Is Lamprey Metamorphosis Regulated by Thyroid Hormones?

Lampreys are one of just a few fishes which have a true (firstor first type) of metamorphosis in their life cycle. In thesea lamprey (Petromyzon marinus), spontaneous metamorphosisis initiated when the size (length and weight), condition factor,and lipid stores reach appropriate levels and coincide withthe postwinter rise in water temperature. The serum levels ofthe thyroid hormones, thyroxine (T₄) and triiodothyronine (T₃),drop dramatically at the onset of metamorphosis and metamorphosiscan be induced with treatment of animals with the goitrogen,KCIO₄, which also results in a decline in serum levels of thyroidhormones. The fact that thyroid hormone treatment can blockspontaneous and induced metamorphosis is support for the viewthat thyroid hormones, particularly T₃, operates like a juvenilehormone in lamprey metamorphosis; this view is counter to therole of thyroid hormones in metamorphosis of other vertebrates.The monodeiodinase pathways, whereby T₄ is converted to T₃ orto the biologically inactive reverse T₃, and even further degradationof T₃, may be a significant mechanism directing metamorphicchange. Lamprey metamorphosis is facultative in that it is initiatedor inhibited depending upon the coordination of a complex integrationof environmental, metabolic and hormonal cues. Thyroid hormonesdo not regulate lamprey metamorphosis in the sense observedin other vertebrate metamorphoses but they are important tothe developmental process. Some of the features of the involvementof thyroid hormones in lamprey metamorphosis may be relatedto the presence of the endostyle in larvae which in turn reflectsthe ancient origins of this vertebrate and perhaps the conservationof an ancient method of induction of metamorphosis. Some cluefor other factors which initiate lamprey metamorphosis may comethrough the examination of inducers of metamorphosis in lowerchordates     

Endocrine Factors Affecting Thyroid Economy of Teleost Fish

SYNOPSIS. This review examines the way in which the activityof the hypothalamo-pituitary-thyroid axis in teleost fishesis modified by pituitary, steroid and amine hormones. Thesefactors may act at the level of the hypothalamus, the pituitarygland (thyrotrops), the thyroid gland, or the peripheral tissues,and affect thyroid hormone synthesis, release, metabolism, ordegradation. With few exceptions, the studies are limited toonly a few species, the results are often fragmentary and contradictory,are based on experiments in which pharmacological, rather thanphysiological, hormone ranges have been used, and there is toolittle information to establish a consistent pattern of responseto endocrines other than those which are components of the hypothalamo-pituitary-thyroidaxis itself. Most studies evaluating the effects of pituitaryhormones on thyroid economy in fish have, of necessity, reliedon mammalian preparations. Some of these (e.g., prolactin andthe gonadotropins) elicit very different responses from thoseof the equivalent semipurified or purified piscine hormone,although mammalian and piscine TSH and GH respectively appearto elicit similar responses in the teleostean thyroid system.The elevation of plasma T₄ levels in response to a challengeof exogenous TSH has been used in several studies as a way ofevaluating the sensitivity of the thyroid to TSH; the responseis modified by photoperiod, season, stage of development, estrogenand corticosteroids.     

Thyroid Hormone Physiology in Galliformes: Age and Strain Related Changes in Physiological Control

SYNOPSIS. Considering thyroid hormone levels in the chicken,we should keep in mind that these concentrations are the integratedresult of production and release on one hand and elimination,tissue utilization or peripheral interconversion on the other.Moreover all these processes can be and are probably influencedby the relative amounts of thyroxine (T₄), triiodothyronine(T₄) and reverse-triiodothyronine (rT₄) as well as by the ratioof free/bound thyroid hormones. Additionally, other hormoneswill act upon the thyroid axis and its peripheral metabolismand vice versa. Environmental factors directly or indirectlymodify or modulate this already complicated picture. In this review, we therefore will first mention the circulatingthyroid hormone levels and the thyroid content of iodohormones.In the second section, the control by the central axis of thyroxineproduction by the thyroid gland as well as some aspects of peripheralmetabolism, with the attention focussed on thyroxine monodeiodinationis discussed. Processes which are known to be of importancein thyroid physiology such as recompartementalization or bileexcretion are hardly studied in avian species. Many questionsabout the control of thyroid functioning remain unanswered becauseof the lack of a homologous TSH radioimmunoassay. It is temptingto assume that the mammalian model can give us the answers;however, too many exceptions in regard to thyroid hormone physiologyas it is known in mammals are described, thereby underscoringthat thyroid physiology in avians should be investigated withouta bias towards research done in mammals.     

Protein synthesis-dependent potentiation by thyroxine of antiviral activity of interferon-gamma

We have studiedthe prenuclear signal transduction pathway by which thyroid hormonepotentiates the antiviral activity of human interferon- (IFN-) inHeLa cells, which are deficient in thyroid hormone receptor (TR). Theaction of thyroid hormone was compared with that of milrinone, whichhas structural homologies with thyroid hormone.L-Thyroxine(T₄),3,5,3'-L-triiodothyronine (T₃), and milrinone enhanced theantiviral activity of IFN- up to 100-fold, a potentiation blocked bycycloheximide. The 5'-deiodinase inhibitor6-n-propyl-2-thiouracil did not blockthe T₄ effect. 3,3',5,5'-Tetraiodothyroacetic acid prevented the effect ofT₄ but not of milrinone. Theeffects of T₄ and milrinone wereblocked by inhibitors of protein kinases C (PKC) and A (PKA) andrestored by PKC and PKA agonists; only the effect ofT₄ was blocked by genistein, atyrosine kinase inhibitor. In separate models, milrinone was shown notto interact with nuclear TR-.T₄ potentiation of the antiviralactivity of IFN- requires PKC, PKA, and tyrosine kinase activitiesbut not traditional TR.

     

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.     

Thyroid Thermogenesis: Regulation of (Na$$K$)-Adenosine Triphosphatase and Active Na,K Transport

SYNOPSIS. Evidence in support of the hypothesis that T₃-inducedenhancement of O₂ consumption in mammalian target tissues isattributable to a stimulation of energy utilization for activetransmembrane Na^$ and K^$ transport is reviewed. The stimulationof target-tissue Na, K transport-dependent respiration followingT₃ treatment is associated with enhanced rates of active Na^$efflux and K^$ influx as well as with an increase in Na, KATPaseenzymaticactivity. The enhancement of Na, K-ATPase activity and enzymeabundance is secondary to a T₃-induced stimulation of Na, K-ATPasea and rß subunit biosynthesis and is probably mediatedby increased abundance of specific messenger RNAs coding forthe subunits of the enzyme. It is proposed that a coordinateaugmentation of Na, KATPase enzymatic activity and enhancementof passive membrane permeability to Na^$ and K^$ are necessaryto maintain the increased rates of active Na, K transport andenergy consumption associated with thyroid thermogenesis.     

Interaction of hyperthyroidism and hindlimb suspension on skeletal myosin heavy chain expression

We examined the novel interaction ofhyperthyroidism and hindlimb suspension on the pattern of myosin heavychain (MHC) expression (mRNA and protein) in skeletal muscles. FemaleSprague-Dawley rats were assigned to four groups:1) normal control (Con);2) thyroid hormone treated[150 µg 3,5,3'-triiodothyronine(T₃) · kg¹ · day¹](T₃);3) hindlimb suspension (HS); or4)T₃-treated and HS(T₃ + HS). Results show for thefirst time the novel observation that the combinationT₃ + HS induces a rapid andsustained, marked (80-90%) downregulation of type I MHC geneexpression that is mirrored temporally by concomitant markedupregulation of type IIb MHC gene expression, as evidenced by the denovo synthesis of type IIb MHC protein in the soleus. The fast type IIxMHC isoform showed a differential response among the experimentalgroups, generally increasing with the separate and combined treatments in both the soleus and vastus intermedius muscles while decreasing inthe plantaris muscles. The fast type IIa MHC was the least responsiveto suspension of the MHCs and reflected its greatest responsiveness toT₃ treatment while also undergoingdifferential adaptations in slow vs. fast muscle (increases vs.decreases, respectively). These results confirm previous findings thatall four adult MHC genes are sensitive toT₃ and suspension in amuscle-specific manner. In addition, we show thatT₃ + HS can interactsynergistically to create novel adaptations in MHC expression thatcould not be observed when each factor was imposed separately.

     

Environmental physiology of the teleostean thyroid gland: a review

Synopsis Recent studies of thyroid hormone function are reviewed as they relate to the environmental physiology of teleost fish. In addition, reports dealing with the apparent interdependence of thyroid gland function with that of other endocrine glands are discussed with emphasis on the interrelated endocrine response associated with changing physiological status of teleosts.Seasonal changes in thyroid gland activity are described in several species. Although seasonal alterations in apparent thyroid status are concomitant with changes in ambient temperature, photoperiod and/or gonadal status, their biological significance is not fully understood and direct relationships are for the most part, not proven. Similarly, most reports of thyroid involvement in gonadal development or maturation are based on indirect evidence of the relationship. The exception to this is a study in immature or hypophysectomized goldfish in which thyroxine (T₄) was shown to promote ovarian development and maturation, possibly acting collateralistically or synergistically with gonadotropin. Even in this study it is not clear whether the T₄ effect is a direct action on the ovarian tissue or an indirect action via the regulation of metabolites necessary for gonad metabolism. Integumentary silvering and retinal porphyropsin formation in salmonids are stimulated by administration of T₄ or thyroid extracts. Administration of T₄ or triiodothyronine (T₃) enhances skeletal and somatic growth in some teleostean species, although the effect on somatic growth is most pronounced when these hormones act synergistically with somatotropin (STH) or androgens. The growth-promoting effects of T₄ and T₃ may be linked to their apparent involvement in lipid, carbohydrate, protein and vitamin metabolism. alterations in apparent thyroid activity concomitant with changes in ambient temperature have been reported (for example correlated with seasonal ambient thermal changes), although there are marked contradictions in data presented by different investigators. Reported temperature-related effects on thyroid function are probably secondary responses of thyroid metabolism to altered temperatures. Evidence of a direct rate of thyroid hormones in the regulation of migration (and associated behavioural modifications), salmonid smoltification, oxygen consumption, and osmotic or ionic regulation although highly suggestive in a number of areas is inconclusive and requires further critical experimental evaluation.The pituitary control (by thyrotropin) of thyroid secretion of T₄ is convincingly shown in several teleosts, and evidence of an inhibitory hypothalamic control of thyrotrop activity is highly suggestive in some species. A thyrotropic effect of somatotropin preparations is well established in several teleostean species; the effect does not appear to be related to contamination of the somatotropin preparations with thyrotropin, and may be an important consideration in explaining the apparently related involvement of T₄ (or T₃) and somatotropin in growth and metabolism. The apparent thyrotropic property of some gonadotropin preparations, shown in several teleostean species, requires further investigation before the doubts regarding hormone preparation purity can be satisfied. Recent studies of effects of prolactin on thyroid function are highly suggestive of an inhibitory role of prolactin in peripheral monodeiodination of T₄ to T₃ which secondarily affects thyroid activity in some species. There is no evidence of a direct involvement of corticotropin, melanotropin or fractions of these molecules on thyroid function in teleosts. Moreover, the little evidence in support of a role of gonadal or adrenocortical steroids in thyroid control is either often contradictory or indirect and needs to be evaluated further.Interlake epizootiological studies of thyroid dysfunction in Great Lakes salmonids provide substantive evidence for the presence of a ubiquitous waterborne goitrogen(s) in the Great Lakes environs. The nature of the goitrogen(s), whether naturally-occurring or a man-introduced toxicant, remains to be determined but the possible existence of waterborne goitrogens in natural water systems and their possible effects on experimental studies of teleostean thyroid function have to be evaluated further. If goitrogens are a common component of aquatic environments their presence could explain some of the data discrepancy among different groups of investigators, and could account for some of the apparent seasonal change in teleost thyroid physiology.     

Quantification of hormone-induced atrophy of large myotubes from C2C12 and L6 cells: atrophy-inducible and atrophy-resistant C2C12 myotubes

Myofiber atrophy is the final outcome of muscle wasting induced by catabolic factors such as glucocorticoids and thyroid hormones. We set up an in vitro system to define the catabolic reaction based on myotube atrophy. Both mouse C₂C₁₂ and rat L6 cells were used. C₂C₁₂ myotube formation was improved by replacing horse serum with the serum substitute Ultroser G. A new method was developed to quantify size changes of large (0.5–1 mm) myotubes only, excluding remaining myoblasts and small myotubes. Dexamethasone reduced myotube size by 30% in L6 but not in C₂C₁₂ myotubes. Expression of the glucocorticoid receptor was twofold higher in L6 myotubes than in C₂C₁₂ myotubes. In both cell lines, 3,3',5-triiodo-L-thyronine (T₃) did not induce a significant size reduction. Expression of the major T₃ receptor (T₃R1) was higher in L6 myotubes. We investigated whether the changes in myotube size are related to changes in atrogin-1 expression, as this enzyme is thought to be a key factor in the initiation of muscle atrophy. Dexamethasone induced a twofold increase of atrogin-1 mRNA; again, only L6 myotubes were susceptible. Interestingly, atrogin-1 expression in Ultroser G-fused C₂C₁₂ myotubes was lower than that in horse serum-fused myotubes. Furthermore, dexamethasone treatment increased atrogin-1 expression only in horse serum-fused myotubes but not in Ultroser G-fused myotubes. Ultroser G-induced fusion may result in atrophy-resistant C₂C₁₂ myotubes. Therefore, C₂C₁₂ myotubes offer an ideal system in which to study skeletal muscle atrophy because, depending on differentiation conditions, C₂C₁₂ cells produce atrophy-inducible and atrophy-resistant myotubes. glucocorticoids; nuclear receptors; atrogin     

Estimation of Chilling Sensitivity and Injury in Gloxinia Leaves by the Thermal Hysteresis of NMR Relaxation Times of Water Protons

Thermal hysteresis of longitudinal relaxation times (T₁) ofwater protons in leaf tissues of gloxinia was investigated bypulse nuclear magnetic resonance (NMR) spectroscopy. The profilesof T₁ hysteresis during a slow cool-warm cycle (20 to 0?C) variedwith the degree of chilling injury. General trends of T₁ changesupon warming were as follows: (1) a small increase of T₁ wasobserved when no chilling injury occurred or the degree of injurywas minor, (2) a large increase of T₁ occurred when injury increased,(3) a large decrease of T₁ occurred from the earlier stage ofwarming when serious injury occured. The sum of T₁ ratio (theratio obtained from the difference between the T₁ value in thecooling process and that of the wanning process for eight stepsof cool-warm cycle, divided by the former) reflecting T₁ increaseor decrease during the warming process was related to varietaldifference and seasonal changes of chilling sensitivity. Therefore,T₁ hysteresis can be used as a diagnostic tool in detectingthe chilling sensitivity and the degree of chilling injury. (Received August 27, 1986; Accepted February 24, 1987)     

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.     

Interaction of Cortisoi and Thyroid Hormone in the Larval Development of Pacific Threadfin

We have conducted three hatchery-scale experiments designedto examine the actions and interactions of cortisoi and thyroidhormones in the development of a larval marine fish. Survivalamong controls varied significantly between the 3 replicateexperiments. The threadfin (Polydactylus sexfilis) consistentlyresponds to 1 hr posthatch immersion in a combination of triiodothyronineand cortisoi (T₃ + F) with accelerated gut development and increasedsurvival compared with untreated controls (C). Survival amonglarvae treated with T₃ or F separately was significantly improvedover controls in one of the three experiments. The frequencyof spinal deformities was reduced by cortisoi treatment, aloneor in combination with T₃. Growth did not vary with treatment,except that variance in larval length was reduced in (T₃ + F)vs. C in all 3 experiments. A hormoneinduced increase in uniformitycould lead to reduced cannibalism, which is a problem in theculture of threadnns. These results suggest interactive hormonalregulation of developmental processes, working within the contextof other biological variables     

Thyroid hormones regulate lipid metabolism in a teleost Anabas testudineus (Bloch)

We compared the long-term action of 3,5,3′-triiodo-l-thyronine (T₃) and 3,5-diiodo-l-thyronine (T₂) on lipid metabolism in a teleost Anabas testudineus. Among the six groups of animals used in this experiment, except for the control group, all received 6-propylthiouracil (6-PTU) to create a hypothyroid state in order to analyse the action of iodothyronines on lipid metabolism. Injections of 6-PTU reduced T₃ concentration in the circulation by 79.6% and injections of iodothyronines enhanced the level of T₃ in the plasma, and a maximum increase was observed in T₃ (500 ng)-treated specimens. Analysis of lipogenic enzymes in liver and heart showed that a tissue-specific variation exists in the action of thyroid hormones and, in many cases, activity is higher in T₂-treated groups. Analysis of various lipid classes showed that long-term administration of T₂ is also effective in producing a comparable effect with that of T₃ on lipid metabolism.     

Egg-Mass Size and Cell Size: Effects of Temperature on Oxygen Distribution

Two processes strongly influence the distribution of oxygenwithin egg masses and cells: the supply of oxygen by diffusionand the consumption of oxygen by embryos and mitochondria. Theseprocesses are differentially sensitive to temperature. The diffusioncoefficient of oxygen depends only weakly on temperature, havinga Q₁₀ of approximately 1.4. In contrast, the consumption ofoxygen depends strongly on temperature, having a Q₁₀ between1.5 and 4.0. Thus, at higher temperatures, the ratio of oxygensupply to demand decreases. I show, by extending a model ofoxygen distribution within metabolizing spheres, that maximalegg-mass sizes and cell sizes are predicted to be smaller athigher temperatures. For egg masses, definitive data are notyet available. For ectothermic cells, this prediction appearsto be supported; cells from a variety of ectothermic organisms,unicellular and multicellular, are smaller when the cells areproduced at warmer temperatures. Establishing a specific connectionbetween this pattern and oxygen distributions requires demonstrationof (1) oxygen concentration gradients within metabolizing spheresand (2) central oxygen concentrations low enough to affect function.Egg masses from a variety of taxa show steep oxygen concentrationgradients and often are severely hypoxic or anoxic in centrallocations. Severe hypoxia appears capable of retarding developmentor killing embryos. Similar kinds of data for ectothermic cellshave not yet been collected, but the literature on oxygen gradientswithin mammalian cells suggests that intracellular gradientsmay be important.     

Inverse relationship between diurnal rhythms in plasma levels of thyroid hormones and pineal arylalkylamine-N-acetyltransferase (AANAT) activity in an air-breathing fish,Clarias gariepinus

Plasma levels of thyroid hormones (T₄ and T₃) and pineal AANAT activity exhibited prominent diurnal rhythms during all the four phases (quiescent, progressive, breeding, and regressive) of the fish, Clarias gariepinus. During all the four phases, we observed that (i) the acrophase (peak) of the circadian rhythm of both T₃ and T₄ occurred around midday, while the acrophase of AANAT activity rhythm was recorded during midnight, (ii) a significant negative correlation existed between plasma levels of the thyroid hormones and AANAT activity, and (iii) a direct relationship existed between water temperature/daylength and plasma levels of thyroid hormones, and an inverse relationship between water temperature/daylength and AANAT activity. On the basis of these findings, we conclude that an inverse relationship exists between the diurnal rhythms in plasma levels of thyroid hormones and pineal AANAT activity. The observed inverse relationship between the levels of thyroid hormones and pineal AANAT activity seems to be maintained by seasonal changes in water temperature and daylength.     

Episodic Breathing in Frogs: Converging Hypotheses on Neural Control of Respiration in Air Breathing Vertebrates

SYNOPSIS. The episodic, or intermittent, breathing of frogsand many ectothermic vertebrates results in important fluctuationsof arterial blood gases. This pattern of breathing differs fromthe rhythmic and continuous alternation of inspiration observedin most homeotherms, which maintain O₂ and CO₂ levels withinnarrow ranges. These differences in pattern of breathing indicatethat the respiratory control systems of ectotherms and homeothermsdiffer substantially. The results of recent studies using invitro brainstemspinal cord preparations of adult frogs and premetamorphictadpoles (Rana catesbeiana and Rana pipiens) demonstrate, however,that the mechanisms for rhythm generation and pattern formationdescribed previously for mammals are also key features of therespiratory control system of frogs. These findings thereforesupport the hypothesis that the respiratory control system ishighly conserved amongst air breathing vertebrates, whetherthey breathe continuously or episodically.     

Growth of Plants in Different Oxygen Concentrations

Dwarf french beans (Phaseolus vulgaris var. Canadian Wonder)were grown in chambers at 25?C with the roots aerated at 20per cent oxygen and tops variously maintained at: T₁ O₂ 0.21;CO₂ 270?10^–6: T₂; O₂ 0.05, CO₂, CO₂ 270?10^–6: T₃;O₂ 0.21; CO₂ 550?10^–6. Experiment 1 (T₁ and T₂) lasted2 weeks: Experiment 2 (T₁ T₂ and T₃) only one week. Hourly estimatesof CO₂ uptake were made by gas analysis and weekly estimatesof fresh weight, dry matter in tops and roots, and leaf area,by sampling. Light intensity was 80 W m^–2 of photosyntheticallyactive radiation. An attempt was made to explain the results in terms of a simplelight absorption model such that where dV/dt is the rate of CO₂ uptake per plant, ßis the photosynthetic efficiency, I₀ is the incident light intensity,f is the fraction of incident light absorbed by unit leaf layerand L is the leaf area index. The analysis showed that ß(T₂)was at least double ß(T₁), whilst f(T₂) was smallerthan f(T₁) at a given leaf area. The results also required thatthroughout the period of the experiment, fL(T₁)=fL(T₂) at anygiven time, i.e. the treatment with the larger leaf area (T₂)has the smaller value of f, and therefore intercepts less lightper unit leaf area. This could be advantageous for plant growth,but requires further experiments. The photosynthetic rates per unit leaf are about 40 per centgreater in T₂ than T₁. Over the relatively short period of the experiment the resultsare adequately described by U=btⁿ, where U is the accumulatedcarbon dioxide uptake, b is related to the photosynthetic efficiency(different for the differing treatments), and n is a constant(similar for all treatments). This relationship with time isbelieved to be a relationship with accumulated radiation, forthe light was constant throughout the experiments. Comparisons of carbon fixed (measured gas uptake) and dry matteraccumulation (sampling) show great scatter with an average valueof 0.43. The first week's results were generally smaller thanthis value and the second week's greater. Energy fixation as a fraction of photosynthetically active radiationon the ground area covered by the plants ranged from 3.5 to10 per cent. The results from treatment T₃ were similar to T₂ suggestingthat increasing CO₂ concentration decreases the growth inhibitionat 21 per cent O₂.     

Differential Effects of Day and Night Temperature on Development to Flowering in Rice

There are conflicting reports with regard to difference in effectsof day temperature (T_D) and night temperatures (T_N) on plantdevelopment. The objective of this study is to determine whetherthere are different effects ofT_DandT_Non development from sowingto flowering in rice (Oryza sativaL.). Plants of 24 rice cultivars were grown in naturally-lightedgrowth chambers at five diurnally constant (22, 24, 26, 28 and32 °C) and four diurnally fluctuating temperatures (26 /22,30 /22, 22 /26 and 22 /30 °C forT_D/T_Nwith 12hd^-1each) witha constant photoperiod of 12hd^-1. The treatments were selectedto enable the separation of effects ofT_DandT_Non developmentrate (DR). The response of DR to constant temperatures was typically nonlinear.This nonlinearity could not explain the difference in floweringdates between fluctuating temperatures with the same mean dailyvalue but oppositeT_D/T_Ndifferences. Differential effects ofT_DandT_NonDR to flowering were detected in all but one cultivar. In mostcases,T_Dexerted a greater influence thanT_N, in contrast withmany previous reports based on the assumption of a linearitybetween DR and temperature. The data were further analysed bya nonlinear model which separated effects ofT_DandT_N. The estimatedvalue for the optimumT_Nwas generally 25 –29 °C, about2 –4 °C lower than the estimated optimumT_Din mostcultivars. The effects ofT_DandT_Non DR were found to be interactivein some cultivars. These results form a new basis for modellingflowering dates in rice. Oryza sativa; rice; flowering; development; day and night temperature; thermoperiodicity     

不同灌溉处理对铁观音茶树光合作用的影响

以田间栽培的2年生铁观音茶树为试验材料,应用叶绿素荧光诱导动力学技术,以不灌溉为对照,分析不同灌溉间隔时间处理［5 d（T₁）、10 d (T₂)、15 d (T₃)、20 d (T₄) 和25 d (T₅)］对铁观音茶树叶片光合作用的影响.结果表明: 随着灌溉间隔期的延长,2年生铁观音茶树叶片水势和叶绿素含量降低; 净光合速率(P_n)先上升后降低,在T₂下达到最大(15.55 μmol·m^-2·s^-1); 光系统Ⅱ(PSⅡ)的原初光能转化效率(F_v/F_m)、可变荧光衰减(ΔF_v)和可变荧光淬灭速率(ΔF_v/F_o)均在T₂下达到最大值,分别为0.844、342.5和4.03.初始荧光(F_o)随着灌溉间隔期的延长而降低,而对照的F_o则呈上升趋势,表明干旱可对茶树叶片PSⅡ造成损害.灌溉间隔期为10 d处理有利于茶树叶片光合电子的传递和CO₂的同化,提高茶树的光合作用效率.     

Polyunsaturated dietary lipids lower the selected body temperature of a lizard

Summary Cold acclimation lowers the selected body temperature (T _b) in many ectothermic vertebrates. This change in behavioural thermoregulation is accompanied by an increase in the proportion of polyunsaturated fatty acids in tissues and cellular membranes. We investigated how diets containing different fatty acids, known to significantly alter the fatty acid composition of animal tissues and membranes, affect the selected T _b of the lizard Tiliqua rugosa. Lizards on a diet containing many polyunsaturated fatty acids (10% sunflower oil) showed a 3–5°C decrease in T _b, whereas T _b in animals on a diet containing mainly saturated fatty acids (10% sheep fat) did not change. Our study suggests that the composition of dietary lipids influences thermoregulation in ectothermic vertebrates and may thus play a role in the seasonal adjustment of their physiology.Abbreviations CST central standard time - T _a air temperature - T _b Body temperature