The Effect of Cooling on Photosynthesis, Amounts of Carbohydrate and Assimilate Export in Sunflower

Sunflower plants (Helianthus annuus L.) grown at 30°C werecooled to 13°C in the light in atmospheric CO² or low CO²,or in darkness. Photosynthetic rate at 30°C after coolingwhole plants in atmospheric CO² for 12 h during a photoperiodwas significantly lower than at the start of the photoperiodcompared to plants cooled at low CO², those cooled in the darkand those maintained at 30°C. Amounts of sucrose, hexosesand starch in leaves at 13°C increased throughout a 14 hphotoperiod to levels higher than in leaves at 30°C, whereamounts of sucrose and hexoses were stable or falling after4 h. Carbohydrate accumulation at 13°C during this photoperiodwas more than twice that at 30°C. After three photoperiodsand two dark periods at 13°C carbohydrate levels in leaveswere still as high as at the end of the first photoperiod, butless carbohydrate accumulated during the photoperiods than duringthe first photoperiod, and more was partitioned as starch. Amountsof soluble carbohydrate in roots were greater after 14 h at13°C than in roots of plants at 30°C. Loss of ¹⁴C fromleaves at 30°C as a proportion of ¹⁴CO2 fixed by them at30°C, decreased after exposure of plants to 13°C inthe light for 30 min prior to ¹⁴CO²feeding. Results indicatean effect of cold on the transport process that was light-dependent.It is inferred that the reduction in the proportion of ¹⁴C lostfrom leaves after 10 h cooling was due to reduced sink demand,whereas the rise in the proportion of ¹⁴C lost from leaves after24 h reflects reduced photosynthetic rate. The coincidence ofreduced photosynthetic rate with raised carbohydrate levelsin leaves maintained at 30°C throughout, whilst the restof the plant was cooled to 13°C in the light implies feedbackinhibition of photosynthesis. This may reduce the imbalancebetween source and sink in sunflower during the first days oflong-term cooling. Key words: Temperature, carbon export, carbohydrates, photosynthesis, sunflower     

Early Effects of Excess Boron on Photosynthesis and Growth of Cucurbita pepo

Summer squash plants {Cucurbita pepo L., cv. ‘Early ProlificStraightneck’) hydroponically cultured under optimal boronnutrition (+ B)³ until age 5 d were transferred to hydroponicsolutions supplemented with excess boron (+ + B)³. As boronaccumulated in the leaves with time, leaf conductance to watervapour, ¹⁴CO₂ fixation, and chlorophyll content of the oldestleaf became significantly less than in the + B-control plantsof the same age; shoot growth essentially ceased. Boron alsoaccumulated in the roots; concomitant inhibition of root elongationand lateral root development resulted. These metabolic changesall occurred prior to the accumulation of sufficient boron tocause the appearance of any symptoms characteristic of borontoxicity. Key words: Boron toxicity, Stomatal conductance, Photosynthesis, Transpiration, Shoot and root growth, Cucurbita pepo     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds     

Translocation of Calcium in Relation to Tomato Fruit Growth

Regulation of the uptake and distribution of calcium in thetomato plant was investigated in plants grown in recirculatingnutrient solutions at electrical conductivities of 2,7,12 and17 millisiemens (mS). Despite an increased calcium content inthe nutrient solution at high conductivity (7–17 mS),the accumulation of calcium by fruit was progressively reducedby increasing salinity, particularly in the distal half. Theincidence of blossom-end rot in fruit (BER) also increased withsalinity. The uptake of water and ⁴⁵Ca by plants was substantially reducedin the high salinity treatment (17 mS) and, to a lesser extent,by high relative humidity (90 per cent r.h. at 20 °C). Further,the translocation of ⁴⁵Ca from roots to shoots was reduced byhigh salinity, while the percentage distribution of ⁴⁵Ca tothe apex was reduced by high humidity. Only approx. 2 per centof the ⁴⁵Ca taken up by a plant was imported by the truss. The uptake of ⁴⁵Ca and its distribution among pedicel, calyxand berry by detached fruit in 24 h showed that fruit from highsalinity plants had a reduced uptake and a lower accumulationof ⁴⁵ Ca in the berry than in the calyx. In addition, plants grown at high conductivity had a lower rateof xylem sap exudation from decapitated plants. The fruit ofthese plants had a smaller xylem cross-sectional area in thefruit pedicel and a smaller calyx than those of the low conductivitytreatment. Calcium, translocation, tomato, fruit, blossom-end rot     

Environmental Effects on the Diurnal Accumulation of 45Ca by Young Fruit and Leaves of Tomato Plants

Diurnal uptake and distribution of ⁴⁵Ca in young fruiting tomatoplants were assessed 12 or 24 h after ⁴⁵Ca was applied to thenutrient solution at the beginning of either the light (12 h)or the dark (12 h) period. During the experiment, the salinityof the nutrient solution (measured as electrical conductivity,EC) was either 2·5 or 17 mS cm^–1 and the relativehumidity (measured as vapour pressure deficit, VPD) was either0·2 or 0·6 kPa The uptake of ⁴⁵Ca by a tomato plant over 12 h was higher inthe light than in the dark but the difference was less at lowhumidity. More ⁴⁵Ca was transported from the roots to the shootin the light than in the dark. More than half of the ⁴⁵Ca inthe shoot was accumulated by the stem; the proportion of ⁴⁵Cain the stem was greater in the dark and was further enhancedby high humidity to more than 80% of the ⁴⁵Ca in the shoot.The accumulation of ⁴⁵Ca by the fruit truss in the dark wasgreater than in the light in all experimental conditions. Underlow humidity the accumulation of ⁴⁵Ca by young leaves was similarin both light and dark. In high humidity there was considerablyless accumulation of ⁴⁵Ca by the young leaves in the dark The uptake of ⁴⁵Ca continued over 24 h but the transport of⁴⁵Ca to individual organs in the second 12 h period was affectedby both light and humidity. Some of the ⁴⁵Ca accumulated byyoung leaves and fruit in the second period appears to havebeen derived from ⁴⁵Ca released from the xylem wall along thetransport pathway in the stem The roles of root pressure and transpiration in the diurnalaccumulation of calcium in young fruit and leaves are discussed Calcium, diurnal translocation, tomato, young fruit and leaves     

Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations,nitrate uptake, and translocation of recently absorbed nitrogen,was investigated in 5-week-old tomato (Lycopersicon esculentumMill cv. Ailsa Craig) plants grown hydroponically in a completenutrient solution. Plants were subjected to dry air (relativehumidity 2–4% for 6 h. The transpiration rate increasedseveral-fold and the shoot water content decreased by almost20%, whereas root water content was unaffected. No effect onin vitro nitrate reductase (NR) activity was detected when usingan EDTA-contraining assay buffer. Replacement of EDTA with Mg²⁺revealed a significant decline in shoot NR activity, which suggestsphosphorylation of the enzyme during the stress treatment. Plantswere grown in a split-root system, in which one root half wasfed ¹⁵N-nitrate during the treatment, in order to determinenitrate uptake and translocation of recently absorbed nitrogenin the plants. Uptake of nitrate was substantially inhibited,but the proportion of absorbed ¹⁵N that was translocated tothe shoots was only slightly affected. In untreated plants,71% of the ¹⁵N recovered in roots had been retranslocated fromthe shoots, whereas in plants subjected to stress the deliveryof ¹⁵N from shoots to roots appeared to be completely inhibited.The data show that lowered humidity in air has significant effectson both uptake of nitrate as well as translocation of nitrogenwithin the plants. Some of these effects appear to be commonwith those observed in plants subjected to reduced water potentialsin the root environment and point to the possibility of theshoot water relations being highly influential on nitrogen uptakeand translocation. Key words: Air humidity, nitrate assimilation, nitrate reductase activity, nitrogen translocation, tomato, water stress     

Potassium Transport in Enteromorpha intestinalis (L.) Link: MEASUREMENT OF INTRACELLULAR K+, EXCHANGE FLUXES AND THERMODYNAMIC ANALYSIS

Potassium transport has been studied in the marine euryhalinealga, Enteromorpha intestimlis cultured in seawater and in low-salinitymedium (Artificial Cape Banks Spring Water, ACBSW; 25·5mol m^–3 Cl^–, 20·4 mol m^–3 Na⁺, 0·5mol m^–3 K⁺). K⁺ fluxes were measured using ⁴²K⁺ and ⁸⁶Rb⁺although ⁸⁶Rb⁺ does not act as an efficient K⁺ analogue in thisplant. ⁴²K⁺ experiments on seawater plants typically exhibiteda single protoplasmic exchange phase whereas ⁸⁶Rb⁺ exhibitedtwo exchange phases. Compartmental analysis of ⁸⁶Rb⁺ effluxexperiments on seawater-grown Enteromorpha plants were usedto deduce the intracellular partition of K⁺ between the cytoplasm(279±38 mMolal) and vacuole (405±68 mMolal). Theplasmalemma K⁺ flux in plants in seawater was greater in thelight than in the dark (563±108 nmol m^–2 s^–1versus 389±66·7 nmol m^–2 s^–1). Inlow-salinity plants, separate cytoplasmic and vacuolar exchangephases were apparent. Analysis of ⁴²K⁺ efflux experiments onlow-salinity plants yielded a cytoplasmic K⁺ of 222±38mMolal and a vacuolar K⁺ of 82±11 mMolal. The plasmalemmaand tonoplast flux was 23±4·5 nmol m^–2 s^–1. The Nernst equation showed that, although K⁺ was close to electrochemicalequilibrium, active accumulation of K⁺ across the plasmalemmaoccurred in plants in seawater and ACBSW both in the light anddark. K⁺ was also actively transported inwards across the tonoplastin low-salinity plants. The electrochemical potential for K⁺across the plasmalemma ranged from 2·41±0·60kJ mol^–1 in plants grown in seawater in the light to 5·79±0·87kJ mol^–1 for plants in ACBSW in the light. Although K⁺is close to electrochemical equilibrium, the flux of K⁺ in plantsin both seawater and ACBSW media is high, hence the power consumptionof K⁺ transport is high. The permeability of K⁺ (P_K⁺) was significantlyhigher in the light than in the dark in plants in seawater (about7·0 versus 2·5 nm s^–1) but in plants inlow-salinity (ACBSW) medium the permeability was independentof light (about 12 nm s^–1). The energy requirements ofactive K⁺ transport by ATP-dependent pumps is discussed. Key words: Enteromorpha, Potassium transport, Ionic relations, Saltwater, Low salinity, Thermodynamics     

Hormonal responses to partial drying of the root system of Helianthus annuus

Plants of Helianthus annuus were pot-grown in soil, with approximately30% of the root system protruding through the base. After 7d, the upper part of the root system of half of the plants wasexposed to drought (internal roots) while the lower part waskept in aerated nutrient solution (protruding root). The treatmentrapidly reduced the internal roots' water content from 26.1to 21.9 g g^–1 dry weight (DW), while in protruding rootsof stressed plants it slowly and continuously decreased from31.9 to 25.2 g g^–1 DW. Leaf water content rapidly decreasedin treated plants from 7.4 to 6.4 g g^–1 DW in the first2d and then reached a plateau. In stressed plants leaf stomatalresistance was significantly higher in the first 3 d while leafwater potential was lower only on the last day. Abscisic acid (ABA) concentration in treated plants increasedsignificantly compared to the controls. In treated internalroots, ABA rose from the first day, reaching a maximum of 1.48±0.49nmol g^–1 DW after 3 d. In treated protruding roots a maximumof 0.99±0.09 nmol g^–1 DW was reached after 1 d.ABA concentration in the xylem sap increased 2 d and 3 d afterthe start of soil drying, with a maximum of 113±12nmoll^–1 during the third day. The ABA rise in the leaves oftreated plants was less significant. Indol-3yl-acetic acid (IAA) concentration in internal rootsof treated plants reached a maximum of 22.54±3.34 nmolg^–1 DW on the third day, then decreased dramatically.The protruding root system of control plants showed a maximumvalue of 16.05±1.77 nmol g^–1 DW on the sixth day. Little difference in cytokinin content of xylem sap was notedbetween control and treated plants. Hormonal variations in different parts of the plant are discussedin relation to drought stress. Key words: Soil drying, roots, ABA, IAA, cytokinins     

Use of the Plastochron Index to Evaluate Effects of Light, Temperature and Nitrogen on Growth of Soya Bean (Glycine max L. Merr.)

The plastochron index (PI) has been compared with leaf growthand biomass accumulation in young soya bean plants of severalcultivars that were grown in controlled environments with differentirradiance levels and durations, temperatures, and nitrogen(N) regimes. Increasing the photoperiod from 10 to 16 h day^–1 increasedthe plastochron rate (PR) and the proportion of axillary growth.Doubling the photosynthetic photon flux density (PPFD) to 1000µmol m^–2S^–1, increased PR and the proportionof roots to total plant weight, but decreased the proportionof stems plus petioles to total. In a series of experiments,the plants were grown in an 8 h photoperiod at constant temperaturesof 17, 20, 26 or 32 °C. As temperature increased, PR increased,but the duration of leaf expansion decreased. Leaves were largestat 20 and progressively smaller at 26, 32 and 17 °C. Biomasswas greatest for a given PI at 20 °C and decreased in theorder of 26, 32, and 17 °C. The proportion of axillary growthalso was greatest at 20 °C. When plants were grown in a15 h photoperiod at temperatures from 17.1 to 26.6 °C, leafsize continued to increase up to the highest temperature. At17 °C, the PR in the 15 h photoperiod (PPFD 390 µmol;m^–2S^–1) was about threefold greater than in 8 h(500 µmol m^–2 S^–1); biomass accumulation perday was about fivefold greater. Increasing N from 3 to 36 mMincreased PR about 10 per cent, altered biomass partitioningamong plant parts, and increased the biomass of the plants.The NO₂ form of N markedly stimulated axillary growth as comparedwith the NH₄⁺ form. Environment or cultivar had little influenceon the duration of leaf expansion in terms of PI. Cultivarsdid not differ consistently in biomass production and allocationin the different environments. Glycine max (L.) Merrill, soybean, soya bean, plastochron index, leaf development, growth analysis, partitioning, light, nitrogen, temperature     

Root Aeration and Respiration in Young Mangrove Plants (Avicennia marina (Forsk.) Vierh.)

The roots of young plants of Avicennia marina (Forsk.) Vierh.grown under simulated tidal conditions were harvested so asto obtain the entire root system. The roots were subdividedand weighed and subsamples taken for manometric determinationof respiration rates at different temperatures. The supply capacityof the above-ground portion of the root system was determinedand the results compared in terms of supply and demand. Theoxygen consumption rate of the roots at 15°C was found tobe 1·69±0·07 µmol kg^–1 s^–1for cable roots and 3·27±0·12 µmolkg^–1 s^–1 for fine roots. The Q₁₀ for respirationwas 2·55 for oxygen consumption in both fine and cableroots, and for carbon dioxide production was 2·66 forfine roots and 3·04 for cable roots. The respiratoryquotient varied with temperature but was less than unity. Concentrationdifferences of between 1·8 mol m^–3 and 3·4mol m^–3 between the inside of root and the air were sufficientto permit aeration of the root system by diffusion alone, andthe aerenchyma contained sufficient oxygen to maintain aerobicconditions while the roots were covered with water. The effectof tide and seasonal temperature change on gas exchange, togetherwith the possibility of some form of carbon dioxide fixationwithin the root, are examined and the implications of theseeffects on growth and development are discussed. Key words: Mangrove, root aeration, respiration, aerenchyma     

The Effect of Light Intensity and Relative Humidity on Growth Rate and Root Respiration of Plantago lanceolata and Zea mays

Plants of Plantago lanceolata L. and Zea mays L., cv. ‘Campo’were grown at two levels of light intensity. Especially in theroots, the rate of dry matter accumulation decreased at lowlight intensity. The carbohydrate content of both roots andshoots of P. lanceolata was not affected by light intensity.The relative contribution of SHAM¹-sensitive respiration, thealternative chain, to total root respiration of both P. lanceolataand Z. mays, was not affected by light intensity during thedaytime. The alternative pathway was somewhat decreased at theend of the dark period, but not in the root tips (0–5mm) where it still contributed 56% in respiration. It was, therefore,concluded that photosynthesis is not a major factor in regulationof root growth in the species investigated. To see whether the effect of light intensity on root growthrate was via transpiration, plants of Z. mays were grown atdifferent air humidities. Both high humidity and low light intensityaffected the root morphology in such a way that the distancebetween the apex and the first laterals on the primary rootaxis increased. It is suggested that this effect on root morphologyis due to transpiration and the subsequent removal of root-producedinhibitors of lateral root growth; although light intensityalso affected the rate of dry matter accumulation of roots andthe rate was not affected by the humidity of the air. It is,therefore, concluded that the effect of light intensity on therate of dry matter accumulation of roots of Z. mays is not viaan effect on transpiration.     

Effects of Growth and Assay Temperatures on Unidirectional K+ Fluxes in Roots of Rye (Secale cereale)

The effects of growth and assay temperature on unidirectionalK⁺ fluxes in excised roots of rye (Secale cereale cv. Rheidol)were studied using ⁸⁶Rb⁺ as a tracer. Both K⁺ influx to thevacuole, estimated as K⁺ uptake between 3 and 12 h after transferof unlabelled roots to radioactive solution, and movement ofK⁺ to the xylem were determined directly. Other fluxes weredetermined on excised roots of plants, which had been labelledwith ⁸⁶Rb⁺ since germination, by conventional triple exponentialefflux analysis. When assayed at 20°C, roots of plants previously grown at20°C(WG roots) had lower rates of net K⁺ uptake than rootsof low temperature-acclimated plants, grown with a temperaturediferential between roots (87°C) and shoots (20°C) eithersince germination (DG roots) or for 3 d prior to experiments(DT roots). This resulted from a greater unidirectional K⁺ effluxacross the plasma membrane and a reduced K⁺ flux to the xylemin WG roots, compared to DG or DT roots, rather than a decreasein unidirectional K⁺ influx or a decrease in the net K⁺ fluxto the vacuole. Indeed, although WG roots had lower rates ofK⁺ influx and K⁺ efflux across the tonoplast at 20°C thanDG or DT roots, roots of plants from all growth temperaturetreatments showed an equivalent net K⁺ flux to the vacuole. Although all unidirectional K⁺ fluxes in roots from plants grownunder all temperature regimes were reduced by lowering the temperatureof the root, these fluxes were differentially affected in rootsof plants from contrasting growth temperature treatments. Rapidcooling to 8°C of WG roots resulted in a lower rate of K+influx and a transient increase in K⁺ efflux across both theplasma membrane and tonoplast, compared to DG and DT roots.Furthermore, since the K⁺ flux to the xylem was lower in WGroots, the net K⁺ uptake at 8°C into WG roots was considerablyreduced compared to DG and DT roots. These results suggest thatlow temperature-acclimation of K⁺ fluxes in rye roots may involvea reduction in the temperature sensitivity of K⁺ influx anda curtailment of K⁺ efflux across both the plasma membrane andtonoplast at low temperatures. Key words: K⁺influx, K⁺ efflux, low temperature, potassium, rye (Secale cereale cv. Rheidol)     

Investigations of Carbon Transport in Plants: II. THE EFFECTS OF LIGHT AND DARKNESS AND SINK ACTIVITY ON TRANSLOCATION

The use of ¹¹C as a tracer has allowed repetitive measurementsof the speed of assimilate translocation to be made on singlemaize plants throughout prolonged periods of light and darkness.The speed appeared to double when the light was switched on.The time required to achieve a maximum speed, usually about3·5 cm min^–1, depended on the duration of the previousdark period. When the plant was transferred to darkness thespeed immediately decreased by about 20 per cent and continuedto decrease over the next 20 h to values of 0·5 to 0·9cm min^–1. The mean speed of translocation in tomato in the light, andother C3 plants, was usually about 1 cm min^–1. It wasreduced by 15–30 per cent when the fruit was removed orcooled from 26 to 10°C.     

Diurnal regulation of NO-3 uptake in soybean plants IV. Dependence on current photosynthesis and sugar availability to the roots

The short-term dependence of NO^–₃ uptake upon photosynthesisand sugar supply to the roots of soybean plants was investigatedin a series of experiments where CO₂ availability, light intensityor conduction of phloem sap to the roots were severely limited.Removal of CO₂ from the atmosphere or girdling of the stem equallyprevented the stimulation of NO^–₃ uptake when plants weretransferred from darkness to the light. The effect of thesetwo treatments can be reversed by CO₂ re-supply or by additionof 10 mM glucose in the nutrient solution, respectively. Glucosewas also more effective in stimulating NO^–₃ uptake byintact plants in darkness than in light. Collectively, theseobservations are interpreted as evidence that the diurnal changesin NO^–₃ uptake are due to decreased phloem transport ofphotosynthates in darkness. Accordingly, the magnitude of thesechanges was much dependent on starch accumulation in the leavesat the end of the photo-period. Shading the plants lowered thisaccumulation, and resulted in an amplification of the diurnalchanges in NO^–₃ uptake. These results are discussed inconnection with the hypothesis that the carbon-dependent plasticityof the night/day ratio of NO^–₃ uptake is an importantfeature of the co-ordination of the acquisition of N and C bythe plant. Key words: Glycine max, light/dark cycle, NO^–₃ uptake, C and N acquisition     

The Effect of Temperature and Light on Gas Exchange and Acid Accumulation in the C3-CAM Plant Clusia minor L

Gas exchange and organic acid accumulation of the C₃-CAM intermediateClusia minor L. were investigated in response to various day/nighttemperatures and two light regimes (low and high PAR). For bothlight levels equal day/night temperatures between 20°C and30°C caused a typical C₃ gas exchange pattern with all CO₂uptake occurring during daylight hours. A day/ night temperatureof 15°C caused a negative CO₂ balance over a 24 h periodfor low-PAR-grown plants while high-PAR-grown plants showeda CAM gas exchange pattern with most CO₂ uptake taking placeduring the dark period. However, there was always a considerablenight-time accumulation of malic acid which increased when thenight-time temperature was lowered and had its maximum (54 mmolm^–2) at day/night temperature of 30/15°C. A significantamount of malic acid accumulation (23 mmol m^–2) in low-PAR-grownplants was observed only at 30/15°C. Recycling of respiratoryCO₂ in terms of malic acid accumulation reached between 2·0and 21·5 mmol m_–2 for high-PAR-grown plants whilethere was no significant recycling for low-PAR-grown plants.Both low and high-PAR-grown plants showed considerable night-timeaccumulation of citric acid. Indeed under several temperatureregimes low-PAR-grown plants showed day/night changes in citricacid levels whereas malic acid levels remained approximatelyconstant or slightly decreased. It is hypothesized that lowand high-PAR-grown plants have different requirements for citrate.In high-PAR-grown plants, the breakdown of citrate preventsphotoinhibition by increasing internal CO₂ levels, whereas inlow-PAR-grown plants the night-time accumulation of citric acidmay function as an energy and carbon saving mechanism. Key words: C. minor, C₃, CAM, citric acid, light intensity     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

Changes in the Distribution of 14C-Labelled Assimilates in Sugar-beet with Variation of Temperature

Plants were allowed to assimilate ¹⁴CO₂ for 30 min at 5, 15,25, and 35 °C. The changes in ¹⁴C content of a mature expandedleaf (Leaf 4), young apical leaves, and storage root, were sequentiallyfollowed over a subsequent period of 24 h in continuous light.In a second experiment plants were transferred after ¹⁴CO₂ assimilationto temperatures of 10, 18, 26, and 34 °C, and the partitionof ¹⁴C between the ethanol-soluble and ethanol-insoluble fractionsof the roots and leaves was followed over a period of 72 h. The specific activities of the apical leaves and of the storageroot increased to a maximum 2 h after labelling at 25 °C,4 h at 15 and 35 °C, and 6 h at 5 °C suggesting thatthe optimum temperature for translocation of photosynthate wasabout 25 °C. The ¹⁴C partition to ethanol-soluble and ethanol-insoluble fractionsof the roots and leaves was largely attained in. 9 h. Littlerepartition of ¹⁴C assimilate fractions occurred as a resultof temperature change or growth. Root ethanol-insoluble activity,however, did increase significantly over the 72-h period : possiblecauses of this slow incorporation and their relevance to themechanism of sugar storage are discussed.     

The Effects of Elevated Atmospheric Carbon Dioxide and Water Stress on Ligth Interception, Dry Matter Production and Yield in Stands of Groundnut (Arachis hypogaea L.

Stands of groundnut (Arachis hypogaea L.), a C₃ legume, weregrown in controlled-environment glasshouses at 28 °C (±5°C)under two levels of atmospheric CO₂ (350 ppmv or 700 ppmv) andtwo levels of soil moisture (irrigated weekly or no water from35 d after sowing). Elevated CO₂ increased the maximum rate of net photosynthesisby up to 40%, with an increase in conversion coefficient forintercepted radiation of 30% (from 1–66 to 2–16g MJ^–1) in well-irrigated conditions, and 94% (from 0–64to 1·24 g MJ^–1) on a drying soil profile. In plantswell supplied with water, elevated CO₂ increased dry matteraccumulation by 16% (from 13·79 to 16·03 t ^–1) and pod yield by 25% (from 2·7 to 3·4t ha^–1).However, the harvest index (total poddry weight/above-grounddry weight) was unaffected by CO₂ treatment. The beneficial effects of elevated CO₂ were enhanced under severewater stress, dry matter production increased by 112% (from4·13 to 8·87 t ha^–1) and a pod yield of1·34t ha^–1 was obtained in elevated CO₂, whereascomparable plotsat 350 ppmv CO₂ only yielded 0·22 t ha^-1.There was a corresponding decrease in harvest index from 0·15to 0·05. Following the withholding of irrigation, plants growing on astored soil water profile in elevated CO₂ could maintain significantlyless negative leaf water potentials (P<0·01) for theremainder of the season than comparable plants grown in ambientCO₂, allowing prolonged plant activity during drought. In plants which were well supplied with water, allocation ofdry matter between leaves, stems, roots, and pods was similarin both CO₂ treatments. On a drying soil profile, allocationin plants grown in 350 ppmv CO₂ changed in favour of root developmentfar earlier in the season than plants grown at 700 ppmv CO₂,indicating that severe waterstress was reached earlier at 350ppmv CO₂. The primary effects of elevated CO₂ on growth and yield of groundnutstands weremediated by an increase in the conversion coefficientfor intercepted radiation and the prolonged maintenance of higherleaf water potentials during increasing drought stress. Key words: Arachis hypogaea, elevated CO₂, water stress, dry matter production     

The Relative Magnitude of the Partial Resistances to Transpirational Water Movement in Sunflower (Helianthus annuus L.)

The effects on leaf water status of cooling entire root systemsor stem and petiolar segments were examined using hydroponicsunflower plants. Leaf water potential (^I) decreased by up to7 x 10⁵ Pa when root temperature was reduced to 10 °C orbelow; complete recovery occurred subsequently provided freezingwas avoided. Leaf water status was unaffected by cooling stemor petiolar segments unless freezing occurred, when severe irreversiblewater stress developed above the cooled zone. The leaves belowthe cooled zone were unaffected, demonstrating that the stressdid not originate from transmitted effects on the roots. Subsequent measurements using small heads of water applied toexcised petioles demonstrated that petiolar resistance was low(c. 0.04 Pa s cm_–4) except in immature and senescent petioleswhere resistance was up to 10 times larger. This trend reflectedthe developmental stage and condition of the xylem. Abrupt increases in evaporative demand, obtained by rapid reductionof relative humidity from 100% to 60 or 40%, induced transientdecreases in of approximately 4.5 and 2.5 x 10⁵ Pa, respectively,which were accompanied by simultaneous large increases in stomatalresistance. No simultaneous transient effects were observedin the stem xylem, demonstrating that the factor responsiblefor the formation of the transient foliar stress resides withinthe lamina. The results are discussed in relation to the relative magnitudeof the various partial resistances to transpirational watermovement.     

Evidence for Symplasmic Ion Transport in Maize Roots

Excised maize roots, placed in saturated water vapour to limitthe external ionic supply, continued to produce exudates attheir basal ends for at least 24 h. The mean rate of fluid exudationfrom roots in water vapour was about 28 per cent of the correspondingrate in ‘control’ roote placed in a solution containing0.1 mil CaCl₂ and 1 mM KC1. Moreover, the net fluxes (mean ±S.E.)of potassium and calcium ions into the exudate were reducedfrom (35.8±3.2) x 10^– and (4.37±0.39) xlO^–9 mole cm^–2 h^– for roots in solution to(10.9±0.6) x 10^–9 and (l.00±0.06)x 10^–9molecm^–2 h^–1 respectively for roots in vapour. It isconsidered that the observation of a prolonged exudation ofwater and ions from the roots placed in water vapour demonstratesthe existence of an alternative ionic supply within the roottissue itself and that this parallel route of ion transportto the exudate constitutes the cortical symplasmic pathway. Pre-treatment of the excised roots with 0.8 M mannitol beforeexudation studies in water vapour and solution led to a significantreduction in the rates of fluid and ion exudation which hadbeen observed in untreated roots under similar conditions. Itis concluded that the plas-molysis, induced by mannitol, disruptedthe symplasmic connections between root cells and that thisperturbation significantly reduced the operation of the symplasmicmode of ion transport into the exudate.