Heat Shock Increases the Tolerance of Plants to UV-B Radiation: 2. Ethylene and Carbon Dioxide Evolution

The effects of heat shock (HS, 45°C) and UV-B radiation (280–320 nm, 18.3 kJ/(m²h)) and the consecutive action of the combination of these factors on ethylene production, gas exchange, and the growth of intact melon (Melo sativusSager.) seedlings were investigated. The changes in ethylene production and carbon dioxide exchange were described by a single-peaked curve. In the course of UV-B irradiation, the time of maximum ethylene and CO₂evolution coincided (the first 5 min) and comprised 0.36 nl/(seedling h) for ethylene and 146.2 l/(seedling h) for carbon dioxide. After HS, the maximum of ethylene production (0.37 nl/(seedling h)) was reached within 10 min, and that of carbon dioxide production (313 l/(seedling h)), within 45 min. The rate of ethylene production (0.22 nl/(seedling h)), carbon dioxide production (97.7 l/(seedling h)), and oxygen consumption (162.5 l/(seedling h)) in the control seedlings did not change in the course of experiment. Throughout the experiment, the respiratory quotient of seedlings was ca. 0.6 regardless on the nature and duration of the acting factor. Preliminary heating at 45°C for 1 h increased the tolerance of seedlings to the subsequent UV-B radiation for 1 h. The protective effect of HS manifests itself in alleviating the inhibiting action of UV-B radiation on seedling growth and development, and this effect was preceded by an increase in ethylene production and respiration. The possible mechanisms of cross-tolerance of plants to overheating and UV-B radiation are discussed.     

Effects of ultraviolet-B enhanced radiation and temperature on growth and photochemical activities inVigna unguiculata

Changes in growth characteristics and photochemical activities inVigna unguiculata L. Walp seedlings maintained at constant temperature of 10, 20, 30 and 40 ‡C under control and ultraviolet-B enhanced radiation (UV-B) were investigated. UV-B retarded the shoot elongation and also leaf expansion to a great extent at 30 ‡C but produced only marginal changes at 20 and 40 ‡C. Similar response was also observed with respect to changes in leaf fresh and dry masses and total chlorophyll (Chl) content under these temperatures. At 10 ‡C the total Chl content was 3-fold higher under the treatment than under control conditions. In seedlings growing at 20 and 30 ‡C the overall photosynthetic electron transport (H₂O -> methyl viologen) showed a significant enhancement during the 36-h UV-B treatment and thereafter a gradual reduction. Although a similar trend was found in photosystem 1 (PS1), the inhibition even after 60 h of UV-B treatment was not statistically significant. Photosystem 2 (PS2) activity was inhibited in seedlings treated for 60 h by UV-B at 20 and 30 ‡C. However, no inhibition was observed at 40 ‡C. No detectable photochemical activity was found in seedlings grown at 10 ‡C under either control or UV-B enhanced irradiation although the chloroplasts contained Chl. This work was supported by a Research Associateship to N.N. from the Council of Scientific and Industrial Research (India) and by a grant from the Ministerio de Education y Ciencia (ref. 5894- AM086772).     

Heat Shock Induced Lipid Changes and Solute Leakage in Germinating Seeds of Pigeonpea

Heat shock (HS) reduced total lipid and phospholipid contents and their synthesis in germinating seeds of pigeonpea [Cajanus cajan (L.) Millspaugh]. Lipid peroxidation was also enhanced with increasing temperature and HS duration. HS influenced lipid metabolism to a higher extent at 45°C than at 40°C. This altered lipid metabolism and lipid peroxidation was associated with the loss of various solutes from the germinating seeds, and modification of growth and development. Pretreatment of germinating seeds at 40°C for 1 h or at 45°C for 10 min followed by incubation at 28°C for 3 h prior to 45°C for 2 h ameliorated solute leakage due to reduced lipid peroxidation and improvement in lipid content and membrane function.     

Growth and morphological responses to different UV wavebands in cucumber (Cucumis sativum) and other dicotyledonous seedlings

We examined the influence of short-term exposure of different UV wavebands on the fine-scale kinetics of hypocotyl growth of dim red light-grown cucumbers (Cucumis sativus L.) and other selected dicotyledonous seedlings to evaluate: (1) whether responses induced by UV-B radiation (280-320 nm) are qualitatively different from those induced by UV-A (320-400 nm) radiation, and (2) whether different wavebands within the UV-B elicit different responses. Responses to brief (30 min) irradiations with 3 different UV wavebands all included transient inhibition of elongation during irradiation followed by wavelength specific responses. Irradiations with proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm) induced inhibition of hypocotyl elongation within 20 min of onset of irradiation, while UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation with a lag of 1-2 h. The response to short wavelength UV-B was persistent for at least 24 h, while the response to long wavelength UV-B lasted only 2-3 h. The UV-A treatment induced reductions in elongation rates of approximately 6-9 h following exposure followed by a continued decline in rates for the following 15-18 h. Short wavelength UV-B also induced positive phototropic curvature in both cucumber and Arabidopsis seedlings, and this response was present in nph-1 mutant Arabidopsis seedlings defective in normal blue light phototropism. Reciprocity was not found for the response to short wavelength UV-B. The short wavelength and long wavelength UV-B responses differed in dose-response relationships and both short wavelength responses (phototropic curvature and elongation inhibition) increased sharply at wavelengths below 300 nm. These results indicate that different photosensory processes are involved in mediating growth and morphological responses to short wavelength UV-B (280-300 nm), long wavelength UV-B (essentially 300-320 nm) and UV-A. The existence of two separate types of hypocotyl inhibition responses to UV-B, with one that depends on the intensity of the light source, provides alternate interpretations to findings in other studies of UV-B induced photomorphogenesis and may explain inconsistencies between action spectra for inhibition of stem growth.     

Prolamellar bodies of oat,wheat, and rye: Structure,lipid composition,and adsorption of saponins

Summary Soybean seedlings (Glycine max) were incubated in narrow temperature regimes to study the effects of heat shock on cell structures. The incubation temperatures used were as follows: 1. 28 °C (2h); 2. 40 °C (2h); 3. 45 °C (2h); 4. 40 °C (2h)45 °C (2h); 5. 47. 5 °C (10 min); 6. 40 °C (2h)47. 5 °C (10 min). Both optical and electron micrographs were taken of the different tissues of root meristems as they responded to heat shock. Cells of roots heated to 45 °C (2h) or 47.5 °C (10 min) with lethal treatment showed drastic heat injuries:e.g., membrane damage, coagulated plasmolysis, protoplasmic contraction, and leakage of cell content. Nucleolar segregation occurred in cells treated at both lethal and supraoptimal temperatures. Seedlings preincubated at 40 °C (2 h) became thermo-tolerant to lethal temperature treatment of 45 °C (2 h) or 47.5 °C (10 min), by protecting the plasmalemma, mitochondria, plastids and nuclei from heat damage. Without preincubation, however, these structures were destroyed.Abbreviations CC Central cylinder - CR Cortex - M Mitochondria - N Nuclei - Nu Nucleoli - P Plastids - RC Root cap - RE Region of elongation - RM Region of meristem     

The Effect of Heat Shock on the Capacity of Wheat Plants to Restore Their Photosynthetic Electron Transport after Photoinhibition or Repeated Heating

The shoots of 16-day-old spring wheat plants (Triticum aestivumL., cv. Albidum 29) were subjected to heat shock (HS) at 40, 41, or 43°C for 10 min. The activity of the Hill reaction in chloroplasts isolated immediately after HS was 83, 61, and 30% of the initial value, respectively. The activity of the Hill reaction was also estimated after plant return to the initial growth conditions for one day. It was completely restored after heating at 40°C and achieved 82 and 30–33% of the initial level after heating at 41 and 43°C, respectively. Thereafter, the shoots were heated repeatedly at 42, 43, or 43.5°C for 10 min, and the activity of the Hill reaction was measured immediately or one day after this heating. Immediately after the second heating, the activity decreased again as compared to its value before heating. The percent of inhibition of the Hill reaction was similar in the control plants not subjected to preliminary HS and HS-treated plants independently of the temperature used. However, after one-day growth under normal conditions, the activity of the Hill reaction was restored almost completely in HS-treated plants but not more than by 10% in the control plants. The conclusion is that different mechanisms underlie the development of the tolerance to HS and recovery. Some plants were tested for the effect of HS (40°C) on their tolerance to photoinhibition. The degree of the Hill reaction inhibition after plant exposure to the light of 65–75 klx for 3 h was essentially similar in detached leaves from the HS-treated and unheated plants and comprised about 40% of the activity before light stress. After the leaves were returned to the low-light conditions for 3 h, the Hill reaction was restored and attained about 75% of that before photoinhibition in both HS-treated and untreated plants. The lack of the HS-induced stimulation of the Hill reaction recovery after photoinhibition is evidently related to the fact that heating and excess light damage different sites of photosystem II, which implies the different pathways for the recovery of its functional activity.     

The effect of hypothermic treatment on the repair or expression of X-ray damage measured in split dose experiments on L5178Y-S cells

Summary The nature of the post-irradiation lesions and processes leading to cellular reproductive death or survival were investigated in mouse lymphoblastic leukemia L5178Y-S (LY-S) cells. Post-(x-)irradiation incubation at 25° C protects LY-S cells against the fixation of biologically expressed damage which takes place at 37° C. An optimal condition for the repair of damage, assayed in split-dose experiments as split-dose recovery (SDR), is 1 h at 37° C followed by 4 h holding at 25° C prior to the second half of a split dose, or 5 h holding at 25° C without a 37° C incubation during the interval between doses. Longer incubations at 37° C resulted in progressively decreased survivals. Postirradiation inhibition of DNA synthesis at 37° C was observed only during the first 30 min; thereafter,³H-dThdR incorporation washigher than in unirradiated controls. Theexcess synthesis effect was removed by shifting irradiated cells to 25° C holding. The inhibition observed at 25° C was reversed by shifting to 37° C. Thus the degree of postirradiation DNA synthesis is inversely related to SDR. DNA filter elution shows complete strand break repair by 20 min at 37° C, and by 3 h at 25° C; DNA double-strand break (DSB) repair plateaus at 80% (37° C) and 60% (25° C) after 90 min. An inverse correlation was found between total strand break repair rate, as assayed by filter elution methods, and cell survival. This work was supported by a grant from The Mathers Charitable Foundation.A preliminary report of this work was presented at the 35th Annual Meeting of the Radiation Research Society, Atlanta, GA 1987, USA     

Enhancement of red-light-induced anthocyanin synthesis in sorghum first internodes by moderate low temperature given in the pre-irradiation culture period

Anthocyanin synthesis in the broom sorghum, Sorghum bicolor Moench cvs. Acme Broomcorn and Sekishokuzairai-Fukuyama, is mediated separately or synergistically by an ultraviolet light-B (UV-B) photoreceptor and phytochrome. When seedlings were exposed to moderate low temperatures ranging from 12 to 20° C before irradiation, only the phytochrome-mediated anthocyanin synthesis was markedly enhanced compared with the control, which was kept throughout at 24° C; synthesis mediated by the UV-B photoreceptor was unaffected. The effectiveness of an exposure to 20° C increased as the duration of exposure increased up to 24 h and as the time of exposure became closer to the time of irradiation. However, when seedlings were exposed to 20° C from after irradiation until harvest, anthocyanin syntheses induced by both UV-B and red light were equally suppressed, probably due to the general reduction of metabolism involved in anthocyanin synthesis that is a consequence of lower temperature. The results support the view that the signal transduction of the pyhtochrome system is different from that of the UV-B photoreceptor, and indicate that the phytochrome system may involve a step or steps which are amplified by a previous exposure to the moderate low temperature.Abbreviations FR far-red light - LT low temperature - MLT moderate low temperature - Pfr far-red-light-absorbing form of phytochrome - R red light - UV ultraviolet light - UV-B ultraviolet light-B We thank Drs. Y. Takeuchi (Shionogi Pharmaceutical Company, Aburahi, Shiga) and K. Hosaka (the Experimental Farm, Kobe University, Kasai) for seeds; Dr. M. Watanabe and Mr. M. Kubota (the National Institute for Basic Biology, Okazaki) for operation of the spectrograph. This work was supported by grants from the Yamada Science Foundation, Ministry of Education (No. 63480015 and 03454048), and the National Institute for Basic Biology (Large Spectrograph grant No. 91-523).     

Light requirement,phytochrome and photoperiodic induction of flowering of Pharbitis nil Chois

For dark-grown seedlings of Pharbitis nil capacity to flower in response to a single inductive dark period was established by 24 h white, far-red (FR) or ruby-red (BCJ) light and by a skeleton photoperiod of 10 min red (R)-24 h dark-10 min R. FR alone was ineffective without a brief terminal (R) irradiation, confirming that the form of phytochrome immediately prior to darkness is a crucial factor for flowering in Pharbitis. The magnitude of the flowering response was significantly greater after 24 h FR or white light (WL) (at 18° C and 27° C) than after two brief skeleton R irradiations, but the increased flowering response was not attributable to photosynthetic CO₂ uptake because this could not be detected in seedlings exposed to 24 h WL at 18° C. Photophosphorylation could have contributed to the increased flowering response as photosystem I fluorescence was detectable in plants exposed to FR, BCJ, or WL, but there were large differences between flowering response and photosystem I capacity as indicated by fluorescence. We conclude that phytochrome plays a major role in photoresponses regulating flowering. There was no simple correlation between developmental changes, such as cotyledon expansion and chlorophyll formation during the 24-h irradiation period, and the capacity to flower in response to a following inductive dark period. Changes in plastid ultrastructure were considerable in light from fluorescent lamps and there was complete breakdown of the prolamellar body with or without lamellar stacking at 27 or 18° C, respectively, but plastid reorganization was minimal in FR-irradiated seedlings.Abbreviations BCJ irradiation from photographic ruby-red lamps - FR far-red light - P_fr far-red-absorbing from of phytochrome - P total phytochrome content - R red light - WL white light from fluorescent lamps     

Acquisition of Thermotolerance in Soybean Seedlings : Synthesis and Accumulation of Heat Shock Proteins and their Cellular Localization

When soybean Glycine max var Wayne seedlings are shifted from a normal growth temperature of 28°C up to 40°C (heat shock or HS), there is a dramatic change in protein synthesis. A new set of proteins known as heat shock proteins (HSPs) is produced and normal protein synthesis is greatly reduced. A brief 10-minute exposure to 45°C followed by incubation at 28°C also results in the synthesis of HSPs. Prolonged incubation (e.g. 1-2 hours) at 45°C results in greatly impaired protein synthesis and seedling death. However, a pretreatment at 40°C or a brief (10-minute) pulse treatment at 45°C followed by a 28°C incubation provide protection (thermal tolerance) to a subsequent exposure at 45°C. Maximum thermoprotection is achieved by a 2-hour 40°C pretreatment or after 2 hours at 28°C with a prior 10-minute 45°C exposure. Arsenite treatment (50 micromolar for 3 hours) also induces the synthesis of HSP-like proteins, and also provides thermoprotection to a 45°C HS; thus, there is a strong positive correlation between the accumulation of HSPs and the acquisition of thermal tolerance under a range of conditions.

During 40°C HS, some HSPs become localized and stably associated with purified organelle fractions (e.g. nuclei, mitochondria, and ribosomes) while others do not. A chase at 28°C results in the gradual loss over a 4-hour period of the HSPs from the organelle fractions, but the HSPs remain selectively localized during a 40°C chase period. If the seedlings are subjected to a second HS after a 28°C chase, the HSPs rapidly (complete within 15 minute) relocalize in the organelle fractions. The relative amount of the HSPs which relocalize during a second HS increases with higher temperatures from 40°C to 45°C. Proteins induced by arsenite treatment are not selectively localized with organelle fractions at 28°C but become organelle-associated during a subsequent HS at 40°C.

     

Effects of solar ultraviolet-B radiation,temperature and CO2 on growth and physiology of sunflower and maize seedlings

The effects of solar UV-B radiation, in combination with elevated temperature (4 °C ) and CO₂ (680 L L^-1 concentration, on sunflower and maize seedlings were studied from May to August in 1991 at the research station Quinta de São Pedro in Portugal (38.7°N). The ambient solar radiation of Portugal was reduced to levels of Central European latitudes by using the ozone filter technique. This radiation served as control, while the ambient solar radiation of Portugal was to simulate intense UV-B treatment (+30%). All plants were grown up to 18 days in 4 climate controlled growth chambers simulating a daily course of temperature with T_max=28 °C or 32 °C , resp., and ambient CO₂ concentrations (340 L L^-1); in one chamber the CO₂ concentration was twice as high (680 L L^-1). Under intense UV-B and at 28 °C (T_max) all growth parameters (height, leaf area, fresh and dry weight, stem elongation rate, relative growth rate) of sunflower and maize seedlings were reduced down to 35% as compared to controls. An increase in growing temperature by 4 °C , alone or in combination with doubled CO₂, compensated or even overcompensated the UV-B effect so that the treated plants were comparable to controls. Chlorophyll content, on a leaf area basis, increased under intense UV-B radiation. This increase was compensated by lower leaf areas, resulting in comparable chlorophyll contents. Similar to growth, also the net photosynthetic rates of sunflower and maize seedlings were reduced down to 29% by intense UV-B calculated on a chlorophyll basis. This reduction was compensated by an increased temperature. Doubling of CO₂ concentration had effects only on sunflower seedlings in which the photosynthetic rates were higher than in the controls. Dark respiration rates of the seedlings were not influenced by any experimental condition. Transpiration and water use efficiency (wue) were not influenced by intense UV-B. Higher temperatures led to higher transpiration rates and lower water use efficiencies, resp.. Doubling of CO₂ reduced the transpiration rate drastically while for wue maximum values were recorded.     

Response of antioxidant defense system to laser radiation apical meristem of Isatis indigotica seedlings exposed to UV-B

To determine the response of antioxidant defense system to laser radiation apical meristem of Isatis indigotica seedlings, Isatis indigotica seedlings were subjected to UV-B radiation (10.08 kJ m⁻²) for 8 h day⁻¹ for 8 days (PAR, 220 µmol m⁻² s⁻¹) and then exposed to He-Ne laser radiation (633 nm; 5.23 mW mm⁻²; beam diameter: 1.5 mm) for 5 min each day without ambient light radiation. Changes in free radical elimination systems were measured, the results indicate that: (1) UV-B radiation enhanced the concentration of Malondialdahyde (MDA) and decreased the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in seedlings compared with the control. The concentration of MDA was decreased and the activities of SOD, CAT and POD were increased when seedlings were subjected to elevated UV-B damage followed by laser; (2) the concentration of UV absorbing compounds and proline were increased progressively with UV-B irradiation, laser irradiation and He-Ne laser irradiation plus UV-B irradiation compared with the control. These results suggest that laser radiation has an active function in repairing UV-B-induced lesions in seedlings.Key words: Isatis indigotica, laser, UV-B lesion     

Impact of silicon on maize seedlings exposed to short-term UV-B irradiation

Enhanced UV-B irradiation is one of the most important abiotic stresses that can influence various aspects of plant morphology, biochemistry and physiology. Silicon as a beneficial element can increase the plant’s tolerance against different abiotic stresses, including UV-B stress. In this work, the effect of silicon supplementation on the sensitivity of young maize (Zea mays L.) seedlings exposed to short-term UV-B radiation was studied. The seedlings were grown with 0 or 5 mM silicon in cultivation medium and on the fifth day of cultivation, they were exposed for 15 and 30 min to UV-B (302 nm) radiation. No significant changes in growth and content of assimilation pigments and the chlorophyll a/b ratio were observed in any of tested irradiation periods in control or Si-treated plants. Under UV-B stress, the content of ROS (hydrogen peroxide and superoxide radical) and TBARS increased in control plants. The oxidative status of Si-treated plants was only slightly affected even after 30 min. Phenolic metabolites (total phenols and flavonoids), important for their screening function under radiation stress, slightly increased after UV-B exposure in control plants, however, only flavonoids increased after 30 min in Si-treated plants. The measured parameters indicated that to some extent silicon supplementation contributes to higher UV-B tolerance of maize seedlings.     

Ultraviolet-B irradiation-induced freezing tolerance in relation to antioxidant system in winter wheat (Triticum aestivum L.) leaves

Seven-day-old seedlings of winter wheat (Triticum aestivum L.) in a growth chamber were exposed to ultraviolet-B (UV-B) irradiation for 20 days with daily biologically effective (BE) UV-B irradiation (UV-B_BE) at low (4.2 kJ m⁻² day⁻¹, L_UVB) and high (7.0 kJ m⁻² day⁻¹, H_UVB) levels. The UV-B irradiated seedlings and the control without UV-B irradiation were then subjected to freezing stress at −6 °C for 6 h and recovered to 20 °C with gradually increased temperature, to investigate the effects of UV-B irradiation on freezing tolerance. During the UV-B exposure, both L_UVB and H_UVB irradiated seedlings had lower half lethal temperature (LT₅₀) values in comparison with the control, and L_UVB more effectively decreased the LT₅₀ values than H_UVB. Moreover, foliar concentrations of thiobarbituric acid reactive substances (TBARS) in the UV-B irradiated seedlings were lower than that of control after recovery from freezing stress. Hydrogen peroxide (H₂O₂) rapidly increased after UV-B exposure, as did activity of superoxide dismutase (SOD). After recovery from freezing stress, activities of catalase (CAT), guaiacol peroxidase (GPX) and glutathione reductase (GR) increased in both L_UVB and H_UVB leaves, whereas activities of ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) significantly increased only in the L_UVB leaves. Furthermore, the ascorbic acid (AsA) concentration and reduced-to-oxidized ascorbate ratio (AsA/DHA) increased in the L_UVB leaves both at the end of UV-B exposure and after recovery from freezing stress. However, the reduced glutathione (GSH) concentration, together with reduced-to-oxidized glutathione ratio (GSH/GSSG) increased in both L_UVB and H_UVB leaves after recovery from freezing stress. UV-B irradiation increased freezing tolerance in winter wheat seedlings, and this response appears to involve the scavenging enzymes and compounds in the antioxidant defense systems, particularly the ascorbate–glutathione cycle.     

Heat-shock-induced enhanced reactivation of UV-irradiated Herpesvirus

The objective of this study was to compare the ability of heat shock (HS) with that of another type of cellular stress, UV irradiation, to cause the induction of enhanced viral reactivation, a process that may represent an SOS-type repair process in mammalian cells. Studies performed to evaluate the effect of HS on growth of Vero cells revealed that HS at 45 degrees C for 45 min caused inhibition of cell growth similar to that caused by UV irradiation at 12 J/m2, but this inhibition was not observed at HS treatment for 5-15 min, or at a UV fluence of 2 J/m2. Enhanced reactivation of UV-irradiated Herpesvirus was observed in cells which had been pretreated by HS for greater than 30 min or UV at 12 J/m2. The synthesis of new proteins following HS for 15 and 45 min and UV at 12 J/m2 was examined by [35S]methionine-labeling experiments. The new synthesis of two HS proteins with molecular weights of 46 000 and 78 000 was induced by both levels of HS, but to a much greater extent at the high dose. These proteins were not detected in response to UV irradiation. These results indicate that, like UV irradiation, HS at levels inhibitory to cell growth induced enhanced viral reactivation in Vero cells. The results also suggest that at least two proteins in the HS protein family are not necessary for this response to occur.     

Activation ofStreptomyces viridochromogenes spores by detergents

Constitutively dormant spores ofStreptomyces viridochromogenes germinate rapidly following treatment with 1.0% of the detergents Tween 80, sodium dodecyl sulfate (SDS), or sodium heptadodecyl sulfate. Six other detergents did not activate the spores. Activation by SDS was studied further. The spores were not activated following treatment with 0.09% or less of SDS for 60 min at 37°C. Activation was complete within 1 to 2 min of treatment with 1.0% SDS. the SDS-activated spores became deactivated during incubation in buffer. Deactivation was slow at 4°C and complete after incubation for 12 h at 25°C or 6 h at 37°C. The endogenous respiratory rate of the spores was increased 3-fold by SDS activation.     

Photoprotection, photosynthesis and growth of tropical tree seedlings under near-ambient and strongly reduced solar ultraviolet-B radiation

Seedlings of two late-successional tropical rainforest tree species, Tetragastris panamensis (Engler) O. Kuntze and Calophyllum longifolium (Willd.), were field grown for 3-4 months at an open site near Panama City (9 degrees N), Panama, under plastic films that either transmitted or excluded most solar UV-B radiation. Experiments were designed to test whether leaves developing under bright sunlight with strongly reduced UV-B are capable of acclimating to near-ambient UV-B conditions. Leaves of T. panamensis that developed under near-ambient UV-B contained higher amounts of UV-absorbing substances than leaves of seedlings grown under reduced UV-B. Photosynthetic pigment composition, content of alpha-tocopherol, CO(2) assimilation, potential photosystem II (PSII) efficiency (evaluated by F(v)/F(m) ratios) and growth of T. panamensis and C. longifolium did not differ between seedlings developed under near-ambient and reduced solar UV-B. When seedlings were transferred from the reduced UV-B treatment to the near-ambient UV-B treatment, a pronounced inhibition of photosynthetic capacity was observed initially in both species. UV-B-mediated inhibition of photosynthetic capacity nearly fully recovered within 1 week of the transfer in C. longifolium, whereas in T. panamensis an about 35% reduced capacity of CO(2) uptake was maintained. A marked increase in UV-absorbing substances was observed in foliage of transferred T. panamensis seedlings. Both species exhibited enhanced mid-day photoinhibition of PSII immediately after being transferred from the reduced UV-B to the near-ambient UV-B treatment. This effect was fully reversible within 1d in T. panamensis and within a few days in C. longifolium. The data show that leaves of these tropical tree seedlings, when developing in full-spectrum sunlight, are effectively protected against high solar UV-B radiation. In contrast, leaves developing under conditions of low UV-B lacked sufficient UV protection. They experienced a decline in photosynthetic competence when suddenly exposed to near-ambient UV-B levels, but exhibited pronounced acclimative responses.     

Solute Leakage in Soybean Seedlings under Various Heat Shock Regimes

The leakage of solute from intact seedlings during incubationunder various heat shock (HS) regimes was studied. ContinuousHS at 40?C did not induce leakage of amino acids, soluble sugarsand electrolytes into the incubation medium, when compared withcontrol incubation at 28?C. Continuous HS at 45?C (lethal treatment)caused leakage to increase continuously and linearly duringa 5-h treatment period. However, brief HS at 47.5?C, (lethaltreatment), unlike continuous HS at 45?C, induced leakage ata slower rate which reached a plateau within 2 to 3 h at 28?C.Preincubation for 2 h at 40?C completely prevented the leakagecaused by the brief HS at 47.5?C, but not that caused by continuous45?C HS. The amount of leakage during 2 h of incubation at 45?Cwas reduced to half by 30 min preincubation at 40?C and wasreduced to a minimal level by 1-h preincubation. Greater reductionof leakage at 45?C HS was observed when an additional 4 h ofincubation at 28?C immediately followed the 40?C preincubation.These results and previous findings (Lin et al. 1984) indicatethat the synthesis and accumulation of HS proteins (HSPs) areimportant for preventing HS-induced leakage from the cells.One of the HSPs, 15 kD in size appeared to be associated withthe plasma membrane. (Received February 12, 1985; Accepted August 30, 1985)     

UV-B response of green and etiolated barley seedlings

7-d-old etiolated and green barley seedlings (Hordeum vulgare L. cv. Alfa) were irradiated with UV-B for 30 min and then kept for 24 h in light or darkness. Chlorophyll (Chl) synthesis was inhibited by about 30 % as a result of UV-B irradiation, but there were no significant changes in photochemical activity measured by variable to maximum fluorescence ratio (F_v/F_m), quantum yield (Φ_PS2) and oxygen evolution rate. Electron transport of etiolated seedlings was similar to that of green ones, nevertheless, the Chl content was more then 2-fold lower. Ribulose-1,5-bisphosphate carboxylase/oxygenase large and small subunits were diminished as a result of UV-B irradiation in etiolated and green plants, especially in those kept in the darkness. Catalase activity decreased and total superoxide dismutase activity increased in green and etiolated plants following UV-B treatment. When benzidine was used as a substrate, an isoform located between guaiacol peroxidases 2 and 3 (guaiacol peroxidase X) appeared, which was specific for UV-B treatment. As a result of irradiation, the contents of UV-B absorbing and UV-B induced compounds increased in green seedlings but not in etiolated seedlings.     

Enhancement of the Tolerance to Oxidative Stress in Cucumber (Cucumis sativus L.) Seedlings by UV-B Irradiation: Possible Involvement of Phenolic Compounds and Antioxidative Enzymes

Cucumis sativus L.) seedlings were irradiated or not irradiated with UV-B for several days in environment-controlled growth chambers. The first leaves irradiated with UV-B were retarded in growth but simultaneously acquired a remarkably high tolerance to oxidative stress, as induced by paraquat treatment, compared with the non-irradiated leaves. This enhanced tolerance was observed within 1d after the start of UV-B irradiation and was maintained during the 12 d period of UV-B treatment. The effects of UV-B on several antioxidative enzymes were examined, and activities of superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase, but not of glutathione reductase, were found to be enhanced. However, activation of these enzymes occurred only from 6 d after the start of irradiation. In contrast, accumulation of phenolic compounds was observed within 1d after the start of UV-B irradiation. HPLC analysis of phenolic compounds showed the distinct enhancement of a substance, which may have antioxidative properties in cucumber seedlings irradiated with UV-B. On the basis of these results, we conclude that not only antioxidative enzymes but also other factors in cucumber seedlings irradiated with UV-B, such as phenolic compounds, may participate in the enhanced tolerance to oxidative stress. Received 10 June 2000/ Accepted in revised form 1 July 2000