Root Elongation Rate Research Articles

Root elongation of seedling peas through layered soil of different penetration resistances

Field soils contain localized zones of larger penetration resistance within peds and compacted layers, while cracks and biopores offer low resistance pathways to roots. Root responses to such localized conditions have not been investigated in detail. This study examined what happens to the root elongation rate when roots grew through a layer of hard soil into a layer of looser soil for a 4 day period.

Root growth and distribution in relation to nutrient availability and uptake

This paper reviews relevant interactions between soil and roots in terms of effects on root growth, and the consequences for nutrient availability. Changes in rates of root elongation, root diameter, and root branching are identified as the most obvious effects on roots, but the production of root hairs and the state of wall thickening in the endodermis with distance behind the root apex can be influenced as a root penetrates the soil. Uptake of nutrients and their movement, together with imbalances in the uptake of anions and cations can result in a change in the availability of nutrients close to the root. Release by roots of organic molecules, particularly organic acids, can change as roots grow through the soil. The acidic compounds can act as chelating agents within the soil so making nutrients such as iron and phosphorus more available. Physiological and morphological characteristics of roots important for describing nutrient uptake are identified. The uptake of a nutrient at the root surface is characterized by the maximum influx (Imax) and the affinity of a carrier in the cell membranes (Km) for the nutrient ion. Imax tends to reflect the concentration of the nutrient in the shoot. Km may be dependent on nutrient supply, and may vary with the age of the root. We conclude that these physiological parameters are particularly important in agricultural soils when coupling them with morphological parameters of root branching to predict uptake when only a part of a root system is in contact with a localized supply of nutrients. A greater importance could well attach to the value of Imax when mycorrhizae are present.Uptake is particularly sensitive to root growth rate and to the radius of the absorbing root. The effective root radius is enhanced by the presence of root hairs due to their small radius, the increased surface they provide for absorption, and the increased volume of soil accessed. Interand intra-specific differences in physiological uptake parameters may become effective when uptake proceeds mainly through root hairs. Roots compete for nutrients when the radial distance of the depletion zone from the root surface approaches the half distance between roots. When roots are more or less uniformly distributed in the soil, the half distance, b, between adjacent roots depends on the root length density Lv. Competition occurs at greater values of b — and hence lower values of Lv — as ionic mobility increases. For ions moving to the root surface by mass flow, the total root length at the time of maximum water extraction from a given soil layer appears to be an important parameter. More account needs to be taken of non-uniform root distributions, especially in the subsoil. The uptake of water and nutrients are likely to be restricted by the clumping of roots, or poor rootsoil contact. All these root parameters will need to be quantified in developing mathematical models to describe the growth of root systems in the field, if nutrient acquisition is to be predicted accurately.

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremely high above-ground dry-weight productivity of 40 Mg ha−1 yr−1. To help understand the below-ground capabilities that support the high above-ground productivity of these Crassulacean acid metabolism plants, roots were studied in the laboratory and in plantations near Mexico City. For approximately 15-year-old plants, the lateral spread of roots from the plant base averaged 1.3 m and the maximal root depth was 0.8 m, both considerably greater than for desert succulents of the same age. Root and shoot growth occurred all year, although the increase in shoot growth at the beginning of the wet season preceded the increase in growth of main roots. New lateral roots branching from the main roots were more common at the beginning of the wet season, which favoured water uptake with a minimal biomass investment, whereas growth of new main roots occurred later in the growing season. The root: shoot dry weight ratio was extremely low, less than 0.07 for 6-year-old plants of both species, and decreased with plant age. The elongation rates of main roots and lateral roots were 10 to 17 mm d−1, higher than for various desert succulents but similar to elongation rates for roots of highly productive C3 and C4 agronomic species. The respiration rate of attached main roots was 32 μmol CO2 evolved kg−1 dry weight s−1 at 4 weeks of age, that of lateral roots was about 70% higher, and both rates decreased with root age. Such respiration rates are 4- to 5-fold higher than for Agave deserti, but similar to rates for C3 and C4 agronomic species. The root hydraulic conductivity had a maximal value of 3 × 10−7 ms−1 MPa−1 at 4 weeks of age, similar to A. deserti. The radial hydraulic conductivity from the root surface to the xylem decreased and the axial conductivity along the xylem increased with root age, again similar to A. deserti. Thus, although roots of A. mapisaga and A. salmiana had hydraulic properties per unit length similar to those of a desert agave, their higher growth rates, their higher respiration rates, and the greater soil volume explored by their roots than for various desert succulents apparently helped support their high above-ground biomass productivity

Effects of depth and time of sowing on emergence of Danthonia richardsonii Cashmore and Danthonia linkii Kunth

Two experiments examined the effects of sowing time and depth (surface and 10, 25, 50 mm) on emergence of Danthonia richardsonii Cashmore and Danthonia linkii Kunth. Experiment 1 was conducted from January to December 1990 on a loam/sand mixture in boxes. Emergence was highest in both species for seeds sown onto the soil surface in summer and autumn (P < 0.05). Sowing at any depth at any time of the year, or surface sowing in winter and spring, markedly reduced emergence. Experiment 2 was conducted in the field at Tamworth, northern New South Wales from September 1991 to August 1992, on a red brown earth and a black earth. This study confirmed that emergence in both species was highest from surface sown seed. Field emergence was lowest in winter, but in contrast to experiment 1, it was higher in spring, particularly on the black earth. Seedling emergence appeared to be related to mean maximum temperature, decreasing in winter as it declined below 20�C, and increasing in spring when it was greater than 23�C. Differences in seed weight were reflected in emergence of D. richardsonii and D. linkii in experiment 1. Similar emergence was recorded for the loamlsand mixture and sand, indicating that there was little effect of texture. Phalaris aquatica L. cv. Sirosa surface sown in December had lower emergence ( P < 0.05) than both Danthonia spp., but emergence of this larger seeded cultivar was higher at depths of 10 and 25 mm. Laboratory studies to determine reasons for the low emergence of D. richardsonii and D. linkii from depth, indicated that neither had an obligate light requirement for germination. Depth, however, reduced germination (P < 0.05) compared with surface sowing of seed. Seedlings at depth also were observed to have slower rates of shoot and root elongation. In the field, the most successful establishments of D. richardsonii and D. linkii seedlings are likely to occur from surface sowings in April and May. Sowing in spring may also be possible if mean maximum soil temperatures exceed 23�C, and seedlings can establish before the onset of hot, dry conditions in summer.

Phosphate nutrition effects on aluminium avoidance of Mucuna pruriens var. utilis

The response of roots of Mucuna pruriens var. utilis to aluminium depends on the duration of exposure and the type of experiment. In a short-term study, the usual reduction of root elongation rate by Al was found. In contrast, a study for a longer period of time, 4 weeks, showed a stimulation of root dry weight and root surface area for 110 or 185 μM Al 3+, compared to a no-Al control. In a split-root experiment, the response of root growth to the presence of Al 3+ in its environment depended on the presence or absence of Al 3+ around other parts of the same root system. It was concluded that Al avoidance, rather than Al toxicity or tolerance, explained the root response of Mucuna to acid subsoil conditions in the field. The question was addressed as to whether the Al-avoidance reaction is related to a local response of plants to P stress, leading to increased branch root development close to a P source. Increasing the P supply to the roots indeed resulted in the disappearance of the Al-avoidance reaction in Mucuna. An experiment with the more Al-tolerant Centrosema pubescens demonstrated that Al avoidance is not restricted to Mucuna species. The Al avoidance described here has an important bearing on selection of crop cultivars for a better root development in acid subsoils. To obtain cultivars with a deep root development in acid subsoils, it is not sufficient to screen for Al tolerance of single roots or intact root systems in a homogeneous medium; it is necessary to test for Al avoidance in heterogeneous media.

Spatial and Temporal Aspects of Growth in the Primary Root of Cotton Seedlings: Effects of NaCl and CaCl2

Seedlings of cotton (Gossypium hirsutum L. cv. Acala SJ-2) were grown in modified Hoagland nutrient solution with various combinations of NaCl and CaCl2. Marking experiments and numerical analysis were conducted to characterize the spatial and temporal patterns of cotton root growth at varied Na/Ca ratios. At 1 mol m−3 Ca, 150 mol m−3 NaCl reduced overall root elongation rate to 60% of the control, while increasing Ca to 10 mol m−3 at the same NaCl concentration restored the elongation rate to 80% of the control. Analysis of the spatial distribution of elongation revealed that the presence of 150 mol m−3 NaCl in the medium shortened the growth zone by about 2 mm from the approximate 10 mm in the control and also reduced the relative elemental elongation rate (i.e. the longitudinal strain rate, defined as the derivatives of displacement velocity of a cellular particle with respect to position on root axis). Supply of 10 mol m−3 Ca at the high salt condition restored partially the relative elemental elongation rate, but not the length of the growth zone. Compared to the control, the growth trajectories showed that at 1 mol m−3 CaCl2 it took more time for a cellular particle to move through the growth zone at 150 mol m−3 NaCl, while at 10 mol m−3 CaCl it took less time and there was no difference between the NaCl treatments

Root Communication Mechanisms and Intracommunity Distributions of Two Mojave Desert Shrubs

Experimental studies using root observation chambers to observe the effects of encounters between individual roots on root elongation rates have revealed that the interactions among roots of Ambrosia dumosa and Larrea tridentata are more complex than simple competition for a limiting resource. Larrea roots inhibited elongation of either Larrea or Ambrosia roots in their vicinity, and Ambrosia roots inhibited elongation of contacted roots on other Ambrosia plants only. The purpose of the study reported here was to test the hypothesized involvement of inhibitory substances released by roots in these interroot encounters by attempting to remove such substances by adsorption to activated carbon. The presence of activity carbon caused a significant decrease in the inhibition of elongation of neighboring roots by Larrea roots, but activated carbon had no effect on the intraspecific responses of Ambrosia roots. These results support the hypotheses that the interaction mechanism of Larrea roots involves the release of a readily diffusible, generally inhibitory substance by Larrea roots into the soil, rather than a simple depletion of water or nutrients from around Larrea roots, and that the intraspecific, self—nonselfish—recognizing interaction mechanism of Ambrosia roots is mediated by contact and is fundamentally different from that of Larrea. These findings may enhance our understanding of Mojave desert community structure. The root—mediated allelopathy of Larrea may play a role in producing and maintaining the commonly occurring, regular distributions of Larrea. The complex communication mechanism of Ambrosia roots appears to constitute a detection and avoidance system that may allow this shrub to grow in clumped intraspecific distributions with little or no intraspecific competition for water. The interspecific interference between Larrea and Ambrosia in the field may be mechanistically asymmetrical due to their different root communication mechanisms.

Effect of Shrubs on Recruitment of Quercus Douglasii and Quercus Lobata in California

Biologically derived safe sites appear to strongly affect spatial and temporal patterns in terrestrial plant communities; however, few experimental field studies have been conducted to investigate the importance of such interactions or the mechanisms by which they operate. Quercus douglasii and Quercus lobata are winter—deciduous oaks endemic to California. I examined association between seedlings of these oaks and several species of shrubs and conducted field experiments that tested for facilitative effects of two shrubs, Salvia leucophylla and Artemisia californica, on seedling survival. Naturally occurring Q. douglasii seedlings were relatively common (280 seedlings/ha) and were strongly associated with shrub canopies. In two experimental plantings, 30% and 55% of Q. douglasii seedlings that emerged under shrubs survived for >1 yr, whereas no seedlings in the open survived in either experiment. Survival of Q. douglasii where shrubs were removed was similar to survival in the open grassland, indicting that association between shrubs and oak seedlings was not due to share microsite requirements. Q. douglasii survival was not different between the two shrub species. Only three Q. lobata seedlings were found in 26 study plots, although reproductive adults were present. The seedlings were under shrub canopies. However, the survival of Q. lobata was not facilitated by shrub cover or shrub simulation and no other evidence was found for nurse—shrub interactions with this oak species. Artificial shade was critical to the survival of Q. douglasii seedlings in simulated shrub environments. When grown in artificial shade, photosynthetic capacities and root elongation rates of Q. douglasii seedlings were significantly higher than those of Q. lobata. These differences may partially explain why nurse—plant interactions exist between shrubs and Q. douglasii seedling mortality differed between the shrub and open grassland habitats. Under shrubs, acorn predation was the primary cause of mortality, whereas mortality due to shoot herbivory was much more frequent in the open grassland. Thus, propagules under shrubs that avoided predation until shoot emergence benefitted from canopy shade and protection from herbivores. The shade tolerance and nurse—plant utilization of Q. douglasii seedlings have parallels with shade tolerance and late successional roles of other oak species in temperature deciduous forests, and, as in these forests, may affect the long—term spatial dynamics in California woodlands.

Effect ofAzospirillum brasilenseinoculation on growth dynamics and hydraulic conductivity of sorghum bicolor roots

Abstract Root growth was monitored in hydroponic systems under greenhouse conditions. Inoculation with Azospirillum brasilense increased the total number and length of adventitious roots of Sorghum bicolor by 33–40% over non‐inoculated controls. This resulted from a higher rate of growth, earlier root appearance and a greater elongation rate of individual sorghum roots. Inoculation also led to a 25–40% increase in hydraulic conductivity of the inoculated root system as compared to controls. In plants subjected to an osmotic stress by the addition of polyethylene glycol (PEG 6000) to the growth medium to produce an osmotic potential of ‐2.0 bar, inoculation significantly diminished most of the drastic adverse effects on root growth. It was concluded that beneficial effects of Azospirillum on the growth and yield of sorghum can be attributed largely to improved growth and function of the root system.

Alteration of Thiol Pools in Roots and Shoots of Maize Seedlings Exposed to Cadmium : Adaptation and Developmental Cost

Roots of intact 5-day-old maize (Zea mays L.) seedlings were exposed to 3 micromolar Cd during a 7-day period. Cysteine, gamma-glutamylcysteine, glutathione (GSH), and Cd-induced acid-soluble thiols (ASTs), including phytochelatins, were quantified in roots and shoots. Adaptation to Cd and its cost to seedling development were evaluated by measuring Cd content, tissue fresh weight, and rate of root elongation. Roots contained 60 to 67% of the Cd in the seedlings between 4 and 7 days of exposure. Exposure to Cd decreased the fresh weight gain in roots from day 4 onward without affecting the shoots. Between days 1.5 and 3.5 of Cd treatment, roots elongated more slowly than controls; however, their growth rate recovered thereafter and exceeded that of controls. Exposure to Cd did not appreciably affect the concentration of cysteine in the seedlings. However, the initial low concentration of gamma-glutamylcysteine increased (after a lag of 6 hours in roots and 2 days in shoots), reaching a plateau by day 6 at 28.5 nanomoles per gram of fresh weight in roots and by day 5 at 19.1 nanomoles per gram of fresh weight in shoots. During the first 9 hours of Cd exposure, the concentration of GSH in roots decreased dramatically (at 31.6 nanomoles per gram of fresh weight per hour) and thereafter decreased more slowly than in controls. The depletion of GSH in the roots (366 nanomoles per gram of fresh weight) matched the synthesis of ASTs (349 nanomoles per gram of fresh weight) during the first 48 hours. The concentration of ASTs in roots increased steadily thereafter to reach 662.2 nanomoles per gram of fresh weight by 6 days of Cd exposure. In shoots, Cd had little influence on the concentration of GSH, but ASTs still accumulated to 173.3 nanomoles per gram fresh weight after 5 days. The molar ratio of thiols in ASTs to Cd increased to a maximum of 10.24 in roots after 4 hours and of 4.25 in shoots after 2 days of Cd exposure. After 4 days, the ratio reached a plateau of approximately 2 in roots and between 2 and 3 in shoots, as if a steady state of Cd chelation had been achieved in both organs. The plateau coincided with recovered root elongation or an adaptation to Cd. The reduced fresh weight gain of the roots during this time, however, indicated that the synthesis of Cd-induced thiols was at a cost to root development.

Effect of Inhibition of Abscisic Acid Accumulation on the Spatial Distribution of Elongation in the Primary Root and Mesocotyl of Maize at Low Water Potentials

Previous work showed that accumulation of endogenous abscisic acid (ABA) acts both to maintain primary root growth and inhibit shoot growth in maize seedlings at low water potentials (psi(w)) (IN Saab, RE Sharp, J Pritchard, GS Voetberg [1990] Plant Physiol 93: 1329-1336). In this study, we have characterized the growth responses of the primary root and mesocotyl of maize (Zea mays L. cv FR27 x FRMo 17) to manipulation of ABA levels at low psi(w) with a high degree of spatial resolution to provide the basis for studies of the mechanism(s) of ABA action. In seedlings growing at low psi(w) and treated with fluridone to inhibit carotenoid (and ABA) biosynthesis, ABA levels were decreased in all locations of the root and mesocotyl growing zones compared with untreated seedlings growing at the same psi(w). In the root, low psi(w) (-1.6 megapascals) caused a shortening of the growing zone, as reported previously. The fluridone treatment was associated with severe inhibition of root elongation rate, which resulted from further shortening of the growing zone. In the mesocotyl, low psi(w) (-0.3 megapascal) also resulted in a shortened growing zone. In contrast with the primary root, however, fluridone treatment prevented most of the inhibition of elongation and the shortening of the growing zone. Final cell length measurements indicated that the responses of both root and mesocotyl elongation to ABA manipulation at low psi(w) involve large effects on cell expansion. Measurements of the relative changes in root and shoot water contents and dry weights after transplanting to a psi(w) of -0.3 megapascal showed that the maintenance of shoot elongation in fluridone-treated seedlings was not attributable to increased water or seed-reserve availability resulting from inhibition of root growth. The results suggest a developmental gradient in tissue responsiveness to endogenous ABA in both the root and mesocotyl growing zones. In the root, the capacity for ABA to protect cell expansion at low psi(w) appears to decrease with increasing distance from the apex. In the mesocotyl, in contrast, the accumulation of ABA at low psi(w) appears to become increasingly inhibitory to expansion as cells are displaced away from the meristematic region.

Influence of a high Mn supply on Norway spruce (Picea abies (L.) Karst.) seedlings in relation to the nitrogen source.

Effects of 3, 25, 100, 200 and 800 microM Mn on biomass and pigment, starch and nitrate concentrations were studied in Norway spruce (Picea abies (L.) Karst.) seedlings grown with either NO(3) (-) or NH(4) (+) as the sole nitrogen source. After 77 days of exposure to 800 microM Mn, shoot growth had ceased in about 50% of the seedlings independently of the N source. Despite high Mn concentrations in roots and shoots of the Mn-treated seedlings, no visible symptoms of Mn toxicity were evident. The rate of root elongation was decreased by treatment with >/= 200 microM Mn when N was supplied as NO(3) (-), but not when it was supplied as NH(4) (+). This difference could be attributed to the higher Mn concentrations in root tips of the NO(3) (-)-grown seedlings compared with the NH(4) (+)-grown seedlings. In Mn-treated seedlings, the concentration of Mg, and to a lesser extent that of Ca, decreased. Depletion of these elements might account for the observed growth depression. Potassium concentrations were similar in the control and Mn-treated seedlings. Treatment of seedlings with 800 microM Mn for 50 days led to several physiological changes: starch accumulated, the concentrations of nitrate and phenolic compounds increased, pigment concentrations decreased, and in vivo nitrate reductase activity in roots was reduced.

Physical restraints underlying short-term inhibition by auxin of root elongation in intact maize seedlings

This report investigates physical changes associated with the short-term inhibition of root elongation in intact maize seedlings (Zea mays L. vs. Halamish) by exogenous auxin. Movement of root tips was assayed by video microscopy in control roots, roots grown for 45 min in 10−6 M indole3-acetic acid (IAA), or roots chilled for 3 min at 11°C. IAA and chilling treatments similarly reduced root elongation rates (from 29 ± 6 μm min−1 to 6 ± 2 μm min−1). Initial rates of root tip contraction induced by 300 mOsmol mannitol were used to calculate tissue contractibility values. These allowed a comparison of effects of IAA and chilling treatments on apparent rates of water transport out of the root tip tissues. Chilling treatment reduced root tip contractibility by 66%, whereas IAA had much less effect (26% reduction). Roots were also exposed to an osmotic jump treatment; the initial osmotically induced increase in elongation rate was used to determine root tip extensibility values. Both IAA and chilling treatments reduced root tip extensibilities by 57%. Inhibition of wall-yielding properties, rather than hydraulic limitations, appeared to be primarily associated with inhibition of intact root tip elongation by exogenous IAA.

Promotive Effects of DCPTA on Seedling Development and Growth of Radish

A radish (Raphanus sativus L. cv. Scarlet turnip white tipped) seedling growth test was developed to examine promotive effects of 2-(3,4-dichlorophenoxy) triethylamine (DCPTA) on seedling vigor and plant development. Compared with controls, seed treatment using 30 μm DCPTA significantly (P = 0.05) enhanced the rates of root and hypocotyl elongation and seedling dry weight. Enhanced hypocotyl development by DCPTA showed a significant linear correlation (r = 0.83) with the increased taproot yield of mature plants grown from DCPTA-treated seeds. The harvestable taproot yield and harvest index of plants grown from seeds treated with 30 μm DCPTA were increased 109% and 38%, respectively, as compared with controls. Incubation of radish seeds in 30 μm DCPTA with actinomycin-D, alpha-amanitin, amisomycin, or cordycepin significantly reduced DCTPA-mediated seedling growth. These results indicate that nuclear gene expression and translation of mRNA on 80S ribosomes are required for the acceleration of seedling development by DCPTA.

Patterns of Root Respiration Associated with the Induction of Aluminium Tolerance inPhaseolus vulgarisL.

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolus vulgaris L. («Dade») was found to be an inducible trait. Upon exposure to 10 μM Al, the rate of root elongation was inhibited in comparison to controls. During the following 72 h, the rate of elongation returned to levels comparable to controls. In contrast, root elongation of an Al-sensitive cultivar («Romano») did not recover after exposure to Al. In Dade, the resumption of root elongation following exposure to Al was accompanied by increased rates of root respiration, whereas respiration rates slowly declined over the 72 h treatment period in Romano

Epidermal cell elongation in sorghum seminal roots exposed to high root-zone temperature

The nature of inhibition of seminal root elongation in sorghum ( Sorghum bicolor Moench) plants grown under different periods of high root-zone temperature (RZT) of 40°C was analyzed by measuring the epidermal cell lengths at fixed points along the growth zone. Cell lengths of roots exposed to high RZT were significantly shorter than those grown at 25°C. Growth in cell lengths became more inhibited as the period of exposure to high temperature was increased. Cell elongation peaked closer to the apex as the period of exposure to high RZT increased. Reduction of the elongation region was 10–16% for every 2 days of growth under high RZT. The total number of epidermal cells added to a cell fine per hour (cell flux), estimated from epidermal cell lengths and root elongation rates, was reduced by 26% for every 2 days of exposure to the high RZT.

Hydraulic Conductivity and Anatomy for Lateral Roots ofAgave desertiDuring Root Growth and Drought-induced Abscission

Hydraulic conductivity (L p ), radial conductivity (L R ), axial conductance (K h ), and related anatomical characteristics for lateral roots of Agave deserti were investigated during root growth and drought-induced abscission. The elongation rate of lateral roots averaged 5 mm d -1 under wet conditions and was reduced 95% by 17 d of drought (ψ soil <-0.5 MPa); 46% of such roots abscised at 21 d of drought. Under wet conditions, L p decreased with age up to 30 d, mainly reflecting suberization of endodermal cells

Ammonium and nitrate uptake by barley genotypes in diurnally fluctuating root temperatures simulating till and no-till conditions

The morphological development and N uptake patterns of spring barley (Hordeum vulgare L.) genotypes of Northern European (Nordic) and Pacific Northwest US (PNW) origin were compared under two diurnally fluctuating root temperature regimes in solution culture. The two regimes, 15/5°C and 9/5°C day maximum/night minimum temperatures, simulated soil temperature differences between tilled vs. heavy-residue, no-till conditions, respectively, observed during early spring in eastern Washington. Previous field experiments indicated that some of the Nordic genotypes accumulated more N and dry matter than the PNW cultivars during early spring under no-till conditions. The objective of this experiment was to determined whether these differences 1) are dependent on the temperature of the rooting environment, and 2) are correlated with genotypic differences in NH4+ and NO3− uptake. Overall, shoot N and dry matter accumulation was reduced by 40% due to lower root temperatures during illumination. Leaf emergence was slowed by 14 to 22%, and tiller production was also inhibited. All genotypes absorbed more ammonium than nitrate from equimolar solutions, and the proportion of total N absorbed as NH4+ was slightly higher in the 9/5°C than the 15/5°C regime. A Finnish genotype, HJA80201, accumulated significantly more shoot N than the PNW cultivars, ‘Clark’ and ‘Steptoe’, and also more than a Swedish cultivar, ‘Pernilla’, in the 9/5°C regime. In the 15/5°C regime Steptoe did not differ in shoot N from the Nordic genotypes, while Clark remained significantly lower. These differences were not correlated to relative propensity for N form. Root lengths of the Nordic genotypes were significantly greater than the PNW genotypes grown under the 9/5°C regime, while the root lengths in the warmer root temperture regime were not significantly different among genotypes. Higher root elongation rates under low soil temperature conditions may be an inherent adaptive mechanism of the Nordic genotypes. Overall, the data indicate that lower maximum daytime temperatures of the soil surface layer likely account for a significant portion of the growth reductions and lower N uptake observed in no-till systems.

Root studies under variable redox potential in soil or soil suspensions using laboratory rhizotrons

This paper describes a set of rhizotrons which allow measurements of plant root growth under controlled redox potential both in soil and in soil suspensions. The redox potential is controlled by a constant flow of various gas mixtures (02-N2 or H2-N2). The maize seedlings (Zea mays cv. McCreedy 7676) used as test plants in this study showed rapid root response to low soil redox potentials. Root elongation rates were 40-55 mm day"1 in control plants, 6 mm day "* for plants grown at the moderate redox potential and zero for plants grown at the lowest redox potential within 2 days. No elongation of primary roots was found at redox potentials below + 200 mV and the primary root tips in the soil died rapidly after this value had been reached. Plant gas exchange data indicated significant reductions in both stomatal conductance and net photosynthesis of maize plants in response to hypoxia. For plants grown in soil, stomatal conductance was reduced from 47 to 19 mmol m~2 s~1 (60% reduction) and net photosynthesis was reduced from 9.96 to 3.42 /?mol C02 m~2 s""1 (66% reduction) in response to hypoxia. Similar responses to hypoxia were found for plants grown in soil suspension.

The recovery of the roots of Zea mays L. from various aluminium treatments: Towards elucidating the regulatory processes that underlie root growth control

Controlled environment studies have demonstrated the potential of roots exposed to Al (5 mg/dm 3) for various times (up to 48 hr) to recover and resume primary growth subsequent to their transfer to Al-free recovery solutions. Ultrastructural investigation of the stages involved in recovery showed that the resumption of root elongation rates during recovery coincided with the presence of a morphologically distinctive secretory activity in peripheral root cap cells. This suggested links for this activity in regulating root growth responses. Changes in the volume of the root cap, attributed to reactivation of the cap meristem initially made inactive by Al, and the recovery of root meristem cells involving nucleolar reaggregation and decreased vacuolation were characteristic of the early stages of root recovery, and preceded by several days a resumption of root elongation rates. Recovery was concomitant with root growth stimulation. Activation of the cells of the quiescent centre to form a new cap was not an early response of roots recovering from Al. In contrast, “continuous” root exposure to Al prevented recovery of the primary root meristem and resulted in severe reductions in root elongation rates and a loss of apical dominance. The stages involved in the recovery of roots from Al are used to discuss root growth regulation and the physiology of Al toxicity.