Xylem Hydraulic Characteristics, Water Relations and Wood Anatomy of the Resurrection PlantMyrothamnus flabellifoliusWelw.

Myrothamnus flabellifoliusWelw. is a desiccation-tolerant (‘resurrection’)plant with a woody stem. Xylem vessels are narrow (14 µmmean diameter) and perforation plates are reticulate. This leadsto specific and leaf specific hydraulic conductivities thatare amongst the lowest recorded for angiosperms (k_s0.87 kg m^-1MPa^-1s^-1;k_l3.28x10^-5kg m^-1MPa^-1s^-1, stem diameter 3 mm). Hydraulic conductivitiesdecrease with increasing pressure gradient. Transpiration ratesin well watered plants were moderate to low, generating xylemwater potentials of -1 to -2 MPa. Acoustic emissions indicatedextensive cavitation events that were initiated at xylem waterpotentials of -2 to -3 MPa. The desiccation-tolerant natureof the tissue permits this species to survive this interruptionof the water supply. On rewatering the roots pressures thatwere developed were low (2.4 kPa). However capillary forceswere demonstrated to be adequate to account for the refillingof xylem vessels and re-establishment of hydraulic continuityeven when water was under a tension of -8 kPa. During dehydrationand rehydration cycles stems showed considerable shrinking andswelling. Unusual knob-like structures of unknown chemical compositionwere observed on the outer surface of xylem vessels. These maybe related to the ability of the stem to withstand the mechanicalstresses associated with this shrinkage and swelling.Copyright1998 Annals of Botany Company cavitation, desiccation, hydraulic conductivity, refilling, resurrection plant, root pressure, xylem anatomy,Myrothamnus flabellifolius     

Acoustic Emissions from Plants: Ultrasonic and Audible Compared

Audible acoustic emissions (AAE) and ultrasonic acoustic emissions(UAE), produced by stem segments during dehydration in air,have been recorded and compared. We hypothesize that cavitationof xylem sap generally results in the production of a broadband acoustic emission (AE) with a lower cut-off frequency determinedby the dimensions of the resonating element. The larger a conduit'sdimensions, the lower is the frequency of its major resonance.Thus the vessels, the largest conduits, can be expected to produceboth AAE and UAE. Fibres and small cavitating elements suchas small tracheids are expected on the other hand to produceonly UAE. Most work utilized Acacia tissues but work was extended to otherplant tissues from a range of species with differing anatomicalcharacteristics. Evidence supporting our hypothesis shows thatAAE and UAE did not coincide in different tissue types or dependsimply upon the degree of dehydration. AAE were detected fromtissue with intact major conduits (vessels) but not in similartissue in which these major conduits had been severed, whereasUAE were detected from both types of tissue. In general, ourhypothesis that larger conduits produced lower frequency signalsand smaller units at the ultrasonic frequencies was supported.We are forced to conclude that some UAE are generated by eventsother than cavitating vessels or fibres. Possible interpretationof our data is discussed in terms of the size of the cavitatingconduits but including differential signal absorption withinthe tissue. Key words: Cavitation, acoustic emission, ultrasound, plants, water stress     

The Acoustic Detection of Cavitation in Fern Sporangia

Cavitation of water in the annular cells of fern sporangia wasmonitored simultaneously by acoustic detection and visual analysisusing a light microscope. The number of cavitable annular cells was found to be 21.3 ±1.7 (n=40). The number and timing of ultrasonic acoustic emissions(UAE) received from a fully hydrated sporangium allowed to dehydrateon the acoustic probe closely approximated the average numberof cavitable annular cells. The number and timing of audibleacoustic emissions (AAE) was found to be lower than the numberof UAE. AAE were associated with cavitation-induced forwardmovements of the sporangium rather than individual cavitationevents within each annular cell. The results are discussed in view of an hypothesis proposedby Ritman and Milburn (1988) which describes the relationshipbetween the frequency of acoustic emission and the size of theresonating element. Key words: Cavitation, acoustic detection, sporangia     

Water relations of a tropical vine-like bamboo (Rhipidocladum racemiflorum): root pressures, vulnerability to cavitation and seasonal changes in embolism

The occurrence of root pressure, the vulnerability of xylemvessels to drought-induced cavitation, and the seasonal changesin hydraulic conductivity due to embolism were studied in theculms of Rhipidocladum racemiflorum (Steud.) McClure, a tropicalvine-like bamboo from central Panama. Positive hydrostatic potentialsup to 120 kPa occurred only during the wet season when the transpirationrate of the plant was low, i.e. at night or during rain events.Although the xylem vessels were large and efficient for conductingwater, they were highly resistant to cavitation. Xylem waterpotentials lower than –4.5 MPa were required to induce50% loss of hydraulic conductivity in culms. The minimum waterpotential reached –3.75 MPa at the end of the 1993 dryseason, so loss of hydraulic conductivity due to embolism remained<10%. The species is adapted to drier habitats both by wayof a low vulnerability to xylem cavitation and by root pressuresin the wet season that could refill vessels that became embolizedduring a severe dry season. Key words: Rhipidocladum racemiflorum, root pressure, cavitation, embolism, water relations     

Xylem conduction and cavitation in Hevea brasiliensis

Clones of Hevea were studied in an attempt to discover the reasonsfor differences in the hydraulic performance of xylem. Differencesbetween clones were determined, including hydraulic conductivityand conduit width and length distributions. However, it hasproved difficult to reconcile anatomical differences with physiologicalperformance for use in future plant breeding programmes. When leaf relative water content (RWC) had been reduced fromabout 95% to 85%, the hydraulic conductivity of petioles decreasedsharply to about 40% of the initial value. This value correspondedwith xylem sap tensions of 1.8–2.0 MPa. Acoustic detectionexperiments revealed that this reduction in hydraulic conductivitycoincided with the greatest occurrence of cavitation. It seemsinescapable that the reduction in hydraulic conductivity wascaused by embolization; thereafter gas bubbles blocked the flowof water inside many of the conduits. There was some indicationthat eventually such bubbles might be dissolved, because thehydraulic conductivity increased again if specimens were fullyrehydrated. Apparently, the incidence of cavitation coincides with the entryof gas bubbles via ultramicroscopic pores into the conduitsthrough the walls according to the air-seeding hypothesis. Whena petiolate leaf is tested in a pressure chamber it is impossibleto make satisfactory measurements of a balancing pressure beyondc. 1.8–2.0 MPa, because air bubbles, mixed with sap andescaping from the conduits, form a persistent froth. Xylem transportin Hevea seems to be disrupted relatively easily under waterstress which is a feature of other tropical species adaptedto rainforest–type environments Key words: Hevea, xylem, cavitation, conduit, hydraulic conductivity     

Cavitation resistance of peduncle,petiole and stem is correlated with bordered pit dimensions in Magnolia grandiflora

Variation in resistance of xylem to embolism among flowers, leaves, and stems strongly influences the survival and reproduction of plants. However, little is known about the vulnerability to xylem embolism under drought stress and their relationships to the anatomical traits of pits among reproductive and vegetative organs. In this study, we investigated the variation in xylem vulnerability to embolism in peduncles, petioles, and stems in a woody plant, Magnolia grandiflora. We analyzed the relationships between water potentials that induced 50% embolism (P₅₀) in peduncles, petioles, and stems and the conduit pit traits hypothesized to influence cavitation resistance. We found that peduncles were more vulnerable to cavitation than petioles and stems, supporting the hypothesis of hydraulic vulnerability segmentation that leaves and stems are prioritized over flowers during drought stress. Moreover, P₅₀ was significantly correlated with variation in the dimensions of inter-vessel pit apertures among peduncles, petioles and stems. These findings highlight that measuring xylem vulnerability to embolism in reproductive organs is essential for understanding the effect of drought on plant reproductive success and mortality under drought stress.     

Vulnerability to cavitation in populations of two desert species, Hymenoclea salsola and Ambrosia dumosa, from different climatic regions

Seeds from different populations of two desert species, Hymenocleasalsola Torr. and A. Gray and Ambrosia dumosa (A. Gray) Payne,were collected along a climatic gradient, germinated in a greenhouseand the plants tested for their vulnerability to cavitationby the air-pressure method. Differences among populations wereevident in A. dumosa, but not in H. salsola. Greenhouse treatmentssimulating regimes in temperature and relative humidity encounteredin different desert environments did not cause appreciable changesin vulnerability to cavitation. It is suggested that a homeostaticmechanism may have helped in maintaining a constant water potentialdrop in the xylem with little need for an adjustment in theresistance to cavitation. Different plant organs had differentvulnerabilities to cavitation, with roots being the most andwoody stems the least susceptible. Young green twigs were intermediate.A simulation model confirmed that low water potentials are mostlikely to cause runaway cavitation in the roots, not in theother organs. It is hypothesized that green twigs are adaptedto the favourable water conditions of the growing season, whilewoody stems are adapted to endure prolonged periods of droughtstress. Key words: Cavitation, xylem embolism, hydraulic conductance, Hymenoclea salsola, Ambrosia dumosa     

Water Flow in Vessels with Simple or Compound Perforation Plates

Hydraulic conductance of vessels was studied in young stem internodesand petioles of five species of dicotyledons that have simpleor compound perforation plates. Measured conductances of Clematisdioscoreifolia, Hardenbergia violacea, and Olea europaea, whichhave simple perforation plates, ranged from 92 to 106% of predictedvalues based on Hagen-Poiseuille flow through ideal capillaries.Petioles of Liquidambar styraciflua and Liriodendron tulipifera,which have compound perforation plates, had measured hydraulicconductances in spring-collected material that matched predictedconductances, but values averaged 60% of predicted in materialfrom late summer, suggesting blockage of vessels with time.An analog model of a vessel with and without compound perforationplates was developed from tygon tubing and steel pins to determinethe effect of obstruction by bars, which produced little flowreduction. Likewise, calculations based on the observed geometryof a scalariform perforation plate suggested that the platetypically reduced flow by only 1%. These data suggest that flowrates in vessels with simple as well as compound perforationplates are essentially equal to flows in ideal capillaries ofthe appropriate diameters. Clematis dioscoreifolia, Hagen-Poiseuille, Hardenbergia violacea, hydraulic conductance, Liquidambar styraciflua, Liriodendron tulipifera, Olea europaea, xylem     

Drought-induced leaf shedding in walnut: evidence for vulnerability segmentation

Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water-stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.     

Influence of Reproduction on the Abscission Process of Cotton (Gossypium barbadense L.) Leaves

Abscission responses of debladed petioles of young and olderleaves were analysed during flowering, fruiting and post fruitingstages of development of G. barbadense plants. Identical abscissionexperiments were performed with materials collected from plantsmaintained in a vegetative condition by removal of flower buds. Inhibition of the abscission of debladed petioles by NAA wasgreater in debudded plants as compared to normal plants andthe extent of inhibition gradually declined during growth. Promotiveeffects of ethrel and abscisic acid were higher in normal plantsthan in debudded plants. The duration of auxin-inhibitablc stage-I of abscission wasextended in debudded plants and it gradually declined with theprogress of development. Debudded plants were characterizedby higher abscission inhibition during stage-I and lower abscissionpromotion during stage-II as a result of application of auxincompounds to the debladed petioles. Laminar tissues of debudded plants contained higher amountsof endogenous IAA and lesser amount of abscisions than did thoseof normal plants and in both cases the levels of these compoundschanged markedly during plant development. Decrease of total RNA content in the distal tissues of the abscissionzones was accompanied by increase in proximal tissues duringabscission in both normal and debudded plants. This tendencywas more pronounced in normally grown plants as compared todebudded plants.     

Distribution and Redistribution of Molybdenum in Black Gram (Vigna mungo L. Hepper) in Relation to Molybdenum Supply

The effect of seven rates of molybdenum (Mo) supply on the distributionand redistribution of Mo in Vigna mungo (black gram) cv. Reguron a Mo-deficient sandy loam was examined from flower bud appearanceto pod set in one experiment and during pod filling to maturityin another. At the three lowest Mo supply rates, N deficiency symptoms typicalof Mo deficiency appeared, and shoot dry matter and shoot nitrogencontent were depressed. Increasing Mo supply increased Mo concentrationsin all plant parts but the response varied with Mo supply andwith plant part. In leaf blades and petioles, Mo concentrationsincreased slightly when the Mo supply increased from severelydeficient to deficient levels but further increases in Mo supplymarkedly increased the Mo concentrations, particularly in immatureand recently matured leaves. In petioles, Mo concentrationsgenerally exceeded those in the blades which they supportedat all levels of Mo supply. At Mo rates greater than that requiredfor maximum growth, Mo concentrations in basal stem segmentsexceeded those in petioles. Molybdenum concentrations in nodulesexceeded those in above ground plant parts except at the highestlevel of Mo supply where the concentrations in basal stem segmentsexceeded those in nodules. In Mo-adequate plants, Mo contents in the trifoliolate leavesdecreased with time suggesting that Mo was readily remobilized.By contrast, in stem segments at all levels of Mo supply, andin trifoliolate leaves in Mo-deficient plants, Mo contents remainedconstant or increased with time suggesting that Mo was not remobilizedin all plant parts or at all levels of Mo supply. Thus, theresults suggest that in black gram Mo was variably mobile, beingphloem immobile at low Mo supply, but phloem-mobile in all plantparts with the possible exception of stem segments at adequateMo supply. The relevance of these results for the developmentof plant tests for Mo deficiency diagnosis is discussed.Copyright1994, 1999 Academic Press Molybdenum, phloem-mobility, redistribution, black gram, Vigna mungo L. Hepper     

Vulnerability of xylem to embolism in relation to leaf water potential and stomatal conductance in Fagus sylvatica f. purpurea and Populus balsamifera

The vulnerability of xylem vessels to water stress-induced cavitationwas studied by measuring hydraulic conductivity and ultrasoundacoustic emissions (AEs) in Fagus sylvatica L. f. purpurea (Ait.)Schneid. and Populus balsamifera L. The occurrence of xylemembolism in summer was investigated in relation to leaf waterpotential and stomatal conductance. Populus was extremely vulnerableto cavitation, losing functional vessels due to embolism atwater potentials lower than –0.7 MPa. Fagus experiencedembolism when water potential fell below –1.9 MPa. Middaywater potentials often approached these threshold values. Whenevaporative demand increased rapidly on sunny days, water lossbecame limited by low stomatal conductance. Thus water potentialsfell only slightly below the threshold values inducing cavitation.Despite the differences in vulnerability, both species tolerateda similar embolism rate of about 10% in the summer. There wasno embolism reversal during prolonged periods of rain. AEs werepredictive of loss in hydraulic conductivity, indicating thatAEs were mainly confined to vessels. Finally, vessel lengthdistribution, vessel diameter (tangential axis), vessel density,and vessel wall thickness had been determined for both speciesinvestigated. Populus had longer and wider vessels than Fagus,whereas vessel wall thickness was similar in both species. Key words: Acoustic emissions, Fagus, Populus, stomatal closure, xylem embolism     

Xylem Cavitation in Nodes and Internodes of Whole Chorisia insignis H.B. et K. Plants Subjected to Water Stress: Relations Between Xylem Conduit Size and Cavitation

Measurements of cavitation occurring in xylem conduits of differentstem parts in whole Chorisia insignis H.B. et. K. plants subjectedto water stress are reported. Pre-stressed plants were shownto undergo cavitation over 10 times greater than watered ones.The most vulnerable parts of plants were one-year-old twigswhere cavitation reached a peak of over 50 acoustic emissions(AE) min^–1 while in two-year-old twigs AE min^–1were about one half this value. Stem zones were found wherecavitation was typically very low even during water stress:these were one-year-old nodes and junctions where branches meet.Measurements of the inside diameters of xylem conduits and distributionof conduit ends in stem parts where AE were detected, showedthat nodes have a significantly larger percentage of narrowxylem conduits than internodes. Similar ‘constricted zones’were found injunctions with respect to two-year-old twigs. Hereabout 50 per cent of the xylem conduits were as narrow as 20to 50 µm in diameter. The distribution of xylem conduitends show about 3 per cent of them ending in the nodes and 1per cent in the internodes of one-year-old twigs. About 11.6per cent of xylem conduits end in the junctions and about ahalf in two-year-old internodes. Our data would give furtherexperimental evidence to the functional concept of ‘plantsegmentation’ into zones (internodes) more efficient inwater conduction, i.e. with wider xylem conduits but more vulnerableto cavitation and others (nodes and junctions) with oppositecharacteristics. Chorisia insignis, acoustic emissions, water stress, nodes, internodes, xylem conduit size, vessel ends     

Effects of Root Temperature and Form of Nitrogen Nutrition on Nitrate Reductase Activity in Oilseed Rape (Brassica napus L.)

The effect of root temperature and form of inorganic nitrogensupply on in vitro nitrate reductase activity (NRA) was studiedin oilseed rape (Brassica napus L. cv. bien venu). Plants weregrown initially in flowing nutrient solution containing 10 µMNH₄NO₃ and then supplied with either nitrate or ammonium for15 d at root temperatures of 3, 7, 11 or 17 °C. Shoot temperatureregime was similar for all plants; 20/15 °C, day/night.Root NRA was highest when roots were grown at 3 and 7 °C.In laminae and petioles NRA was highest when roots were 11 or17 °C. The plants supplied with ammonium had much lowerlevels of NRA in roots after 5 d than the plants supplied onlywith nitrate. NRA in the laminae of plants supplied with ammoniumwas low relative to that in plants supplied with nitrate onlywhen root temperature was 11 or 17 °C. Values of the apparent activation energy (E_a) of NR, calculatedfrom the Arrhenius equation, in laminae and petioles were differentfrom roots suggesting difference in enzyme conformation. Evidencethat the temperature at which roots were growing affected E_awas equivocal. Oilseed rape, Brassica napus L., activation energy, ammonium, Arrhenius equation, nitrate, root temperature, nitrate reductase     

Biomass Allocation of Erythronium americanum Populations in Different Irradiance Levels

The forest herb Erythronium americanum was collected from threeNew Jersey habitats and grown in 100%, 43%, or 23% glasshouselight to determine if biomass allocation responses to irradiancediffered between populations. Shaded plants had longer petioles,produced fewer runners, and had greater specific leaf areasthan unshaded plants; these responses did not differ significantlyamong populations. All populations produced smaller bulbs whengrown in shade. Over all light treatments, mean leaf area ofplants from a stable, established site was greater than thatof plants from others. In 100 % light, these plants had thegreater bulb growth. The simple architecture of Erythroniummay constrain morphological responses to the light environment. Erythronium americanum(Ker.), forest herb, biomass allocation, light intensity     

Different Aspects of Cavitation Resistance in Ceratonia siliqua, a Drought-Avoiding Mediterranean Tree

Potted plants of Ceratonia siliqua L., growing in a greenhouse,were used to detect xylem cavitation (in terms of ultrasoundacoustic emissions AE) in internodes and node-to-petiole (N-P)junctions, after different periods of drought (9, 16 and 23d). Diurnal AE were only 100 in internodes of watered (W) plantsbut 320, 1250 and 2460 in 9-, 16- and 23-d stressed ones. InN-P junctions, AE were only 15 to 20% with respect to internodes. Stem perfusion with dye allowed measurement of the percentageof xylem conduit transverse area blocked by cavitation. Thiswas 2% in internodes of W-plants and 5.2, 13.8 and 40.4% inthose of 9-, 16- and 23-d stressed ones. In N-P junctions, 18.5%of the xylem conduit transverse area was blocked in the 23-dstressed plants only. The major resistance to cavitation exhibitedby the N-P junctions is interpreted in terms of their greaternumber of narrow xylem conduits. The percentage of blocked xylemconduits within a range of diameters showed that the narrowera xylem conduit, the less likely it was that cavitation wouldoccur. After rewatering, the release of the xylem blockage causedby cavitation occurred within 2 h. Our data suggest that C.siliqua can be considered to be a cavitation avoider, especiallyin its stem-to-leaf transition zones. Ceratonia siliqua L., Carob tree, cavitation avoidance, xylem architecture, ultrasonic acoustic emissions     

Cation and Chloride Partitioning through Xylem and Phloem within the Whole Plant of Ricinus communis L. under Conditions of Salt Stress

Uptake and partitioning through the xylem and phloem of K⁺,Na⁺, Mg²⁺ , Ca²⁺ and Cl^– were studied over a 9 d intervalduring late vegetative growth of castor bean (Ricinus communisL.) plants exposed to a mean salinity stress of 128 mol m^–3NaCl. Empirically based models of flow and utilization of eachion within the whole plant were constructed using informationon ion increments of plant parts, molar ratios of ions to carbonin phloem sap sampled from petioles and stem internodes andpreviously derived information on carbon flow between plantsparts in xylem and phloem in identical plant material. Salientfeatures of the plant budget for K⁺ were prominent depositionin leaves, high mobility of K⁺ in phloem, high rates of cyclingthrough leaves and downward translocation of K⁺ providing theroot with a large excess of K⁺ . Corresponding data for Na⁺showed marked retention in the root, lateral uptake from xylemby hypocotyl, stem internodes and petioles leading to low intakeby young leaf laminae and substantial cycling from older leavesback to the root. The partitioning of the anionic componentof NaCl salinity, Cl^–, contrasted to that of Na⁺ in thatit was not substantially retained in the root, but depositedmore or less uniformly in stem, petiole and leaf lamina tissues.The flow pattern for Mg²⁺ showed relatively even depositionthrough the plant but some preferential uptake by young leaves,generally lesser export than import by leaf laminae, and a returnflow of Mg²⁺ from shoot to root considerably less than the recordedincrement of the root. Ca²⁺ partitioning contrasted with thatof the other ions in showing extremely poor phloem mobility,leading to progressive preferential accumulation in leaf laminaeand negligible cycling of the element through leaves or root.Features of the response of Ricinus to salinity shown in thepresent study were discussed with data from similar modellingstudies on white lupin (Lupinus albus L.) and barley (Hordeumvulgare L.) Key words: Ricinus communis L, potassium, sodium, chloride, calcium, magnesium, phloem, xylem, transport, partitioning, salinity     

The Effect of a Grafted Bud on Vascularization of the Petiole in Phaseolus, Datura and Lycopersicon

The radially-organized petiole of Phaseolus and the dorsiventralpetioles of Datura and Lycopersicon were used as stocks in budgrafting. Petiolar structure was examined after some weeks'growth of the scion, during which the grafted petiole came tofunction as a stem in supporting normal, vigorous shoot growthand was not abscised even at the end of the growing season.Below the graft union, reactivated petiolar cambium producedmassive amounts of secondary tissue with greatly enlarged vessels.Cambial activity was confined to the existing vascular bundlesexcept for the development of a little inter-fascicular cambiumin young, grafted Phaseolus petioles. Datura petioles woundedbelow the graft union, by a cut into the petiolar are from eitherthe abaxial or the adaxial surface and removal of a 1 cm lengthof tissue, responded by restoring the vascular are (abaxialwounds) or almost completing a vascular ring (adaxial wounds).In grafted, wounded Lycopersicon petioles the presence of deadxylem caused the separation of cambium regenerated from thecut vascular are and that arising in relation to the centralwound surface. A similar response, in which stimulated internalphloem plays an important part, occurred in certain woundedLycopersicon stems. The results are discussed in terms of thegradient induction hypothesis. graft, petiole, wound, cambium, xylem, phloem, Phaseolus multiflorus, Datura stramonium, Lycopersicon esculentum, bean, thornapple, tomato     

Testing 'Economy in Design' in Plants: are the Petioles and Rachises of Leaves 'Designed' According to the Principle of Uniform Strength?

Fifteen petioles and rachises from three dicotyledon species(Acer saccharum, A. negundo, and Aesculus hippocastanum), apalm (Chamaedorea erumpens), and a fern (Cyrtomium falcatum)were used to test the hypothesis of 'economy in design' in termsof the design principle of uniform strength, i.e. a beam inwhich the section modulus (Z) varies along beam-length (L) inthe same proportion as the bending moment (M). Such a beam is'economical' regarding the amount of material used in its 'construction'because each of its cross section has the minimum transversearea required to satisfy the conditions of strength. The extentto which the morphology of a petiole or rachis conformed withthis design principle was initially evaluated by normalizingZ (measured at a distance, x, from the tip of a petiole or rachis)with respect to the magnitude of Z measured at the base of thepetiole. The normalized values were plotted against normalizedpetiole-rachis length (x/L). The design principle was judgedto be demonstrated when such a plot was found to be isometric,i.e. when the plot had a slope of unity. This procedure wastested further by plotting M/Z vs. x/L for representative leavesof C. erumpens and A. saccharum, and judged adequate. The allometriesof all six simple/palmate leaves were found not agree with thedesign principle. The taperings of nine petioles and rachisesfrom pinnate leaves were consistent with the design principle.This was interpreted to provide circumstantial evidence for'economy in design' in the petioles of some pinnate leaves andevidence that the mechanical 'design' of the petioles of somesimple/palmate leaves differs substantially from that of pinnateleaves.Copyright 1993, 1999 Academic Press Leaf biomechanics, plant adaptation, petioles, rachises     

Biomechanical Responses of Chamaedorea and Spathiphyllum Petioles to Tissue Dehydration

Data are presented for the mechanical responses to dehydrationof petioles from two monocotyledons (Chamaedorea erumpens andSpathiphyllum Clevelandii). These data were used to test thehypothesis that the mechanical properties (elastic modulus Eand flexural stiffness EI) of petioles from C. erumpens arealtered significantly less by dehydration than those of petiolesfrom Spathiphyllum. Dehydration, resulting from either dryingat room temperature or from submergence in various concentrationsof mannitol solutions, produced significant increases in E anddecreases in EI (due to geometric distortions) in both youngand mature Spathiphyllum petioles. Similar trends were observedfor young petioles of C. erumpens, but significantly less sofor mature petioles of this species. Regardless of petiolarage, E increased allometrically as a linear function of tissuedensity, which in turn correlated with the volume fraction oflignified tissues in petioles; however, the proportional increaseof E as a function of tissue density was significantly greaterfor C. erumpens petioles than for Spathiphyllum. Anatomicalanalyses of petiolar transections indicated that Chamaedoreapetioles had larger volume fractions of lignified tissues thanthose of Spathiphyllum and that these tissues were located tomaximize stiffness. These data (and previously reported allometricrelationships between EI and petiolar length) shed light onthe difficulties in evaluating the ‘costs’ of committingtissues to mechanical support. Petioles, biomechanics, leaf anatomy, monocotyledons