas ABA and its acidic metabolites were found mainly in the extravacuolar fractions. Our results, together with a number of studies on the increase of ABA-Glc as a response to stress, allow us to propose that ABA-Glc is irreversibly compartmented in the vacuoles of plant cells.Abbreviations ABA abscisic acid - ABA-Glc -D-glucopyranosyl ester of ABA - DPA 4-dihydrophaseic acid; nigellic acid=3-methyl-5-(1-hydroxy-2-hydroxymethyl-6-dimethyl-4-oxo-cyclohex-2-enyl)-penta-2Z, 4E-dienoic acid - PA phaseic acid     

Monitoring the mitochondrial transmembrane potential with the JC-1 fluorochrome in programmed cell death during mesophyll leaf senescence

Summary. Analysis of the mitochondrial transmembrane potential (_m) with the help of the JC-1 fluorochrome (5,5,6,6-tetrachloro-1,1,3,3-tetraethylbenzimidazolcarbocyanine iodide) during mesophyll leaf senescence was performed in order to determine whether a reduction of _m takes place during mesophyll senescence and whether plant mitochondria, like mammalian ones, might be involved in the induction of programmed cell death. Fluorescence analysis of mesophyll protoplasts of Pisum sativum in a confocal microscope, fluorescent spectra analysis and time dependence of fluorescence intensity of monomers and of J-aggregates revealed that JC-1 is incorporated and accumulated specifically in plant mitochondria. Analysis of _m during mesophyll protoplast senescence revealed that two subpopulations of mitochondria which differ in _m exist in all analyzed stages of leaf senescence. The first subpopulation contains mitochondria with red fluorescence of J-aggregates due to an unperturbed high _m. The second subpopulation comprises mitochondria with green fluorescence of monomers due to a low _m, proving total depolarization of mitochondrial membranes. Fluorescence analysis demonstrated that even in the latest analyzed stages of leaf senescence, mitochondria with a high _m still exist. Fluorometric measurements revealed that the fluorescence intensity of J-aggregates decreases with the age of plants, which indicates that a reduction of _m during the mesophyll senescence process takes place; however, it does not take place within the whole population of mitochondria of the same protoplast. The reason of this can be due to a dramatic reorganization of mitochondria in mesophyll cells and the appearance of large mitochondria with local heterogeneity of _m in the oldest analyzed stages. All mitochondria in every stage of senescence maintained their membrane organization even when their size, distribution, and spatial organization in protoplasts changed dramatically. We stated that the reduction of _m does not directly induce programmed cell death in mesophyll cells, as opposed to animal apoptosis.Correspondence and reprints: Department of Plant Anatomy and Cytology, Institute of Experimental Biology of Plants, Warsaw University, Miecznikowa 1, 02-096 Warszawa, Poland.     

Changes in leaf water potential and CAM inSempervivum montanum andSedum album in response to water availability in the field

Summary The short term effects of irrigation on diurnal changes in leaf and titratable acidity were examined both inSempervivum montanum and inSedum album, a facultative CAM plant, in the Spanish Pyrenees. InSemperivivum, leaf responded rapidly to irrigation and, in both the control and irrigated plants, increased during the day and decreased during the night and early morning. By contrast, leaf inSedum responded more slowly to irrigation and showed a decrease during the day and an increase in the period between evening and early morning. Under the conditions of the short-term experiments, changes in acid metabolism were not observed in either species following irrigation. The results suggest that transpirational water loss together with redistribution of water within the plant are more important than the osmotic concentration of malic acid in determining leaf in both species and that daytime water loss is greater inSedum than inSempervivum.The effect of long-term water stress on leaf and acid levels was also assessed in both species over a 3-week period. Both leaf and acidification inSempervivum decreased over this time period but could, at least partially, be reversed by irrigation. InSedum, leaf also declined but a more gradual reduction in acidification occurred than inSempervivum. Irrigation inSedum at least partially reversed the decline in leaf but produced a complex pattern of acid metabolism. Nocturnal acidification in the irrigated plants was lower than in the non-irrigated control when preceded by a cool day but showed complete recovery following a hot day. It is suggested inSedum album that C₃ photosynthesis during the preceding light period, as determined by light intensity and leaf temperature, may be important in determining the extent of nocturnal acidification under field conditions.     

Carbon fixation in eucalypts in the field

Summary The rate of CO₂ assimilation at light saturation and an intercellular CO₂ concentration of 350 l l^-1 (photosynthetic capacity), measured in leaves of Eucalyptus pauciflora, E. behriana, E. delegatensis and Acacia melanoxylon, declined over the course of cloudless days under naturally varying environmental conditions as well as under constant optimal conditions for high CO₂ uptake. Since the capacity did not recover during the light period, it was different from the midday depression of gas exchange. The change appeared to be caused neither by the diurnal variation of total leaf water potential, by photoinhibition of redox-reaction centres in photosystems nor by changes in the intrinsic properties of Ribulose-bisphosphate carboxylase-oxygenase. The decline was more pronounced in winter than in summer. It was related to the duration of illumination or the cumulative carbon gain. It was reversible in the following dark phase, and it did not occur on changeable days with short peaks of high light.Despite the decline in photosynthetic capacity, the initial slope of the CO₂ response of net photosynthesis, as obtained at low intercellular CO₂ concentrations, remained constant during the day, but declined at night when photosynthetic capacity recovered. In all cases stomatal conductance varied in parallel with photosynthetic capacity. The relevance of changes in photosynthetic capacity for the intercellular CO₂ concentration is discussed.Abbreviations and symbols A CO₂ assimilation - ABA abscisic acid - A_c350 photosynthetic capacity at c_i=350l l^-1 - c_i intercellular CO₂ concentration - g leaf conductance to water vapour - I photon flux density (irradiance) - P air pressure - P_i inorganic phosphate - R_d net CO₂ release at _* - Rubisco Ribulose-bisphosphate carboxylase-oxygenase - RuBP Ribulose-bisphosphate - T leaf temperature - w leaf-to-air water vapour concentration difference - A/c_i carboxylation efficiency at low c_i - _* light-independent CO₂ compensation point - total leaf water potential     

Hexose transport and membrane depolarization in Riccia fluitans

In the aquatic liverwort Riccia fluitans, the uptake of ¹⁴C-labeled 3-O-methyl glucose (3-OMG) and membrane depolarization ( _m ) caused by different hexoses has been studied as a function of time and concentration of hexose, K⁺ and H⁺, respectively. The rate of uptake of the non-metabolized 3-OMG shows two components: (A)A pH-dependent saturable uptake with a k_m value around 0.1 mM which saturates at 2.1 and 7.2 mol G _DW ^-1 h^-1 at pH 6.8 and 5.0, respectively; and (B) a pH-insensitive uptake component which increases linearly with the external 3-OMG concentration and does not saturate 4 mM. Hexoses rapidly depolarize the plasmalemma of the thallus cell and increase its electrical conductance. The maximal _m was 60±2 mV, the concentrations (mM) for half-maximal _m were 0.24 glucose, 0.32 galactose, 0.37 2-deoxy glucose, 0.38 3-OMG, 0.57 mannose, and 34 fructose. In terms of a hexose carrier model and an equivalent circuit for the hexose-induced depolarized state of the membrane, it is proposed that a hexose carrier operates either electrogenically in its protonated, pH-and voltage-sensitive state, or by transmembrane diffusion of its uncharged state.Symbols and Abbreviations _m membrane potential (mV) - g _m membrane (slope) conductance (Sm^-2) - 3-OMG 3-O-methyl glucose     

Control of the rate of cell enlargement: Excision,wall relaxation,and growth-induced water potentials

A new guillotine thermocouple psychrometer was used to make continuous measurements of water potential before and after the excision of elongating and mature regions of darkgrown soybean (Glycine max L. Merr.) stems. Transpiration could not occur, but growth took place during the measurement if the tissue was intact. Tests showed that the instrument measured the average water potential of the sampled tissue and responded rapidly to changes in water potential. By measuring tissue osmotic potential ( _s ), turgor pressure ( _p ) could be calculated. In the intact plant, _s and _p were essentially constant for the entire 22 h measurement, but _s was lower and _p higher in the elongating region than in the mature region. This caused the water potential in the elongating region to be lower than in the mature region. The mature tissue equilibrated with the water potential of the xylem. Therefore, the difference in water potential between mature and elongating tissue represented a difference between the xylem and the elongating region, reflecting a water potential gradient from the xylem to the epidermis that was involved in supplying water for elongation. When mature tissue was excised with the guillotine, _s and _p did not change. However, when elongating tissue was excised, water was absorbed from the xylem, whose water potential decreased. This collapsed the gradient and prevented further water uptake. Tissue _p then decreased rapidly (5 min) by about 0.1 MPa in the elongating tissue. The _p decreased because the cell walls relaxed as extension, caused by _p , continued briefly without water uptake. The _p decreased until the minimum for wall extension (Y) was reached, whereupon elongation ceased. This was followed by a slow further decrease in Y but no additional elongation. In elongating tissue excised with mature tissue attached, there was almost no effect on water potential or _p for several hours. Nevertheless, growth was reduced immediately and continued at a decreasing rate. In this case, the mature tissue supplied water to the elongating tissue and the cell walls did not relax. Based on these measurements, a theory is presented for simultaneously evaluating the effects of water supply and water demand associated with growth. Because wall relaxation measured with the psychrometer provided a new method for determining Y and wall extensibility, all the factors required by the theory could be evaluated for the first time in a single sample. The analysis showed that water uptake and wall extension co-limited elongation in soybean stems under our conditions. This co-limitation explains why elongation responded immediately to a decrease in the water potential of the xylem and why excision with attached mature tissue caused an immediate decrease in growth rate without an immediate change in _p Abbreviations and symbols L tissue conductance for water - m wall extensibility - Y average yield threshold (MPa) - _o water potential of the xylem - _p turgor pressure - _s osmotic potential - _w water potential of the elon gating tissue     

Root and stem xylem embolism,stomatal conductance,and leaf turgor in Acer grandidentatum populations along a soil moisture gradient

The objective of this study was to determine how adjustment in stomatal conductance (g ^s) and turgor loss point (_tlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993–1994), the slope site had substantially lower xylem pressures (_px) and g ^s than the riparian site, particularly during the drought year of 1994. The _tlp was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday _px and the critical pressure triggering catastrophic xylem embolism (_pxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from _pxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g _s=9±2 versus 79±15 mmol m^-2 s^-1 at riparian site) and made any further osmotic adjustment of _tlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.     

Transport of phenylalanine into vacuoles isolated from barley mesophyll protoplasts

The energy-dependent transport of phenylalanine into isolated vacuoles of barley (Hordeum vulgare L.) mesophyll protoplasts has been studied by silicone-layer floatation filtering. The uptake of this aromatic amino acid into the vacuolar compartment is markedly increased by MgATP, showing saturation kinetics; the K _m values were 0.5 mM for MgATP and 1.2 mM for phenylalanine. V _max for phenylalanine transport was estimated to 140 nmol phenylalanine·(mg·Chl)^-1·h^-1. The transport shows a distinct pH optimum at 7.3 and is markedly inhibited by 40 mM nitrate. Azide (1 mM) and vanadate (400 M) had no or little effect on rates of transport while p-fluorophenylalanine seemed to be an effective inhibitor, indicating a possible competition at an amino-acid carrier. Ionophores such as valinomycin, nigericin or gramicidin were strong inhibitors of phenylalanine transport, indicating that this process is coupled to both the transmembrane pH gradient (pH) and the transmembrane potential ().Abbreviations and symbols BSA bovine serum albumin - Chl chlorophyll - Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid - pH transmembrane pH gradient - transmembrane potential