Decreased Growth-Induced Water Potential (A Primary Cause of Growth Inhibition at Low Water Potentials)

Cell enlargement depends on a growth-induced difference in water potential to move water into the cells. Water deficits decrease this potential difference and inhibit growth. To investigate whether the decrease causes the growth inhibition, pressure was applied to the roots of soybean (Glycine max L. Merr.) seedlings and the growth and potential difference were monitored in the stems. In water-limited plants, the inhibited stem growth increased when the roots were pressurized and it reverted to the previous rate when the pressure was released. The pressure around the roots was perceived as an increased turgor in the stem in small cells next to the xylem, but not in outlying cortical cells. This local effect implied that water transport was impeded by the small cells. The diffusivity for water was much less in the small cells than in the outlying cells. The small cells thus were a barrier that caused the growth-induced potential difference to be large during rapid growth, but to reverse locally during the early part of a water deficit. Such a barrier may be a frequent property of meristems. Because stem growth responded to the pressure-induced recovery of the potential difference across this barrier, we conclude that a decrease in the growth-induced potential difference was a primary cause of the inhibition.     

Partitioning Control by Water Potential Gradient: Evidence for Compartmentation Breakdown in Grape Berries

Xylem exudate was obtained from berries of Riesling grapes atdifferent stages of development after the onset of ripeningusing a pressure bomb technique. The osmotic potential of theexudate bore a 1:1 relationship to that of juice from the sameberries which were afterwards crushed and centrifuged. Thisresult provides the first direct evidence of compartmentationbreakdown in grape berries after the onset of ripening. Changesin berry deformability which occur at the same time and measurementsof the dynamics of exudation flow lead to the same conclusionregarding compartmentation breakdown. The breakdown in compartmentation occurs at the same time asthe rate of phloem translocation to the fruit suddenly increases.A mechanism was recently proposed to account for this increase.It required the existence of a water potential difference betweensource and sink such as would result from compartmentation breakdownin the sink tissues. The results, therefore, may be taken toindicate that this mechanism is indeed involved in the controlof assimilate partitioning in Vitis. Evidence in other publicationssuggests that the mechanism may be reasonably widespread inplants. Key words: Assimilate partitioning, phloem translocation mechanism, Vitis vinifera L., water potential gradients     

The Regulation of the Water Potential Gradient in the Host and Parasite Relationship betweenSorghum bicolorandStriga hermonthica

A glasshouse experiment was carried out to investigate the factorscontrolling the abstraction of xylem fluid from its host bythe parasiteStriga hermonthica(Scrophulariaceae).Strigahad amean daily transpiration rate far exceeding that of its hostsorghum (Sorghum bicolor), with infestation byStrigaalso shownto lower the transpiration rate of the host. Stopping the host'stranspiration was shown to decrease the transpiration rate ofthe parasite. Stopping the parasite's transpiration only gavean initial increase in the host's transpiration rate which wasnot sustained. The parasite had a lower water potential thanits host, values being -0.42 MPa and -0.23 MPa, respectively,and an accompanying higher osmotic pressure of 0.68 MPa against0.51 MPa for sorghum. Modifying the water potential gradientby bagging both partners together showed that the differentialin osmotic pressure and water potential was largely maintainedby the parasite's higher rate of transpiration. A favourablewater potential gradient towards the parasite still existedfollowing the cessation of transpiration, this being generatedby the haustorial resistance to hydraulic conductivity whichwas found to be some 1.5–4.5 times greater than that offeredby the parasite shoot. Both the high rate of transpiration andthe increased resistance across the haustoria would appear tobe necessary means to facilitate the diversion of host resourcesto the parasite.Copyright 1997 Annals of Botany Company Striga hermonthica; sorghum; water relations; haustorium; root parasite     

Denitrification Potential in Lake Sediment Increases Across a Gradient of Catchment Agriculture

Intensification of catchment agriculture has increased nutrient loads and accelerated eutrophication in some lakes, often resulting in episodic harmful algal blooms or prolonged periods of anoxia. The influence of catchment agriculture on lake sediment denitrification capacity as a nitrogen (N) removal mechanism in lakes is largely unknown, particularly in contrast to research on denitrification in agricultural streams and rivers. We measured denitrification enzyme activity (DEA) to assess sediment denitrification potential in seven monomictic and three polymictic lakes that range in the proportion of agriculture in the catchment from 3 to 96% to determine if there is a link between agricultural land use in the lake catchment and sediment denitrification potential. We collected sediment cores for DEA measurements over 3 weeks in austral spring 2008 (October–November). Lake Okaro, with 96% catchment agriculture, had approximately 15 times higher DEA than Lake Tikitapu, with 3% catchment agriculture (232.2 ± 55.9 vs. 15.9 ± 4.5 μg N gAFDM⁻¹ h⁻¹, respectively). Additionally, sediment denitrification potential increased with the proportion of catchment in agriculture (R ² = 0.85, P < 0.001). Our data suggest that lakes retain a high capacity to remove excess N via denitrification under increasing N loads from higher proportions of catchment agriculture. However, evidence from the literature suggests that despite a high capacity for denitrification and longer water residence times, lakes with high N loads will still remove a smaller proportion of their N load. Lakes have a denitrification potential that reflects the condition of the lake catchment, but more measurements of in situ denitrification rates across lake catchments is necessary to determine if this capacity translates to high N removal rates.     

Stem Water Potential is a Sensitive Indicator of Grapevine Water Status

Dawn leaf water potential (dawn     

Frictional Potential Losses and Total Water Potential in Plants: a Re-evaluation

A theoretical treatment is given of isothermal potential lossesdue to frictional resistances against water flow in plants.It becomes apparent that in the past two main difficulties havebeen widely ignored: the proper choice of dimensions for fluxesand resistances deserves consideration, and the original vanden Honert concept seems to be especially prone to misinterpretation.A revised equation is presented for total water potential ata certain point in the plant, and some implications of thisapproach are outlined.     

Demonstration of Voltage Dependency of Light-induced Potential Change in Chara

The electromotive force (E_m) of the plasma membrane of the tonoplast-freecell of Chara australis decreased when the electrogenic pumpwas stopped by removing ATP or Mg²⁺ from the cell. Such a cellshowed a rapid light-induced potential change (rLPC). Threefactors were considered to be responsible for the generationof rLPC; removal of Mg-ATP, stoppage of the electrogenic pumpand membrane depolarization per se. Tonoplast-free cells having enough Mg-ATP occasionally showedsmall E_m (–87— –116 mV) due to stoppage ofthe electrogenic pump. Since the rLPC was induced in such cells,removal of Mg-ATP cannot be the factor. Cells having large E_mdue to active pump activity also showed rLPC when the potentialdifference across the plasma membrane (V_m) was depolarized byan outward electric current; evidence that not the stoppageof the pump but membrane depolarization is a necessary conditionfor the generation of rLPC. In the rLPC Vm always changed in the negative direction. However,calculation of E_m revealed the existence of a reversal potential[ (E_m)rev] toward which E_m converged from either more negativeor more positive values. The (E_m)rev approximately coincidedwith the equilibrium potential for K⁺ across the plasma membrane.Intracellular anions occupying lower positions in the lyotropicseries inhibited rLPC. (Received February 9, 1981; Accepted May 16, 1981)     

Water and Carbon Fluxes Along an Elevational Gradient in a Sagebrush Ecosystem

Ecosystems - Differences in water and carbon fluxes along a climate/elevation gradient within a sagebrush ecosystem are quantified, and inferences are made about climate warming using a network of...     

Distribution Patterns of Tropical Dry Forest Trees Along a Mesoscale Water Availability Gradient

Tropical dry forests (TDFs) host a large diversity of tree species but little is known of potential mechanisms that contribute to its maintenance. Given the paramount importance of water availability in such forests, tree species would be expected to show nonrandom patterns along water availability gradients, as well as differential individual species responses. In this work we explored whether that was true for 50 dominant tree species. Within a total area of 5.2 ha, divided into 26 transects each with ten 20 × 10 m plots, we registered presence–absence of these tree species with diameter at breast height ≥5 cm. We assessed the response of trees to four environmental variables differentially related to water availability in three steps: (1) identifying the shape of the response to individual environmental variables, (2) testing for artifacts in previous patterns due to spatial autocorrelation of presence–absence, and (3) identifying the environmental variable or combination of variables that best explained the pattern. We then classified the species with respect to their probability of occurrence along the gradient, and explored which components of the water cycle were likely to be driving the observed patterns. We found that 14 species were generalists, 16 drought tolerant, 9 intermediate, 3 water demanding and 8 showed mixed responses. Lateral flow and access to ground water most likely underlie such patterns. Our results confirm the key role played by water availability in tree species distribution. Water‐related niche differentiation seems to be crucial for maintaining the high diversity of this TDF. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

Low Redox Decreases Potential Phosphorus Limitation on Soil Biogeochemical Cycling Along a Tropical Rainfall Gradient

Ecosystems - Humid tropical forests on highly weathered soils are often characterized by low bioavailable phosphorus (P) concentrations. These ecosystems also often experience low and fluctuating...     

A Robust Water Potential Parameterisation

Abstract

We compare molecular dynamics simulation results for the properties of liquid water predicted by four novel water potential models. These models are designed as a combination of parameters taken from the dedicated but brittle TIP3P water potential, and the more flexible but less accurate parameterisations such as the Dreiding and Universal force fields. We find that a hybrid of Dreiding and TIP3P delivers the best results, yielding a density, diffusion coefficient and radial distribution function in good agreement with experiment, performing in some respects even better than the dedicated reference TIP3P model. Another Dreiding based force field predicts semi-quantitative results for the water structure and dynamics while the Universal force field based models are incapable of simulating a condensed phase of water at all, continuing to expand indefinitely. These observations are useful for selecting and designing robust water force field parameterisations that can be used for general simulation purposes.     

Direct gene transferState of the Art and Future Potential

Incubation of freshly isolated protoplasts with a gene under the control of plant expression signals leads to high levels of stable integrative transformation. The gene is transmitted to sexual offspring and inherited in a Mendelian pattern. The foreign gene is in most cases stably maintained through several sexual cycles. However, cases of somatic and meiotic instability are also found. A partially optimized protocol routinely yields transformation frequencies in the percentage range. Host range limitations were not expected and also not found in transformation experiments with protoplasts from a series of herbaceous dicots and a graminaceous monocot. Co-transformation experiments with a selectable and a non-selectable gene on separate plasmids yielded high levels of cotransfer of the non-selectable gene.     

Demonstration of a Direct Interaction between σ-1 Receptors and Acid-Sensing Ion Channels

The σ-1 receptor is a widely expressed protein that interacts with a variety of ion channels, including the acid-sensing ion channel (ASIC) 1a. Here we used atomic force microscopy to determine the architecture of the ASIC1a/σ-1 receptor complex. When isolated His₈-tagged ASIC1a was imaged in complex with anti-His₆ antibodies, the angle between pairs of bound antibodies was 135°, consistent with the known trimeric structure of the channel. When ASIC1a was coexpressed with FLAG/His₆-tagged σ-1 receptor, ASIC1a became decorated with small particles, and pairs of these particles bound at an angle of 131°. When these complexes were incubated with anti-FLAG antibodies, pairs of antibodies bound at an angle of 134°, confirming that the small particles were σ-1 receptors. Of interest, we found that the σ-1 receptor ligand haloperidol caused an ∼50% reduction in ASIC1a/σ-receptor binding, suggesting a way in which σ-1 ligands might modulate channel properties. For the first time, to our knowledge, we have resolved the structure of a complex between the σ-1 receptor and a target ion channel, and demonstrated that the stoichiometry of the interaction is 1 σ-1 receptor/1 ASIC1a subunit.     

Changes in Membrane Potential as a Demonstration of Selective Pore Formation in the Plasmalemma by Poly-l-Lysine Treatment

A technique which allows determination of solute pool concentrations in the cytosol was developed exploiting the interaction between a polycation and the anionic sites of the plasmalemma. It was shown that treatment of Nicotiana tabacum, cv Xanthi, cells in suspension culture with an appropriate concentration of poly-l-lysine induced pore formation selectively in the plasmalemma. The data presented in this paper shows that the plasmalemma of all the cells was affected while the tonoplast remained undamaged. This conclusion is based on the facts that treatment of the cells with the minimum amount of poly-l-lysine which just abolishes the electrogenic potential (similarly to carbonyl cyanide-p-trifluormethoxyphenylhydrazone and NaN₃) induces the leakage of only a small fraction of the K⁺ present in the cells. These effects of poly-l-lysine differ from the effects of polymyxin B which induces total leakage of low molecular weight solutes (R. Weimberg, H. R. Lerner, A. Poljakoff-Mayber 1983 J Exp Bot 34: 1333-1346) and therefore affects also the tonoplast.     

Effect of Excision on Leaf Water Potential

Improvements of thermocouple hygrometric techniques for in situleaf water potential measurement in the field now allows forcontinuous monitoring of water potential in response to an externalperturbation, such as leaf excision. Using Citrus jambhiri plants,measured leaf water potentials of completely excised leaf portionsimmediately increased when the petiole was excised or incisionswere made either transverse or parallel to the midrib. Incisionsparallel to the midrib were on the side nearest the hygrometeror opposite it if preceded by a petiole excision. Midrib incisionswere 100–150 mm long with the nearest cut edge being 20–50mm from the hygrometer cavity. All excisions were such thatleaf tissue was removed from the leaf with water potential onone of the leaf portions being measured continuously prior toand after excising. The peak increase in measured water potentialof the excised leaf portions ranged between 20 kPa and 80 kPabut averaged 50 kPa. In uncovered leaves, particularly underfield conditions with the associated high evaporative demand,measured leaf water potential declined rapidly after the initialincrease. The increase in measured water potential immediatelyfollowing various types of excision was confirmed for dark andlight conditions (laboratory and field respectively) using bothpsychrometric and dewpoint modes and occurred for secondaryexcisions, but to a lesser extent. Discovery of this phenomenonimplies that water potential measured on detached leaves maynot always represent accurately in situ leaf water potential. Key words: Leaf water potential, Thermocouple hygrometers, Leaf excision effects