Rapid and tissue-specific changes in ABA and in growth rate in response to salinity in barley leaves

The addition of 100 mM NaCl to the root medium of barley plantscaused the rapid cessation of elongation of the growing leafthree, followed by a sudden resumption of growth during thefollowing hour. The idea that resumption of growth is precededand mediated by rapid and tissue-specific changes in ABA concentrationand by changes in transpiration was tested. Leaf elongationvelocity was recorded continuously using linear variable displacementtransducers (LVDT), ABA was determined by immunoassay, and transpirationand stomatal conductivity were measured gravimetrically andby porometry, respectively. Within 10 min following additionof salt, ABA increased 6-fold in the distal portion of the leafelongation zone; in the proximal portion, ABA accumulated witha delay. In the portion of the growing blade that had emergedABA increased 3-fold and remained elevated during the following20 min. This preceded a decrease in transpiration and stomatalconductivity, which, in turn, coincided with growth resumption.Twenty hours following the addition of salt, the ABA concentrationshad returned to the level before stress. Leaf elongation velocitywas still reduced. It is concluded that NaCl causes a rapidincrease in ABA in the transpiring portion of the growing leaf.This leads to a decrease in transpiration. As a result, xylemwater potential is expected to rise. The moment that the waterpotential gradient between the xylem and the peripheral cellsin the growth zone favours water uptake again into the latter,leaf elongation resumes. The results suggest that ABA causesdifferent responses in different leaf regions, all aimed atpromoting the resumption of leaf growth. Key words: Abscisic acid, cell elongation, Hordeum vulgare, leaf growth, salinity, water relations.     

Participation of Plant Hormones in Growth Resumption of Wheat Shoots Following Short-Term NaCl Treatment

The effects of sodium-chloride salinity on the leaf elongation rate, transpiration rate, cell sap osmolality, and phytohormone content in 7-day-old shoots of durum wheat (Triticum durum L.) were studied. Leaf growth was suppressed under the salinity stress and resumed 1 h after NaCl removal. The resumption of leaf growth coincided with a decrease in the transpiration rate due to the rapid ABA accumulation in the differentiation leaf zone. The increased IAA concentration in the growing leaf zone promoted the formation of the attraction signal. The authors concluded that the changes in phytohormonal status in wheat plants occurred already following short-term (up to 1 h) salinity and were directed to the maintenance of plant growth under these conditions.     

Does night-time transpiration provide any benefit to wheat (Triticum aestivum L.) plants which are exposed to salt stress?

The study aimed to test whether night-time transpiration provides any potential benefit to wheat plants which are subjected to salt stress. Hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150, and 200 mM NaCl) for 5–8 days prior to harvest (day 14–18). Salt stress caused large decreases in transpiration and leaf elongation rates during day and night. The quantitative relation between the diurnal use of water for transpiration and leaf growth was comparatively little affected by salt. Night-time transpirational water loss occurred predominantly through stomata in support of respiration. Diurnal gas exchange and leaf growth were functionally linked to each other through the provision of resources (carbon, energy) and an increase in leaf surface area. Diurnal rates of water use associated with leaf cell expansive growth were highly correlated with the water potential of the xylem, which was dominated by the tension component. The tissue-specific expression level of nine candidate aquaporin genes in elongating and mature leaf tissue was little affected by salt stress or day/night changes. Growing plants under conditions of reduced night-time transpirational water loss by increasing the relative humidity (RH) during the night to 95% had little effect on the growth response to salt stress, nor was the accumulation of Na⁺ and Cl⁻ in shoot tissue altered. We conclude that night-time gas exchange supports the growth in leaf area over a 24 h day/night period. Night-time transpirational water loss neither decreases nor increases the tolerance to salt stress in wheat.     

Rapid and tissue-specific accumulation of solutes in the growth zone of barley leaves in response to salinity

The aim of the present study was to test whether rapid accumulation of solutes in response to salinity in leaf tissues of barley (Hordeum vulgare L.) contributes to recovery and maintenance of residual elongation growth. Addition of 100 mM NaCl to the root medium caused an immediate reduction close to zero in elongation velocity of the growing leaf 3. After 20–30 min, elongation velocity recovered suddenly, to 40–50% of the pre-stress level. Bulk osmolality increased first, after 60 min, significantly in the proximal half of the elongation zone. Over the following 3 days, osmolality increases became significant in the distal half of the elongation zone, the adjacent, enclosed non-elongation zone and finally in the emerged portion of the blade. The developmental gradient and time course in osmolality increase along the growing leaf was reflected in the pattern of solute (Cl, Na and K) accumulation in bulk tissue and epidermal cells. The partitioning of newly accumulated solutes between epidermis and bulk tissue changed with time. Even though solute accumulation does not contribute to the sudden and partial growth recovery 20–30 min after exposure to salt, it does facilitate residual growth from 1 h onwards. This is due to a high sink strength for solutes of the proximal part of the growth zone and its ability to accumulate solutes rapidly and at high rates.Abbreviations EDX analysis Energy-dispersive X-ray analysis - LEV Leaf elongation velocity - LVDT Linear variable differential transformer - REGR Relative elemental growth rate     

Water Relations and Growth of the flacca Tomato Mutant in Relation to Abscisic Acid

The flacca mutant in tomato (Lycopersicon esculentum Mill. cv Rheinlands Ruhm) was employed to examine the effects of a relatively constant diurnal water stress on leaf growth and water relations. As the mutant is deficient in abscisic acid (ABA) and can be phenotypically reverted to the wild type by applications of the growth substance, inferences can be made concerning the involvement of ABA in responses to water stress. Water potential and turgor were lower in leaves of flacca than of Rheinlands Ruhm, and were increased by ABA treatment. ABA decreased transpiration rates by causing stomatal closure and also increased the hydraulic conductance of the sprayed plants. Osmotic adjustment did not occur in flacca plants despite the daily leaf water deficits. Stem elongation was inhibited by ABA, but leaf growth was promoted. It is concluded that, in some cases, ABA may promote leaf growth via its effect on leaf water balance.     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

Sequence of drought response of maize seedlings in drying soil

Leaf elongation in monocotyledonous plants is sensitive to drought. To better understand the sequence of events in plants subjected to soil drying, leaf elongation and transpiration of maize seedlings ( Zea mays L.) of 4 cultivars were monitored continuously and the diurnal courses of the root and leaf water relations were determined. Results from this study indicate the following sequence of drought response: Leaf elongation decreased before changes in the leaf water relations of non‐growing zones of leaf blades were detected and before transpiration decreased. Reductions in leaf elongation preceded changes in the root water potential (ψ_w). Root ψ_w was not a very sensitive indicator of soil dryness, whereas the root osmotic potential (ψ_s) and root turgor (ψ_p) were more sensitive indicators. The earliest events observed in drying soil were a significant increase in the largest root diameter class (1 720 to 1 960 gm) and a decrease in leaf elongation ( P = 0.08) 2 days after withholding water. Significant increases in root length were observed 2 days later. Soil drying increased the number of fine roots with diameters of <240 µm. Slight increases in soil strength did not affect leaf elongation in the drying soil.     

Efficient leaf ion partitioning,an overriding condition for abscisic acid-controlled stomatal and leaf growth responses to NaCl salinization in two legumes

Two tree medics contrasting in salt tolerance, Medicago arborea and Medicago citrina, were compared to evaluate the relative importance of abscisic acid on leaf growth and stomatal responses to salt stress. Plants were grown for 30 d in solution culture with 1, 50, 100 or 200 mM NaCl. Salinized plants of M. citrina had lower Na+ and Cl- uptake and maintained better leaf growth than M. arborea. In M. citrina, stomatal conductance was only slightly affected by salt and, in consequence, the salt treatment had no significant influence, neither on the CO2 fixation rate nor the transpiration rate in these plants. Moreover, leaf photosynthetic pigments and soluble protein in M. citrina were increased by the presence of NaCl, while a decrease of both parameters with salt was found in M. arborea. However, leaf and xylem ABA increased only in salt-treated M. citrina, while no differences were found among treatments in M. arborea. The role of ion compartmentation, gas exchange parameters and ABA concentrations in relation to salt tolerance in M. arborea and M. citrina is discussed.     

A Tolerant Behavior in Salt-Sensitive Tomato Plants can be Mimicked by Chemical Stimuli

Lycopersicon esculentum plants exhibit increased salt stress tolerance following treatment with adipic acid monoethylester and 1,3-diaminepropane (DAAME), known as an inducer of resistance against biotic stress in tomato and pepper. For an efficient water and nutrient uptake, plants should adapt their water potential to compensate a decrease in water soil potential produced by salt stress. DAAME-treated plants showed a faster and stronger water potential reduction and an enhanced proline accumulation. Salinity-induced oxidative stress was also ameliorated by DAAME treatments. Oxidative membrane damage and ethylene emission were both reduced in DAAME-treated plants. This effect is probably a consequence of an increase of both non-enzymatic antioxidant activity as well as peroxidase activity. DAAME-mediated tolerance resulted in an unaltered photosynthetic rate and a stimulation of the decrease in transpiration under stress conditions without a cost in growth due to salt stress. The reduction in transpiration rate was concomitant with a reduction in phytotoxic Na⁺ and Cl⁻ accumulation under saline stress. Interestingly, the ABA deficient tomato mutant sitiens was insensitive to DAAME-induced tolerance following NaCl stress exposure. Additionally, DAAME treatments increased the ABA content of leaves, therefore, an intact ABA signalling pathway seems to be important to express DAAME-induced salt tolerance. Here, we show a possibility of enhance tomato stress tolerance by chemical induction of the major plant defences against salt stress. DAAME-induced tolerance against salt stress could be complementary to or share elements with induced resistance against biotic stress. This might be the reason for the observed wide spectrum of effectiveness of this compound.Key Words: adipic acid monoethyl ester, 1,3-diaminepropane, Lycopersicon esculentum, salt stress, oxidative stress, ethylene, chemical induced tolerance     

Control of Leaf Expansion Rate of Droughted Maize Plants under Fluctuating Evaporative Demand (A Superposition of Hydraulic and Chemical Messages?)

We have analyzed the possibility that chemical signaling does not entirely account for the effect of water deficit on the maize (Zea mays L.) leaf elongation rate (LER) under high evaporative demand. We followed time courses of LER (0.2-h interval) and spatial distribution of elongation rate in leaves of either water-deficient or abscisic acid (ABA)-fed plants subjected to varying transpiration rates in the field, in the greenhouse, and in the growth chamber. At low transpiration rates the effect of the soil water status on LER was related to the concentration of ABA in the xylem sap and could be mimicked by feeding artificial ABA. Transpiring plants experienced a further reduction in LER, directly linked to the transpiration rate or leaf water status. Leaf zones located at more than 20 mm from the ligule stopped expanding during the day and renewed expansion during the night. Neither ABA concentration in the xylem sap, which did not appreciably vary during the day, nor ABA flux into shoots could account for the effect of evaporative demand. In particular, maximum LER was observed simultaneously with a minimum ABA flux in the droughted plants, but with a maximum ABA flux in ABA-fed plants. All data were interpreted as the superposition of two additive effects: the first involved ABA signaling and was observed during the night and in ABA-fed plants, and the second involved the transpiration rate and was observed even in well-watered plants. We suggest that a hydraulic signal is the most likely candidate for this second effect.     

Growth,Water Relations,and Accumulation of Organic and Inorganic Solutes in Roots of Maize Seedlings during Salt Stress

Seedlings of maize (Zea mays L. cv Pioneer 3906), hydroponically grown in the dark, were exposed to NaCl either gradually (salt acclimation) or in one step (salt shock). In the salt-acclimation treatment, root extension was indistinguishable from that of unsalinized controls for at least 6 d at concentrations up to 100 mM NaCl. By contrast, salt shock rapidly inhibited extension, followed by a gradual recovery, so that by 24 h extension rates were the same as for controls, even at 150 mM NaCl. Salt shock caused a rapid decrease in root water and solute potentials for the apical zones, and the estimated turgor potential showed only a small decline; similar but more gradual changes occurred with salt acclimation. The 5-bar decrease in root solute potential with salt shock (150 mM NaCl) during the initial 10 min of exposure could not be accounted for by dehydration, indicating that substantial osmotic adjustment occurred rapidly. Changes in concentration of inorganic solutes (Na+, K+, and Cl-) and organic solutes (proline, sucrose, fructose, and glucose) were measured during salt shock. The contribution of these solutes to changes in root solute potential with salinization was estimated.     

Salt-stress-induced ABA accumulation is more sensitively triggered in roots than in shoots

Salt-stress-induced ABA accumulation in maize root tissues was compared with that in leaf tissues. While salt stress with NaCl resulted in a significant ABA accumulation in root tissues (up to 10-fold), the same stress only led to a small ABA accumulation in leaf tissues (about 1-fold). Pretreatment with ethylene glycol (EG), a permeable and inert monomer of PEG, could prevent the shrinkage of cell volume and completely block the ABA accumulation in leaf tissues under salt stress, but substantial salt-induced ABA accumulation was still observed in root tissues following such pretreatment. Hypotonic salt solutions, i.e. below 100 mM NaCl, still induced a significant ABA accumulation (more than 3-fold) in roots, but showed no effect on that in leaf tissues. Results suggest that the salt-stress-induced ABA accumulation in roots may also be triggered by an osmosensing mechanism, which is in addition to the perception of the changes in reduced cellular volume or plasmalemma tension that leads to ABA accumulation in leaves. When leaf and root tissues were immersed into salt solutions, salt entered into the cells as a function of time and salt concentrations. Such entrance apparently led to a loss of sensitivity of leaf tissues to accumulate ABA under the salt stress, and also prevented the leaf tissues from responding to further air-drying in terms of ABA accumulation. Roots showed no such responses. Results suggest that the entrance of salt into leaf cells brought about some toxic effect that might have reduced the capability of leaf cells to produce ABA under dehydration.     

Abscisic acid alters carbohydrate accumulation induced by differential response to sodium salts in the halophyte Prosopis strombulifera

Abstract

The role of abscisic acid (ABA) was analyzed in roots and leaves of the halophyte Prosopis strombulifera in response to low osmotic potential (Ψo: ?1.0, ?1.9, and ?2.6?MPa) induced by sodium chloride (NaCl), sodium sulfate (Na₂SO₄), and the iso-osmotic combination of both compounds (NaCl?+?Na₂SO₄). P. strombulifera plants were sprayed with ABA, as well as with inhibitors of ABA biosynthesis (sodium tungstate and fluridone). Different parameters were measured, including total plant height, leaf number, root length, root and shoot biomass, water content, transpiration rate, and total soluble carbohydrates, specific carbohydrates and ABA concentrations. Results showed that sodium salts affected growth parameters in varying ways, depending on the type of salts used as well as the osmotic potentials. ABA-sprayed plants displayed the lowest transpiration values. These plants had a higher content of total soluble carbohydrates in roots, greater root biomass and length and increased root/shoot rate. This study shows that ABA triggers different biochemical and physiological responses after the perception of a stressful condition, and that the interaction between different concentrations and types of salts, and the addition of ABA or its inhibitors generates responses that affect development and growth in the halophyte P. strombulifera.     

Hormonal Control of the Shoot-to-Root Ratio Is Not Related to Water Deficiency in Wheat Plants under Mineral Deficiency

We measured the content of hormones, the rate of growth, and some parameters of water regime (water content, transpiration, and stomatal and hydraulic conductivities) one and two days after wheat plant transfer from 10 to 1% Hoagland-Arnon nutrient medium. It was shown that, a day after dilution of nutrient solution, the content of various cytokinin forms decreased in the xylem sap, shoots, and roots. This decrease was most pronounced in the case of zeatin in the xylem sap and zeatin riboside in the mature zone of the first leaf. ABA was found to accumulate in shoots. A day after dilution of nutrient solution, we observed root elongation evidently induced by mineral nutrient deficiency, and this accelerated root growth was maintained later. Two days after dilution of nutrient solution, we observed the slowing of shoot weight accumulation, whereas root weight remained unchanged. Plant growth response could be related to ABA accumulation in shoots and cytokinin depletion in the whole plant. A reduced hydraulic conductivity and water content in the growing leaf zone was detected only two days after dilution of nutrient solution. Thus, changes in the growth rates and hormone contents could not result from disturbances in water regime induced by mineral nutrient deficiency.     

Physiological responses to salt stress in young umbu plants

Soil salinity affects plant growth and development due to harmful ion effects and water stress caused by reduced osmotic potential in the soil solution. In order to evaluate the effects of salt stress in young umbu plants, research was performed in green house conditions at the Laboratory of Plant Physiology at Federal Rural University of Pernambuco, Brazil. Growth, stomatal behaviour, water relations, and both inorganic and organic solutes were studied aiming for a better understanding of the responses of umbu plants to increasing salinity. Plants were grown in washed sand with Hoagland and Arnon nutrient solution with 0, 25, 50, 75, and 100 mM NaCl. Growth, leaf water potential, transpiration, and diffusive resistance were evaluated. Na⁺, K⁺, Cl⁻, soluble carbohydrates, and free amino acid contents were measured in several plant organs. Most variables were affected with salinity above 50 mM NaCl showing decreases in: number of leaves, plant height, stems diameter, and dry masses, and increases in root-to-shoot ratio. Reductions in ψ_pd were observed in plants grown under 75 and 100 mM NaCl. All salt levels above zero increased Na⁺ and Cl⁻ contents in leaves. However, K⁺ content was not affected. Na⁺ and Cl⁻ in stems and roots reached saturation in treatments above 50 mM NaCl. Organic solute accumulation in response to salt stress was not observed in umbu plants. These results suggest that umbu plants tolerate salt levels up to 50 mM NaCl without showing significant physio-morphological alterations.     

NaCl抑制棉花幼苗生长的机理—盐离子效应

75和150 mmol/L NaCl处理.降低棉花幼苗叶面积、叶相对扩展率,蒸腾和木质部汁液K~ 浓度;而增大叶细胞质膜透性、渗透势、叶Na~ 含量和木质部汁液Na~ 和Cl~-的浓度。生长在75mmol/L NaCl加压(根际)和不加压条件下的棉花,叶面积、叶相对扩展率、蒸腾、叶质膜透性和渗透势的变化基本一样。这些结果表明棉花幼苗的拒盐能力不大,盐害的原因是盐的原初效应,而不是盐的次生效应。另外,盐对棉花幼苗叶相对扩展率和质膜透性的效应在生长后期降低,表明棉花幼苗也具有一定的耐盐能力。     

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

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

Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporative demand

We studied the possible involvement of ABA in the control of water relations under conditions of increased evaporative demand. Warming the air by 3°C increased stomatal conductance and raised transpiration rates of hydroponically grown Triticum durum plants while bringing about a temporary loss of relative water content (RWC) and immediate cessation of leaf extension. However, both RWC and extension growth recovered within 30 min although transpiration remained high. The restoration of leaf hydration and growth were enabled by increased root hydraulic conductivity after increasing the air temperature. The use of mercuric chloride (an inhibitor of water channels) to interfere with the rise on root hydraulic conductivity hindered the restoration of extension growth. Air warming increased ABA content in roots and decreased it in shoots. We propose this redistribution of ABA in favour of the roots which increased the root hydraulic conductivity sufficiently to permit rapid recovery of shoot hydration and leaf elongation rates without the involvement of stomatal closure. This proposal is based on known ability of ABA to increase hydraulic conductivity confirmed in these experiments by measuring the effect of exogenous ABA on osmotically driven flow of xylem sap from the roots. Accumulation of root ABA was mainly the outcome of increased export from the shoots. When phloem transport in air-warmed plants was inhibited by cooling the shoot base this prevented ABA enrichment of the roots and favoured an accumulation of ABA in the shoot. As a consequence, stomata closed.     

Transpiration, CO2 assimilation, WUE, and stomatal aperture in leaves of Viscum album (L.): Effect of abscisic acid (ABA) in the xylem sap of its host (Populus x euamericana).

Leaves of the mistletoe Viscum album (L.) show a high rate of transpiration, even when the host is under severe drought stress. The hypothesis that a strong control of ABA influx from the xylem sap of the host into the mistletoe prevents stomatal closure in mistletoe leaves was tested under the following conditions: sections of poplar twigs carrying a mistletoe were perfused with artificial xylem sap that contained different ABA concentrations and both transpiration and ABA levels were analysed in mistletoe leaves. Despite variation by a factor of 10(4), the ABA content of the host xylem did not affect ABA levels, leaf transpiration, CO(2) assimilation, WUE, or the degree of stomatal aperture in mistletoe leaves. These observations support the hypothesis of a strong control of ABA influx from the host of the xylem into the mistletoe, although degradation of ABA before it enters the mistletoe leaves cannot be excluded. This mechanism may ensure a water and nutritional status favourable for the mistletoe, even if the water status of the host is impaired. Despite the lack of short-term sensitivity of ABA levels in mistletoe leaves to even strong changes of ABA levels in the xylem sap of the host, ABA levels in mistletoe leaves were relatively high compared to ABA levels in the leaves of several tree species including poplar. Since significant transpiration of the mistletoe leaves was observed despite high ABA levels, a diminished sensitivity of the stomata of mistletoe leaves to ABA has to be concluded. The stomatal density of adaxial Viscum leaves of 89+/-23 stomata per mm is lower than those reported in a study performed at the end of the 19th century.     

The Effects of NaCl Treatment on Water Relations, Growth, and ABA Content in Barley Cultivars Differing in Drought Tolerance

Changes in transpiration and stomatal conductance and other characteristics of water relations, growth rate, and ABA content have been followed in short- and long-term experiments in two barley cultivars (cv. Michaelovsky and cv. Prairie) with contrasting drought resistance characteristics. The aim of this work was to reveal the importance of stomatal behavior in salt tolerance and also the involvement of ABA in its control. Salinity stress brought about a reduction in stomatal conductance in both cultivars, but the effect was initially more pronounced in the drought-tolerant cv. Prairie than in the drought-sensitive cv. Michaelovsky. The difference between the two cultivars changed with time, and later on transpiration and stomatal conductance became higher in Prairie than in Michaelovsky. In both the short and the long term, the extent of stomatal closure due to salinity correlated with the level of ABA accumulation in the leaves of the plants. Fast stomatal closure was likely to be responsible for growth resumption after an initial arrest by salt treatment and for the maintenance of extension growth later on, thus enabling its higher rate in Prairie than in Michaelovsky plants. Leaves of Prairie accumulated less toxic chloride ions, which may be the result of a lower transpiration rate observed during the first phase of salt treatment. A subsequent increase in stomatal conductance observed in Prairie is likely to ameliorate their gas exchange and maintain photosynthesis and growth. Thus, differences between the cultivars in the stomatal response to salinity changed with time, which may be why there are discrepancies in the attempts to relate stomatal conductance to salt tolerance observed in literature.