The effect of growth at low temperature on photosynthetic characteristics and mechanisms of photoprotection of maize leaves

We examined the photosynthetic properties, the activity of antioxidantenzymes and the amount of caroten-oids of two maize genotypescharacterized by different sensitivity to low temperature. Plantsof the low-temperature-sensitive genotype A-619 and of the low-temperature-resistantgenotype VA-36 were grown at 25/20C (control plants) and at16/14C (plants grown at low temperature). Twenty-five daysafter seeding, the exposure to low temperature caused, in allplants, a reduction of leaf chlorophyll and carotenoid content,but an increase in the activity of the oxygen-detoxifying enzymes,superoxide dismutase and ascorbate per-oxidase. However, theresponse of photosynthesis, stomatal conductance and the fluorescenceproperties to light and temperature were not affected by growthat low temperature. Fifty days after seeding, photosynthesis, stomatal conductance,and fluorescence properties of A-619 leaves grown at low temperaturewere significantly reduced with respect to control plants atall temperatures and light intensities. In the leaves of A-619plants grown at low temperature, the electron transport ratewas not tightly down-regulated by carbon metabolism and an excessof electrons was shown by the increased ratio between the quantumyield of electron transport of photosystem II and the quantumyield of CO2 assimilation. On the contrary, VA-36 leaves grownat 16C maintained the same photosynthetic characteristics andphotochemical properties as control plants. The chlorophyllcontent of both genotypes and carotenoid content of A-619 plantswere lower in leaves of plants maintained at 16C than in thosegrown at 25C. In contrast, the carotenoid content of VA-36leaves of plants grown at low temperature were higher than inplants grown at 25C. The activity of superoxide dismutase and ascorbate peroxidaseof VA-36 plants grown at low temperature were higher than incontrols. In A-619 plants grown at fow temperature the activityof superoxide dismutase was higher than in controls, but theactivity of ascorbate peroxidase was lower than in controls.Our findings suggest that when maize plants are grown at lowtemperature the electron transport rate may be in excess ofcarbon metabolism and electrons may be used to reduce oxygen.A co-ordinate increase of pigment amounts and of the activityof oxygen-detoxifying enzymes is necessary to protect maizeleaves from the accumulation of oxygen radicals at low temperature.In A-619 plants, the carotenoid content did not increase andthe activity of ascorbate peroxidase was low when plants wereexposed to low temperature for 50 d. As a result, the photochemicalapparatus of A-619 leaves was damaged and photo-oxidation occurred.These experiments also indicated that when photosynthesis wasreduced by a transitory reduction of temperature, the electrontransport was still tightly down-regulated by carbon metabolismand the photosynthetic apparatus of both genotypes was not damaged. Key words: Photosynthesis, electron transport, antioxid-ants, carotenoids, low temperature.     

Rapid acclimation of root hydraulic conductivity to low temperature

Root hydraulic conductance of many species is substantially reduced by exposure to low temperatures. The objective of this research was to investigate the decrease and recovery of root hydraulic conductivity in spinach (Spinacia oleracea L.) root systems upon exposure to low temperature. Root hydraulic conductivity (Lp) was determined for detached whole root systems as the slope of the flux and an applied pressure gradient. Water flux (Jv), of root systems grown at 20C, decreased immediately upon exposure to 5C. After 2-5 h Jv recovered and reached a stable value after 12 h exposure to 5°C. In separate experiments, the root Lp of plants acclimated for 7 d at 5°C was 125% greater than that of isolated root systems acclimated for 12 h at 5°C. Lp of plants grown and measured at 5°C was about 50% of the Lp of plants grown and measured at 20°C. The rapid acclimation to low temperatures observed in detopped root systems was also indicated in intact plants at 20/5°C (shoot/root temperatures) using mass flow porometry. Acclimation of the root system after exposure to 5°C was apparent by recovery of stomatal opening. These results indicate that spinach root systems have the ability to acclimate rapidly to changes in temperature and to continue acclimating during prolonged exposure to low temperature.     

Changes in the Activity of Antioxidant Enzymes in Wheat Leaves and Roots as a Function of Nitrogen Source and Supply

The activity of enzymes participating in the systems of antioxidant protection was assayed in the second leaf and roots of 21-day-old wheat seedlings (Triticum aestivum L.) grown in a medium with nitrate (NO^– ₃ treatment), ammonium (NH⁺ ₄ treatment), or without nitrogen added (N-deficiency treatment). The activities of superoxide dismutase (SOD), peroxidase, ascorbate peroxidase, glutathione reductase, and catalase in the leaves and roots of the NH⁺ ₄ plants was significantly higher than in the plants grown in the nitrate medium. The activity of SOD decreased and ascorbate peroxidase markedly increased in leaves, whereas the activity of ascorbate peroxidase increased in the roots of N-deficient plants, as compared to the plants grown in nitrate and ammonium. Low-temperature incubation (5°, 12 h) differentially affected the antioxidant activity of the studied plants. Whereas leaf enzyme activities did not change in the NH⁺ ₄ plants, the activities of SOD, peroxidase, ascorbate peroxidase, and catalase markedly increased in the NO^– ₃ plants. In leaves of the N-deficient plant, the activity of SOD decreased; however, the activity of other enzymes increased. In response to temperature decrease, catalase activity increased in the roots of NO^– ₃ and NH⁺ ₄-plants, whereas in the N-deficient plants, the activity of peroxidase increased. Thus, in wheat, both nitrogen form and nitrogen deficiency changed the time-course of antioxidant enzyme activities in response to low temperature.     

The temperature dependence of the stimulation of photosynthesis by elevated carbon dioxide in wheat and barley

The temperature dependencies of the solubility of carbon dioxide and oxygen in water and the temperature dependency of the kinetic characteristics of the ribulose-1,5 bisphosphate carboxylase/oxygenase (Rubisco) enzyme result in the short-term stimulation of photosynthesis with a doubling of carbon dioxide from 350 to 700 mol mol^-1 usually decreasing from about 90% at 30C to about 25% at 10C at high photon flux. In field-grown wheat and barley, the expected values at 30°C were observed, but also values as high as 60% at 10°C. The much larger than expected stimulation at cool temperatures in these species also occurred in plants grown at 15°C, but not at 23°C in controlled environment chambers. Gas exchange analysis indicated that an unusually high diffusive limitation was not an explanation for the large response. Assessment of the apparent in vivo specificity of Rubisco by determining the carbon dioxide concentration at which carboxylation equalled carbon dioxide release from oxygenation, indicated that growth at low temperatures altered the apparent enzyme specificity in these species compared to these species grown at the warmer temperature. Inserting the observed specificities into a biochemical model of photosynthesis indicated that altered Rubisco specificity was consistent with the observed rates of assimilation. Whether altered apparent Rubisco specificity is caused by altered stoichiometry of photorespiration or an actual change in enzyme specificity, the results indicate that the temperature dependence of the stimulation of photosynthesis by elevated carbon dioxide may vary greatly with species and with prior exposure to low temperature.Keywords: Barley, carbon dioxide, photosynthesis, temperature, wheat.      

The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum Link and H. glaucum Steud.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.We are grateful to Zeneca Agrochemicals, Jealotts Hill, Berkshire, UK who provided [¹⁴C]paraquat. E.P. was supported through a Ph.D. scholarship from the Australian International Development Assistance Bureau and C.P. was the recipient of an Australian Research Council Postdoctoral Fellowship.     

Over-expression of chloroplast-targeted Mn superoxide dismutase in cotton (Gossypium hirsutum L., cv. Coker 312) does not alter the reduction of photosynthesis after short exposures to low temperature and high light intensity

Transgenic cotton plants from several independently-transformed lines expressing a chimeric gene encoding a chloroplast-targeted Mn superoxide dismutase (SOD) from tobacco exhibit a three-fold increase in the total leaf SOD activity, strong Mn SOD activity associated with isolated chloroplasts, and a 30% and 20% increase in ascorbate peroxidase and glutathione reductase activities, respectively. The Mn SOD plants did exhibit a slightly enhanced protection against light-mediated, paraquat-induced cellular damage but only at 0.3 µM paraquat. In addition, photosynthetic rates at 10°C and 15°C were similar to those of controls, and the immediate recovery of photosynthesis after a 35-min exposure to 5°C and full sun was only slightly better than that for wild-type plants. The recovery for longer exposure times was comparable for both genotypes as was the deactivation of the H₂O₂-sensitive, Calvin-cycle enzyme, stromal fructose 1,6-bisphosphatase (FBPase). Compared to the controls, Mn SOD plant leaves in full sun prior to chilling stress had a lower activation of FBPase, a higher ratio of oxidized to reduced forms of ascorbate, and a higher total glutathione content. After 35 min at 5°C in full sunlight, total glutathione had risen in control leaves to 88% of the Mn SOD plant values, and oxidized to reduced ascorbate ratios were higher for both genotypes. However, an 80% increase in the ratio of oxidized to reduced glutathione occurred for Mn SOD plant leaves with no change for controls. This increased demand on the ascorbate-glutathione cycle is circumstantial evidence that high Mn SOD activity in the chloroplast leads to increased H₂O₂ pools that could, in some manner, affect photosynthetic recovery after a stress period. We postulate that the pool sizes of reduced ascorbate and glutathione may restrict the ability of the ascorbate-glutathione cycle to compensate for the increased activity of SOD in cotton over-producing mitochondrial Mn SOD in chloroplasts during short-term chilling/high light stress.     

Effect of temperature on secondary metabolites production and antioxidant enzyme activities in Eleutherococcus senticosus somatic embryos

Somatic embryos of Eleutherococcus senticosus were exposed at 12, 16, 24 and 30 °C for duration of 45 days in bioreactor. The effects of such treatments on the growth, eleutheroside B, E, E₁, total phenolics, flavonoids, chlorogenic acid concentrations and antioxidant enzymes activities were investigated. The results revealed that low (12 and 18 °C) and high (30 °C) temperature caused significant decrease in fresh weight (FW), dry weight (DW), total phenolics, flavonoids and total eleutheroside accumulation, while low temperature increased eleutheroside E accumulation in somatic embryos. Low temperature significantly increased superoxide dismutase (SOD), catalase (CAT), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) activities whereas a strong increase in ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) activity was obtained at 12 °C grown somatic embryos. In contrast, high temperature significantly decreased antioxidant enzymes activities and even guaiacol peroxidase (G-POD) activity also decreased at low temperature in comparison to 24 °C grown embryos. These data suggest that low and high temperature treatment provoked an oxidative stress in E. senticosus embryos, as shown by the increase in lipid peroxidation. The increase in lipid peroxidation was paralleled by a rise in lipoxygenase (LOX) activity and hydrogen peroxide (H₂O₂) content. However, this stress was more prominent at high temperature than low temperature grown embryos. This result suggests that the reduced growth of embryo at 30 °C was concomitant with reduced efficiency of these protective enzymes. On the other hand, increases in antioxidant activities at 12 and 18 °C could also be a response to the cellular damage; however, this increase could not stop the deleterious effects of low temperature, but reduced stress severity thus allowing embryo growth to occur.     

How do changes in temperature during growth affect leaf pigment composition and photosynthesis in Zea mays genotypes differing in sensitivity to low temperature?

The changes in photosynthetic activity and composition of pigments induced by changes in temperature were examined in the third leaf of three chilling-tolerant and three chilling-sensitive genotypes of maize (Zea mays L.). The plants were grown under a controlled environment at a photon flux density of 550 mol m^-2 s^-1, a 12 h photoperiod and at a suboptimal temperature of 14/12 C (day/night) until the full expansion of the third leaf. After this treatment, the chilling-tolerant genotypes, when compared with the sensitive ones, displayed a higher photosynthetic activity, a higher content of chlorophyll (Chl) a+b, a higher Chl a/b ratio, a larger total carotenoid pool size as well as a different carotenoid composition. When temperature was subsequently increased to 24/22 C for 3 d the composition of the pigments changed, but the chilling-sensitive genotypes, while adjusting their lower Chl a/b ratio and their different carotenoid composition, were unable to adjust their lower content of chlorophyll, their smaller total carotenoid pool size or their lower photosynthetic performance. Moreover, while the chilling-tolerant genotypes converted the most part of zeaxanthin to violaxanthin in the xanthophyll cycle, the chilling-sensitive genotypes retained high amounts of zeaxanthin. The changes in pigment composition that occurred over the 3 d at 24/22 °C were largely conserved when the plants were returned to 14/12 °C, but photosynthetic activity decreased and zeaxanthin accumulated again. The results suggest that the capability of the chilling-tolerant genotypes, when compared with the sensitive ones, to retain high amounts of pigments and to form a competent photosynthetic apparatus at low temperature is the basis for their more vigorous growth in cool climates.     

Chlorophyll fluorescence as a selection tool for cold tolerance of photosynthesis in maize (Zea mays L.)

The possibility of using quenching analysis of chlorophyll a fluorescence as a selection tool for improving the cold tolerance of maize was investigated in six genotypes differing greatly in the ability to develop a competent photosynthetic apparatus at low temperature. Upon gradual cooling measurements of the quantum yield of electron transport (PSII) indicated that leaves of tolerant genotypes, that developed at suboptimal temperature (15C), maintained higher rates of electron transport than leaves of sensitive genotypes. This difference was largely due to the ability of the tolerant plants to keep higher efficiency of excitation energy capture by open photosystem II reaction centres (F'v/F'm). The absence of genotypic differences in leaves that developed at optimal temperature indicates that the trait is not expressed constitutively, but relies on adaptation mechanisms. Furthermore, the genotypic difference was not expressed under increasing illumination at 15C and 25°C suggesting that the trait is also low-temperature-specific and is not expressed solely in response to increasing excess light energy. Applying the method to flint and dent breeding population led to a substantial increase (up to 31%) in the photosynthetic capacity of hybrids between selected F3 inbreeding families grown at suboptimal temperature, demonstrating that the method is an efficient selection tool for improving the cold tolerance of maize through breeding.     

Kinetics of dormancy release and the high temperature germination response in Aesculus hippocastanum seeds

The kinetics of primary dormancy loss were investigated in seeds of horse chestnut (Aesculus hippocastanum L.) harvested in four different years. Freshly collected seeds from 1991 held for up to 1 year at temperatures between 2C and 42C exhibited two peaks in germination (radicle growth), representing a low temperature (2-8°C) and a high temperature response (31-36°C). Germination at 36°C generally occurred within 1 month of sowing, but was never fully expressed in the seedlots investigated. At low temperatures (2-8°C), germination started after around 4 months. Generally, very low levels of termination were observed at intermediate temperatures (11-26°C). Stratification at 6°C prior to germination at warmer temperatures increased the proportion of seeds that germinated, and the rate of germination for all seedlots. Within a harvest, germination percentage (on a probit scale) increased linearly with stratification time and this relationship was independent of germination temperature (16-26°C). However, inter-seasonal differences in the increases in germination capacity following chilling were observed, varying from 0.044 to 0.07 probits d^-1 of chilling at 6°C. Increased sensitivity to chilling was associated with warmer temperatures during the period of seed filling. The estimated base temperature for germination, Tb, for newly harvested seeds varied slightly between collection years but was close to 25°C. For all seedlots, Tb decreased by 1°C every 6 d of chilling at 6°C. This systematic reduction in Tb with chilling ultimately facilitated germination at 6°C after dormancy release.     

Assimilate transport in maize (Zea mays L.) seedlings at vertical low temperature gradients in the root zone

Even moderate chilling temperatures may cause important modifications in assimilate movement in maize seedlings from the shoot to the roots, but there is no information on long-distance transport of assimilates in plants subjected to vertical gradients of moderately low temperatures in the root zone. Seedlings of a chilling-tolerant (KW1074) and a chilling-sensitive inbred line (CM109) of maize were grown in a system that allowed the maintenance of temperature gradients between the topsoil (0-10 cm) and the subsoil (10-50 cm). After pregrowth at 24C until the third-leaf stage, plants were subjected to chilling-stress regimes for 6 d (17/17/17C, 17/17/12°C, 12/12/12°C, 12/12/17°C, air/topsoil/subsoil). The time taken for the assimilates to enter the phloem from the second leaf increased at low temperatures for both lines, but to a much greater extent in CM109. Although mainly influenced by air and topsoil temperature, low temperature in the subsoil also affected this trait in CM109. The speed of assimilate transport between the second leaf and the mesocotyl in KW1074 was strongly reduced by cool temperatures in the shoot and topsoil as well as by 12°C in the subsoil in CM109, because the latter line had a larger portion of its root system in the subsoil as compared to KW1074. The portion of assimilates allocated to the root decreased at low temperatures in both lines, but to a greater extent in CM109, and was controlled mostly by the subsoil temperature. After rewarming, values of all measured parameters of assimilate transport returned to near pregrowth levels within a few days.Keywords: Assimilate transport, low temperature stress, root growth, vertical soil temperature gradients, Zea mays L.      

Alteration of transpiration rate, by changing air vapour pressure deficit, influences leaf extension rate transiently in Miscanthus

A controlled environment chamber for whole plants is described in which vapour pressure deficit (VPD) and temperature can be controlled independently. Plant responses to changes in VPD at constant temperature were measured in terms of leaf extension and plant transpiration rates. Manipulation of VPD independently of temperature was shown to be capable of altering the leaf extension rates of the C4 grass Miscanthus x giganteus grown in hydroponics. The effects of VPD on leaf extension are attributed to changes in transpiration rate and hence leaf water status. It was found that, at a temperature of 20C, the influence of a fixed change in VPD was proportionally less than those observed at temperatures which are close to the threshold for growth (between 6 and 10C). These responses are discussed in relation to our current understanding of the mechanisms of cell growth. The fact that the VPD effects on leaf expansion rates were largely transient suggest that simple models driven by temperature alone are adequate to predict leaf expansion within the temperature range 6-20°C, for this genotype of Miscanthus, in the field.Key words: Leaf growth, Miscanthus x giganteus, temperature, vapour pressure deficit, C4 plants.      

The effect of rhizosphere dissolved inorganic carbon on gas exchange characteristics and growth rates of tomato seedlings

The possibility that an enhanced supply of dissolved inorganic carbon (DIC=CO2+HCO3^-) to the root solution could increase the growth of Lycopersicon esculentum (L.) Mill. cv. F144 was investigated under both saline and non-saline root medium conditions. Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO2 concentrations in the range between 0 and 5000 mol mol^-1. The biomass of both control and salinity-stressed plants grown at high temperatures (daily maximum of 37C) and an irradiance of 1500 mol m^-2 s^-1 was increased by up to 200% by enriched rhizosphere DIC. The growth rates of plants grown with irradiances of less than 100 mol m^-2 s^-1 were increased by elevated rhizosphere DIC concentrations only when grown at high shoot temperatures (35C) or with salinity 28°C). At high light intensities, the photosynthetic rate, the CO2 and light-saturated photosynthetic rate (jmax) and the stomatal conductance of plants grown at high light intensity were lower in plants supplied with enriched compared to ambient DIC. This was interpreted as 'down-regulation' of the photosynthetic system in plants supplied with elevated DIC. Labelled organic carbon in the xylem sap derived from root DI¹⁴C incorporation was found to be sufficient to deliver carbon to the shoot at rates equivalent to 1% and 10% of the photosynthetic rate of the plants supplied with ambient- and enriched-DIC, respectively. It was concluded that organic carbon derived from DIC incorporation and translocated in the xylem from the root to the shoot may provide a source of carbon for the shoots, especially under conditions where low stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.     

Genetic analysis of cold-tolerance of photosynthesis in maize

The genetic basis of cold-tolerance was investigated by analyzing the quantitative trait loci (QTL) of an F₂:3 population derived from a cross between two lines bred for contrasting cold-tolerance using chlorophyll fluorescence as a selection tool. Chlorophyll fluorescence parameters, CO₂ exchange rate, leaf greenness, shoot dry matter and shoot nitrogen content were determined in plants grown under controlled conditions at 25/22 °C or 15/13 °C (day/night). The analysis revealed the presence of 18 and 19 QTLs (LOD > 3.5) significantly involved in the variation of nine target traits in plants grown at 25/22 °C and 15/13 °C, respectively. Only four QTLs were clearly identified in both temperatures regimes for the same traits, demonstrating that the genetic control of the performance of the photosynthetic apparatus differed, depending on the temperature regime. A major QTL for the cold-tolerance of photosynthesis was identified on chromosome 6. This QTL alone explained 37.4 of the phenotypic variance in the chronic photoinhibition at low temperature and was significantly involved in the expression of six other traits, including the rate of carbon fixation and shoot dry matter accumulation, indicating that the tolerance to photoinhibition is a key factor in the tolerance of maize to low growth temperature. An additional QTL on chromosomes 2 corresponded to a QTL identified previously in another population, suggesting some common genetic basis of the cold-tolerance of photosynthesis in different maize germplasms.     

Arbuscular mycorrhizal influence on growth,photosynthetic pigments,osmotic adjustment and oxidative stress in tomato plants subjected to low temperature stress

The influence of the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, on characteristics of growth, photosynthetic pigments, osmotic adjustment, membrane lipid peroxidation and activity of antioxidant enzymes in leaves of tomato (Lycopersicon esculentum cv Zhongzha105) plants was studied in pot culture under low temperature stress. The tomato plants were placed in a sand and soil mixture at 25°C for 6 weeks, and then subjected to 8°C for 1 week. AM symbiosis decreased malondialdehyde (MDA) content in leaves. The contents of photosynthetic pigments, sugars and soluble protein in leaves were higher, but leaf proline content was lower in mycorrhizal than non-mycorrhizal plants. AM colonization increased the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) in leaves. The results indicate that the AM fungus is capable of alleviating the damage caused by low temperature stress on tomato plants by reducing membrane lipid peroxidation and increasing the photosynthetic pigments, accumulation of osmotic adjustment compounds, and antioxidant enzyme activity. Consequently, arbuscular mycorrhiza formation highly enhanced the cold tolerance of tomato plant, which increased host biomass and promoted plant growth.     

Protective systems against active oxygen species in spinach: response to cold acclimation in excess light

Spinach (Spinacia oleracea L.) plants were acclimated to 1° C or maintained at 18° C under the same light regime (260–300 mol photons·m^–2·s^–1). The cold acclimation led to several metabolic and biochemical changes that apparently include improved protection of the photosynthetic apparatus against active oxygen species. In particular, cold-acclimated leaves exhibited a considerably higher ascorbate content and significantly increased activities of superoxide dismutase, ascorbate peroxidase, and monodehydroascorbate reductase in the chloroplasts. The level of dehydroascorbate reductase did not alter. Catalase activity decreased. The photosynthetic pigment composition of cold-acclimated spinach was characterized by increased levels of the xanthophylls lutein + zeaxanthin and violaxanthin. The observed changes are discussed in terms of their possible relevance for plant resistance to photoinhibition at chilling temperatures.Abbreviations DHA dehydroascorbate - GSH reduced glutathione - MDA monodehydroascorbate - SOD superoxide dismutase The authors thank the Deutsche Forschungsgemeinschaft for financial support of this study.     

Effects of root zone temperature and paraquat in the induction of oxidative stress in <Emphasis Type="Italic">Trichosanthes cucumerina</Emphasis> L.

We investigated the influence of root zone temperature (RZT) and the aerial application of paraquat on stress defence mechanisms of Trichosanthes cucumerina L. To achieve this objective, T. cucumerina cv Green was grown with roots at 25 and 30°C root zone temperature and maintained at 20 ± 1°C air temperature in a growth chamber. These RZT and air temperature had earlier been shown to favor growth and fruit production in T. cucumerina. Plants at each RZT were subjected to paraquat treatment (+P) and without paraquat treatment (−P). Paraquat (0.2 mmol/L) was applied as aerial spray. Results showed that the individual main effects of RZT and paraquat treatments significantly affected the chlorophyll fluorescence and gas exchange parameters, while the interaction of both treatments had no significant effect. Results showed that the total phenolics and ascorbic acid contents of T. cucumerina at 30°C were significantly higher than at 25°C. The T. cucumerina plants in +P treatment recorded significantly lower maximum photochemical efficiency (F _v/F _m), net photosynthesis (A), transpiration rate (E), intercellular CO₂ concentration (C _i) and stomatal conductance (g ₁) compared to untreated plants. Also, plants raised at 30°C recorded significantly higher F _v/F _m, A, E, C _i and g ₁ compared to plants raised at 25°C. Plants that were sampled at 48 h after paraquat treatment recorded a higher degree of oxidative damage compared to those sampled at 24 h after treatment. We showed that the degree of damage suffered by T. cucumerina, when treated with paraquat either at 25 or 30°C RZT was similar at 48 h after treatment. We concluded that either at 25 or 30°C, exposure of T. cucumerina to paraquat would impose the same degree of oxidative damage.     

Chloroplasts and mitochondria have multiple heat tolerant isozymes of SOD and APX in leaf and inflorescence in Chenopodium album

Thermal stability of antioxidant defense enzymes superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (APX, EC 1.11.1.11) was studied in chloroplasts and mitochondria of leaf and inflorescence in heat adaptive weed Chenopodium album. Leaf samples were taken in March (31 °C/14 °C) and young inflorescence (INF) was sampled at flowering in April (40 °C/21 °C). Leaf and INF chloroplast and mitochondrial fractions were subjected to elevated temperatures in vitro (5–100 °C) for 30′. SOD and APX showed activity even after boiling treatment in both chloroplast and mitochondria of leaf and INF. SOD was more heat stable than APX in both chloroplasts and mitochondria in both the tissues. Chloroplast contained more heat stable SOD and APX isozymes than mitochondria in both leaf and INF. To the best of our knowledge this is the first report showing presence of thermostable APX isozymes (100 °C for 30′) in chloroplasts and mitochondria in C. album. Heat stable isozymes of SOD and APX in chloroplasts and mitochondria in leaves and inflorescence may contribute to heat tolerance in C. album.     

Effects of Cold Acclimation and Salicylic Acid on Changes in ACC and MACC Contents in Maize during Chilling

The effect of 0.5 mM salicylic acid (SA) pretreatment and of growing at hardening temperatures on chilling-induced changes in 1-aminocyclopropane-1-carboxylic acid (ACC) and malonyl 1-aminocyclopropane-1-carboxylic acid (MACC) was investigated in young maize (Zea mays L.) plants grown in hydroponic solution at 22/20 °C. Chilling at 5 °C caused an increase in ACC content;however, this increase was less pronounced in plants cold acclimated at 13/11 °C 4 d before the chilling treatment, and in those which were pretreated with SA for 1 d before the cold stress. Changes in MACC at low temperature showed no correlation with chilling tolerance in maize.     

Abscisic acid and mefluidide in the regulation of cold hardiness development in Solanum tuberosum L. potato

Plants of Solanum tuberosum L. potato do not cold acclimate when exposed to low temperature such as 5°C, day/night. When ABA (45 M) was added to the culture medium, stem-cultured plantlets of S. tuberosum, cv. Red Pontiac, either grown at 20°C/15°C, day/night, or at 5°C, increased in cold hardiness from –2°C (killing temperature) to –4.5°C. The increase in cold hardiness could be inhibited in both temperature regimes if cycloheximide (70 M) was added to the culture medium at the inception of ABA treatment. Cycloheximide did not inhibit cold hardiness development, however, when it was added to the culture medium 3 days after ABA treatment.When pot-grown plants were foliar sprayed with mefluidide (50 M), ABA content increased from 10 nmol to 30 nmol g^–1 dry weight and plants increased in cold hardiness from –2°C to about –3.5°C. The increases in free ABA and cold hardiness occurred only in plants grown at 20°C/15°C; neither ABA nor cold hardiness increased in plants grown at 5°C.The results suggest that an increase in ABA and a subsequent de novo synthesis of proteins are required for the development of cold hardiness in S. tuberosum regardless of temperature regime, and that the inability to synthesize ABA at low temperature, rather than protein synthesis, appears to be the reason why S. tuberosum does not cold acclimate.