Control of pulmonary surfactant secretion: an evolutionary perspective

Pulmonary surfactant, a mixture consisting of phospholipids (PL) and proteins, is secreted by type II cells in the lungs of all air-breathing vertebrates. Virtually nothing is known about the factors that control the secretion of pulmonary surfactant in nonmammalian vertebrates. With the use of type II cell cultures from Australian lungfish, North American bullfrogs, and fat-tailed dunnarts, we describe the autonomic regulation of surfactant secretion among the vertebrates. ACh, but not epinephrine (Epi), stimulated total PL and disaturated PL (DSP) secretion from type II cells isolated from Australian lungfish. Both Epi and ACh stimulated PL and DSP secretion from type II cells of bullfrogs and fat-tailed dunnarts. Neither Epi nor ACh affected the secretion of cholesterol from type II cell cultures of bullfrogs or dunnarts. Pulmonary surfactant secretion may be predominantly controlled by the autonomic nervous system in nonmammalian vertebrates. The parasympathetic nervous system may predominate at lower body temperatures, stimulating surfactant secretion without elevating metabolic rate. Adrenergic influences on the surfactant system may have developed subsequent to the radiation of the tetrapods. Furthermore, ventilatory influences on the surfactant system may have arisen at the time of the evolution of the mammalian bronchoalveolar lung. Further studies using other carefully chosen species from each of the vertebrate groups are required to confirm this hypothesis.     

The composition and function of the pulmonary surfactant system during metamorphosis in the tiger salamander Ambystoma tigrinum

Mammalian lungs secrete a mixture of surface-active lipids (surfactant), which greatly reduces the surface tension of the fluid coating the inner lung surface, thereby reducing the risk of collapse upon deflation and increasing compliance upon inflation. During foetal lung maturation, these lipids become enriched in the primary and active ingredient, a disaturated phopholipid. However, disaturated phospholipids exist in their inactive gellike form at temperatures below 37°C and thus are inappropriate for controlling surface tension in the lungs of many ectotherms. We examined the development of the composition and function of the surfactant system of the tiger salamander (Ambystoma tigrinum) during metamorphosis from the fully aquatic larva (termed stage I) through an intermediate air-breathing larval form (stage IV) to the terrestrial adult (stage VII). Biochemical analysis of lung washings from these three life stages revealed a decrease in the percentage of disaturated phospholipid per total phospholipid (23.03 versus 15.92%) with lung maturity. The relative cholesterol content remained constant. The increased level of phospholipid saturation in the fully aquatic larvae may reflect their generally higher body temperature and the higher external hydrostatic compression forces exerted on the lungs, compared to the terrestrial adults. Opening pressure (pressure required for initial lung opening) prior to lavage decreased from larval to adult salamanders (7.96 versus 4.69 cm H₂O), indicating a decrease in resistance to opening with lung development. Opening pressure increased after lavage in older aquatic (stage IV) larvae (5.36 versus 9.80 cm H₂O) and in the adults (4.69 versus 7.65 cm H₂O), indicating that the surfactant system in salamanders may have an antiglue function which prevents apposing epithelial surfaces from adhering together.Abbreviations bm body mass - Chol cholesterol - DSP disaturated phospholipid - PC phosphatidylcholine - PL phospholipid - postlav postlavage - prelav prelavage - P-V pressure-volume - RH relative humidity - T_b body temperature - USP unsaturated phospholipid - WL wet lung mass     

The roles of cholesterol in pulmonary surfactant: insights from comparative and evolutionary studies

In most eutherian mammals, cholesterol (Chol) comprises approximately 8-10 wt.% or 14-20 mol.% of both alveolar and lamellar body surfactant. It is regarded as an integral component of pulmonary surfactant, yet few studies have concentrated on its function or control. Throughout the evolution of the vertebrates, the contribution of cholesterol relative to surfactant phospholipids decreases, while that of the disaturated phospholipids (DSP) increases. Chol generally appears to dominate in animals with primitive bag-like lungs that lack septation, in the saccular lung of snakes or swimbladders which are not used predominantly for respiration, and also in immature lungs. It is possible that in these systems, cholesterol represents a protosurfactant. Cholesterol is controlled separately from the phospholipid (PL) component in surfactant. For example, in heterothermic mammals such as the fat-tailed dunnart, Sminthopsis crassicaudata, and the microchiropteran bat, Chalinolobus gouldii, and also in the lizard, Ctenophorus nuchalis, the relative amount of Chol increases in cold animals. During the late stages of embryonic development in chickens and lizards, the Chol to PL and Chol to DSP ratios decrease dramatically. While in isolated lizard lungs, adrenaline and acetylcholine stimulate the secretion of surfactant PL, Chol secretion remains unaffected. This is also supported in isolated cell studies of lizards and dunnarts. The rapid changes in the Chol to PL ratio in response to various physiological stimuli suggest that these two components have different turnover rates and may be packaged and processed differently. Infusion of [3H]cholesterol into the rat tail vein resulted in a large increase in Chol specific activity within 30 min in the lamellar body (LB) fraction, but over a 48-h period, failed to appear in the alveolar surfactant fraction. Analysis of the limiting membrane of the lamellar bodies revealed a high (76%) concentration of LB cholesterol. The majority of lamellar body Chol is, therefore, not released into the alveolar compartment, as the limiting membrane fuses with the cell membrane upon exocytosis. It appears unlikely, therefore, that lamellar bodies are the major source of alveolar Chol. It is possible that the majority of alveolar Chol is synthesised endogenously within the lung and stored independently from surfactant phospholipids. The role of cholesterol in the limiting membrane of the lamellar body may be to enable fast and easy processing by maintaining the membrane in a relatively fluid state.     

Membrane cholesterol extraction decreases Na+ transport in A6 renal epithelia

In this study, we have investigated the dependence of Na⁺ transport regulation on membrane cholesterol content in A6 renal epithelia. We continuously monitored short-circuit current (I_sc), transepithelial conductance (G_T), and transepithelial capacitance (C_T) to evaluate the effects of cholesterol extraction from the apical and basolateral membranes in steady-state conditions and during activation with hyposmotic shock, oxytocin, and adenosine. Cholesterol extraction was achieved by perfusing the epithelia with methyl--cyclodextrin (mCD) for 1 h. In steady-state conditions, apical membrane cholesterol extraction did not significantly affect the electrophysiological parameters; in contrast, marked reductions were observed during basolateral mCD treatment. However, apical mCD application hampered the responses of I_sc and G_T to hypotonicity, oxytocin, and adenosine. Analysis of the blocker-induced fluctuation in I_sc demonstrated that apical mCD treatment decreased the epithelial Na⁺ channel (ENaC) open probability (P_o) in the steady state as well as after activation of Na⁺ transport by adenosine, whereas the density of conducting channels was not significantly changed as confirmed by C_T measurements. Na⁺ transport activation by hypotonicity was abolished during basolateral mCD treatment as a result of reduced Na⁺/K⁺ pump activity. On the basis of the findings in this study, we conclude that basolateral membrane cholesterol extraction reduces Na⁺/K⁺ pump activity, whereas the reduced cholesterol content of the apical membranes affects the activation of Na⁺ transport by reducing ENaC P_o. epithelial Na⁺ channel; Na⁺-K⁺-ATPase activity; short-circuit current; methyl--cyclodextrin; channel open probability     

Temperature-dependent development in marine copepods: comments on choices of models

Guerrero et al (1994) promote Tauti's equation, rate = a exp(b temp), as simple to fit by log conversion (which may be formallyinappropriate), empirically adequate, and having ‘appropriatebiological characteristics’. No function is justifiablefrom reductionist theories, but Belehrádek's, rate =a (temp – T₀)^b, with b fixed for the taxon of interest,fits equally well, and singularly distinguishes differencesattributable to temperature adaptation (T₀, often misunderstoodas ‘biological zero’), and to size or other species-dependentproperties (a).     

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.     

The Influence of Stress Conditions on Chlorophyll Content of Two Range Grasses with Contrasting Photosynthetic Pathways

The influence of various treatments and temperature regimeson total chlorophylls and on the chlorophyll a:b ratio of westernwheatgrass and blue grama plants was investigated at differenttime intervals during the 120-day growth period. Western wheatgrass,a C₃ species, accumulated greater amounts of chlorophyll thandid blue grama plants, a C₄ species. Maximum concentrations(mg gd wt^–1) of chlorophylls in western wheatgrass andin blue grama were recorded at the lower (13/7°C) and higher(30/18°C) temperature regimes. Nitrogen fertilizer alonedecreased the chlorophyll content in both species. The chlorophylla:b ratio in blue grama ranged from an average of 2·00under irrigated plus fertilized conditions to 3·00 undercontrol and fertilized conditions. On the other hand, the chlorophylla:b ratio in western wheatgrass remained constant at 3·00throughout the growing season under various treatments and temperatureregimes.     

Dibromothymoquinone (DBMIB) Replaces the Function of QA at 77 K in the Isolated Photosystem II Reaction Center (D1-D2-Cytochrome b559) Complex: Difference Spectrum of the P680+(DBMIB-) State

Transient absorbance changes of the primary electron donor chlorophylla (P680) and acceptor pheophytin a (H) were measured at 77 Kby nanosecond laser spectroscopy in the D1-D2-cytochrome b₅₅₉photosystem II reaction center complex containing dibromomethylisopropylbenzoquinone (DBMIB). After the laser excitation of the reactioncenter in the presence of DBMIB, only the P680⁺-(DBMIB^-) statewas detected. P680⁺ mainly decayed with a t_1/e of 11 ms. Inthe absence of DBMIB, the excitation produced the P680⁺H^- radicalpair. The radical pair produced the triplet state (P680^T) witha t_1/e of 50 ns, and P680^T then decayed with a t_1/e of 2.1 ms.It was concluded that H_- was oxidized by DBMIB in a time rangefaster than the detecting time resolution (3.5 ns) even at 77K. The rapid oxidation of H^- by DBMIB was also confirmed bythe suppression of delayed fluorescence with a decay t_1/e of50 ns. The P680⁺(DBMIB^-)/P680(DBMIB) difference spectrum exhibiteda Q_y, band with a peak at 682 nm with a shoulder at 673 nm.The spectral shape was almost temperature insensitive between77 and 265 K. The feature of this spectrum in the wavelengthrange between 330 and 720 nm was compared with that of P680_T/P680or H^-/H at 77 K. (Received May 8, 1996; Accepted June 24, 1996)     

Studies on cytochromes in photosynthetic electron transport system I. photoreduction and photo-oxidation of cytochrome b559 by photosystem II in spinach chloroplasts

Light-induced redox-reactions of cytochrome b₅₅₉ in spinachchloroplasts were investigated. Illumination of chloroplastsinduced photoreduction of cytochrorne b₅₅₉ Red light (650 nm)was more effective than far-red light (725 nm), indicating thatthe photoreduction is a photosystem II-mediated reaction. Onaddition of DCMU, the photoreduction was eliminated and a photooxidationof cytochrome b₅₅₉ was observed. The rate of this photooxidationwas faster with photosystem II light than with photo-systemI light. On addition of Mn⁺⁺ the photooxidation was partly suppressed;far-red light became as effective as red light in inducing photooxidationof cytochrome b₅₉₉, in the presence of DCMU and Mn⁺⁺. Ascorbate completely suppressed photooxidation of cytochromeb⁵⁵⁹ In the presence of ascorbate, however, photooxidation wasobserved in the presence of inhibitors or after inhibitory treatmentsof chloroplasts which affected the oxidizing side of systemII. These inhibitors and inhibitory treatments, but not DCMU,decreased the redoxpotential of cytochrome b₅₅₉. Reactivationof Hill reaction in Tris-washed chloroplasts by indophenol-ascorbatetreatment was not accompanied by an abolishment of photooxidationof cytochrome b₅₅₉. A possible mechanism is proposed to account for these reactionsof cytochrome b₅₅₉ in the photosynthetic electron transportin chloroplasts. (Received April 4, 1972; )     

Simulation and Prediction of Plant Phenology for Five Crops Based on PhotoperiodxTemperature Interaction

This paper presents a plant phenological model based on genotypextemperaturexphotoperiodinteraction (GPTmodel). In the model, rate of development towardsa specified stage (e.g. flowering) for a given genotype is composedof three components: the genotype's maximum rate of development;any delay due to a non-optimal temperature; and any delay dueto a photoperiod response. It is assumed that development tothe specified stage is an autonomous process established bymost, if not all, genes other than the vernalization genes andthe photoperiod genes; and that this autonomous process is delayedby any activity of the photoperiod genes. Since all physiologicalprocesses are modulated by temperature, any photoperiod responseis inevitably a photoperiodxtemperature interaction. This interactionis simulated by assuming that the photoperiod gene activityoccurs only beyond a critical photoperiod (P_c) and is enlargedby temperature above a base temperature (T_bp) that allows thephotoperiod gene activity. The model is written asR=1/D_b-S_t(T-T_opt)²-S_p(T-T_bp)|P-P_c|, whereRis the expected rate of development to the specifiedstage under any combination of temperature (T) and photoperiod(P). The other model parameters are:S_p, the sensitivity to adelaying photoperiod;T_opt, the optimum temperature for developmentin the absence of the photoperiod response;S_t, the sensitivityto a non-optimum temperature; andD_b, the basic duration to thespecified stage (or intrinsic earliness), the inverse of whichis the maximum rate of development.D_bis observable only ifT=T_optandsimultaneouslyP     

Are Mistletoes Shade Plants? CO2Assimilation and Chlorophyll Fluorescence of Temperate Mistletoes and their Hosts

Mistletoes usually have slower rates of photosynthesis thantheir hosts. This study examines CO₂assimilation, chlorophyllfluorescence and the chlorophyll content of temperate host–parasitepairs (nine hosts parasitized by Ileostylus micranthus and Carpodetusserratus parasitized by Tupeia antarctica). The hosts of I.micranthus had higher mean annual CO₂assimilation (3.59 ±0.41 µmol m^-2 s^-1) than I. micranthus(2.42 ± 0.20µmol m^-2 s^-1), and C. serratus(2.41 ± 0.43 µmolm^-2 s^-1) showed higher CO₂assimilation than T. antarctica(0.67± 0.64 µmol m^-2 s^-1). Hosts saturated at significantlyhigher electron transport rates (ETR) and light levels thanmistletoes. The positive relationship between CO₂assimilationand electron transport suggests that the lower CO₂assimilationrates in mistletoes are a consequence of lower electron transportrates. When photosynthetic rates, ETR and chlorophyll a /b ratioswere adjusted for photosynthetically active radiation, hostsdid not have significantly higher CO₂assimilation (3.21 ±0.37 µmol m^-2 s^-1) than mistletoes (2.54 ± 0.41µmol m^-2 s^-1), but still had significantly higher ETRand chlorophyll a / b ratios. The electron transport rates,saturating light and chlorophyll a / b ratios of sun leavesfrom mistletoes were similar to host shade leaves. These responsesindicate that in comparison with their hosts, mistletoe leaveshave the photosynthetic characteristics of the leaves of shadeplants. Copyright 2000 Annals of Botany Company CO₂assimilation, photosynthetic active radiation (PAR), chlorophyll fluorescence, electron transport rate (ETR), photochemical quenching (q_p), non-photochemical quenching (q_n), sun and shade leaves, chlorophyll content, Ileostylus micranthus, Tupeia antarctica, New Zealand     

The Effects of Priming and Ageing on Seed Vigour in Tomato

A comparison was made of the effects of seed priming or ageingtreatments on the performance of tomato (Lycopersicon esculentumMill. cv. UC204C) seeds according to a number of indices ofseed vigour. A single lot of tomato seeds was primed in 120mol m^–3 K2HPO4 + 150 mol m^–3 KNO₃ for 5 d at 20?C, or aged at 13% moisture content (dry weight basis) and 50?C for 6 d. Germination percentage (>98%) was unaffectedby priming and reduced to 85% by ageing. X-ray photographs andlongitudinal sections revealed the formation of free space surroundingthe embryo in dry primed seeds, which was not evident in controlor aged seeds. Priming increased the rate of germination atall temperatures above the base temperature (T_b), while ageingdecreased it. T_b was unaffected by priming and only slightlyincreased by ageing. The variation in individual times to germinationwas approximately doubled in both primed and aged seed comparedto the control, based upon the slopes of probit germinationpercentage versus log thermal time curves. Root growth aftergermination tests and seedling growth in both greenhouse andfield tests were not influenced by either priming or ageing.The conductivity test was found to be unreliable as a vigourtest for tomato seeds. The results identify several indiceswhich can be used to quantify seed vigour in tomato. They alsoillustrate that seed priming can enhance seed performance accordingto some criteria, while having no effect or decreasing qualityaccording to other criteria. Seed vigour can apparently be separatedinto various components which can be independently influencedby seed enhancement treatments. Key words: Tomato, seed germination rate, seed priming, seed vigour     

K+ transport and capacitance of the basolateral membrane of the larval frog skin

Skin from larval bullfrogs was mounted in an Ussing-type chamberin which the apical surface was bathed with a Ringer solution containing 115 mM K⁺ and thebasolateral surface was bathed with a Ringer solution containing 115 mMNa⁺. Ion transport was measured asthe short-circuit current(I_sc) with alow-noise voltage clamp, and skin resistance(R_m) wasmeasured by applying a direct current voltage pulse. Membrane impedance was calculated by applying a voltage signal consisting of 53 sine wavesto the command stage of the voltage clamp. From the ratio of theFourier-transformed voltage and current signals, it was possible tocalculate the resistance and capacitance of the apical and basolateralmembranes of the epithelium(R_a andR_b,C_a and C_b,respectively). With as the anion,R_m decreasedrapidly within 5 min following the addition of 150 U/ml nystatin to theapical solution, whereasI_sc increasedfrom 0.66 to 52.03 µA/cm² over a60-min period. These results indicate that nystatin becomes rapidlyincorporated into the apical membrane and that the increase inbasolateral K⁺ permeabilityrequires a more prolonged time course. Intermediate levels ofI_sc were obtainedby adding 50, 100, and 150 U/ml nystatin to the apical solution. Thisproduced a progressive decrease in R_a andR_b whileC_a andC_b remainedconstant. With Cl as theanion, I_sc valuesincreased from 2.03 to 89.57 µA/cm² following treatment with150 U/ml nystatin, whereas with gluconate as the anionI_sc was onlyincreased from 0.63 to 11.64 µA/cm². This suggests that theincrease in basolateral K⁺permeability produced by nystatin treatment, in the presence of morepermeable anions, is due to swelling of the epithelial cells of thetissue rather than the gradient for apicalK⁺ entry. Finally,C_b was notdifferent among skins exposed toCl,, or gluconate, despite the largedifferences inI_sc, nor didinhibition of I_scby treatment with hyperosmotic dextrose cause significant changes inC_b. These resultssupport the hypothesis that increases in cell volume activateK⁺ channels that are alreadypresent in the basolateral membrane of epithelial cells.

     

准噶尔荒漠早春短命植物的光合特性及生物量分配特点

准噶尔荒漠分布的早春短命植物不仅具有十分独特的生物学特点,而且在荒漠植物群落演替、物种多样性维持及土壤改良与防治水土流失等方面具有重要的生态学价值。该文运用Li-6400开放式气体交换光合作用测定系统,对分布于准噶尔荒漠的16种早春短命植物生长盛期的净光合速率(P_n)、蒸腾速率(T_r)、水分利用效率(WUE)等特征进行了测定,并对其中7种植物与生长相关的生物量分配特征进行了分析。结果表明:1)16种植物的最大P_n、 最大T_r及WUE分别为8.07~35.96 μmol CO₂·m^-2·s^-1、3.16~29.64 mmol H₂O·m^-2·s^-1、0.54~4.26 μmol CO₂·mmol^-1H₂O;种间最大P_n与最大气孔导度(Stomatal conductance, G_s)之间存在正相关关系,其相关系数为0.77(p<0.05),线性回归斜率为26.36 μmol·mmol^-1;从光合速率对胞间CO₂浓度及光量子通量密度的响应曲线来看,这类植物的表观CO₂补偿点均在4~5 Pa之间(28~30 ℃),表观羧化效率为0.64~1.86 μmol CO₂·m^-2·s^-1·Pa^-1,表观量子效率为0.05~0.06。2)从生物量分配来看,所测植物的个体生物量为0.05~0.39 g;单株总叶面积为 3.24~51.40 cm²;单位叶面积干重为0.40~0.77 g·m^-2,根在总生物量中所占比例为5.72%~19.43%,单株叶面积比在2.92~9.00 m²·kg^-1之间。种间根所占生物量的比与对应的WUE之间的比较分析结果表明,二者之间存在显著的正相关关系,其相关系数r为0.93(p<0.01)。这些结果表明,所观测的早春短命植物具有典型的C₃植物特征,相比其它类型的荒漠植物具有较高的单位叶面积P_n、高T_r及低WUE,并且在生长发育过程中表现出很低的根/地上生物量比、较高的叶面积比和单位叶面积干重,说明它们具有相对高的生长速率,这与其生长发育节律相一致,反映了它们与准噶尔荒漠环境相适应的特点。     

In situ Effects of Elevated Atmospheric CO2on Leaf Freezing Resistance and Carbohydrates in a Native Temperate Grassland

The objectives of this study were to quantify changes in leaffreezing resistance and carbohydrate concentrations caused bylong-term (6 years) exposure to elevated CO₂(ambient: 360 µll^-1, elevated: 600 µl l^-1) in five dominant plant speciesgrowing in situ in a native temperate grassland. Across allfive species tested from three functional groups, the mean temperatureat which all leaves were damaged (T₁₀₀) significantly (P = 0.016)increased from -9.6 to -8.5 °C under elevated CO₂ , anda similar marginally significant (P = 0.079) reduction was observedfor the mean temperature that caused 50% leaf damage (T₅₀),from -6.7 to -6.0 °C. The mean temperature at which initialleaf damage was observed (T₀) was not significantly influencedby elevated CO₂ . Although concentrations of soluble sugars(+25%,P = 0.042), starch (+53%, P < 0.001), and total non-structuralcarbohydrates (TNC, +40%, P < 0.001) were significantly higherunder elevated CO₂ , leaf freezing resistance actually decreasedunder elevated CO₂ . Concentrations of soluble sugars were positivelycorrelated with freezing resistance when viewed across all fivecommunity dominants, but within any individual species, no suchrelationships were found. We also found no evidence for ouroriginal hypothesis that increased concentrations of solublesugars increase freezing resistance. Thus, future atmosphericCO₂levels may instead increase the risk of late spring freezingdamage. Furthermore, the strong differences in freezing resistanceobserved among the species, along with decreased freezing resistance,may increase the risk of losing species that have inherentlyweak freezing resistances from the plant community. Copyright2001 Annals of Botany Company CO₂enrichment, frost hardiness, sugar, starch, total non-structural carbohydrates (TNC)     

Photoinhibition and the diurnal variation of phytoplankton photosynthesis--I. Development of a photosynthesis--irradiance model from studies of in situ responses

Diurnal series of fluorescence and photosynthesis assays wereconducted in high altitude (3803 m), tropical (16°), LakeTiticaca (Peru/Bolivia). Near-surface diurnal thermoclines formedon typical days of high photon flux density (PFD, {small tilde}2000 µE m^–2 s^–1). In the depth range of diurnalstratification profiles of in vivo fluorescence, both without(F_a and with (F_b DCMU, exhibited a mean decrease of 64% frommorning to mid-day, but little change (mean increase of 1.5%)through the afternoon. Three times during the day surface, mid-depth(3–5 m) and deep (15–20 m) phytoplankton sampleswere incubated with H¹⁴CO₃^– under short (<2 h) exposuresto a range of in situ PFDs. Comparison of phytoplankton in differentsamples (ANOVA) showed identical photosynthetic response insunrise (isothermal) samples but a significant drop in surfaceand mid-depth photosynthesis at all PFDs during times of diurnalstratification. Similarly, both low-light () and light-saturated(P² _max photosynthetic parameters were lower in mid-day surfacesamples compared to deep samples. In addition, previously photoinhibitedsamples had a higher threshold intensity for photoinhibition,I_T. These results, together with diurnal time series of fluorescencefrom in situ incubations, demonstrate that recovery from extendedepisodes of photoinhibition during diurnal stratification isslower than suggested by previous observations in vitro. Photosynthesisby near-surface phytoplankton is different in light increasingup to I_T than light decreasing from I_T. This effect can be modeledby reducing and P_max as a function of the maximum photoinhibitingPFD in the diurnal light history. ¹Present address: Division of Molecular Plant Biology, Universityof California, Berkeley, Berkeley, CA 94720, USA     

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     

Thyroxine in Early Strobilation in Aurelia aurita

Radiochromatographic studies of ¹³¹I-treated Aurelia polypsrevealed synthesis of three compounds tentatively identifiedas monoiodotyrosine (MIT), diiodotyrosine (DIT), and thyroxine(T₄). One compound, MIT, is found within 8 hr after ¹³¹I administration,before the detection of DIT or T₄ which appear within 24 hr.T₄ is not usually detected after 48 hr although MIT and DITwere found up to the segmentation period. None of the compoundswere detected in ephyrae treated with ¹³¹I for 24 hr. Administration of low dosages of the goitrogens, thiourea,,propylthiouracil, and potassium thiocyanate, in conjunctionwith iodide, prevented strobilation induction. Radiochromatographyof jellyfish given the goitrogens and ¹³¹I revealed a reduceduptake of iodide and an impairment of the synthesis of the iodinatedcompounds. Jellyfish use thyroxine directly for strobilatioa inductionas demonstrated by ¹³¹I - labeled T₄ administration. The T₄was detected in the polyps up to the 48-hr period of strobilationduring which time some of the T₄ was excreted into the medium,as was some ¹³¹I. The fact that T₄, synthesis has thus far been found only instrobilating forms of Aurelia suggests that T₄ is involved primarilywith the differentiation of new structures which occurs duringstrobilation.     

Effects of osmotic shrinkage on voltage-gated Ca2+ channel currents in rat anterior pituitary cells

Increased extracellular osmolarity ([Os]_e) suppresses stimulated hormone secretion from anterior pituitary cells. Ca²⁺ influx may mediate this effect. We show that increase in [Os]_e (by 18–125%) differentially suppresses L-type and T-type Ca²⁺ channel currents (I_L and I_T, respectively); I_L was more sensitive than I_T. Hyperosmotic suppression of I_L depended on the magnitude of increase in [Os]_e and was correlated with the percent decrease in pituitary cell volume, suggesting that pituitary cell shrinkage can modulate L-type currents. The hyperosmotic suppression of I_L and I_T persisted after incubation of pituitary cells either with the actin-disrupter cytochalasin D or with the actin stabilizer phalloidin, suggesting that the actin cytoskeleton is not involved in this modulation. The hyperosmotic suppression of Ca²⁺ influx was not correlated with changes in reversal potential, membrane capacitance, and access resistance. Together, these results suggest that the hyperosmotic suppression of Ca²⁺ influx involves Ca²⁺ channel proteins. We therefore recorded the activity of L-type Ca²⁺ channels from cell-attached patches while exposing the cell outside the patch pipette to hyperosmotic media. Increased [Os]_e reduced the activity of Ca²⁺ channels but did not change single-channel conductance. This hyperosmotic suppression of Ca²⁺ currents may therefore contribute to the previously reported hyperosmotic suppression of hormone secretion. L-type Ca²⁺ channels; osmosensitivity; mechanosensitivity; osmolarity; hyperosmolarity     

Growth rates of dominant planktonic ciliates in two freshwater bodies of different trophic degree

The in situ growth rates of dominant ciliate species were studiedduring and shortly after phytoplankton peaks in two water bodies:the eutrophic Rímov Reservoir (South Bohemia, Czech Republic)and the oligo-mesotrophic Piburger See (Tyrol, Austria). Growthrate estimates based on changes in ciliate abundances in incubatedpre-screened samples (E_N) were compared with those derived fromthe ciliate cell volume and ambient temperatures (E_T). The valuesof E_N were always rather lower than those of E_T. During thestudies, the food supply limited the ciliate growth dependingon the ciliate feeding mode. An ecological grouping into filterfeeding versus raptorial feeding (‘hunting’) species,on the one hand, and attached/crawling (browsing) versus freeswimming species, on the other hand, clearly affected experimentalestimation. Both fine filter feeders (namely attached) and browsersexhibited a calculated E_N closer to the theoretical (maximum)E_T than did hunters and coarse filter feeders. It was apparent,for example, comparing E_N and E_T (day^–1) of the followingspecies: filter feeders Halteria grandinella (E_N = 0.42; E_T>1.4), Strobilidium hexakinetum (0.34;>1.9), Pelagohalteriaviridis (0.27;>0.9), Vorticella aquadulcis complex (0.75;>1.0);raptorial Balanion planctonicum (0.65;>1.5), Urotricha furcata(in Rímov Reservoir 0.65;>2.1; in Piburger See 0.20;>1.5),Rhabdoaskenasia minima (0.22;>1.0), Askenasia acrostomia(0.12;>0.6); opportunistic Cyrtolophosis mucicola (0.42;>1.6)and Cinetochilum margaritaceum (0.86;>1.4). Predation byrotifers apparently affected measurements in several samplescontaining {small tilde}400 rotifers l^–1 however, it seemedto be of little importance in the water column.