The effect of lateral illumination on growth oscillations of pine seedling hypocotyls (Pinus silvestris L.)

Phototropic sensitivity of forest wood seedlings to lateral illumination was proposed as an early assay for distinquishing various ecotypes of woody plants of the same species. Statistical analysis showed that results were significantly influenced by heterogenity of experimental material caused by an interference of phototropic movements and natural oscillations of hypocotyl. Both movements of pine seedlings (Pinus silvestris L.) were registered by phase photography and their mechanism was analyzed. The apical part of growing hypocotyl illuminated from above oscillates in a space spiral with frequency 3.3 h at mean growth rate 0.66 mm h^?1. The mean size of spiral amplitudes is 2.9 mm. The oscillation rhythm is disturbed after the lateral illumination and a phase shift was observed as a result. A new oscillation rhythm with frequency 3.9 h and mean growth rate 0.69 mm h^?1 was stabilized after a period of time equal to one nutation turn. Oscillation amplitudes were increased to 4.3 mm. In addition to the radiation intensity the appearance of the phototropic response to light pulse is first of all effected by the actual position of the apical part of hypocotyl in the course of endogenous circumnutations. A uniform plant material for the early assay may be obtained by selection of seedlings which are at the beginning of lateral illumination in the same phase of nutation rhythm. Under such conditions the deviations of longitudinal axis of oscillating spirals characterize the actual phototropic curvature.     

Elongation and circumnutation oscillations of hypocotyl of pine seedlings (Pinus silvestris L.)

The elongation of pine seedlings (Pinus silvestris L.) is associated with nutation movements. Trajectories of these growth oscillations were recorded by film technique in horizontal and vertical projection during a three day period of growth. On the basis of these data the parameters of elongation and nutation oscillations,i.e. rate, amplitudes and frequency of oscillations, were calculated and their changes during plant development compared. Oscillation trajectories are circular or elliptic spirals the amplitudes of which are increasing with the age of hypocotyl from 1.5 mm to 7.5 mm. The frequencies of nutations are decreasing during the growth from 0.5 to 0.2 rev. h^?1. On the other hand, the growth rate of hypocotyl increased from values near to 10^?3 mm h^?1 at the beginning of the experiment to 4×10^?1 mm h^?1 recorded at the end of the third day. The zone of nutation curvature was slightly transferred from the middle of the hypocotyl toward the apex and its location has not been identical with that of elongation. This indicates that the system controlling nutation oscillations need not be identical with that controlling direction of elongation. At a certain stage of development behaviour of the decapitated pine hypocotyl is analogical to that of the root without the centre of georeception. A possibility of analogy of the system controlling direction of hypocotyl growth and of the system proposed for geotropical control of root growth is discussed.     

Diurnal oscillatory movements of growing leaves of tobacco

The analysis of diurnal oscillatory movements of tobacco leaves was used in the diagnosis of viral infection of plants. The oscillatory helices circumscribed by a growing leaf of a healthy plant were regular, but some deviations, particularly in the transition points, were recorded. In order to make clear the cause of these irregularities of trajectory, the course of elongation of leaf petiole and blade in relation to localization and shift of zones of elongation during ontogenesis was analysed. The present analysis is similar to that described by the author's earlier experiments with pea roots. Oscillatory curves circumscribed by petiole, projected on a horizontal plane, were compared with curves circumscribed by the blade tip. The analysis of the leaves of different ages enabled us to study this process in dependence on growth rate. It was confirmed that oscillations are a result of elongation; the extent of oscillations is quantitatively dependent on the growth rate. An analysis of the zones of growth showed that in petiole the active meristems are localized near to its base while in the leaf lamina they move gradually during the ontogenesis from the apical to the basal part of the leaf blade. Active meristems of young rapidly growing leaves are localized approximately in the middle of the blade while those of old leaves were found in close proximity to the base of the lamina. The growth rate of petiole can be expressed in hundreds of mm per hour (4.8×10^?2 mm h^?1); half of this value was recorded for its apical part. The growth rate of leaf blade was found approximately ten times higher (3.2×10^?1 mm h^?1). The oscillatory movements of growing leaf consists of two integrate components: of oscillations originating in the base of the petiole and of oscillations of leaf blade the centrum of which is localized in the basal third of the blade. The arrangement of the experiments did not enable us to determine to what extent the phototropic response of leaf blade participates in leaf movements. The movements of leaves of an intact plant are evidently affected by rhythmic stem oscillations. Stem is an integral part of a system which participates in the transfer of information in plants.     

Growth and nutation parameters of primary root of pedunculated oak (Quercus robur L.)

The elongation of primary roots of pedunculated oak (Quercus robur L.), which is associated with nutation oscillations, was registered by a time lapse cinematography. The following characteristics were expressed graphically: relative growth rate [mm h-¹], frequency [rev. h-¹] and amplitudes [mm] of nutation oscillations, their direction and location of nutation curvatures. The growth-oscillation mechanism is interpreted as a part of the geocontrol system of the root and was evaluated on the basis of its physiological efficiency. Comparison of nutation periods showed that the control system of pedunculated oak is three times more and two times less efficient than that of pea and maize, respectively. Comparison of the oscillation amplitudes of growing roots gives similar results.     

Relationship between Growth and Electric Oscillations in Bean Roots

Extracellular and intracellular electric potentials in bean roots are known to show electric oscillations along the longitudinal axis with a period of several minutes. The relationship between growth and the electric oscillations was studied using roots of adzuki (Phaseolus chrysanthos). We measured surface electric potentials with a multielectrode apparatus while simultaneously measuring elongation using a CCD camera and monitor. Roots having an electric oscillation grew faster than roots with no oscillation. Furthermore, elongation rate was higher in roots with higher oscillation frequency. Oscillation frequency had a strong dependence on temperature; i.e. Q₁₀ was estimated at 1.7. These results suggest a correlation between electric oscillation and elongation.     

Diurnal and ultradiurnal oscillations of growing organs within the framework of the information system of the plant

The analysis of growth and movements of seedling organs of kidney bean (Phaseolus vulgaris L.) provides a pattern of periodic phases of activity and relaxation. The existence of a central organ which would control the phase relationships, is not anticipated in the integrity of the plant. The cyclic activity of individual organs shows itself by growth associated with oscillation movements. One and the same organ may simultaneously accomplish oscillatory movements with a diurnal and ultradiurnal frequency. These rhythms originate during the organ development; the first pair of kidney bean leaves at first executes oscillation movements with a diurnal frequency and only after it is fully developed it exhibits a diurnal cycle with the photophil phase upwards and the scotophil downwards, the oscillations with an ultradiurnal oycle being maintained. The movements of the two leaves are synchronous, but there occur short sections with a desynchronous cycle. Simultaneously with these oscillations, in which the leaf petiole takes part, the adult leaf performs oscillatory movements perpendicular to the longitudinal leaf axis, the so-called side swings, controlled by periodical changes of the joint attaching the leaf blade. Their frequency is practically identical with that of the ultradiurnal cycle. Thus the periodic growth activity of the kidney bean results in growth oscillations passing in the diurnal cycle with a frequency of 0.043 rev.h^-1, their ascending and descending phases consisting of periodical ultradiurnal oscillations in cycles of 0.73–0.59 rev.h^-1. The epicotyl growth shows a similar pattern: into the basic diurnal nutation cycle with a frequency of 0.042 rev.h^-1 ultradiurnal oscillation cycles are incorporated having a similar frequency to that revealed in leaves (0.69–0.64 rev.h^-1). The diurnal oscillatory cycles belong to a system established on the basis of periodicity of day and night and other geophysical cycles. The ultradiurnal rhythmic oscillations are presumed to be an expression of the geocontrol system of root and shoot growth direction and orientation of the organ in space. The shape of their trajectories in bean leaves is contradictory to this; they are not spatial helices, as the kybernetic model would presuppose, but have a vertical, upwards and downwards course in one plane. Since these oscillatory movements with an ultradiurnal cycle cease after petiole excision from the stem and after shoot apex amputation, one may presume that they are coupled with the low-frequency oscillatory system of the epicotyl.     

Parameters of spiral oscillations as indicating the efficiency of control system of growing roots

The efficiency of the geocontrol system of pea and. maize roots was compared on the basis of their oscillation movements. The rate of elongation of maize roots is 3.6 times higher, the amplitudes of oscillation deviations are 10 times lower and their frequency is 5 times lower than the corresponding values recorded for pea roots. The period of time which elapses between the stimulus and growth response, i.e. the geotropic reaction time, is 4 times shorter in maize roots than in pea roots. The maize root is able to correct the direction of its elongation 3.5 to 5 times more frequently than the root of pea. This characterize the difference between the both compared control systems from the point of view of physiological economy.     

Suberin lamellae in the hypodermis of maize (Zea mays) roots; development and factors affecting the permeability of hypodermal layers

Abstract The development of suberin lamellae in the hypodermis of Zea mays cv. LG 11 was observed by electron microscopy and the presence of suberin inferred from autoliuorescence and by Sudan black B staining in nodal (adventitious) and primary (seminal) root axes. Suberin lamellae were evident at a distance of 30–50 mm from the tip of roots growing at 20°C and became more prominent with distance from the tip. Both oxygen deficiency and growth at 13°C produced shorter roots in which the hypodermis was suberized closer to the root tip. There were no suberin lamellae in epidermal cells or cortical collenchyma adjacent to the hypodermis. Plasmodesmata were not occluded by the suberin lamellae: there were twice as many of them in the inner tangential hypodermal wall (1,14 μn^?2) as in the junction between the epidermis and hypodermis (0.54 μm^?2). Water uptake by seminal axes (measured by micropotometry) was greater at distances more than 100 mm from the root lip than in the apical zone where the hypodermis was unsuberized. In the more mature zones of roots grown at 13°C rates of water uptake were greater than in roots grown at 20°C even though hypodermal suberization was more marked. Sleeves of epidermal/hypodermal cells (plus some accessory collenchyma) were isolated from the basal 60 mm of nodal axes by enzymatic digestion (drisclase). The roots were either kept totally immersed in culture solution or had the basal 50 mm exposed to moist air above the solution surface. In both treatments the permeabilities to tritiated water and ₈₆Rb were low (circa 10^?5mms^?1) in sleeves isolated from the extreme base. In roots grown totally immersed, however, the permeability of sleeves increased 10 to 50-fold over a distance of 40 mm. In roots exposed to moist air the permeability remained at a low level until the point where the root entered the culture solution and then increased rapidly (> 50-fold in a distance of 8 mm). Growth of roots in oxygen depleted (5% O₂) solutions promoted the development of extensive cortical aerenchymas. These developments were not associated with any reduction in permeability of sleeves isolated from the basal 40 mm of the axis. It was concluded that the presence of suberin lamellae in hypodermal walls does not necessarily indicate low permeability of cells or tissues to water or solutes. The properties of the walls (lamellae?) can be greatly changed by exposure to moist air, perhaps due to increased oxygen availability.     

Aerenchyma formation and associated oxygen movement in seminal and nodal roots of wheat

Abstract The present paper describes the effects of growth of roots of wheat (Triticum aestivum cv. Gamenya) in hypoxic nutrient solutions on acrenchyma formation and O₂ movement from shoots to roots. Two types of roots were investigated: (1) seminal roots of 4–7-d-old seedlings, and (2) seminal and nodal roots of 10–28-d-old plants. Gas-filled porosity of seminal and nodal roots increased from 3 to 12% and from 5–7 to 11–15%, respectively, when the roots emerged in stagnant or N₂-flushed solutions (0.003 mol m ^?3 O₂) compared with growth in continuously acrated solutions (0.26 mol m ^?3 O₂). However, neither root type increased in porosity when they were longer than 100–200 mm at the start of the exposure to these stagnant or N₂-flushed treatments. A vernier microscope and cylindrical platinum-electrode were used to examine the relationship between root extension and transport of O₂ from shoots to roots via the gas spaces. Measurements were made when the roots were in an anoxic medium and were dependent solely on O₂ supplied from the shoots. For seminal roots of 5–7-d-old seedlings raised in stagnant solutions (90–100 mm), internal O₂ transport was sufficient to support a rate of root elongation in the O₂-free medium of between 0.03 and 0.17 mm h^?1. When the O₂ pressure around the shoots was increased from 20 to 100 kPa O₂, the O₂ concentrations at the walls of the expanding zone (2–7 mm from the tip) of these roots increased from 0.006 mol m^?3 to between 0.04 and 0.26 mol m^?3, and the rate of root extension increased five-fold. Oxygen transport to roots grown continuously in acrated solutions was considerably less than for roots raised in stagnant solutions; this difference was greater for seminal than for nodal roots. When the acrated seminal roots were longer than 100 mm and transferred to an O₂-free root medium, O₂ concentration became zero at the root tip causing elongation to cease. After 24 h of anoxia, none of these roots were able to resume elongation following a return to acrated solutions.     

Analysis of movement in primary maize roots

Studying plant root kinematics is important for understanding certain aspects of root growth and movement, which are strictly correlated in plants. However, there is little available data on autonomous movements in plant roots, such as nutations, and the data that are available are poorly described. We investigated the autonomous movements during growth in primary maize roots by estimating the main kinematic parameters of nutations (i.e., the period of duration and amplitude) and the growth rate. The estimations of nutation parameters were performed by developing dedicated methods, which are based on the analysis of root tip displacement and tip velocity. The data relative to the tip displacements were obtained using tip tracing software developed by our team specifically for this purpose. The results confirmed that the nutational phenomenon covers the continuous range of periods and amplitudes, with certain dominant period-amplitude types, which we clustered into three groups: i) amplitudes less than 0.1 mm and 4–16 min periods, ii) amplitudes less than 0.1 mm and 20–120 min periods, and iii) amplitudes greater than 0.1 mm and 24–120 min periods.     

Gravitational compensation and the phototropic response of oat coleoptiles

Avena seedlings were germinated and grown while continuously rotated on the horizontal axis of a clinostat. The coleoptiles of these gravity-compensated plants were phototropically more responsive than those of plants rotated on a vertical axis. When the plants were compensated after unilateral irradiation, phototropic curvature of the shoot progressed for the next 6 hours, with the rate of curving decreasing about 3 hours after irradiation. The decrease in rate was less in the plants gravity-compensated before irradiation than in those vertically rotated. In the period 70 to 76 hours after planting, the growth rate of the compensated coleoptiles was significantly less than that of the vertically rotated seedlings. The greater phototropic curvature, the decreased growth rate, and the slower rate of straightening of the curved, compensated shoot can be correlated with several consequences of compensation: an increase in sensitivity to auxin, a lowering of auxin content in the coleoptile tip, and possibly, from an interaction between compensation and phototropic stimulation, an enhanced difference in auxin transport between the illuminated and shaded halves of the unilaterally irradiated shoot.

The phototropic response of the vertically rotated seedling was significantly different from that of the vertical stationary, indicating the importance of vertically rotated controls in clinostat experiments.

     

Absorption of nitrate and ammonium ions by Lolium perenne from flowing solution cultures at low root temperatures

Lolium perenne L. cv. 23 (perennial ryegrass) plants were grown in flowing solution culture and acclimatized over 49 d to low root temperature (5°C) prior to treatment at root temperatures of 3, 5, 7 and 9°C for 41 d with common air temperature of 20/15°C day/night and solution pH 5·0. The effects of root temperature on growth, uptake and assimilation of N were compared with N supplied as either NH₄ or NO3 at 10 mmol m^?3. At any given temperature, the relative growth rate (RGR) of roots exceeded that of shoots, thus the root fraction (Rf) increased with time. These effects were found in plants grown with the two N sources. Plants grown at 3 and 5°C had very high dry matter contents as reflected by the fresh weight: freeze-dried weight ratio. This ratio increased sharply, especially in roots at 7 and 9°C. Expressed on a fresh weight basis, there was no major effect of root temperature on the [N] of plants receiving NHJ but at any given temperature, the [N] in plants grown with NHJ was significantly greater than in those grown with NO₃. The specific absorption rate (SAR) of NH⁺₄ was greater at all temperatures than SAR-NO₃. In plants grown with NH⁺, 3–5% of the total N was recovered as NH⁺₄, whereas in those grown with NO^?₃ the unassimilated NO^?₃ rose sharply between 7 and 9°C to become 14 and 28% of the total N in shoots and roots, respectively. The greater assimilation of NH⁺₄ lead to concentrations of insoluble reduced N (= protein) which were 125 and 20% greater, in roots and shoots, respectively, than in NO^?₃-grown plants. Plants grown with NH⁺₄ had very much greater glutamine and asparagine concentrations in both roots and shoots, although other amino acids were more similar in Concentration to those in NO^?₃ grown plants. It is concluded that slow growth at low root temperature is not caused by restriction of the absorption or assimilation of either NH⁺₄ or NO^?₃. The additional residual N (protein) in NH⁺₄ grown plants may serve as a labile store of N which could support growth when external N supply becomes deficient.     

Spontaneous oscillations of extracellular electrical potentials measured on Lepidium sativum L. roots

Abstract. Electrical potential differences (PDs) were measured continuously at six points in line on the surface of primary roots of cress ( Lepidium sativum L.). The root-to-hypocotyl transition was used as the reference. A gradient of extracellular potentials existed along the root axis. The most negative potentials were observed at the root tip. Whenever the growth rate declined to 0·10 mm h⁻¹ or less, the PDs were nearly constant in space and time. Otherwise, the PDs showed different types of changes with time. Roots with growth rates between 0·65 and 1·10 mm h⁻¹ preferably showed PDs characterized by approximately sinoidal oscillations. Maximal amplitudes appeared in the middle of the elongation zone. Roots which were exposed to a N₂-atmosphere exhibited a reversible disappearance of the electrical oscillations and a decrease in the PD-gradient. Similar effects were obtained by the application of the metabolic inhibitor azide (N₃⁻). These results proved that the electrical oscillations are dependent on the aerobic cell metabolism.     

Empirical evidence of a convection–diffusion model for pH patterns in the rhizospheres of root tips

A recently formulated convection–diffusion model predicted that root growth plus diffusion of protons in the neighbouring soil would lead to particular pH patterns around the moving root tip. To test the predictions of this theory, pH was measured at differing radial distances from the root surface after 24 h of growth in a medium with low diffusivity (sandy soil) and after a shorter period (55 min of growth) in a medium with high diffusivity (agar). In agreement with the theory, the growth zone was found to influence the pH of the soil for distances less than 1 mm from the root surface (even after many hours) and the pH of the agar for a distance of at least 5 mm (after only 1 h). The axial pattern of pH along the surface of soil‐grown Zea mays L. root tips was found to be the same for roots growing at different rates under different temperatures (2·23  mm h^?1 at 26 °C or 1·27 mm h^?1 at 20 °C). Thus, the plant can synchronize proton flux with growth to maintain a particular surface pH pattern within the growth zone. This implies that root tips growing at different rates in response to different temperatures can carry the same microenvironment of pH through a homogeneous soil.     

Signal transmission during gravitropic curvature of primary roots of Zea mays

Primary roots of Zea mays cv. Ageotropic are nonresponsive to gravity and elongate approximately 0.80 mm h^?1. Applying mucilage-like material (K-Y Jelly) to the terminal 1.5 cm of these roots induces graviresponsiveness and slow elongation 28% (i.e. from 0.80 to 0.58mm h^?1). Applying mucilage-like material to one side of the terminal 1.5 cm of the root induces curvature toward the mucilage, irrespective of the root's orientation to gravity. Applying a 2-mm-wideband of mucilage-like material to a root's circumference 8 to 10 mm behind the root cap neither induces gravicurvature nor affects elongation significantly. Similarly, applying mucilage-like material to only the root cap does not significantly affect elongation or graviresponsiveness. Gravicurvature of mutant roots occurs only when mucilage-like material is applied to the root/root-cap junction. Reversing the caps of wild-type and mutant roots produces gravitropic responses characteristic of the root cap rather than the host root. These results are consistent with the suggestion that gravitropic effectors are growth inhibitors that move apoplastically through mucilage between the root cap and root.     

Studies on the growth and development in Spirogyra

The phototropic response of Spirogyra sp. filaments and its relation with the different phases of their diurnal movements were studied. The filaments rapidly responded to unilateral irradiation; curvature towards the light source began in their tip region but shifted down to more basal regions. However, this typical and steady phototropic curvature could be observed only in the GnSt phase of the diurnal movement. In the other phases competitive states between the phototropic movement and other kinds of movement were evident. Thus, from the results of our previous and present studies it is proposed that the diurnal movement of Spirogyra filaments is composed of various individual movements, including a phototropic one; among these movements there exists a certain balance determined by the culture conditions and the time of day, and the phototropic movement is relatively inferior to the other movements.Abbreviations Gn negative gravitropism - Gp positive gravitropism - Sh shrinking (spiralling) growth - St straight form of growth V=Tanaka et al. (1983)     

Fine-root morphological and growth traits in a Turkey-oak stand in relation to seasonal changes in soil moisture in the Southern Apennines, Italy

We investigated the effects of seasonal changes in soil moisture on the morphological and growth traits of fine roots (<2?mm in diameter) in a mature Turkey-oak stand (Quercus cerris L.) in the Southern Apennines of Italy. Root samples (diameter:?<0.5, 0.5?C1.0, 1.0?C1.5, and 1.5?C2.0?mm) were collected with the Auger method. Mean annual fine-root mass and length on site was 443?g?m^?2 (oak fine roots 321?g?m^?2; other species 122?g?m^?2) and 3.18?km?m^?2 (oak fine roots 1.14?km?m^?2; other species 2.04?km?m^?2), respectively. Mean specific root length was 8.3?m?g^?1. All fine-root traits displayed a complex pattern that was significantly related to season. In the four diameter classes, both fine-root biomass and length peaked in summer when soil water content was the lowest and air temperature the highest of the season. Moreover, both fine-root biomass and length were inversely related with soil moisture (p?<?0.001). The finest roots (<0.5?mm in diameter) constituted an important fraction of total fine-root length (79?%), but only 21?% of biomass. Only in this root class, consequent to change in mean diameter, specific root length peaked when soil water content was lowest showing an inverse relationship (p?<?0.001). Furthermore, fine-root production and turnover decreased with increasing root diameter. These results suggest that changes in root length per unit mass, and pulses in root growth to exploit transient periods of low soil water content may enable trees to increase nutrient and water uptake under seasonal drought conditions.     

Effects of NaCl on ion relations and carbohydrate status of roots and on osmotic regulation of roots and shoots of Atriplex amnicola

Abstract Atriplex amnicola was grown at 25, 200 or 400 mol m³ NaCl. Root tissues at different stages of development were investigated for concentrations of K⁺, Na⁺ and Mg²⁺, and in some cases for Cl^?. Sugar and starch concentrations were measured for plants grown at 25 or 400 mol m³ NaCl. In the ‘slightly vaeuolated’ root tips, Na⁺ was only 40 mol m^?3 at an external concentration of 400 mol m^?3 NaCl. The concentrations of K⁺ were not affected substantially by external NaCl between 25 mol m^?3 and 400 mol m^?3. The ‘highly vacuolated’ root tissues had substantially higher concentrations of K⁺, Na⁺ and Cl^? in plants grown at 200 and 400 mol m ³ NaCl than in plants grown at 25 mol m^?3 NaCl. Concentrations of Cr and of the sum of the cations in recently expanded tissue were similar to those in the bulk of the roots, consisting mainly of old cells. However, the K⁺: Na⁺ decreased with age; at 400 mol m^?3 external NaCl with a K⁺: Na⁺ of 0.012, the K⁺: Na⁺ in recently expanded 12 mm root tips was as high as 1.6, compared with 0.7 for the bulk of the roots. These ion data were used to estimate cytoplasmic and vacuolar concentrations of K⁺ and Na ⁺. Such calculations indicated that between 25 mol m³ and 400 mol m^?3 external NaCl the concentration of the sum of (Na⁺+K⁺) in the cytoplasm was maintained at about 180–200 mol m^?3 (cell water basis). In contrast, the (Na⁺+ K⁺) concentration in the vacuole was 170 mol m^?3 for plants grown at 25 mol m^?3 NaCl and 420 mol 400 mol m^?3 NaCl. The expanding root (issues exhibited greatly decreased soluble sugars and starch between dusk and dawn. Ai both times, sugar and starch concentrations in these tissues were 2.5–4.0 times greater in plants grown at 400 mol m^?3 NaCl compared with plants grown at 25 mol m^?3 NaCl. In contrast, carbohydrate concentrations in expanded root tissues were very similar at 25 and 400 mol m^?3 and showed little diurnal fluctuation. This paper considers the causes for the slower growth of A. amnicola at 400 than at 25 mol m”³ NaCl, using the data for the roots described here, and those for the shoots presented in the preceding paper (Aslam et al., 1986). There is no support for possible adverse effects by high internal ion concentrations. Instead, there may be deficiencies in supply of organic solutes for osmotic regulation; during part of the night a limited supply of such solutes may well restrict the rate of expansion of cells in plants growing at 400 mol m^?3 NaCl. There is insufficient evidence to decide whether this limitation in the expanding tissues is particularly prominent for the roots or for the shoots.     

Rhizotoxicity of aluminate and polycationic aluminium at high pH

Although monomeric Al species are often toxic in acidic soils, the effects of the aluminate ion (Al(OH) ₄ ⁻ ) on roots grown in alkaline media are still unclear. Dilute, alkaline (pH 9.5) nutrient solutions were used to investigate the effects of Al(OH) ₄ ⁻ on root growth of mungbean (Vigna radiata L.). Root growth was reduced by 13% after 3 d growth in solutions with an Al(OH) ₄ ⁻ activity of 16 μM and no detectable polycationic Al (Al₁₃). This decrease in root growth was associated with the formation of lesions on the root tips (due to the rupturing of the epidermal and outer cortical cells) and a slight limitation to root hair growth (particularly on the lateral roots). When roots displaying these symptoms were transferred to fresh Al(OH) ₄ ⁻ solutions for a further 12 h, no root tip lesions were observed and root hair growth on the lateral roots improved. The symptoms were similar to those induced by Al₁₃ at concentrations as low as 0.50 μM Al which are below the detection limit of the ferron method. Thus, Al(OH) ₄ ⁻ is considered to be non-toxic, with the observed reduction in root growth in solutions containing Al(OH) ₄ ⁻ due to the gradual formation of toxic Al₁₃ in the bulk nutrient solution resulting from the acidification of the alkaline nutrient solution by the plant roots.     

Salinity and the Hydraulic Conductance of Roots

The effect of salinity on hydraulic conductance of intact roots of tomato (Lycopersicon esculentum Mill.) and sunflower (Helianthus annuus L.) was determined in split-root experiments using salinized nutrient solutions. Experiments were conducted in controlled climate chambers under two or three relative humidity levels and four solution osmotic potential levels. The relationship between water flux through roots (J_v) and total water potential difference between the leaves and the root medium (Δψ) was linear, usually with a small intercept. Thus, the root hydraulic conductance (L) was not affected by salinity within the range of fluxes obtained in these experiments, with L= 0.036 mm h^?1 bar^?1 for tomato and L= 0.0167 mm h^?1 bar^?1 for sunflower. Our results agreed with theoretical analysis of coupled water and ion uptake. From Cl^? and Na⁺ uptake data, the reflection coefficient (o) for tomato roots was calculated as 0.956, which was compatible with the near-zero intercept. A large intercept for sunflower could not be readily explained. Relative humidity strongly affected root growth, with more rapid growth under low humidity conditions. Transpiration of sunflower plants was reduced by 20% when the relative humidity was increased from 34% to 84%, whereas transpiration in tomato was reduced 50%.