Change in XET activities,cell wall extensibility and hypocotyl elongation of soybean seedlings at low water potential

In dark-grown soybean (Glycine max [L.] Merr.) seedlings, exposing the roots to water-deficient vermiculite (_w=–0.36 MPa) inhibited hypocotyl (stem) elongation. The inhibition was associated with decreased extensibility of the cell walls in the elongation zone. A detailed spatial analysis showed xyloglucan endotransglucosylase (XET; EC 2.4.1.207) activity on the basis of unit cell wall dry weight was decreased in the elongation region after seedlings were transplanted to low _w. The decrease in XET activity was at least partially due to an accumulation of cell wall mass. Since cell number was only slightly altered, wall mass had increased per cell and probably led to increased wall thickness and decreased cell wall extensibility. Alternatively, an increase in cell wall mass may represent a mechanism for regulating enzyme activity in cell walls, XET in this case, and therefore cell wall extensibility. Hypocotyl elongation was partially recovered after seedlings were grown in low-_w vermiculate for about 80 h. The partial recovery of hypocotyl elongation was associated with a partial recovery of cell wall extensibility and an enhancement of XET activity in the hypocotyl elongation zone. Our results indicate XTH proteins may play an important role in regulating cell wall extensibility and thus cell elongation in soybean hypocotyls. Our results also showed an imperfect correlation of spatial elongation and XET activity along the hypocotyls. Other potential functions of XTH and their regulation in soybean hypocotyl growth are discussed.     

Hydraulic resistance to radial water flow in growing hypocotyl of soybean measured by a new pressure-perfusion technique

Hydraulic resistances to water flow have been determined in the cortex of hypocotyls of growing seedlings of soybean (Glycine max L. Merr. cv. Wayne). Data at the cell level (hydraulic conductivity, Lp; half-time of water exchange, T _1/2; elastic modulus, ; diffusivity for the cell-to-cell pathway, D _c) were obtained by the pressure probe, diffusivities for the tissue (D _t) by sorption experiments and the hydraulic conductivity of the entire cortex (Lp_r) by a new pressure-perfusion technique. For cortical cells in the elongating and mature regions of the hypocotyls T _1/2=0.4–15.1 s, Lp=0.2·10^-5–10.0·10^-5 cm s^-1 bar^-1 and D _c=0.1·10^-6–5.5·10^-6 cm² s^-1. Sorption kinetics yielded a tissue diffusivity D _t=0.2·10^-6–0.8·10^-6 cm² s^-1. The sorption kinetics include both cell-wall and cell-to-cell pathways for water transport. By comparing D _c and D _t, it was concluded that during swelling or shrinking of the tissue and during growth a substantial amount of water moves from cell to cell. The pressure-perfusion technique imposed hydrostatic gradients across the cortex either by manipulating the hydrostatic pressure in the xylem of hypocotyl segments or by forcing water from outside into the xylem. In segments with intact cuticle, the hydraulic conductance of the radial path (Lp_r) was a function of the rate of water flow and also of flow direction. In segments without cuticle, Lp_r was large (Lp_r=2·10^-5–20·10^-5 cm s^-1 bar^-1) and exceeded the corticla cell Lp. The results of the pressure-perfusion experiments are not compatible with a cell-to-cell transport and can only the explained by a preferred apoplasmic water movement. A tentative explanation for the differences found in the different types of experiments is that during hydrostatic perfusion the apoplasmic path dominates because of the high hydraulic conductivity of the cell wall or a preferred water movement by film flow in the intercellular space system. For shrinking and swelling experiments and during growth, the films are small and the cell-to-cell path dominates. This could lead to larger gradients in water potential in the tissue than expected from Lp_r. It is suggested that the reason for the preference of the cell-to-cell path during swelling and growth is that the solute contribution to the driving force in the apoplast is small, and tensions normally present in the wall prevent sufficiently thick water films from forming. The solute contribution is not very effective because the reflection coefficient of the cell-wall material should be very small for small solutes. The results demonstrate that in plant tissues the relative magnitude of cell-wall versus cell-to-cell transport could dependent on the physical nature of the driving forces (hydrostatic, osmotic) involved.Abbreviations and symbols D _c diffusivity of the cell-to-cell pathway - D _t diffusivity of the tissue - radial flow rate per cm² of segment surface - Lp hydraulic conductivity of plasma-membrane - Lp_r radial hydraulic conductance of the cortex - T _1/2 half-time of water exchange between cell and surroundings - volumetric elastic modulus     

黄果龙葵幼苗对镉胁迫的生理生长响应

黄果龙葵（Solanum diphyllum）是茄科的小灌木,已有的研究对黄果龙葵的镉（Cd）响应知之甚少。该研究采用水培控制试验,在160μmol·L^-1 Cd胁迫下,测定了黄果龙葵幼苗在不同胁迫时间点（0、0.5、1、3、7、19 d）的生长、耐性系数（TI）、Cd富集特性、超氧化物歧化酶（SOD）活性、渗透调节物质和光合色素等指标。结果表明：（1）在胁迫0～3 d时Cd对黄果龙葵幼苗的生长无显著影响。胁迫3～19 d时表现为明显抑制,且毒害随胁迫时间延长而加重。与对照组相比,Cd胁迫使黄果龙葵幼苗的生物量降低13.28%～62.40%。（2）黄果龙葵幼苗的最大Cd积累量为60.14μg·plant^-1,其地上部份的Cd积累量占植株的15.46%～35.24%。（3）丙二醛（MDA）含量呈上升趋势,根的MDA含量最大增幅为对照组的5.25倍。SOD活性、游离脯氨酸、可溶性糖、叶绿素a、叶绿素b和类胡萝卜素的含量均呈先升后降的趋势。（4）黄果龙葵幼苗的耐性系数TI随胁迫时间延长而降低,最小值为0.64。综上结果认为,黄果龙葵是一种潜在的Cd...     

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     

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     

Effects of a long chain aliphatic alcohol mixture on growth and solute accumulation in water stressed wheat seedlings under laboratory conditions

Polyethylene glycol (PEG)-induced water stress adversely affected the germination and seedling growth of Sonalika and WL2265 cultivars of wheat (Triticum aestivum L.). Application of a mixture of long-chain aliphatic alcohols having the composition C-24 Tetracosanol (10%), C-26 Hexacosanol (16%), C-28 Octacosanol (15%), C-30 Tricontanol (30%), C-32 Dotriacontanol (15%) and C-34 Tetratriacontanol (14%), partially ameliorated these effects and promoted both percent germination and seedling growth. Application also stimulated the activities of the hydrolases - and -amylase and acid invertase, so increasing free sugar accumulation. A role for long chain aliphatic alcohols in the regulation of carbohydrate metabolism is suggested. The alleviation of moisture stress by application of this mixture suggest that long chain aliphatic alcohols may be most effective at low water potentials.     

NaCl胁迫对小黄花菜生长及相关生理指标的影响

为探讨小黄花菜的耐盐机理,选育良好的耐盐植物以缓解土壤盐渍化问题,该文选取小黄花菜(Hemerocallis minor)为试材,采用砂培法,研究不同浓度NaCl(50、100、150、200、250 mmol·L-1)胁迫对小黄花菜的生长性状、细胞质膜透性和有机渗透调节物质含量等的影响.结果表明:(1)小黄花菜在10...     

Rapid alterations in growth rate and electrical potentials upon stem excision in pea seedlings

Excision of the epicotyl base of pea (Pisum sativum L.) seedlings in air results in a fast drop in the growth rate and rapid transient membrane depolarization of the surface cells near the cut. Subsequent immersion of the cut end into solution leads to a rapid, transient rise in the epicotyl growth rate and an acropetally propagating depolarization with an amplitude of about 35 mV and a speed of approx. 1 mm · s^–1. The same result can be achieved directly by excision of the pea epicotyl under water. Shape, amplitude and velocity of the depolarization characterize it as a slow-wave potential. These results indicate that the propagating depolarization is caused by a surge in water uptake. Neither a second surge in water uptake (measured as a rapid increase in growth rate when the cut end was placed in air and then back into solution) nor another cut can produce the depolarization a second time. Cyanide suppresses the electrical signal at the treated position without inhibiting its transmission through this area and its development in untreated parts of the epicotyl. The large depolarization and repolarization which occur in the epidermal and subepidermal cells are not associated with changes in cell input resistance. Both results indicate that it is a transient shut-down of the plasma-membrane proton pump rather than large ion fluxes which is causing the depolarization. We conclude that the slow wave potential is spread in the stem via a hydraulic surge occurring upon relief of the negative xylem pressure after the hydraulic resistance of the root has been removed by excision.Abbreviations and Symbols GR growth rate - P_x xylem pressure - R_in cell input resistance - SWP slow wave potential - V_m membrane potential - V_s surface potential This work was supported by grants to D.J.C. from the National Science Foundation and the U.S. Department of Energy.     

Osmoregulation of the Australian freshwater crocodile,Crocodylus johnstoni,in fresh and saline waters

An unusual saltwater population of the freshwater crocodilian, Crocodylus johnstoni, was studied in the estuary of the Limmen Bight River in Australia's Northern Territory and compared with populations in permanently freshwater habitats. Crocodiles in the river were found across a large salinity gradient, from fresh water to a salinity of 24 mg · ml^-1, more than twice the body fluid concentration. Plasma osmolarity, concentrations of plasma Na⁺, Cl^-, and K⁺, and exchangeable Na⁺ pools were all remarkably constant across the salinity spectrum and were not substantially higher or more variable than those in crocodiles from permanently freshwater habitats. Body fluid volume did not vary; condition factor and hydration status of crocodiles were not correlated with salinity and were not different from those of crocodiles from permanently fresh water. C. johnstoni clearly has considerable powers of osmoregulation in waters of low to medium salinity. Whether this osmoregulatory competence extends to continuously hyperosmotic environments is not known, but distributional data suggest that C. johnstoni in hyperosmotic conditions may require periodic access to hypoosmotic water. The study demonstrates a physiological capacity for colonisation of at least some estuarine waters by this normally stenohaline freshwater crocodilian.Abbreviations ANOCOVA analysis of covariance - BW body weight - CF condition factor - ExNa exchangeable sodium - HF hydration factor - SVL snout-vent length - TBW total body water - THO tritiated water     

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     

土壤水分含量和接种摩西斗管囊霉 (Funneliformis mosseae)对伯乐树幼苗生长的影响

该研究测定了伯乐树在不同水分处理(40%、60%、80%、100%)和时间节点下的菌根侵染率,不同水分处理前后接种组和对照组幼苗株高、地径、叶片数等形态指标以及叶片丙二醛含量、幼苗成活率指标。结果表明:(1)相对含水量60%条件下,菌丝、丛枝和总的侵染率最高,40%低水分含量和80%以上高水分含量都不利于菌根真菌对幼苗的侵染。(2)在40%和60%低水分量含量条件下,接种幼苗的丙二醛含量显著低于对照,接种摩西斗管囊霉可提高幼苗对干旱胁迫的抗耐性,但80%和100%高水分含量条件下接种对幼苗的作用不明显。(3)幼苗在自然和接种条件下适宜生长的土壤相对含水量为80%和60%。(4)相同水分条件下,接种幼苗成活率高于对照,其中相对含水量60%下接种幼苗的成活率最高(90%),在40%低水分含量或100%饱和水分含量下,幼苗的成活率都相对较低。     

Osmoregulation and water balance in the springhare (Pedetes capensis)

Springhares are large rodents that live in arid and semi-arid regions of Africa. We deprived springhares of water for periods of up to 7 days to determine what physiological adaptations, if any, enable them to survive in arid regions without drinking. During water deprivation, springhares lost up to 30% body weight and produced a mean maximum urine concentration of 2548 mosmol kg⁻¹ with a maximum of 3076 mosmol kg⁻¹ in an individual animal. Haematocrit and plasma sodium and potassium concentrations were well regulated throughout water deprivation at 47.5 ± 3.8% and 132.6 ± 7.4 mmol l⁻¹ and 3.5 ± 0.7 mmol l⁻¹, respectively, while plasma osmolality increased slightly from 293 ± 12.5 mosmol kg⁻¹ to 324 ± 7.3 mosmol kg⁻¹. Springhares thus appeared to be good osmoregulators and were able to maintain plasma volume during 7 days of water deprivation. In addition to the production of a relatively concentrated urine, water loss was limited by the lowered solute load and faecal water loss achieved by a reduction in food consumption and by the production of very dry faeces. These abilities, together with a favourable burrow microclimate and nocturnal activity pattern, enable them to survive in arid regions. Accepted: 8 September 1998     

Kinetics of wound-induced hydraulic signals and variation potentials in wheat seedlings

Displacement transducers were used to demonstrate that localised wounding causes a rapid and systemic increase in leaf thickness in seedlings of wheat (Triticum durum Desf. cv. Iva). These increases are interpreted as reflecting wound-induced hydraulic signals. The duration of the wound-induced increase was found to be about 1 h or more, and it was shown that repeated wounds could induce repeated responses. The increase occurred even when plants had no access to an external water supply. Change in leaf thickness was shown closely to reflect change in leaf water potential. The velocity and kinetics of the wound-induced hydraulic signal were measured using multiple transducers ranged along a single leaf. The front of the signal was shown to travel through the plant at rates of at least 10 cm · s^–1. Development of the increase in leaf thickness was found to be relatively faster furthest from the wound. Onset of the change in leaf thickness in leaves remote from the wound was shown to precede onset of changes in surface electrical potential (variation potential) which are also induced by wounding. In contrast to reports from other species, variation potentials in wheat were here shown to spread extremely rapidly, at rates similar to that of the hydraulic signal. These data support the view that wound-induced hydraulic signals are the trigger for variation potentials in wheat.Symbol: _w water potential Grateful thanks are due to Paul Springer of the HRI (Wellesbourne) mechanical workshop for building equipment, and to H.G. Jones for helpful discussion.     

不同核桃品种的耐寒性及其渗透调节机制

以‘香玲’、‘鲁W06-1’、‘西洛3号’3个核桃品种（优系）为材料,对低温胁迫各材料的耐受情况及相应渗透调节物质含量和总蛋白质表达的变化情况进行分析,以揭示低温胁迫下不同核桃品种（优系）的渗透调节机制和蛋白质组分差异。结果显示:（1）3个核桃品种（优系）休眠枝条解剖结构分析表明,‘鲁W06-1’一年生休眠枝条的木质部比率最大,‘西洛3号’的韧皮部厚度显著大于其他两个品种（优系）。（2）随胁迫温度降低和胁迫时间的延长,枝条相对电导率（REC）逐渐升高,且‘西洛3号’的相对电导率均大于‘鲁W06-1’和‘香玲’。（3）核桃枝条可溶性糖、可溶性蛋白和游离脯氨酸含量均随温度降低先增加后减小,但各品种(优系)增至最大值的温度不同,其中‘鲁W06-1’枝条积累可溶性蛋白的速度和幅度较大,且各低温胁迫处理后的枝条可溶性糖和游离脯氨酸的含量也更高;3种渗透调节物质含量之间均呈极显著正相关关系,可溶性糖与游离脯氨酸含量的相关系数达0.844,表明二者对低温胁迫的应答反应密切相关。（4）在不同胁迫温度和胁迫时间处理后,核桃枝条的蛋白质表达谱带总体相似,但检测出6条表达量明显增加的差异条带,它们的分子量范围为38.9～87.9 kD。研究表明,3个核桃品种（优系）的耐寒性强弱为‘鲁W06-1’>‘香玲’>‘西洛3号’;低温胁迫下,核桃枝条迅速积累可溶性蛋白,然后积累可溶性糖和游离脯氨酸,并以耐寒性较强品种（优系）积累速度更快、积累量更大,且耐寒性较强的品种（优系）枝条蛋白谱带出现较多表达量增加的条带。     

肉桂醛对NaCl胁迫下番茄幼苗生长特性的影响

为探明肉桂醛对番茄幼苗耐盐性的影响,该研究以番茄‘合作903’为试验材料,探究肉桂醛(100μg·L^-1)对处于100 mmol·L^-1 NaCl胁迫下番茄种子萌发与幼苗生长特性的影响。在试验中设置4组处理：对照(CK,蒸馏水)、NaCl (100 mmol·L^-1)、NaCl+CA(100 mmol·L^-1 NaCl+100μg·L^-1 CA)、CA(100μg·L^-1)。结果表明：(1)在100 mmol·L^-1 NaCl胁迫下,添加100μg·L^-1的肉桂醛,番茄种子的发芽势和发芽率均有所提升,番茄幼苗的根长和鲜重均显著提高,说明添加肉桂醛对NaCl胁迫下番茄种子的萌发与幼苗生长均有一定缓解作用。(2)肉桂醛能够有效降低NaCl胁迫导致的番茄幼苗根尖总活性氧(reactive oxygen species, ROS)的过量累积,缓解膜脂过氧化程度从而降低细胞死亡率。综上表明,肉桂醛处理能够通过缓解幼苗的氧化...     

Effect of water restriction on energy and water balance and osmoregulation of the fruit bat Rousettus aegyptiacus

The energy budget, water balance and osmoregulation of the fruit bat, Rousettus aegyptiacus, were studied during normal hydration and during water restriction (oven-dried apple diet). The water input and output were balanced during both normal hydration and water restriction. The kidney of the fruit bat is well adapted to handle the water load from its fruit diet by excreting large volumes (14% of the body mass per day) of dilute urine (113±25 mosmol·kg H₂P^-1) as well as reducing urine volume (-95%) and increasing urine osmotic concentration (555±280 mosmol·kg H₂O^-1) during water restriction. The haematocrit, plasma haemoglobin and total protein concentrations did not increase during water restriction and heat exposure, suggesting the conservation of plasma volume. Gross energy intake was not alfected by water restriction. However, digested energy intake and digestibility were significantly reduced. The effective regulation of energy and water budgets during water restriction suggests that the fruit bat can cope with seasonal climatic changes and with variable fruit supply during various seasons.Abbreviations BM body mass - DEI digested energy intake - EWL evaporative water loss - GEL gross energy intake - NH normal hydration - T _a ambient temperature - WR water restriction     

Leaf and root growth dynamics in Eucalyptus globulus seedlings grown in drying soil

Summary Seedlings of Eucalyptus globulus growing in soil columns were subjected to a 24 day soil drying treatment. Water and solute potentials of both young expanding and fully expanded leaves declined under reduced soil water availability, while slightly higher turgor was sustained by the fully expanded leaves. Although leaf area of unwatered seedlings was smaller, the corresponding leaf dry weight was quite similar to that of well-watered seedlings. Soon after rewatering, leaf area of plants experiencing water shortage was comparable to that of well-watered plants. It seems that a difference in wall properties between juvenile and mature leaves allows for an effective pattern of water use by eucalypt plants growing in drying soil. Some stomatal opening is sustained and therefore, presumably, some carbon may be fixed, keeping the carbon balance of the whole plant positive, and allowing a continuous cell division despite the limited water supply. The highest root density of both well-watered and unwatered plants was found in the upper soil layers. However, root growth of unwatered seedlings was gradually increased in the deeper soil layers, where thicker root apices and higher soil water depletion rates per unit root length were recorded. As a consequence, root absorbing surface area was as large in unwatered plants as in well-watered plants.     

Osmoregulation in Rhizobium meliloti: inhibition of growth by salts

A defined medium of low osmolarity was developed permitting growth of Rhizobium meliloti with generation times of approximately 2.8 h doubling^-1. The effects of sodium, potassium, magnesium, ammonium, chloride, sulfate, phosphate, bicarbonate and acetate ions on the growth rate of R. meliloti were determined. Sodium, potassium and ammonium ions had little effect on growth at concentrations of 100 mEq or less; magnesium ion inhibited growth severely at concentrations of 50 mEq (25 mM). Of the anions, chloride and sulfate appeared to have little effect while phosphate, bicarbonate, and acetate inhibited growth at concentrations of as little as 25 mEq. The addition of proline, glutamate, or betaine to cells growing in inhibitory concentrations of NaCl did not relieve the inhibition. When grown in the presence of inhibitory levels of NaCl, the intracellular concentration of glutamate but not of proline or gamma amino butyric acid increased 5-fold.     

Biological seed priming mitigates the effects of water stress in sunflower seedlings

The sunflower (Helianthus annuus L. cv. PAC 36) seedlings were inoculated with plant growth promoting rhizobacteria (PGPR), viz. Azotobacter chroococcum (A+), Bacillus polymyxa (B+), separately and in combination of the two (AB+). Relative water content and seedling growth were maximum in AB+ seedlings under control. Water stress significantly decreased the RWC, growth and dry mass of non-inoculated seedlings. However, inoculated seedlings maintained higher growth even under water stress. Pigments and protein contents decreased under water stress, but higher amount of the same was observed in stressed AB+ seedlings. Enhanced activity of nitrate reductase was recorded in AB+ seedlings with maximum in control. Water stress significantly decreased the nitrate reductase activity. A significant increase in the activity of superoxide dismutase (SOD) in leaves was recorded under water stress except in B+ with maximum increase in non-inoculated seedlings. Catalase (CAT) activity decreased in stressed non-inoculated seedlings while increased in the leaves of A+ and AB+ seedlings. Almost similar trends were recorded for both leaves and cotyledons. PGPR improved the water status in stressed seedlings and thereby physiological and biochemical parameters and thus ameliorated the severe effects of water stress.     

Osmoregulation and cell composition in salt-adaptation of Nostoc muscorum

Adaptation to salt in the cyanobacterium Nostocmuscorum, is composed of a few mechanisms which together lead to the generation of a salt-tolerant cell. The initial mechanism combines a stimulation of photosynthetic activity with the accumulation of sucrose as an osmoregulator. The secondary mechanism involves the adaptation of N₂ fixation activity and protein biosynthesis. The adaptation is most efficient in response to NaCl-induced stress and functions only partially under stress induced by either KCl or a nonionic osmoticum such as mannitol.