Senescence in Leaves of Acer pseudoplatanus L. and Parthenocissus tricuspidata Planch: 1. Changes in Some Leaf Constituents during Maturity and Senescence

Data are recorded of the changes in chlorophylls, carotene,sugar, shikimic acid, and anthocyanin in leaves of sycamore(Acer pseudoplatanus L.) and Virginia creeper (Parthenocissustricuspidata Planch.) during leaf maturity and senescence. InParthenocissus the losses of chlorophylls, carotene, sugar,and shikimic acid during senescence were closely correlated,and were inversely related to the accumulation of anthocyanin.The losses of chlorophylls, carotene, and sugar by Acer leaveswere also closely correlated. No evidence was found to supportthe suggestion that anthocyanin formation was caused by accumulationof sugar during senescence, but marked differences in shikimicacid content were found between leaves of Acer, which did notform anthocyanin, and Parthenocissus. which did. It is suggestedthat autumn senescence of these leaves involves the rapid senescenceof an increasing proportion of the leaf tissue during a periodof 80 days, and that measurements of the content of constituentsgive an estimate of the proportion of the leaf tissue whichhas senesced.     

Acclimation in Seedlings of a Tropical Tree, Bischofia javanica, Following a Stepwise Reduction in Light

Acclimation of fully developed leaves of Bischofia javanicaBlume to shadelight was examined. Seedlings were grown undersimulated daylight (1000 µmol m^–2 s^–1), thentransferred to a simulated shadelight (40 µmol m^–2s^–1). When a high-light leaf was transferred to low light, large negativenet photosynthetic rates (P_m) were recorded. This decrease wasrapid, but within 7 d the rate increased and became equal tothe low-light control leaf. These changes in photosynthesisdid not follow the pattern of changes in stomatal conductance(g_s). Transfer to the low light resulted in a dramatic decreasein leaf weight per unit area (L_w), and most of the decreasesin L_w occurred within 3 d of transfer when the P_m of the transferredleaf was well below that of the low-light control leaf. There was a significant decrease in chlorophyll a in the transferredleaf without an appreciable change in chlorophyll b resultingin a large decrease in the chlorophyll a to chlorophyll b ratio.Leaf chlorophylls per unit area were higher in the transferredleaf than the low-light control leaf. Maximum photosyntheticrate in the transferred leaf was decreased by 40% compared tothat for the high-control leaf, but was almost at the same extenthigher than the low-light control leaf The results are discussedin the context of carbon gain capacity of its seedlings underlight-limiting forest understorey habitats. Bischofia, chlorophylls, light, photosynthesis, shade acclimation, tree seedlings, tropical tree     

Environmental relevance of heavy metal-substituted chlorophylls using the example of water plants

Following experiments which studied the substitution of thecentral ion of isolated chlorophylls by heavy metal ions invitro, in vivo experiments with submersed water plants werecarried out. It was discovered that the substitution of thecentral atom of chlorophyll, magnesium, by heavy metals (mercury,copper, cadmium, nickel, zinc, lead) in vivo is an importantdamage mechanism in stressed plants. This substitution preventsphotosynthetic light-harvesting in the affected chlorophyllmolecules, resulting in a breakdown of photosynthesis. The reactionvaries with light intensity. In low light irradiance all thecentral atoms of the chlorophylls are accessible to heavy metals,with heavy metal chlorophylls being formed, some of which aremuch more stable towards irradiance than Mg-chlorophyll. Consequently,plants remain green even when they are dead. In high light,however, almost all chlorophyll decays, showing that under suchconditions most of the chlorophylls are inaccessible to heavymetal ions. Key words: Heavy metal chlorophylls, submersed water plants, antenna pigments, copper, zinc     

Photoinhibition and Light Acclimation in Seedlings of Bischofia javanica, a Tropical Forest Tree from Asia

The aim of this study was to examine the potential for lightacclimation in shade grown seedlings of Bischofia javanica Blume.The seedlings were grown under simulated forest shade light(40 µmol m^–2 s^–1), and after transfer to ahigher light level (1200 µmol m^–2 s^–1), chlorophyllfluorescence induction kinetics, net photosynthesis, and changesin leaf chlorophylls and leaf anatomy were examined in leavesthat were fully developed prior to the transfer. The low-light (LL) leaf displayed photoinhibition immediatelyafter transfer to high-light (HL). This photo-inhibition wassubstantial, and continued for several days. Chlorophyll bleachingoccurred only after a certain degree of photoinhibition hadproceeded. Photosynthetic light acclimation commenced immediatelyafter severe photoinhibition. An increase in chlorophylls perunit leaf area was also immediate after severe bleaching. Thechanges in leaf chlorophylls over time were consistent withthe visual observations of bleaching and recovery. The leafweight per unit leaf area increased gradually on transfer toHL and finally it approached that of the newly formed HL leaf.Although fully expanded prior to transfer to HL, the leaf thicknesswas increased by about 45% and the leaf tissues became denserwithout changing the leaf area and the stomatal density. Finally,the net photosynthetic rate per unit leaf area was higher thanthat before exposure by 75% but less than that of newly formedHL leaf by more than 30%. Moreover, leaf movements were observedafter exposure to HL and also the formation of short epicormicshoots with a cluster of small leaves on the lower part of thestem during light acclimation. It is concluded that the fully expanded shade leaf has a wideacclimation plasticity. In addition to leaf acclimation, wholeplant responses such as leaf movements, the formation of epicormicshoots and the production of new ‘sun-type’ leavesunder HL may be of crucial importance to the success of thespecies following opening of the canopy. Photoinhibition, light acclimation, photosynthesis, fluorescence, tropical trees, shade, Bischofia javanica     

An analysis of relative elemental growth rate, epidermal cell size and xyloglucan endotransglycosylase activity through the growing zone of ageing maize leaves

The effect of development on leaf elongation rate (LER) andthe distribution of relative elemental growth rate (REGR), epidermalcell length, and xyloglucan endotransglycosylase (XET) activitythrough the growing zone of the third leaf of maize was investigated.As the leaf aged and leaf elongation slowed, the length of thegrowing zone (initially 35 mm) and the maximal REGR (initially0.09 mm mm^–1 h^–1) declined. The decline in REGRwas not uniform through the growth profile. Leaf ageing sawa maintenance of REGR towards the base of the leaf. Epidermalcell size was not constant at a given position in the growingzone, but was seen to increase as the leaf aged. There was apeak of XET activity close to the base of the growing zone.The peak of XET activity preceded the zone of maximum REGR.XET activity declined as leaves aged and their elongation rateslowed. When leaf elongation was complete a distinct peak ofXET activity remained close to the base of the leaf. Key words: Leaf elongation rate (LER), relative elemental growth rate (REGR), xyloglucan endotransglycosylase (XET)     

Observations on pigments and morphology of Gyrodinium aureolum Hulburt, a marine dinoflagellate containing 19' -hexanoyloxyfucoxanthin as the main carotenoid

Gyrodinium aureolum, a common "red tide" dinoflagellate in Europeanwaters often associated with fish mortality, was isolated fromthe Oslofjord, Norway, and analysed for chlorophylls and carotenoids.Besides chlorophyll a and c the following carotenoids were characterizedby thin-layer chromatography, visible light spectrophotometryand mass spectrometry: ß,-carotene, ß,ß-carotene,djatoxanthin, diadinoxanthin, 19'-hexanoyloxyfucoxanthin and3 xanthophylls which could not be correlated with hitherto structurallyknown carotenoids from dinoflagellates. G. aureolum deviatesfrom most dinoflagellates by the lack of peridinin, but showsaffinity with Gyrodinium sp.-A by the possession of 19'-hexanoyloxyfucoxanthin. Preliminary light microscopical observations on the internalstructure indicate that G. aureolum is uni-nucleate with a typicaldinokaryotic nucleus containing continually condensed chromosomes.The chloroplasts seem to possess an internal pyrenoid like someother dinoflagellates with deviating carotenoid pigmentation.The similarity in carotenoid pigmentation and chloroplast structureof Emiliania huxleyi (Prymnesiophyceae) and Gyrodinium sp.-Aand G.aureolum (Dinophyceae) is pointed out. The potential chemotaxonomicvalue of the carotenoid composition in establishing identitywith morphologically similar and ichthyotoxic dinoflagellatesis briefly discussed.     

Changes in Chlorophyll Content and Organization During Senescence of the Primary Leaves of Phaseolus vulgaris L. in Relation to Photosynthetic Electron Transport

A large decrease was observed in the chlorophyll content ofthe primary leaves of Phaseolus vulgaris during senescence.Chloroplasts isolated from mature and senescent leaves gavevery similar light saturation curves for electron transportreactions involving either PS I or PS II, indicating that theaverage number of chlorophyll molecules associated with eachreaction centre did not change during senescence. It is concludedthat the reaction centres ceased to function at the same timeas, or perhaps before, their antenna chlorophylls were lostfrom the thylakoid membrane, and that the percentage decreasein the number of functional reaction centres per leaf was atleast as great as the percentage decrease in the leaf chlorophyllcontent. The chlorophyll-protein composition of thylakoid membrane preparationswas examined by electrophoresis of samples treated with sodiumdodecyl sulphate. In older leaves a smaller proportion of thechlorophyll applied to polyacrylamide gels was associated withthe P700- chlorophyll a-protein complex. There was also a declinein emission at 734 nm in the 77 °K fluorescence spectrumof intact leaf tissue during senescence. These results indicatethat older leaves contained a smaller proportion of chlorophyllsassociated with PS I, and this is consistent with the decreaseobserved in the leaf chlorophyll a/b ratio during senescence.The effect of these changes in chlorophyll content on the capacityof the chloroplast to carry out photosynthetic electron transportis discussed.     

Microclimate, Canopy Structure and Photosynthesis in Canopies of Three Contrasting Temperate Forage Grasses: II. Microclimate and Canopy Structure

Profiles of shortwave radiation, net radiation and temperaturewere measured in swards of three grasses of contrasting structureLolium perenne cv. S24, L. perenne cv. Reveille and Festucaarundinacea cv. S170. Measurements were also made of the reflectionof shortwave radiation, leaf water potential and stomatal resistance.Differences in canopy structure influenced the absorption andreflection of radiation by the varieties. The absorption ofnet radiation and its influence on air temperature inside thecanopy was shown to vary with canopy structure. Calculationsshowed that diurnal changes in the reflection and transmissionof light (400–700 nm) would have little effect on canopyphotosynthesis. No clear relationship between leaf extensionrate, temperature and leaf water potential could be established,although decreases in water potential did appear to reduce thepotential response of leaf extension rate to temperature.     

Effect of Excision on Leaf Water Potential

Improvements of thermocouple hygrometric techniques for in situleaf water potential measurement in the field now allows forcontinuous monitoring of water potential in response to an externalperturbation, such as leaf excision. Using Citrus jambhiri plants,measured leaf water potentials of completely excised leaf portionsimmediately increased when the petiole was excised or incisionswere made either transverse or parallel to the midrib. Incisionsparallel to the midrib were on the side nearest the hygrometeror opposite it if preceded by a petiole excision. Midrib incisionswere 100–150 mm long with the nearest cut edge being 20–50mm from the hygrometer cavity. All excisions were such thatleaf tissue was removed from the leaf with water potential onone of the leaf portions being measured continuously prior toand after excising. The peak increase in measured water potentialof the excised leaf portions ranged between 20 kPa and 80 kPabut averaged 50 kPa. In uncovered leaves, particularly underfield conditions with the associated high evaporative demand,measured leaf water potential declined rapidly after the initialincrease. The increase in measured water potential immediatelyfollowing various types of excision was confirmed for dark andlight conditions (laboratory and field respectively) using bothpsychrometric and dewpoint modes and occurred for secondaryexcisions, but to a lesser extent. Discovery of this phenomenonimplies that water potential measured on detached leaves maynot always represent accurately in situ leaf water potential. Key words: Leaf water potential, Thermocouple hygrometers, Leaf excision effects     

Wheat Cultivars Respond Differently to a Drying Top Soil and a Possible Non-Hydraulic Root Signal

Research has shown that when plant roots are exposed to a dryingsoil a non-hydraulic (chemical) signal is produced in the rootand transported to the shoot, causing stomatal closure and growthretardation. This study was designed to reveal genetic diversityin wheat response to soil conditions which elicit a root signal,as the first step in the investigation of the genetic controlof the production of and the response to the root signal. Five spring wheat (Triticum aestivum L.) cultivars were establishedin the growth chamber in soil-filled polyvinyl chloride tubes,120 cm long and of an internal diameter of 10·2 cm. Soilwas well fertilized and wet to field capacity at emergence whentwo treatments were imposed: (1) tubes were watered from thetop as needed to eliminate stress (control); and (2) tubes hada constant water table at a soil depth of 100 to 120 cm, withno applied water. Measurements were performed on five dateson leaf water status and stomatal diffusive resistance. Above-groundbiomass and grain yield per plant were determined at maturity. The water table treatment resulted in dry and hard top soilconditions which were previously indicated to elicit a possibleroot signal. Under these experimental conditions, cultivarsdiffered in their leaf water status, stomatal diffusive resistance(R_s) and plant production. In the control treatment, R_s of cultivarsincreased with reductions in their relative water content (RWC)and leaf water potential (LWP), indicating the expected controlof R_s by leaf water status. Under conditions of a drying topsoil, relative water content (RWC) and leaf water potential(LWP) increased in cultivars that had a higher R_s, indicatingthat stomatal activity was controlling leaf water status. Itwas therefore suggested that the drying top soil elicited aroot signal which caused stomatal closure and reduced plantproduction. Under such conditions, two cultivars (Bethlehemand V748) consistently maintained relatively low R_s and highplant production, despite their relatively lower RWC and LWP,as compared with cvs C97, V747 and V652. Limited observationssuggest that in these two cultivars relatively fewer roots mayhave been exposed to the drying top soil, as compared with theother three cultivars. Key words: Triticum aestivum, cultivars, soil moistrue, drought stress, root, root signal, stomata, relative water content, leaf water potential, biomass, yield     

Relationships Between Nitrogen and Water Concentration in Shoot Tissue ofMolinia caeruleaDuring Shoot Development

Plants ofMolinia caeruleawere supplied with either a low (0.2mol m^-3) or high (10 mol m^-3) supply of nitrogen over two growingseasons. A total of 14 destructive plant harvests were made:when plants were in an over-wintering state prior to the secondseason; immediately following bud burst; and on 12 further occasionsthroughout the second season. The relationships between shootnitrogen concentration on a dry mass basis, shoot water contentand plant developmental stage were investigated. Shoot nitrogenconcentration on a dry mass basis fell as the growing seasonprogressed. In contrast, the concentration of nitrogen in tissuewater after bud burst showed only a slight reduction. The concentrationof nitrogen both on a dry mass basis and in tissue water wasgreater for plants receiving the higher supply of nitrogen.Shoot water content was highest immediately following bud burstthen declined as the season progressed, with plants receivingthe low nitrogen supply having slightly greater shoot watercontents. It was concluded that the decline in shoot nitrogenconcentration ofM. caeruleaon a dry mass basis as the mass increasedwas mainly explained by changes in shoot water content. Theobserved increase in the rate of decline of both shoot nitrogenconcentration and water content with increased shoot mass coincidedwith the cessation of leaf tissue production and was thereforedue to a switch from the production of leaves to other tissues.Copyright1999 Annals of Botany Company Molinia caerulea(L.), purple moor grass, nitrogen, water content, shoot development.     

Inhibition of photosynthesis in olive trees (Olea europaea L.) during water stress and rewatering

The effect of high levels of natural light on leaf photosynthesisin olive trees (Olea europaea L. var. Coratina), grown in potsoutdoors in the summer and subjected to water, stress, was studied.Net photosynthetic rates reached maximum values early in themorning in both control and stressed plants and subsequentlydeclined gradually. This inactivation of photosynthetic activitywas accompanied by changes in the fluorescence characteristicsof the upper intact leaf surface. The maximum fluorescence yield(Fp) and the ratio Fv/Fp decreased at midday especially in water-stressedplants, but the initial fluorescence (Fo) rose to a maximumvalue at midday and declined again in the afternoon. In controlplants the values of maximum fluorescence Fp and the ratio Fv/Fpincreased again in the afternoon and had recovered almost completelyby 8 p.m. as the leaf water potential recovered. In stressedplants this diurnal recovery was not complete, so that the photosyntheticrates and the ratio Fv/Fp declined gradually during the developmentof water stress. These results indicate that in olive treessubjected to severe water stress the non-stomatal componentof photosynthesis was affected and perhaps a light-dependentinactivation of the primary photochemistry associated with photosystemII (PSII) occurred. Four to five days after rewatering severelystressed plants, the predawn leaf water potential, net photosyntheticrates and chlorophyll fluorescence indices recovered only partially. Key words: Olea europaea, photosynthesis, water stress, chlorophyll a fluorescence, inhibition of photosynthesis     

Chlorophyll a forms in mesophyll and bundle sheath chloroplasts of Zea mays leaves grown at low light intensity

Mesophyll and bundle sheath chloroplasts were prepared fromleaves of Zea mays grown at light intensities of 1.1 and 240µW/cm², respectively. The mesophyll chloroplasts thatdeveloped at the low intensity and bundle sheath chloroplatsthat developed at both low and high intensities showed higherratios of chlorophyll a/b and P700/chlorophylls compared withthe normal ratios found for the mesophyll chloroplasts thathad developed at the high intensity. Derivative absorption spectrophotometryat 77?K revealed that the low intensity mesophyll chloroplastscontained more of chlorophyll a forms with longer wavelengthred bands than high intensity mesophyll chloroplasts. More ofthe longer wavelength forms of chlorophyll a were also presentin the bundle sheath chloroplasts that had developed at lowand high intensities. All these four types of chloroplasts showedtwo peaks of fluorescence, one at 687 hra and the other at 733or 738 nm. In addition to these peaks, the high intensity mesophyllchloroplasts showed a shoulder at 697 nm, and the two typesof bundle sheath chloroplasts showed a shoulder at 680 nm. (Received June 17, 1974; )     

Action spectrum for light activation of pyruvate, phosphate dikinase in maize leaves

Light activation of pyruvate, phosphate dikinase was investigatedusing leaf discs of dark pretreated maize (Zea mays L. var.Golden Gross Bantam) leaves. The action spectrum resembled theabsorption spectrum of the leaf, having two peaks at 436 and671 nm and a minimum at 503 nm. This similarity suggests involvementof chlorophyll in the light activation as a photoreceptor. Lightactivation was markedly inhibited by 3-(p-chlorophenyl)-1,1-dimethyl urea. We thus inferred that activation of the enzymewas closely correlated to the photosynthetic electron transportsystem. (Received July 3, 1974; )     

Sclerophylly and Plant Water Relations in Three Mediterranean Quercus Species

The possible role in drought resistance played by sclerophyllywas studied in the Mediterranean oaks Quercus ilex, Q. suberand Q. pubescens. Studies were conducted on leaves at 30, 50and 80% of their final surface area, as well as on mature leavesof the current year's growth in June and September and on 1-year-oldleaves. Leaves of different ages of the three species showed quite differentdegrees of sclerophylly (DS). Q. ilex leaves reached the definitiveDS of 1.75 g dm^–2 during leaf expansion; Q. pubescensleaves hardened at the end of their expansion, with a finalDS of 0.93 g dm^–2; Q. suber showed the lowest DS of 0.76g dm^–2. Leaf conductance to water vapour (g₁) of 1-year-old leaves ofQ. ilex, measured in the field, showed a duration of the g₁peak values about twice that of the other two species. The minimumleaf relative water content (RWC), however, was near the samein the three species, indicating that water loss was recoveredpartly by Q. ilex leaves. This was apparently due to the higherbulk modulus of elasticity (     

Epicuticular Phenolics Over Guard Cells: Exploitation for in situ Stomatal Counting by Fluorescence Microscopy and Combined Image Analysis

Guard cells emit an alkali-induced, blue fluorescence upon excitationby ultraviolet radiation (emission maximum energy at 365 nm).Fluorescence emission of guard cells was brighter than thatof the neighbouring epidermal cells in a number of wild andcultivated plants including conifers, but the relative fluorescenceintensity and quality was species-dependent. Three representativeplants possessing stomatal complexes which differed morphologicallywere studied: Olea europaea, Vicia faba and Triticum aestivum.Immersing leaves of these plants in chloroform for 30 s (therebyremoving epicuticular waxes) significantly reduced the intensityof the fluorescence emitted by guard cells. This indicates thatguard cell fluorescence could be due to either an increasedconcentration of fluorescing compounds (probably wax-bound phenolics),or a thicker cuticular layer covering the guard cells. Giventhat the alkali-induced blue fluorescence of the guard cellsis a common characteristic of all plants examined, it couldbe used as a rapid and convenient method for in situ measurementsof the number, distribution and size of stomatal complexes.The proposed experimental procedure includes a single coatingof the leaf surface by, or immersion of the whole leaf in, a10% solution of KOH for 2 min, washing with distilled water,and direct observation of the leaf surface under the fluorescencemicroscope. Fluorescence images were suitable for digital imageanalysis and methodology was developed for stomatal countingusing Olea europaea as a model species. Copyright 2001 Annalsof Botany Company Cuticle, epicuticular waxes, fluorescence microscopy, image analysis, phenolics, stomata     

Light Gradients and the Transverse Distribution of Chlorophyll Fluorescence in Mangrove andCamelliaLeaves

The light gradient and transverse distribution of chlorophyllfluorescence in mangrove andCamellialeaves, which have differentmorphological characteristics, were examined using a micro-fluorescenceimaging system reported previously (Takahashiet al., Plant,Cell and Environment17: 105–110, 1994). Epidermal cellsscattered light strongly, resulting in an increase in the fluencerate in epidermal cells. For theCamellialeaf, a light gradientwas formed by absorption of light by photosynthetic pigmentsassociated with the induction of chlorophyll fluorescence. Forthe mangrove leaf, a light gradient was formed by backward scatteredlight within a thick layer of non-assimilatory cells. Lightwith a low absorption coefficient (515 nm) penetrated deeperthan that with a higher absorption coefficient (477 nm and 488nm) in theCamellialeaf, while light of both wavelengths showedsimilar profiles in the mangrove leaf. In the mangrove leaves,scattered light declined significantly in the non-assimilatorycell layer which is in front of the assimilatory cells. Light,the intensity of which was reduced to approx. 10% of the maximum,was well scattered and induced a considerable amount of chlorophyllfluorescence in the assimilatory cells, which appear to be wellorganized to capture weak light.Copyright 1998 Annals of BotanyCompany fluorescence, intact leaf, light gradient, mangrove (Rhizophora mucronataLamk.),Camellia japonicaL.     

The Influence of Leaf Water Status and ABA on Leaf Growth and Stomata of Phaseolus Seedlings with Hypoxic Roots

The objective of this study was to assess the relative rolesof leaf water status and root-sourced signals in mediating beanleaf responses to root hypoxia. To do so, the roots of beanplants under varied VPD (0.95 kPa to 0.25 KPa) were made hypoxic.Under all conditions, leaf growth rates and stomatal conductanceswere reduced. There was a transitory decline in leaf water potentialat high VPD which accounted for the initial reduction in leafgrowth rates and stomatal conductance. At low VPD, no waterdeficits were detected. Leaf growth inhibition and reduced stomatalconductance under low VPD treatments were unrelated to leafwater status and must be induced by some other factor. In vitrogrowth of leaf discs was reduced by xylem sap collected fromhypoxic roots. Exogenously applied ABA, at high concentrationsin KCl and sucrose, or at low concentrations diluted in xylemsap from aerated plants, inhibited in vitro growth of leaf discs.Applications of ABA in the transpiration stream reduced stomatalconductance.     

Exploring Phenotypic Plasticity in the LichenRamalina capitata: Morphology, Water Relations and Chlorophyll Content in North- and South-facing Populations

The present work analyses the morphology, anatomy, water relationsand chlorophyll content of thalli of the lichenRamalina capitatavar.protectafromtwo different populations exposed to contrasting microclimaticconditions due to differences in the orientation of the rocksurface. The population on the north-facing rock surface (NFS)was exposed to lower photosynthetic photon flux densities (PPFD),remained at high relative humidities for longer periods of timeand was exposed to lower temperatures than the population onthe south-facing surface (SFS). We proposed the hypothesis thatthe shadier the habitat the greater the ecological advantagefor enhanced light harvesting. Thalli from the SFS had shorterand wider lacinia, thicker thalli, mostly due to increased medullathickness, a higher water-retention capacity, a higher percentageof thallus volume occupied by the algal cells and a higher chlorophyllcontent than thalli from NFS. The phenotypic plastic responseof the traits studied inR. capitatavar.protectawas not directlyrelated to differences in the light availability, at least forthe range of PPFD experienced by the two populations studied,since the population exposed to higher PPFD exhibited largeramounts of light harvesting pigments. Both populations exhibitedthe same intrathalline distribution of algal cells and chlorophylls,which were more abundant in the apical than in the basal zonesof all thalli studied. Periods of water-induced metabolic activitywere shorter in the SFS than in the NFS, and structural andchlorophyll data indicated that thalli from the SFS were betterprepared for the photosynthetic exploitation of these brieferperiods and for maintaining thallus hydration into dry periods.These results suggest that differences in selective pressurebetween the two populations ofR. capitatavar.protectastudiedinvolved maximization of the photosynthetic exploitation ofthe periods of metabolic activity when they are brief, as hasbeen described for certain vascular plants from xeric environments. Ramalina capitatavar.protecta; algal cells; chlorophylls; water relations; microclimate; morphology; intrathalline variation; lichen; phenotypic plasticity     

An Ecophysiological Comparison of Measurements of the Diurnal Rhythm of the Leaf Elongation and Changes of the Leaf Thickness of Salt-resistant Dicotyledonae and Monocotyledonae

Rozema, J., Arp, W., van Diggelen, J., Kok, E. and Letschert,J. 1987. An ecophysiological comparison of measurements of thediurnal rhythm of the leaf elongation and changes of the leafthickness of salt-resistant Dicotyledonae and Monocotyledonae.—J.exp. Bot. 38: 442–453. The continuous measurement of leaf elongation and leaf thicknesswith the use of a rotation potentiometer set up revealed a rapidand sensitive reaction of halophytic plants to conditions affectingthe plant's water relations. At increased salinity (450 molm^–3 NaCl) the rate of leaf elongation decreased both inAster tripolium and in Sparlina anghca. Increased shrinkageduring the day and a long period for recovery swelling at nightin leaves of Aster iripolium at increased salinity illustratesthat water shortage is part of the cause of salinity-inducedgrowth reduction. All dicotyledonous species analysed (Aster tripolium, A triplexhastata, A. littoralis, Suaeda maritima and Beta vulgaris) showeda day/night ratio of the leaf elongation rate lower than 1,while this ratio was higher than or equal to 1 in Monocotyledons(Spartina anglica, Juncus gerardii, J. maritimus, Festuca rubrassp. litoralis, Elymus pycnanthus). With the exception of Triglochinmaritima none of the monocotyledonous halophytes tested (Sparlinaanglica, Juncus gerardii, J. maritimus, Festuca rubra ssp. litoralis,Elymus pycnanthus) exhibited a diurnal rhythm of leaf thicknesschanges, such as was observed for all dicotyledonous speciesstudied (Aster tripolium, Atriplex hastata, A. littoralis, Salicorniabrachyslachya, Suaeda maritima, Glaux maritima, Odontites vernassp. serotina). The diurnal pattern of the leaf elongation rateand the leaf thickness changes can be explained by variationof photosynthetic rate and transpiration water losses by stomatalclosure in the dark and opening in the light such as shown forthe dicotyledon species Glaux maritima. This difference betweendicot and monocot species in diurnal variation of the leaf elongationrate and leaf thickness may partly be explained in terms ofthe different position of the growth zone and possibly by adifference in elasticity of the tissue of halophytic monocotyledonsand dicotyledons. The consequences of these differences arediscussed. Key words: Leaf elongation rate, leaf thickness, water relations, salt resistance, Dicotyledonae, Monocotyledonae