Respiration of Aquatic and Terrestrial Amphibian Embryos

Respiratory constraints on the structure of single eggs andegg masses have affected the mode of amphibian reproductionin water and in air. Aquatic eggs generally require less oxygen,develop faster, and hatch earlier, but these characteristicsare related to small ovum size. A comparison of two speciesof aquatic and terrestrial breeding frogs with similarly sizedova shows no differences in hatching stage, maximum rate ofoxygen uptake, oxygen conductance of the egg capsule, or Po₂difference across the capsule. However, the aquatic speciesdevelops about 2.4 times faster and tolerates lower environmentalPo₂, suggesting adaptation for development in ephemeral water.Modelling of diffusive oxygen transport into a single aquaticegg shows that a large amount of jelly (or a boundary layer)around the capsule may not greatly restrict gas exchange, ifthe inner radius of the capsule is large. However, gelatinousegg masses that contain other embryos that compete for oxygenare therefore limited in size, unless the eggs are ventilatedby convection of water among them. Aquatic egg are often suspendedin masses above the substrate, promoting oxygen movement intothe mass from all directions. Terrestrial egg masses are morediffusion limited, because gravity and surface tension collapsethem, preventing convection between the eggs, and restrictingthe source for oxygen diffusion. Terrestrial embryos are oftenlarger than their aquatic counterparts and have higher demandsfor oxygen. Terrestrial conditions have selected for adaptationsthat reduce respiratory competition between embryos, for example,separating of embryos by large volumes of jelly or reducingthe number of eggs in a clutch. The size of foam nests is unlimited,because oxygen for each embryo is supplied directly from thefoam.     

Dormancy in Seeds of Charlock: IV. INTERRELATIONSHIPS OF GROWTH, OXYGEN SUPPLY AND CONCENTRATION OF INHIBITOR

Respiration measurements were made over a period of 24 h at25 °C on seeds and excised embryos maintained in Warburgflasks with partial pressures of oxygen ranging from 0 to 1atm. In the initial phase (0 to 4 h), the rate of oxygen uptake(Q_O2 of excised embryos increased linearly with external oxygenconcentration (C_O from 0 to 0.1 atm O₂ from 0.1 to 0.2 atm O₂the relation was curvilinear, and from 0.2 to 1.0 atm O₂ uptakewas independent of concentration. In later stages the relationbetween Q_O2 and C_o changed, and from 20 to 24 h the rate ofoxygen uptake increased with concentration to 1.0 atm O₂. Thechanges with time were associated with increase in rate of respiration,increase in cell size and cell number, and the oxidation offats. The decline in concentration of oxygen from the surfaceto the centre of embryos was calculated to be relatively smallat each external oxygen concentration. Althugh the rate of diffusionfailed to keep pace with consumption, the main parameters whichdetermined the internal oxygen status of the embryos were thesurface concentrations and the permeability of the seed coat.The resistance of the seed coat to diffusion of oxygen was foundto be very high, the coefficient of diffusion being about 10^–7mm² s^–1. The concentration of oxygen and in air were estimatedto be approximately 0.04 and 0.02 atm O₂, respectively. Sincea smaller concentration of oxygen (0.012 atm O₂) in the tissueswas found to be sufficient for growth, the dormancy of the seedswas not due to lack of oxygen. Dormancy appeared to be due tothe activity of growth-inhibiting substances, the concentrationof which increased with decrease in oxygen supply; below 0.1atm O₂ their rate of production increased with decrease in theoxygen concentration of the tissues. They accumulated withinthe testas of dormant seeds and prevented cell elongation. Extractsof the inhibitory substances were partially purifield by partitioningthe aqueous fraction with ether and separating chromatographically.The active principle(s) was not abscisic acid ((+)—AbscisinII, ‘Dormin’) or the mustard oil, allylisothiocyanate.     

Studies in the Respiration of Apple at Various Pressures of Oxygen

The results of experiments on the effect of high pressures ofoxygen on apples are outlined. It is shown that partial pressuresof the order of 500 per cent, oxygen are toxic to the tissuesand that there is no evidence of an initial increase in CO₂,output over that in 1 atmosphere 100 per cent, oxygen. It isfurther shown that the actual pressure (up to 20 atmospheres)is not responsible for the disorganization of the cells butthat there is specific oxygen toxicity.     

Role of blood-oxygen transport in thermal tolerance of the cuttlefish, Sepia officinalis

Mechanisms that affect thermal tolerance of ectothermic organismshave recently received much interest, mainly due to global warmingand climate-change debates in both the public and in the scientificcommunity. In physiological terms, thermal tolerance of severalmarine ectothermic taxa can be linked to oxygen availability,with capacity limitations in ventilatory and circulatory systemscontributing to oxygen limitation at extreme temperatures. Thepresent review briefly summarizes the processes that definethermal tolerance in a model cephalopod organism, the cuttlefishSepia officinalis, with a focus on the contribution of the cephalopodoxygen-carrying blood pigment, hemocyanin. When acutely exposedto either extremely high or low temperatures, cuttlefish displaya gradual transition to an anaerobic mode of energy productionin key muscle tissues once critical temperatures (T_crit) arereached. At high temperatures, stagnating metabolic rates anda developing hypoxemia can be correlated with a progressivefailure of the circulatory system, well before T_crit is reached.However, at low temperatures, declining metabolic rates cannotbe related to ventilatory or circulatory failure. Rather, wepropose a role for hemocyanin functional characteristics asa major limiting factor preventing proper tissue oxygenation.Using information on the oxygen binding characteristics of cephalopodhemocyanins, we argue that high oxygen affinities (= low P₅₀values), as found at low temperatures, allow efficient oxygenshuttling only at very low venous oxygen partial pressures.Low venous PO₂s limit rates of oxygen diffusion into cells,thus eventually causing the observed transition to anaerobicmetabolism. On the basis of existing blood physiological, molecular,and crystallographical data, the potential to resolve the roleof hemocyanin isoforms in thermal adaptation by an integratedmolecular physiological approach is discussed.     

Temperature and Antarctic plankton community respiration

Antarctic plankton community respiration rates were determinedfrom in vitro changes in dissolved oxygen. Oxygen consumptionrates, measured at in situ temperatures between 0 and 6°C,were found to lie in the range 0.3–3.7 µmol O₂ l^–1per 24 h. Water samples were collected between East FalklandIsland and South Georgia, South Atlantic Ocean, and incubatedshipboard in the dark at up to 36 temperatures between –2and 14–C. A respiration rate at each temperature was thendetermined and used to calculate the temperature coefficient(Q₁₀) of Antarctic planktonic community respiration from theArrhenius equation. Fourteen Q₀ values lay in the range 1–3,with four further values >5. This range of temperature coefficientvalues for community respiration is comparable to the publishedrange of values for plankton photosynthesis. Frequency distributionsof temperature coefficients for the two processes show similarmodal Q₁₀5 of 2–3. Thus, this study does not lend supportto the hypothesis of a differential response of photosynthesisand community respiration to low temperature.     

Binding Affinities of Oxidized and Reduced Forms of Tetrahalogenated Benzoquinones to the QB Site in Oxygen-Evolving Photosystem II Particles from Synechococcus elongatus

Binding affinities of the Q_B site for four tetrahalogenatedbenzoquinones (THBQs) were investigated by measuring their abilityto serve as electron acceptors or act as inhibitors of oxygenevolution in Synechococcus photosystem II particles. Iodanil,bromanil and chloranil but not fluoranil induced a rapid oxidationof Q_A^– and doubled the area over the fluorescence inductioncurve, indicating dark oxidation of Q₄₀₀. Analyses of thesetwo THBQ-induced reactions and inhibition of the acceleratedQ_A^– oxidation by DCMU yielded binding constants of thequinones comparable to those determined from measurements ofoxygen evolution. Generally, THBQs bound tightly to the Q_B site.However, the binding affinity varied in a wide range with THBQs.The Q_B site bound iodanil with an extremely high affinity butfluoranil relatively weakly. The hydroquinone forms of the THBQsalso bound to the Q_B site and inhibited Q_A^– oxidationby Q_B. The concentrations of the hydroquinones required for50% inhibition of Q_A^– oxidation suggest that the Q_B sitebinds the hydroquinones more weakly than the corresponding quinonesexcept for fluoranil, which binds to the Q_B site more tightlyin its reduced form than in oxidized one. The abilities of THBQsto function as electron acceptors or inhibitors of oxygen evolution,and as oxidants of Q₄₀₀ in the dark, are discussed in relationto the binding affinities of the quinones to the Q_B site. ⁴Present address: Department of Biology, Faculty of Science,Toho University. Miyama 2-2-1, Funabashi, 274 Japan     

Growth, metabolism and growth efficiency of a euphausiid crustacean Euphausia pacifica in the southern Japan Sea, as influenced by temperature

Growth (assessed from intermolt period and molt increment) andmetabolism (oxygen consumption) of the post-larva of Euphausiapacifica from the southern Japan Sea were determined at sevengraded temperatures ranging from 1 to 25°C. The intermoltperiod shortened progressively as temperature increased from1 to 20°C, but an effect of temperature on molt-to-moltgrowth increment was not seen. Oxygen consumption rates wereaccelerated by the increase in temperature up to 20°C. Beyond20°C, E.pacifica exhibited reduced oxygen consumption anddied within 1 day without molting. After removing the effectof body size, the relationships between growth rate and temperature,and between oxygen consumption rate and temperature, were established.The carbon budget was calculated as a function of temperature.Because of differential effects of temperature on growth andmetabolism, the net growth efficiency [K₂ growthx100/(growth+metabolism)]changed with temperature. The optimum temperature at which E.pacificaattained the maximum K₂ was 11.4°C, which was derived fromcalculation of cumulative carbon invested in growth and metabolismin this animal. In an alternative method, the optimum temperaturewas obtained mathematically by solving a set of differentialequations. The biological and ecological significance of theoptimum temperature which leads to the maximum K₂ is discussed.     

Effects of Temperature and Water Loss on Terrestrial Locomotor Performance in Land Crabs: Integrating Laboratory and Field Studies

SYNOPSIS. Terrestrial and semi-terrestrial crustaceans are exposedto fluctuations in ambient temperature and conditions that favorevaporative water loss. These environmental stresses alter performancelimits in the laboratory and behavior in the field. The maximalrate of oxygen consumption, maximum aerobic speed, and endurancecapacity are greater at a body temperature (T_b) of 24°Cthan at 15°C or 30°C in the ghost crab, Ocypode quadrata.The total metabolic cost to move at the same relative speedis greater at a T_b of 24°C than at 15°C. Slower aerobickinetics at 15°C result in a smaller relative contributionof oxidative metabolism to total metabolic cost. However, therelative contributions from accelerated glycolysis are similarat both temperatures. When locomotion is intermittent, the totaldistance traveled before fatigue can be similar at Tbs of 15and 24°C but result from different movement and pause durationsat these temperatures. Performance limits of the ghost crabare negatively affected by dehydration and are sensitive torates of water loss. In the laboratory, endurance capacity ofthe fiddler crab, Uca pugilator, is greater at a T_b of 30°Cthan at 25°C. In the field, freely moving fiddler crabswith a T_b of 30°C travel at faster mean preferred speeds,as determined by motion analysis, than crabs at 25°C. Datafor land crabs support and advance general ectothermic modelsfor the effects of temperature and dehydration on locomotorperformance.     

Functional Structure of the Oxygen-Evolving Unit of Photosystem II as Determined by Radiation Inactivation

The size of the complex that is essential for the electron-transferactivity from the oxygen-evolving center to the secondary electronacceptor, Q_B, is about 250 kDa, as determined by target-sizeanalysis after the radiation inactivation of functions of photosystemII (PS II). Inter-Chl tranfer of excitation energy was insensitiveto the radiation inactivation indicating that the masses ofCP47, CP43, and light-harvesting Chi a/b proteins are not includedin the functional size of the oxygen-evolving PS II complex.The transfer of electrons from the secondary electron donor,Z, to Q_B was catalyzed by a unit of only 65 kDa. The sizes ofthe complexes involved in these light-induced functions of PSII were dependent on the intensity of actinic light. Under saturatingintensities of light, the functional size of the complex fortransfer of electrons from Z to Q_B was 38 kDa, with a correspondingdecrease in the size of the oxygen-evolving PS II from 250 kDato 125 kDa [Takahashi, Mano and Asada (1985) Plant Cell Physiol.26: 383]. The protein of about 30 kDa functions in the photoreductionof the pheophytin molecule, as well as in the electron transferfrom Z to Q_A. Under low-intensity light, complexes having thesame sizes as those of the basal functional complexes undersaturating-intensity light are further required, probably tostabilize separated charges in the PS II reaction center andthe oxygen-evolving center. (Received June 20, 1990; Accepted September 18, 1990)     

Effect of Oxygen on Photosynthetic 14CO2 Fixation in Chroomonas sp. (Cryptophyta) III: Effect of Oxygen on Photosynthetic Carbon Metabolism

Photosynthetic carbon metabolism was studied with Chroomonassp. cells in which the rate of photosynthesis was inhibitedunder both an anaerobic condition and high concentrations ofoxygen. The time course of ¹⁴C-incorporation into photosyntheticproducts showed that 3-phosphoglycerate was the initial productof photosynthetic CO₂ fixation in Chroomonas sp. cells. During5-min photosynthesis, a considerable amount of ¹⁴C was incorporatedinto the insoluble fraction (mostly cryptomonad starch), andoxygen predominantly affected ¹⁴C-incorporation into this fraction.Although ¹⁴C-incorporation into intermediates of the photorespiratorypathway increased with increasing O₂ concentration, the amountswere much less than expected from the degree of oxygen inhibition.It is noteworthy that ¹⁴C-dihydroxyacetone phosphate accumulatedduring photosynthesis only under the anaerobic condition, whereasthe levels of the other phosphate esters were scarcely affectedby the oxygen concentration. Ribulose-1,5-bisphosphate carboxylase from Chroomonas sp. wascompetitively inhibited by oxygen, and its K_m(CO₂) value wassimilar to those of terrestrial C₃ plant enzymes. (Received November 19, 1984; Accepted May 20, 1985)     

Simultaneous changes in cell quotas of microcystin, chlorophyll a, protein and carbohydrate during different growth phases of a batch culture experiment with Microcystis aeruginosa

The cell quotas of microcystin (Q_mcyst), protein (Q_prot), chlorophylla (Q_chloro) and carbohydrate (Q_carbo), as well as the net productionrates of these parameters, were determined during the exponentialand stationary phases in nine batch cultures of Microcystisaeruginosa (CYA 228) at light regimes from 33 to 53 µmolphotons m^–2 s^–1. The following results were obtained.(i) A parallel pattern was found in the changes of Q_mcyst, Q_prot,Q_chloro and Q_carbo during the entire growth cycle and significantcorrelations were recorded between Q_mcyst and Q_prot, Q_chloroand Q_carbo. (ii) The net microcystin production rate (µ_mcyst)was positively correlated with the specific cell division rate(µ_c), the chlorophyll production rate (µ_chloro)and the protein production rate. (iii) A significant inverselinear relationship was found between µ_c and Q_mcyst, i.e.cultures with a positive µ_c had a Q_mcyst between 110 and400 fg microcystin cell^–1, while declining cultures hadQ_mcyst values >400 fg microcystin cell^–1. Maximum variationin Q_mcyst within cultures was 3.5-fold. Collectively, the resultsshow that cells produced microcystin at rates approximatingthose needed to replace losses to daughter cells during divisionand that microcystin was produced in a similar way to proteinand chlorophyll, indicating a constitutive microcystin production.     

Organic Acid Metabolism of Sedum praealtum

1. The organic acids present are citric, isocitric, and l-malic,with a small residue of unidentified acids.
2. The diurnal variationin acidity is due chiefly to changes,in malic acid, with aparallel fluctuation shown by citric acid.Under these conditionsisocitric acid shows little change.
3. The importance of carbondioxide during acidification is confirmed,and it is shown thatat room temperatures or higher the CO₂produced in respirationis sufficient to produce maximum acidification.At lower temperaturesthe supply of CO₂ limits acid production.
4. In the absence ofoxygen no acidification occurs, but even smallquantities (approx.1 per cent.) are sufficient to cause someacid production.
5. Completebalance-sheets are presented for acids, carbohydrates,CO₂ andoxygen for leaves maintained in the dark at high andlow temperatures.As acids are produced there is a correspondingloss of carbohydrate(chiefly starch). A scheme of reactionsis suggested to explainthe experimental results.

     

美洲斑潜蝇实验种群生命表的研究

在15、20、25、30和35℃五种温度下,观察了美洲斑潜蝇Liriomyza sativae Blanchard实验种群的发育历期、存活率、产卵量、寿命等,组建了实验种群生命表,估测了种群参数。结果表明：随温度升高发育历期缩短,在试验的温度范围内,卵、幼虫、蛹的历期变化分别为7.6～2.0天,12.7～2.9天,34.3～6.8天;成虫寿命变化为17.3～6.5天。卵、幼虫、蛹的发育起点温度分别为7.5℃,9.8℃,11.5℃,10.9℃,有效积温为3.9,52.7,128.5,229.9日度。在五种温度下,卵的存活率均在84.2%以上。幼虫除在15℃时存活率为66.7%外,其它温度均在94.6%以上,蛹的存活率变化幅度最大,25℃时为80.3%,35℃时仅为10.0%。30℃时种群的内禀增长力r_m最大,加倍时间最短,净生殖率最高。在20～35℃四种温度下种群的稳定年龄组配中未成熟阶段所占比例均在97%以上。     

The role of temperature in the seed germination of two species of the Solanum nigrum complex

The germination behaviour of S. nigrum L. and S. physalifoliumRusby var. nitidibacatum (Bitter) Edmonds is compared, basedon temperature requirements during imbibition. Three seed lotsof S. nigrum had base temperatures (T_b) between 7.5C and 10C,showing a lower T_b when the period of freezing days, duringwhich each population was collected, was reduced. S. physalifoliumhas a higher value for T_b at constant temperatures (21C) thatcan be interpreted as a dormancy constraint. This constraintis released by alternating temperatures at amplitudes exceeding5C and with the high temperature above 21C by apparently reducingT_b to 12.5C. This implies that for S. physaiifolium temperature has a dualeffect on germination. It is the driving force for changes indormancy, but germination also depends on the temperature. Therole of temperature for S. nigrum is simpler: each populationcollected showed differences in the thermal time required forgermination that could be related to the temperature regimenof the original environment. Key words: Dormancy, S. nigrum, S. physalifolium, thermal time     

The Effect of Petiolar Temperature upon the Translocation Rate of 137Cs in the Phloem of Nymphoides peltata

A fixed length of the petiole of Nymphoides peltata was subjectedto a variety of temperature treatments. A Q₁₀ is calculatedfor the velocity of flow, from the mass transport rate of ¹³⁷Csin the phloem at temperatures between 0 and 50?C. Associatedwith lowered temperatures there was a temporary interruptionof transport overriding the longer term temperature dependence.The characteristics of the interruption were dependent uponthe size of the temperature drop and the magnitude of the ‘new’temperature.     

The Effect of Temperature on the Supply of Oxygen to Embryos ofIntact Acer pseudoplatanus L. Seeds

The effect of temperature on various factors affecting O₂ supplyto enclosed embryos of A. pseudoplatanus L. seeds was investigatedusing a simple model for O₂ consumption by intact seeds. Lowtemperatures which induced germination had little effect onthe diffusivity of the testa although an increase in diffusivityoccurred immediately prior to radicle emergence. The O₂ concentrationsurrounding an enclosed embryo was found to be 5–10% thatof the solution bathing the seeds at storage temperatures. Therespiratory capacity of the embryos was found to be stronglydependent on temperature and, largely as a result of this, atchilling temperatures the O₂ concentration surrounding the enclosedembryo approached that in the ambient solution. The apparentArrhenius activation energy for O₂ consumption was considerablygreater below 17 °C than above this temperature. Althoughinsufficient evidence in themselves for a relationship betweenO₂ availability and seed germination, the results do provideevidence for a mechanism by which such a relationship may beassociated with seeds requiring chilling to induce germination.     

Thermal Dependence of Air Convection Requirement and Blood Gases in the Snake Coluber constrictor

In this report I discuss ventilatory and circulatory adjustmentsthat prov for increased O₂transport associated with increasedbody temperature in the snake Coluber constrictor. Also includedis the effect of temperature upon acid-base status. Minute ventilationincreases with rising body temperature but does not keep pacewith the increment in resting O₂ consumption. The decrease inair convection requirement (i.e., ventilation ÷ oxygenconsumption) causes lung pO₂ and arterial oxygen contentto falland lung pCO₂ to rise. With the rise in lung pCO₂, systemicarterial pCO₂ and H⁺; concentration increase while plasma bicarbonateconcentration does not change. The effect of temperature uponair convection requirement, arterial pCO₂, and pH are most pronouncedat body temperatures above about 27°C whereColuber behavesapproximately as an alphastat pH regulator. Despite the inverserelationship between temperature and lung pO₂, systemic arterialpO₂ is about 80 torr lower at 15°C than at 35°C. Thisdecline in arterial pO₂ as temperature falls is explained byleft shifting the oxygen dissociation curve in the presenceof aconstant right-to-left intracardiac shunt.     

The Responsiveness to Temperature of the Extension Rates of Leaves of Wheat Growing in the Field under Different Levels of Nitrogen Fertilizer

The response of the rates of extension (LER) of wheat leaves(Triticum aestivum cv. Gamenya) to temperatures maintained fora short period was measured by changing the temperature of theextension zone and recording the changes in leaf length. Therange of temperatures used was from 4-38 °C. The LER ofall leaves responded to increases in temperature as field temperatureswere suboptimal. The data obtained from several series of measurementsover different ranges of temperature were combined to producea general response curve. The minimum temperature for LER wasconsidered to be approximately 0 °C, the optimum was 28.4°C, while the maximum was in excess of 38 °C. The responsivenessof LER to temperature, measured by the Q₁₀, declined exponentiallyfrom >6 at 5 °C to 2 at 20 °C. The Q₁₀ at 15 °Cwas not affected by nitrogen supply.     

Seed Viability Constants for Lettuce

Seeds of two lettuce cultivars (Lactuca sativa L., cv. Meikoninginand cv. Grand Rapids) were hermetically stored with constantmoisture contents ranging between 3.6 and 17.9 per cent (freshweight basis) at constant temperatures ranging between 5 and75 °C. The decline with time in percentage germination andpercentage normal seedlings was determined for each storagetreatment. The data were fitted to an equation which containsthe constants: K₁, the probit of the initial percentage germinationor normal seedlings; K_E, a species constant; C_W, the constantof a logarithmic moisture term; C_H, the constant of a lineartemperature term and C_Q, the constant of a quadratic temperatureterm. Regression analysis of data from storage periods up to5.5 years at temperatures of 5–75 °C and seed moisturecontents of 3.6–13.6 per cent yielded the following values:K_E= 8.218, C_W=4.797±0.163, C_H=0.0489±0.0050 andC_Q=0.000365±0.000056. Although this equation consistentlyprovided a better fit, simplified equations, assuming eithera log-linear relationship between seed longevity and temperature,or a log-linear relationship between seed longevity and bothmoisture content and temperature, accounted for more than 94per cent of the variation at the restricted temperature rangeof 5–40 °C. Longevity of the same seed lots at sub-zero temperatures (–5,–10 and –20 °C) was studied in separate tests.Freezing damage, resulting in abnormal seedlings in the germinationtest, occurred at –20 °C when the moisture contentof the seeds exceeded 12 per cent. No decline in percentagenormal seedlings was observed after a storage period of 18 monthsor longer at –20 °C, provided the seed moisture contentdid not exceed 9.5 per cent. For seeds stored at –5 and–10 °C with 9.6–12.5 per cent moisture content,the observed rate of decline of percentage normal seedlingswas adequately predicted by the viability equation, using theabove values for the constants. This suggests that for low moisturecontents the viability equation can be applied to estimate longevityat sub-zero temperatures. Lettuce, Lactuca sativa (L.), seed longevity, seed storage, viability constants, storage conditions     

Comparison of Atrazine-Resistant and -Susceptible Biotypes of Senecio vulgaris L.: Effects of High and Low Temperatures on the in vivo Photosynthetic Electron Transfer in Intact Leaves

The effects of temperature on the yield of in vivo modulatedchlorophyll fluorescence were measured in intact leaves of atrazineresistantand -susceptible biotypes of the weed Senecio vulgaris L. At25 ?C, the photochemical quenching (q_Q of steady-state chlorophyllfluorescence was reduced by around 30% in the atrazine-resistantmutant as compared to the susceptible wild type, indicatinga higher reduction state of the primary electron acceptor Q_Aof photosystem II in the former biotype. Moderately elevatedtemperatures (above 30 ?C) further increased the steady-stateconcentration of reduced in the mutant. Analysis of the temperature dependence of both the photochemicalquenching q_Q and the initial fluorescence level F_o clearly indicateda drastically enhanced heat-sensitivity of the photochemicalapparatus in the atrazine-resistant Senecio leaves. The heat-inducedchanges in F₀ and q_Q were closely correlated, suggesting thatthe phenomenon responsible for the rise in F₀ was also involvedin the inhibition of the photosynthetic electron flux. Low temperaturesalso affected reoxidation but, in contrast to heating, no apparent differences were observed in the behaviourof the two biotypes exposed to chilling stress. Key words: Atrazine-resistance, chlorophyll fluorescence, heat stress, Senecio vulgaris