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 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     

Osmotic adjustment and the inhibition of leaf,root, stem and silk growth at low water potentials in maize

The expansion growth of plant organs is inhibited at low water potentials ( _w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The _w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the _w in the region of cell expansion decreased in each organ. The _w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these _w and ceased only when _w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the _w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the _w was lower in the expanding regions than in the mature tissues, indicating that there was a _w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a _w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because _w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular _w changed about the same amount as the soil _w. Therefore, the gradient in _w favored water uptake by roots but not leaves at low _w. The data show that expansion growth responds to low _w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of _w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the _w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and _w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.Symbols _s osmotic potential - _w water potential     

Winter water relations of a deciduous timberline conifer,Larix lyallii Parl.

Summary The degree of winter desiccation resistance exhibited by Larix lyallii Parl. was assessed by determination of water potential components and content of buds, xylem pressure potential ( _xylem) of twigs and amount of damage through winter at timberline in the Rocky Mountains of Canada. Comparative measurements were made on sympatric evergreen tree species to evaluate differences in winter desiccation avoidance and tolerance between evergreen and deciduous trees. Total () and osmotic plus matric potentials ( ₊ ) of L. lyallii buds were lowest in December (-5.0 to-5.3 MPa and-6.6 to-7.0 MPa, respectively) when temperatures were lowest. Bud and water content increased in late winter while _xylem of twigs continued to decline until March. The buds were isolated from the xylem from October through February, as indicated by large differences in water potential between the two organs during this time. Buds thus avoided desiccation as water was lost from the twigs. At the same time the buds were tolerant of very low and ₊ , a characteristic which is an important component of freezing damage resistance. Desiccation damage to buds of L. lyallii was much less than that to buds of similar-sized nearby trees of Abies lasiocarpa, although _xylem of both species was similar. The deciduous habit apparently confers a significant advantate to L. lyallii, which dominated the upper timberline sites, in reduced susceptibility to winter desiccation damage. Other deciduous timberline species might also benefit from this advantage where winter conditions are desiccating.Seedlings of L. lyallii were also studded for their winter desiccation resistance because they have a large component of non-deciduous (wintergreen) needles that are photosynthetically active through two growing seasons and must overwinter as mature tissue. Experimental exposure of these needles, which are normally protected by the snowpack, caused nearly complete mortality of the wintergreen needles when twig _xylem was only-3.9 MPa. The buds on these twigs were undamaged.     

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.     

Osmoregulation and the control of phloem-sap composition in Ricinus communis L.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K⁺:Na⁺ ratio and low levels of Ca²⁺ and free H⁺. Sucrose and K⁺ (together with its associated anions) accounted for 75% of the phloem-sap solute potential (_s). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K⁺ levels. Measurements of phloem-sap _s and xylem water potential () indicated that this resulted in a partial maintenance of phloem turgor pressure _p. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K⁺ levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H⁺ co-transport mechanism of phloem loading.Abbreviations water potential - _s solute potential - _p pressure potential     

Phloem turgor and the regulation of sucrose loading in Ricinus communis L.

The influence of plant water relations on phloem loading was studied in Ricinus communis L. Phloem transport was maintained in response to bark incisions even at severe water deficits. Water stress was associated with a net increase in the solute content of the sieve tubes, which resulted in maintenance of a positive phloem turgor pressure _p. There was a significant increase in solute flux through the phloem with decreasing xylem water potential (). In addition, sugar uptake by leaf discs was examined in media adjusted to different water potentials with either sorbitol (a relatively impermeant solute) or ethylene glycol (a relatively permeant solute). The limitations in this experimental system are discussed. The results nevertheless indicated that sucrose uptake can be stimulated by a reduction in cell _p, but that it is little affected by cell or solute potential _s. On the basis of these data we suggest that sucrose loading is turgor-pressure dependent. This may provide the mechanism by which transport responds to changes in sink demand in the whole plant.Abbreviations water potential - _s solute potential - _p pressure potential     

Stomatal response to leaf water potential in almond trees under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L. cv. Garrigues) were grown in the field under drip irrigated and non irrigated conditions. Leaf water potential () and leaf conductance (g₁) were determined at three different times of the growing season (spring, summer and autumn). The relationships between and g₁ in both treatments showed a continuous decrease of g₁ as decreased in spring and summer. Data from the autumn presented a threshold value of (approx. –2.7 MPa in dry treatment, and approx. –1.4 MPa in wet treatment) below which leaf conductance remained constant.     

Acclimation of photosynthesis in Zea mays to low water potentials involves alterations in protoplast volume reduction

Effects of water-stress treatment of Zea mays L. plants on protoplast volume and photosynthesis in leaf slices exposed to solutions of different osmotic potential ( _s) were studied. Decreased photosynthetic capacity in the leaf slices at low tissue _w was associated with dehydration-induced protoplast-volume reduction. Leaf slices from plants exposed to in-situ water deficits exhibited greater photosynthetic capacity and relative protoplast volume at low water potential ( _w) invitro than tissue from control plants.In-situ water stress induced osmotic adjustment of the leaf tissue as determined by pressure/volume analysis. It is concluded that plant acclimation to low leaf _w may involve a reduced degree of cell shrinkage at a given _w. This acclimation would allow for the maintenance of relatively higher photosynthetic capacity at low water protentials.Symbols _s Osmotic potential - _w water potential New Jersey Agricultural Experiment Station Publication No. 12149-6-87     

The membrane potential in whole cells of Methanobacterium thermoautotrophicum

of whole cells of Methanobacterium thermoautotrophicum was estimated under varying conditions using an electrode sensitive to the lipophilic cation tetraphenylphosphonium chloride (TPP⁺). Since was found to be extremely sensitive to air, a special reaction vessel was developed to maintain strict anaerobiosis. The cells took up TPP⁺ under energization by H₂ and CO₂ thus allowing to calculate the from the distribution of TPP⁺ inside and outside the cells. The unspecific uptake of deenergized cells was around 10% of the total uptake of energized cells. TPP⁺ itself slightly diminished the , but had no effect on the formation of methane. Typical values of were in the range of-150 to-200 mV. showed a quantitative dependence on both the electron donor H₂ and the electron acceptor CO₂. NaCl stimulated the extent of the , whereas KCl slightly diminished it. Valinomycin resulted in a linear decline of , whereas the methane production rate was only slightly affected. In contrast, monensin reduced both methanogenesis and .Abbreviations pmf proton motive force - membrane potential - TPP⁺ tetraphenylphosphonium (chloride salt) - TPMP⁺ triphenylmethylphosphonium (chloride salt, if not otherwise indicated) - d.w. dry weight - t _d doubling time - PVC polyvinylchloride     

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.     

Water relations of crok-oak (Quercus suber L.) under natural conditions

Daily and annual courses of leaf transpiration, stomatal conductance and shoot water potential of four Quercus suber individuals were compared in a semi-natural stand in southwest Portugal, from spring 1989 to early summer 1990.The trees investigated showed annual patterns typical of evergreen sclerophyllous species but varied in their range of stomatal operation. This appeared to be related to differences in hydraulic conductivity in the root-to-leaf pathway.Maximum stomatal conductance and transpiration rates occurred from March to June.Water stress was found to be moderate and winter cold stress due to low air and soil temperatures appeared to have an influence on plant water balance through their effects on flow resistances.Abbreviations g_sw stomatal conductance - g_max maximum stomatal conductance - PAR photosynthetically active radiation - RH relative humidity of the air - T leaf transpiration - Ta air temperature - TL leaf temperature - T_max maximum leaf transpiration - W air-to-leaf vapor pressure difference - shoot water potential - PD predawn shoot water potential - MIN minimum shoot water potential     

Effects of Osmotic Drought Stress Induced by a Combination of NaCl and Polyethylene Glycol on Leaf Water Status,Photosynthetic Gas Exchange,and Water Use Efficiency of Pistacia khinjuk and P. mutica

Leaf water potential, leaf osmotic potential, chlorophyll a and b contents, stomatal conductance, net photosynthetic rate, and water use efficiency were determined in two pistachio species (Pistacia khinjuk L. and P. mutica L.) grown under osmotic drought stress induced by a combination of NaCl and polyethylene glycol 6000. A decrease in values for all mentioned variables was observed as the osmotic potential of the nutrient solution (_s) decreased. The osmotic adjustment () of the species increased by decreasing _s. Thus P. khinjuk had a higher osmotic drought stress tolerance than P. mutica.     

Seasonal water potential changes and proline accumulation in mediterranean shrubland species

We studied the water relations of 6 shrub and 3 tree species typical of the mediterranean climate region of central Spain to identify differential responses to water stress between and within species, and to determine if free proline concentration in leaves could be used as a water stress indicator. Predawn and midday water potentials (w) on a seasonal basis, relative water content (RWC), leaf mass per area, foliar nitrogen and free proline concentrations were measured. The lowest water potentials were observed at the end of the summer, with recovery to higher water potentials in the fall and winter seasons. Species differed regarding the annual w fluctuation. Thymus zygis, Halimium viscosum, Genista hirsuta and Juniperus oxycedrus exhibited the most negative midday and predawn w (both less than -6 MPa) with a large magnitude of response to changing conditions in soil moisture of the upper horizon of the soil. Lavandula pedunculata and Cistus ladanifer showed a moderate response. Quercus rotundifolia, Quercus faginea and Retama sphaerocarpa showed a modest response. The w of different size individuals of Quercus rotundifolia and Cistus ladanifer were compared. The annual w fluctuation was greater in small individuals as compared to large individuals. In every species, there was an increase in proline concentration of bulk leaf tissues when predawn w dropped below -5 MPa. Small plants of Cistus ladanifer reached lower water potentials and also higher concentration of proline than bigger plants. Proline could possibly be used as a drought stress indicator in every species except Q. rotundifolia. It is suggested that in addition to water stress avoidance due to deep root systems, some mechanisms of water stress tolerance may operate among shrub and tree species of central Spain.     

Effects of moisture stress on the multiplication and expansion of cells in leaves of sugar beet

Summary Sugar beets were subjected to moisture stress by decreasing the water potential of the culture solution osmotically with polyethylene glycol by a known amount, , and, alternatively by applying matric potential, , at the plant roots. Lowering the water potential at the root surface less than 200 millibars by either method resulted in significant decreases in the rate of cell multiplication. The final number of cells per leaf at = -372 mb the final was 165% of that at = -473 mb ( = –101 mb); similarly at = –15 mb the final cell number was 198% of that at = –196 mb ( = –181 mb). The mean cell volume of leaves was not significantly affected by these levels of moisture stress.     

Leaf water relations characteristics of Lupinus angustifolius and L. cosentinii

Summary Lupins (Lupinus angustifolius and L. cosentinii) growing in 321 containers in a glasshouse were exposed to drought by withholding water. Leaf water potential (₁), and leaf osmotic potential (_s) were measured daily as soil water became depleted. Leaf water relations were further assessed by a pressure-volume technique and by measuring _s and relative water content of leaves after rehydration. Analysis by pressure-volume or cryoscopic techniques showed that leaf osmotic potential at saturation (_s100) decreased from -0.6 MPa in well watered to -0.9 MPa in severely droughted leaves, and leaf water potential at zero turgor (_zt) decreased from about -0.7 to -1.1 MPa in well watered and droughted plants, respectively. Relative water content at zero turgor (RWC_zt) was high (88%) and tended to be decreased by drought. The ratio of turgid leaf weight to dry weight was not influenced by drought and was high at about 8.0. The bulk elastic modulus () was approximately halved by drought when related to leaf turgor potential (_p) and probably mediated turgor maintenance during drought. The latter was found to be negatively influenced by rate of drought. Supplying the plants with high levels of K salts did not promote adjustment or turgor maintenance.     

Subcellular heterogeneity of mitochondrial membrane potential in anther tapetum

Studies on animal material have revealed that changes in the mitochondrial permeability transition pore (PTP), which cause a reduction in the mitochondrial transmembrane potential (_m) followed by release of cytochrome c, belong to the earliest manifestations of some types of apoptosis. We have attempted to monitor the _m of mitochondria during programmed cell death (PCD) of the secretory tapetum using JC-1, a fluorochrome dye that detects mitochondrial membrane potential and to relate changes in this potential to mitochondrial ultrastructure. Analysis of tapetal cells isolated from Ornithogalum virens anthers revealed that the _m of mitochondria in the tapetal cells alters during development; the change, however, is not uniform in the mitochondrial population within a single tapetal cell. In young tapetal cells, at the tetrad stage, we detected only the red fluorescence of JC-1 aggregates in all tapetal mitochondria, which indicates highly negative _m. In an advanced stage of PCD at the late microspore stage, in each tapetal cell we detected both mitochondria with red (as formerly) and mitochondria with green fluorescence. The green fluorescence of JC-1 monomers indicates mitochondria with depolarised membranes. These changes in _m are related to observed changes in mitochondria ultrastructure. This is the first documentation of intracellular heterogeneity of _m during anther tapetum development. Alteration in _m suggests a relationship between mitochondrial function and PCD processes in tapetal cells.     

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.     

Xylem cavitation in excised leaves of Malus sylvestris Mill. and measurement of leaf water status with the pressure chamber

Summary A change occurs in the relationship between xylem water potential (_x) measured with the pressure chamber and leaf water potential ((_w)) when a period of post-excision water loss is allowed before measurement of (_x) and (_x). When this occurs, water stress is over-estimated by the pressure chamber measurement. Over the same period of water loss, cavitation vibrations have been detected acoustically in excised leaves. It is suggested that the measurement of (_x) is affected by emptying of some of the xylem vessels due to cavitation. This would require that additional pressure be applied to a leaf in the pressure chamber in order to measure (_x).     

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