Changing proton concentrations at the surfaces of gravistimulated Phleum roots

The pH patterns at the surfaces of both vertically growing roots of Phleum pratense L. and roots tilted by 45° were recorded using H ⁺-sensitive microelectrodes. During vertical growth the root cap exhibited lower pH values than the meristematic zone. The highest pH values were found at the border between meristematic and elongation zones. In the apical part of the elongation zone the values strongly decreased basipetally. They reached a minimum value of pH 5.4–5.5 (medium pH of about 6.0) at a distance of 700 m from the root tip. This region of strongest acidification usually coincided with that of the highest relative rates of elongation. The region of the first visible curvature following gravistimulation was positioned at 100–200 m more apically. The pH values increased in the basal elongation zone towards the mature zone. The H⁺-flux pattern around a vertically growing Phleum root was characterized by high influxes in the meristematic zone and smaller effluxes in the elongation zone. Tilting the root by 45° induced changes in the pH values of the upper and lower sides of a Phleum root. At a distance of 300–500 m from the root tip, the upper side was strongly acidified while the pH of the lower side slightly increased compared with the values during vertical orientation. pH differences of up to 0.9 pH units between the two sides of a root were detected. These differences decreased basipetally and could not be measured more distant than 700–800 m from the tip. Compared with a vertically growing root, the H⁺-flux pattern of a Phleum root tilted by 45° exhibited effluxes on the entire upper organ flank while the pattern was scarcely altered on the lower side. The curvature-initiating zone in Phleum roots is positioned within that section of the root in which pH changes occur after tilting. The region of highest pH differences, however, is nearer to the apex of the root than the curvature-initiating zone. The pH changes began 8.2 min after a root had been tilted. The bending process started after 17.2 min, i.e. after double the time needed for differential acidification. After reorienting a root, which had just begun to bend, to its previous vertical position the inversion of the pH gradient could be measured within the same mean time of about 8 min. This is again significantly earlier than the beginning of the rebending process. The results indicate that, during the graviresponse, ionic movements occur much earlier than the changes in hormonal activities reported in the literature.Abbreviation CIZ curvature-initiating zone A preliminary report was presented at the 29th Plenary Meeting of the Committee on Space Research (COSPAR) in Washington D.C., USA, 28 Aug – 5 Sept 1992 (Zieschang and Sievers 1993)This work was supported by the Deutsche Forschungsgemeinschaft. We thank Professor H. Felle (Botanisches Institut, Universität Gießen, Gießen, FRG) for practical instructions concerning the method of H⁺-sensitive microelectrodes and Professor W. Simonis (Botanisches Institut, Universität Würzburg, Würzburg, FRG) for allowing to use the microelectrode amplifier.     

Acclimation of potassium influx in rye (Secale cereale) to low root temperatures

The influx of K⁺(⁸⁶Rb⁺) into intact roots of rye (Secale cereale L. cv. Rheidal) exposed to a differential temperature (DT) between the root (8° C) and shoot (20° C) is initially reduced compared with warm-grown (WG) controls with both shoot and root maintained at 20° C. Over a period of 3 d, however, K⁺-influx rates into DT plants are restored to levels similar to or greater than those of the WG controls, the absolute rates of K⁺ influx being strongly dependent upon the shoot/root ratio. Acclimation in DT plants results in a reduction of K⁺ influx into the apical (0–2 cm) region of the seminal root which is associated with a compensatory increase in K⁺ influx into the more mature, basal regions of the root. Values of V _max and apparent K _m for K⁺ influx into DT plants were similar to those for WG plants at assay temperatures of 8° C and 20° C except for an increase in the apparent K _m at 8° C. The influx of K⁺ from solutions containing 0.6 mol·m^-3 K⁺ into both WG and DT plants was found to be linearly related to assay temperature over the range 2–27° C, and the temperature sensitivity of K⁺ influx to be dependent upon shoot/root ratio. At high shoot/root ratios, the ratio of K⁺ influx at 20° C:K⁺ influx at 8° C for WG plants approached a minimum value of 1.9 whereas that for DT plants approached unity indicating that K⁺ influx into DT plants has a large temperature-insensitive component. Additionally, when plants were grown in solutions of low potassium concentration, K⁺ influx into DT plants was consistently greater than that into WG plants, in spite of having a greater root potassium concentration ([K⁺]int). This result indicates some change in the regulation of K⁺ influx by [K⁺]int in plants exposed to low root temperatures. We suggest that K⁺ influx into rye seedlings exposed to low root temperatures is regulated by the increased demand placed on the root system by a proportionally larger shoot and that the acclimation of K⁺ influx to low temperatures may be the result of an increased hydraulic conductivity of the root system.Abbreviations DT differential temperature pretreatment - [K⁺]int root potassium concentration - [K⁺]ext potassium concentration of nutrient medium - WG warm-grown pretreatment     

Distribution and redistribution of extension growth along vertical and horizontal gravireacting maize roots

Horizontal primary roots of Zea mays L. were photographed during the course of their gravireaction and during a preceding growth period in the vertical orientation. The displacement, by root elongation, of marker particles on the root surface was recorded. The particle-displacement rates were used to estimate the distribution of elemental elongation rates along opposite sides of the growing root apex. In the temperature range 21–25°C there was a stimulation of local elongation rates along the upper side of a gravireacting root and a reduction (and sometimes a cessation) of elongation along the lower side. Elemental elongation rates have been related to the development of root curvature, and the magnitude of the differential growth between upper and lower sides required for a particular rate of bending has also been estimated. The results complement, and are compatible with, findings relating to the distribution of certain endogenous growth regulators believed to participate in the gravireaction.Abbreviation RELEL relative elemental rate of elongation     

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeño and South American 5) maize genotypes. In a growth solution containing 6 M Al³⁺, Tuxpeño and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al³⁺ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.Abbreviations SA3 South American 3, an Al-tolerant maize cultivar - SA5 South American 5, an Al-sensitive maize cultivar The authors would like to express their appreciation to Drs. John Thompson, Ross Welch and Mr. Stephen Schaefer for their training and guidance in the use of the chromatography systems. This work was supported by a Swiss National Science Foundation Fellowship to Didier Pellet, and U.S. Department of Agriculture/National Research Initiative Competitive Grant 93-37100-8874 to Leon Kochian. We would also like to thank Drs. S. Pandey and E. Ceballos from the CIMMYT Regional office at CIAT Cali, Colombia for providing seed for the maize varieties and inbred line.     

The quiescent center in roots of maize: initiation,maintenance and role in organization of the root apical meristem

Summary Using roots of maize, we tested the hypothesis that the origin and maintenance of the quiescent center (QC) are a consequence of polar auxin supply. Exposing roots to the polar auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA), or to low temperature (4 °C, with subsequent return to 24 °C), enhances mitotic frequency within the QC. In both treatments, the QC most typically is activated at its distal face, and the protoderm/dermatogen undergoes several periclinal divisions. As a result, the root body penetrates and ruptures the root cap junction and the characteristic closed apical organization changes to open. A QC persists during these changes in apical organization, but it is diminished in size. The data from the TIBA-treated roots suggest a role for auxin in the origin and maintenance of the QC, and further, that alterations in QC dimensions are a consequence of polar auxin supply. We hypothesize that the root cap, and specifically the root cap initials, are important in regulating polar auxin movements towards the root apex, and hence are important in determining the status of the QC.Abbreviations QC quiescent center - TIBA 2,3,5-triiodobenzoic acid Dedicated to the memory of Professor John G. Torrey     

Pectin esterification is spatially regulated both within cell walls and between developing tissues of root apices

Monoclonal antibodies recognizing un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin have been used to locate these epitopes by indirect immunofluorescence and immunogold electron microscopy in the root apex of carrot (Daucus carota L.). Both antibodies labelled the walls of cells in all tissues of the developing root apex. Immunogold labelling observed at the level of the electron microscope indicated differential location of the pectin epitopes within the cell walls. The un-esterified epitope was located to the inner surface of the primary cell walls adjacent to the plasma membrane, in the middle lamella and abundantly to the outer surface at intercellular spaces. In contrast, the epitope containing methyl-esterified pectin was located evenly throughout the cell wall. In root apices of certain other species the JIM5 and JIM7 epitopes were found to be restricted to distinct tissues of the developing roots. In the root apex of oat (Avena sativa L.), JIM5 was most abundantly reactive with cell walls at the region of intercellular spaces of the cortical cells. JIM7 was reactive with cells of the cortex and the stele. Neither epitope occurred in walls of the epidermal or root-cap cells. These pattern of expression were observed to derive from the very earliest stages of the development of these tissues in the oat root meristem and were maintained in the mature root. In the coleoptile and leaf tissues of oat seedlings, JIM5 labelled all cells abundantly whereas JIM7 was unreactive. Other members of the Gramineae and also the Chenopodiaceae are shown to express similar restricted spatial patterns of distribution of these pectin epitopes in root apices.Abbreviations CDTA 1,2-diaminocyclohexane tetraacetic acid - RG rhamnogalacturonan J.P.K. was supported by the Agricultural and Food Research Council Cell Signalling and Recognition Programme. We thank J. Cooke and N. Stacey for technical assistance, H.A. Schols, Drs. P. Albersheim and A. Darvill for pectic polysaccharides, and Dr. R.R. Selvendran and M. McCann for useful discussions.     

Chloroplast thylakoid membrane fluidity and its sensitivity to temperature

In order to investigate membrane fluidity, the hydrophobic probe, 1,6-diphenyl-1,3,5-hexatriene (DPH), has been incorporated into intact isolated thylakoids and separated granal and stromal lamellae obtained from the chloroplasts of Pisum sativum. The steady-state polarization of DPH fluorescence was measured as a function of temperature and indicated that at physiological values the thylakoid membrane is a relatively fluid system with the stromal lamellae being less viscous than the lamellae of the grana. According to the DPH technique, neither region of the membrane, however, showed a sharp phase transition of its bulk lipids from the liquid-crystalline to the gel state for the temperature range -20° to 50° C. Comparison of intact thylakoids isolated from plants grown at cold (4°/7°C) and warm (14°/17° C) temperatures indicate that there is an adaptation mechanism operating which seems to maintain an optimal membrane viscosity necessary for growth. Using a modified Perrin equation the optimal average viscosity for the thylakoid membrane of the chill-resistant variety used in the study (Feltham First) is estimated to be about 1.8 poise.Abbreviations DPH 1,6-diphenyl-1,3,5-hexatriene - Hepes N-(2-hydroxyethyl)-1-piperazineethanesulphonic acid     

Expression and localization of polygalacturonase during the outgrowth of lateral roots in Allium porrum L.

The presence of polygalacturonase and its correlation with the formation of lateral roots in leek (Allium porrum L.) seedlings have been investigated. During root growth, a steady increase in polygalacturonase activity was associated with that of the lateral root primordia. Fractionation of root extract by fast protein liquid chromatography resolved at least two polygalacturonase isoforms. One of the isoforms, a 75-kdalton protein, strongly reacted on Western blots probed with a polyclonal antibody raised against tomato polygalacturonase. It also reacted with both polyclonal and monoclonal antisera raised against Fusarium moniliforme polygalacturonase. In situ localization with these three antibodies showed that polygalacturonase was present over the meristems of lateral root primordia. Antibodies against pectins (Knox et al. 1990, Planta 181, 512–521) detected large amounts of pectic material filling the area between the apex of the primordium and the mother root tissues. We suggest that a polygalacturonase plays an important role in leek root morphogenesis, particularly during lateral root outgrowth.Abbreviations FPLC fast protein liquid chromatography - RGU one unit of polygalacturonase activity - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis The Authors are grateful to Dr. Dean Della Penna (Department of Vegetable Crops, University of California, Davis, Calif., USA) for generously providing the polyclonal antibody raised against the tomato polygalacturonase. This research was supported by National Research of Italy, Special project RAISA, Subproject N2, N360.     

Influence of root temperature on growth of Pinus sylvestris,Fagus sylvatica,Tilia cordata and Quercus robur

One-year-old tree seedlings were incubated in a greenhouse from April to July, under natural daylight conditions, with their root systems at constant temperatures of 5, 10, 15, 20, 25, 30 and 35 °C and with the above ground parts kept at a constant air temperature of 18–20 °C. The course of height growth, total mass increment, root, shoot and leaf weight as well as leaf areas were measured. The results indicate that clear differences exist in the optimal root zone temperatures for various growth parameters in different tree species. Pinus sylvestris had a maximal height increment at about 5–10 °C and maximal total mass increment at 15 °C root temperature. In contrast, the optimum for Quercus robur was at 25 °C. Tilia cordata and Fagus sylvatica had their optima for most growth parameters at 20 °C. The root temperature apparently indirectly influenced photosynthesis (dry weight accumulation) and respiration loss. From the observed symptoms and indications in the literature it seems probable that a change in hormone levels is involved as the main factor in the described effects. Variation of root temperature had only an insignificant effect on bud burst and the time at which the shoots sprouted. Apparently species of northern origin seem to have lower root temperature optima than those of more southern origin. This is to be verified by investigation of other tree species.     

Rates of cell division in the young prefloral shoot apex ofChrysanthemum segetum L.

Summary The rate of cell division was determined by the colchicine induced metaphase-accumulation technique in the young prefloral shoot apex of the quantitative long-day plantChrysanthemum segetum L. growing under conditions favourable to flowering (16-hour photoperiod; 124Em^–2s^–1; 22 °C). Cell cycle duration was evaluated in relation to the location of the cells in the intact apex. The cell cycle durations were 53.5 hours, 47.4 hours, and 97.7 hours in the axial, lateral and subapical central cells respectively. Compared with previous results, these data give evidence of the major role played by the early increase in cell division rate of axial cells in the new pattern of the prefloral shoot apex at its initial stage of development. By comparison with the vegetative shoot apex, the cell cycle duration was preferentially shortened in the axial zone; it was only slightly altered in the lateral zone while it was lengthened in the vacuolating subapical central cells. In the three zones within the prefloral shoot apex, the duration of mitosis was constant (3.2 to 3.3 hours) and the same as in the vegetative shoot apex.     

Reduction of the nodulation barrier inMedicago laciniata by alteration of the root temperature

Medicago laciniata, an annual leguminous plant of Saharo-Sindian origin, is particularly refractory to root nodulation by most strains ofRhizobium meliloti. Using a series of such bacterial strains belonging to the 8 groups of Brockwell and Hely, and a variety of environmental conditions, it was noted that several normally non-nodulating strains (at 20°C) produced ineffective nodules at root temperatures of 24°C to 28°C. Nodulation at 20°C failed to occur in the presence of a wide variety of test compounds and physical conditions. No phytoalexins or anti-Rhizobium growth inhibitors were isolated from inoculated root tissue at any temperature. Temperature shift experiments indicated no infection of the root hairs at 20°C, and infection threads produced at the permissive root temperature failed to elongate after transfer to 20°C. However, if meristematic activity had been initiated in the inner root-cortical cells as a result of infection thread penetration at 28°C, no blockage of nodule maturation occurred upon subsequent transfer to 20°C root temperature. Nodules produced at 28°C were completely devoid of nitrogenase activity, although the apical (but not the distal) regions contained normal-appearing bacteriods, surrounded by enclosing membranes, and possessed a fully functional leghaemoglobin. A shortage of metabolic energy did not appear to be involved in the ineffective response. A hypothesis to explain the nodulation phenomenon observed was based on the observation in the roots of 2 factors present at 20°C but not at 28°C.     

Extracellular ice and cell shape in frost-stressed cereal leaves: A low-temperature scanning-electron-microscopy study

Low-temperature scanning electron microscopy was used to examine transverse fracture faces through cereal leaf pieces subjected to frost. Specimens were studied before and after sublimation of the ice. The position of extracellular ice in the leaf was inferred from the difference between the specimen before and after sublimation and from ridges and points which occurred in the extracellular ice during sublimation. Steps in the fracture surface indicated that the fracture plane passed through the extracellular ice crystals as well as through cells and also helped identify extracellular ice. The cells in controls were turgid and extracellular ice was absent. Leaf pieces from hardened rye were excised and frost-stressed to-3.3°,-21° and-72°C, cooling at 2–12°·h^-1. Cell collapse and extracellular ice were evident at-3.3°C and increased considerably by-21° C. At-21° and-72°C the leaf pieces were mainly filled with extracellular ice and there were few remaining gas spaces. The epidermal and mesophyll cells were laterally flattened, perpendicular to their attachment to adjacent cells, and phloem and vascular sheath cells were more irregularly deformed. Leaf pieces from tender barley were cooled at 2°C·min^-1 to-20° C; they were then mainly filled with extracellular ice, and the cells were highly collapsed as in the rye. In rye leaves frozen to-3.6° C before excision, ice crystals occurred in peri-vascular, sub-epidermal and intervening mesophyll spaces. In rye leaf pieces frozen to-3.3° C after excision or to-3.6° C before excision, mesophyll cells were partly collapsed even when not covered by ice, indicating that collapse of the cell wall, as well as the enclosed protoplast, was driven by dehydration. No gas or ice-filled spaces were found between wall and the enclosed protoplast. It is suggested that this can be explained without invoking chemical bonding between cell wall and plasma membrane: when the wall pores are filled by water, the pore size would reduce vapour pressure so making penetration of the wall by ice or gas less likely.Abbreviations SEM scanning electron microscopy     

Root-zone temperature and salinity: Interacting effects on tillering,growth and element concentration in barley

The effects of root-zone salinity (0, 30, and 60 mmol L^–1 of NaCl) and root-zone temperature (10, 15, 20, and 25°C) and their interactions on the number of tillers, total dry matter production, and the concentration of nutrients in the roots and tops of barley (Hordeum vulgare L.) were studied. Experiments were conducted in growth chambers (day/night photoperiod of 16/8 h and constant air temperature of 20°C) and under water-culture conditions. Salinity and root temperature affected all the parameters tested. Interactions between salinity and temperature were significant (p<0.05) for the number of tillers, growth of tops and roots, and the concentration of Na, K, P in the tops and the concentration of P in the roots. Maximum number of tillers and the highest dry matter were produced when the root temperature was at the intermediate levels of 15 to 20°C. Effect of salinity on most parameters tested strongly depended on the prevailing root temperature. For example, at root temperature of 10°C addition of 30 mmol L^–1 NaCl to the nutrient solution stimulated the growth of barley roots; at root temperature of 25°C, however, the same NaCl concentration inhibited the root growth. At 60 mmol L^–1, root and shoot growth were maximum when root temperature was kept at the intermediate level of 15°C; most inhibition of salinity occurred at both low (10°C) and high (25°C) root temperatures. As the root temperature was raised from 10 to 25°C, the concentration of Na generally decreased in the tops and increased in the roots. At a given Na concentration in the tops or in the roots, respective growth of tops or roots was much less inhibited if the roots were grown at 15–20°C. It is concluded that the tolerance of barley plant to NaCl salinity of the rooting media appears to be altered by the root temperature and is highest if the root temperature is kept at 15 to 20°C.     

Evaluation of host and pathogen responses for screening soil amendments to control Sclerotium rolfsii

Three strains of Bradyrhizobium, 280A, 2209A and 32H1, that nodulated peanuts (Arachis hypogaea L.), were tested for their ability to grow and survive at elevated temperatures of up to 42°C in laboratory culture. Strain 32H1 was unable to grow at 37°C and was more sensitive to elevated temperatures than the other two strains. All three produced heat-shock proteins of molecular weights 17 kDa and 18 kDa. Two greenhouse experiments were conducted to determine the effect of high root temperature on nodulation, growth and nitrogen fixation of peanut. Two peanut varieties (Virginia cv NC7 and Spanish cv Pronto) were inoculated and exposed to root temperatures of 30°, 37° and 40°C. Nodulation and nitrogen fixation were strongly affected by root temperature but there was no variety × temperature interaction. At a constant 40°C root temperature no nodules were formed. Nodules were formed when roots were exposed to this temperature with diurnal cycling but no nitrogen fixation occurred. Highest plant dry weight, shoot nitrogen content and total nitrogen were observed at a constant root temperature of 30°C. Increasing root temperature to 37°C reduced average nitrogen content by 37% and total nitrogen by 49% but did not reduce nodulation. The symbiotic performance of the strains corresponded to their abilities to grow and survive at high temperature in culture.     

Immunological assay of phytochrome in small sections of roots and other organs of maize (Zea mays L.) seedlings

Phytochrome was determined in small sections of maize (Zea mays L.) seedlings by means of a highly specific double sandwich enzyme immunoassay which uses a monoclonal anti-phytochrome antibody for binding phytochrome and anti-phytochrome serum to detect the bound phytochrome. The distribution of phytochrome in maize seedlings was followed from germination to the 7th d after soaking the caryopses. Regions of high phytochrome accumulation were found in the coleoptile tip, the root cap and the shoot apex: the values for 5-d-old seedlings were 120, 80 and 70 g phytochrome per g fresh weight (or 0.91, 0.61 and 0.53 nmol·g^-1), respectively. The mesocotyl and the leaves contained relatively low amounts of phytochrome (less than 10 g·g^-1FW), which were almost uniformly distributed throughout these organs. As might be expected, regions of these organs adjacent to the shoot apex showed higher levels. The root, other than root tip, was almost devoid of phytochrome (0.2 to 0.5 g·g^-1). The general distribution of phytochrome in organs did not change during the development of seedlings. The amount of phytochrome, however, did fluctuate: up to the 5th or 6th d after soaking the caryopses, the levels increased in the regions of high phytochrome accumulation but thereafter decreased. After the 6th d the roots were 15 cm or longer and the coleoptiles became prone to penetration by primary leaves. The tips of adventitious roots, emerging after the 6th d, were also found to contain phytochrome. When the root cap was illuminated (4.3 W·m^-1), phytochrome was degraded as in illuminated shoots. Degradation of phytochrome in coleoptile, mesocotyl and shoot apex started with a lag phase but phytochrome degradation in the root cap and the leaves started without a lag. In contrast to shoot phytochrome, which was almost completely degraded under continuous illumination, about 3% of initial phytochrome was measured in root caps after 24 h continuous illumination. Some of the data, obtained by immunological measurements, may indicate differences between phytochrome, or its synthesis or degradation, in the root cap and shoots. The results are discussed with a view to different red-light-mediated responses of grass seedlings.Abbreviations ABTS 2,2-azino-bis(3-ethylbenz-thiazoline)-sulfonic acid - EIA enzyme immunoassay - PBS phosphatebuffered saline - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis     

Biophysics of the growth responses of pea roots to changes in penetration resistance

Two experiments were performed in simplified soil-less systems to study how roots respond to changes in mechanical impedance. In the first the increases in root force and diameter that occur when a pea root was impeded mechanically inside a hole with rigid conical walls were determined. The experiment was performed at 8°C and at 25°C, and the root growth pressures generated were calculated during periods of 12, 24, 48 and 72 hours. The maximum growth pressures generated were approximately the same at both temperatures, although the maximum pressure was achieved approximately twice as quickly at 25°C than at 8°C, being reached within 15–20 hours. In the second set of experiments a new technique was developed to measure simultaneously the elongation rate and the force exerted by the roots of seedlings grown in moist air. A constant force was exerted by a force transducer on a pea radicle using a system of pulleys, and the elongation rate of the pea root was monitored using a linear variable differential transformer (LVDT). The changes in root elongation rate were recorded that occurred in response to increases and decreases in the applied force. Root elongation rate decreased by more than 50% within 30 min of increasing the applied force by 100 mN. A similarly fast, but smaller increase in growth rate occurred when the force was removed. The interpretation of results from both studies will be discussed in terms of a modified form of the Lockhart model of growth.     

Pollen selection for low temperature adaptation in tomato

Summary Pollen selection experiments were conducted in tomato to determine the effects of low temperature conditions during pollination on the rate of root elongation of the progeny. Pollen was harvested from an F₁ interspecific hybrid between a high altitude Lycopersicon hirsutum accession and the cultivated tomato L. esculentum. The pollen was applied to stigmas of malesterile L. esculentum plants maintained in growth chambers set at either 12°C/7°C or 24°C/18°C. BC₁ seeds from the low and normal temperature crosses were germinated and root elongation rate was measured at either 9°C or 24°C. At 9°C, the rate of root elongation for progeny of the low temperature crosses was higher than for progeny of crosses at normal temperatures; at 24°C the rate of root elongation was similar for the two crossing treatments. To compare the temperature responses of the two backcross populations we also calculated the relative inhibitory effect of low temperature on the rate of root elongation: the ratio between the rate of root elongation at 9°C to that at 24°C. Root elongation of seedlings from the low temperature crosses was less inhibited by the cold than root elongation for progeny of the normal temperature crosses. These results suggest a relationship between pollen selection at low temperatures and the expression of a sporophytic trait under the same environmental stress.     

Nodulation and N2 fixation of guar at high root temperature

Guar (Cyamopsis tetregonoloba (L.) may be grown when soil temperatures are potentially high enough at the time of planting to inhibit nodulation and N₂ fixation. An experiment was conducted using controlled conditions to determine the influence of high root temperature on growth and N₂ fixation of guar. The experiment included two strains of rhizobia, two varieties of guar, two mineral N treatments, and root temperatures of 34, 37, and 40°C. Plants were grown for 44 days. The root temperature of 40°C reduced N fixation by at least 80% and nodule weight by more than 50%. Significant interactions occurred between most factors in influencing nodulation, N₂ fixation and dry matter production. Guar, nodulated by rhizobial strain GAR022-1 and fully dependent on N₂ fixation or provided with starter mineral N (25 mg pot^–1), was not influenced by the root temperature of 37°C as compared to 34°C. Nodulation and N₂ fixation by strain 32H1 was reduced by at least 40% when no starter mineral N was provided and the root temperature was 37°C. Providing starter mineral N to one variety of guar doubled the quantity of N₂ fixed by strain 32H1 at both 34 and 37°C but N₂ fixation was lower at the higher root temperature. It appears that root temperatures between 37° and 40°C bracketed the critical root temperature for N₂ fixation by nodulated guar and that the critical root temperature for guar dependent on mineral N was above 40°C.     

Long-term chilling of young tomato plants under low light and subsequent recovery

The influence of unfavourable climatic conditions at the onset of the growth period on chilling-sensitive tomato (Lycopersicon esculentum Mill., cv. Abunda) was studied by exposing young plants to combinations of low temperature and low light (60–100 mol quanta · m^–2 · s^–1) for several weeks. When the temperature did not decrease below a critical point (8 ° C) no loss of developmental capacity of the plants was detected. However, while new leaves were readily formed upon return to normal growth conditions (22/18 °C, day/night, in a greenhouse), net accumulation of biomass showed a lag phase of approximately one week. This delay was accompanied by a strong, irreversible inhibition of photosynthesis in the fully expanded leaves which had been exposed to the chilling treatment. When plants were subjected to temperatures below 8 ° C, survival rates decreased after three weeks at 6 ° C and irreversible damage of apical meristematic tissue occurred. Drought-hardening prior to chilling ensured survival at 6 ° C and protected the plants against meristem loss.Abreviation Chl chlorophyll Thanks are due to G.P. Telkamp for technical assistance. This research is financially supported by the Netherlands Technology Foundation (STW, Utrecht, The Netherlands), and is coordinated by the Foundation for Biological Research (BION, 's-Gravenhage, The Netherlands).