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     

Responses of the Root Systems of Sunflower and Maize to Unidirectional Stem Flexure

Plants of two contrasting species of herbaceous annuals, thedicot sunflower (Helianthus annuusL.) and the monocot maize(Zea maysL.), grown in the glasshouse were subjected to regularunidirectional stem flexure. Differences in morphology and mechanicalproperties of roots and shoots were then investigated. Rootsystems were divided into quadrants around the axis of stimulationand differences in root morphology and mechanics between thezones were investigated. There were considerable differencesbetween roots in the leeward and windward zones compared withroots perpendicular to the axis of stimulation. First-orderlateral roots in both species were thicker, more rigid and morenumerous. These results suggest that plant roots respond locallyto mechanical stimulation. There were, however, also differencesin the responses of the two species. In sunflower, the tap rootand stem base became elliptical in cross section with the majoraxis lying in the plane of stimulation. The lateral roots offlexed sunflowers in both the leeward and windward zones showedsimilar growth responses: roots were thicker, more numerousand weighed more than those in the perpendicular zones. However,only leeward roots showed significant differences in their mechanicalproperties; roots were more rigid, stronger and stiffer. Incontrast, the leeward roots of maize were thicker and more numerous,with a greater biomass than the windward roots. However, onlyroots in the windward zone were stiffer than those in the perpendicularzone. These differences between sunflower and maize are relatedto their contrasting anchorage mechanics.Copyright 1998 Annalsof Botany Company Anchorage, biomechanics, adaptive growth, roots, thigmomorphogenesis,Helianthus annuusL.,Zea maysL.     

Germination and Early Growth of Plants Studied Using Neutron Radiography

Seed swelling, germination, root extension, lateral root initiationand shoot growth were studied in soils of different water contents,using non-destructive, serial neutron radiography. Seeds fromthree varieties of soya beans (Glycine max L.) and one varietyeach of maize (Zea mays L.) and vetch (Vicia sativa L.) wereused. The seeds germinated when they had increased in size bya certain amount, if germination is taken as the time when theradicle first appears. The rate at which roots and shoots extendalso depend on soil water content. Glycine max L., Vicia sativa L., Zea mays L., Soya bean vetch, maize, seed germination, root extension, lateral root initiation, neutron radiography     

Wetting and drying cycles in the maize rhizosphere under controlled conditions. Mechanics of the root-adhering soil

Mechanical properties of the topsoil (sandy Podsol and silty Luvisol, FAO) adhering to maize (Zea mays L.) roots and its bulk soil counterpart were studied as a function of soil texture and final soil water suction at harvest, with three soil water suction values of approximately 30, 50 and 60 kPa. Two scales of observation were also selected: the whole soil:root system and the root-adhering soil aggregates. Three methods were used to characterize the stability of the soil:root system: mechanical shaking in air, and dispersion by low-power ultrasonication, with or without preliminary immersion of the soil:root system in water. Soil disruption kinetics, which were fitted with first-order kinetics equations, were analyzed and discussed. For example, silty soil ultrasonication kinetics, without preliminary water-immersion, could be divided into two parts: the first faster part, which was characterized by a mean rate K value of 6.8–7.2 mJ^-1, is attributed to soil slaking, whereas the second slower part, which was characterized by a mean rate K value of 1.5–1.6 mJ^-1, was attributed to the rupture of the `firmly root-adhering soil' from the roots. A clear plant effect was observed for both aggregate tensile strength and friability, with higher aggregate strength for the root-adhering silty soil (450–500 kPa) than for its bulk silty soil counterpart (410–420 kPa), and lower friability (coefficient of variation of the aggregate strength) for the root-adhering silty soil (e.g. 67% at a soil water suction value of 30 kPa) than for its bulk silty soil counterpart (e.g. 49% at asoil water suction value of 30 kPa). These effects were attributed to root exudation, which was significantly higher for the driest silty topsoil than for the wetter ones. In conclusion, the mechanical properties of the silty topsoil adhering to the maize roots are attributed to both physical and biological interactions occurring in the maize rhizosphere. This revised version was published online in June 2006 with corrections to the Cover Date.     

Development of Golgi Apparatus in the Root Cap Cells of Maize (Zea mays L.) as Affected by Compacted Soil

Effects of soil mechanical impedance on the development of Golgiapparatus in the root cap cells of maize were studied undercontrolled soil-water conditions Heavily compacted soil (bulkdensity = 1.50 g cm^–2) had 3.3 to 3.4 times greater mechanicalimpedance than control soil (bulk density = 1.33 g cm^–3),but their oxygen diffusion rates were not significantly differentThe number of dictyosomes and the number and area of secretoryvesicles per unit area of tangentially sub-peripheral root capcells in the heavily compacted soil increased compared to thosein the control These results suggest that secretory activityof the root cap cells is promoted by soil mechanical impedance Dictyosome, Golgi apparatus, maize, mucilage, root cap, secretory activity, secretory vesicles, soil mechanical impedance, Zea mays L     

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     

The spontaneous growth response of maize coleoptile segments and the change in tissue 8

Decapitated segments from maize (Zea mays L.) coleoptiles orientedvertically in an upright position show a strong spontaneousgrowth response (SGR) 3 h after decapitation. The latent periodof the SGR is markedly reduced when these segments are orientedin an inverted position. Coleoptile segments with intact tipsexhibit a weak and transient SGR in the vertical upright orientation.However, in the inverted orientation, these segments show atypical SGR. The data are inconsistent with the current hypothesisthat the SGR is caused by a time-dependent increase in tissuesensitivity to auxin. The parallel increase in membrane potentialdifference and growth rate during the time-course of the SGRindicates a possible role for PM H⁺-ATPase in the establishmentof the SGR in maize coleoptile segments. Key words: Auxin, spontaneous growth response, membrane potential, plasma membrane H⁺-ATPase, Zea mays L.     

The Responses of Field-grown Sunflower and Maize to Mechanical Support

The effects of mechanical support on two contrasting speciesof herbaceous annual, the dicot sunflower (Helianthus annuusL.) and the monocot maize (Zea mays L.), were investigated bycomparing the growth and mechanical properties of supportedplants and those which were left to sway freely in the wind. Providing support had its greatest effect on the more highly-stressedbasal areas of the plants, such as the lower stem and the baseof the lateral roots. The diameter of the stem bases of bothspecies was approx. 10% lower in supported plants, but therewas no difference between treatments in the diameter of thestem above 50 cm. Roots of both species also showed a reductionin rigidity and bending strength of 40–50% in the supportedplants compared with freely swaying plants. There was a significantreduction in the partitioning of biomass to the root systemsof supported plants of both species. There were differences in the way in which sunflower and maizeresponded to the provision of support; in sunflower, the reductionin lateral diameter was about twice that in maize, whereas inmaize the decrease in the number of first-order laterals wastwice that of sunflower. This study suggests that thigmomorphogenesismay be a localized response, but that different species canrespond in different ways to mechanical stimulation. Wind; support; anchorage; thigmomorphogenesis; Helianthus annuus L.; sunflower; Zea mays L.; maize     

Effect of Root and Shoot Meristem Temperature on Shoot to Root Dry Matter Partitioning and the Internal Concentrations of Nitrogen and Carbohydrates in Maize and Wheat

Maize (Zea mays L.) and spring wheat (Triticum aestivum L.)were grown in nutrient solution at uniformly high air temperature(20 °C), but different root zone temperatures (RZT 20, 16,12 °C). To manipulate the ratio of shoot activity to rootactivity, the plants were grown with their shoot base includingthe apical meristem either above (i.e. at 20 °C) or withinthe nutrient solution (i.e. at 20, 16 or 12 °C). In wheat, the ratio of shoot:root dry matter partitioning decreasedat low RZT, whereas the opposite was true for maize. In bothspecies, dry matter partitioning to the shoot was one-sidedlyincreased when the shoot base temperature, and thus shoot activity,were increased at low RZT. The concentrations of non-structuralcarbohydrates (NSC) in the shoots and roots were higher at lowin comparison to high RZT in both species, irrespective of theshoot base temperature. The concentrations of nitrogen (N) inthe shoot and root fresh matter also increased at low RZT withthe exception of maize grown at 12 °C RZT and 20 °Cshoot base temperature. The ratio of NSC:N was increased inboth species at low RZT. However this ratio was negatively correlatedwith the ratio of shoot:root dry matter partitioning in wheat,but positively correlated in maize. It is suggested that dry matter partitioning between shoot androots at low RZT is not causally related to the internal nitrogenor carbohydrate status of the plants. Furthermore, balancedactivity between shoot and roots is maintained by adaptationsin specific shoot and root activity, rather than by an alteredratio of biomass allocation between shoot and roots.Copyright1994, 1999 Academic Press Wheat, Triticum aestivum, maize, Zea mays, root temperature, shoot meristem temperature, biomass allocation, shoot:root ratio, carbohydrate status, nitrogen status, functional equilibrium     

Root and Shoot Growth of Plants Treated with Abscisic Acid

Young seedlings of Capsicum annum L., Commelina communis L.and maize (Zea mays L.) were subjected to a mild water-stressingtreatment and/or treated with abscisic acid (ABA). Plants rootedin soil received a soil-drying treatment and their leaves weresprayed with a 10–⁴ M solution of ABA. Plants grown insolution culture were stressed by the addition of polyethyleneglycol (PEG) to the rooting medium and ABA was also added tothe rooting medium, either with or without PEG. The effectsof both treatments on the growth of roots and shoots and theultimate root: shoot dry weight ratio were very similar. Shootgrowth was limited both by water stress and by ABA application;while there was some evidence that mild water stress and/orABA application may have resulted in a stimulation of root growth.More severe water stress reduced the growth of roots but theoverall effect of stress was to increase the ratio of rootsto shoots. Capsicum annum L., Commelina communis L., Zea mays L., water stress, abscisic acid     

Influence of soil edaphic factors and their critical limits on seedling emergence of corn (Zea mays L.)

Summary In the present investigation the influence of various soil physical parameters on seeding emergence of corn (Zea mays L.) and their critical limits was studied in order to obtain a proper crop stand. The effect of soil compaction, resulting into various bulk densities on some soil physical properties was also studied. Seedling emergence decreased with increasing bulk density, soil suction, seed placement depth, soil strength and with decreasing oxygen diffusion rates. The critical values for these soil physical parameters were found to be 1.4 g/cm³, 3 atm., 4 cm, 1.0 kg/cm² and 40×10^-8 g O₂/cm²/min bulk density, soil suction, seed placement depth, soil strength and oxygen diffusion rates respectively. Increasing bulk density decreased the soil water content, oxygen diffusion rates and increased the soil strength. re]19751117 Soil Science Department     

Somatic Embryogenesis and Plant Regeneration from Cultured Leaf Explants of Zea mays

Young leaf segments of Zea mays L. seedlings were cultured onMurashige and Skoog's basal nutrient medium supplemented with2 mg l^–1 2, 4-D and sub-cultured on medium containing8 mg l^–1 2,4-D. Two types of callus tissues appeared—embryogenicand non-embryogenic. The embryogenic callus tissue producednumerous somatic embryos which on transfer to media containinglow amounts of 2,4-D or ABA produced plantlets. Callus tissuesexhibited embryogenic potential for more than 1 year. Zea mays L. cv. Ageti-76, Zea mays L. cv. N-L-D-Comp., maize, leaf, callus, somatic embryogenesis, regeneration     

Plant Growth Analysis: Growth and Yield Component Responses to Population Density in Forage Maize

Forage maize (Zea mays L.) was grown in monocultures at populationdensities ranging from 4·9 to 11·1 plants m^–2.Data for plant growth analysis were obtained from six harvestscarried out from 21 to 115 d after planting. Conventional plantgrowth analysis indicated that improvements in forage productivityper unit land area by high population density resulted directlyfrom increased plant presence. Reduction in dry weight per shootat high population density was associated with reduced unitleaf rate. Leaf area ratio was little affected, which may implythat competition for soil nutrients or oxygen was the chiefcause of plant interference. Yield component analysis demonstratedthe increasing importance of population density treatments asa source of variation as growth progressed. Direct relationshipsbetween variation in yield per plant and variation in two yieldcomponents, stem diameter and the inverse of leaf area ratio,were demonstrated. Both conventional plant growth analysis andyield component analysis indicated complex physiological andmorphological adjustments to species population density. Plant growth analysis, yield component analysis, Zea mays L     

Does ABA in the Xylem Control the Rate of Leaf Growth in Soil-Dried Maize and Sunflower Plants?

Maize (Zea mays L.) and sunflower (Helianthus annuus L.) plantswere grown in large volumes of soil and leaf growth rate wasmonitored on a daily basis. Half the plants were given a soildrying treatment and when they showed a significant restrictionof growth rate (compared to both their daily growth rate beforedrying and the average growth rate of well-watered plants onthe same day), leaf water relations were measured and xylemsap was extracted using several techniques. There was a significant negative log-linear relationship betweenthe rate of leaf growth and the concentration of ABA in thexylem for both species. There was no clear relationship betweenleaf growth rate and leaf water potential or turgor for eitherspecies. Assessment of different methods for sampling xylemsap suggests that exudates collected from stem stumps or samplescollected by pressurizing the whole root system are suitablefor estimating ABA concentration in xylem, at least with largeplants of maize or sunflower, provided the first few hundredcubic millimetres of collected sap are used for the assay. Centrifugationof sections of stems resulted in dilution of ABA in the xylemsap with sap squeezed from parenchyma tissue. This is because,at least in plants subjected to mild soil drying, the concentrationof the ABA in the xylem is far higher than that in the cellsap of stem tissue. Results support the proposal that ABA plays a major role asa chemical signal involved in the root-to-shoot communicationof the effects of soil drying. The non-hydraulic restrictionof leaf growth by a chemical signal can be explained by theextra root-sourced ABA in the xylem and may be an importantcomponent of the modification of growth and development whichresults from prolonged soil drought. Key words: Soil drying, ABA, leaf growth, Zea mays L., Helianthus annuus L.     

Conocarpus biochar as a soil amendment for reducing heavy metal availability and uptake by maize plants

The objective of this study was to assess the use of Concarpus biochar as a soil amendment for reducing heavy metal accessibility and uptake by maize plants (Zea mays L.). The impacts of biochar rates (0.0, 1.0, 3.0, and 5.0% w/w) and two soil moisture levels (75% and 100% of field capacity, FC) on immobilization and availability of Fe, Mn, Zn, Cd, Cu and Pb to maize plants as well as its application effects on soil pH, EC, bulk density, and moisture content were evaluated using heavy metal-contaminated soil collected from mining area. The biochar addition significantly decreased the bulk density and increased moisture content of soil. Applying biochar significantly reduced NH₄OAc- or AB-DTPA-extractable heavy metal concentrations of soils, indicating metal immobilization. Conocarpus biochar increased shoot dry biomass of maize plants by 54.5–102% at 75% FC and 133–266% at 100% FC. Moreover, applying biochar significantly reduced shoot heavy metal concentrations in maize plants (except for Fe at 75% FC) in response to increasing application rates, with a highest decrease of 51.3% and 60.5% for Mn, 28% and 21.2% for Zn, 60% and 29.5% for Cu, 53.2% and 47.2% for Cd at soil moisture levels of 75% FC and 100% FC, respectively. The results suggest that biochar may be effectively used as a soil amendment for heavy metal immobilization and in reducing its phytotoxicity.     

Dynamics of Architectural Development of Isolated Plants of Maize (Zea mays L.), in a Non-limiting Environment: The Branching Potential of Modern Maize

The maximal architectural development of a modern maize cultivar(Zea mays L. ‘Dea’) was studied in fully isolatedand carefully irrigated plants. Under these favourable conditions,this (usually) non-tillering and non-prolific cultivar displayeda large amount of branching (down to the third order), includinglarge basal tillers and prolific ear shoots. This developmentwas analysed by combining: (1) architectural analysis, initiallydeveloped for trees; and (2) quantitative analysis of tilleringkinetics, designed for other grass species. The architecturalunit of maize included a main long axis (A1) and lateral shortshoots bearing a terminal spike (A2). The basal tillers (notedA1') resulted from a complete and sylleptic reiteration. Itskinetics were consistent with tillering models, but with a particularlylong delay in branch emergence (seven phyllochrons), comparedto other grass species. This delay is likely to enhance regulationby leaf (or root) density in stands and explains the inhibitionof branch development, as usually observed in field conditions,even at low density. Similarly, the suppression of the basalreiteration in secondary branches observed in isolated plantsprobably results from increased intra-tussock density. In isolatedplants, androgenous axes combining A1' and A2 morphologies werealso produced in intermediate positions. It is shown that theycan result from a basipetal sequence of A2 differentiation reachingbuds in the course of their A1' development. The consequencesof these unexpected results are discussed in terms of maizedevelopment and architectural analysis of grasses. Copyright1999 Annals of Botany Company Plant architecture, development, maize (Zea mays L.), tillering, branching, shoot, prolificacy, phyllochron.     

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     

Osmotic Adjustment and Stomatal Response to Water Deficits in Maize

A pot experiment was carried out using five maize {Zea maysL.) cultivars under three soil moisture levels (MPa 0 to –0.05,–0.3 to –0.9 and –1.2 to –1.5) to investigatethe effects of water deficits on osmotic adjustment and stomatalconductance. The degree of leaf rolling and the sugar and nutrientconcentrations in leaf cell sap were measured. Leaf water potential and osmotic potential decreased and stomatalconductance decreased with increasing water deficits. Stomatalconductance correlated positively with leaf water potentialand osmotic potential. Degree of leaf rolling was lower in cultivarswhich maintained higher turgor. Osmotic adjustment of 0.08 to0.43 MPa was found under the lowest soil moisture level in fivecultivars used. Sugar and K were the major osmotic substancesin the maize plant. Sugar, K and Mg concentrations increasedunder water deficit, and correlated negatively with a decreasein osmotic potential. Key words: Zea mays L., leaf water relations, leaf rolling, osmotic adjustment, stomatal conductance, water deficit     

Ammonium Tolerance and Carbohydrate Status in Maize Cultivars

Four maize (Zea mays L.) hybrids were grown hydroponically for4 weeks with 20 mM ammonium or nitrate as the sole nitrogensource. Dry matter production was strongly depressed by ammoniumnutrition in the hybrid Helga relative to plants grown on nitrate,and moderately decreased in the hybrid Melina. Ammonium hadno inhibitory effect on total yield in the other two hybrids(Ramses and DK 261). The relative growth rate (RGR) of rootsand shoots of the sensitive hybrid Helga decreased significantlyunder ammonium nutrition during the first 2 weeks of the experiment,while at the end of the experiment nitrogen form had no effecton the RGR in any of the four hybrids. The strong reductionin RGR of Helga in the early seedling stage was correlated withthe accumulation of twice the concentration of free ammoniumin the shoot tissue relative to the other hybrids. Helga wastherefore unable to sufficiently detoxify ammonia in the roots.Root concentrations of water soluble carbohydrates (WSC) inHelga and Melina in the early seedling stage did not differunder ammonium and nitrate nutrition. In contrast, Ramses andDK 261 both had elevated WSC concentrations in ammonium-fedroots. It is hypothesized that a sufficient supply of carbonskeletons for ammonium assimilation in the roots is requiredfor maximum growth under high ammonium concentrations, and thatthere is genotypic variability in this physiological trait. Ammonium; carbohydrates; growth rate; maize; nitrate; roots; Zea mays L     

Turgor Differences and Water Stress in Maize and Sorghum Leaves During Drought and Recovery

The pressure potentials (turgor pressure) in leaves of maize(Zea mays L.) and grain sorghum (Sorghum vulgare Pers.) plantssubjected to water stress in a controlled environment were estimatedfrom measurements of water and osmotic potentials. Changes inturgor pressure were larger in sorghum than in maize duringthe development of water stress and after re-watering. It issuggested that this indicates a lower cell wall elasticity insorghum than in maize. This fact may affect some of the physiologicalactivities of sorghum