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

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

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. in Flowing Solution Culture: I. GROWTH

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. in flowing solution culture. I. Growth.—J.exp. Bot. 38: 42–52 Oilseed rape (Brassica napus L. cv. Bien venu) was grown for49 d in flowing nutrient solution at pH 6?0 with root temperaturedecrementally reduced from 20?C to 5?C; and then exposed todifferent root temperatures (3, 5, 7, 9, 11, 13,17 or 25?C)held constant for 14 d. The air temperature was 20/15?C day/nightand nitrogen was supplied automatically to maintain 10 mmolm^–3 NH₄NO₃ in solution. Total dry matter production wasexponential with time and similar at all root temperatures givinga specific growth rate of 0?0784 g g^–1 d^–1. Partitioningof dry matter was influenced by root temperature; shoot: rootratios increased during treatment at 17?C and 25?C but decreasedafter 5 d at 3?C and 5?C. The ratio of shoot specific growthrate: root specific growth rate increased with the ratio ofwater soluble carbohydrates (shoot: root). Concentrations ofwater soluble carbohydrates in shoot and root were inverselyrelated to root temperature; at 3, 5 and 7?C they increasedin stem + petioles throughout treatment, coinciding with a decreasein the weight of tissue water per unit dry matter. These resultssuggest that the accumulation of soluble carbohydrates at lowtemperature is the result of metabolic imbalance and of osmoticadjustment to water stress. Key words: Brassica napus, oilseed rape, root temperature, specific growth rate     

Adaptation of potassium translocation into the shoot of maize (Zea mays) to shoot demand: Evidence for xylem loading as a regulating step

In order to manipulate the shoot demand for mineral nutrients per unit root weight, maize ( Zea mays L.) seedlings were grown in nutrient solution with different temperatures in the root zone and at the shoot base. The aerial temperature was kept uniform at 24/20°C day/night. At a root zone temperature (RZT) of 24°C, shoot growth was reduced by decreasing the shoot base temperature (SBT) to 12°C; at a RZT of 12°C, shoot growth was increased by raising the SBT to 24°C. At both RZT root growth was not affected by the SBT. Thus, the shoot demand for nutrients per unit root was either increased by raising, or decreased by lowering the SBT. The net uptake rate of potassium (K), as determined from accumulation rates between sequential harvests, was not affected within the first 3 days after lowering the SBT, whereas net translocation rates of K into the shoot and translocation rates in the xylem exudate of decapitated plants were markedly reduced. Obviously, translocation of K into the shoot seems to be regulated independently from K uptake into the root cells. Translocation rates of K in the xylem exudate of decapitated plants were markedly reduced when the nutrient solution was replaced by CaCl₂ solution during exudation. But, depending on the SBT before decapitation, significant differences remained in the translocation rates of K even when K uptake from the nutrient solution was prevented.
From the results it is suggested that xylem loading of K is regulated separately from K uptake from the external solution and that the adaptation of K translocation to shoot demand is coupled with an altered capacity of the root for xylem loading.     

Effects of Low Temperature on Growth and Nutrient Accumulation in Rye (Secale cereale) and Wheat (Triticum aestivum)

Rye (Secale cereale cv. Rheidol) and wheat (Triticum aestivumcv. Mardler) were grown at shoot/root temperatures of 20/20°C (warm grown, WG plants), 8/8 °C (cold grown, CG plants)and 20/8 °C (differential grown, DG plants). Plants fromcontrasting growth temperature regimes were standardized andcompared using a developmental timescale based on accumulatedthermal time (°C d) at the shoot meristem. Accumulationof dry matter, nitrogen and potassium were exponential overthe time period studied (150–550 °C d). In rye, therates of plant dry matter and f. wt accumulation were linearlyrelated to the temperature of the shoot meristem. However, inwheat, although the rates of plant dry matter and f. wt accumulationwere temperature dependent, the linear relationship with shootmeristem temperature was weaker than in rye. The shoot/rootratio of rye was stable irrespective of growth temperature treatment,but the shoot/root ratio of wheat varied with growth temperaturetreatment. The shoot/root ratio of DG wheat was 50% greaterthan WG wheat. In both cereals, nutrient concentrations anddry matter content tended to be greater in organs exposed directlyto low temperatures. The mean specific absorption rates of nutrientswere calculated for the whole period studied for each species/temperaturecombination and were positively correlated with both plant shoot/rootratio and relative growth rate. The data suggest that nutrientuptake rates were influenced primarily by plant demand, withno indication of specific nutrient limitations at low temperatures. Nutrient accumulation, relative growth rate (RGR), rye, Secale cereale cv. Rheidol, temperature, thermal time, Triticum aestivum cv. Mardler, wheat     

Root Zone Temperature Effects on Growth and Phosphate Absorption in Rape Brassica napus cv. Emerald

Solution culture experiments with fodder rape (Brassica napuscv. Emerald) show that reduced root temperatures appear to havelittle effect on phosphate inflow over a wide range of P concentration.At a cool root temperature (10 ?C) plant growth rate was reducedbut this was compensated for by a low root: shoot ratio, sothat inflow remained relatively steady. An increased inflowper unit length of root was only achieved at an elevated roottemperature of 35 ?C. The minimum phosphate concentration towhich plants could lower the culture solution (C_mln) rangedfrom 0.15 to 2.5 mmol m^–3 according to whether roots wereat a low (5 ?C) or high (35 ?C) temperature respectively. Thetotal phosphorus concentration in tissues was affected by rootzone temperature and at low root temperatures this could bea growth limiting factor. The organic (assimilated) fractionof P in shoot tissues was smaller in low temperature plants.These showed visual symptoms of apparent P deficiency. Levelsof inorganic P in roots may also be a factor in feedback ofcontrol of inflow. Key words: Temperature, Roots, Phosphate, Rape (Brassica napus)     

Distinct Influence of Root and Shoot Temperature on Nitrogen Fixation by White Clover

White clover (Trifolium repens L.) was grown in controlled environmentsto determine the distinct effects of root and shoot temperatureon the accumulation of total and fixed (¹⁵ N dilution) nitrogenat two levels of nitrate (10 and 75 mM). Nitrogen fixation(BNF) showed a positive response to higher shoot temperature(23 vs. 13 C day temperature), irrespective of whether or notroot temperature was increased in parallel. Low root temperature(5 C) caused a marked reduction in the accumulation of totalnitrogen at both nitrate levels, and led to a lower proportionof N derived from BNF. The temperature response of BNF was attributedfor the major part to an adaptation to the demand for fixedN. It is therefore concluded that BNF is not primarily responsiblefor the reduced clover growth at low temperatures. White clover, Trifolium repens L., temperature, nitrogen fixation, nitrate, root, shoot     

Cytokinin Activity in Lupinus albus

The cytokinin content of the root exudate and leaves of fruiting white lupin plants (Lupinus albus L.) was investigated at 2 weekly intervals after anthesis of the lowest flower on the primary inflorescence. Up to 8 weeks after anthesis the level of cytokinins in the root exudate increased. However, at 10 weeks after anthesis insufficient sap was produced for analysis. Cytokinins co-eluting with zeatin and zeatin riboside were detected in the root exudate after fractionation on Sephadex LH-20. The cytokinin levels in the mature leaves steadily increased up to 8 weeks after anthesis and thereafter remained relatively constant. Three peaks of activity, co-eluting with zeatin, zeatin riboside and the glucoside cytokinins were recorded in the leaf extracts. The level of glucoside cytokinins in the leaves was high at 8 and 10 weeks after anthesis. Paper chromatography of extracts of fruits collected at 2 weeks after anthesis indicated that as fruit development proceeded there was a build up of cytokinin in this region of the plant. It is suggested that, in the white lupin, the cytokinins translocated to the shoot are accumulated in the leaves and in the fruits and that it is only later when there is a considerable decrease in sap (10 weeks after anthesis) production that a decreasing supply of cytokinins leads to shoot senescence.     

Cultivars of Hexaploid Wheat of Contrasting Stature and Chlorophyll Retention Differ in Cytokinin Content and Responsiveness

The work reported here compared cytokinin content and sensitivityin a selection of hexaploid wheat (Triticum aestivum L.) cultivarsusing the following measurements: leaf cytokinins at three timepoints during light-growth and at four 24 h intervals afterlight-grown plants were transferred to darkness; sensitivityof root growth to direct applications of isopentenyl adenosine([9R]iP); and, sensitivity of germination and subsequent rootand shoot growth to 18 h imbibition of seeds in benzyladenine(BA). Accumulation of zeatin riboside-type cultivars was greatestduring light-growth in Tibet Dwarf, a wheat with an extremedwarf phenotype, intermediate in Omar standard and dwarf cultivars,and lowest in the standard and dwarf versions of Itana. Cytokininlevels were otherwise not directly correlated to plant staturein these wheats. There were no cultivar-associated qualitativedifferences in the types of cytokinins detected in this study.During the 16 h light period, the content of zeatin riboside-typecytokinins increased up to tenfold and then declined to basallevels during dark growth. Chlorophyll retention during dark-growthwas correlated with leaf cytokinin content. Data collected ata restricted number of sampling points during dark-growth suggesteda cyclic accumulation of [9R]iP-type cytokinins and the apparentcycle in Tibet Dwarf was offset by 24 h. Tibet Dwarf showedthe greatest root growth inhibition after exposure of seedlingroots to [9R]iP or imbibition of seeds in BA. Neither of thesetreatments affected shoot growth in any of the cultivars. Wheat; Triticum aestivum ; cytokinin; zeatin riboside; benzyladenine; root inhibition     

Effect of Heat Stress on Photosynthetic Activity and Chloroplast Ultrastructure in Correlation with Endogenous Cytokinin Concentration in Maize Seedlings

The effect of heat stress (47.5?C for 2 min given to the roots)on photosynthetic activity and chloroplast ultrastructure wasstudied in correlation with the endogenous cytokinin concentrationin maize (Zea mays L. "Fronica") seedlings. Shoot and root growth were visibly inhibited by the heat treatment.In etiolated leaf segments there was a clear-cut inhibitionof photosynthetic activity, chlorophyll accumulation and chloroplastdevelopment. This inhibition was reversible by benzyladenine.In green tissue heat stress only affected photosynthetic activityand chlorophyll accumulation. Again this inhibition could bereversed by benzyladenine. In the primary leaves of green seedlings heat stress loweredcytokinin concentration severely; in etiolated leaves no effectof heat stress could be observed, since the cytokinin concentrationwas below the detectable level. Our results provide an indication for a correlation betweenhormonal control and chloroplast biogenesis. (Received July 5, 1984; Accepted October 12, 1984)     

Does Cytokinin Transport from Root-To-Shoot in the Xylem Sap Regulate Leaf Responses to Root Hypoxia?

We have examined the hypothesis that cytokinins transportedfrom roots to shoots affects leaf growth, stomatal conductance,and cytokinin concentration of leaves of Phaseolus and a hybridpoplar (Populus trichocarpa x Populus deltoides) with hypoxicroots. Because cytokinins may interact with other substances,potassium and calcium concentrations were determined in xylemsap of Populus plants with hypoxic and aerated roots while gibberellin(GA) concentrations were measured in shoot tissues. Root hypoxiadecreased leaf growth and closed stomata in both species. Inboth species, fluxes of cytokinins out of the roots were reduced,but no differences in bulk leaf concentrations were measuredbetween the hypoxic and aerated plants. Shoots with aeratedroots contained slightly higher concentrations of GA₁ and GA₃than shoots from hypoxic plants. There were no differences incalcium or potassium concentrations in xylem sap between aerationtreatments. Exogenously applied cytokinins did not alleviatethe growth or stomatal responses caused by root hypoxia. Informationon the site(s) and mechanism(s) of cytokinin action and theways in which cytokinins are compartmentalized within plantcells will be required to understand the physiological significanceof cytokinin transport in the transpirational stream. Key words: Cytokinins, hypoxia, Populus, Phaseolus     

Internal Root Anatomy of Maize Seedlings (Zea mays L.) as Influenced by Temperature and Genotype

The objectives of this investigation were to determine: (a)the general effect of temperature on internal root anatomy;(b) whether genotypic differences in such root traits exist;and (c) the association between internal root traits and shootgrowth, lateral root branching and cold tolerance of maize (Zeamays L.). Seedlings of 20 central European hybrids were grownunder high or low temperature (25/22·5 °C or 15/12·5°C day/night temperatures) until the third leaf was fullyexpanded. Light microscopy of cross sections revealed a largerdiameter of primary roots at low temperature which was due toa larger stele diameter and a thickening of the cortex. Concurrently,an increase in total cross sectional area of metaxylem elementswas obtained. It is assumed that the modification of the internalroot structure by temperature has an effect on both axial andradial water flow capacity. For all anatomical traits studied,variability between genotypes was apparent under both growingconditions. Furthermore, different genotypic responses to temperaturewere observed. However, basic differences between cold-tolerantand cold-sensitive genotypes did not exist. While at high temperatureroot traits and shoot growth were significantly and positivelycorrelated, at low temperature the correlation coefficient wasinsignificant. Consequently, it was not possible to characterizethe performance of the shoot at low temperature based on anatomicaltraits of the root. Moderate, positive correlation coefficientswere obtained between internal root traits and lateral rootbranching. The potential use of root anatomical traits as indirectselection criteria is discussed. Chilling tolerance, genotypes, root anatomy, Zea mays L     

Effects of Low Temperature on the Development and Morphology of Rye (Secale cereale) and Wheat (Triticum aestivum)

Seedlings of Secale cereale cv. Rheidol and Triticum aestivumcv. Mardler were grown at shoot/root temperatures of 20/20 °C,20/8 °C and 8/8 °C. During vegetative growth both cerealsproduced leaves, tillers and roots in a defined pattern, ata species-specific rate which was linearly related to the temperatureof the shoot meristem. Thus, plant development could be standardizedon a temperature x time (°C d) basis despite contrastinggrowth-temperature treatments. When compared at a similar developmentalstage, the cooling of whole plants or of plant roots resultedin an increase in the d. wt: f. wt ratio of both shoot and roottissues, a decrease in the length of both the longest shootand root, and the development of broader and thicker leaves.Although the effects of temperature on developmental characteristicscould be accurately predicted by an empirical relationship,the effects on morphological characteristics could not. Development, phyllochron, rye, Secale cereale cv. Rheidol, temperature, thermal time, Triticum aestivum cv. Mardler, wheat     

Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

Barley plants (Hordewn vulgare L. cv. Atem) were grown fromseed for 28 d in flowing solution culture, during which timeroot temperature was lowered decrementally to 5?C. Plants werethen subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17or 25 ?C, with common air temperature of 25/15 ?C (day/night).Changes in growth, plant total N, and NO₃^– levels, andnet uptake of NH₄⁺ and NO₃^– from a maintained concentrationof 10 mmol m^–3 NH₄NO₃ were measured over 14 d. Dry matterproduction increased 6-fold with increasing root temperaturebetween 3–25 ?C. The growth response was biphasic followingan increase in root temperature. Phase I, lasting about 5 d,was characterized by high root specific growth rates relativeto those of the shoot, particularly on a fresh weight basis.During Phase I the shoot dry weight specific growth rates wereinversely related to root temperature between 3–13 ?C.Phase 2, from 5–14 d, was characterized by the approachtowards, and/or attainment of, balanced exponential growth betweenshoots and roots. Concentrations of total N in plant dry matterincreased with root temperature between 3–25 ?C, moreso in the shoots than roots and most acutely in the youngestfully expanded leaf (2?l–6?9% N). When N contents wereexpressed on a tissue fresh weight basis the variation withtemperature lessened and the highest concentration in the shootwas at 11 ?C. Uptake of N increased with root temperature, andat all temperatures uptake of NH₄⁺, exceeded that of NO₃^–,irrespective of time. The proportions of total N uptake over14 d absorbed in the form of NH₄⁺ were (%): 86, 91, 75, 77,76, 73, 77, and 80, respectively, at 3, 5, 7, 9, Il, 13, 17,and 25 ?C. At all temperatures the preference for NH₄⁺ overNO₃^– uptake increased with time. An inverse relationshipbetween root temperature (3–11 ?C) and the uptake of NH₄⁺as a proportion of total N uptake was apparent during PhaseI. The possible mechanisms by which root temperature limitsgrowth and influences N uptake are discussed. Key words: Hordeum vulgare, root temperature, ammonium, nitrate, ion uptake, growth rate     

Effect of Root Zone Temperature and Shoot Demand on Uptake and Xylem Transport of Macronutrients in Maize (Zea mays L.)

The effect of shoot demand for nutrients on nutrient uptakeand translocation in the xylem exudate was studied in maizegrowing in nutrient solution at uniform shoot zone (24/2C,day/night), but different root zone temperatures (RZT: 12C,18C, 24C). The shoot base (apical shoot meristem and zoneof leaf extension) was either kept within or lifted above thecooling zone. In plants with their shoot base above the coolingzone (RZT: 12C and 18C) shoot growth was significantly increasedbut not root growth. Therefore, at suboptimal RZT shoot freshweight increment d^–1 g^–1 root fresh weight, whichwas taken as a parameter for the shoot demand for nutrientsper unit root, varied strongly depending on the temperatureof the shoot base (shoot base temperature, SBT). In short-term studies (2 h and 1 d after onset of temperaturetreatment) rates of nutrient (nitrogen, N; potassium, K; phosphorus,P; calcium, Ca) uptake or translocation in the xylem exudatewere markedly decreased at suboptimal RZT (12C, 18C), irrespectiveof the SBT. In long-term studies (3, 5, and 10 d after onsetof temperature treatment) uptake and translocation of K, N,and Ca, but not P, increased in plants at suboptimal RZT whenthe shoot demand was high (shoot base above the cooling zone)but decreased when the shoot demand was low (shoot base withinthe cooling zone). These results suggest, that the increase of translocation ratesof N, K, and Ca after long-term exposure to suboptimal RZT wasa consequence of a higher shoot demand per unit root fresh weightand not due to a direct temperature effect on the nutrient uptakesystem. Key words: Xylem exudate, nutrient translocation, root zone temperature, shoot demand, nutrient circulation     

Ontogenetic variation of four cytokinins in soybean root pressure exudate

Cytokinins exported from the root may be involved in the correlative control of plant development. To test this hypothesis in soybean ((Glycine max [L.] Merr. cv. McCall, cv Chippewa 64, and cv Hodgson 78), cytokinins were intercepted en route from the root to the shoot by collecting root pressure exudate from detopped roots. The quantities of four cytokinins in the exudate were studied throughout the development of plants grown in the field and in controlled environment chambers. Zeatin, zeatin riboside, and their dihydro derivatives, dihydrozeatin and dihydrozeatin riboside, were isolated and quantitated using high-performance liquid chromatography.

Cytokinin fluxes (pmoles per plant per hour) were independent of exudate flux (grams per plant per hour). All fluxes are averages for a 6- or 8-h collection period. The ribosides accounted for the majority of the observed cytokinin transport. The fluxes of zeatin riboside and dihydrozeatin riboside increased from low levels during vegetative growth to maxima during late flowering or early pod formation. Before the seeds began rapid dry matter accumulation, zeatin riboside and dihydrozeatin riboside fluxes decreased and remained at low levels through maturation. The fluxes of zeatin and dihydrozeatin were low throughout development.

No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).

The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.

     

Effects of constant and fluctuating soil temperature on growth,development and nutrient uptake of maize seedlings

Summary The effect of constant and fluctuating soil temperature and two soil moisture regimes on the growth, development, transpiration and nutrient uptake by maize seedlings was studied in a greenhouse investigation. The constant root temperatures were maintained at 30, 34, 35, 36, 37, and 38°C for both 250 and 750 cm of soil moisture suctions. The fluctuating root temperature, for 250 cm of soil moisture suction only, of 30–35, 30–39, 30–40, 30–45 and 30–48°C were maintained to simulate the soil temperature regime under field conditions. The constant root temperature of 35°C and fluctuating temperature between 30–40°C significantly decreased the shoot and root growth and transpiration rate. On the average, there was 1.3 and 0.7 g decrease in fresh shoot weight and 0.36 and 0.30 g in fresh root weight per degree increase in root temperature for 250 and 750 soil moisture suction, respectively. In general, the effect of high soil moisture suction on maize seedlings was more severe when at high root temperature. The shoot and root concentration of N, P, and K decreased while that of B increased with increase in root temperature. The root concentration of Zn also decreased with increase in root temperature.     

Differences between maize and wheat in growth-related nutrient demand and uptake of potassium and phosphorus at suboptimal root zone temperatures

The effects of low root zone temperatures (RZT) on nutrient demand for growth and the capacity for nutrient acquisition were compared in maize and wheat growing in nutrient solution. To differentiate between direct temperature effects on nutrient uptake and indirect effects via an altered ratio of shoot to root growth, the plants were grown with their shoot base including apical shoot meristem either within the root zone (low SB), i.e. at RZT (12°, 16°, or 20°C) or, above the root zone (high SB), i.e. at uniformly high air temperature (20°/16° day/night).At low SB, suboptimal RZT reduced shoot growth more than root growth in wheat, whereas the opposite was true in maize. However, in both species the shoot growth rate per unit weight of roots, which was taken as parameter for the shoot demand for mineral nutrients per unit of roots, decreased at low RZT. Accordingly, the concentrations of potassium (K) and phosphorus (P) remained constant or even increased at low RZT despite reduced uptake rates.At high SB, shoot growth at low RZT in both species was higher than at low SB, whereas root growth was not increased. At high SB, the shoot demand per unit of roots was similar for all RZT in wheat, but increased with decreasing RZT in maize. Uptake rates of K at high SB and low RZT adapted to shoot demand within four days, and were even higher in maize than in wheat. Uptake rates of P adapted more slowly to shoot demand in both species, resulting in reduced concentrations of P in the shoot, particularly in maize.In conclusion, the two species did not markedly differ in their physiological capacity for uptake of K and P at low RZT. However, maize had a lower ability than wheat to adapt morphologically to suboptimal RZT by increasing biomass allocation towards the roots. This may cause a greater susceptibility of maize to nutrient deficiency, particularly if the temperatures around the shoot base are high and uptake is limited by nutrient transport processes in the soil towards the roots.     

A comparative study of the response of the roots and shoots of sunflower and maize to mechanical stimulation

Despite numerous studies of the effects of mechanical stimulationon plant shoots, the response of roots to mechanical stimulationhas largely been neglected. In this study the effects of shootflexure on the morphology and mechanics of two contrasting speciesof herbaceous angiosperm, growing in a glasshouse were compared:maize (Zea mays), a monocot; and sunflower (Helianthus annuusL.) a dicot. Mechanical stimulation affected the root more than the shootcomponents. Root systems of mechanicallystressed sunflowershad a greater angle of spread and increased root number. Aswell as large morphological and weight effects, with increasesover the control of 33% in the length of rigid root and 38%in the dry weight of lateral roots, in sunflowers, there werealso mechanical effects. In both species roots of flexed plantswere more rigid, stronger and composed of stiffer material andtheir root systems also provided greater anchorage strength.In contrast, there was only a small reduction in shoot weightand shoot height in flexed plants and no effects on mechanicalproperties. There were differences in behaviour between species; maize rootmorphology responded less than that of sunflowers to mechanicalstimulation. The basal diameter of roots increased by only 8%compared with 16% in sunflowers, though the roots of both speciesshowed similar increases in material stiffness. This differenceis related to the lack of secondary thickening in the monocotscompared with the dicot sunflowers. Key words: Thigmomorphogenesis, Helianthus annuus L., Zea mays, anchorage, lodging     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. cv. Bien venu in Flowing Solution Culture: II. UPTAKE FROM SOLUTIONS CONTAINING NH4NO3

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. CV. Bien venu in flowing solution culture.II. Uptake from solutions containing NH₄NO₃.—J. exp. Bot.38: 53–66 The effects of root temperature on uptake and assimilation ofNH₄⁺ and NO₃^– by oilseed rape (Brassica napus L. CV. Bienvenu) were examined. Plants were grown for 49 d in flowing nutrientsolution at pH 6?0 with root temperature decrementally reducedfrom 20?C to 5?C; and then exposed to different root temperatures(3, 5, 7, 9, 11, 13, 17 or 25?C) held constant for 14 d. Theair temperature was 20/15?C day/night and nitrogen was suppliedautomatically to maintain 10 mmol m^–3 NH₄NO₃ in solution.Total uptake of nitrogen over 14 d increased threefold between3–13?C but was constant above 13?C. Net uptake of NH₄⁺exceeded that of NO₃^– at all temperatures except 17?C,and represented 47–65% of the total uptake of nitrogen.Unit absorption rates of NH₄⁺ and of 1?5–2?7 for NO₃^–suggested that NO₃^– absorption was more sensitive thanNH₄⁺ absorption to temperature. Rates of absorption were relativelystable at 3?C and 5?C compared with those at 17?C and 25?C whichincreased sharply after 10 d. Tissue concentration of N in theshoot, expressed on a fresh weight basis, was independent ofroot temperature throughout, but doubled between 3–25?Cwhen expressed on a dry weight basis. The apparent proportionof net uptake of NO₃^– that was assimilated was inverselyrelated to root temperature. The results are used to examinethe relation between unit absorption rate adn shoot:root ratioin the context of short and long term responses to change ofroot temperature Key words: Brassica napus, oilseed rape, root temperature, nitrogen uptake     

Correlative Changes in Endogenous Hormone Levels and Shoot Growth Induced by Short Heat Treatments to the Root

Heat treatments of two minutes at 46 to 47°C to root systems of Nicotiana rustica and Phaseolus vulgaris affected roots and shoots. Xylem exudate of Phaseolus was collected, and it was found that heat treatment reduced cytokinin levels and increased abscisic acid levels in the exudate. Shoot and root growth of both species was reduced. Root membrane integrity of Nicotiana was measured and was found to be impaired. It is suggested that the changes in hormone activity due to heat treatment regulate the reduction in shoot growth. Cell wall metabolism and glucosyl transferases making β-glucans were investigated in Phaseolus leaves. Incorporation of ¹⁴C from ¹⁴CO₂ into wall constituents was slightly inhibited but neither photosynthesis nor extracted β-glucan synthetases were affected during the first 12 hours after treatment.