Effects of blue light deficiency on acclimation of light energy partitioning in PSII and CO2 assimilation capacity to high irradiance in spinach leaves

Blue light effects on the acclimation of energy partitioningcharacteristics in PSII and CO₂ assimilation capacity in spinachto high growth irradiance were investigated. Plants were grownhydroponically in different light treatments that were a combinationof two light qualities and two irradiances, i.e. white lightand blue-deficient light at photosynthetic photon flux densities(PPFDs) of 100 and 500 µmol m^–2 s^–1. The CO₂assimilation rate, the quantum efficiency of PSII (PSII) andthermal dissipation activity / in young, fully expanded leaves were measured under 1,600 µmol m^–2 s^–1white light. The CO₂ assimilation rate and PSII were higher,while / was lower in plants grown under high irradiancethan in plants grown under low irradiance. These responses wereobserved irrespective of the presence or absence of blue lightduring growth. The extent of the increase in the CO₂ assimilationrate and PSII and the decrease in / by high growth irradiance was smaller under blue light-deficient conditions. These resultsindicate that blue light helps to boost the acclimation responsesof energy partitioning in PSII and CO₂ assimilation to highirradiance. Similarly, leaf N, Cyt f and Chl contents per unitleaf area increased by high growth irradiance, and the extentof the increment in leaf N, Cyt f and Chl was smaller underblue light-deficient conditions. Regression analysis showedthat the differences in energy partitioning in PSII and CO₂assimilation between plants grown under high white light andhigh blue-deficient light were closely related to the differencein leaf N.     

Long-term acclimatization of Scenedesmus bicellularis to high-frequency intermittent lighting (100 Hz). II. Photosynthetic pigments, carboxylating enzymes and biochemical composition

The long-term responses of the green microalga, Scenedesmusbicellularis, to a 100 Hz quasi-square wave (intermittent light,IL) were assessed after a 4 week acclimatization period. Atthe end of this period, the photosynthetic pigments, carboxylatingenzyme activities and biochemical composition of algae grownunder IL were compared to those of algae acclimatized to a lightflux without fluctuations (continuous light, CL). Differencesbetween IL and CL treatments were small. IL cells grown underlimiting irradiance had characteristics close to CL cells grownat the same daily irradiance. At saturating irradiance, thecharacteristics of IL cells resembled those of CL cells at thesame instantaneous irradiance, mostly because of the flattenedresponse of microalgae to increased irradiance. The flickeringof any artificial lighting system (100–120 Hz) can thusbe neglected when such light is used to grow algae.     

Effects of Nitrogen Nutrition on Photosynthesis in Cd-treated Sunflower Plants

Increased nitrogen supply stimulates plant growth and photosynthesis.Since it was shown that heavy metals may cause deficienciesof essential nutrients in plants the potential reversal of cadmiumtoxicity by increased N nutrition was investigated. The effectson photosynthesis of low Cd (0, 0.5, 2 or 5 mmol m^-3) combinedwith three N treatments (2, 7.5 or 10 mol m^-3) were examinedin young sunflower plants. Chlorophyll fluorescence quenchingparameters were determined at ambient CO₂and at 100 or 800 µmolquanta m^-2 s^-1. The vitality index (R_fd) decreased approx. three-timesin response to 5 mmol m^-3Cd, at 2 and 10 mol m^-3N. The maximumphotochemical efficiency of PSII reaction centres (F_v/ F_m) wasnot influenced by Cd or N treatment. The highest Cd concentrationdecreased quantum efficiency of PSII electron transport (_II)by 30%, at 2 and 10 mol m^-3N, mostly due to increased closureof PSII reaction centres (q_P). Photosynthetic oxygen evolutionrates at saturating CO₂were decreased in plants treated with5 mmol m^-3Cd, at all N concentrations. The results indicatethat Cd treatment affected the ribulose-1,5-bisphosphate (RuBP)regeneration capacity of the Calvin cycle more than other processes.At the same time, the amounts of soluble and ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco) protein increased with Cd treatment.Decreased photosynthesis, but substantially increased Rubiscocontent, in sunflower leaves under Cd stress indicate that asignificant amount of Rubisco protein is not active in photosynthesisand could have another function. It is shown that optimal nitrogennutrition decreases the inhibitory effects of Cd in young sunflowerplants. Copyright 2000 Annals of Botany Company Helianthus annuus L., cadmium, nitrogen, photosynthesis, Rubisco, sunflower     

Antenna Sizes of Photosystem I and Photosystem II in Chlorophyll b-Deficient Mutants of Rice. Evidence for an Antenna Function of Photosystem II Centers That are Inactive in Electron Transport

Light-harvesting capacities of photosystem I (PSI) and photosystemII (PSII) in a wild-type and three chlorophyll b-deficient mutantstrains of rice were determined by measuring the initial slopeof light-response curve of PSI and PSII electron transport andkinetics of light-induced redox changes of P-700 and Q_A, respectively.The light-harvesting capacity of PSI determined by the two methodswas only moderately reduced by chlorophyll b-deficiency. Analysisof the fluorescence induction that monitors time course of Q_Aphotoreduction showed that both relative abundance and antennasize of PSII_a decrease with increasing deficiency of chlorophyllb and there is only PSII_rß in chlorina 2 which totallylacks chlorophyll b. The numbers of antenna chlorophyll moleculesassociated with the mutant PSII centers were, therefore, threeto five times smaller than that of PSII_a in the wild type rice.Rates of PSII electron transport determined on the basis ofPSII centers in the three mutants were 60–70% of thatin the normal plant at all photon flux densities examined, indicatingthat substantial portions of the mutant PSII centers are inactivein electron transport. The initial slopes of light-responsecurves of PSII electron transport revealed that the functionalantenna sizes of the active populations of PSII centers in themutants correspond to about half that of PSII in the wild typerice. Thus, the numbers of chlorophyll molecules that serveas antenna of the oxygen-evolving PSII centers in the mutantsare significantly larger than those that are actually associatedwith each PSII center. It is proposed that the inactive PSIIserves as an antenna of the active PSII in the three chlorophyllb-deficient mutants of rice. In spite of the reduced antennasize of PSII, therefore, the total light-harvesting capacityof PSII approximately matches that of PSI in the mutants. (Received July 29, 1994; Accepted February 7, 1996)     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. II. A general review

The causes of interspecific differences in the µ-l relationshipare examined in the context of a mechanistic model which relatesµ to irradiance in terms of six factors:, k_c photosyntheticquotient (PQ), Chl a:C, respiration and excretion. The effectof cell size on the light saturated growth rate is also considered.It is shown that photosynthetic efficiency and PQ exhibit remarkablylittle interspecific variability, and average 0.024 ±0.005 µg C(µg Chl a)^–1 h^–1 (µEm^–2 s^–1)^–1 and 1.5 ± 0.2 mol 02 molC^–1 (when NO₃^– is the nitrogen source) respectively.Two useful relationships were derived: (i) between growth efficiency,_g and Chl a:C at µ. = 0; (ii) between the compensationintensity, I_c and the Chl a-specific maintenance respirationrate. Both relationships were independent of temperature anddaylength. Species best adapted to growth at low light werefound to exhibit high Chl a:C ratios and low maintenance respirationrates. As a group, diatoms were consistently the best adaptedfor growth at low irradiance. Chiorophytes, haptophytes, chrysophytesand cryptophytes were intermediate in their performance at lowirradiance. Dinoflagellates exhibited extreme diversity, withspecies spanning the spectrum from very good performance atlow irradiance to very poor. A new µ_max-cell carbon relationshipis given based on growth rates normalized to 15°C. Evidenceis presented to show that noise in this relationship can besignificantly reduced by using only carbon-specific growth ratesand using only data for species grown at the same daylength.     

Response to Saturation Deficit of Leaf Extension in a Stand of Pearl Millet (Pennisetum typhoides S. & H.)II: II. DEPENDENCE ON LEAF WATER STATUS AND IRRADIANCE

This paper describes how the dominant relation between leafextension and temperature in pearl millet is modified by atmosphericsaturation vapour pressure deficit (SD) and irradiance. Standsof plants were grown at two levels of SD and soil moisture content.Leaf extension, water potential (₁) and stomatal conductancewere all reduced at high SD, ₁ was more closely related to transpirationrate than to SD itself. Leaf extension rate (R) was poorly correlatedwith ₁, even after correction for temperature differences, owingto variation in solute potential between leaves. However, Rin individual leaves was linearly related to turgor potential,except after periods of low irradiance. The thermal time conceptwas modified to incorporate turgor potential and used to showthat the ‘turgor thermal rate of extension’ decreasedsharply at low irradiances, presumably due to assimilate shortage. Key words: Extension, Saturation deficit, Millet     

Molecular dynamics simulations of high-mannose oligosaccharides

Conformations of several high-mannose-type oligosaccharidesthat are generated during the biosynthetic degradation of Man₉GlcNAc₂to Man₅GlcNAc₂ have been studied by molecular dynamics (MD).Simulations were performed on NCI-FCRDC's Cray Y-MP 8D/8128supercomputer using Biosym's CVFF force field for 1000 Ps withdifferent initial conformations. The conformations of the two1,3- and the two 1,6-linkages in each oligomannose were different,suggesting that deriving oligosaccharide conformations basedon the conformational preferences of the constituent disaccharidefragments will not always yield correct results. Unlike otheroligomannoses, Man₉GlcNAc₂ appears to take more than one distinctconformation around the core 1,6-linkage. These various conformationsmay play an important role in determining the processing pathways.Using the data on the preferred conformations of these oligomannosesand the available experimental results, possible pathways forprocessing Man₉GlcNAc₂ to Man₅GlcNAc₂ by 1,2-linkage-specificmannosidases have been proposed. Conformational analysis ofMan₅GlcNAc₂ indicates that the addition of ß1,2-GlcNActo the 1,3-linked core mannose, besides serving as a prerequisitefor mannosidase II action as suggested earlier, may also preventthe removal of 1,3-mannose. The MD simulations also suggestthat the processing of the precursor oligosaccharide duringAsn-linked complex and hybrid glycan biosynthesis proceeds ina well-defined pathway involving more than one 1,2-linkage-specificmannosidase. Knowledge of the conformation of the processingintermediates obtained from the present study can be used todesign highly specific substrate analogues to inhibit a particularmannosidase, thereby blocking one processing pathway withoutinterfering with the others. carbohydrates conformation glycosidase inhibitors mannosidase oligosaccharide processing     

Stoichiometries of Photosystem I and Photosystem II in Rice Mutants Differently Deficient in Chlorophyll b

Stoichiometries of photosystem I (PSI) and photosystem II (PSII)reaction centers in a cultivar of rice, Norin No. 8, and threechlorophyll b-deficient mutants derived from the cultivar wereinvestigated. Quantitation of PSI by photooxidation of P-700and chromatographic assay of vitamin K₁ showed that, on thebasis of chlorophyll, the mutants have higher concentrationsof PSI than the wildtype rice. Greater increases were observedin the PSII contents measured by photoreduction of Q_A, bindingof a radioactive herbicide and atomic absorption spectroscopyof Mn. Consequently, the PSII to PSI ratio increased from 1.1–1.3in the wild-type rice to 1.8 in chlorina 2, which contains noChl b, and to 2.0–3.3 in chlorina 11 and chlorina 14,which have chlorophyll a/b ratios of 9 and 13, respectively.Measurement of oxygen evolution with saturating single-turnoverflashes revealed that, whereas at most 20% of PSII centers areinactive in oxygen evolution in the wildtype rice, the non-functionalPSII centers amount to about 50% in the three mutant strains.The fluorescence induction kinetics was also analyzed to estimateproportions of the inactive PSII in the mutants. The data obtainedsuggest that plants have an ability to adjust the stoichiometryof the two photosystems and the functional organization of PSIIin response to the genetically induced deficiency of chlorophyllb. (Received July 29, 1994; Accepted February 7, 1996)     

Nutrient Dilution by Starch in CO2-enriched Chrysanthemum

Increasing growth irradiance and CO₂ generally decreases foliarnutrient concentration on a dry weight basis and increases foliarstarch concentration. However, the extent to which starch concentrations‘dilute’ foliar nutrient concentrations when thelatter are expressed on a dry weight basis is not known. Todetermine the importance of differential starch accumulationin calculating nutrient concentrations on a dry weight basis,leaf nutrient and starch concentrations were measured in Chrysanthemum? morifolium ‘Fiesta’ (Ramat.) cuttings grown atthree irradiance levels and two CO₂ levels for eight weeks inboth winter and spring. On a dry weight basis, foliar concentrationsof most nutrients were lower in both seasons as a result ofthe elevated CO₂ and irradiance levels, and total dry weightswere higher. Per cent starch was greater at the high CO₂ levelin both seasons but was only greater at higher irradiances inthe winter experiment. When starch was subtracted from the leafdry weights, the differences between CO₂ and irradiance treatmentsdisappeared with respect to N, P, K, Ca, Mg, S, and B but notfor Fe, Mn, Zn, and Cu. Key words: CO₂ enrichment, starch, nutrients, irradiance     

Inhibition of Gibberellic Acid-induced Starch Mobilization by Salicylhydroxamic acid in Avena fatua L. Seed

Inhibition of GA₃-induced endosperm mobilization in Avena fatuaL. by salicylhydroxamic acid (SHAM), a widely used alternativerespiration inhibitor, was studied. SHAM strongly inhibitedthe GA₃-induced release of reducing sugars in the incubationmedium by 3 mm de-embryonated endosperm segments; at 4 mM SHAM,GA₃-induced sugar release was inhibited by 66–79 per cent.Extracts prepared from segments incubated in 0.05 mM GA₃ with2, 5 and 10 mM SHAM showed 30, 53 and 71 per cent lower -amylaseactivity, respectively, compared to the GA₃-alone treatment.Addition of SHAM (0.5–5 mM) during the enzyme assay hadno effect on the activity of -amylase. Thus, the inhibitionof starch mobilization in endosperm by SHAM is due to inhibitionof the production and not the activity of -amylase. The inhibitionof Avena fatua seedling growth by SHAM reported earlier may,in part, be due to its effect on endosperm mobilization. Since (1) Avena fatua seeds have been shown to have little orno SHAM-sensitive respiration, and (2) concentrations of SHAMnecessary for inhibiting endosperm mobilization were significantlyhigher than those generally necessary for inhibiting alternativerespiration, the inhibition of endosperm mobilization by thiscompound does not appear to involve its effect on alternativerespiration. Avena fatua L., wild oat, -amylase, endosperm, gibberellic acid, salicylhydroxamic acid, seed     

Light Harvesting and Utilization by Phytoplankton

In this study we use a model based on target theory to analyzesteady-state photosynthesis-irradiance relationships in continuouslight. From the average turnover time () of photosynthetic units(PSU_O2), numerical analyses of the model coefficients, and measurementsof the light field and cell absorptivity, apparent absorptioncrosssections of photosystem II (PSII) were determined for three species of marine unicellular algaegrown at different irradiance levels. These cross-sections generally,but not always, increased with decreased growth irradiance.Additionally, the ratios of photosystem I/photosystem II reactioncenters were calculated from measurements of oxygen flash yieldsand chlorophyll/P₇₀₀ ratios. From the ratios of the reactioncenters, cell absorptivity and the apparent absorption cross-sectionof photosystem II, the apparent absorption cross-sections ofphotosystem I (PSI) were also calculated. Finally, on the basis of our calculated absorptioncross-sections, we estimated the minimum quantum requirementsfor O₂ evolution. Our results suggest that the absorption cross-sectionsof PS I and PS II vary independently and the minimum quantumrequirements for O₂ vary by more than twofold, increasing from9.1 to 20.6 quanta/O₂, as growth irradiance increases. The increasein quantum requirement corresponds to larger apparent cross-sectionsfor photosystem I and higher carotenoid/chlorophyll ratios. (Received October 15, 1985; Accepted July 17, 1986)     

Effects of Gibberellins and Prohexadione on the Activities of Oryzain and {alpha}-Amylase in Rice Seeds

Oryzains, cysteine proteinases of rice seeds, are induced byGA₃ in germinating rice seeds [Abe et al. (1987) Agric. Biol.Chem. 51: 1509]. The effects of GA₁, GA₃, GA₄, GA₉, and GA₂₀on the production of oryzain and -amylase were investigatedin embryoless half- and whole-seeds of rice (cv. Nipponbare).When gibberellins (GAs) were incubated with embryoless half-seeds,GA₁, GA₃ and GA₄ induced oryzain and -amylase, but GA₉, andGA₂₀ did not. GA₉ and GAM induced oryzain and -amylase productionin whole seeds, but this production was inhibited by the simultaneousapplication of prohexadione, an inhibitor of 2ß- and3ß-hydroxylation of GAs. Prohexadione did not inhibitthe activities of oryzain and -amylase induced by GA₁. Theseresults suggest that GAs possessing the 3ß-hydroxylgroup induce activities of oryzain and -amylase in rice seedsand that GA₉ and GA₂₀ have activity only after they are convertedmetabolically to active GAs, probably GA₄ and GA₁, respectively.GA₁, was more active than GA₄ in both half seeds and wholeseeds incubation. Oryzain and -amylase activities induced byGA₄ were significantly inhibited in the presence of 10^–4M prohexadione. This suggests that the conversion of GA₁, toGA₄ (13-hydroxylation) might be inhibited at a high dose ofprohexadione in whole seeds. ⁴Present address: Institute of Food Development, Kyung Hee University,Suwon 449-701, Korea     

Effect of Supra-High Irradiation on the Photosynthetic System of the Cyanophyte Synechocystis PCC 6714

Stability of thylakoid components under supra-high irradiancewas studied with the cyanophyte Synechocystis PCC 6714. Theactivity of overall photosynthesis was quickly inactivated (T_1/2=20min) under supra-high irradiance (300 W m^–2, white light).In parallel with the inactivation of photosynthesis, QA in PSII was also inactivated. Both inactivations were acceleratedby chloramphenicol (CAP) addition. The reactivation of PS IIrequired weak irradiation and was suppressed by CAP. However,PS I measured as P700 was very stable. The level of PS I measuredas P700 was not significantly reduced by the irradiation for12 h even in the presence of CAP while the level of Cyt b₅₅₉,component of PS II, was decreased markedly. The function ofPS I before and after supra-high irradiation with CAP was examinedby comparing sizes of P700 oxidation induced by a short flash,by a continuous light, and by determination of O₂-and ferredoxin-reduction.No difference was observed in PS I actions before and afterthe irradiation treatment. These results indicate that the PSI complex is very tolerant of supra-high irradiation. However,the cells grown under supra-high irradiance contained much fewerPS I and PS II complexes than Cyt b₆–f complexes. Theformer levels were reduced to a half to one fourth of thosebefore growth while the level of Cyt b₆–f complex wasnot reduced so much. A possible mechanism for changes in thylakoidcomposition under supra-high irradiation was discussed. (Received February 16, 1991; Accepted June 12, 1991)     

Physiological Responses of Phytoflagellates to Dissolved Organic Substrate Additions. 2. Dominant Role of Autotrophic Nutrition in Pyrenomonas salina (Cryptophyceae)

The enhancement of algal growth by organic substrate assimilationis a common laboratory observation, yet few studies have addressedthe interaction of dissolved organic compounds and environmentalfactors for controlling the relative contribution of heterotrophyand autotrophy to the nutrition of these algae. The effectsof light intensity and glycerol addition on the growth, cellvolume, pigmentation, and carbon uptake of the facultative heterotroph,Pyrenomonas salina Santore, were examined. Glycerol additionto cultures growing at a limiting light intensity increasedthe growth rate, increased the average cell volume and cellularstarch content, decreased the cellular phycoerythrin to chlorophyll ratio, and had no effect on the CO₂ fixation rate cell^–1.Glycerol addition to cultures growing at a moderate light intensitythat was saturating for photo-autotrophic growth increased theaverage cell volume and cellular starch content but had no effecton the CO₂ fixation rate cell^–1. The results indicatethat autotrophy was the major process for carbon acquisitionduring the growth of P. salina, but that carbon acquisitionfrom glycerol catabolism also was used to partially supportgrowth of the alga at the limiting light intensity. In addition,glycerol presumably was used to fulfill the energy and/or reductantrequirements of the alga, and to increase the reserve carbohydrate(starch). ¹ Current address and address for correspondences: Horn PointEnvironmental Laboratories, University of Maryland, PO Box 775,Cambridge, Maryland 21613, U.S.A. (Received October 29, 1990; Accepted May 31, 1991)     

Fluid pressure in human dermal fibroblast aggregates measured with micropipettes

Previous studies indicated that connective tissue cells in dermis are involved in control of interstitial fluid pressure (P_if). We wanted to develop and characterize an in vitro model representative of loose connective tissue to study dynamic changes in fluid pressure (P_f) over a time course of a few minutes. P_f was measured with micropipettes in human dermal fibroblast cell aggregates of varying size (<100- and >100-µm diameter) and age (days 1-4) kept at different temperatures (15, 25, and 35°C). Pressures were measured at different depths of micropipette penetration and after treatment with prostaglandin E₁ isopropyl ester (PGE₁), latanoprost (PGF₂), and ouabain. P_f was positive (more than +2 mmHg) during control conditions and increased with increasing aggregate size (day 2), age (day 4 vs. day 1), temperature, and depth of micropipette penetration. P_f decreased from 2.9 to 2.0 mmHg during the first 10 min after application of 10 µl of 1 mM PGE₁ (P < 0.001). P_f increased from 3.0 to 4.8 mmHg (P < 0.01) after administration of 10 µl of 1.4 µM ouabain and from 3.1 to 4.4 mmHg after addition of 5 µl of 1.42 mM PGF₂ (P > 0.05). In conclusion, we have developed and validated a new in vitro method for studying fluid pressure in loose connective tissue elements with the advantage of allowing reliable and rapid screening of substances that have a potential to modify P_f and studying in more detail specific cell types involved in control of P_f. This study also provides evidence that fibroblasts in the connective tissue can actively modulate P_f. micropuncture; prostaglandin E₁; prostaglandin F₂; ouabain; integrins     

Carbon Metabolism in Seagrasses: I. THE UTILIZATION OF EXOGENOUS INORGANIC CARBON SPECIES IN PHOTOSYNTHESIS

Four species of seagrasses, Halophila stipulacea, Thalassodendronciliatum, Halodule uninervis, and Syringodium isoetifolium,were investigated for their ability to utilize and CO₂ as exogenous carbon sources for photosynthesis. Ratesof photosynthesis were measured as rates of O₂ evolution ina closed system in which the pH was continuously controlled.A computer program was written to calculate the concentrationsof different carbon species as a function of pH and other specifiedexperimental conditions. Bicarbonate as well as CO₂ were readily assimilated by all fourseagrass species. Saturating concentrations of , at saturating light intensities, were 0.5–1.8 mM dependingon the species. Rates of photosynthesis under such conditionswere 0.1–0.55 µmol O₂ min^–1 mg^–1 chlorophyll.At saturating CO₂ concentrations, i.e. 0.5–1.3 mM, ratesof photosynthesis were 0.22–1.4 µmol CO₂ min^–1mg^–1 chlorophyll. Photosynthetic rates in each specieswere considerably higher when CO₂ rather than was supplied at saturating concentrations. The concentration of in natural seawater was found to be saturating, and that of CO₂ insufficient forconsiderable photosynthetic rates in these plants under thegiven conditions It was thus concluded that is the major carbon source for photosynthesis in seagrasses.     

Pumpkin (Cucurbita sp.) seed globulin V. Proteolytic activities involved in globulin degradation in ungerminated seeds

Two proteolytic activities I and II involved in the globulindegradation were detected in pumpkin seeds. Activity I, hydrolyzing and ß subunits of the globulin to form F_ß,was found in both dry seeds and cycloheximide-treated cotyledons,and decreased during germination. Activity II, hydrolyzing F_ßto produce small peptides and amino acids, was not observedin dry seeds but found in cycloheximide-treated cotyledons,increased up to 4 days, and gradually decreased during germination. Activity I gave limited hydrolytic products from the globulinand the chain, but not from F_ß, the chain and some animal proteins. It was inhibitedby EDTA. On the other hand, activity II hydrolyzed F_ßand the chain faster than the globulin, the chain and some animal proteins. It was inhibitedby EDTA and p-chloromer-curibenzoate, and activated by ß-mercaptoethanol,dithiothreitol and CoCl₂. Optimum pH's were at about 6.8 andat 6.0 to 6.8 for activities I and II, respectively. The degradation process of the globulin can be divided intotwo steps: the first step is the conversion of globulin to F_ßand the second step, F_ß to small peptides and aminoacids. (Received November 9, 1979; )     

Phytoplankton production in subarctic lake and river ecosystems: development of a photosynthesis-temperature-irradiance model

We investigated the size-dependent temperature response of naturalphytoplankton communities from a lake and a river in the Canadiansubarctic. Photosynthesis by total, <2 m and >2 µmsize fractions was determined at 11 irradiances (1–109%of ambient solar radiation) and five temperatures (5–25C)in outdoor solar incubators. Temperature had no effect on photosynthesisat low irradiance, but strongly regulated the photosyntheticresponse at saturating and inhibiting irradiances. For the riverphytoplankton, low temperatures lowered EK values (onset oflight saturation) and shifted photosynthesis in the water columnfrom light dependence to temperature depend ence. A photosynthesis-temperature-irradiance(P-T-E) model was developed to describe the varied temperatureresponse of photosynthesis across the full range of limiting,saturating and inhibiting irra diances. The P-T-E model explained74–95% of the variation in photosynthesis for all sizefractions (total community, >2 µ fraction and <2µm fraction). Picoplankton (<2 µm) had greaterphotosyn thetic rates (P_max) at all temperatures than did thetotaland >2 µm communities. The picoplankton fraction wasalso more responsive to increasing temperature than larger cells,implying a greater sensitivity to diurnal or longer term changesin lake water temperature.     

Mechanisms of EGF-induced stimulation of sodium reabsorption by alveolar epithelial cells

We investigated theeffects of epidermal growth factor (EGF) on activeNa⁺ absorption by alveolarepithelium. Rat alveolar epithelial cells (AEC) were isolated andcultivated in serum-free medium on tissue culture-treated polycarbonatefilters. mRNA for rat epithelial Na⁺ channel (rENaC) -, -,and -subunits and Na⁺ pump₁- and₁-subunits were detected inday 4 monolayers by Northern analysisand were unchanged in abundance in day5 monolayers in the absence of EGF. Monolayerscultivated in the presence of EGF (20 ng/ml) for 24 h fromday 4 to day5 showed an increase in both₁ and₁Na⁺ pump subunit mRNA but noincrease in rENaC subunit mRNA. EGF-treated monolayers showed parallelincreases in Na⁺ pump₁- and₁-subunit protein by immunoblotrelative to untreated monolayers. Fixed AEC monolayers demonstratedpredominantly membrane-associated immunofluorescent labeling withanti-Na⁺ pump₁- and₁-subunit antibodies, withincreased intensity of cell labeling for both subunits seen at 24 hfollowing exposure to EGF. These changes inNa⁺ pump mRNA and protein precededa delayed (>12 h) increase in short-current circuit (measure ofactive transepithelial Na⁺transport) across monolayers treated with EGF compared with untreated monolayers. We conclude that EGF increases activeNa⁺ resorption across AECmonolayers primarily via direct effects onNa⁺ pump subunit mRNA expressionand protein synthesis, leading to increased numbers of functionalNa⁺ pumps in the basolateralmembranes.

     

Brassinosteroid-Induced Bending of the Leaf Lamina of Dwarf Rice Seedlings: An Auxin-Mediated Phenomenon

A synthetic brassinosteroid (BR), 2,3,22ß,23ß-tetrahydroxy-24ß-methyl-B-homo7-oxa-5-cholestan-6-one, an isomer of the growth promoter brassinolide,when applied to seedlings of dwarf rice Oryza sativa var. Tan-ginbozuand Waito-C, induced a significant bending of the second leaflamina at 100 ng/plant and higher dosages. Promotion of thesecond leaf sheath elongation, the characteristic response ofdwarf rice varieties to gibberellins, was significantly butmodestly enhanced by BR at a dosage of 10,000 ng/plant, fiveorders of magnitude higher than the minimal dosage responseto GA₃. Gibberellin A₃ had no significant effect on the bendingof the second leaf lamina, nor did any synergism exist betweenBR and GA₃ in leaf lamina bending or leaf sheath elongation.Neither ethylene nor (2-chloroethyl)phosphonic acid (ethephon)caused the bending of the second leaf lamina, and neither synergizedthe BR effect. However, IAA and -naphthaleneacetic acid causedsignificant bending at 5,000 ng/plant, and both auxins significantlysynergized the effect of BR on the bending, IAA being effectiveat 500 ng/ plant in this regard. The antiauxins, 2,3,5-triiodobenzoicacid (TIBA) and -(p-chlorophenoxy)isobutyric acid (PCIB) completelynullified both the BR-induced bending and the BR$IAA-synergizedbending. The BR-induced bending response may thus be mediatedthrough endogenous auxin. (Received May 11, 1982; Accepted August 25, 1982)