Effects of mechanical stress and plant density on mechanical characteristics, growth, and lifetime reproduction of tobacco plants

Plastic increases in stem elongation in dense vegetation are generally believed to be induced by canopy shading, but because plants protect each other from wind, shielding (reduced mechanical stress) could also play a role. To address this issue, tobacco Nicotiana tabacum plants were subjected to two levels of mechanical stress, 0 (control) or 40 (flexed) daily flexures, and grown solitarily, in a dense monostand (with plants of only one mechanical treatment), or in a mixed stand (flexed and control plants grown together). Flexed plants produced shorter and thicker stems with a lower Young's modulus than control plants, while dense-stand plants had relatively taller and thinner stems than solitary ones. Flexing effects on stem characteristics were independent of stand density. Growth, reproduction, and survival of solitary plants were not affected by flexing, while in the monostand growth was slightly reduced. But in the mixed stand, flexed plants were readily shaded by controls and had considerably lower growth, survival, and reproduction rates. These results suggest that wind shielding indeed plays a role in the plastic increase in stem elongation of plants in dense vegetation and that this response can have important consequences for competitive ability and lifetime seed production.     

Effects of Vibration on Mechanical Properties and Biomass Allocation Pattern ofCapsella bursa-pastoris(Cruciferae)

The herbaceous dicot speciesCapsella bursa-pastoris(Cruciferae)was used to determine the influence of chronic mechanical perturbationon the biomass allocation pattern (i.e. dry weight distributionamong roots, stems and reproductive structures) and the mechanicalproperties of roots and stems (i.e. tensile breaking stressand Young's modulus). It was hypothesized that mechanicallystimulated plants would allocate more of their total biomassto root systems and less to shoots compared to control plantsand that the breaking stress (a measure of strength) and Young'smodulus (a measure of material stiffness) would increase forroots and decrease for stems because these responses would adaptivelyreduce the bending moment at the base of shoots and increasethe anchorage strength of root systems. It was also hypothesizedthat mechanical perturbation would maladaptively reduce therelative fitness of individuals by reducing biomass allocationto their reproductive organs and the ability to broadcast seedsby means of elastic stem flexure. These hypotheses were testedby vibrating cultivated plants for 60 s every day during thecourse of growth to maturity and comparing their dry weightdistributions and the mechanical properties of their body parts(measured in tension) to those of undisturbed control plants.Based on a total of 51 experimentally manipulated and 44 controlplants for which mechanical properties were successfully tested,chronic organ flexure resulted in more massive root systemsand less massive vegetative shoots, increased the magnitudesof root breaking stress and Young's modulus and had the reverseeffect on stems, reduced the dry weight of reproductive structuresat maturity, delayed the formation of the first mature flowerand fruit, and accelerated the on-set of plant senescence comparedto control plants. These responses to chronic organ flexureare interpreted to be vegetatively adaptive, since they reducethe probability of stem and root failure as a consequence ofwind-pressure or foraging, and to be reproductively maladaptive,since they reduce reproductive effort and the ability to mechanicallydischarge seeds.Copyright 1998 Annals of Botany Company Adaptation, biomass allocation, biomechanics, elastic properties, roots, stems, thigmomorphogenesis.     

Ethylene sensitivity affects changes in growth patterns, but not stem properties, in response to mechanical stress in tobacco

Plant responses to mechanical stress (e.g. wind or touch) involve a suite of physiologic and developmental changes, collectively known as thigmomorphogenesis, including reductions in height increment, Young's modulus of stems, shoot growth, and seed production, and increased stem girth and root growth. A role of the phytohormone ethylene in thigmomorphogenesis has been proposed but the extent of this involvement is not entirely clear. To address this issue, wild-type (WT) and ethylene-insensitive transgenic (Tetr) tobacco ( Nicotianum tabacum ) plants were subjected to three levels of mechanical stress: 0, 25 and 75 daily flexures. Flexed plants produced shorter, thicker stems with a lower Young's modulus than non-flexed ones, and these responses occurred independently of genotype. This suggests that ethylene does not play a role in thigmomorphogenesis-related changes in stem characteristics in tobacco. The effect of mechanical stress on dry mass increment (growth), on the other hand, differed between the genotypes: in the WT plants, shoot growth but not root growth was reduced under mechanical stress, resulting in reduced total growth and increased root mass fractions. In the Tetr plants, neither shoot nor root growth were affected. This suggests that ethylene is involved in the inhibition of tobacco shoot growth under mechanical stress.     

The Growth Response of the Stems of Genetically Modified Tobacco Plants (Nicotiana tabacum'Samsun') to Flexural Stimulation

Genetically modified tobacco plants (Nicotiana tabacum‘Samsun’)with antisense cinnamyl alcohol dehydrogenase DNA, produce secondaryxylem of a reduced tensile stiffness. These plants were grownalongside control plants. The stems of the plants were flexedor protected from flexing over a period of several weeks. Thetensile moduli and second moments of areas of the differenttissues inside the stems were measured and used to calculatethe bending stiffness of the plants. In tobacco, the cylinderof xylem was found to be the most important tissue in determiningthe bending stiffness of the plants. The thickness of the xylemtissue cylinder increased when plants were subjected to flexuralstimulation. This increased the bending stiffness of the stems.The response to mechanical stimulation was found to be correlatedwith tissue strain and the genetically modified plants wereable to exactly compensate for the reduced modulus of theirxylem tissue by increasing the thickness of the xylem tissuecylinder more than in control plants.Copyright 1999 Annals ofBotany Company. Tobacco plants, stem bending, xylem tissue, second moment of area, thigmomorphogenesis, mechanical strain.     

The Effects of Infection by Rust (Puccinia lagenophorae Cooke) on the Growth of Groundsel (Senecio vulgaris L.) Cultivated under a Range of Nutrient Concentrations

Groundsel (Senecio vulgaris L.), healthy or infected with therust fungus Puccinia lagenophorae Cooke, was grown at a rangeof nutrient concentrations in sand culture. There were statisticallysignificant interactions between the effects of infection andnutrient supply upon the dry weights of stems, leaves, rootsand reproductive tissues, leaf area and cumulative capitulumproduction. This interaction occurred since infection causedsignificant inhibitions of growth only at moderate or high nutrientconcentrations. At low concentrations rusted plants were similarto or slightly larger than controls. Both in controls and rustedplants root: shoot ratios increased as nutrient supply declined.The ratio of root: shoot dry weight was consistently reducedby infection whilst root length: leaf area ratio was relativelyunchanged. More detailed investigations confirmed that infection had littleeffect on plant growth under nutrient deficient conditions despitesuppression of the host's ability to increase root: shoot ratiosin response to nutrient stress. This reflected the inhibitionof relative growth rates in rusted plants at high but not lownutrient concentrations, which in turn reflected reduced netassimilation rates (NAR). Increases in leaf-area ratio (LAR)often ameliorated the decline in NAR in rusted plants. Senecio vulgaris L., Puccinia lagenophorae Cooke, nutrient deficiency, growth, root: shoot ratio     

The effect of mechanical stimulation on root and shoot development of young containerised Quercus robur and Robinia pseudoacacia trees

Thigmomorphogenesis is a well-studied process in agricultural crops and coniferous trees. Nevertheless, the effects on both shoot and root characteristics for deciduous woody species received little attention so far. In this study, the objective was to understand the effect of aboveground flexing treatments on the development of structural, mechanical and physiological root and shoot characteristics for two deciduous tree species, Black locust (Robinia pseudoacacia L.) and English oak (Quercus robur L.). Flexing treatments were performed using an electromechanical device with a rotating arm touching and bending the plants at regular intervals. A wide range of stem, shoot as well as root system characteristics was measured. The different flexing treatments altered above- and belowground plant development for both species, with strongest effects on Quercus and most significant differences between the control and the unidirectional flexing treatment. Some responses are in accordance with previous findings, such as stem eccentricity and reduced shoot elongation under unidirectional flexing, but others are renewing, such as the lower stomatal density and larger epidermal cell surface for the Quercus plants under variable flexing direction. Despite some common responses, both species frequently differed in the way they were affected. Belowground, Quercus plants under unidirectional flexing invested relatively more in their first order root and deeper second order roots, whereas Robinia plants allocated relatively more to fine root biomass and horizontal shallow roots. Both strategies potentially increased pull-out as well as overturning resistance in their own way. The presented findings are valid for young trees grown in small containers. Based on practical know-how and shortcomings experienced in the course of this experiment, methodological recommendations are formulated. We finally stress the complex variability in growth responses, especially for root systems, observed in different studies and related to dissimilarity in species, soil conditions, plant history or type of mechanical perturbation.     

Mechanics and Growth Form of the MossHylocomium splendens

The mossHylocomium splendenshas two different growth forms.Sympodial growth occurs where the apical meristem ceases activityannually and growth is continued by a lateral bud. Sympodialplants are vertical and self-supporting. Monopodial growth occurswhen the apex continues growth. Monopodial plants are prostrate.The aims of the study were to examine stem mechanics of thedifferent growth forms and to compare mechanical propertiesalong stems. Stems of annual segments were subjected to threepoint bending tests. In sympodial plants the stiffness of thestem material increased significantly with segment age. Flexuralrigidity increased significantly with age in segments from 1to 4 years old, and then declined. Segment diameter decreasedsignificantly with age in sympodial plants. Monopodial plantsshowed no significant effects of segment age on the diameter,material stiffness or flexural rigidity of stems. Sympodialsegments were significantly wider, stronger, more rigid andcomposed of stiffer material with a higher stress at yield thanmonopodial segments, but did not have a larger proportion ofstrengthening material. Sympodial stems had significantly morecellulose than monopodial stems. The mossHylocomium splendensshowsa range of mechanical adaptations, as a self-supporting or aprostrate plant, which suit it to life in very different environments.Copyright1998 Annals of Botany Company. Hylocomium splendens, growth form, mechanics, cellulose, stiffness, flexural rigidity, bending, monopodial, sympodial, adaptation, anatomy, stems, plant.     

The Mechanical Roles of Clasping Leaf Sheaths: Evidence fromArundinaria tecta(Poaceae) Shoots Subjected to Bending and Twisting Forces

Representative shoot segments of the grass speciesArundinariatéctaconsisting of one intact internode and its subtendingnode and clasping leaf sheath were tested to determine the mechanicalinfluence of the leaf sheath on the ability of stems to resistbending and twisting forces. These segments were also used tomeasure shoot morphometry and composite tissue Young's and shearmoduli (EandG,respectively) to simulate the global deformationpatterns attending bending and twisting by means of finite elementanalyses. On average, leaf sheaths contributed 33% of the overallbending stiffness and 43% of the overall torsional stiffnessof stem segments. Comparisons betweenEandGof isolated internodesand leaf sheaths indicated that sheaths were composed of stiffertissues measured either in bending or twisting. Thus, leaf sheathscould act as an external cylindrical brace composed of stiffermaterials than those of the internodes they enveloped. The magnitudesof internodalEandGwere correlated with internodal shape suchthat the ability of internodes to resist twisting relative tothe ability to resist bending forces decreased as internodesbecame more slender or developed thinner walls (both of whichoccur in an acropetal direction from the base to the tip ofshoots). Finite element simulations predicted that, in bending,the leaf sheath laterally braces internodal walls as they tendto ovalize in cross section and push against its inner surfacewhich ovalizes to a lesser extent in the plane normal to thecurvature of shoot flexure. In twisting, the successive ovalizedtransections of internodal walls assumed a helical pattern alongthe length of shoot segments. This helical deformation patternwas attended by an inner lateral contraction of internodal wallsthat was less developed in the leaf sheath that thus provideddecreasing mechanical support to the internode as the lateralcontraction of internodal walls amplified. The twisting of internodesand sheaths was also predicted to concentrate tensile and shearstrains in the nodal diaphragm. Here stress intensities sufficientto produce tissue shear failure were concentrated at two opposingpoints on the surface of the diaphragm. Finite element analysesthus identified a potential weak point in the mechanical constructionof hollow, septate shoots that are, nevertheless, more thanadequately stiff to support their own weight, yet sufficientlyflexible to twist without irreparable damage in normal winds.Copyright1998 Annals of Botany Company Plant stems; nodes; internodes; leaf sheaths; elastic moduli; wind lodging; biomechanics.     

Deep Phenotyping of Coarse Root Architecture in R. pseudoacacia Reveals That Tree Root System Plasticity Is Confined within Its Architectural Model

This study aims at assessing the influence of slope angle and multi-directional flexing and their interaction on the root architecture of Robinia pseudoacacia seedlings, with a particular focus on architectural model and trait plasticity. 36 trees were grown from seed in containers inclined at 0° (control) or 45° (slope) in a glasshouse. The shoots of half the plants were gently flexed for 5 minutes a day. After 6 months, root systems were excavated and digitized in 3D, and biomass measured. Over 100 root architectural traits were determined. Both slope and flexing increased significantly plant size. Non-flexed trees on 45° slopes developed shallow roots which were largely aligned perpendicular to the slope. Compared to the controls, flexed trees on 0° slopes possessed a shorter and thicker taproot held in place by regularly distributed long and thin lateral roots. Flexed trees on the 45° slope also developed a thick vertically aligned taproot, with more volume allocated to upslope surface lateral roots, due to the greater soil volume uphill. We show that there is an inherent root system architectural model, but that a certain number of traits are highly plastic. This plasticity will permit root architectural design to be modified depending on external mechanical signals perceived by young trees.     

The effects of air flow and stem flexure on the mechanical and hydraulic properties of the stems of sunflowers Helianthus annuus L

Many studies have shown that wind affects plant development, causing them to develop shorter and usually stronger stems. Many of these effects have been shown to be due to a response to mechanical flexing of the stem which is known as thigmomorphogenesis. However, it is not known how wind affects the hydraulic properties of stems, nor have the effects of air flow past leaves been examined in isolation from mechanical flexing. This study, therefore, used a factorial experiment to distinguish between the effects of stem flexing and air flow, and examined the morphology, hydraulics and mechanics of developing sunflowers Helianthus annuus. It was found that flexure and air flow had opposite effects on several aspects of development; air flow increased plant height and length-specific stem hydraulic conductivity, k(h), and reduced stem rigidity and strength, while flexing did the reverse. There was also a clear trade-off between hydraulic and mechanical capability: as one increased the other decreased. A plant's response to wind must, therefore, be a complex response to at least two different stimuli and this might help explain why it varies with species and environment.     

Studies in the Nutrition of Apple Rootstocks: 1. Effects of Deficiencies of Iron and Magnesium on Growth

Plants of the apple rootstock M.VII were grown for a singleseason by spraying their roots continuously with nutrient solutions:(a) complete, (b) low iron, (c) low magnesium, and (d) low ironand low magnesium. Detailed records were taken throughout theseason of fresh weight, shoot length, diameter, and leaf area,while the fresh and dry weights of component parts were determinedon harvested samples. The leaves of the iron-deficient plants were chlorotic and hada lower dry weight and higher water content per unit area thanthose of the control plants. Growth and net assimilation ratewere both reduced but the distribution of assimilates was similarto that in the controls. Total growth was also markedly reduced by the low-magnesiumtreatment, but the leaf symptoms were different and the plantswere morphologically distinct from the control plants. Verylittle chlorosis occurred, but necrosis and severe defoliation,progressing up the shoot, reduced the leaf area ratio and probablylargely accounted for the low net assimilation rate and relativegrowth rate. The diameter of the shoot was affected much morethan its length and the shoot/root ratio was greatly increased. Plants deficient in both iron and magnesium showed less defoliationthan when magnesium alone was low. More dry matter accumulatedin the old stem and less in the new shoot than would have occurredwith a simple combination of the single deficiencies.     

Root Confinement and its Effects on the Water Relations, Growth and Assimilate Partitioning of Tomato (Lycopersicon esculentum Mill)

Although it is well established that the root growth in manyspecies is very sensitive to mechanical impedance or to confinementin small volumes, little is known about the consequent effectson growth of the whole plant and the mechanisms involved. Thiswork investigated the effects of root confinement on the waterrelations, growth and assimilate partitioning of tomato (Lycopersiconesculentum Mill) grown in solution culture. Six-week old plants were transferred to either 4500 ml or 75ml containers filled with nutrient solution, and allowed togrow for 14 d. Transpiration, leaf-air temperature differences,and leaf diffusive resistances were measured frequently. Leaf,stem and shoot dry masses, leaf area and root length, were estimatedwhen the treatments were imposed and at the end of the experiment.After 14 d growth the root and shoot hydraulic resistances wereestimated from measurements of leaf water potential and transpirationrate, using a steady-state technique. Confining root growth to the small containers substantiallyreduced shoot and root growth and increased the proportion oftotal dry matter present in the stems. These effects were dueto drought stress. The hydraulic resistance of the root systemwas greatest in the confined plants. This led to more negativeleaf water potentials, increased leaf diffusive resistance,and reduced the net assimilation rate by a factor of 2.5. Transpirationper unit leaf area was less affected. However, cumulative transpirationwas also reduced by a factor of 2.5. mostly because of the smallerleaf area on the confined plants. Root hydraulic resistivitywas measured at 3.1 x 10¹²s m^–1 in the control treatment,but increased to 3.9 x 10¹² s m^–1 for roots in the smallcontainer. The mechanisms by which root confinement caused drought stressand disrupted the pattern of assimilate partitioning are discussedin detail. Assimilate partitioning, Lycopersicon esculentum, root confinement, plant growth, root growth, root resistance, shoot resistance, tomato, transpiration, water-use efficiency     

Interaction of Ancymidol and benzyladenine in control of growth of juvenile Hedera helix

The growth of the juvenile form of Hedera helix L. was greatly suppressed by soil drench treatment with the growth retardant, Ancymidol. Treated plants were smaller in stature, had fewer nodes and shorter internodes, accumulated less dry weight, had reduced leaf area, and lowered shoot: root ratios than untreated plants. In contrast, shoot tip application of the cytokinin, benzyladenine, increased stem diameter and shoot:root ratio over that of controls.
When Ancymidol and benzyladenine were applied together, reductions in plant growth were not evident – plants appeared as the untreated controls. The implications of this interaction are discussed.     

On Arborescence in Gahnia clarkei and Gahnia sieberiana

Specimens of Gahnia sieberiana from Brisbane, Queensland, andof Gahnia clarkei from near Orbost, Victoria, were collectedand examined both morphologically and anatomically. The speciesgrow in wet areas and are of interest because they representthe largest arborescent species known in the Cyperaceae. Stemdiameters up to 120 mm and stems up to 10 m long have been observed.Such long stems tend to be supported by nearby vegetation. Althoughfresh stems are tough and woody, they are brittle. Branchingof the stems is sympodial, and numerous branches are producedby plants growing in exposed habitats. There is less branchingin plants from shaded habitats. Basal shoots may also occur.Adventitious roots develop basally on most plants, but withG. sieberiana, some adventitious roots form near the shoot apexand grow in and around leaf bases. Anatomical features of interestare an endodermoid layer composed of sclereids with elongate,undulated, outer tangential walls that are lignified and suberized,short vessel elements with horizontal to oblique simple perforationplates, and relatively short sclereids surrounding vessel elementsin the vascular bundles. Some vascular bundles are bipolar.The presence of short vessel elements here is in marked contrastto the longer tracheary elements in other arborescent monocotyledons. Arborescence, stem anatomy, Cyperaceae, Gahnia, saw sedge, Monocotyledon, bipolar bundles, morphology, endodermoid layer     

Physiology of the parasitic association between maize and witchweed (Striga hermonthica): is ABA involved?

Striga hermonthica (Del.) Benth. is an obligate hemiparasiticangiosperm which can cause severe losses of yield in cerealcrops in the semi-arid tropics. The effects of this parasiteon the growth and stomatal conductance of three varieties ofmaize (Zea mays L.) during the first 6 weeks of the associationhave been studied. From 24 d after planting (DAP), infectedplants were significantly shorter than uninfected controls.When the plants were harvested 45 DAP, infected plants had fewerfully expanded leaves, less leaf biomass and less pseudo-stembiomass than uninfected controls. However, the parasitized plantshad more root biomass and hence a higher root:shoot ratio thanuninfected controls. The stomatal conductance of infected hostswas severely inhibited by comparison with that in uninfectedplants. The possibility that abscisic acid (ABA) may be involved inthe regulation of the parasitic association was investigated.ABA concentrations in leaf tissue of maize (cv. Cargimontana)and S. hermonthica were determined by radioimmunoassay. Whilethere was a difference between cultivars in the extent of theresponse, the concentrations of ABA were significantly higherin infected maize plants than in the uninfected controls. InS. hermonthica, leaf tissue ABA concentration was found to bean order of magnitude higher than in the host leaf tissue. Detachedleaves of S. hermonthica which were dehydrated at room temperatureuntil they had lost 10–20% of their fresh weight containedthree times the ABA concentration of control leaves. This suggeststhat leaves of S. hermonthica can synthesize or re-mobilizeABA in response to water deficit. It is not yet known whetherthis contributes to the higher concentration in infected hosts,but the results suggest that ABA has a role in this parasiticassociation. Key words: Striga hermonthica, abscisic acid, growth, parasitic angiosperm, stomatal conductance     

Nutrient-limited growth rates: quantitative benefits of stress responses and some aspects of regulation

Young sunflower plants (Helianthus annuus L.) under stress oflow nitrate or phosphate availability exhibited increases inroot: shoot ratio and in kinetic parameters for uptake. Theyshowed no significant changes in photosynthetic utilizationof either nutrient. Increases in root: shoot ratio were achievedby early and persistent suppression of shoot growth, but notroot growth. Affinity for phosphate uptake, 1/K_m(P), increasedwith phosphate stress, as did affinity for nitrate uptake, 1/K_m(N),with nitrate stress. Maximal uptake rate, V_max, for phosphateuptake increased with phosphorus stress; V_max for nitrate didnot increase with nitrogen stress. Phosphate V_max was relatedstrongly to root nutrient status. Decreases in V_max with plantage were not well explained by changes in age structure of roots.Estimated benefits of acclimatory changes in root: shoot ratioand uptake kinetics ranged up to 2-fold increases in relativegrowth rate, RGR. The relation of RGR to uptake physiology followedpredictions of functional balance moderately well, with somesystematic deviations. Analyses of RGR using growth models implyno significant growth benefit from regulating Vmax, specifically,not from down-regulating it at high nutrient availability. Quantitativebenefits of increases in root: shoot ratio and uptake parametersare predicted to be quite small under common conditions whereinnutrient concentrations are significantly depleted by uptake.The root: shoot response is estimated to confer the smallestbenefit under non-depleting conditions and the largest benefitunder depleting conditions. Even then, the absolute benefitis predicted to be small, possibly excepting the case of heterogeneoussoils. Depleting and non-depleting conditions are addressedwith very different experimental techniques. We note that atheoretical framework is lacking that spans both these cases,other than purely numerical formulations that are not readilyinterpreted. Key words: Nutrient stress, nutrient uptake, nutrient use efficiency, relative growth rate, Helianthus annuus     

A comparative study of the response of the roots and shoots of sunflower and maize to mechanical stimulation

Despite numerous studies of the effects of mechanical stimulationon plant shoots, the response of roots to mechanical stimulationhas largely been neglected. In this study the effects of shootflexure on the morphology and mechanics of two contrasting speciesof herbaceous angiosperm, growing in a glasshouse were compared:maize (Zea mays), a monocot; and sunflower (Helianthus annuusL.) a dicot. Mechanical stimulation affected the root more than the shootcomponents. Root systems of mechanicallystressed sunflowershad a greater angle of spread and increased root number. Aswell as large morphological and weight effects, with increasesover the control of 33% in the length of rigid root and 38%in the dry weight of lateral roots, in sunflowers, there werealso mechanical effects. In both species roots of flexed plantswere more rigid, stronger and composed of stiffer material andtheir root systems also provided greater anchorage strength.In contrast, there was only a small reduction in shoot weightand shoot height in flexed plants and no effects on mechanicalproperties. There were differences in behaviour between species; maize rootmorphology responded less than that of sunflowers to mechanicalstimulation. The basal diameter of roots increased by only 8%compared with 16% in sunflowers, though the roots of both speciesshowed similar increases in material stiffness. This differenceis related to the lack of secondary thickening in the monocotscompared with the dicot sunflowers. Key words: Thigmomorphogenesis, Helianthus annuus L., Zea mays, anchorage, lodging     

Growth of Zea mays L. plants with their seminal roots only. Effects on plant development, xylem transport, mineral nutrition and the flow and distribution of abscisic acid (ABA) as a possible shoot to root signal

Maize (Zea mays L.) was grown in quartz sand culture eitherwith a normal root system (controls) or with seminal roots only(‘single-rooted’). Development of adventitious rootswas prevented by using plants with an etiolated mesocotyl andthe stem base was positioned 5–8 cm above the sand. Eventhough the roots of the single-rooted plants were sufficientlysupplied with water and nutrients, the leaves experienced waterdeficits and showed decreased transpiration as trans plrationalwater flow was restricted by the constant number of xylem vesselspresent in the mesocotyl. As a consequence of this restriction,transpirational water flow velocities in the metaxylem vesselsreached mean values of 270 m h^–1 and phloem transportvelocities of 5.2 m h^–1. Despite limited xylem transportmineral nutrient concentrations in leaf tissues were not decreasedin single-rooted plants, but shoot and particularly stem developmentwas somewhat inhibited. Due to the lack of adventitious rootsthe shoot:root ratio was strongly increased in the single-rootedplants, but the seminal roots showed compensatory growth comparedto those in control plants. Consistent with decreased leaf conductance,ABA concentrations in leaves of single-rooted plants were elevatedup to 10-fold, but xylem sap ABA concentrations in these plantswere lower than in controls, in good agreement with the well-wateredconditions experienced by the seminal roots. Surprisingly, however,ABA concentrations in tissues of the seminal roots of the single-rooted plants were clearly increased compared to the controls,presumably due to increased ABA import via phloem from the water-stressedleaves. The results are discussed in relation to the role ofABA as a shoot to root signal. Key words: Zea mays, seminal roots, plant development, xylem transport, mineral nutrition, ABA, shoot-to-root signal     

The Effect of Grain Number per Ear (Sink Size) on Source Activity and its Water-Relations in Wheat

Blum, A., Mayer, J. and Golan, G. 1988. The effect of grainnumber per ear (sink size) on source activity and its water-relationsin wheat.–J. exp. Bot. 39: 106–114. Work was done to evaluate the nature of sink-source relationshipsin wheat (Triticum aestivum L.), when the strength of the sinkwas modified by the removal of half of the grain from the earat about anthesis. The main hypothesis was that sink-sourcerelationship would be modified by water stress and that a weakersink would improve the drought resistance of the source. Two experiments were performed. The first experiment evaluatedthe effect of de-graining in two wheat varieties grown in thefield. The second experiment (in the greenhouse) evaluated theeffect of de-graining in plants subjected to water stress afteranthesis by immersing the root system in a solution of polyethyleneglycol (6000), as compared with non-stressed controls. In bothexperiments measurements were performed after de-graining toprovide data on leaf gas exchange, leaf water potential, osmoticadjustment of leaves and ears (greenhouse), the percent of stemweight loss as an index of stem reserve mobilization, finalroot weight (greenhouse) and ear weight components. De-graining caused a decrease in flag leaf stomatal conductance,carbon exchange rate (CER) and transpiration and an increasein flag leaf water potential. These effects were stronger withwater stress. De-graining did not affect osmotic adjustmentin the flag leaf but induced better adjustment in glumes andawns. De-graining decreased the percent of stem weight lossand increased final root weight, especially under drought stress. A weaker sink was, therefore, considered to improve plant droughtresistance in terms of the maintenance of higher leaf waterpotential, a larger root, a better osmotic adjustment in theear and, possibly, increased flag leaf longevity. The ‘cost’of this improved drought resistance was in reduced flag leafCER and reduced stem (and root?) reserve mobilization. Key words: Drought resistance, carbon exchange rate, stomata, transpiration, osmotic adjustment, leaf water potential, root, awns, yield     

Structural development of the shoot and root systems of two winter wheat cultivars, Triticum aestivum L.

The structural development of the stems and basal anchorageroots of Galahad and Hereward winter wheat cultivars (Triticumaestivum L.) were investigated and related to their mechanicalfunction. Stem and root morphology, anatomy and mechanical propertieswere examined from tillering (March) up to maturity (August),together with plant weight distribution. This allowed us tocalculate a ‘factor of safety’ against root andstem failure throughout development. As the plants grew taller the stem and the anchorage ‘coronalroots’ increased in bending strength countering the increasingmechanical demands. The bending strength, in turn, was correlatedwith the amount of lignified material around the stem and rootperimeter. Structural development ceased by ear emergence, whenthe plant was at its tallest, but because the ear weight continuedto rise the ‘self-weight’ moment pushing the plantover continued to increase. This meant that the ‘safetyfactors’ of both cultivars against both root and stemmechanical failure decreased throughout development. In bothcultivars the safety factors against root failure were lowerthan for stem failure, and Galahad had lower factors of safetythan Hereward. All these findings were consistent with resultsof field trials; failure tends to occur late in development,during grain filling, and is localized to the root system, whilstGalahad is more prone to lodging than Hereward. The pattern of mechanical development of winter wheat seemsto be one which would maximize its reproductive success, maintainingits structural integrity especially early in development whileinvesting in a minimum of structural material. Key words: Safety factor, anchorage, lodging, biomechan-ics, structural development