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.     

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。     

Dynamics of Competition in Populations of Carrot (Daucus carota)

Populations of carrot (Daucus carota) were raised over a widerange of densities (79–5763 plants m^-2) to examine thedynamics of competition in terms of yield–density relationshipsand size variability, and to investigate the effects of nutrientsupply on competition. While the relationship between shootyield and density was asymptotic, the relationship between rootand total yield and density tended to be parabolic. For a giventime and density series the relationship between yield per unitarea and density could best be described by the model: y=w_mD_(1+aD)^b wherey is the yield per unit area,D is density,w_m, a andb arefitted parameters. The parametersw_m anda increased over timebut nutrient availability affected onlyw_m. An extension of thebasic yield-density model is proposed to describe the dynamicsof the yield–density relationship over time: y=kD_{[1+cexp(-rt)]{1+}     

Estimation of Photorespiration of Green Plants and of their Mesophyll Resistance to CO2 Uptake

An estimate of photorespiration is obtained from the relationshipbetween the net exchange of CO₂ of the leaf and the internalCO₂ concentration, i.e. within the mesophyll intercellular spaces.The latter is obtained by calculation, taking into account thecombined epidermal and boundary-layer resistances between thebulk atmosphere and the mesophyll intercellular spaces. Thelinear part of this relationship (at low CO₂ concentrations)is extrapolated to zero internal concentration, at which noneof the intercellular photorespired CO₂ is available for reassimilation.The calculated output of CO₂ under such conditions providesan estimate of photorespiration, but, by failing to take intoaccount intracellular reassimilation of photorespired CO₂ underestimatesactual photorespiration. As the slope of this linear relationshiprepresents the mesophyll (intracellular) resistance to CO₂ uptake,this procedure was used to recalculate published data on effectsof light intensity and of oxygen concentration on net photosynthesis.The analysis showed that increased oxygen concentration anddecreased light intensity reduced photosynthesis largely byincreasing mesophyll resistance to CO₂ uptake. It is suggestedthat the CO₂ compensation point () is a function of both photorespiration(L) and mesophyll resistance (r_m): = L. r_m.     

Analysis by {delta}13C Measurement on Mechanism of Cultivar Difference in Leaf Photosynthesis of Rice (Oryza sativa L.)

Our previous paper showed that cultivar difference of flag leafphotosynthesis (LPS) in rice (Oryza sativa L.) was attributedto the difference in mesophyll resistance (r_m). In this paper,we tried to divide r_m into CO₂ transfer resistance (r_m) andCO₂ fixation resistance (r_c) for further analysis of r_m by meansof     

Nutrient Solution pH Control using Dipolar Buffers in Studies of Trifolium repens L. Nitrogen Nutrition

In studies of Trifolium repens nitrogen nutrition, the controlof nutrient solution pH using dipolar buffers, was evaluatedin tube culture under sterile conditions. Five buffers; MES,ADA, ACES, BES and MOPS with pK₂s (20 °C) of 6.15, 6.60,6.90, 7.15 and 7.20 respectively, at a concentration of 2.0mol m^–3, were provided to inoculated Trifolium repensgrowing in nutrient solution containing 7.13 mol m^–3 nitrogenas (NH₄)₂SO₄. Initial pH of each solution was adjusted to theappropriate buffer pK₂ Two buffers, ADA and ACES completelyinhibited plant growth. The remaining buffers had little effectin limiting pH change, although plant dry matter was higherand nodule numbers lower in the presence of these buffers. MESand MOPS were supplied to nutrient solutions with and without7.13 mol m^–3 (NH₄)₂SO₄, at concentrations ranging from0–12 mol m^–3. MES at 9 mol m^–3 and 12 molm^–3 reduced growth of plants reliant on the symbiosisfor providing nitrogen. The provision of MES to plants providedwith NH₄⁺ significantly increased plant yield and reduced nodulenumber at all concentrations. MOPS did not affect plant yieldor nodule number. The use of dipolar buffers in legume nitrogennutrition studies is considered in terms of buffering capacity,and the side effects on plant growth and symbiotic development. Key words: Ammonium, Dipolar buffer, Nitrogen nutrition, pH control, Symbiosis, Trifolium repens     

The Influence of Temperature on Photosynthesis and Transpiration in ten Temperate Grass Varieties Grown in four Different Environments

The influence of temperature on photosynthesis and transpirationwas studied in ten varieties of Lolium perenne, L. multiflorum,Dactylis glomerata, and Festuca arundinacea from three climaticorigins grown in three different controlled environments (15?C, 72 W m^-2 visible irradiation, 16-h photoperiod; 25 ?C, 72W m^-2 visible irradiation, 16-h photoperiod; and 25 ?C, 180W m^-2 visible irradiation, 16-h photoperiod) and in the glasshousein July/August. The optimum temperature for photosynthesis was influenced primarilyby growth environment; growth at low temperature (15 ?C) resultedin a low optimum temperature, which differed little from varietyto variety. The maximum CO₂-exchange rate was influenced bygrowth environment and by variety. Within a variety, plantsgrown at higher light intensity or lower temperature had a greaterCO₂-exchange rate. Seven varieties showed a negative correlationbetween the optimum leaf temperature and the maximum CO₂-exchangerate. Activation energies for photosynthesis were influencedby growth environment only. There were marked varietal differences in the values of leafresistances (r_a + r_t) obtained from transpiration data at theoptimum leaf temperature for CO₂ exchange. In Lolium, and Dactylisthe Mediterranean varieties had higher leaf resistances thanthe Northern varieties with the maritime varieties intermediate.In general the Dactylis varieties had higher resistances thanthe corresponding Lolium and Festuca varieties. Only at highgrowth temperatures was (r_a+r_l) insensitive to temperature;otherwise an activation energy of about 10 kcal/mole was observed.A negative correlation was found between mean varietal diffusionresistances (r_a+r_l), and corresponding maximum CO₂-exchangerates.     

Effects of light intensity and quality on the relative N and P requirement (the optimum N:P ratio) of marine planktonic algae

The relative requirement of N and P (the optimum N:P ratio)by Dunaliella tertiolecta, Phaeodactylum tricornutum, Prymnesiumparvum and Thalassiosira pseudonana was studied under variouslight intensities and spectra. The ratio was determined as theratio of the minimum cell N and P concentrations (q₀N and q₀pwhen either nutrient was limiting. The ratio varied widely amongspecies; under light-saturation for growth (116 µEin ^m–2s^–1 it ranged from 11.8 in ^D. tertiolecta to 36.6 in P.tricornutum. The ratio appeared to be higher at a sub-saturatingintensity (24 µEin m^–2 s^–1 in all except P.tricornutum, mainly because of higher q_oN with little changein q_oP. In T. pseudonana Q_oP also increased, resulting in aninsignificant change in the ratio. The ratio varied little withinthe range of saturation intensity. Light quality affected q_oNand q_oP as well as the ratio, and the pattern of change variedfrom species to species. The optimum ratio of individual specieswas linearly correlated to their q_oN except in P. tricornutum.q_oN for all species showed a linear correlation with cell proteinconcentrations irrespective of light conditions. The changeof optimum N:P ratios in the three species thus appears to berelated to changes in cell protein contents. The ratio of carbohydratesto protein remained constant regardless of light intensity orquality and was higher in P-limited cultures. We conclude thatchanges in light regime can strongly influence algal nutrientrequirements and species interrelationships by altering theoptimum cellular N:P ratio.     

Photoinhibition under Atmospheric O2, the Activation State of RuBP Carboxylase and the Content of Photosynthetic Intermediates in Soybean and Wheat

Ward, D. A. and Drake, B. G. 1987. Photoinhibition under atmosphericO₂, the activation state of RuBP carboxylase and the contentof photosynthetic intermediates in soybean and wheat.—J.exp. Bot. 38: 1937–1948. Associations between photosynthesis, the activation state ofRuBP carboxylase and the contents of photosynthetic intermediateswere compared in soybean and wheat leaves before and after exposureto photoinhibitory treatments in the presence of atmosphericO₂. Exposing attached leaves to a supra-saturating irradiance(3 800 µmol quanta m^{– 2} s^–1) for 2 h in CO_2-freeair decreased carboxylation efficiency and the light-saturatedphotosynthetic rate in air by approximately 50%. Exposure tothe photoinhibitory treatment for periods in excess of 2 h didnot cause a further decrease of photosynthesis in soybean. Althoughphotosynthesis was reduced, the initial and total (fully-activated)activities of ribulose 1,5-bisphosphate carboxylase (RuBPCase)in leaf extracts were unaltered in each species by the photoinhibitorytreatment. This was true for leaves sampled under both air andat a rate-limiting intercellular CO₂ partial pressure (C_i) of75 µPa Pa^–1. The contents of ribulose l,5-bisphosphate(RuBP) and 3-phosphoglyceric acid (3-PGA) were reduced by thephotoinhibitory treatment in soybean leaves sampled in air andat a rate-limiting C_i, although the RuBP/3-PGA ratio was unaffected.The relative reduction of RuBP content in soybean leaves atrate-limiting C₁ was similar to the corresponding reductionof carboxylation efficiency. For wheat,the relative reductionof RuBP content at rate-limiting C_i (–19%) caused by thephotoinhibitory treatment was considerably less than the correspondingdecrease of carboxylation efficiency (–49%).The RuBP/3-PGAratio of wheat was also increased significantly by the photoinhibitorytreatment The significance of these observations to the regulationof CO₂-limited photosynthesis in leaves experiencing photoinhibitionunder atmospheric oxygen is discussed. Consideration is alsogiven to the previous contention that contemporary measurementsof initial activity in crude extracts may provide a spuriousindication of the amount of the enzyme-CO₂-Mg_{2 +} form of RuBPcarboxylase present in the leaf. Key words: Carboxylation efficiency, RuBP carboxylase, photoinhibition, RuBP, 3-PGA     

Leaf Characteristics and Net Gas Exchange of Diploid and Autotetraploid Citrus

The effect of tetraploidy on leaf characteristics and net gasexchange was studied in diploid (2x ) and autotetraploid (4x) ‘Valencia’ sweet orange (Citrus sinensis (L.)Osb.) and ‘Femminello’ lemon (Citrus limon (L.)Burm. f.) leaves. Comparisons between ploidy levels were madeunder high irradiance (I) in a growth chamber or low total Iin a glasshouse. Tetraploids of both species had thicker leaves,larger mesophyll cell volume and lower light transmittance thandiploids regardless of growth I. Mesophyll surface area perunit leaf area of 2x leaves was 5–15% greater than on4x leaves. Leaf thickness and mesophyll cell volume were greaterin high I leaves than low I leaves. In high I, average leafarea was similar for 2x and 4x leaves, whereas in low I it was30% greater in 4x than in 2x leaves. Nitrogen and chlorophyllconcentration per cell increased with ploidy level in both growthconditions. The ratio of chlorophyll a:b was 25% greater in2x than in 4x leaves. When net CO₂assimilation rate (A_CO2) wasbased on leaf area, 4x orange leaves had 24–35% lowerA_CO2than their diploids. There were no significant differencesin A_CO2between 2x and 4x orange or lemon leaves when expressedon a per cell basis. Overall, lower A_CO2per unit leaf area oftetraploids was related to increase in leaf thickness, largermesophyll cell volume, the decrease in mesophyll area exposedto internal air spaces, and the lower ratio between cell surfaceto cell volume. Such changes probably increased the resistanceto CO₂diffusion to the site of carboyxlation in the chloroplasts. Cell volume; chlorophyll; irradiance; leaf thickness; nitrogen; photosynthesis; ploidy; Citrus limon ; C. sinensis ; ‘Valencia’ sweet orange; ‘Femminello’ lemon     

Studies on the Movement of Water Through Apple Trees

Resistances to the flow of water through young potted appletrees were estimated by measuring the transpiration rate oftrees with and without root systems. Root system resistanceswere obtained by difference. Whole-plant resistances were ofthe order 10 x 10¹³ Pa s m^–3 and there was some evidencethat root resistances (R_r) varied with transpiration rate; theratio R_r:R_x (where R_x is resistance to water flow in the stemsystem) altered from 2:1 at relatively high transpiration ratesto 1:1 at lower rates. The trunk of a 9-year-old orchard tree (trunk diameter {smalltilde}7 cm, height {small tilde}2.5 m) was cut under water andestimates of the flow resistances in this tree were obtained.These were much lower than the resistances to flow in the pottedtrees. Capacitance (defined as the change in stored water content perunit change in plant water potential) values were calculatedfor the small trees and the large tree from measurements ofweight and water potential changes after the trees were removedfrom water. They were very similar on a weight basis (approx.2.0 x 10^–8 kg kg^–1 Pa^–1). Leaf capacitancevalues ({small tilde}1 x 10^–8 kg Pa^–1 m^–2)were also obtained. Stomatal conductances decreased with water potential and increasedwith short-wave radiation, but the relationships were not definitive.Estimates of boundary layer conductance in a greenhouse (verylow wind speeds) were of the same order ({small tilde}5 mm s^–1)as values obtained previously.     

Photosynthesis and Light Activation of Ribulose 1, 5-bisphosphate Carboxylase in the Presence of Starch

Owing to a typographical error three equations were omittedfrom page 1294. The correct paragraphs are set out below. The component K₁ corrected for the difference in temperaturebetween the enzyme assay and the leaf and was calculated accordingto the Arrhemus equation. where v10 and v18 are the reaction velocities of carboxylationat 10?C and 18?C, respectively and A is the activation energy(A = 90 kJ mol^–1, as determined for purified wheat RuBPCOby M?chler, Keys and Cornelius, 1980) The components K2 corrected for the difference in CO₂ partialpressure between enzyme assay and leaf and for competitive inhibitionof carboxylation by O₂ and was calculated according to the modifiedMichaelis Menten equation where v_c, is the carboxylation velocity under leaf conditions,V_c. is the maximum carboxylation velocity as determined in theenzyme assay, K_c, and K_o are the Michaelis constants for carboxylationand oxygenation, respectively (K_o = 159 Pa CO₂. K_o = 35.3 kPaO₂, as interpolated for 18?C from spinach data as determinedby Jordan and Ogren, 1984), O is oxygen partial pressure inair and C₁ is intercellular CO₂ partial pressure in leaves (C₁= 29.1 ? 0.8 Pa (? s c , n = 15)) The component K3 corrected for the decrease in CO₂ fixationin leaves due to photorespiration and was calculated accordingto equation 3 Equation 3 is denved from the equation for the substrate specificityof RuBPCO, S= v_c/v_oC (Laing, Ogren, and Hageman, 1974), andfrom the equation for the stoichiometry of photorespiratoryCO₂ release, F=v_c–1/2 v_o, where v_c, and v_c are reactionvelocities of carboxylation and oxygenation, O and C are partialpressures of 02 and intercellular CO₂, F is net photosynthesisand S is the substrate specificity of RuBPCO (S= 3061 Pa/Pa,as interpolated for 18?C from spinach data as determined byJordan and Ogren, 1984)     

不同温湿度条件下飞虱虫疠霉对桃蚜生殖力和内禀增长力的影响

对7日龄桃蚜 Myzus persicae用高剂量飞虱虫疠霉Pandora delphacis接种后,饲养于不同温(10～30℃)、湿(74%～100%RH)度组合条件下观察其生殖力及内禀增长力(r_m) 的变化。结果表明,被接种个体生殖力比对照显著降低,且下降幅度受温度、湿度及温湿互作的影响。在各处理组合中,尤其以20～30℃和95%～100%RH的条件下下降更为明显。各处理r_m在不同温度间差异显著,均随温度呈抛物线形变化,25℃下达最高,但不同湿度间差异不显著。与对照相比,被接种蚜的r_m>除10℃与各湿度组合间无显著差异外,其余温湿组合下均显著低于对照。上述分析表明,飞虱虫疠霉的侵染能显著影响桃蚜生殖力及r_m,尤其是在桃蚜繁殖的最适温度范围内效果最为明显。     

Limitations of Gas Exchanges in Intact Leaves during Ontogeny of Field-grown Chickpeas

The ontogenic changes in several component processes of photosynthesiswere measured in chickpeas. Gas exchange characteristics ofintact leaves were studied to analyse the effects of ambientconditions under which chickpeas are usually grown. The CO₂assimilation rate per unit leaf area remained fairly high duringthe vegetative stage, reaching a peak at early pod-fill anddeclining subsequently throughout pod development. The intercellularCO₂ partial pressure (C₁) remained more or less constant (195µbar) during vegetative growth and the early stages ofseed-filling. With falling RWC and PAR interception, the stomatalconductance declined more rapidly than the CO₂ assimilationrate resulting in a value of C₁ less than that normally existingunder ambient conditions. From the A/C₁-analysis, CO₂ assimilationduring pod-filling appears to be limited by the RuBP-regenerationcapacity because the carboxylation efficiency and in vitro RuBPCaseactivity were initially unaffected. However, as leaves aged,the carboxylation efficiency and in vitro RuBPCase activitydecreased abruptly with increasing leaf temperatures above 30°C, and the C₁ was greater than normally existing values(195 µbar), suggesting an increased mesophyll limitationof photosynthesis. It is suggested that a decline in the CO₂assimilation rate of leaves during pod development and an acceleratedsenescence are induced by adverse ambient conditions, particularlyplant water stress and high leaf temperature. Key words: Cicer arietinum L., gas exchange, photosynthesis, ribulose-1,5-bisphosphate carboxylase     

Limitations to net photosynthesis as affected by nitrogen status in jack pine (Pinus banksiana Lamb.) seedlings

Relative limitations of nitrogen (N) status on the processescontributing to photosynthetic rate (A) were investigated. Jackpine {Pinus banksiana Lamb.) seedlings from seeds grown in sandculture were supplied with four different N treatments for 6weeks, which resulted in a needle N content ranging from 50–85mmol m^–2 (14–32 mg g^–1 dry weight). Leaf gasexchange at varying CO₂ levels was measured and limitationson A₃₅₀ (A at ambient CO₂ level) caused by finite, limitingcarboxylation efficiency (c.e.), maximum A (A_max)and stomatalconductance were estimated from an analysis of the responseof A to internal CO₂ concentration. Although c.e. and A_max decreasedlinearly with the decline in needle N, the magnitudes of theirchanges relative to A₃₅₀ differed. A_max varied with A₃₅₀ andalways exceeded A₃₅₀ by 37–38% c.e., however, declinedfaster than A₃₅₀, as needle N level decreased. Consequently,relative limitation on A₃₅₀ caused by inefficient A_max remainedconstant, but limitations caused by c.e. increased by 10–15%at low N levels. In contrast, the limitation by stomatal conductancedeclined initially, but remained stable when N content droppedbelow 75 mmol m^–2. The results suggest: (1) a decreasein biochemical capacity, but not stomatal conductance, contributedto the reduction of A₃₅₀ induced by N-deficiency in jack pineseedlings; and (2) the capacity of carboxylation appeared tobe impaired more than that of electron transport and/or photophosphorylationand its reduction may be the major reason for the reductionin A₃₅₀. Key words: A–C_i analysis, carboxylation efficiency, electron transport, nitrogen deficiency, stomatal conductance     

Estimating the Effects of Varying Temperature on the Rate of Development of Plants

Experiments on the effects of temperature on plant developmentusually yield measurements of the times at which recognizablestages of development are reached, and a record of the varyingtemperature patterns during development. Two complementary methodsfor analyzing such data to reveal the underlying relationshipsbetween rate of development and temperature are discussed. Inboth methods it is assumed that there exists a unique relationshipbetween current rate of plant development and current temperature. In the ‘discrete-rate’ method, the rate of developmentfunction, r(T), is approximated by a set of discrete valuesr₁, r₂...r_i...r_n, each applicable over a given temperature range.The fractional state of development of a plant, S, is writtenas the sum of the products of these rates, r_i, times the fractionof total time during development that temperature was withineach range, f_i, i.e. S = r_i. Systems of such equations can besolved for the r_i, values, and this enables an assessment ofthe rate-temperature relationship without preconceived notionson the form of the plant response. This assessment can then be used as a basis for selecting asuitable interpolation function to be fitted to the data usingthe ‘rate-function’ method. Here a mathematicalform for the function r(T) is selected at the outset. The temperaturescale is divided into discrete intervals and a set of equations,similar to the above, is developed. These equations are solvedfor best values of the parameters of the r(T) function. These methods are illustrated by application to controlled-environmentdata on times from sowing to flowering in soybeans and pigeonpeas,and on rates of leaf appearance in maize. Advantages claimed for the two methods are that they providea systematic approach for relating development to plant temperatureand that they are based upon an explicit physiological hypothesis. Phenology, Zea mays, Cajanus cajan, Glycine max     

Selectivity of ion channels: competitive catalysis versus independent electrodiffusion

In order to express selectivity of ion channels, it is commonto determine the reversal voltages, V_r in the presence of differention species, and to convert these voltages to permeability ratiosby constant field equations, which are based on independentelectro-diffusion. In the case of individual ion channels, competitionrather than independence can be expected. This situation hasbeen described here in terms of a catalytic reaction cycle fora channel which may translocate K⁺ as well as Ca²⁺ . An exampleis given, where the fundamental rate constants for the K⁺ cycleand for the Ca²⁺ cycle are the same, and opposing 1:10 concentrationgradients for Ca²⁺ and for K⁺ are applied. In this case, thereversal voltage of the resultant total current is so closeto the equilibrium voltage of Ca²⁺, that constant field calculationwould yield an overestimated permeability ratio P_Ca:P_K of about10. Two limiting cases are pointed out. In the case of dilutesolution, the ions move independently and the system can obeythe constant field theory. At high concentrations, the systemapproaches complete dependence and will consequently behaveas an antiporter. Key words: Antiport, calcium permeability, channel selectivity, competitive catalysis, enzyme kinetics     

The electromotive force of the Chara membrane during the hyperpolarizing response

The hyperpolarizing response of the Chara internodal cell wasstudied by applying the voltage clamp and constant current techniques.By assuming the membrane as an electromotive force (emf) inseries with a resistance r (which is the sum of a series resistancer_s and the membrane resistance r_m), it was shown that the hyperpolarizingresponse was brought about not only by the increase in membraneresistance but by the increase in membrane emf. The time dependentcurrent-voltage (I–V_m) curve obtained under the voltageclamp during the hyperpolarizing response showed a negativeresistance. The hyperpolarizing response is also an excitation,since it is a transition process of the membrane across a negativeresistance region. (Received July 22, 1974; )     

The Growth and Yield Responses ofFragaria ananassato Elevated CO2and N Supply

Strawberry plants (Fragaria ananassaDuchesne var. Elsanta) weregrown in pots at two concentrations of carbon dioxide (partialpressures of 39 and 56 Pa) and with three rates of nitrogensupply (0.04, 0.4 and 4 mMas nutrient solution) to study theirindividual and interactive effects on plant growth and fruityield. Nitrogen deficiency reduced total dry biomass and relativegrowth rate (RGR), mainly through reductions in leaf area ratio(LAR) and plant N concentration (PNC), although both the netassimilation rate (NAR) and root weight ratio (RWR) increased.Elevated CO₂increased the N productivity (NP) but reduced theLAR. High CO₂increased the fruit yield by 42% at high N supplyand by 17% at low N supply. The CO₂yield enhancement occurredthrough an increase in the flower and fruit number of individualplants. This resulted in an increase in the fruit weight ratio(FWR) of plants at high CO₂. Nitrogen deficiency reduced thefruit yield by about 50% through decreases in fruit size, fruitset and the number of fruits. However, N deficiency increasedthe proportion of total plant dry biomass allocated to fruits.There were no significant interactions between CO₂and N supplyon yield.Copyright 1998 Annals of Botany Company Nitrogen; carbon dioxide; strawberry (Fragaria ananassaDuchesne); fruit yield.     

Preliminary Studies on the Photosynthetic Structures of Trifolium alpinum L. as Related to Productivity

Trifolium alpinum L. is a high-quality alpine forage plant growingspontaneously from 1900 to 2800 m above sea level and is widelydistributed in Piedmont and the Valle d'Aosta (Italy), whereit can reach population frequencies of 90 per cent. Yields weredetermined on forage harvested in the Valle dell'Orco (Piedmont)and were comparable to cultivated clovers from higher latitudes;yields decreased progressively as the elevation increased. Thechemical and nutritional characteristics of the forage, thoughcomparable to clovers cultivated in the Po valley (Italy), were,however, more constant. The structure of the leaf lamina asrelated to elevation was investigated using light microscopy,TEM and SEM. This is complemented by data on chlorophyll concentration,succulence, specific leaf weight and area. At all elevationsT. alpinum lacks, apart from bundle sheath cell chloroplastsin a centrifugal arrangement, the structural characteristicsof C₄ plants. The chlorophyll a:b ratio (less than four) istypical of a C₂ plant. Succulence indices (S and S_m) were verylow, making CAM pathway photosynthesis unlikely. Unusual anddifficult to interpret structures included: small functionalchloroplasts in both the epidermises, stomata present almostexclusively in the upper epidermis and mitochondria enveloped(or enclosed) by chloroplasts. It was observed that, as theelevation increases, populations are selected which are well-adaptedfor gas exchange (increase in specific leaf area, stomatal densityand intercellular spaces) and characterized by a decrease inthe grana thylacoid:integrana thylacoid ratio (consistent withthe increase in the chlorophyll a:b ratio), the per cent water,S_m and the specific leaf weight. Trifolium alpinum L., alpine trefoil, leaf structures, photosynthesis, yield, elevation, C₂, C₄