Cooperation of Ethylene and Auxin in the Growth Regulation of Rice Coleoptile Segments

Elongation of coleoptile segments, having or not having a tip,excised from rice (Oryza sativa L. cv. Sasanishiki) seedlingswas promoted by exogenous ethylene above 0.3 µl l^–1as well as by IAA above 0.1 µM. Ethylene production ofdecapitated segments was stimulated by IAA above 1.0µM,and this was strongly inhibited by 1.0 µM AVG. AVG inhibitedthe IAA-stimulated elongation of the decapitated segment witha 4 h lag period, and this was completely recovered by ethyleneapplied at the concentration of 0.03 µl l^–1, whichhad no effect on elongation without exogenous IAA. The effectsof IAA and ethylene on elongation were additive. These factsshow that ethylene produced in response to IAA promotes ricecoleoptile elongation in concert with IAA, probably by prolongingthe possible duration of the IAA-stimulated elongation, butthat they act independently of each other. Moreover, AVG stronglyinhibited the endogenous growth of coleoptile segments withtips and this effect was nullified by the exogenous applicationof 0.03 µl l^–1 ethylene. These data imply that theelongation of intact rice coleoptiles may be regulated cooperativelyby endogenous ethylene and auxin in the same manner as foundin the IAA-stimulated elongation of the decapitated coleoptilesegments. Key words: oryza sativa, Ethylene, Auxin, Coleoptile growth     

Control of leaf cell elongation in barley. Generation rates of osmotic pressure and turgor, and growth-associated water potential gradients

In a previous study on the effects of N-supply on leaf cell elongation, the spatial distribution of relative cell elongation rates (RCER), epidermal cell turgor, osmotic pressure (OP) and water potential (Ψ) along the elongation zone of the third leaf of barley was determined (W. Fricke et al. 1997, Planta 202: 522–530). The results suggested that in plants receiving N at fixed relative addition rates (N-supply limitation of growth), cell elongation was rate-limited by the rate of solute provision, whereas in plants growing on complete nutrient solution containing excessive amounts of N (N-demand limitation), cell elongation was rate-limited by the rate of water supply or wall yielding. In the present paper, these suggestions were tested further. The generation rates of cell OP, turgor and Ψ along the elongation zone were calculated by applying the continuity equation of fluid dynamics to the previous data. To allow a more conclusive interpretation of results, anatomical data were collected and bulk solute concentrations determined. The rate of OP generation generally exceeded the rate of turgor generation. As a result, negative values of cell Ψ were created, particularly in demand-limited plants. These plants showed highest RCER along the elongation zone and a Ψ gradient of at least −0.15 MPa between water source (xylem) and expanding epidermal cells. The latter was similar to a theoretically predicted value (−0.18 MPa). Highest rates of OP generation were observed in demand-limited plants, with a maximum rate of 0.112 MPa · h⁻¹ at 16–20 mm from the leaf base. This was almost twice the rate in N-supply-limited plants and implied that the cells in the leaf elongation zone were capable of importing (or synthesising) every minute almost 1 mM of osmolytes. Potassium, Cl⁻ and NO₃ ⁻ were the main inorganic osmolytes (only determined for demand-limited plants). Their concentrations suggest that, unlike the situation in fully expanded epidermal cells, sugars are used to generate OP and turgor. Anatomical data revealed that the zone of lateral cell expansion extended distally beyond the zone of cell elongation. It is concluded that leaf cell expansion in barley relies on high rates of water and solute supply, rates that may not be sustainable during periods of sufficient N-supply (limitation by water supply: Ψ gradients) or limiting N-supply (limitation by solute provision: reduced OP-generation rates). To minimise the possibility of growth limitation by water and osmolyte provision, longitudinal and lateral cell expansion peak at different locations along the growth zone. Received: 15 October 1997 / Accepted: 12 March 1998     

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

A Stepping Motor Auxanometer to Record Rhizome Elongation Continuously

Utilizing a stepping motor combined with a micrometer tablea contact auxanometer capable of recording 2.5 µm incrementsof growth has been developed. This has been adopted to recordcontinuously the pattern of elongation in rhizomes of Agropyronrepens (L.) Beauv. for periods of 1 week. The system has beenshown to be accurate over a range of temperature and humidityconditions. Data presented to support the precision of the auxanometerindicate some of the physiological factors which influence theelongation record. Changes in the light regime around the plantcause transitory changes in the elongation rate. A fluctuationin the elongation record with a period of about 1 h is provisionallyattributed to circumnutational movement of the rhizome apex.     

Cell elongation, turgor and osmotic pressure in developing sunflower hypocotyls

The relationship between cell elongation, change in turgor andcell osmotic pressure was investigated in the sub-apical regionof hypocotyls of developing sunflower seedlings (Helianthusannuus L.) that were grown in continuous white light. Cell turgorwas measured with the pressure probe. The same hypocotyl sectionswere used for determination of osmotic pressure of the tissuesap. Acceleration of cell elongation during the early phaseof growth was accompanied by a 25% decrease in both turgor andosmotic pressure. During the linear phase of growth both pressuresremained largely constant. The difference between turgor andosmotic pressure (water potential) was –0.10 to –0.13MPa. Excision of one cotyledon had no effect on growth, turgorand osmotic pressure. However, after removal of both cotyledonscell elongation ceased and a substantial decrease in both pressureswas measured. In addition, we determined the longitudinal tissuepressure in seedlings from which one or both cotyledons hadbeen removed. Tissue pressure and turgor were very similar quantitiesunder all experimental conditions. Our results demonstrate thatturgor and cell osmotic pressure show a parallel change duringdevelopment of the stem. Cessation of cell elongation afterremoval of the cotyledons is attributable to a decrease in turgor(tissue) pressure, which provides the driving force for growthin the hypocotyl of the intact plant. Key words: Cell elongation, Helianthus annuus, osmotic pressure, tissue pressure, turgor     

Dynamic aspects and enhancement of leaf elongation in rice

Some dynamic aspects of leaf elongation in rice were studied. Under both well watered and water-deficient conditions, leaf elongation rates were 15 to 30% greater during the day than during the night. Night temperatures below 27 C limited the rate of elongation at night but when night temperatures exceeded 27 C, night elongation rates exceeded rates during the day. The diurnal pattern of elongation was opposite to the pattern of bulk leaf turgor which was lower during the day than at night.     

Measurement of Yield Threshold and Cell Wal Extensibility of Intact Wheat Roots under Different Ionic, Osmotic and Temperature Treatments

Yield stress threshold (Y) and volumetric extensibility () arethe rheological properties that appear to control root growth.In this study they were measured in wheat roots by means ofparallel measurement of the growth rate (r) of intact wheatroots and of the turgor pressures (P) of individual cells withinthe expansion zone. Growth and turgor pressure were manipulatedby immersion in graded osmoticum (mannitol) solutions. Turgorwas measured with a pressure probe and growth rate by visualobservation. The influence of various growth conditions on Yand was investigated; (a) At 27 °C.In 0.5 mol m^–3 CaCl₂ r, P, Y and were20.7±4.6 µm min^–1, 0.77±0.05 MPa,0.07±0.03 MPa and 26±1.9 µm min^–1MPa^–1 (expressed as increase in length), respectively.Following 24 h growth in 10 mol m^–3 KC1 these parametersbecame 12.3±3.5 µm min^–1, 0.72±0.04MPa, 0.13±0.01 MPa and 21±0.7 µm min^–1MPa^–1. After 24 h osmotic adjustment in 150 mol m^–3mannitol/0.5 mol m^–3 CaCl₂ r= 19.6±4.2 µmmin^–1, P = 0.68±0.05 MPa and Y and were 0.07±0.04MPa and 30±0.2 µm min^–1 MPa^–01, respectively.After 24 h growth in 350 mol m^–3 mannitol/0.5 mol m^–3CaCl₂ r= 13.3±4.1 µm min^–1, P= 0.58±0.07MPa, Y=0.12±0.01 MPa and ø 32±0.2 tim min^–1MPa^–1. During osmotic adjustment in 200 mol m^–3mannitol/0.5 mol m^–3 CaCl₂, with or without KCl, the recoveryof growth rate corresponded to turgor pressure recovery (t1/2approximately 3 h). (b) At 15 °C. Lowered temperature dramatically influencedthe growth parameters which became r= 8.3±2.8 um min^–1,P=0.78 MPa, r=<0.2 MPa and =15±0.1 µm min^–1MPa^–1. Therefore, Y and are influenced by 10 mol m^–3 K⁺ ionsand low temperature. In each case the effective pressure forgrowth (P-Y) was large indicating that small fluctuations ofsoil water potential will not stop root elongation. Key words: Yield threshold, cell wall extensibility, wheat root growth, temperature, turgor pressur     

Short-term response of Pisum stem segments to indole-3-acetic acid

The short-term response of green pea stem segments to indole-3-aceticacid (IAA) was investigated by continuously recording stem elongationwith a differential transformer. Stem segment elongation promotedby IAA began following a latent period after application. Thelatent period was more effectively shortened by raising thetemperature rather than the concentration of IAA; it was reducednearly to 0 min by treatment at 40?C. The length of the latentperiod was not affected by turgor pressures of stem cells, thoughthe rate of stem growth was diminished at lower turgor pressures.Stems pretreated with actinomycin D for 60 min, cycloheximidefor 30 min or colchicin for 6 hr were similar to untreated stemsin their short term response to IAA. This implies that the initiallypromoted elongation does not result from the activity of enzymessynthesized during the latent period by the action of IAA. (Received April 5, 1973; )     

Biophysical limitation of leaf cell elongation in source-reduced barley

The biophysical basis of reduced leaf elongation rate in source-reduced barley ( Hordeum vulgare L. cv Golf) was studied. Reduction in source strength was achieved by removing the blade of leaves 1 and 2 at the time leaf 3 had emerged 3.0-6.7 cm from the encircling sheath. Third leaves of source-reduced plants elongated at 10-36% lower velocities than those of control plants. Removal of source leaves had no significant effect on maximum relative elemental growth rates (REGRs) and the length of the elongation zone (42-46 mm) but caused a shift of high REGR towards the basal portion of the elongation zone. Cell turgor was similar between treatments in the zone of maximal REGR (16-24 mm from base), but was significantly lower in source-reduced plants in the distal part of the elongation zone, where REGR was also lower. Throughout the elongation zone, osmolality and growth-associated water potential gradients were significantly smaller in source-reduced plants; bulk concentrations of sugars (hexoses, sucrose) were also lower. However, even in control plants, sugars contributed little to bulk osmotic pressure (6-11%). The most likely biophysical limitation to leaf (cell) elongation in source-reduced barley was a reduction in turgor in the distal half of the elongation zone. It is proposed that in the proximal half, increase in average tissue hydraulic conductance enabled source-reduced plants to maintain turgor and REGR at control level, while spending less energy on solute transport.     

Effects of growth temperture on photosynthetic carbon metabolism in green plants I. Photosynthetic activites of various plants acclimatized to varied temperatures

In wheat, pea, barley, broad bean and rape, the photosyntheticcapacities in the leaves of plants acclimatized to a highertemperature (20–25?C) were much higher than those in thesame plants grown at low outdoor temperatures in winter (meanairtemperature, 5–7?C). On transferring the cold-grown wheat plants (vats, 5–7?C)to the higher temperature (25/20?C), the Photosynthetic rateof laves remained unchanged for the first 8 hr, then graduallyincreased to attain after 16 hr a maximum level which was thesame as that in wheat plants grown at the higher temperature(growth box, 20–25?C). When intact wheat without soilwas taken from the cold winter field and the attached leaveswere placed in the chamber at 25?C, the Photosyntheic rate ofthe attached leaves increased sluggishly for 2 hr, then remarkablyincreased to attain the maximum level 9 hr afer transferralto the chamber. When the leaves detached from wheat grown inthe winter field were transferred to the assimilation chamberkept at 25?C, the rate started to increase without a lag periodand attained the maximum level within 3 hr. The optimum temperaturefor photosynthesis shifted in parallel with the rise of airtemperature under which the wheat had been grown. The slopes of increase in the photosynthetic rates with increasingintensity of illumination wwere practically the same in wheatgrown under different temperatures. It was found that the total amount of sucrose in wheat grownat low temperatures in the winter was higher than that in thosegrown at the high temperature. ¹Present address: Department of Botany, Faculty of Science,University of Tokyo, Tokyo, Japan. (Received July 19, 1973; )     

Critical study of coleoptile elongation controlled by IAA and ABA I. Growth kinetics and distribution

The elongation rate of wheat coleoptiles, treated with IAA andABA, was already affected during the first 8 hr of culture.The most sensitive zone of the material—for hormonal treatments—wasfirst localized and then comparatively cultured both in situand in vitro. Growth stimulation by IAA was nearly proportionalto its concentration up to 10^–4 M, while ABA always inducedan significant inhibition. (Received January 31, 1977; )     

Why do leaves and leaf cells of N-limited barley elongate at reduced rates?

The objective of the present study was to assess whether, in barley, nitrogen supply limits the rate of leaf elongation through a reduction in (relative) cell elongation rate and whether this is attributable to a reduced turgor, a reduced availability of osmolytes or, by implication, changed wall properties. Plants were grown on full-strength Hoagland solution (“Hoagland”-plants), or on N-deficient Hoagland solution while receiving N at a relative addition rate of 16 or 8% N · plant-N⁻¹ · d⁻¹ (“16%-” and “8%-plants”). Hoagland-plants were demand-limited, whereas 16%- and 8%-plants were supply-limited in N. Third leaves were analysed for leaf elongation rate and final epidermal cell length, and, within the basal growing region, for the spatial distribution of relative segmental elongation rates (RSER, pin-pricking method), epidermal cell turgor (cell-pressure probe), osmotic pressure (OP, picolitre osmometry) and water potential (Ψ). During the development of the third leaf, plants grew at relative growth rates (relative increase in fresh weight ) of 18.2, 15.6 and 8.1% · d⁻¹ (Hoagland-, 16%- and 8%-plants, respectively). Final leaf length and leaf elongation rate were highest in Hoagland plants (ca. 34.1 cm and 2.33–2.60 mm · h⁻¹, respectively), intermediate in 16%- plants (31.0 cm and 1.89–1.96 mm · h⁻¹) and lowest in 8%-plants (29.4 cm and 1.41–1.58 mm · h⁻¹). These differences were accompanied by only small differences in final cell length, but large differences in cell-flux rates (146, 187 and 201 cells · cell-file⁻¹ · d⁻¹ in 8%-, 16%- and Hoagland-plants, respectively). The length of the growth zone (32–38 mm) was not much affected by N-levels (and nutrient technique). A decrease in RSER in the growth zone distal to 10 mm produced the significant effect of N-levels on leaf elongation rate. In all treatments, cell turgor was almost constant throughout the growing region, as were cell OP and Ψ in 16%- and 8%-plants. In Hoagland-plants, however, cell OP increased by ca. 0.1 MPa within the zone of highest elongation rates and, as a consequence, cell Ψ decreased simultaneously by 0.1 MPa. Cell Ψ increased considerably where elongation ceased. Within the zone where differences in RSERs were highest between treatments (10–34 mm from base) average turgor was lowest, OP highest and Ψ most negative in Hoagland- compared to 8%- and 16%-plants (P < 0.001), but not significantly different between 8%- and 16%-plants. Received: 9 January 1997 / Accepted: 6 March 1997     

Simultaneous measurement of root force and elongation for seedling pea roots

A new method was developed to measure simultaneously, continuously,and non-destructively the elongation rate and the force exertedby the roots of seedlings grown in moist air. A pea (Pisum sativumL. cv. Helka) seedling was suspended inside a modified sampletube on one side of a pulley, with the tip of the radicle pushingon to a force transducer through a hole in the tube. The forceon the root tip was monitored by the force transducer and couldbe adjusted by adding or removing mass from the counterweighton the other side of the pulley. As the root grew, the sampletube was raised and the elongation of the root was monitoredusing a linear variable differential transformer (LVDT) attachedto the thread connecting the sample tube and counterweight.The changes in elongation rate were recorded which occurredin response to increases and decreases in the applied force.Forces of up to 125 mN were exerted on the root, correspondingto forces per unit final cross-sectional area (i.e. root growthpressures) of up to 0.1 MPa. As soon as the force on the root was changed there was a rapidreversible compression or extension of the root. Superimposedon this elastic/viscoelastic deformation, the root elongationrate slowed by more than 50% within 30 min of increasing theforce applied to the root by 100 mN. A similarly fast but smallerincrease in growth rate occurred when the force was removed.Both of these ‘fast’ responses were followed bya longer period of more gradual change in the root elongationrate over a period of 20 h or longer. Both ‘fast’and ‘slow’ responses may be explained in terms ofa modified Lockhart model of growth. The initial ‘fast’response of the root is probably due to the immediate changein the effective pressure (i.e. the turgor pressure minus theyield stress and external resisting pressure) available to drivecell elongation. The reason for the second slower adjustmentof the elongation rate is not known, but is probably due tosome combination of a decrease in the rate of cell productionand/or a stiffening of the cell walls in the longitudinal directionwith increasing mechanical resistance. The increase in rootdiameter in response to mechanical impedance decreased the rootgrowth pressure that the root exerted, but was associated witha slower root elongation rate. Key words: Compaction, mechanical impedance, penetration resistance, root diameter, soil strength     

Control of Wheat Root Elongation Growth: I. EFFECTS OF IONS ON GROWTH RATE, WALL RHEOLOGY AND CELL WATER RELATIONS

Pritchard, J., Tomos, A. D. and Wyn Jones, R. G. 1987. Controlof wheat root elongation growth. I. Effects of ions on growthrate, wall rheology and cell water relations.—J. exp.Bot. 38: 948–959. The nature of the ions in the bathing medium of hydroponicallygrown wheat seedlings strongly influenced root growth rate.In 0·5 mol m^–3 CaSO₄ the growth rate was 32 mm24 h^–1 (used as 100% control rate). K⁺ and SO^– ions(10 mol m^–3) each inhibited extension growth (to about40% and 70% of the control value respectively). In the absenceof K⁺, Cl^– greatly reduced the inhibition due to SO₄^2–.Measurement of tissue plasticity and elasticity in the expandingzone with an Instron-type tensiometer indicated that both werea function of growth rate although relationship of plasticityto growth rate was the steeper and the more pronounced. Turgor pressure at the proximal end of the expanding zone wasnot correlated to growth, being approximately 0·65 MPain all treatments. In mature tissue turgor pressure varied withtreatment, but was also not related to growth rate. Cell membranehydraulic conductivity (5 x 10^–7 ± 1·3 (10)m s^–1 MPa^–2) was not influenced by the presenceof K⁺. We propose that K⁺ and SO₄^{2 –} influence root growthrates by modulating the rheological properties of the wallsof the expanding cell. The physiological significance of these properties is discussed. Key words: Growth, wall extensibility, turgor pressure, wheat roots     

An architectural analysis of the elongation of field-grown sunflower root systems. Elements for modelling the effects of temperature and intercepted radiation

The effects of photosynthetic photon flux density (PPFD) andsoil temperature on root system elongation rate have been analysedby using an architectural framework. Root elongation rate wasanalysed by considering three terms, (i) the branch appearancerate, (ii) the individual elongation rates of the taproot andbranches and (iii) the proportion of branches which stop elongating.Large ranges ofPPFD and soil temperature were obtained in aseries of field and growth chamber experiments. In the field,the growth of root systems experiencing day-to-day natural fluctuationof PPFD and temperature was followed, and some of the plantsunder study were shaded. In the growth chamber, plants experiencedcontrasting and constant PPFDs and root temperatures. The directeffect of apex temperature on individual root elongation ratewas surprisingly low in the range 13–25C, except forthe first days after germination. Root elongation rate was essentiallyrelated to intercepted PPFD and to distance to the source, bothin the field and in the growth chamber. Branch appearance ratesubstantially varied among days and environmental conditions.It was essentially linked to taproot elongation rate, as theprofile of branch density along the taproot was quite stable.The length of the taproot segment carrying newly appeared brancheson a given day was equal to taproot elongation on this day,plus a 'buffering term' which transiently increased if taprootelongation rate slowed down. The proportion of branches whichstopped elongating a short distance from the taproot rangedfrom 50–80% and was, therefore, a major architecturalvariable, although it is not taken into account in current architecturalmodels. A set of equations accounting for the variabilitiesin elongation rate, branch appearance rate and proportion ofbranches which stop elongating, as a function of interceptedPPFD and apex temperature is proposed. These equations applyfor both field and growth chamber experiments. Key words: Sunflower, root system, model, temperature, radiation     

Growth responses of seminal roots of wheat seedlings to a reduction in the water potential of vermiculite

We examined the elongation rate, water status and solute accumulation in the seminal roots of wheat seedlings (Triticum aestivum L.) that were growing in vermiculite with a water potential (Ψ_w) ranging from −0 03 to −1 10 MPa. The elongation rate of the primary seminal root was similar to that of the first pair of seminal roots but that of the second pair of seminal roots was lower at all values of Ψ_w tested. The elongation rate was highest in vermiculite with a Ψ_w of −0.03 MPa but did not decrease significantly until the Ψ_w was reduced to −0.15 MPa. Further reductions in Ψ_w reduced the elongation rate markedly. The Ψ_w of mature tissues was always similar to that of vermiculite. The osmotic potential (Ψ_o) decreased to the same extent as the decrease in Ψ_w. Thus, the turgor pressure (Ψ_p) remained unchanged even in vermiculite with a low Ψ_w. In elongating tissues, Ψ_w and Ψ_o were far lower than they were in mature tissues and, thus, reductions in turgor were not significant. Even when the Ψ_w of vermiculite changed, there were no consistent changes in terms of a difference in Ψ_w between elongating plus mature tissues and vermiculite. There were also no consistent changes in levels of osmotica, calculated using the van’t Hoff’s law, in the elongating tissues but the levels in mature tissues increased in vermiculite with a low Ψ_w. Our results suggest that (1) reductions in root elongation in vermiculite with a low Ψ_w were caused by reductions in the extensibility and/or increases in the yield threshold of cell walls and by reductions in the hydraulic conductivity of the tissues; and (2) a seminal root regulates its growth to keep turgor pressure unchanged.     

The Elongation Rate of Wheat Leaves: I. ELONGATION RATES DURING DAY AND NIGHT

A measuring device is described with which it is possible torecord the elongation rate (in mm h^-1) of monocotyledonous leavesduring the whole growing period. By means of inductive displacementtransducers connected to a computer, the elongation rate isdetermined every 10 min during day and night for 3–8 d.A time period of 30 min proved to be suitable for calculatingthe elongation rate at any given moment. Shorter periods emphasizethe experimental error and longer time periods tend to concealsudden changes in the elongation rate. Elongation rate curvesof wheat leaves are presented. They revealed that the elongationrate. Elongation rate curves of wheat leaves are presented.They revealed that the elongation rates during the night periodwere significantly lower than during the day (approximately60–70% of the rates by day). This reduction is not causedby a reduction of the plant temperature. If the night periodwas extended, the elongation rate decreased almost to zero after14 h darkness. The length of this dark period until decreasebegins was dependent on the previous day's light intensity.The elongation rates of the 3rd wheat leaf under the given conditionswere for 5 d about 3 mm h^-1, and during the nights about 2 mmh^-1. During the 7th day, the elongation rate decreased and remainedzero from the 8th day onwards.     

Nitrate Accumulation and its Relation to Leaf Elongation in Spinach Leaves

The leaf elongation rate (LER) of spinach leaves during theday was twice that during the night when grown at a photon fluxdensity of 145 µmol m^–2 s^–1. All leaves showedthe same LER-pattern over 24 h. Due to low turgor, LER was lowin the afternoon and in the first hours of the night until wateruptake restored full turgor. Osmotic potential remained constantdue to increased nitrate uptake and starch degradation in thisperiod. LER increased to high rates in the second part of thenight and in the morning. The lower rate in the dark comparedto the light was not caused by the lower night temperatures,as increased photon flux density during growth resulted in equalrates in the light and the dark. Increased relative humiditydecreased LER and afternoon rates were most sensitive to waterstress. A ‘low light’ night period did not changeLER-pattern during the night or on the following day. We concludethat nitrate is not an obligatory osmoticum during the nightand can be exchanged for organic osmotica without decreasingLER. During the night the turgor is first restored by increasingwater uptake, nitrate uptake and starch degradation. This resultedin increased leaf fresh weight in this period. Thereafter, elongationincreased by simultaneous uptake of nitrate and water. Nitrateconcentration was, therefore, constant in the older leaves.In the younger leaves nitrate concentration increased to replacesoluble carbohydrates. The vacuoles of the old leaves were filledwith nitrate before those of the young leaves. Key words: Spinacia oleracea L., nitrate accumulation, osmotic potential, organic acids     

Frond elongation rates of shallow waterMacrocystis pyrifera (L.) Ag. In northern Baja California,Mexico

Macrocystis pyrifera (L.) Ag.frond elongation rates were measured during autumn-winter, spring and summer in a shallow water (7.5 m depth) kelp bed in Bahía Papalote, northern Baja California. Frond elongation was maximum during spring and minimum during winter. Frond growth was significantly correlated with solar radiation, and was highest in the smallest size fronds (0–2 m). Average frond elongation rate varied between 0.3–11% d^–1 during the study period. The relationship between average frond length and elongation rate followed an exponential curve with a negative slope during autumn-winter and summer, but was best described by a straight line during spring. Standard growth rates were obtained by a graphic method. Standard growth rates had intermediate values between those reported for southern California and southern Baja California.M. pyrifera growth cycle shows a different trend from what has been previously reported.     

Patterns of Root Respiration Associated with the Induction of Aluminium Tolerance in Phaseolus vulgaris L.

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolusvulgaris L. (‘Dade’) was found to be an inducibletrait. Upon exposure to 10 µM Al, the rate of root elongationwas inhibited in comparison to controls. During the following72 h, the rate of elongation returned to levels comparable tocontrols. In contrast, root elongation of an Al-sensitive cultivar(‘Romano’) did not recover after exposure to Al.In Dade, the resumption of root elongation following exposureto Al was accompanied by increased rates of root respiration,whereas respiration rates slowly declined over the 72 h treatmentperiod in Romano. When partitioned into growth and maintenanceexpenditures, a larger proportion of root respiration of Dadeexposed to Al was allocated to maintenance processes, potentiallyreflecting diversion of energy to metabolic pathways that offsetthe adverse effects of Al toxicity. Romano did not show sucha pattern and respiration associated with both growth and maintenancewas reduced after exposure to Al. Root and shoot growth of bothcultivars were also measured to determine the effects of long-term(21 d) exposure to 10 µM Al. Dade plants exposed to Alexhibited enhanced growth in comparison to controls, whereasRomano plants were characterized by reduced shoot and root growth.Modelling the time-course of root respiration and measuringthe long-term growth responses to Al is a valuable method ofelucidating respiratory costs of stress tolerance. Key words: Aluminium, differential tolerance, maintenance respiration, Phaseolus vulgaris, root respiration