Thermal Kinetics of Glutathione Reductase and their Relation to Thermotolerance in Diverse Cultivars of Maize

The characteristics of glutathione reductase from a number ofmaize cultivars with contrasting thermotolerance have been investigated.The apparent K_m (Michaelis constant) for oxidised glutathione(GSSG) was measured between 10 and 45°C at constant pH.The enzyme from highland cultivars adapted to cool environmentshad a slightly lower apparent K_m for GSSG than that from lowlandtropical cultivars at low assay temperatures. Similarly theenzyme from lowland tropical cultivars had a lower apparentK_m for GSSG at high assay temperatures. However these effectswere small and regression lines plotted through the data werenot significantly different in slope or intercept. There wasa strong correlation (r = 0·939) between apparent K_mand V_max (maximum initial velocity) as assay temperature wasvaried. The interpretation of apparent K_m/temperature relationshipsis discussed with hypothetical examples of the effects of temperatureon enzyme activity/substrate concentration plots. It is demonstratedthat an increase in apparent K_m at higher assay temperaturesneed not necessarily reflect any temperature-dependent impairmentof enzyme function. The apparent K_m for GSSG of glutathionereductase from maize increased over four-fold when the temperaturewas raised from 10 to 45°C, but it is concluded that invivo rates of reaction are likely to be increased rather thandecreased by this same change in temperature. Glutathione reductasewould thus appear to be equally well adapted to function atall these temperatures. This suggests that the potential ofenzyme thermal kinetics to predict thermotolerance may be limited.Copyright1994, 1999 Academic Press Zea mays L., maize, glutathione reductase, thermal kinetics, thermotolerance     

Nitrate Effects on Growth of the First Four Main Stem Leaves of a Range of Temperate Cereals and Pasture Grasses

The effects of different applied NO₃^– concentrations onextension growth and final length and area of leaves 1–4of five cereals and six pasture grasses of temperate originwere examined. Increased applied NO₃^– in the range 0.1–0.5.0mol m^–3 caused decreased duration of growth but increasedgrowth rate and final length of leaves 2–4 of the cerealsAvena saliva, Hordeum vulgare, Secale cereale, x Triticosecaleand Triticum aestivum. For all cereals, increased NO₃^–resulted in increased area of leaves 1–4. Pasture grasseswere supplied either 0.5 or 50 mol m^–3 NO₃^–. Increasedapplied NO₃^– (0.5–5.0 mol m^–3) resulted indecreased duration of growth and increased growth rate and finalarea of leaves 1–4 of Bromus wiltdenowii, leaves 2–4ofFestuca arundinaceae and leaves 3 and 4 of Lolium muitiflorum.In addition, length of leaves 3 and 4 of B. witidenowii increasedwith increased NO₃^–. Increased NO₃^– resulted inincreased area of leaves 2–4 of Dactylis gtomerata andLolium perenne and leaves 3 and 4 of Phalaris aquaiica but hadno effect on extension growth of all three species. Avena sativa L, oat, Hordeum vulgare L, barley, Secale cereale L, rye, x Triticosecale Wittm, triticale, Triticum aestivum L, wheat, Bromus willdenowii Kunth, prairie grass, Dactylis gtomerata L, cocksfoot, Festuca arundinaceae Shreb, tall fescue, Lolium multijlorum Lam, Italian ryegrass, Lolium perenne L, perennial ryegrass, Phalaris aquatica L, nitrate, leaf extension, leaf expansion     

Seed Viability Constants for Lettuce

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

No Light Activation and High Malate Sensitivity of Phosphoenolpyruvate Carboxylase in Guard Cell Protoplasts of Commelina communis L.

Guard cell protoplasts (GCP) were prepared from leaves of Commelinacommunis L. and phosphoenolpyruvate carboxylase (PEPc) activityrecorded after injection of the protoplasts directly into theassay medium. The GCP were lysed immediately by the presenceof Triton X-100 and a lowered osmotic concentration in the assaycuvette enabling PEPc activity to be measured with ‘nascent’enzyme. There was no light activation of the enzyme with K_mPEP (about 3.7 mol m^–3) and Vmax being similar in light-ordark-treated protoplasts. Illumination of the GCP in the presenceof CO₂-free air and KCI, a treatment which is known to swellGCP, did not change the kinetics. PEPc activity at saturating PEP was very sensitive to malateinhibition, 20 mmol m^–3 (the I₅₀ value) inhibiting activityby about 50%. Inhibition was similar in light- or dark-treatedprotoplasts. Malate inhibition was, however, much less (I₅₀= 500 mmol m^–3) if the enzyme source was a protoplastextract kept in the absence of glycerol. Inclusion of 20% glycerolin the extraction medium maintained the enzyme in the malate-sensitiveform as occurred in the in vivo assays. The high apparent K_mPEP and the high sensitivity to malate inhibition of GCP PEPcare features unlike those observed with PEPc from leaf tissuesof C₄ and CAM plants and from GCP extracts. PEPc activity increased slightly in the presence of KCI in theassay medium up to about 10 mol m^–3 and thereafter activityslowly declined as KCI concentrations increased further. Key words: Guard cell protoplasts, phosphoenolpyruvate carboxylase     

A Cytochrome P450 Mediated Naringenin 3'-Hydroxylase from Sweet Orange Cell Cultures

A microsomal flavonoid 3'-hydroxylase (F3'H) catalyzing themetabolism of naringenin to eriodictyol in Citrus sinensis (L.)Osbeck cv. ‘Hamlin’ cell suspension cultures wasshown to be a cytochrome P450 enzyme. This reaction requiredO₂ and NADPH and was inhibited by CO, with partial reversalof CO-inhibition by light at 450 nm. Cytochrome P450 contentranged from 10–20 pmol (mg microsomal protein)^–1.The F3'H reaction was shown to be linear in regard to proteinconcentration between 2.5 and 25 µg of microsomal protein.The optimum pH for the reaction was 7.4–7.6 and the temperatureoptimum was between 30 and 37°C. The apparent K_m and V_maxfor naringenin were 24 µM±3.2 and 81.4±7.9pmol eriodictyol min^–1 (mg protein)^–1, respectively.The microsomal F3'H was also capable of forming dihydroquercetinfrom dihydrokaempferol (40 pmol min^–1 (mg protein)^–1)and of quercetin from kaempferol (3.25 pmol min^–1 (mgprotein^–1). Cytochrome c and ketoconazole were the bestinhibitors of WH activity followed by piperonyl butoxide anda-naphthoflavone. Light was shown to be an inducer of the F3'Halmost doubling the specific activity and increasing the microsomalcytochrome P450 content by 30% over that of dark grown cells.F3'H activity was also confirmed in microsomal preparationsof young (new flush) leaves from ‘Hamlin’ treesand flavedo of ‘Hamlin’ oranges, ‘Marsh’grapefruit, and ‘Lisbon’ lemon. No activity wasobserved in older, hardened leaves and albedo of all the fruittested. Initiation of embryogenesis in the ‘Hamlin’cell suspension cultures by switching from a sucrose mediumto a glycerol-based medium resulted in the down-regulation ofF3'H. ¹Mention of a trademark, warranty, proprietary product, or vendordoes not constitute a guarantee by the U.S. Department of Agricultureand does not imply its approval to the exclusion of other productsor vendors that may also be suitable.     

Respiratory Efflux in Relation to Temperature of Simulated Swards of Perennial Ryegrass with Contrasting Soluble Carbohydrate Contents

Fully light-intercepting simulated swards of S24 perennial ryegrasswere exposed to contrasting environmental conditions in a growthroom for 4 days. Half experienced 20 h days of 120 Wm^–2(400–700. nm) and 5 °C, and came to have a WSC (watersoluble carbohydrate) content of 235 mg g^–1 and half 4h days of 20 Wm^–2 and 25 °C leading to a WSC of 25mg g^–1. Their rates of CO₂ efflux were monitored at anumber of temperatures during an 8 h dark period; half experiencedincreasing (5–30 °C) and half decreasing (30–5°C) temperatures. The ‘high’ WSC swards hadrespiration rates of 3.7 mg CO₂ g^–1 (d. wt) h^–1at 15 °C, and the ‘low’ swards 0.8 mg CO₂ g^–1h^–1. The order in which the temperatures were experiencedwas immaterial. Even the ‘low’ WSC swards showedno evidence of a respiratory decline during the dark periodthat could be attributed to substrate shortage. The relationshipbetween temperature and CO₂ efflux was best represented by logisticcurves. Even so, a Q₁₀ of 2 fitted the data reasonably well,at least up to 20 °C, and has practical advantages wheninterpolating estimated between measured values of respirationin the construction of a carbon balance sheet. Lohum perenne L., ryegrass, respiration, temperature, Q₁₀, soluble carbohydrate content, simulated sward     

Nitrate Effects on Growth of the First Four Main Stem Leaves of a Range of Temperate Cereals and Pasture Grasses

The effects of different applied NO₃^– concentrations onextension growth and final length and area of leaves 1–4of five cereals and six pasture grasses of temperate originwere examined. Increased applied NO₃^– in the range 0.1–50mol m^–3; caused decreased duration of growth but increasedgrowth rate and final length of leaves 2–4 of the cerealsAvena saliva, Hordeum vulgare, Secale cereale x Triticosecaleand Triticum aestivum. For all cereals, increased NO₃^–resulted in increased area of leaves 1-4. Pasture grasses weresupplied either 0.5 or 50 mol m^–3; NO₃^–. Increasedapplied NO₃^– (0.5–50 mol m^–3) resulted indecreased duration of growth and increased growth rate and finalarea of leaves 1–4 of Bromus willdenowii leaves 2–4of Festuca arundinaceae and leaves 3 and 4 of Lolium multiflorum.In addition, length of leaves 3 and 4 of B. willdenowii increasedwith increased NO₃. Increased NO₃ resulted in increased areaof leaves 2–4 of Daciylis glomerata and Lolium perenneand leaves 3 and 4 of Phalaris aquatica but had no effect onextension growth of all three species. Avena saliva L., oat, Hordeum vulgare L., barley, Secale cereaie L., rye, x Triticosecale Wittm, triticale, Triticum aestivum L., wheat, Bromus willdenowii Kunth, prairie grass, Dactylis glomerata L., cocksfoot, Festuca arundinaceae Shreb, tall fescue, Lolium multiflorum Lam, Italian ryegrass, Lolium perenne L, perennial ryegrass, Phalaris aquatica L, nitrate,, leaf extension, leaf expansion     

The Effect of External pH on the Apparent CO2 Affinity of Chlorella saccharophila

The carbon dioxide compensation point of the unicellular greenalga, Chloretla saccharophila, was determined in aqueous mediumby a gas chromatographic method. Compensation points decreasedmarkedly from 63 cm³ m^–3 at an external pH of 4.0 to 3.2cm³ m^–3 at pH 8.0 and were not affected by the O₂ concentrationof the medium. The calculated CO² concentration required tosupport the half-maximum photosynthetic rate of the algal cellsranged from 6.0 mmol m_–3 at an external pH of 60 to 1.5mmol m^–3 at pH 8.0 and these values were not affectedby O₂ concentration. The K_m(CO₂) of nbulose-l,5-bisphosphatecarboxylase isolated from cells grown either at pH 4.0 or pH8.0 was determined to be 64 mmol m^–3. These results indicatethat loss of CO₂ by photorespiration does not occur in C. saccharophilacells at acid pH and the disparity between the apparent affinityfor CO₂ of the intact cells and that of the carboxylase indicatesthe operation of a ‘CO₂ concentrating mechanism’in this alga at acid pH. Key words: Acidophilic alga, bicarbonate transport, Chlorella saccharophila, compensation point, CO₂ affinity, PH, RuBP carboxylase     

The optical properties of a turbid reservoir and its phytoplankton in relation to photosynthesis and growth (Mount Bold Reservoir, South Australia)

In situ light measurements were used to obtain information oninherent and apparent optical properties. The average verticalattenuation coefficient K_d(ave) varied from 1.1 to 4.6 In unitsm^–1 During three periods the variation in K_d(ave) correlatedwith changes in chlorophyll a concentration and specific attenuationcoefficients K_s, of 0.013, 0.014 and 0.022 m² mg Chl a^–1were calculated. Chlorophyll-specific diffuse absorption coefficients(A,) for these periods were 0.012. 0.013 and 0.017 m² mg Chla^–1 and only varied significantly from estimates of K_sin the period when scattering was intense. Absorption coefficientsa(z^mid) and scattering coefficients b(z^mid) calculated for themid-point of the euphotic zone ranged between 0.45 and 2.9 mand 3.5–52.0 m respectively. Chlorophyll-specific absorptioncoefficients K_a, of 0.005, 0.006 and 0.007 m² mg Chl a^–1and scattering coefficients K_b of 0.05. 0.09 and 0.191 m² mgChl a^–1 were measured during the three periods. The highK_b value occurred when gas-vacuolate cyanobactena were dominant.Algal photosynthesis and light absorption were related throughthe maximum quantum yield _m which varied between 0.019 and 0.11mol C Einstein^–1 while average quantum yields _a, variedbetween 0.006 and 0.024 with a mean of 0.013 mol C Einstein^–1A comparison of changes in the mean irradiance of the mixedzone and chlorophyll concentration indicated that growth waslight limited below 0.04–0.05 Einsteins absorbed mg Chla^–1 day^–1.     

Nitrate Transport into Chara corallina Cells using 36ClO-3 as an Analogue for Nitrate: I. INTERACTION BETWEEN 36ClO-3 AND NO-3 AND CHARACTERIZATION OF 36CIO-3/NO-3 INFLUX

The use of chlorate as an analogue for NO^–₃ during nitrateuptake into Chara corallina cells has been investigated. NO^–₃inhibits ³⁶C1O^–₃ influx into Chara over the concentrationrange 0–1000 mmol m^–3. Lineweaver-Burke plots ofthe data are characteristic of competitive inhibition by NO^–3in the low concentration range (0–300 mmol m^–3 ClO^–₃)and apparent K_INO3 is 140 mmol m^–3 which is of a similarorder of magnitude as apparent K_mCIO3- 180 mmol m^–3. Athigher substrate concentrations the inhibition by NO^–₃was not characteristic of competitive or uncompetitive inhibition. ³⁶C1O^–₃/NO^–₃ influx was dependent on K⁺ and Ca²⁺in the external medium and inhibited by FCCP. NO^–₃ pretreatmentor N starvation increased subsequent ³⁶C1O^–₃/NO^–₃influx into Chara. A comparison between rates of net NO^–₃uptake and ³⁶C1O^–₃/NO^–₃ influx supported the previoushypothesis that NO^–₃ efflux is an important componentin the determination of overall uptake rates. Key words: Nitrate, Chara, ³⁶CIO^–₃     

Flowering Competence after Exposure to Naturally Fluctuating Winter Temperatures in a Perennial Grass, Deschampsia caespitosa (L.) Beauv

Tussocks of three clones of the perennial grass Deschampsiacaespitosa were exposed to natural temperatures, for successiveperiods with increments of one week, throughout each seasonin four consecutive autumn/winter seasons. After exposure theywere returned to a warm, illuminated glasshouse and the incidence,intensity and timing of flowering were recorded. The year-to-year variations in exposure time requirement forfull flowering were very highly correlated (r² > 0.98) withmean air temperature in the period between mid-September andearly December. The relationship with the natural logarithmof accumulated air temperature was similarly good. Daily meanair temperature was also demonstrated to be a good indicatorof daily mean tussock temperature (r² = 0.91), in a series ofmeasurements in the autumn of 1976. More prolonged exposure increased the proportion of apices ina tussock which flowered, but the maximum value at the end ofa whole winter was only about 15 per cent. The cumulative vernalizationeffects of naturally fluctuating temperature regimes are analysedand the significance of these findings for the maintenance ofperenniality in clonal grasses is considered. Deschampsia caespitosa (L.) Beauv., tufted hairgrass, flowering, vernalization, temperature, perenniality     

Translocation and Temperature

The apparent diffusion constant, K, derived from profiles ofradioactivity during labelled sucrose translocation, is temperature-dependentwith a Q₁₀ of 2.9 in Pteridium. This value is similar to thoseobtained by other authors for mass transfer rates. The behaviourof K with temperature supports a translocation mechanism ofthe ‘activated-diffusion’ type and is consistentwith the Canny and Phillips translocation model which also suggestsa basis for the 25-30° C temperature optimum in translocation.     

UV-induced mortality of zoea I larvae of brown shrimp Crangon crangon (Linnaeus, 1758)

Zoea I larvae of the brown shrimp Crangon crangon (Decapoda)were exposed to varying levels of UV radiation in a sunshinesimulator. ‘Short-term exposures’ (0–8 h)were used to determine the highest UV dose with no significanteffect (NOEC; defined by limit of detection) and the lethaldose of 10 and 50% mortality (LD₁₀ and LD₅₀). Crangon crangonshowed a relatively high sensitivity to UVB radiation (NOEC= 10 kJ m^–2, LD₁₀ = 15 kJ m^–2, LD₅₀ = 24 kJ m^–2)compared to other crust-acean species. LD values (1997–1998)showed no adaptation to seasonal light regimes. ‘Long-termexposures’ (0–10 days) were carried out to assessthe range where the ‘law of reciprocity’ is valid.The larvae were exposed to UV levels of 0.2, 0.4 and 0.7 J m^–2for appropriate time intervals, always cumulating in a sublethaldose of 5 kJ m^–2 day^–1. Results reflect a possiblethreshold (0.2–0.4 J m^–2 UVB) in the effect of thedifferent UVB doses used; thus, a proportional relationshipof intensity and exposure time can only be shown at UVB levelsabove this threshold intensity.     

Potassium-Ammonium Uptake Interactions in Tobacco Seedlings

Short-term (< 12 h) uptake experiments were conducted with6–7-week-old tobacco (Nicotiana tabacum L. cv. Ky 14)seedlings to determine absorption interactions between K⁺ andNH₄⁺. At equal solution concentrations (0.5 mol m^–3) netK⁺ uptake was inhibited 30–35% by NH₄⁺ and NH₄⁺ uptakewas decreased 9–24%. Removal of NH₄⁺ resulted in completerecovery in K⁺ uptake rate, but NH⁴⁺ uptake rate did not recoverwhen K⁺ was removed. In both cases, inhibition of the uptakerate of one cation saturated as the concentration of the othercation was increased up to 0.5 mol m^–3. The relative effectof K⁺-NH₄⁺ interactions was not altered when Cl- was replacedwith SO₄^2–, but the magnitudes of the uptake rates wereless in the absence of Cl-. The V_max for NH₄⁺ uptake was reducedfrom 128 to 105 µmol g^–1 dry wt. h^–1 in thepresence of 0.5 mol m^–3 K⁺ and the K_m for NH₄⁺ doubledfrom 12 to 27 mmol m^–3 in the presence of K⁺. The resultsof these K⁺-NH₄⁺ experiments are interpreted as mixed-noncompetitiveinteractions. However, an enhanced efflux of K⁺ coupled to NH₄⁺influx via an antiporter cannot be ruled out as contributingto the decrease in net K⁺ uptake. Key words: Nicotiana tabacum, K⁺, NH₄⁺, Uptake interactions     

Evidence for Essential Maintenance Respiration of Leaves of Xanthium Strumarium at High Temperature

Mesophyll resistance to photosynthetic carboxylation (r'_m) wasused as a criterion for leaf integrity. It was measured, at25 °C, in the light, before and after periods of high temperature(3 h at 38 °C) in the dark. During the high temperatureperiods, respiration (R_D) of attached leaves of Xanthium strumariumwas suppressed from 27%-36% by either low [O₂] (1.04% or 0.21%v.v.) or high [CO₂] (840 µl 1^–1) in the ambientair. Neither treatment affected rates of R_D or photo-respirationduring the second period at 25 °C. There was no significant increase of r'_m when R_D was not suppressedduring the high temperature treatment. When R_D was suppressedat high temperatures, r'_m increased from about 3s cm^–1before, to about 26 s cm^–1 after the high temperaturetreatment. The increase depended upon the degree of suppression. It is concluded that increased R_D at high temperature in Xanthiumleaves is partly the result of an increase of energy demandingmaintenance. The subsequent rate of carbon dioxide fixationis reduced when this increase of maintenance-induced respirationis inhibited.     

Acclimation of NO-3 fluxes to low root temperature by Brassica napus in relation to NO-3 supply

Acclimation of NO^–₃ transport fluxes (influx, efflux)in roots of oilseed rape (Brassica napus L. cv. Bien venu) andtheir sensitivity to growth at low root temperature was studiedin relation to external NO^–₃ supply, defined by constantconcentrations ranging from sub- to supra-optimal with respectto plant growth rate. Plants were grown from seed in flowingnutrient solutions containing 250 mmol m^–3 NO^–₃at 17°C for 20d, and solution temperature in half the cultureunits was then lowered decrementally over 3 d to 7°C. Threedays later plants were supplied with NO^–₃ at 1, 10, 100or 1000 mmol m^–3 maintained for 18 d. Dry matter productionwas decreased more by low root zone temperature than low [NO^–₃]_e. Root specific growth rates were inversely related to [NO^–₃]_eand shoot:root ratios increased with time at [NO^–₃]_e between10–1000 mmol m^–3. Net uptake of NO^–₃ at 17°Cwas twice that at 7°C, and at both temperatures it doubledwith increasing [NO^–₃]_e between 1–10 mmol m^–3with further small increases at higher [NO^–₃]_e. Mean unitabsorption rates of NO^–₃ between 0–6 d and 6–14d were linearly related (r² of 0.79–0.99) to log₁₀[NO].Steady-state Q₁₀ (7–17°C) for uptake between 0–6d were 0.91, 1.62, 1.27, and 1.10, respectively, at [NO^–₃]_eof 1, 10, 100, and 1000 mmol m^–3, compared with correspondingvalues of 0.98, 1.38, 1.68, and 1.89 between 6–14 d. Thedata indicated that net uptake rates at 7 and 17°C divergedover time at high [NO^–₃]_e. Short-term uptake rates from1 mol m^–3 NO^–₃ measured at 17°C were higherin plants grown with roots at 7°C than at 17°C; for7°C plants there was a strong inverse linear relationship(r²=0.94) between uptake rate and treatment log₁₀ [NO^–₃]_ewhilst rates in 17°C plants were independent of prior [NO^–₃]_e. Rates of NO^–₃ influx and efflux under different steady-stateconditions of NO^–₃ supply and root temperature were calculatedfrom dilution of ¹⁵N added to culture solutions. Efflux wassubstantial relative to net uptake in all treatments, and wasinversely related to [NO^–₃]_e at 17°C but not at 7°C.Ratios of influx: efflux ranged from 1.6–2.9 at 17°Cand 1.3–1.8 at 7°C, indicating the proportionatelygreater impact of efflux at low root temperature. Ratios ofefflux: net uptake were 0.53–1.56 at 17°C and 1.21–3.58at 7°C. The apparent sensitivities of influx and effluxto steady-state root temperature varied with [NO^–₃]_e.Both fluxes were higher at 17°C than 7°C in the presenceof 100–1000 mmol m^–3 NO^–₃ but the trend wasreversed at 1–10 mmol m^–3 NO. Concentrations oftotal N measured in xylem exudate were at least 2-fold higherat 7°C compared with 17°C, attributable mainly to higherconcentrations of NO^–₃ glutamine and proline. The resultsare discussed in terms of acclimatory and other responses shownby the NO^–₃ transport system under conditions of limitingNO^–₃ supply and low root temperature. Key words: Brassica napus, nitrate supply, efflux, influx, root temperature, xylem exudate     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Components of Growth and Dark Respiration of Kikuyu (Pennisetum clandestinum Chiov.) at Various Temperatures

Growth and dark respiration were measured in dense, miniatureswards of kikuyu grass grown at constant temperatures of 15,20, 25 and 30 °C. Total respiration over the first 12 hof darkness was very high and CO² efflux per unit surface areavaried from 2.4 to 3.9 g CO₂ m^–2 h^–1 at 15 and 30°C respectively. Such rates were consistent with the correspondinglyhigh net growth rates of 24 and 63 g d. wt m^–2 d^–1and the heavy yields of herbage. When plants were kept in thedark, CO₂ efflux subsequently declined rapidly to a lower, constantrate which was taken to be the maintenance respiration rate.The half-life of the declining phase of respiration averaged10.9 and 6.0 h at 15 and 30 °C respectively, and was curvilinearlyrelated to the specific maintenance respiration rate (m). Therapid decline in respiration was consistent with the low concentrationsof total soluble carbohydrate and starch in the herbage. Valuesof m for lamina and top growth increased with temperature witha Q₁₀ of 2.6 and 1.42 respectively, but m of stems alone wasnot affected by temperature. Using results from this study forkikuyu and from McCree (1974) for sorghum and white clover,it was noted that all three species have similar m when grownat temperatures which are near their respective optimums forgrowth. Kikuyu, Pennisetum clandestinum, growth, respiration, temperature     

Growth and development of Neocalanus flemingeri/plumchrus in the northern Gulf of Alaska: validation of the artificial-cohort method in cold waters

In situ growth and development of Neocalanus flemingeri/plumchrusstage C1–C4 copepodites were estimated by both the artificial-cohortand the single-stage incubation methods in March, April andMay of 2001–2005 at 5–6°C. Results from thesetwo methods were comparable and consistent. In the field, C1–C4stage durations ranged from 7 to >100 days, dependent ontemperature and chlorophyll a (Chl a) concentration. Averagestage durations were 12.4–14.1 days, yielding an averageof 56 days to reach C5, but under optimal conditions stage durationswere closer to 10 days, shortening the time to reach C5 (fromC1) to 46 days. Generally, growth rates decreased with increasingstage, ranging from 0.28 day^–1 to close to zero but weretypically between 0.20 and 0.05 day^–1, averaging 0.110± 0.006 day^–1 (mean ± SE) for single-stageand 0.107 ± 0.005 day^–1 (mean ± SE) forartificial-cohort methods. Growth was well described by equationsof Michaelis–Menten form, with maximum growth rates (G_max)of 0.17–0.18 day^–1 and half saturation Chl a concentrations(K_chl) of 0.45–0.46 mg m^–3 for combined C1–3,while G_max dropped to 0.08–0.09 day^–1 but K_chl remainedat 0.38–0.93 mg m^–3 for C4. In this study, in situgrowth of N. flemingeri/plumchrus was frequently food limitedto some degree, particularly during March. A comparison withglobal models of copepod growth rates suggests that these modelsstill require considerable refinement. We suggest that the artificial-cohortmethod is the most practical approach to generating the multispeciesdata required to address these deficiencies.     

Potassium Transport Across the Membranes of Chara: I. THE RELATIONSHIP BETWEEN RADIOACTIVE TRACER INFLUX AND ELECTRICAL CONDUCTANCE

Smith, J. R., Smith, F. A. and Walker, N. A. 1987. Potassiumtransport across the membrane of Chara. I. The relationshipbetween radioactive tracer influx and electrical conductance.—J.exp. Bot. 38:731–751. The ⁴²K influx () and the electrical conductance (G_m) were measured simultaneously for the ‘membrane’of internodal cells of Chara australis as a function of theexternal [KCl] (K?. In bathing solutions of pH = 5?0, progressively increased from 20?5to 430?60 nmol m^–2 s^–1 and G_m increased from 0?36?0?02to 3?8?0?8 S m^–2 when K? was increased from 0?1 to 10mol m^–3. The resting membrane potential difference (p.d.)was approximately -135 mV for low K? and approached the expectedNernst equilibrium p.d. for K⁺ ions when K? > 1?0 mol m^–3.Measurements of ³⁶Cl influx suggested that the ⁴²K influx waspredominantly electrogenic. The equivalent Goldman permeabilityto K⁺ ions (P_k) was approximately 20–30 nm s^–1 anddid not vary significantly with increasing K?. The equivalentconductance attributable to the electrogenic transport of K⁺ ions was calculated from assuming passive, independent diffusionof K⁺ ions and the ratio was found to be typically close to one. It was also found that themagnitudes of and G_m measuredsimultaneously for each individual cell were also well correlatedfor K? 1?0 mol m^–3, and that the slope of the line ofbest fit was close to one. For each K? it was found that theconductance not attributable to K⁺ translocation and presumablyassociated primarily with the transport of protons or theirequivalents was typically 0?2–0?5 Sm^–2. For K? >1?0 mol m^–3 the results indicated that the transport ofK⁺ ions was essentially independent, i.e. there was no evidencefor flux interactions. The results also indicated that the equivalentconductance derived from the measured ⁴²K influx could usefullyindicate the fraction of the electrical conductance attributableto the translocation of K⁺ ions. Key words: Potassium, conductance, influx