Temperature-induced Changes in the Synthesis of Unsaturated Fatty Acids by Acanthamoeba castellanii

ABSTRACT. Major fatty acid components of Acanthamoeba castellanii lipids extracted after growth at 30°C include myristate, palmitate, stearate and the polyunsaturates linoleate, eicosadienoate, eicosatrienoate and arachidonate, with oleate as the sole major monounsaturated fatty acid. By comparison, growth at 15°C gave increased linoleate, eicosatrienoate and arachidonate, but decreased oleate and palmitate. When the growth temperature was shifted downwards from 30°C to 15°C, increased lipid unsaturation occurred over a period of 24 h; thus decreases of oleate and eicosadienoate were accompanied by increases in linoleate, eicosatrienoate, arachidonate and eicosapentaenoate. An upwards shift from 15°C to 30°C gave negligible alterations in fatty acid composition over a similar period. At 15°C organisms rapidly use [1-¹⁴C] acetate for de novo fatty acid synthesis; stearate is converted via oleate to further desaturation and chain elongation products. Similar short term experiments at 30°C indicate only de novo synthesis and Δ9-desaturation; synthesis of polyunsaturates was a much slower process. Rapid incorporation of [1-¹⁴C] oleate at 30°C was not accompanied by metabolic conversion over two hours, whereas at 15°C n-6 desaturation to linoleate was observed. Temperature shift of organisms from 15°C to 30°C in the presence of [1-¹⁴C] acetate revealed that over half of the fatty acids in newly-synthesised lipids were saturated, but the proportions of unsaturated fatty acids increased with time until the total polyenoate components reached 17% after 22 h. A shift of temperature in the reverse direction gave a corresponding figure of 60% for polyunsaturated fatty acids. These results emphasize the importance of n-6 desaturation in the low temperature adaptation of Acanthamoeba castellanii .     

Balancing carboxylation and regeneration of ribulose-1,5- bisphosphate in leaf photosynthesis: temperature acclimation of an evergreen tree, Quercus myrsinaefolia

Changes in the temperature dependence of the photosynthetic rate depending on growth temperature were investigated for a temperate evergreen tree, Quercus myrsinaefolia . Plants were grown at 250 μ mol quanta m^–2 s^–1 under two temperature conditions, 15 and 30 °C. The optimal temperature that maximizes the light-saturated rate of photosynthesis at 350 μ L L^–1 CO₂ was found to be 20–25 and 30–35 °C for leaves grown at 15 and 30 °C, respectively. We focused on two processes, carboxylation and regeneration of ribulose-1,5-bisphosphate (RuBP), which potentially limit photosynthetic rates. Because the former process is known to limit photosynthesis at lower CO₂ concentrations while the latter limits it at higher CO₂ concentrations, we determined the temperature dependence of the photosynthetic rate at 200 and 1000 μ L L^–1 CO₂ under saturated light. It was revealed that the temperature dependence of both processes varied depending on the growth temperature. Using a biochemical model, we estimated the capacity of the two processes at various temperatures under ambient CO₂ concentration. It was suggested that, in leaves grown at low temperature (15 °C), the photosynthetic rate was limited solely by RuBP carboxylation under any temperature. On the other hand, it was suggested that, in leaves grown at high temperature (30 °C), the photosynthetic rate was limited by RuBP regeneration below 22 °C, but limited by RuBP carboxylation above 22 °C. We concluded that: (1) the changes in the temperature dependence of carboxylation and regeneration of RuBP and (2) the changes in the balance of these two processes altered the temperature dependence of the photosynthetic rate.     

Effect of Temperature on the Formation of Microsclerotia of Verticillium dahliae

Microsclerotium formation by six isolates of Verticillium dahliae was studied at different temperatures both in vitro and in Arabidopsis thaliana . In vitro mycelial growth was optimal at 25°C, but microsclerotium formation was greatest at 20°C (two isolates) or 15–20°C (one isolate). Seedlings of A. thaliana were root-dipped in a conidial suspension, planted, and either placed at 5, 10, 15, or 25°C, or left at 20°C until the onset of senescence, after which some of the plants were placed at 5, 10, 15, or 25°C. The amount of microsclerotia per unit of shoot weight was assessed in relation to isolate and temperature. The optimal temperature for production of microsclerotia was 15–25°C. Two isolates each produced about 10 times more microsclerotia than each of the other four isolates. For these isolates, high R ²_adj.-values of 0.77 and 0.66 were obtained, with temperature and its square as highly significant (P   < 0.001) independent variables. R ²_adj.-values for the other isolates varied between 0.28 and 0.39. Moving plants to different temperatures at the onset of senescence led to microsclerotial densities that were intermediate between densities on plants that had grown at constantly 20°C and plants grown at other temperatures. This suggests that vascular colonization rate and rate of microsclerotium formation are similarly affected by temperature. The senescence rate of plants appeared unimportant except for plants grown at 25°C, which showed the highest amounts of microsclerotia per unit of plant weight in the most rapidly senescing plants.     

Chilling sensitivity of the frost-tolerant potato Solanum commersonii

Frost tolerance has been reported in the shoots of wild, tuberiferous potato species such as Solanum commersonii when the plants are grown in either field or controlled conditions. However, these plants can survive as underground tubers and avoid unfavorable environmental conditions altogether. As such, leaf growth and photosynthesis at low temperature may not be required for survival of the plants. In order to determine the temperature sensitivity of S. commersonii shoots, we examined leaf growth, development and photosynthesis in plants raised at 20/16°C (day/night). 12/9°C and 5/2°C. S. commersonii leaves grown at 5°C exhibited a marked decrease in leaf area and in total chlorophyll (Chl) content per leaf area when compared with leaves grown at 20°C. Furthermore, leaves grown at 5°C did not exhibit the expected decrease in either water content or susceptibility to low-temperature-induced photoinhibition that normally characterizes cold acclimation in frost-tolerant plants. Measurements of CO₂-saturated O₂ evolution showed that the photosynthetic apparatus of 5°C plants was functional, even though the efficiency of photosystem II photochemistry was reduced by growth at 5°C. A decrease in the resolution of the M-peak in the slow transients for Chl a fluorescence in leaves grown at 12 and 5°C and in all leaves exposed to high light at 5°C indicated that low temperature significantly affected processes on the reducing side of Q_A, the primary quinone electron acceptor in photosystem II. Thus S. commarsonii exhibits the characteristics of a plant that is limited by chilling temperatures. Although S. commersonii can tolerate light frosts, its sensitivity to chilling temperatures may result in shoot dieback in winter in its native habitat. The plants may avoid both chilling and freezing temperatures by overwintering as underground tubers.     

Kinetic resolution of different recovery phases of photoinhibited photosystem II in cold-acclimated and non-acclimated spinach leaves

Leaf discs from spinach were exposed to a photon flux density of 1250 μmol m⁻²s⁻¹ at 5°C for 2 or 3 h in ambient air. Photoinhibition of photosystem II (PS II) was measured by means of chlorophyll fluorescence. Recovery of photosystem II was followed at 6°C and 20°C in low light or darkness for periods up to 12 h.
The experimental setup allowed kinetic resolution of different phases of recovery. The experiments revealed a temperature dependent dark recovery phase and two distinct light- and temperature dependent phases: (1) A relatively fast, light dependent recovery phase occurred in parallel with partial recovery of basic fluorescence at 6°C and 20°C. A population of PS II centers with very slow fluorescence induction kinetics, which had accumulated during photoinhibition treatment, disappeared during this phase. This fast recovery phase is proposed to represent reactivation of photoinhibited PS II, without dissassembly or incorporation of new D₁-protein. (2) A relatively slow light-dependent recovery phase took place at 20°C, but not at 6°C. In the presence of the chloroplast translation inhibitor streptomycin, part of the 2nd phase was inhibited. This phase is proposed to involve assembly of new Photosystem II centers, which is partly dependent on de novo synthesis of D₁-reaction center protein, but presumably is also using a preexisting pool of D₁-protein. Cold acclimation of the leaves resulted in a decreased sensitivity for photoinhibition of photosystem II. Recovery of photoinhibited photosystem II at 6°C of the cold-acclimated leaves was faster than in non-acclimated leaves, but this effect can be ascribed to diminished photoinhibitory damage.     

Respiratory Q10 of marigold (Tagetes patula) in response to long-term temperature differences and its relationship to growth and maintenance respiration

Acclimation of respiration to temperature is not well understood. To determine whether whole plant respiration responses to long-term temperature treatments can be described using the Q₁₀ concept, the CO₂ exchange rate of marigolds ( Tagetes patula L. 'Queen Sophia'), grown at 20°C or 30°C, was measured for 62 days. When plants of the same age were compared, plants grown at 20°C consistently had a higher specific respiration (R_spc) than plants grown at 30°C (long-term Q₁₀= 0.71–0.97). This was due to a combination of greater dry mass at 30°C and a decrease in R_spc with increasing mass. When plants of the same dry mass were compared, the long-term Q₁₀ was 1.35–1.55; i.e. R_spc was higher at 30°C than at 20°C. Whole plant respiration could be accurately described by dividing respiration into growth and maintenance components. The maintenance respiration coefficient was higher at 30°C than at 20°C, while the growth respiration coefficient was lower at 30°C, partly because of temperature-dependent changes in plant composition. These results suggest difficulties with interpreting temperature effects on whole plant respiration, because conclusions depend greatly on whether plants of the same age or mass are compared. These difficulties can be minimized by describing whole plant respiration on the basis of growth and maintenance components.     

The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

Low temperature-induced fatty acid desaturation in Brassica napus: thermal deactivation and reactivation of the process.

When Brassica napus plants are grown at low temperatures (e.g., 5 degrees C) the rate of desaturation in leaves of newly formed fatty acids in both chloroplastic (MGDG) and cytosolic (PC) diacylglycerols is higher or more rapid than in plants grown at higher temperatures (e.g., 30 degrees C). This low temperature-induced increase in the rate of desaturation is lost within hours if plants are transferred to higher temperatures. However, if plants are then returned to low temperatures they regain the ability to rapidly desaturate fatty acids. This process is restored relatively slowly (over days) in contrast to the more rapid loss at high temperatures. This has important physiological consequences on the level of unsaturated fatty acids in plant membranes and the process of temperature control of the fatty acid composition of membrane lipids.     

TETREUTREPTIA POMQUETENSIS (EUGLENOPHYTA), A PSYCHROPHILIC SPECIES: GROWTH AND FATTY ACID COMPOSITION

Tetreutreptia pomquetensis McLachlan, Fritz et Seguel, a quadriflagellated euglenoid isolated from a shallow-water embayment of the southeastern Gulf of St. Lawrence, is a common winter species where habitats are ice covered for 4 to 5 months. Growth in culture, which was similar to that of many euryhaline species, was limited at normal seawater salinities to temperatures between ∼0° and 7° C, and the alga could not be maintained at 10° C; thus, it is a strict psychrophile. The maximum growth rate (μ≅ 0.35·days⁻¹) was around 5° C and, given its size (∼100 μm length), high productivities are indicated. The major fatty acids (FAs) identified were 18:4ω3, 16:4ω3, 20:5ω3, and 16:0. The overall FA spectrum does not align closely to other euglenoids but may reflect more the low growth temperature. The presence of 18:5ω3 is unusual, as this FA had been known previously only from dinoflagellates and haptophytes. The two most abundant sulfoquinovosyl diacylglycerols (SQDGs) had pairings of [18:4/16:0] and [18:4/18:4] and together constituted 80% of the total SQDGs. The latter pairing is unique, not having been previously recognized. The FA data support the hypothesis that the degree of unsaturation is indicative of low temperatures and membrane integrity.     

A comparison of low temperature growth vs low temperature shifts to induce resistance to photoinhibition in spinach (Spinacia oleracea)

Plants of Spinacia oleracea L. cv. Savoy grown under cold-hardening (5°C) and nonhardening (16°C) conditions were exposed to a photoinhibitory irradiance of 1300 μmol rrr^: m-² S-¹ 5°C for 12 h. Plants grown at 5°C exhibited a greater resistance to photoinhibition at low temperature in comparison to plants grown at 16°C as measured by the photochemical efficiency of photosyslem II. In contrast, tuily expanded leaves of plants grown at 16°C and then shifted to 5°C for 10 days did not exhibit increased resistance to photoinhibition. This was observed irrespective of the phoioperiod experienced during the shift to a lower temperature. Furthermore, spinach grown at 16°C and subsequently exposed to a stepped, daily decrease in temperature from 16 to 1°C over 10 days w ith a concomitant reduction in photoperiod. also did not exhibit any change in susceptibility to photoinhibition. Thus, a decrease in photoperiod accompanied by either an abrupt or stepped low temperature shift cannot induce increased resistance to photoinhibition. This confirms the hypothesis that growth and development at cold-hardening temperature are absolute requirements for the acquisition of resistance to photoinhibition at low temperature.     

Photosynthetic response of Eragrostis tef to temperature

Photosynthetic characteristics of leaves of tef, Eragrostis tef (Zucc.) Trotter, plants, grown at 25/15°C (day/night), were measured at temperatures from 18 to 48°C. The highest carbon exchange rates (CER) occurred between 36 and 42°C. and averaged 27 μmol m⁻² s⁻¹. At lower or higher temperatures, CER was reduced, but the availability of CO₂ to the mesophyll, measured as internal CO₂ concentration, was highest when temperatures were above or below the optimum for CER. In addition, CER and stomatal conductance were not correlated, but residual conductance was highly correlated with CER (r = 0.98). In additional experiments, relative ¹³C composition for leaf tissue grown at 25, 35 and 45°C averaged -14.4 per mille, confirming that tef is a C₄ grass species. Dry matter accumulation was higher at 35 than at 25, and lowest at 45°C. Leaf CER rates increased hyperbolically with increased light when measured from 0 to 2000 μmol m⁻² s⁻¹ PPFD. The highest CER, 31.8 μ-mol m_-2 s⁻¹, occurred at 35°C and 2000 μmol m⁻² s⁻¹ PPFR. At high light, CER at 25 and 35°C were nearly equal because of higher stomatal conductance at 25°C. Residual conductance was, however, clearly highest at 35°C compared to 25 and 45°C treatments. Stomatal conductance and residual conductance were not correlated in either set of experiments, yet residual conductance was always highest when temperatures were between 35 and 42°C across experiments, suggesting that internal leaf photosynthetic potential was highest across that temperature range.     

Anaerobic production of polyunsaturated fatty acids by Shewanella putrefaciens strain ACAM 342

Abstract The fatty acid composition of cultures of Shewanella putrefaciens strain ACAM 342 grown aero-bically and anaerobically at 15°C and 25°C were analysed by capillary gas chromatography. The bacterium was found to produce the polyunsaturated fatty acids (PUFA) 18:2ω3, 18:3ω3 and 20:5ω3 under aerobic and anaerobic conditions at both growth temperatures. This result suggests that the bacterium possesses both the aerobic and anaerobic pathways for unsaturated fatty acid synthesis, where an alternate terminal electron acceptor(s) is utilised in the absence of oxygen.     

Effect of paclobutrazol and chilling on leaf membrane lipids in cucumber seedlings

The effects of paclobutrazol on the leaf membrane lipid composition of seedlings of cucumber ( Cucumis sativus L. cv. Victory) subjected to chilling temperatures were assessed. At a non-injurious temperature (12.5°C), there was no difference in the polar lipid fatty acid composition or in the glycolipid, phospholipid or free sterol content of leaves from treated vs untreated seedlings, regardless of whether paclobutrazol was administered 1 or 7 days prior to analysis. In the latter case (7 days pretreatment), there were clear effects of the bioregulator on plant growth and morphology as well as on leaf chlorophyll content. At an injurious chilling temperature (5°C), desaturation of leaf polar lipid fatty acids was markedly reduced in both treated and untreated seedlings. Chilling at 5°C resulted in losses of fresh weight and membrane lipids in leaves of both groups of plants. These losses were either reversible or irreversible, depending upon the duration of chilling and of pretreatment with paclobutrazol. Seedlings pretreated with 10 μg ml⁻¹ paclobutrazol generally sustained less chilling injury than untreated controls, as judged by the extent of wilting, necrosis and desiccation. This correlated with reduced losses of leaf fresh Weight and membrane lipids.     

Daily photosynthetic and C-export patterns in winter wheat leaves during cold stress and acclimation

Diurnal patterns of whole-plant and leaf gas exchange and ¹⁴C-export of winter wheat acclimated at 20 and 5°C were determined. The 5°C-acclimated plants had lower relative growth rates, smaller biomass and leaf area, but larger specific leaf weight than 20°C plants. Photosynthetic rates in 20°C and 5°C-acclimated leaves were similar; however, daytime export from 5°C-acclimated leaves was 45% lower. Photosynthesis and export remained steady in 20°C and 5°C-acclimated leaves during the daytime. By comparison, photosynthesis in 5°C-stressed leaves (20°C-acclimated plants exposed to 5°C 12 h before and during measurements) declined from 70 to 50% of the 20°C-acclimated leaves during the daytime, while export remained constant at 35% of the 20°C-acclimated and 60% of the 5°C-acclimated leaves. At high light and CO₂, photosynthesis and export increased in both 20°C and 5°C-acclimated leaves, but rates in 5°C-stressed leaves remained unchanged. At all conditions daytime export was greater than nighttime export. Taken together, during cold acclimation photosynthesis was upregulated, whereas export was only partially increased. We suggest that this reflects a requirement of cold-acclimated plants to both sustain an increased leaf metabolic demand while concomitantly supporting translocation of photoassimilates to overwintering sinks.     

Effects of temperature and gibberellin on the activity of endogenous cytokinin-like substances in leaves of Begonia x cheimantha

Changes in activity of endogenous cytokinin-like substances were examined in intact plants and excised leaves of Begonia x chemantha Everett cv. Prinsesse Astrid (Christimas Begonia) by means of the tobacco callus bioassay. Cytokinin activity in the leaves of intact plants was higher in plants grown at 18°C than in those grown at 21° or 24°C. In excised leaves, an increase in cytokinin activity was observed during the first 4 days following leaf detachment. However, after the seventh day cylokinin activity decreased again. This decrease was more profound in leaves exposed to 24°C than in those exposed to 18°C.
Treatment of detached leaves with gibberellic acid (2.8 m M ) caused an increase in measurable cytokinin activity. This increase was more profound in the zones of activity which correspond with zeatin glucosides on paper and Sephadex LH-20 chromatography. Additional zones of activity appeared after Sephadex chromatography. These were of a more slow moving nature with elution volumes corresponding to N^b-(Δ²-isopentenyl)adenine and its derivaties. Water-treated control leaves had higher activity in the regions corresponding to zeatin and zeatin riboside.     

Geibberllin and temperature influence carbohydrate content and flowering in Phalaenopsis

When Phalaenopsis amabilis is grown under high temperature (30/25°C, day/night), flowering is blocked, and this can be reversed by gibberellin A₃ (GA₃) treatment. Associated with GA₃ treatment under high temperature are increases in sucrose, glucose and fructose as compared with warm-treated plants. Spraying with sucrose solution alone caused leaf epinasty in plants grown under high temperature. Epinasty was released by about 9 days of GA₃ treatment. In GA₃-treated plants under high temperatures, sucrose application to the source leaves led to an increase in sugar content in both leaves and inflorescence. In contrast, although in warm-treated plants sucrose application to the source leaves increased sugar content in the leaves, it did not increase sucrose content in the inflorescence. These results corroborate our hypothesis that in Phalaenopsis GA₃ stimulates sink activity in the apical meristem and promotes the translocation of sucrose from source leaves to the apex of the inflorescence, where it accumulates. GA₃ treatment led to an increase in sucrose synthase activity and had no effect on invertase activity.     

Effect of frozen storage on fatty acid composition and changes in lipid content of Scomberomorus commersoni and Carcharhinus dussumieri

Investigated were the changes in fatty acid composition, oxidation and enzymatic deterioration of lipids in frozen (−30°C) fish fillets from the Persian Gulf. The narrow barred Spanish mackerel ( Scomberomorus commersoni ) and white cheek shark ( Carcharhinus dussumieri ) were tested with storage times of 0, 1, 2, 3, 4, 5 and 6 months at −18°C. Statistical results showed that the major fatty acids among the saturated and monounsaturated fatty acids of each fish species were palmitic (C16:0) and oleic (C18:1n-9) acids, respectively. Both linoleic acid (C18:2n-6) and arachidonic acid (AA) (C20:4n-6) were predominant in total n-6 polyunsaturated fatty acids in both mackerel and shark. The EPA (eicosapentaenoic acid; C20:5 n-3) and DHA (docosahexaenoic acid; C22:6 n-3) acids were the major fatty acids among total n-3 acids in both fishes. During frozen storage, the PUFA (40.1 and 23.94%), n-3 (48 and 42.83%), ω 3/ ω 6 (41.36 and 50%), PUFA/SFA (56 and 42.23%) and EPA + DHA/C16 (55.55 and 46.66%) contents decreased in S. commersoni and C. dussumieri , respectively. Also peroxide, thiobarbituric acid (TBA) and free fatty acid (FFA) values significantly increased (P < 0.01) with the time of storage.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .     

A cotyledon screen for resistance to scab (Cladosporium cucumerinum) in cucumber (Cucumis sativus) seedlings

An efficient protocol for differentiating cucumber (Cucumis sativus) seedlings resistant and susceptible to scab (Cladosporium cucumerinum) has been developed by examining the interacting factors of spore density and cotyledon growth stage. The procedure permitted both resistant and susceptible plants to be recovered. Seedlings were grown at 20°C and inoculated 1 day after emergence by placing a 5 μl inoculum droplet (2 × 10⁶ spores/ml) on the newly expanded cotyledons. Seedlings were incubated in a darkened dew chamber for 48 h at 20°C and 100% r.h., and then grown for 6 days at 20°C. Plants could be rated as resistant or susceptible 8 days after inoculation.     

Does grafting provide tomato plants an advantage against H2O2 production under conditions of thermal shock?

Non-grafted tomato plants ( Lycopersicon esculentum L. cv. Tmknvf₂) and grafted tomato plants ( L. esculentum L. cv. Tmknvf₂ ×  L. esculentum L. cv. RX-335) were grown for 30 days at three different temperatures (10°C, 25°C and 35°C). In the leaves of these plants, the enzymatic activities of SOD, GPX, CAT, APX, DHAR and GR were analysed, as were the concentrations of total H₂O₂, ascorbate and glutathione as well as foliar DW. Regardless of whether the plant was grafted or not, our results indicate that the thermal stress occurred mainly at 35°C, with the following effects: (1) high SOD activity; (2) H₂O₂ accumulation; (3) foliar-biomass reduction; (4) low GPX, CAT, APX, DHAR and GR activities; and (5) high concentrations of ascorbate and glutathione. In addition, our data show these effects to be much weaker in grafted than in non-grafted plants, directly reflected in greater biomass production. Therefore, the use of grafted plants under excessively high temperatures may offer an advantage over non-grafted plants in terms of resistance against thermal shock.