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An experiment was conducted to investigate the mechanism of the regulation of salicylic acid (SA) and 2,4-epibrassionolide (EBR) on photosynthesis in cucumber (Cucumis sativa) seedlings under low temperature stress. Taking cucumber cultivar 'Youbo1-5' as material, the seedlings were pre-treated with 1 mmol·L -1 SA or 0.1 μmol·L -1 EBR (sprayed once a day), and then were exposed to chilling temperature (10 ℃/5 ℃, PFD 80 μmol·m -2 ·s -1 ) after being pre-treated 2 days. The results showed that the growth and net photosynthetic rate (P n ) of cucumber seedlings were decreased under low temperature stress. However, the P n , maximal photochemical efficiency of PS2 (F v /F m ), actual photochemical efficiency of PS2 (Φ PS 2 ) and photochemical quenching coefficient (q P ) were significantly improved in SA- and EBR-pretreated seedlings, and the increase range of non-photochemical quenching coefficient (NPQ) was decreased. Moreover, the activities of ribulose 1,5-biphosphate carboxylase/oxygenase (Rubisco), sedoheptulose-1,7-bisphosphatase (SBPase), fructose-1,6-bisphosphate aldolase (FBA) and transketolase (TK) were signi-ficantly increased. These findings suggested that SA and EBR improved photosynthesis of cucumber seedlings by promoting the activities of key enzymes and increased low temperature tolerance.

'Jinyan No. 4'cucumber was used as experimental material. The growth of cucumber radicles treated with different concentrations of p-hydroxybenzoic acid (0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 mmol·L -1 ) was investigated firstly, and 10 mmol·L -1 p-hydroxybenzoic acid was chosen for the further experiment. To investigate if melatonin alleviate p-hydroxybenzoic acid stress on cucumber radical elongation, 0, 1, 10, 25, 50, 75 and 100 μmol·L -1 melatonin were used to pretreat cucumber seeds for 24 h before 10 mmol·L -1 p-hydroxybenzoic acid treatment. The results suggested that exogenous melatonin could alleviate the radicle growth inhibition induced by p-hydroxybenzoic acid, and 75 μmol·L -1 melatonin showed the best effect. The growth of cucumber radical was significantly inhibited with an increase in p-hydroxybenzoic acid concentration, which was accompanied with lower activity of amylase. The melatonin pretreatment could significantly increase the activities of amylase and antioxidant enzymes in cucumber seeds resulting in lower accumulation of O 2 -· , H 2 O 2 and MDA. Exogenous melatonin effectively reduced the PHBA stress on cucumber germination, which might be due to its function in decreasing oxidative stress and promoting starch catabolism.

The effects of 24-epibrassinolide (EBR) on antioxidant system and photosynthesis of cucumber seedlings were studied under suboptimal temperature, light and salt environment. Compared with the control, the leaf H2O2 content, lipid peroxidation and cell membrane permeability significantly increased, and the leaf net photosynthetic rate (Pn), stomatal conductance (g(s)), intercellular CO2 concentration (Ci), transpiration rate (Tr) and dry matter accumulation decreased by 39.3%, 40.0%, 21.2%, 47.2% and 35.9% in the suboptimal environment, respectively. Applying EBR could further improve the antioxidant enzyme activity, reduce the H2O2 content and membrane permeability of seedlings, alleviate the drop range of Pn, g(s) and Tr, improve the growth of seedlings and increase the dry matter accumulation by 25.9%. Therefore, EBR treatment could keep higher photosynthetic performance to effectively promote cucumber seedlings growth through adjusting the protective enzyme activity and reducing membrane lipid peroxide level under suboptimal temperature, light and salt environment.

With wheat as the donor plant and cucumber as the receptor plant, this study investigated the effects of root exudates from wheat cultivars with different allelopathic potentials (positive or negative) and companion cropping with wheat on soil fungal community structure by PCR-DGGE method and cucumber growth. Results showed that the wheat root exudates with positive allelopathic potential increased height and stem diameter of cucumber seedlings significantly, compared to the control seedlings (W) after 6 days and 12 days treatment, respectively. Also, wheat root exudates with both positive and negative allelopathic potential increased the seedling height of cucumber significantly after 18 days treatment. The wheat root exudates with different allelopathic potentials decreased the band number, Shannon and evenness indices of soil fungal community significantly in cucumber seedling rhizosphere, and those in the soil with the control seedlings (W) were also significantly higher than that in the control soil without seedlings (Wn) after 6 days treatment. The band number, Shannon and evenness indices in all the treatments were significantly higher than those in the control soil without seedlings (Wn) after 18 days treatment. Companion cropping with negative allelopathic potential wheat decreased the Shannon and evenness indices of soil fungi community significantly in the cucumber seedling rhizosphere, suggesting the wheat root exudates and companion cropping with wheat changed soil fungal community structure in the cucumber seedling rhizosphere. The results of DGGE map and the principal component analysis showed that companion cropping with wheat cultivars with different allelopathic potentials changed soil fungal community structure in cucumber seedling rhizosphere.

The present study evaluated the effects of exogenous silicon on growth and physiological characteristics of hydroponically cultured cucumber seedlings under ammonium stress. The results showed that the growth, especially the aerial part growth of cucumber seedlings cultured with ammonium were significantly inhibited than those with nitrate, especially after treatment for 10 d, the aerial part fresh mass of cucumber seedlings were reduced 6.17 g per plant. The accumulation of reactive oxygen species (ROS) was also promoted in cucumber seedlings under ammonium, and the contents of O2*- and H2O2 were significantly increased in cucumber leaves. With the exogenous silicon treatment, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) were significantly improved, the ability to remove reactive oxygen species was enhanced, the contents of O2*- and H2O2 were significantly reduced in cucumber leaves, decreasing the reactive oxygen damage to the cell membrane, and the ratio of electrolyte leakage and the content of MDA in cucumber leaves. Also, with exogenous silicon treatment, the plasma membrane and activity of vacuolar membrane H(+)-ATP was significantly increased, transport capacity of intracellular proton was improved, and the level of ammonium in cucumber body was significantly reduced, thereby reducing the toxicity of ammonium. In conclusion, exogenous silicon could relieve ammonium stress, by increasing the antioxidant enzyme activity, H(+)-ATP activity, and decreasing the ammonium content in cucumber seedlings.

This paper studied the effects of 2,4-epibrassinolide (EBR) on the photosynthesis and antioxidant system of cucumber seedling leaves under NaCl stress. As compared with the control, NaCl stress increased the leaf superoxide anion production rate, hydrogen peroxide content, malondialdehyde content, and cell membrane permeability while decreased the leaf net photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration significantly, and inhibited the seedlings growth significantly. Applying EBR could increase the activities of leaf superoxide dismutase, peroxidase, and catalase, decrease the leaf superoxide anion production rate, hydrogen peroxide content, malondialdehyde content, and cell membrane permeability, make the leaves keep a higher photosynthetic rate, and thus, promote the seedlings growth, being able to effectively alleviate the damage of NaCl stress.

A water culture experiment with 1/2 Hoagland solution was carried out to study the effects of exogenous 24-epibrassinolide (EBR) on the leaf photosynthetic characteristics and polyamines (PAs) content of cucumber seedlings under hypoxia stress. Under the stress, the leaf net photosynthetic rate (Pn), stomatal conductance (g(s)), transpiration rate (Tr), and intercellular CO2 concentration (Ci) decreased significantly while the chlorophyll content was in adverse, and the seedling growth was inhibited. Hypoxia stress increased the leaf Put, Spd, and Spm contents and Put/PAs ratio significantly, but decreased the (Spd+Spm)/Put. Under the stress, exogenous EBR not only increased the Pn, g(s), Tr, and chlorophyll content significantly, but also increased the contents of free Spm, conjugated Spd and Spm, and bound Put, Spd, and Spm significantly. Meanwhile, the PAs content and (Spd + Spm)/Put ratio were further increased, and the Put/PAs ratio decreased. The results suggested that exogenous EBR could regulate the formation and form transformation of PAs in cucumber seedling leaf, made the leaf keep a higher photosynthetic performance, promote the increase of leaf area and dry mass, and alleviate the damage of hypoxia stress.

Based on the salt-tolerance identification of a series of cucumber rootstock varieties, the cucumber cultivar 'Xintaimici' grafted on different salt-tolerant rootstock varieties 'Sherpa', 'Shintosa' , 'Tielizhen' , and 'Figleaf gourd' was selected to study its seedlings physiological responses to NaCl stress, taking the self-rooted ones as the control. Under the stress of 100 mmol NaCl x L(-1), the leaf electrolyte leakage rate and malondialdehyde (MDA) content of the grafted seedlings were significantly lower than those of self-rooted seedlings, and the seedlings grafted on 'Figleaf gourd' showed the lowest electrolyte leakage rate and malondialdehyde (MDA) content, followed by the seedlings grafted on 'Tielizhen', 'Shintosa' , and 'Sherpa'. The leaf proline and soluble sugar contents and peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) activities of the grafted seedlings were significantly higher than those of the self-rooted seedlings, and the POD, SOD, CAT, and APX activities were the highest for the seedlings grafted on 'Figleaf gourd' and the lowest for the seedlings grafted on 'Sherpa', but had no significant differences for the seedlings grafted on 'Tielizhen' and 'Shintosa'. The leaf Na+ content of the seedlings grafted on different rootstock varieties ranked as 'Figleaf gourd' < 'Tielizhen' < 'Shintosa' < 'Sherpa', while the leaf K+ content had little difference for the seedlings grafted on 'Figleaf gourd', 'Tielizhen', and 'Shintosa' but was significantly higher than that for the seedlings grafted on 'Sherpa'. The self-rooted seedlings had the highest leaf Na+ content but the lowest leaf K+ content. The leaf Na+/K+ ratio of grafted seedlings was significantly lower than that of self-rooted ones, and the seedlings grafted on 'Figleaf gourd' had the lowest leaf Na+/K+ ratio.

Taking cucumber cultivar 'Jinchun 4' as test material, and by the method of foliar spraying, this paper studied the effects of exogenous melatonin (MT) on the ascorbic acid (AsA) metabolism system in cucumber seedlings under high temperature stress. Under the stress of high temperature, the leaf hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents of the cucumber seedlings increased obviously, ascorbic acid (AsA) and glutathione (GSH) contents had a persistent decrease while dehydroascorbate (DHA) and oxidized-glutathione (GSSG) contents had a gradual increase, and AsA/DHA and GSH/GSSG decreased greatly. The activities of ascorbate peroxidase (APx), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) in the seedling leaves under high temperature stress increased markedly, with the maximum after 12 h stress. Foliar spraying MT could effectively restrain the accumulation of H2O2 and MDA in seedling leaves, increase the leaf AsA and GSH contents and the activities of ascorbic acid metabolizing enzymes APx, MDHAR, GR and DHAR, and accordingly, enhance the H2O2-scavenging ability, inhibit the production of active O2, maintain the stability of cell membrane, reduce the damage of high temperature to the plants, and improve the ability of cucumber seedlings against high temperature stress.

Taking the seedlings of cucumber cultivar 'Jinchun 4' as test material, this paper studied the effects of foliar application of exogenous melatonin (MT) on the active oxygen metabolism of the seedlings under high temperature stress. Under the stress, the exogenous MT could significantly decrease the leaf superoxide radical (O2-*) production rate, hydrogen peroxide (H2O2) content, cell membrane permeability (electrolyte leakage), and malonaldehyde (MDA) content, but increase the activities of superoxide radical (SOD), peroxidase (POD), and catalase (CAT), and the contents of ascorbic acid (AsA), glutathione (GSH), and soluble proteins, illustrating that pretreatment with MT could inhibit the reactive oxygen species (ROS) production, raise the antioxidative enzyme activities and antioxidant contents, and decrease the membrane lipid peroxidation and electrolyte leakage to protect the integrity of lipid membranes, being able to alleviate the damage of high temperature stress to the seedlings.

By the method of hydroponic culture, this paper studied the alleviation effects of LaCl3 on the photosynthetic characteristics of cucumber seedlings under nitrate (140 mmol NO3(-) x L(-1)) stress, with the related mechanisms discussed. Under nitrate stress, the seedlings leaf chlorophyll and carotenoids contents decreased significantly, and the leaf Mg2+-ATPase, Ca2+-ATPase activities also decreased. On the 7th day of nitrate stress, the decrease of seedlings photosynthetic rate was mainly due to stomatal limitation; but on the 12th day of nitrate stress, the decrease was mainly due to no-stomatal limitation. Supplement with LaCl3 could make the cucumber seedlings keep relatively higher leaf Mg2+-ATPase and Ca2+-ATPase activities and chlorophyll and carotenoids contents, and applying 20 micromol x L(-1) of LaCl3 could increase the carotenoids content significantly. LaCl3 could also improve the leaf gas exchange, and alleviate the decrease of leaf Fv/Fm, PhiPSII, AQY, CE, and qp under nitrate stress, which helped the leaves making good use of light energy and maintaining higher CO2 assimilation capacity. An additional 20 micromol x L(-1) of LaCl3 could alleviate the nitrate stress on the photosynthesis of cucumber seedlings efficiently, but an additional 200 micromol x L(-1) of LaCl3 only had the alleviation effect at the initial period of nitrate stress. Our results could benefit to the improvement of greenhouse soil.

By a solution culture experiment, cucumber seedlings were treated with different concentration (0, 25, 50 and 75 mmol x L(-1)) NaCl and NaHCO3, and their physiological responses were studied. The results showed that with increasing NaCl and NaHCO3 stress, the shoot and root growth, the leaf chlorophyll content and relative water content of the seedlings decreased dramatically, and the decrements were greater under NaHCO3 than under NaCl stress. The shoot Na content increased significantly with increasing NaCl and NaHCO, stress, while shoot K+ content was in adverse. At the same concentration of Na+ in culture solution, the addition of NaHCO3 induced a greater decrement of shoot K+ content, and thus, a lower shoot K+/Na+ ratio. Under NaHCO3 stress, the leaf electrolyte leakage rate and the leaf MDA, proline, and soluble sugar contents increased much greater, compared with those under NaCl stress. Under NaCl and NaHCO3 stress, the leaf superoxide dismutase (SOD), ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) activities were induced significantly, while the leaf peroxidase (POD) activity was inhibited obviously.

The study with water culture showed that nitrate stress had significant inhibitory effects on the plant growth, especially the shoot growth of cucumber seedlings. On the 7th day under nitrate stress, the shoot fresh mass per plant decreased by 12.77 g, leaf SOD, POD and CAT activities increased, while leaf APX, DHAR and GR activities decreased significantly, compared with the control. The addition of low concentration (0.05 mmol L(-1)) LaCl3 increased the shoot fresh mass per plant by 35%, enhanced the leaf thermally stable protein content and SOD, POD, CAT, APX, DHAR and GR activities, and decreased the leaf MDA content and electrolytic leakage, resulting in a definite alleviation of the inhibitory effects of nitrate. However, an addition of high concentration (0.5 mmol L(-1)) LaCl3 had no obvious alleviation effect. It was concluded that the addition of certain concentration LaCl3 could alleviate the nitrate stress on cucumber via increasing anti-oxidative enzymes activities and thermally stable protein content.

Under the conditions of root substrate injection and foliar spray of different concentration spermidine, this paper measured the disease index, defense enzyme activities, and contents of disease resistance-related substances in cucumber (Cucumis sativus L.) seedlings infected by Botrytis cinerea Pers., aimed to investigate the roles of spermidine in induced resistance of cucumber seedlings to B. cinerea. The results showed that after treated with spermidine, the disease index decreased by 25.4%, phenylalanine ammonia lyase (PAL), peroxidase (POD) and polyphenol oxidase (PPO) activities increased by 48.4%, 635.9% and 38.9%, respectively, and total polyphenol, flavonoid and lignin contents increased significantly, compared with the control. All of these suggested that spermidine could induce the disease resistance of cucumber seedlings, and its optimum concentration was 150 mg L(-1).

By the method of water culture, this paper studied the effects of NaCl stress on the seedlings root growth and polyamine metabolism of two cucumber cultivars 'Changchun mici' and 'Jin-chun No. 2'. The results showed that under NaCl stress, root growth was inhibited, and lipid peroxidation and electrolyte leakage increased. These changes were more obvious in salt-sensitive 'Jin-chun No. 2' than in salt-tolerant 'Changchun mici'. NaCl stress caused an increase of arginine decarboxylase (ADC), ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC) activities in cucumber roots, which was much greater in 'Changchun mici' than in 'Jinchun No. 2'. At the highest value, ADC, ODC and SAMDC activities increased by 149.3%, 60.1% and 69.4% in 'Changchun mici' and 118.6%, 56.2% and 50.6% in 'Jinchun No. 2', respectively, in comparison with the control. Diamine oxidase activity increased in 'Changchun mici' but not in 'Jinchun No. 2', and the increment of polyamine oxidase activity in 'Changchun mici' was smaller than that in 'Jinchun No. 2'. As a result, the contents of free spermidine and spermine and of conjugated and bound polyamines in 'Changchun mici' increased significantly, while that of free putrescine had a significant increase in 'Jinchun No. 2'. It could be concluded that higher levels of free spermidine and spermine and of conjugated and bound polyamines and lower level of free putrescine in roots could improve the adaptability of cucumber seedlings to salt stress.

With a Japanese salt-tolerant pumpkin cultivar 'King Shintosa' (Cucurbita maxima x C. moschata) as rootstock and a cucumber (Cucumis sativus) cultivar 'Xintaimici' as cion, this paper studied the temporal dynamics of different form polyamines contents in the leaves of grafted and own-rooted cucumber plants under 100 mmol x L(-1) NaCl stress. The results showed that the free putrescence (Put) content of graftedplant leaves (G2) was significantly higher than that of own-rooted plant leaves (O2), except on the second day of NaCl stress. During the whole period of NaCl stress, the free spermidine (Spd) and spermine (Spm) contents of G2 were significantly higher than those of O2, and the total free polyamine content of G2 peaked on the fourth day of stress. The free Put/PAs value of G2 was significantly lower than that of O2 except on the fourth day of stress, but (Spd + Spm)/Put value was significantly higher than that of O2 during the whole period of stress. In the test period, the contents of conjugated and bound Put, Spd, Spm of G2 were significantly higher than those of O2. The total conjugated and bound polyamine contents of G2 peaked on the sixth day of stress. Both conjugated Put/PAs and conjugated (Spd + Spm)/Put values had a similar changing trend to free polyamine. The bound Put/PAs value of G2 was significantly lower than that of O2 except on the sixth day of stress, while the bound (Spd + Spm)/Put value was significantly higher than that of O2 during the whole period of stress. All of these demonstrated that grafted cucumber plant possessed a stronger salt tolerance.

With root injection and foliar spray, this paper studied the effects of different concentrations salicylic acid, brassinolide, chitosan and spermidine on the growth, morphogenesis, and physiological and biochemical characters of cucumber ( Cucumis sativus L. ) seedlings under 200 mmol x L(-1) NaCl stress. The results showed that at proper concentrations, these four exogenous chemical activators could markedly decrease the salt stress index and mortality of cucumber seedlings, and the decrement induced by 0. 01 mg x L (-1) brassinolide was the largest, being 63. 0% and 75. 0% , respectively. The activities of superoxide dismutase (SOD) , peroxidase (POD) and catalase (CAT) increased significantly, resulting in a marked decrease of malondialdehyde (MDA) content and electrolyte leakage. The dry weight water content and morphogenesis of cucumber seedlings improved, and the stem diameter, leaf number, and healthy index increased significantly. All of these suggested that exogenous chemical activators at proper concentrations could induce the salt resistance of cucumber, and mitigate the damage degree of salt stress. The salt resistance effect of test exogenous chemical activators decreased in the sequence of 0.005 -0.05 mg (L-1) brassinolide, 150 -250 mg x L (-1) spermidine, 100 -200 mg x L(-1) chitosan, and 50 -150 mg x L(-1) salicylic acid.

By a solution culture experiment, this paper studied the effects of excess Mn on the growth, chlorophyll content, chlorophyll fluorescence parameters and photosynthesis of cucumber under different light intensity. The results indicated that excess Mn inhibited plant growth, which was more obvious under high light intensity than under low light intensity. The primary maximum photochemical efficiency of PSII (v/Fm), quantum efficiency of non-cyclic electron transport of PSII (phiPSII), and photochemical quenching (qP) were significantly decreased in excess Mn treatment under high light intensity, while no significant effects on Fv/Fm and qP were observed under low light intensity. Excess Mn, particularly under high light intensity, decreased net photosynthetic rate (Pn) and stomatal conductance (Gs). Excess Mn increased intracellular CO2 (Ci) under high light intensity and decreased Ci under low light intensity, while stomatal limitation value (Ls) was just reverse to Ci. It could be concluded that the decrease of Pn in excess Mn treatment was not resulted from stomatal limitation under high light intensity, but was true under low light intensity.

Studies on the effects of different concentrations of triadimefon on cucumber seedlings growth and their resistance to chilling injury showed that 20-100 mumol.L-1 triadimefon inhibited the shoot or root length of cucumber seedlings, and increased their root weight or root/shoot ratio and chlorophyll content significantly. After at low temperature for 3 days, a higher chlorophyll content, lower membrane relative permeability and higher SOD activity were observed in the leaves of cucumber seedlings soaked with 10-50 mumol.L-1 triadimefon. 50 mumol.L-1 triadimefon could retard the accumulation of MDA at low temperature stress. It was also observed that 50 mumol.L-1 triadimefon enhanced the accumulation of praline at the first day of chilling treatment. However, higher concentrations(> 100 mumol.L-1) of triadimefon were unfavorable for cucumber seedlings to defense against low temperature. It was thus concluded that triadimefon protected the cucumber seedlings against low temperature.