The mechanics of anchorage in seedlings of sunflower, Helianthus annuus L.

Forces applied to plants will subject many of the roots to tension, which must be transferred to the soil via shear if uprooting is to be prevented. The stress distribution will depend on the relative stiffnesses of the earth and root, and the mode of failure will depend on the relative strength of the soil and of the root soil bond. This study of the anchorage of sunflower radicles combined uprooting tests performed by a tensile testing machine with mechanical tests on the roots and soil.
The maximum extraction force increased with length to an asymptotic value and was reached at a very low displacement. Root hairs and soil particles covered the tapered top 20 mm of extracted root, but the lower cylindrical region was bare. The soil was stiffer than the root, so shear stress was initially concentrated at the top of the root, soil strength over the top 20 mm resisting uprooting. Lower regions of the root were stressed later, their sparser root hairs being sheared off, and resist uprooting only by friction. In a further lest upper and lower regions of radicles were uprooted separately. As predicted, the upper region generated much greater resistance to uprooting per unit length, and at much lower displacements than the lower region.
The top of the radicle is well adapted for anchorage, the profuse root hairs and mucigel it produces glueing the root to the soil. The lower regions are thus protected from damage.     

An Experimental Investigation of the Resistance of Model Root Systems to Uprooting

The architecture of a tree root system may influence its abilityto withstand uprooting by wind loading. To determine how theroot branching pattern may alter the anchorage efficiency ofa tree, artificial model root systems with different topologiesand branching angles were built. The root systems were embeddedat various depths in wet sand and the pull-out resistance measured.A model to predict the uprooting resistance from the data collectedwas designed, allowing predictions of anchorage strength withregards to architecture. The dominant factors influencing pull-outresistance were the depth and length of roots in the soil. Themost efficient type of branching pattern predicted by the programwas one with an increased number of roots deep in the soil.The optimum branching angle most likely to resist pull-out isa vertical angle of 90° between a lateral and the main axis.The predicted mechanically optimal radial angle between a lateralbranch and its daughter is between 0 and 20°. Values ofbranching angle are compared with those measured in real woodyroot systems of European larch and Sitka spruce. Root architecture; root anchorage; pull-out resistance; windthrow; Picea sitchensis ; Larix decidua     

The anchorage mechanics of deep rooted larch, Larix europea L. japonica

The anchorage of deep rooted 16-year-old larch trees, Larixeuropea japonica, has been studied by combining winching testswith analyses of strain around the base of the trunk and rootsystem and mechanical tests on individual roots. These showedthat anchorage is provided by the laterals which emerge fromaround the stem base, sinker roots which emerge along theirlength, and tap roots positioned directly underneath the bole.During anchorage failure the leeward laterals are bent and eventuallybreak close to their base, whilst the windward laterals arepulled out of the ground, with their sinker roots intact. Afterinitially being confined by the soil and bending, the tap rootrotates in the soil. Anchorage failure is similar when the soilis dry as when it is wet, but failure occurs closer to the trunk.Strain measurements along the lateral roots revealed that thestresses were highest close to the trunk and that these regionsof the roots contribute most to tree stability. The two major components of anchorage were found to be the resistanceof leeward laterals to bending and the resistance of tap rootsand windward sinkers to uprooting. Bending tests on leewardlaterals revealed that they provide around 25% of tree anchorage.Almost 75% of the anchorage strength must, therefore, be providedby the windward sinkers and tap roots. Anchorage strength ofroots was positively correlated to their cross-sectional area.The vertical orientation of the sinkers makes the anchoragesystem of larch more efficient than the plate system formedby Sitka spruce on waterlogged soils and means that no root-soilplate is formed. Key words: Anchorage, root architecture, sinker roots, root bending strength, windthrow     

The Mechanics of Anchorage in Wheat Triticum aestivum L.: I. THE ANCHORAGE OF WHEAT SEEDLINGS

Model of the mechanics of uprooting lead to the identificationof ‘optimal’ anchorage systems which can withstanda given upward force at a minimum construction cost. Such systemshave many downward-pointing fibrous roots which are strengthenedprogressively towards the base. A study of the anchorage systemof 7- and 21-d-old wheat (Triticum aestivum L.) plants showedthat the plants possessed five seminal roots, of which onlythree pointed vertically. Each root was well suited for anchorage,being convered in root hairs and strengthened progressivelytowards the base by lignification of the stele. Strength andstiffiness of roots but not their mass per unit length increasedwith age. There was little interaction between roots when plantswere uprooted; the three vertical roots broke while the twohorizontal ones pulled out, as occurred when roots were pulledout singly, Uprooting forces increased with age and the rootsystem could withstand uprooting forces greater than those requiredto pull out upper leaves, so reducing the chances of the plantbeing uprooted by a herbivore, By 3 weeks a stiff adventitiousroot system, which would later help prevent the wheat lodging,was developing.     

The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees

The anchorage mechanics of mature buttressed trees of Aglaiaand Nephelium, and of non-buttressed Mallotus wrayi have beeninvestigated by combining a study of the morphology of theirroot systems with a series of anchorage tests. Both types possessed tap roots, but only buttressed trees possessedsinker roots, which branched from the ends of the buttresses.The anchorage strength of the buttressed trees was almost double(10.6 kNm) that of the unbuttressed ones (4.9 kNm), and themaximum moment was generated at lower angles. In but tressedtrees, the leeward buttresses were pushed into the soil beforebending and eventually breaking towards their tip, whilst thewindward buttresses pulled out of the soil or delaminated ifthey possessed sinker roots. The tap root rotated in the soilto windward. In contrast, during failure of unbuttressed treesthe tap root both moved and bent towards the leeward, the windwardroots were pulled out of the soil, and the leeward lateralssimply buckled. Strains along but tresses were much higher thanalong the laterals of unbuttressed trees. These results suggest that buttresses act in both tension andcompression and make a much larger contribution to anchoragethan the thin laterals of non-buttressed trees. The relativecontribution of the but tresses was determined by carrying outa further series of anchorage tests in which both buttressedand unbuttressed trees were pulled over after all their lateralshad been cut away. These trees were therefore only anchoredby their taproot. Failure of both types was similar to intactunbuttressed trees, and they had similar anchorage strengthstoeach other, 4 kNm, around 80% of the value for intact non-buttressedtrees, but only 40% of the strength of intact buttressed trees.Buttresses therefore contribute around 60% of the anchorageof buttressed trees, producing around six times more anchoragethan the thin laterals of unbuttressed trees. Key words: Anchorage, root architecture, sinker roots, tap roots, root bending strength, buttresses     

A generic 3D finite element model of tree anchorage integrating soil mechanics and real root system architecture

Understanding the mechanism of tree anchorage in a forest is a priority because of the increase in wind storms in recent years and their projected recurrence as a consequence of global warming. To characterize anchorage mechanisms during tree uprooting, we developed a generic finite element model where real three-dimensional (3D) root system architectures were represented in a 3D soil. The model was used to simulate tree overturning during wind loading, and results compared with real data from two poplar species (Populus trichocarpa and P. deltoides). These trees were winched sideways until failure, and uprooting force and root architecture measured. The uprooting force was higher for P. deltoides than P. trichocarpa, probably due to its higher root volume and thicker lateral roots. Results from the model showed that soil type influences failure modes. In frictional soils, e.g., sandy soils, plastic failure of the soil occurred mainly on the windward side of the tree. In cohesive soils, e.g., clay soils, a more symmetrical slip surface was formed. Root systems were more resistant to uprooting in cohesive soil than in frictional soil. Applications of this generic model include virtual uprooting experiments, where each component of anchorage can be tested individually.     

The Mechanics of Root Lodging in Winter Wheat, Triticum aestivum L.

The anchorage of winter wheat, Triticum aestivum L., is providedby a cone of rigid coronal roots which emerge from around thestem base. During root lodging this cone rotates at its windwardedge below the soil surface, the soil inside the cone movingas a block and compressing the soil beneath. A theoretical modelof anchorage suggested that lodging resistance should be dependenton the diameter of the root-soil cone, coronal root bendingstrength and soil shear strength. We tested the predictions of the anchorage model by carryingout two series of experiments. In the first, varieties of contrastinglodging resistances were artificially lodged. The moment requiredto rotate plants into the soil, the diameter of the root-soilcone, and the bending strength of the coronal roots were recorded.The lodging moment was correlated with the size of the soilcone, as predicted. Generally, differences in anchorage strengthbetween varieties were due to differences in root-soil conediameter, although coronal root strength was also important. A second series of tests was carried out using model plantsanchored by plastic discs. The behaviour of the models duringartificial lodging supported the anchorage model; the forceresisting lodging was similar to that of plants with root-soilcones of the same size and the resisting force was dependenton the soil strength. These results suggest that root lodging resistance might beimproved by increasing both the angle of spread and the bendingstrength of the coronal roots. Key words: Anchorage, root-soil cone, coronal roots, lodging, wheat     

The Increase in Anchorage with Tree Size of the Tropical Tap Rooted TreeMallotus wrayi, King (Euphorbiaceae)

The mechanical development of the anchorage system of the taprooted tropical speciesMallotus wrayiKing (Euphorbiaceae) wasinvestigated by pulling over and examining trees with a diameterat breast height (d_bh) of 4.2 cm to 14.3 cm. The mode of mechanicalfailure depended upon the size of the tree: thicker trees (d_bhapprox.9 cm) failed in the ground with their tap roots pushing intothe soil on the winchward side; in smaller trees (d_bhapprox.7 cm) the trunk snapped before anchorage failure; and in verysmall trees (of d_bh<6 cm) neither type of failure occurredand the trees returned to their original upright position undamagedafter the test. The anchorage strength of the trees was correlatedwith the second power of trunk diameter rather than with thethird power that theory suggests is optimal because tap rootsdid not show an isometric increase in length or diameter. Thereforeas trees grow larger the ‘factor of safety’ againstanchorage failure falls, making them prone to fail in theirroots. These results suggest that only relatively small treespecies can rely solely on the tap root to prevent uprooting.It may be for this reason that most larger trees develop thicklateral roots.Copyright 1998 Annals of Botany Company Anchorage, tap roots, scaling,Mallotus wrayi, isometric growth, functional development, windthrow, root systems.     

Root architecture and tree stability

Summary Root anchorage is discussed with a view to determining the optimum use of root material for enhanced stability. Field observations were made on Sitka spruce root systems while lateral forces were applied to the stem with a winch to pull the tree over. Measurements included the applied force, angles of inclination, soil and root movement, timing of the sound of root breakage using buried microphones, weight and shape of the root-soil plate and damage to the roots.Components of anchorage include the dimensions and mass of the root-soil plate levered from the ground by the displaced stem, and tensile strength of roots and soil beneath the plate; root and soil tensile strength and root/soil resistance on the windward perimeter; and on the lee side the stiffness of the hinge at the fulcrum.Strength properties of roots and soil are reviewed. Models devised for landslip are extended to consider behaviour under tension, of roots singly and in groups, and the concept is developed of a critical rooting density at which root/soil resistance exceeds soil strength, giving rise to the characteric root-soil plate on uprooted trees. The lee side part of the root-soil plate acts as a cantilevered beam and determines the distance of the fulcrum from the tree. Physical laws defining the reduced stiffness of beams as a result of subdivision, indicate the importance of the number/size distribution of roots and weakening effects of branching.On the windward side upward movement of the root-soil plate causes sequential breakage of soil and roots. Under an increasing applied load, failure occurs in parts of the soil-root system before the maximum force for uprooting is achieved. A preliminary approach is made to modelling where the changing contributions of the components of anchorage are allowed for throughout the uprooting process.     

The Mechanics of Anchorage in Wheat Triticum aestivum L.: II. ANCHORAGE OF MATURE WHEAT AGAINST LODGING

The root system of mature wheat Triticum aestivum Marts Doveis dominated by the 7 to 15 adventitious roots which emergefrom the perimeter of the stem base, pointing radially outwardsand downwards. The basal, coronal region of these roots is thickand unbranched, attached to a rhizosheath of earth by a densecovering of root hairs and stiffened in bending by lignificationof outer layers of the cortex. Root lodging of plants involves bending of the coronal rootsat their base and axial movement of leeward and windward rootsthrough the soil; their resistance to these motions providemoments resisting lodging. A model of anchorage was producedby summing the resistance of each root to both forms of motionto give two anchorage components. The model was tested in aseries of mechanical experiments in which simulated lodgingwas followed by loading of individual roots; results supportedthe anchorage model and suggested that in the experimental conditionsthe two components of anchorage were approximately equal inmagnitude. The stem was about 30% stronger than the anchoragesystem. The coronal anchorage roots made up 4.4% of total dry mass;it is suggested that anchorage could be improved either by increasinginvestment in this region or by altering root orientation. Sequentialdevelopment of seminal and adventitious root systems is relatedto the changes in anchorage requirement with age.     

Understanding the impact of root morphology on overturning mechanisms: a modelling approach

BACKGROUND AND AIMS: The Finite Element Method (FEM) has been used in recent years to simulate overturning processes in trees. This study aimed at using FEM to determine the role of individual roots in tree anchorage with regard to different rooting patterns, and to estimate stress distribution in the soil and roots during overturning. METHODS: The FEM was used to carry out 2-D simulations of tree uprooting in saturated soft clay and loamy sand-like soil. The anchorage model consisted of a root system embedded in a soil block. Two root patterns were used and individual roots removed to determine their contribution to anchorage. KEY RESULTS: In clay-like soil the size of the root-soil plate formed during overturning was defined by the longest roots. Consequently, all other roots localized within this plate had no influence on anchorage strength. In sand-like soil, removing individual root elements altered anchorage resistance. This result was due to a modification of the shape and size of the root-soil plate, as well as the location of the rotation axis. The tap root and deeper roots had more influence on overturning resistance in sand-like soil compared with clay-like soil. Mechanical stresses were higher in the most superficial roots and also in leeward roots in sand-like soil. The relative difference in stresses between the upper and lower sides of lateral roots was sensitive to root insertion angle. Assuming that root eccentricity is a response to mechanical stresses, these results explain why eccentricity differs depending on root architecture. CONCLUSIONS: A simple 2-D Finite Element model was developed to better understand the mechanisms involved during tree overturning. It has been shown how root system morphology and soil mechanical properties can modify the shape of the root plate slip surface as well as the position of the rotation axis, which are major components of tree anchorage.     

The role of root system architecture and root hairs in promoting anchorage against uprooting forces in Allium cepa and root mutants of Arabidopsis thaliana.

The role played by lateral roots and root hairs in promoting plant anchorage, and specifically resistance to vertical uprooting forces has been determined experimentally. Two species were studied, Allium cepa (onion) which has a particularly simple root system and two mutants of Arabidopsis thaliana, one without root hairs (rhd 2-1) and another with reduced lateral root branching (axr 4-2). Maximum strength of individual onion roots within a plant increased with plant age. In uprooting tests on onion seedlings, resistance to uprooting could be resolved into a series of events associated with the breakage of individual roots. Peak pulling resistance was explained in a regression model by a combination of a measure of plant size and the extent to which the uprooting resistance of individual roots was additive. This additive effect is termed root co-operation. A simple model is presented to demonstrate the role played by root co-operation in uprooting resistance. In similar uprooting tests on Arabidopsis thaliana, the mutant axr 4-2, with very restricted lateral development, showed a 14% reduction in peak pulling resistance when compared with the wild-type plants of similar shoot dry weight. The uprooting force trace of axr 4-2 was different to that of the wild type, and the main axis was a more significant contributor to anchorage than in the wild type. By contrast, the root hair-deficient mutant rhd 2-1 showed no difference in peak pulling resistance compared with the wild type, suggesting that root hairs do not normally play a role in uprooting resistance. The results show that lateral roots play an important role in anchorage, and that co-operation between roots may be the most significant factor.     

Model and whole-plant studies on the anchorage capabilities of bulbs

Though the resistance to uprooting of cylindrical roots and root systems has been extensively investigated, almost no research has been performed on the factors that influence the uprooting resistance of bulbs. However, engineers have modelled bulb-like foundations and have investigated their resistance to upward movements. This study combined engineering theory with practical biology, using model bulbs of different shapes and sizes, embedding them at different depths in different soil media, and pulling them out while recording the uprooting force. Uprooting resistances of the models was compared to those of real onion and garlic bulbs with and without their root systems. Cone shaped models resisted uprooting best at all embedment depths and in both soil types, always followed by bulb shaped and cylindrical models. These results are explicable in terms of engineering theory. Cones resisted uprooting best because their maximum diameter is embedded deepest. A bulb shape is an ideal compromise as it has no sharp edges, and also allows easy downward movement. In sand uprooting resistance increased faster with depth than with bulb diameter, whereas in agricultural soils, the uprooting force was proportional both to the depth and the diameter of the model. The tests on the plants showed that real bulbs anchor plants by similar mechanisms and amounts to the models. The bulbs accounted for between 15% and 50% of the uprooting resistance of the plant, so they can make an important contribution to anchorage, particularly towards the end of the season.     

The Anchorage Mechanics of Maize, Zea mays

The anchorage system of mature maize Zea mays was investigatedby combining morphological and anatomical study of the rootsystem with mechanical tests on roots and with studies in whichplants were pulled over. The root system is dominated by 20–30adventitious roots which emerge in rings from the stem basepointing radially downwards and outwards, approximately 30°from the vertical. Roots are strengthened near their base bya heavily lignified exodermis which makes them rigid in bending;distally, strength and rigidity both decrease because rootsbecome thinner and less lignified. When plants were pulled over,a maximum anchorage moment of 5–20 Nm was mobilized atangles of 8–10°, larger plants having stronger anchorage.Movement was initially centred on the leeward side of the stem,anchorage being due to the resistance of both windward and leewardroots to axial motion through the soil and to bending. At displacementsover 10°, however, leeward roots buckled under combinedbending and compression and the centre of rotation shifted tothe windward perimeter of the root system; subsequent movementof the cone of roots and soil was resisted only by the bearingstrength of the soil beneath it. The differences between anchorage failure in balsam and sunflowersand that in maize probably results from the lower angular spreadand the weakness in compression of the maize roots which preventsthe leeward side of the root system from bearing large downwardloads. The system behaves more like that of wheat; these resultssuggest that the lodging resistance of both plants may be improvedby increasing the bending strength and angle of spread of theadventitious roots. Key words: Zea mays, roots, anchorage     

Anchorage Mechanics of the Tap Root System of Winter-sown Oilseed Rape (Brassica napus L.)

The anchorage mechanics of mature winter-sown oilseed rape (‘Envol’)were investigated by combining a morphological and mechanicalstudy of the root system with anchorage tests on real and modelplants. Oilseed rape plants were anchored by a rigid tap root;the few laterals all emerged below the centre of rotation ofthe root system (approx. 30 mm below the soil surface). Whenplants were pulled over, the tap root bent and the top 30 mmmoved in the soil towards the direction of pull, creating acrevice on the opposite side. The maximum anchorage moment was2.9 ± 0.36 N m. Two main components of anchorage wereidentified: the bending resistance of the tap root and the resistanceof the soil on the near side to compression. The relative importanceof these components was determined by measuring both the bendingresistance of the tap root, and the resistance of metal tubesof varying diameter, inserted to various depths in the soil,to being pulled over. These tests showed that the tap root bendingmoment at failure could account for around 40% of anchoragemoment, while soil resistance could account for around 60%.The model tests on the tubes also help to shed light on theway in which the dimensions of tap roots will influence theiranchorage capability. Copyright 2001 Annals of Botany Company Anchorage, lodging, root bending resistance, mechanical properties, oilseed rape, Brassica napus L     

A Comparative Study of the Anchorage Systems of Himalayan Balsam Impatiens glandulifera and Mature Sunflower Helianthus annuus

The anchorage systems of Himalayan balsam Impatiens glanduliferaand mature sunflowers Helianthus annuus were investigated bycombining morphological and anatomical study of the root systemswith mechanical tests on roots and with studies in which matureplants were pulled over. The root system of balsam is dominated by large numbers of fleshytapering adventitious roots which point downwards from theirorigin at the wide stem base. Sunflowers, in contrast, havea tapering tap-root from which 20–30 well-branched lateralsemerge, pointing radially outwards and downwards. Roots of eachspecies have contrasting anatomy: those of balsam resemble stems,having a central watery pith and being strengthened peripherallyby lignification of vascular tissue; roots of sunflowers arestrengthened by a solid woody stele. Roots of both species arerigid in tension and, towards the base, in bending. Both species exhibited similar behaviour to that known for treessuch as Sitka spruce; when pulled over they rotated about ahinge leeward of the stem base and a root-soil ball was pulledout of the surrounding soil. Anchorage was resolved into threecomponents which, in order of decreasing magnitude, were (i)the resistance to pulling of the roots on the windward sideof the plant (and, for sunflower, the tap-root); (ii) the resistanceof roots and soil at the leeward hinge to rotation; and (iii)the weight of the root-soil ball. Sunflower had stronger anchoragebut achieved it at a greater cost in terms of the dry mass ofits root system. In each species, the morphology, anatomy and mechanical propertiesof the root system can be related to those of the stem. Thewide stem base of balsam allows large numbers of mechanicallyefficient fleshy roots to be attached whereas in sunflowersa woody tap-root system is necessary to anchor the much narrowerstem. Key words: Impatiens, Helianthus, roots, anchorage     

The effect of root architecture and root loss through trenching on the anchorage of tropical urban trees (Eugenia grandis Wight)

Eugenia grandis (Wight) is grown in urban environments throughout Malaysia and root systems are often damaged through trenching for the laying down of roads and utilities. We investigated the effect of root cutting through trenching on the biomechanics of mature E. grandis. The force necessary to winch trees 0.2 m from the vertical was measured. Trenches were then dug at different distances (1.5, 1.0 and 0.5 m) from the trunk on the tension side of groups of trees. Each tree was winched sideways again and the uprooting force recorded. No trenches were made in a control group of trees which were winched until failure occurred. Critical turning moment (TM_crit) and tree anchorage rotational stiffness (TARS) before and after trenching were calculated. Root systems were extracted for architectural analysis and relationships between architectural parameters and TM_crit and TARS were investigated. No differences were found between TM_crit and trenching distance. However, in control trees and trees with roots cut at 1.5 m, significant relationships did exist between both TM_crit and TARS with stem dimensions, rooting depth and root plate size. TARS was significantly decreased when roots were cut at 0.5 m only. Surprisingly, no relationships existed between TM_crit and TARS with any root system parameter when trenching was carried out at 0.5 or 1.0 m. Our study showed that in terms of TARS and TM_crit, mechanical stability was not greatly affected by trenching, probably because rooting depth close to the trunk was a major component of anchorage.     

Linking plant morphological traits to uprooting resistance in eroded marly lands (Southern Alps, France)

In marly catchments of the French Southern Alps, soils are subjected to harsh water erosion that can result in concentrated flows uprooting small plants. Evaluating and predicting plant resistance to uprooting from simple plant traits is therefore highly important so that the most efficient plant strategy for future restoration of eroded slopes can be defined. Twelve species growing on marly land were studied. For each species, in-situ lateral uprooting tests were conducted and morphological plant traits were measured on small plants at the early stages of their development. The results show that maximum uprooting force was most positively correlated with stem basal diameter. Resistance to uprooting depends on a combination of several traits. Tap root length, the proportion of fine lateral roots and root topology were the best predictors of anchorage strength.     

The Effects of Soil Bulk Density on the Morphology and Anchorage Mechanics of the Root Systems of Sunflower and Maize

The effects of soil bulk density and hence strength on two contrastingspecies of herbaceous annuals, the dicot sunflower (HelianthusannuusL.) and the monocot maize (Zea maysL.), were investigatedby comparing the morphology and mechanics of field-grown plantsin soil with a low and high bulk density. Soil with a low bulkdensity had a significantly lower penetration resistance (118±4.4kPa) than the high bulk density soil (325±12.2 kPa;P<0.0001).Soil strength affected shoot and root systems of both speciesbut had no significant effect on shoot height. In both speciesroots were thicker closer to the stem base in strong soil comparedto those in weaker soil. Sunflower tap-roots growing in strongsoil tapered more rapidly than those in weak soil. Only in maize,however, were roots growing in weak soil stiffer than thosein strong soil. Despite only small absolute differences in thepenetration resistance of the soil both species growing in strongsoil had greater anchorage strength than those in weak soil.As a consequence more plants in weak soil lodged compared withthose growing in strong soil. This study shows that plants can,to a small extent, respond to changes in soil strength, butthat changes do not appear to compensate fully for alterationsin soil conditions. Furthermore it may be possible, by manipulatingsoil strength, to control lodging.Copyright 1999 Annals of BotanyCompany Roots, compaction, soil strength, anchorage mechanics, bulk density, thigmomorphogenesis, lodging,Helianthus annuusL.,Zea maysL.     

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