The influence of supra-optimal root-zone temperatures on growth and stomatal conductance in Capsicum annuum L

Pepper (Capsicum annuum L.) plants were grown aeroponically in a Singapore greenhouse under natural diurnally fluctuating ambient shoot temperatures, but at two different root-zone temperatures (RZTs): a constant 20 +/- 2 degrees C RZT and a diurnally fluctuating ambient (A) (25-40 degrees C) RZT. Plants grown at 20-RZT had more leaves, greater leaf area and dry weight than A-RZT plants. Reciprocal transfer experiments were conducted between RZTs to investigate the effect on plant growth, stomatal conductance (gs) and water relations. Transfer of plants from A-RZT to 20-RZT increased plant dry weight, leaf area, number of leaves, shoot water potential (psi shoot), and gs; while transfer of plants from 20-RZT to A-RZT decreased these parameters. Root hydraulic conductivity was measured in the latter transfer and decreased by 80% after 23 d at A-RZT. Transfer of plants from 20-RZT to A-RZT had no effect on xylem ABA concentration or xylem nitrate concentration, but reduced xylem sap pH by 0.2 units. At both RZTs, gs measured in the youngest fully expanded leaves increased with plant development. In plants with the same number of leaves, A-RZT plants had a higher gs than 20-RZT plants, but only under high atmospheric vapour pressure deficit. The roles of chemical signals and hydraulic factors in controlling gs of aeroponically grown Capsicum plants at different RZTs are discussed.     

Root-zone temperature effects on photosynthesis, <Superscript>14</Superscript>C-photoassimilate partitioning and growth of temperate lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis> cv. ‘Panama’) in the tropics

The effect of root growth temperature on maximal photosynthetic CO₂ assimilation (P _max), carbohydrate content, ¹⁴C-photoassimilate partitioning, growth, and root morphology of lettuce was studied after transfer of the root system from cool root-zone temperature (C-RZT) of 20 °C to hot ambient-RZT (A-RZT) and vice versa. Four days after RZT transfer, P _max and leaf total soluble sugar content were highest and lowest, respectively, in C-RZT and A-RZT plants. P _max and total leaf soluble sugar content were much lower in plants transferred from C-to A-RZT (C→A-RZT) than in C-RZT plants. However, these two parameters were much higher in plants transferred from A-to C-RZT (A→C-RZT) than in A-RZT plants. A-RZT and C→A-RZT plants had higher root total soluble sugar content than A→C-RZT and C-RZT plants. Leaf total insoluble sugar content was similar in leaves of all plants while it was the highest in the roots of C-RZT plants. Developing leaves of C-RZT plants had higher ¹⁴C-photoassimilate content than A-RZT plants. The A→C-RZT plants also had higher ¹⁴C-photoassimilate content in their developing leaves than A-RZT plants. However, more ¹⁴C-photoassimilates were translocated to the roots of A-RZT and C→A-RZT plants, but they were mainly used for root thickening than for its elongation. Increases in leaf area, shoot and root fresh mass were slower in C→A-RZT than in C-RZT plants. Conversely, A→C-RZT plants had higher increases in these parameters than A-RZT plants. Lower root/shoot ratio (R/S) in C-RZT than in A-RZT plants confirmed that more photoassimilates were channelled to the shoots than to the roots of C-RZT plants. Roots of C-RZT plants had greater total length with a greater number of tips and surface area, and smaller average diameter as compared to A-RZT plants. In C→A-RZT plants, there was root thickening but the increases in its length, tip number and surface area decreased. The reverse was observed for A→C-RZT plants. These results further supported the idea that newly fixed photoassimilates contributed more to root thickening than to root elongation in A-RZT and C→A-RZT plants.     

Limitations to photosynthesis of lettuce grown under tropical conditions: alleviation by root-zone cooling.

Aerial parts of lettuce plants were grown under natural tropical fluctuating ambient temperatures, but with their roots exposed to two different root-zone temperatures (RZTs): a constant 20 degrees C-RZT and a fluctuating ambient (A-) RZT from 23-40 degrees C. Plants grown at A-RZT showed lower photosynthetic CO2 assimilation (A), stomatal conductance (gs), midday leaf relative water content (RWC), and chlorophyll fluorescence ratio Fv/Fm than 20 degrees C-RZT plants on both sunny and cloudy days. Substantial midday depression of A and g(s) occurred on both sunny and cloudy days in both RZT treatments, although Fv/Fm did not vary diurnally on cloudy days. Reciprocal temperature transfer experiments investigated the occurrence and possible causes of stomatal and non-stomatal limitations of photosynthesis. For both temperature transfers, light-saturated stomatal conductance (gs sat) and photosynthetic CO2 assimilation (A(sat)) were highly correlated with each other and with midday RWC, suggesting that A was limited by water stress-mediated stomatal closure. However, prolonged growth at A-RZT reduced light- and CO2-saturated photosynthetic O2 evolution (Pmax), indicating non-stomatal limitation of photosynthesis. Tight temporal coupling of leaf nitrogen content and P(max) during both temperature transfers suggested that decreased nutrient status caused this non-stomatal limitation of photosynthesis.     

Soil temperature and flooding effects on two species of citrus

Summary Rough lemon (Citrus jambhiri Lush.) and sour orange (C. aurantium L.) seedlings were grown at constant soil temperatures of 16, 24, and 33 C for 3 months. Shoot and root growth of rough lemon was greatest at 33 C while growth of sour orange was greatest at 24 C. There were no significant effects of soil temperature on shoot: root ratio, leaf water potential or stomatal conductance. The hydraulic conductivity of intact root systems of both species was highest when seedlings were grown at 16 C. Thus, acclimation through greater root conductivity at low soil temperature may have compensated for decreased root growth at 16 C and negated effects of soil temperature on plant water relations. Half the plants growing at each soil temperature were subsequently flooded. Within 1 week, the soil redox potential (Eh) dropped below zero mV, reaching a minimum Eh of –250mV after 3 weeks of flooded conditions. Flooded plants exhibited lower root conductivity, a cessation of shoot growth, lower leaf water potentials, lower stomatal conductances, and visual sloughing of fibrous roots. Decreases in root conductivity in response to flooding were large enough to account for the observed decreases in stomatal conductance.Florida Agricultural Experiment Stations Journal Series No. 4080.     

Abscisic acid accumulation maintains maize primary root elongation at low water potentials by restricting ethylene production

Previous work showed that primary root elongation in maize (Zea mays L.) seedlings at low water potentials (psi(w)) requires the accumulation of abscisic acid (ABA) (R.E. Sharp, Y. Wu, G.S. Voetberg, I.N. Saab, M.E. LeNoble [1994] J Exp Bot 45: 1743-1751). The objective of the present study was to determine whether the inhibition of elongation in ABA-deficient roots is attributable to ethylene. At a psi(w) of -1.6 MPa, inhibition of root elongation in dark-grown seedlings treated with fluridone to impose ABA deficiency was largely prevented with two inhibitors of ethylene synthesis (aminooxyacetic acid and aminoethoxyvinylglycine) and one inhibitor of ethylene action (silver thiosulfate). The fluridone treatment caused an increase in the rate of ethylene evolution from intact seedlings. This effect was completely prevented with aminooxyacetic acid and also when ABA was supplied at a concentration that restored the ABA content of the root elongation zone and the root elongation rate. Consistent results were obtained when ABA deficiency was imposed using the vp5 mutant. Both fluridone-treated and vp5 roots exhibited additional morphological symptoms of excess ethylene. The results demonstrate that an important role of ABA accumulation in the maintenance of root elongation at low psi(w) is to restrict ethylene production.     

Root-zone CO2 and root-zone temperature effects on photosynthesis and nitrogen metabolism of aeroponically grown lettuce (Lactuca sativa L.) in the tropics

Effects of elevated root-zone (RZ) CO₂ concentration (RZ [CO₂]) and RZ temperature (RZT) on photosynthesis, productivity, nitrate (NO₃ ^?), total reduced nitrogen (TRN), total leaf soluble and Rubisco proteins were studied in aeroponically grown lettuce plants in a tropical greenhouse. Three weeks after transplanting, four different RZ [CO₂] concentrations (ambient, 360 ppm, and elevated concentrations of 2,000; 10,000; and 50,000 ppm) were imposed on plants at 20°C-RZT or ambient(A)-RZT (24–38°C). Elevated RZ [CO₂] resulted in significantly higher light-saturated net photosynthetic rate, but lower light-saturated stomatal conductance. Higher elevated RZ [CO₂] also protected plants from both chronic and dynamic photoinhibition (measured by chlorophyll fluorescence Fv/Fm ratio) and reduced leaf water loss. Under each RZ [CO₂], all these variables were significantly higher in 20°C-RZT plants than in A-RZT plants. All plants accumulated more biomass at elevated RZ [CO₂] than at ambient RZ [CO₂]. Greater increases of biomass in roots than in shoots were manifested by lower shoot/root ratios at elevated RZ [CO₂]. Although the total biomass was higher at 20°C-RZT, the increase in biomass under elevated RZ [CO₂] was greater at A-RZT. Shoot NO₃ ^? and TRN concentrations, total leaf soluble and Rubisco protein concentrations were higher in all elevated RZ [CO₂] plants than in plants under ambient RZ [CO₂] at both RZTs. Under each RZ [CO₂], total leaf soluble and Rubisco protein concentrations were significantly higher at 20°C-RZT than at A-RZT. Our results demonstrated that increased P _Nmax and productivity under elevated [CO₂] was partially due to the alleviation of midday water loss, both dynamic and chronic photoinhibition as well as higher turnover of Calvin cycle with higher Rubisco proteins.     

Primary and secondary induction requirements for flowering of Festuca rubra

Root and shoot temperatures were varied independently to determine the importance of root temperature during cold acclimation. Spinach ( Spinacia oleracea L. cvs Harbin and Bloomsdale) plants were subjected to 20/20°C. 20/5°C, 5/20°C, and 5/5°C (shoot/root) temperature treatments. Leaf freezing tolerance, water potential, stomatal resistance, osmotic potential, and water content were measured at 0.25. 1.25, 3.25, and 7.25 days of treatment. There was no change in freezing tolerance or the water relations of the 20/20°C treated plants during the course of the experiment. Freezing tolerance was increased by the 5°C shoot temperature treatments, but was not enhanced by water stress induced by the low root temperature. Leaf water potential and water content decreased and stomatal resistance increased within 6 h in the 20/5°C plants. By day 3, osmotic potential began decreasing in the 20/5°C plants. Leaf water content, osmotic potential, and water potential decreased more gradually in plants grown with 5°C shoot temperature, irrespective of root temperature. Decreased water content and osmotic potential were not correlated with increased freezing tolerance as reported for other herbaceous crop plants.     

Limitations of the use of plant material grown in Petri dishes for physiological experiments

Roots of plants growing "aeroponically" (AP) on moistened filter paper in Petri dishes for a few days are fairly often used for physiological experiments (e.g. measurement of root growth), for ion or herbicide uptake tests, before the establishment of hydroponic or aseptic cultures although their hormonal status is markedly different from that of the hydroponic (HP) control. On the 4th day of germination the ethylene production of cucumber (Cucumis sativus L. cv. Budai csemege) roots growing in AP under controlled conditions increased considerably and exhibited a maximum curve, HP roots evolved ethylene much more constantly. The morphological changes in AP roots (e.g. inhibited elongation and swelling of primary roots, and increased formation of root hairs), resembling those caused by exogenously applied ethylene, can be prevented with 10(-5) M Ag+, an inhibitor of ethylene action. In roots of one-week-old AP seedlings, the amount of an acidic inhibitor, which as judged from the Rf values is likely to be abscisic acid (ABA), is about twice as high as in HP seedlings. An elevated ethylene or ABA level of AP roots may result in a reduced elongation of the primary roots. Counteraction of this inhibition by Ag+ suggests that the effect of ethylene is the primary event in the reduction of root length. When using plant material grown in Petri dishes the possibility of similar changes in hormonal status of the roots must be taken into consideration.     

The influence of root-zone temperature on growth of Betula pendula Roth.

We have examined shoot and root growth and the concentration of carbohydrates in seedlings of a northern (67°N) and a southern (61°N) ecotype of Betula pendula Roth. cultivated at root-zone temperatures of 2, 6, 12 and 17°C. Three hydroponic experiments were conducted in controlled environments. We used three different pretreatments before seedlings were subjected to the experimental temperature treatments. Actively growing seedlings that were acclimated to the hydroponic solution for 3 weeks at a root temperature of 17°C, continued to grow at all the experimental temperatures, with an expected increase in growth from 2 to 17°C. However, if we started with ecodormant cold stored plants or used seedlings grown actively in perlite, no growth was observed at 2°C and only minor growth was found at 6°C. The highest root temperature always produced the best growth. The concentration of nonstructural carbohydrates was higher in seedlings grown at 2°C than at 17°C, and this is probably due to extensive incorporation of carbohydrates into cell walls and other structural elements at 17°C. We found no evidence for differences between the two ecotypes in root growth, in timing of bud burst, but shoot growth terminated in the northern ecotype in the first experiment because the natural photoperiod was below the critical value. Our study highlights the importance of post-transplantation stress (planting check) related to root growth, and that root threshold temperatures may change according to the way plants are pretreated.     

Suppression and promotion of growth by ethylene in rice seedlings depends on ambient humidity

We examined the effect of ethylene on the growth of rice seedlings (Oryza sativa L.) at various degrees of humidity. Ethylene significantly suppressed the growth of shoots when applied to seedlings grown under 30% relative humidity (RH), but promoted the growth of shoots when applied to seedlings grown under 100% RH. The application of gibberellic acid (GA₃) promoted the elongation of shoots in seedlings grown under 30% and 100% RH. Ethylene inhibited the shoot elongation induced by GA₃ at 30% RH, but enhanced the elongation induced by GA₃ at 100% RH. These results indicate that ethylene can either promote or suppress the growth of rice shoots depending on ambient humidity, and that these actions of ethylene may be mediated through modulating the responsiveness of shoots to gibberellin.     

Root-zone temperature effects on the early development of maize

Summary Maize plants were grown in sand culture under greenhouse conditions from emergence to the 4-leaf stage at root-zone temperature of 12.5°, 15° and 17.5°C in one experiment, and grown to the 6-leaf stage at root zone temperatures of 15°, 20°, and 25°C in a second experiment. Attention was given to plant part differentiation as determined by leaf appearance, and to growth as determined by dry tissue accumulation, at specified growth stages.For anyone growth-stage interval the number of days required for that interval increased with decreasing root-zone temperature. Dry weights of both roots and shoots at the various growth stages decreased with increasing root-zone temperature. Root zone temperature had a direct influence on the meristematic region of the shoots of young maize plants because of the close proximity of this region to the ground surface and thereby regulated plant development during the period of leaf initiation.Increased root-zone temperature enhanced plant development rate relative to plant growth rate thus reducing the ultimate yield of maize at the 4- and 6- leaf stages.It was concluded that because of the direct influence of root-zone temperature on the shoot meristem and hence on the nutrient demands of the shoot, due consideration should be given to this factor in studies concerned with soil temperature.Agronomy Department Paper No. 709.     

Relationships among Rhizosphere Oxygen Deficiency,Root Restriction,Photosynthesis, and Growth in Baldcypress (Taxodium Distichum L.) Seedlings

Seedlings of baldcypress (Taxodium distichum L.) grown in sealed containers containing nutrient solution were subjected to root-zone oxygen deficiency, physical restriction, and the combined stresses in a greenhouse. After six weeks of treatments (Phase I), half of the plants were harvested. The remaining half were allowed to continue (Phase II) under various treatments except plants that had restricted roots were freed thus allowing free expansion of roots into the nutrient solution. Oxygen deficiency and root physical restriction inhibited plant gas exchange parameters. Net photosynthetic rate (PN) was significantly higher in aerated unrestricted root (AUR) plants than in aerated root restricted (AR) plants and in anaerobic root unrestricted (FUR) plants than in anaerobic root restricted (FR) plants. After Phase I, FUR plants' shoot and root biomasses were 57.0 and 30.6 % lower than those of AUR plants, and AUR plants showed 3.3 and 3.8 times greater shoot and root biomasses than the AR plants, respectively. During Phase II, PN recovered rapidly in plants under aerated conditions, but not in plants under anaerobic conditions. The removal of physical root restriction under both aerated and anaerobic conditions resulted in rapid shoot and root growth in seedlings. Hence, root restriction or root-zone anaerobiosis, reductions in plant gas exchange, and biomass production in baldcypress were closely interrelated. In addition, root release from restriction was related to the regain of photosynthetic activity and biomass growth. The results support the previously proposed source-sink feed-back inhibition of photosynthesis in plants subjected to root-zone oxygen deficiency or physical restriction.     

Physiological,morphological, and growth responses ofPlatanus occidentalis seedlings to flooding

Summary Flooding ofPlatanus occidentalis L. seedlings for up to 40 days induced several changes including early stomatal closure, greatly accelerated ethylene production by stems, formation of hypertrophied lenticels and adventitious roots on submerged portions of stems, and marked growth inhibition. Poor adaptation ofPlatanus occidentalis seedlings to soil inundation was shown in stomatal closure during the entire flooding period, inhibition of root elongation and branching, and death of roots. Some adaptation to flooding was indicated by (1) production of hypertrophied lenticels which may assist in exchange of dissolved gases in flood water and in release of toxic compounds, and (2) production of adventitious roots on stems which may increase absorption of water. These adaptations appeared to be associated with greatly stimulated ethylene production in stems of flooded plants. The greater reduction of root growth over shoot growth in flooded seedlings will result in decreased drought tolerance after the flood waters recede. The generally low tolerance to flooding of seedlings of species that are widely rated as highly flood tolerant is emphasized.     

Influence of root restriction and ethylene exposure on apicaldominance of petunia (Petunia xhybrida Hort. Vilm.-Andr.)

Reducing rooting volume restricted root growth during theproduction of Petunia x hybrida'Orchid and resulted in an unfavorable increase in apicaldominance. Exposing young petunia seedlings to ethylene counteracted theeffects of root restriction. Rooting volumes of 9, 28, 58, or 160mL restricted the development of lateral shoots, therebyincreasing apical dominance compared to plants grown in 162 mLrooting volumes. Ethephon, an ethylene-producing compound, increased thedevelopment of lateral shoots of seedlings grown in rooting volumes rangingfrom 28 mL to 160 mL. At a rooting volume of 9mL, ethylene exposure was not capable of reducing the growth ofthe main shoot; apical dominance remained strong in both the control andethephon-treated plants. Because lateral shoot development was not restrictedby rooting volumes greater than 160 mL, exposing these plants toethylene did not result in supplementary lateral shoot development. Levels ofindole-3-acetic acid (IAA), isopentenyladenosine (iPA), and zeatin riboside(ZR) decreased on a whole shoot basis as rooting volume decreased from 162 to58 mL. Indoleacetic acid levels in ethephon-exposed plantsdecreased 20% compared to the control. The cytokinins iPA and ZR showedno response to ethylene exposure; however, the ratio of auxin/cytokinindecreased 24% compared to the control. The decrease in theauxin/cytokinin ratio was associated with an increase in the number and lengthof lateral shoots.     

Ethylene Evolution from Maize (Zea mays L.) Seedling Roots and Shoots in Response to Mechanical Impedance

The effect of mechanical impedance on ethylene evolution and growth of preemergent maize (Zea mays L.) seedlings was investigated by pressurizing the growth medium in triaxial cells in a controlled environment. Pressure increased the bulk density of the medium and thus the resistance to growth. The elongation of maize primary roots and preemergent shoots was severely hindered by applied pressures as low as 10 kilopascals. Following a steep decline in elongation at low pressures, both shoots and roots responded to additional pressure in a linear manner, but shoots were more severely affected than roots at higher pressures. Radial expansion was promoted in both organs by mechanical impedance. Primary roots typically became thinner during the experimental period when grown unimpeded. In contrast, pressures as low as 25 kilopascals caused a 25% increase in root tip diameter. Shoots showed a slight enhancement of radial expansion; however, in contrast to roots, the shoots increased in diameter even when growing unimpeded. Such morphological changes were not evident until at least 3 hours after initiation of treatment. All levels of applied pressure promoted ethylene evolution as early as 1 hour after application of pressure. After 1 hour, ethylene evolution rates had increased 10, 32, 70, and 255% at 25, 50, 75, and 100 kilopascals respectively, and continued to increase linearly for at least 10 hours. When intact corn seedlings were subjected to a series of hourly cycles of pressure, followed by relaxation, ethylene production rates increased or decreased rapidly, illustrating tight coupling between mechanical impedance and tissue response. Seedlings exposed to 1 microliter of ethylene per liter showed symptoms similar to those shown by plants grown under mechanical impedance. Root diameter increased 5 times as much as the shoot diameter. Pretreatment with 10 micromolar aminoethoxyvinyl glycine plus 1 micromolar silver thiosulfate maintained ethylene production rates of impeded seedlings at basal levels and restored shoot and root extension to 84 and 90% of unimpeded values, respectively. Our results support the hypothesis that ethylene plays a pivotal role in the regulation of plant tissue response to mechanical impedance.     

Gibberellin-ethylene interaction controls radial expansion in citrus roots

The involvement of gibberellins (GAs) and ethylene in the process of root radial expansion was studied in young seedlings of Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.]. The GA inhibitors cycocel, paclobutrazol, and tetcyclacis enhanced radial expansion of the root tip (up to 2.3-fold) as a result of increases in stele diameter and inner cortex width. The GA deficiency increased cell number and width, and changed the polarity of growth, generating wider and shorter cortical cells in the elongation zone. In the presence or absence of GA inhibitors, GA₃ decreased root tip width and reduced all parameters related to radial expansion. The ethylene inhibitors (aminooxyacetic acid; cobalt ions, CoCl₂; silver thiosulfate) suppressed swelling induced by GA deficiency, generating thinner cells just as GA₃ did. In contrast to GA₃, ethylene inhibitors produced longer cells strongly resembling those of the untreated seedlings. Ethylene released by ethephon did not modify root tip width in control plants, while root diameter behind the root tip was increased. In the presence of low and ineffective concentrations of cycocel, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid increased radial expansion of root tips (1.3-fold) and changed the polarity of growth, producing wider and shorter inner cortical cells as GA inhibitors did. These observations imply, first, that ethylene is the hormonal effector of the process of root radial expansion and, second, that the endogenous GAs modulate the promotive response of ethylene. Received: 4 October 1996 / Accepted: 25 December 1996     

Effects of different cultivation temperatures on plasma membrane ATPase activity and lipid composition of sugar beet roots.

Sugar beet seedlings (Beta vulgaris L. cv. Monohill) were cultivated for 3 weeks at different root and shoot temperatures and the plasma membranes (PM) from roots were purified by aqueous two-phase partitioning and analyzed for lipid composition and ATPase activities. Lipid analyses, undertaken immediately after PM purification from the roots, showed that a low root zone temperature (10 degrees C) decreased the ratio between the major lipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). A low temperature in the root environment increased the mol% of PE and decreased the mol% of phosphatidic acid (PA), independent on the shoot growth temperature. A low temperature also decreased the mol% of linoleic acid (18:2) and increased mol% of linolenic acid (18:3) in the analyzed lipid classes, especially in PC and PE. The ratio between acyl chain lipids and protein generally increased in PM from roots grown at 10 degrees C, compared with higher temperature. The changes in lipid composition correlated with changes in ATPase activities, detected as hydrolyses of MgATP. The kinetic parameters, K(m) and V of the PM H(+)ATPase in roots increased at a low cultivation temperature, independent on shoot temperature. Moreover, Arrhenius analyses showed that the transition temperature was independent of both root or shoot growth temperature at 10-24 degrees C, whereas the activation energy of the ATPase was dependent on the growth temperature of the root, and independent on shoot temperature. Thus, acclimation processes can take place in roots, irrespective of the shoot temperature.     

Response of spring rape (Brassica napus var. oleifera L.) to inoculation with plant growth promoting rhizobacteria containing 1-aminocyclopropane-1-carboxylate deaminase depends on nutrient status of the plant

Responses of rape (Brassica napus var. oleifera L.) to inoculation with plant growth promoting rhizobacteria, Pseudomonas putida Am2, Pseudomonas putida Bm3, Alcaligenes xylosoxidans Cm4, and Pseudomonas sp. Dp2, containing 1-aminocyclopropane-l-carboxylate (ACC) deaminase were studied using growth pouch and soil cultures. In growth pouch culture, the bacteria significantly increased root elongation of phosphorus-sufficient seedlings, whereas root elongation of phosphorus-deficient seedlings was not affected or was even inhibited by the bacteria. Bacterial stimulation of root elongation of phosphorus-sufficient seedlings was eliminated in the presence of a high ammonia concentration (1 mM) in the nutrient solution. Bacterial effects on root elongation of potassium-deficient and potassium-sufficient seedlings were similar. The bacteria also decreased inorganic phosphate content in shoots of potassium- and phosphorus-sufficient seedlings, reduced ethylene production by phosphorus-sufficient seedlings, and inhibited development of root hairs. The effects of treatment with Ag+, a chemical inhibitor of plant ethylene production, on root elongation, ethylene evolution, and root hair formation were similar to bacterial treatments. The number of bacteria on the roots of phosphorus-deficient seedlings was not limited by phosphorus deficiency. In pot experiments with soil culture, inoculation of seeds with bacteria and treatment with aminoethoxyvinylglycine, an inhibitor of ethylene biosynthesis in plants, increased root and (or) shoot biomass of rape plants. Stimulation of plant growth caused by the bacteria was often associated with a decrease in the content of nutrients, such as P, K, S, Mo, and Ba, in shoots, depending on the strain used. The results obtained show that the growth-promoting effects of ACC-utilizing rhizobacteria depend significantly on the nutrient status of the plant.     

Effects of root restriction and growth regulator treatments on the growth of carrot (Daucus carota L.) seedlings

The growth of carrot (Daucus carota L.) seedlings in clear polystyrene vials (7.5 × 2.5 cm diameter) which were either perforated or unperforated and suspended in a large container containing nutrient solution was compared with that of similar seedlings grown directly in the main container. After 28 days plants grown in the perforated, aerated vials had smaller shoots, tap roots (c 20 percent reduction) and fibrous roots (c 60 percent reduction) than unrestricted plants. In the absence of adequate aeration, the effects of root restriction were greater. Foliar applications of ethephon to the leaves of unrestricted plants reduced shoot weights but had little effect on either tap root or fibrous root weights, or even increased tap root weight at lower concentrations. Silver thiosulphate, aminovinylglycine and a highly mobile cytokinin (SD8339) all slightly enhanced shoot growth of restricted plants but had no effect on either tap roots or fibrous root development. It is suggested that although root restriction effects may be partly mediated by endogenous hormones, it seems unlikely that such effects are due solely to stimulation of ethylene production or a reduction in endogenous cytokinin levels.Abbreviations AVG Aminovinylglycine - BA N⁶-Benzyladenine - Ethephon 2-Chloroethylphosphonic Acid - SD8339 6-Benzylamino-9-(Tetrahydropyran-2-yl)-Adenine - STS Silver Thiosulphate     

Anomalous Temperature Dependence of Seedling Development in Some Soybean (Glycine max [L.] Merr.) Cultivars: Role of Ethylene

In Clark and Shelby soybean (Glycine max [L.] Merr.) seedlings, hypocotyl elongation was inhibited and hypocotyl swelling and root dry weight were increased by a temperature of 25 C. At 20 and 30 C, development was normal, as was development of Hawkeye and Mandarin soybean seedlings at all three temperatures. Dry matter distribution at 9 days indicates that inhibition of hypocotyl elongation is not due to a lack of translocation from cotyledons, but to a diversion of dry matter from hypocotyl to root. Ethylene evolution by Clark seedlings at 25 C exceeds that at 20 and 30 C. At all three temperatures, Mandarin seedlings' ethylene evolution is at the same low rates as those of Clark at 20 and 30 C. Clark's enhanced rate at 25 C precedes the deceleration of hypocotyl elongation occurring at 5 days. The abnormal effects of a temperature of 25 C on Clark seedlings' development is partially reversed by CO₂ and GA₃ and can be duplicated in Mandarin by applying ethylene. In Clark, effects of the temperature can be further accentuated by indoleacetic acid, which stimulates ethylene evolution, and by applying ethylene to the seedlings. It is concluded that the temperature-induced symptoms, similar in most respects to the well known “triple response” of legume seedlings, are caused by abnormally high levels of ethylene in tissues of the anomalous cultivars.