Kinematics and Dynamics of Sorghum (Sorghum bicolor L.) Leaf Development at Various Na/Ca Salinities (I. Elongation Growth)

In many salt-sensitive species, elevated concentrations of Ca in the root growth media ameliorate part of the shoot growth reduction caused by NaCl stress. The physiological mechanisms by which Ca exerts protective effects on leaf growth are still not understood. Understanding growth inhibition caused by a stress necessitates locating the leaf expansion region and quantifying the profile of the growth reduction. This will enable comparisons and correlations with spatial gradients of probable physiologically inhibiting factors. In this work we applied the methods of growth kinematics to analyze the effects of elevated Ca concentrations on the spatial and temporal distributions of growth within the intercalary expanding region of salinized sorghum (Sorghum bicolor [L.] Moench, cv NK 265) leaves. NaCl (100 mM) caused a decrease in leaf elongation rate by shortening the leaf growing zone by 20%, as well as reducing the peak value of the longitudinal relative elemental growth rate (REG rate). Increasing the Ca concentrations from 1 to 10 mM restored the length of the growing zone of both emerged and unemerged salinized leaves and increased the peak value of the REG rate. The beneficial effects of supplemental Ca were, however, more pronounced in leaves after their appearance above the whorl of encircling older leaf sheaths. Elevated Ca then resulted in a peak value of REG rate higher than in the salinized leaves. The peak value of unemerged leaves was not increased, although it was maintained over a longer distance. The duration of elongation growth associated with a cell during its displacement from the leaf base was longer in salinized than control leaves, despite the fact that the elongation zone was shorter in salinity. Although partially restoring the length of the elongation region, supplemental Ca had no effect on the age of cessation of growth. Elongation of a tissue element, therefore, ceased when a cellular element reached a certain age and not a specific distance from the leaf base.     

Salinity-induced calcium deficiencies in wheat and barley

Salinity-calcium interactions, which have been shown to be important in plants grown in dryland saline soils of the Canadian prairies, were studied in two species differing in salt tolerance. In solution culture, wheat showed a greater reduction in growth and a higher incidence of foliar Ca deficiency symptoms than barley when grown under MgSO₄ or Na₂SO₄ plus MgSO₄ salt stress. Amendment of the saline solution with Ca to increase the Ca/(Na+Mg) ratio ameliorated the effects of salt, but more so in wheat than in barley. At least part of the difference in salt tolerance between the two species must therefore relate to species differences in the interaction of salinity and Ca nutrition. The greater response of wheat to Ca was not due to a lower Ca status in leaf tissue; on the contrary, although Ca amendments improved tissue Ca/(Na+Mg) ratios in both species, salinized wheat had equivalent or higher Ca content, and higher Ca/(Na+Mg) ratios than did barley. The higher Ca requirement of wheat is apparently specific to a saline situation; at low salinity, wheat growth was not reduced as extensively as that of barley as Ca/(Na+Mg) ratio was decreased. High night-time humidity dramatically improved wheat growth under saline conditions, but increasing the Ca concentration of the saline solution had no effect on growth in the high humidity treatment. Membrane leakage from leaf tissue of wheat grown under saline conditions was increased compared to tissue from non-saline plants. Plants grown in Ca-amended saline solutions showed no increase in membrane leakage. These results confirm the importance of Ca interaction with salinity stress, and indicate differences in species response.     

Leaf development,transpiration and ion uptake and distribution in sugarcane cultivars grown under salinity

The effects of salinity on leaf growth, initiation and senescence, on transpiration rates, on leaf water potential and on uptake and distribution of several ions were studied in two sugarcane cultivars differing in salinity sensitivity. Plants, growing in a growing mixture in pots, were exposed to salinized irrigation water for 68 days, starting 60 days after planting. EC values of the irrigation water were 1.0, 2.0, 4.0, 8.0 and 12 dS/m, obtained by using a mixture of NaCl and CaCl2. Plants were also grown in nutrient solution and were at a similar age when exposed to a salinity level of 3 dS/m for 30 days followed by 6.0 dS/m for an additional 30 days. Two Na:Ca ratios of 18:1 and 1:2 were used for salinization of the nutrient solution. Both leaf dry weight and area decreased with increasing salinity, but in the more salinity tolerant cultivar H69-8235, the decrease was moderate. Salinity hardly reduced average area per leaf in H69-8235, while the number of leaves declined sharply. This decline was caused by enhanced senescence of mature leaves and not by a decreased rate of leaf initiation. In the more sensitive cultivar, H65-7052, leaf area and initiation of new leaves were sharply reduced by salinity while leaf senescence was less affected. Leaf water potential decreased during the early stages of salinity exposure, and the reduction in water potential was larger in H69-8235. Salinity also decreased the rate of transpiration rate but to a lesser extent than leaf development and growth. The accumulation of Cl and Na in the TVD (top visible dewlap) leaf of the tolerant cultivar H69-8235 was greater than in the sensitive cultivar H65-7052. The concentration of Cl in the TVD leaf was more than 10 times that of Na in both cultivars. The concentration of both ions, but not of K, increased during the early stages of salinity exposure and then remained constant. A gradient in concentration of Cl and Na over the plant was found in both cultivars at all salinity levels, and was steepest between the TVD and younger leaves. No specific Na effect on leaf growth or transpiration could be detected. The accumulation of Cl and Na but not of K occurred primarily in the roots rather than in the leaves and stalks. This revised version was published online in August 2006 with corrections to the Cover Date.     

Salinity tolerance of 'Valencia' orange trees on rootstocks with contrasting salt tolerance is not improved by moderate shade

The effects of shading in combination with salinity treatments were studied in citrus trees on two rootstocks with contrasting salt tolerance to determine if shading could reduce the negative effects of salinity stress. Well-nourished 2-year-old 'Valencia' orange trees grafted on Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citrange (Carr, relatively salt sensitive), were grown either under a 50% shade cloth or left unshaded in full sunlight. Half the trees received no salinity treatment and half were salinized with 50 mM Cl- during two 9 week salinity periods in the spring and autumn interrupted by an 11 week rainy period. The shade treatment reduced midday leaf temperature and leaf-to-air vapour pressure deficit regardless of salinity treatments. In non-salinized trees, shade increased midday CO2 assimilation rate (A(CO2)) and stomatal conductance, but had no effect on leaf transpiration (E(lf)). Shade also increased leaf chlorophyll and photosynthetic water use efficiency (A(CO2)/E(lf)) in leaves on both rootstocks and increased total plant dry weight in Cleo. The salinity treatment reduced leaf growth and leaf gas exchange parameters. Shade decreased Cl- concentrations in leaves of salinized Carr trees, but had no effect on leaf or root Cl- of trees on Cleo. There were no significant differences in leaf gas exchange parameters of shaded and unshaded salinized plants but the growth reduction from salinity stress was actually greater for shaded than for unshaded trees. Shaded trees on both rootstocks had higher leaf Na+ than unshaded trees after the first salinity period, and this shade-induced elevated leaf Na+ persisted after the second salinity period in trees on Carr. Thus, shading did not alleviate the negative effects of salinity on growth and Na+ accumulation.     

Supplemental manganese improves the relative growth, net assimilation and photosynthetic rates of salt-stressed barley

Previous results in our laboratory indicated that a reduced Mn concentration in the leaves of barley was highly correlated with the reduced relative growth and net assimilation rates of salt-stressed plants. If Mn deficiency limits the growth of salt-stressed barley, then increasing leaf Mn concentrations should increase growth. In the present study, the effect of supplemental Mn on the growth of salt-stressed barley ( Hordeum vulgare L. cv. CM 72) was tested to determine if a salinity-induced Mn deficiency was limiting growth. Plants were salinized with 125 mol m⁻³ NaCl and 9.6 mol m⁻³ CaCl₂. Supplemental Mn was applied in 2 ways: 1) by increasing the Mn concentration in the solution culture and 2) by spraying Mn solutions directly onto the leaves. Growth was markedly inhibited at this salinity level. Dry matter production was increased 100% in salt-stressed plants treated with supplemental Mn to about 32% of the level of nonsalinized controls. The optimum solution culture concentration was 2.0 mmol m⁻³, and the optimum concentration applied to the leaves was 5.0 mol m⁻³. Supplemental Mn did not affect the growth of control plants. Further experiments showed that supplemental Mn increased Mn concentrations and uptake to the shoot. Supplemental Mn increased the relative growth rate of salt-stressed plants and this increase was attributed to an increase in the net assimilation rate; there were no significant effects on the leaf area ratio. Supplemental Mn also increased the net photosynthetic rate of salt-stressed plants. The data support the hypothesis that salinity induced a Mn deficiency in the shoot, which partially reduced photosynthetic rates and growth.     

Polyribosome metabolism, growth and water status in the growing tissues of osmotically stressed plant seedlings

Relationships between growth of osmotically stressed intact seedlings and polyribosome levels and water status of growing tissues were examined. Sudden exposure of barley (Hordeum vulgare L. cv. Arivat) roots to a solution of ?0.8 MPa polyethylene glycol caused leaf growth to stop almost immedately, but growth resumed at a much lower rate after 0.5–1 h. In the growing region of leaves, the polyribosome: total ribosome ratio of free (non-membrane-bound) ribosomes was significantly reduced after 15 min stress, but a decrease in the large polyribosome:total polyribosome ratio occurred only after 1–2 h. Membrane-bound and free polyribosome levels both decreased to 70% of unstressed control values after 4 h stress. Recovery of total polyribosomes occurred within 1 h after relief of 4 h stress, but required 3 h after relief of 24 h stress. Stress detectably reduced the water potential and osmotic potential of growing tissue within 0.5–1.0 h, and osmotic adjustment continued for up to 10 h. Recovery of water status was incomplete after 1 h relief of a 4 h stress. In contrast, expanded blade tissues of stressed plants underwent minor changes in water status and slow decreases in polyribosomes levels. These results confirm that growing tissues of barley leaves are selectively responsive to stress, and suggest that changes in growth, water status and polyribosome levels may be initiated by the same signal. Measurements of seedling growth, polyribosome levels and water status of growing tissues of barley and wheat (Triticum aestivum L. cv. Zaragoza) leaves, etiolated pea (Pisum sativum L. cv. Alaska) epicotyl and etiolated squash (Cucurbita pepo L. cv. Elite) hypocotyl stressed with polyethylene glycol solutions of ?0.3 to ?0.8 MPa for 12 h or more showed that polyribosome levels were highly correlated with seedling growth rate as well as with tissue water and osmotic potentials, while turgor remained unchanged. These results suggest that long-term growth of osmotically stressed plants may be limited by a reduced capacity for protein synthesis in growing tissues and is not dictated by turgor loss.     

Leaf gas exchange and solute accumulation in the halophyte Salvadora persica grown at moderate salinity

The domestication of halophytes has been proposed as a strategy to expand cultivation onto unfavorable land. However, halophytes mainly have been considered for their performance in extremely saline environments, and only a few species have been characterized in terms of their tolerance and physiological responses to moderately high levels of salinity. Salvadora persica is an evergreen perennial halophyte capable of growing under extreme conditions, from very dry environments to highly saline soils. It possesses high potential economic value as a source of oil and medicinal compounds. To quantify its response to salinity, S. persica seedlings were exposed to 200 mM NaCl for 3 weeks, and growth, leaf gas exchange and solute accumulation were measured. The presence of NaCl induced a 100% increase in fresh weight and a 30% increase in dry weight, relative to non-salinized controls. Increases in fresh weight and dry weight were not associated with higher rates of net CO(2) assimilation, however. Analysis of ion accumulation revealed that S. persica leaves accumulated Na(+) as a primary osmoticum. The concentration of Na(+) in leaves of salinized plants was approximately 40-fold greater than that measured in non-salinized controls, and this was associated with significant reductions in leaf K(+) and Ca(2+) concentrations. In addition, a significant accumulation of proline, probably associated with osmotic adjustment and protection of membrane stability, occurred in roots of salinized plants.     

Physiological basis of differential response to salinity in rice cultivars

Growth analyses of rice Oryza sativa L. seedlings in salinized nutrient solution condition were conducted with 24 cultivars and lines after genetic purification. Cultivar differences in relative growth rate in salinized conditions were chiefly dependent on differences in shoot Na content. The shoot Na content was affected by Na selectivity in the root and by the leaf area ratio (LAR, leaf area per total dry weight). The contribution of LAR was equally important to that of root cultivar selectivity against Na uptake under a higher salinization condition where root selectivity against Na may be decreased due to reduced root activity. Cultivar differences in salt tolerance in highly salinized conditions were mainly attributed to differences in these two factors. A more convenient and efficient screening method for salt tolerance is proposed.     

Photosynthetic capacity is related to the cellular and subcellular partitioning of Na+, K+ and Cl- in salt-affected barley and durum wheat

The capacity of plants to tolerate high levels of salinity depends on the ability to exclude salt from the shoot, or to tolerate high concentrations of salt in the leaf (tissue tolerance). It is widely held that a major component of tissue tolerance is the capacity to compartmentalize salt into safe storage places such as vacuoles. This mechanism would avoid toxic effects of salt on photosynthesis and other key metabolic processes. To test this, the relationship between photosynthetic capacity and the cellular and subcellular distribution of Na+, K+ and Cl- was studied in salt-sensitive durum wheat (cv. Wollaroi) and salt-tolerant barley (cv. Franklin) seedlings grown in a range of salinity treatments. Photosynthetic capacity parameters (Vcmax, Jmax) of salt-stressed Wollaroi decreased at a lower leaf Na+ concentration than in Franklin. Vacuolar concentrations of Na+, K+ and Cl- in mesophyll and epidermal cells were measured using cryo-scanning electron microscopy (SEM) X-ray microanalysis. In both species, the vacuolar Na+ concentration was similar in mesophyll and epidermal cells, whereas K+ was at higher concentrations in the mesophyll, and Cl- higher in the epidermis. The calculated cytoplasmic Na+ concentration increased to higher concentrations with increasing bulk leaf Na+ concentration in Wollaroi compared to Franklin. Vacuolar K+ concentration was lower in the epidermal cells of Franklin than Wollaroi, resulting in higher cytoplasmic K+ concentrations and a higher K+ : Na+ ratio. This study indicated that the maintenance of photosynthetic capacity (and the resulting greater salt tolerance) at higher leaf Na+ levels of barley compared to durum wheat was associated with the maintenance of higher K+, lower Na+ and the resulting higher K+ : Na+ in the cytoplasm of mesophyll cells of barley.     

Physiological behaviour of loquat and anger rootstocks in relation to salinity and calcium addition

The salinity tolerance of two commercial rootstocks used for loquat plants (Eribotrya japonica Lindl.), loquat and anger, was studied in a pot experiment. The plants were irrigated using solutions containing 5 and 50mM NaCl and 5 and 25mM calcium acetate for 4 months. The growth, tissue mineral content, water status, and leaf gas exchange responses to salt treatment with and without additional calcium were examined. Plant growth was not modified by salinity in anger (50mM), but was reduced in loquat; leaf biomass and stem diameter were particularly affected. However, Cl(-) levels leaf increased with salinity to a greater extent in anger, while the Na(+) content increased to the same extent in both species, indicating that ion transport from root to leaves was not inhibited in either species. Additional calcium (25mM) reduced Na(+) and Cl(-) concentrations in both species, but did not minimise the effects of salinity on the growth of salt-treated loquat plants. The decrease in K(+) concentrations had no effect on growth, as anger was the most tolerant rootstock and had lowest leaf K(+) content. Salinity reduced the Ca(2+) concentration in the roots of both species. However, when calcium was added, the concentration of Ca(2+) increased in the roots of salinised plants. Leaf water potential at pre-dawn decreased significantly in both species under saline conditions. Leaf gas exchange, stomatal conductance and, in particular, net CO(2) assimilation, decreased with salinity only in loquat, indicating that photosynthesis could be the growth-limiting factor in this species.     

Chloroplast Protein Synthesis During Barley Leaf Growth and Senescence: Effect of Leaf Excision

Chloroplast protein synthesis was measured during the expansion,maturity and senescence of the oldest leaf of barley, Hordeumvulgare L., var. Hassan. A maximum rate of protein synthesisoccurred near the end of the expansion stage 9 d after sowing.Protein synthesis increased again at the beginning of senescenceand reached a new maximum at day 14 after sowing. Detachmentand incubation of leaves in the dark stimulated chioroplastprotein synthesis by fully expanded or by senescent leaves butnot by expanding leaves. If the detached leaves were kept inthe light, chloroplast protein synthesis was stimulated in fullyexpanded but not in senescent leaves. Short treatments (18 h)of leaf segments with growth substances in either light or indarkness, significantly changed the rate of protein synthesisshown by chloroplasts. The relationship between chloroplastprotein synthesis and leaf senescence is discussed. Key words: Hormones, light, maturity     

An investigation of boron toxicity in barley using metabolomics

Boron (B) is an essential micronutrient that affects plant growth at either deficient or toxic concentrations in soil. The aim of this work was to investigate the adaptation of barley (Hordeum vulgare) plants to toxic B levels and to increase our understanding of B toxicity tolerance mechanisms. We used a metabolomics approach to compare metabolite profiles in root and leaf tissues of an intolerant, commercial cultivar (cv Clipper) and a B-tolerant Algerian landrace (cv Sahara). After exposure to elevated B (200 and 1,000 microM), the number and amplitude of metabolite changes in roots was greater in Clipper than in Sahara. In contrast, leaf metabolites of both cultivars only responded following 1,000 microM treatment, at which B toxicity symptoms (necrosis) were visible. In addition, metabolite levels were dramatically altered in the tips of leaves of the sensitive cultivar Clipper after growth in 1,000 microM B compared to those of Sahara. This correlates with a gradual accumulation of B from leaf base to tip in B-intolerant cultivars. Overall, there were always greater differences between tissue types (roots and leaves) than between the two cultivars. This work has provided insights into metabolic differences of two genetically distinct barley cultivars and information about how they respond metabolically to increasing B levels.     

Cold-induced accumulation of protein in the leaves of spring and winter barley cultivars

Electrophoretic pattern and quantitative changes in soluble proteins were determined in the leaves of spring and winter cultivars of barley (Hordeum vulgare L., cv. Makouei and cv. Reyhan, respectively) exposed to 4 degrees C for 14 d. Seedlings were grown in a controlled growth chamber for 2 weeks at a constant air temperature of 20 degrees C and then transferred to constant 4 degrees C for 14 d followed by returning to 20 degrees C (cold treatment), or they were maintained throughout at 20 degrees C during the experimental period of 40 d (control treatment). Plants were sampled every 48 h for leaf fresh weight measurements. Total leaf soluble proteins were extracted and their concentration was either determined by a colorimetric method, or size-fractionated on SDS-PAGE. Low temperature-induced increases in protein amount occurred over the second week of exposure to cold treatment irrespective of cultivar: the winter cultivar was 2 d prior in this response. The protein patterns and their density showed differences between-cultivars and between-temperature treatments. A new cold-induced polypeptide was recognized in the leaves of winter barley cultivar on day 22 (8 d at 4 degrees C) compared to the control. This polypeptide was produced earlier over the first 48 h of low temperature in the winter cultivar compared with the spring one, recognizing in the leaves of cold-treated seedling until day 26. This more rapid response to a low temperature by the winter barley cultivar indicates a more sensitive response compared with the spring barley, probably cold-shock protein is a component of this cold-induced response.     

Intercellular Compartmentation of Ions in Barley Leaves in Relation to Potassium Nutrition and Salinity

X-ray microanalysis was used to determine the distributionsof several nutrient elements between vacuoles of epidermal andmesophyll cells in barley leaves and these distributions wererelated to shoot nutrient concentrations. Under the growth conditionsused, P was found only in mesophyll vacuoles, never in the epidermis.In contrast, Cl and Ca were located almost exclusively in theepidermis while K and Na were more evenly distributed betweenthe two cell types. The compartmentation of Ca and Cl in theepidermis was maintained over a wide range of tissue concentrationsof these ions. In particular, Cl was excluded from the mesophyllof salt-grown barley until the tissue concentration reachedabout 170 mol m^–3 and then it appeared in the vacuolesof these cells, but only at low concentrations. In contrast,Na was not excluded from the mesophyll of salt-grown or K-deficientbarley and there was evidence that this ion was preferentiallyaccumulated in the mesophyll. Nutrients were evenly distributedbetween the adaxial and abaxial epidermal layers, except K.which was at slightly higher concentrations in the adaxial epidermis.There was considerable variation in the concentrations of ionsin adjacent epidermal cells. The results indicate that intercellularcompartmentation of nutrients occurs in barley leaves and therole of this phenomenon in responses to nutrient deficienciesand salinity is discussed Key words: Compartmentation, barley, salinity, ions, potassium nutrition     

Changes in the Water Balance and Photosynthesis of Onion, Bean and Cotton Plants under Saline Conditions

The osmotic concentration (osmotic potential) of onion leaf sap did not adjust to chloride salinity, and consequently water potential, turgor, stomatal aperture and transpiration were reduced. Although osmotic concentration of bean and cotton leaf sap did adjust to a saline root medium and turgor was no less in the salinized plants than in the controls, stomata of the salinized plants remained only partly open and transpiration was reduced. Net photosynthesis of onion plants was reduced by salinity (this effect being much enhanced in a hot dry atmosphere) but it could be rapidly raised to the level of the controls by inducing elevated leaf turgor. Stomatal closure was initially responsible for most of the ～30 % reduction in photosynthesis of salinized beans. This was due to interference with CO₂ diffusion and could be overcome by raising the CO₂ concentration in the air. At a later stage of growth, salinity affected the light reaction of bean photosynthesis, and elevation of the air CO₂ had little effect. Closure of stomata of salinized cotton plants had only a relatively small effect on net photosynthesis. Light intensity and CO₂ concentration experiments showed that salinity was reducing the photosynthesis of cotton leaves mainly by affecting the light reaction of photosynthesis. It is concluded that chloride salinity does affect the water balance and rate of photosynthesis of plants and that the nature and degree of the effects will depend upon climatic conditions and may be very different between plant species and in the same species at different periods of growth.     

Effect of divalent cations on ion fluxes and leaf photochemistry in salinized barley leaves

Photosynthetic characteristics, leaf ionic content, and net fluxes of Na(+), K(+), and Cl(-) were studied in barley (Hordeum vulgare L) plants grown hydroponically at various Na/Ca ratios. Five weeks of moderate (50 mM) or high (100 mM) NaCl stress caused a significant decline in chlorophyll content, chlorophyll fluorescence characteristics, and stomatal conductance (g(s)) in plant leaves grown at low calcium level. Supplemental Ca(2+) enabled normal photochemical efficiency of PSII (F(v)/F(m) around 0.83), restored chlorophyll content to 80-90% of control, but had a much smaller (50% of control) effect on g(s). In experiments on excised leaves, not only Ca(2+), but also other divalent cations (in particular, Ba(2+) and Mg(2+)), significantly ameliorated the otherwise toxic effect of NaCl on leaf photochemistry, thus attributing potential targets for such amelioration to leaf tissues. To study the underlying ionic mechanisms of this process, the MIFE technique was used to measure the kinetics of net Na(+), K(+), and Cl(-) fluxes from salinized barley leaf mesophyll in response to physiological concentrations of Ca(2+), Ba(2+), Mg(2+), and Zn(2+). Addition of 20 mM Na(+) as NaCl or Na(2)SO(4) to the bath caused significant uptake of Na(+) and efflux of K(+). These effects were reversed by adding 1 mM divalent cations to the bath solution, with the relative efficiency Ba(2+)>Zn(2+)=Ca(2+)>Mg(2+). Effect of divalent cations on Na(+) efflux was transient, while their application caused a prolonged shift towards K(+) uptake. This suggests that, in addition to their known ability to block non-selective cation channels (NSCC) responsible for Na(+) entry, divalent cations also control the activity or gating properties of K(+) transporters at the mesophyll cell plasma membrane, thereby assisting in maintaining the high K/Na ratio required for optimal leaf photosynthesis.     

Exogenous spermidine affects polyamine metabolism in salinity-stressed Cucumis sativus roots and enhances short-term salinity tolerance

We investigated the effects of short-term salinity stress and spermidine application to salinized nutrient solution on polyamine metabolism and various stress defense reactions in the roots of two cucumber (Cucumis sativus L.) cultivars, Changchun mici and Jinchun No. 2. Seedlings grown in nutrient solution salinized with 50 mM NaCl for 8 d displayed reduced relative water content, net photosynthetic rates and plant growth, together with increased lipid peroxidation and electrolyte leakage in the roots. These changes were more marked in cv. Jinchun No. 2 than in cv. Changchun mici, confirming that the latter cultivar is more salinity-tolerant than the former. Salinity stress caused an increase in superoxide and hydrogen peroxide production, particularly in cv. Jinchun No. 2 roots, while the salinity-induced increase in antioxidant enzyme activities and proline contents in the roots was much larger in cv. Changchun mici than in cv. Jinchun No. 2. In comparison to cv. Jinchun No. 2, cv. Changchun mici showed a marked increase in arginine decarboxylase, ornithine decarboxylase, S-adenosylmethionine decarboxylase and diamine oxidase activities, as well as free spermidine and spermine, soluble conjugated and insoluble bound putrescine, spermidine and spermine contents in the roots during exposure to salinity. On the other hand, spermidine application to salinized nutrient solution resulted in alleviation of the salinity-induced membrane damage in the roots and plant growth and photosynthesis inhibition, together with an increase in polyamine and proline contents and antioxidant enzyme activities in the roots of cv. Jinchun No. 2 but not of cv. Changchun mici. These results suggest that spermidine confers short-term salinity tolerance on cucumber probably through inducing antioxidant enzymes and osmoticants.