Freezing tolerance of citrus, spinach, and petunia leaf tissue : osmotic adjustment and sensitivity to freeze induced cellular dehydration

Seasonal variations in freezing tolerance, water content, water and osmotic potential, and levels of soluble sugars of leaves of field-grown Valencia orange (Citrus sinensis) trees were studied to determine the ability of citrus trees to cold acclimate under natural conditions. Controlled environmental studies of young potted citrus trees, spinach (Spinacia pleracea), and petunia (Petunia hybrids) were carried out to study the water relations during cold acclimation under less variable conditions. During the coolest weeks of the winter, leaf water content and osmotic potential of field-grown trees decreased about 20 to 25%, while soluble sugars increased by 100%. At the same time, freezing tolerance increased from lethal temperature for 50% (LT₅₀) of −2.8 to −3.8°C. In contrast, citrus leaves cold acclimated at a constant 10°C in growth chambers were freezing tolerant to about −6°C. The calculated freezing induced cellular dehydration at the LT₅₀ remained relatively constant for field-grown leaves throughout the year, but increased for leaves of plants cold acclimated at 10°C in a controlled environment. Spinach leaves cold acclimated at 5°C tolerated increased cellular dehydration compared to nonacclimated leaves. Cold acclimated petunia leaves increased in freezing tolerance by decreasing osmotic potential, but had no capacity to change cellular dehydration sensitivity. The result suggest that two cold acclimation mechanisms are involved in both citrus and spinach leaves and only one in petunia leaves. The common mechanism in all three species tested was a minor increase in tolerance (about −1°C) resulting from low temperature induced osmotic adjustment, and the second in citrus and spinach was a noncolligative mechanism that increased the cellular resistance to freeze hydration.     

Effect of exogenous spermidine on polyamine metabolism in water hyacinth leaves under mercury stress

The influence of exogenous spermidine (Spd) on arginine decarboxylase (ADC), ornithine decarboxylase (ODC), polyamine oxidase (PAO) activities and polyamines (PAs), proline contents in water hyacinth leaves under Mercury (Hg) stress was investigated after 6 days treatment. The results showed that free putrescine (Put) content increased, the contents of free spermidine (Spd) and spermine (Spm) and the (Spd + Spm)/Put ratio in water hyacinth leaves decreased significantly with the increase of the Hg concentrations. Hg stress also disturbed the activities of ADC, ODC and PAO and caused changes on proline content. Compared to the Hg-treatment only, exogenous Spd (0.1 mM) significantly reduced the accumulation of free Put, increased the contents of free Spd and Spm and the ratio of (Spd + Spm)/Put in water hyacinth leaves. Furthermore, exogenous Spd enhanced the activities of ADC, ODC and PAO and significantly increased proline content. The PS-conjugated PAs and PIS-bound PAs changed in the same trend as free PAs. These results suggest that exogenous Spd can alleviate the metabolic disturbance of polyamines caused by Hg in water hyacinth leaves.     

Effect of cold acclimation on bulk tissue electrical impedance: I. Measurements with birdsfoot trefoil at subfreezing temperatures

The resistive and reactive components of electrical impedance were measured for birdsfoot trefoil (Lotus corniculatus L.) stems at freezing temperatures to −8°C. As temperature decreased the specific resistance at frequencies between 49 hertz and 1.11 megahertz of stems from cold acclimated plants increased more rapidly than from nonacclimated plants. This temperature dependence of specific resistance could be characterized by an Arrhenius activation energy; cold acclimated stems had a larger Arrhenius activation energy than nonacclimated stems. The low frequency resistance is believed to characterize the extracellular region of the stems and the high frequency resistance is believed to characterize the intracellular region of the stems. Cold acclimation increased the intracellular but not the extracellular resistance at nonfreezing temperatures. Cold acclimated stems were not injured by freezing to −8°C and thawing, but nonacclimated stems were injured by freezing to temperatures between −2.2 and −5.6°C and thawing. Injury to nonacclimated stems at freezing temperatures below −2.2°C was indicated by a decrease in the ratio of resistance at 49 Hz to that at 1.11 megahertz.     

Exogenous polyamines enhance copper tolerance of Nymphoides peltatum

The protective effects of polyamines (PAs) against copper (Cu) toxicity were investigated in the leaves of Nymphoides peltatum. Cu treatment increased the putrescine (Put) level and lowered spermidine (Spd) and spermine (Spm) levels, thereby reducing the (Spd+Spm)/Put ratio in leaves. Exogenous application of Spd or Spm markedly reversed these Cu-induced effects for all three PAs and partially restored the (Spd+Spm)/Put ratio in leaves. It also significantly enhanced the level of proline, retarded the loss of soluble protein, decreased the rate of O2*- generation and H2O2 content, and prevented Cu-induced lipid peroxidation. Furthermore, exogenous Spd and Spm reduced the accumulation of Cu and effectively maintained the balance of nutrient elements in plant leaves under Cu stress. These results suggest that exogenous application of Spd or Spm can enhance the tolerance of N. peltatum to Cu by increasing the levels of endogenous Spd and Spm as well as the (Spd+Spm)/Put ratio.     

Effect of cold acclimation on intracellular ice formation in isolated protoplasts

When cooled at rapid rates to temperatures between −10 and −30°C, the incidence of intracellular ice formation was less in protoplasts enzymically isolated from cold acclimated leaves of rye (Secale cereale L. cv Puma) than that observed in protoplasts isolated from nonacclimated leaves. The extent of supercooling of the intracellular solution at any given temperature increased in both nonacclimated and acclimated protoplasts as the rate of cooling increased. There was no unique relationship between the extent of supercooling and the incidence of intracellular ice formation in either nonacclimated or acclimated protoplasts. In both nonacclimated and acclimated protoplasts, the extent of intracellular supercooling was similar under conditions that resulted in the greatest difference in the incidence of intracellular ice formation—cooling to −15 or −20°C at rates of 10 or 16°C/minute. Further, the hydraulic conductivity determined during freeze-induced dehydration at −5°C was similar for both nonacclimated and acclimated protoplasts. A major distinction between nonacclimated and acclimated protoplasts was the temperature at which nucleation occurred. In nonacclimated protoplasts, nucleation occurred over a relatively narrow temperature range with a median nucleation temperature of −15°C, whereas in acclimated protoplasts, nucleation occurred over a broader temperature range with a median nucleation temperature of −42°C. We conclude that the decreased incidence of intracellular ice formation in acclimated protoplasts is attributable to an increase in the stability of the plasma membrane which precludes nucleation of the supercooled intracellular solution and is not attributable to an increase in hydraulic conductivity of the plasma membrane which purportedly precludes supercooling of the intracellular solution.     

Effect of cold acclimation on the incidence of two forms of freezing injury in protoplasts isolated from rye leaves

The freezing tolerance and incidence of two forms of freezing injury (expansion-induced lysis and loss of osmotic responsiveness) were determined for protoplasts isolated from rye leaves (Secale cereale L. cv Puma) at various times during cold acclimation. During the first 4 weeks of the cold acclimation period, the LT₅₀ (i.e. the minimum temperature at which 50% of the protoplasts survived) decreased from −5°C to −25°C. In protoplasts isolated from nonacclimated leaves (NA protoplasts), expansion-induced lysis (EIL) was the predominant form of injury at the LT₅₀. However, after only 1 week of cold acclimation, the incidence of EIL was reduced to less than 10% at any subzero temperature; and loss of osmotic responsiveness was the predominant form of injury, regardless of the freezing temperature. Fusion of either NA protoplasts or protoplasts isolated from leaves of seedlings cold acclimated for 1 week (1-week ACC protoplasts) with liposomes of dilinoleoylphosphatidylcholine also decreased the incidence of EIL to less than 10%. Fusion of protoplasts with dilinoleoylphosphatidylcholine diminished the incidence of loss of osmotic responsiveness, but only in NA protoplasts or 1-week ACC protoplasts that were frozen to temperatures over the range of -5 to -10°C. These results suggest that the cold acclimation process, which results in a quantitative increase in freezing resistance, involves several different qualitative changes in the cryobehavior of the plasma membrane.     

多胺与香蕉抗寒性的关系的研究

人工气候箱中模拟寒潮对香蕉苗造成低温伤害,低温胁迫前用1 mmol·L-1多胺(PAs)和D-精氨酸(D-Arg)喷洒香蕉叶片。结果表明,低温胁迫后香蕉叶片内源腐胺(Put)含量下降,亚精胺(Spd)含量明显增多,精胺(Spm)比较稳定;D-Arg处理的内源多胺总量明显降低。外源Spd和Spm可以提高受冷胁迫的香蕉叶片中过氧化物酶(POD)活性、降低电解质渗漏率、增加可溶性糖和脯氨酸的含量,有助于提高香蕉的抗寒力;Put对香蕉抗寒力没有明显影响;D-Arg则有不利作用。     

外源多胺对铜胁迫下水鳖叶片多胺代谢、抗氧化系统和矿质营养元素的影响

采用营养液水培的方法,研究了外源亚精胺(Spd)和精胺(Spm)对Cu胁迫下水鳖叶片3种形态多胺(PAs)、抗氧化系统及营养元素的影响。结果表明:(1)Cu胁迫使水鳖叶片腐胺(Put)急剧积累,Spd和Spm明显下降,从而使(Spd+Spm)/Put比值也随之下降。外源Spd和Spm显著或极显著逆转Cu诱导的PAs变化,抑制Put的积累,缓解Spd和Spm的下降,从而提高了(Spd+Spm)/Put比值。(2)外源Spd和Spm抑制了Cu胁迫诱导的多胺氧化酶(PAO)的增加,缓解了二胺氧化酶(DAO)的下降。(3)与单一Cu胁迫相比,Spd和Spm显著或极显著提高了超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX)和谷胱甘肽还原酶(GR)活性和抗坏血酸(AsA)、谷胱甘肽(GSH)、游离脯氨酸(Pro)含量,从而降低了超氧阴离子(O2.-)产生速率和过氧化氢(H2O2)含量,极显著降低了丙二醛(MDA)含量,缓解了Cu诱导的氧化胁迫。(4)外源Spd和Spm显著或极显著缓解了Cu胁迫下矿质营养元素吸收平衡的紊乱。以上结果均说明了外施Spd和Spm可增加水鳖对Cu胁迫的耐受性。     

Temperature response of plasma membranes in tuber-bearing solanum species

Permeability coefficients (Kp) of nonelectroytes were determined at several temperatures for nonacclimated and acclimated plasma membranes of the frost sensitive Solanum tuberosum and the frost resistant Solanum commersonii. In nonacclimated membranes, Kp were equal at 25°C for the two species. Kp decreased with decreased temperature in both species; however, the decrease was much greater in the frost sensitive S. tuberosum than in frost resistant S. commersonii.

Kp increased with cold acclimation. After acclimation the temperature sensitivity of Kp or activation energy (Ea) for S. tuberosum was similar to that of S. commersonii; however, Kp for S. tuberosum were lower than those for S. commersonii at all temperatures.

The differences in Kp and activation energy indicate plasma membrane differences between the species before acclimation. After acclimation there was still a difference between the plasma membranes of the two species as well as a difference between the nonacclimated and acclimated membranes of the same species.

     

6-BA+B9对龙眼花芽生理分化期内源多胺含量变化的影响

在龙眼花芽生理分化期喷施6-BA 200 mg/L+B₉ 2000 mg/L,跟踪测定其内源多胺含量变化。研究结果表明,龙眼花芽生理分化期内源腐胺(Put)、亚精胺(Spd)和精胺(Spm)含量均出现积累,喷施6-BA+B₉能明显提高内源多胺含量,特别是Put和Spd的含量。内源多胺与花芽分化有一定的内在联系,生长调节剂对花芽分化的调控有可能通过多胺来实现或协同作用。     

The role of ABA in freezing tolerance and cold acclimation in barley

The role of ABA in freezing resistance in nonacclimated and cold‐acclimated barley ( Hordeum vulgare L.) was studied. Eleven nonacclimated cultivars differed in their LT₅₀, ranging from −10.8 to −4.8°C. Sugars, free proline, soluble proteins and ABA were analyzed in nonacclimated cultivars and during cold acclimation of one cultivar. There was an inverse correlation between LT₅₀ and both ABA and sucrose contents. Exogenous ABA caused a decrease in the freezing point of leaf tissue in the cultivar with the lowest level of endogenous ABA, but not in the cultivar with the highest level, suggesting that ABA in the latter may be near the optimum endogenous level to induce freezing tolerance. Plants of cv. Aramir treated with ABA or allowed to acclimate to cold temperature increased their soluble sugar content to a similar level. The LT₅₀ of leaves of cold‐acclimated cv. Aramir decreased from −5.8 to −11.4°C, with biphasic kinetics, accumulating proline and soluble sugars with similar kinetics. The biphasic profile observed during cold acclimation could be a direct consequence of cryoprotectant accumulation kinetics. ABA and soluble protein accumulation showed a single step profile, associated mainly with the second phase of the LT₅₀ decrease. Thus, a significant increase in endogenous ABA is part of the response of barley to low temperature and may be required as a signal for the second phase of cold acclimation. Endogenous ABA contents in the nonacclimated state may determine constitutive freezing tolerance.     

Stress-dependent accumulation of spermidine and spermine in the halophyte Mesembryanthemum crystallinum under salinity conditions

We studied the effects of chloride salinity (300 and 500 mM NaCl) on the content of free polyamines (PAs) from putrescine (Put) family in Mesembryanthemum crystallinum L. leaves and roots. The contents of Put and spermidine (Spd) in leaves increased temporarily, achieving the highest values on the third day of salinity treatment; thereafter (by days 7–14), they dropped sharply. The content of spermine (Spm) increased gradually, and its high level was maintained until the end of experiment. The dynamics of Spm accumulation in leaves under salinity conditions resembled that of phosphoenolpyruvate carboxylase (PEPC), a key enzyme of the water-saving CAM pathway of photosynthesis. This indicates indirectly the involvement of Spm in the common ice plant adaptation to salinity. A decrease in the molar ratios of Spd to Spm in the leaves under salinity conditions could point to the acceleration of Spm biosynthesis (accumulation) during plant adaptation, whereas the levels of Spm precursors, Put and Spd, did not increase. This phenomenon could be explained by an accelerated conversion of Spd into Spm, an active liberation of free Spm from its conjugates, or changes in the rates of studied PA biosynthesis and degradation under salinity. At the same time, the intracellular concentration of ethylene rose under these conditions. It was supposed and then demonstrated, that the pathway of ethylene biosynthesis and that of the synthesis of Put family PAs compete under severe salinity conditions. This competition might be based on the disturbances in sulfur metabolism and a decrease in the methionine content, an immediate precursor of S-adenosyl-L-methionine.     

Effect of spermine treatment on the functioning of Thellungiella salsuginea antioxidant system

The effect of spermine (Spm) treatment on the content of polyamines (PAs) and activities of antioxidant enzymes in the roots and leaves of Thellungiella salsuginea (Pall.) O.E. Schulz plants grown under optimal conditions were studied. The genes encoding three forms of ascorbate peroxidase (APX; APX1, APX2, and APX4) and genes of key enzymes of proline metabolism (Pro, P5CS1, 1P5CD) were identified, and their expression intensity was measured. Six-day-old plants were treated with Spm (1 and 2 mM) and with the inhibitor of polyamine oxidase (PAO) activity, N,N-(2-hydroxyethyl)hydrazine (HEH, 1 and 2 mM) separately or in combination by adding these compounds to nutrient medium. Roots and leaves responded differently to Spm treatment. In the leaves, the content of PAs reduced due to a decreased in the spermidine (Spd) content, whereas in the roots the total pool of PAs increased due to putrescine (Put) and Spd accumulation. Treatment with Spm activated PAO in the roots but not in the leaves; HEH removed this increase, but the intercellular Spm concentration was not substantially changed. It was suggested that treatment with Spm suppressed the biosynthesis of intracellular Spm and, on the other hand, stimulated the reverse conversion of Spm into Spd and further into Put due to the activation of one of the PAO isoforms. Plant treatment with Spm was not accompanied by a noticeable activation enzymes degrading hydrogen peroxide in the roots: APX, (except of peroxidase II), and catalase. However, the activity of Cu/Zn-SOD doubled and the activity of Mn-SOD reduced. In the leaves, slight activation of peroxidases I and III, the inhibition of Cu/Zn- and Mn-SOD, differential changes in the time-coursed of gene expression of three APX isoforms, and activated gene expression of key enzymes of Pro metabolism were observed. At the same time, the level of MDA did not increase either in the leaves or in the roots. This indicates that treatment of Th. salsuginea plants with Spm under optimal growing conditions did not enhance ROS generation and did not manifest prooxidant properties.     

Induction of Freezing Tolerance in Spinach during Cold Acclimation

Spinach (Spinacia oleracea L.) seedlings, grown in soil or on an agar medium in vitro, became cold acclimated when exposed to a constant 5°C. Plants subjected to cold acclimation, beginning 1 week postgermination, attained freezing tolerance levels similar to that achieved by seedlings that were cold acclimated beginning 3 weeks after sowing. Seedlings at 1 week of age had only cotyledonary leaves, while 3-week-old seedlings had developed true leaves. Plants grown in vitro were able to increase in freezing tolerance, but were slightly less hardy than soil-grown plants. These results suggest that spinach, a cool-season crop that begins growth in early spring when subzero temperatures are likely, can undergo cold acclimation at the earliest stages of development following germination. Axenic seedlings, grown in vitro, were used to develop a noninjurious radiolabeling technique. Leaf proteins were radiolabeled to specific activities of 10⁵ counts per minute per microgram at 25°C or 5 × 10⁴ counts per minute per microgram at 5°C over a 24 hour period. The ability to radiolabel leaf proteins of in vitro grown plants to high specific activities at low temperature, without injury or microbial contamination, will facilitate studies of cold acclimation.     

Induction of freezing tolerance in spinach is associated with the synthesis of cold acclimation induced proteins

Spinach (Spinacia oleracea L. cv Bloomsdale) seedlings cultured in vitro were used to study changes in protein synthesis during cold acclimation. Seedlings grown for 3 weeks postsowing on an inorganic-nutrient-agar medium were able to increase their freezing tolerance when grown at 5°C. During cold acclimation at 5°C and deacclimation at 25°C, the kinetics of freezing tolerance induction and loss were similar to that of soil-grown plants. Freezing tolerance increased after 1 day of cold acclimation and reached a maximum within 7 days. Upon deacclimation at 25°C, freezing tolerance declined within 1 day and was largely lost by the 7th day. Leaf proteins of intact plants grown at 5 and 25°C were in vivo radiolabeled, without wounding or injury, to high specific activities with [³⁵S]methionine. Leaf proteins were radiolabeled at 0, 1, 2, 3, 4, 7, and 14 days of cold acclimation and at 1, 3, and 7 days of deacclimation. Up to 500 labeled proteins were separated by two-dimensional gel electrophoresis and visualized by fluorography. A rapid and stable change in the protein synthesis pattern was observed when seedlings were transferred to the low temperature environment. Cold-acclimated leaves contained 22 polypeptides not found in nonacclimated leaves. Exposure to 5°C induced the synthesis of three high molecular weight cold acclimation proteins (CAPs) (M_r of about 160,000, 117,000, and 85,000) and greatly increased the synthesis of a fourth high molecular weight protein (M_r 79,000). These proteins were synthesized during day 1 and throughout the 14 day exposure to 5°C. During deacclimation, the synthesis of CAPs 160, 117, and 85 was greatly reduced by the first day of exposure to 25°C. However, CAP 79 was synthesized throughout the 7 day deacclimation treatment. Thus, the induction at low temperature and termination at warm temperature of the synthesis of CAPs 160, 117, and 85 was highly correlated with the induction and loss of freezing tolerance. Cold acclimation did not result in a general posttranslational modification of leaf proteins. Most of the observed changes in the two-dimensional gel patterns could be attributed to the de novo synthesis of proteins induced by low temperature. In spinach leaf tissue, heat shock altered the pattern of protein synthesis and induced the synthesis of several heat shock proteins (HSPs). One polypeptide synthesized in cold-acclimated leaves had a molecular weight and net charge (M_r 79,000, pI 4.8) similar to that of a HSP (M_r 83,000, pI 4.8). However, heat shock did not increase the freezing tolerance, and cold acclimation did not increase heat tolerance over that of nonacclimated plants, but heat-shocked leaf tissue was more tolerant to high temperatures than nonacclimated or cold-acclimated leaf tissue. When protein extracts from heat-shocked and cold-acclimated leaves were mixed and separated in the same two-dimensional gel, the CAP and HSP were shown to be two separate polypeptides with slightly different isoelectric points and molecular weights.     

Accumulation of Free Proline in Citrus Leaves during Cold Hardening of Young Trees in Controlled Temperature Regimes

Free proline increased in leaves of orange (Citrus sinensis [L.] Osb. cv. Valencia) and grapefruit (Citrus paradisi Macfad. cv. Star Ruby) trees on a wide range of citrus rootstocks during cold hardening. Increases in sugars accompanied proline accumulation. During cold hardening, the rate of proline accumulation was greater in old than in young leaves. In leaves of grapefruit trees kept in the dark during cold hardening, neither proline nor sugars increased and the degree of cold hardiness was less than in trees exposed to light. Like sugar accumulations, proline accumulation does not reflect specific degrees of cold hardiness in citrus cultivars.     

水分胁迫下山黧豆多胺代谢与β_N_草酰_L_α,β_二氨基丙酸积累相关性的研究

Seedlings of grass pea (Lathyrus sativus L.) were treated with polyethylene glycol (PEG), in addition with putrescine (Put) and α-difluoromethylarginine (DFMA) in order to study the relation between polyamine metabolism and accumulation of β-N-oxalyl-L-α, β-diaminopropionic acid (ODAP) under water stress. The results indicated that Put, spermidine (Spd) and spermine (Spm) contents of leaves gradually increased along with the extension of PEG-treatment time. Especially, the increase in Spm contents was significant together with a gradual increase of ODAP contents. Put and Spd contents of leaves markedly increased along with the extension of PEG+Put treatment time. There was little effect on Spm contents and ODAP contents in leaves with PEG+Put treatment. The accumulation of Put, Spd, Spm and ODAP in leaves was inhibited by PEG+DFMA treatment. The inhibition of DFMA for Put and Spd biosynthesis was partially overcome by adding exogenous Put, but little effect for the inhibition of DFMA on Spm biosynthesis by adding exogenous Put. The accumulation of ODAP was inhibited with PEG+DFMA+Put treatment in leaves. This indicated that there was a relationship between the increase of Spm contents and the accumulation of ODAP in leaves under water stress.     

‘神马’菊花花芽分化与内源多胺的关系

用薄层层析-荧光法测定菊花'神马'花芽分化期间顶芽和叶片中多胺的动态变化,分析了菊花花芽分化与多胺的关系.结果表明,花芽分化起始期顶芽中的腐胺(Put)含量急剧下降,此后在低水平上波动;叶片内Put则于总苞鳞片分化初期大幅上升,其后各阶段处于较低的水平.顶芽中精胺(Spm)与亚精胺(Spd)含量呈平行波动上升趋势,顶芽中Spin在小花原基分化初期直到花冠分化中期处于优势地位,而顶芽中Spd并无明显变化.顶芽、叶片中的Spin变化趋势相反,顶芽中Spm、Spd的含量变化趋势十分相似,但叶片中却呈交替性变化.结果显示,菊花花芽分化过程中,Put含量的降低有利于启动菊花花芽分化,后期Spm的增加有利于小花的分化,叶片可能向顶芽提供Spm,顶芽和叶片中的Spd与小花原基分化有密切关系.     

External spermine prevents UVA-induced damage of <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 via increased catalase activity and decreased H<Subscript>2</Subscript>O<Subscript>2</Subscript> and malonaldehyde levels

Common polyamines, putrescine (Put), spermidine (Spd), and spermine (Spm), are cationic compounds known as beneficial factors for many cellular processes including cell division, proliferation, differentiation, and stress response in all living organisms. Effects of exogenous Spm on the protective responses of Synechocystis sp. PCC 6803 exposed to UVA were investigated. The presence of 0.5 mM Spm in the culture medium significantly reduced cell growth after 60 min under white light condition but protected the cells after growing for 60 min under UVA. The stress-tolerant response of Synechocystis cells represented by the ratio of putrescine/spermidine (Put/Spd) showed about a 6-fold increase after 60 min UVA in the presence of Spm. In addition, those levels of chlorophyll a, carotenoids, and photosynthetic oxygen evolution were increased by Spm supplementation in UVA-treated cells. Exogenous Spm induced the activity of catalase but not superoxide dismutase in cells under UVA treatment. On the other hand, Spm treatment enabled cells to apparently decrease the intracellular free radical H₂O₂ and malonaldehyde (MDA) levels. Overall results suggested that Spm supplementation could protect Synechocystis sp. PCC 6803 cells via the increase of Put/Spd ratio and the reduction of both H₂O₂ and MDA levels in conjunction with the induction of catalase activity. Interestingly, UVA-treated cells as compared to non-treated cells with exogenous Spm showed a decrease of Spm with an increase of Put and no change in Spd. This suggested the back conversion of Spm to Spd and finally to Put as cellular mechanism in response to UVA.