Photosynthesis, Water Relations, Temperature and Canopy Structure as Factors Influencing the Growth of Sainfoin (Onobrychis viciifolia Scop.) and Lucerne (Medicago sativa L.)

Measurements of the growth of sainfoin and lucerne were madein the field after cutting on 31 May 1977. Sainfoin reacheda total above-ground dry weight of 408 g m^–2 over thegrowing period of 48 days compared with 598 g m^–2 in lucerne.Final leaf area indices (LAIs) were 2.8 in sainfoin and 6.1in lucerne. The specific leaf areas (SLAs) for sainfoin wereapproximately half those of lucerne throughout the regrowthperiod. The maximum rates of leaf appearance were 0.12 leavesper day in sainfoin and 0.85 leaves per day in lucerne. Themaximum mean rate of plant extension growth for lucerne of 2.12mm h^–1 occurred during the night, whereas, in sainfointhe maximum rate of 1.72 mm h^–1 occurred during the day. Measurements of extinction coefficients for PAR ranged from0.45 to 0.89 in sainfoin and from 0 42 to 0.57 in lucerne. Asthe lucerne crop increased in size leaf water potentials andsolute potentials became more negative. In sainfoin leaf waterpotentials remained remarkably high throughout the growth period,solute potentials decreased and turgor potentials increased.The stomatal conductances of the two species were similar. The photosynthetic capacities and rates of dark respirationper unit leaf area in both species were similar. The rate ofcanopy ‘gross’ photosynthesis at 295 W m^–2was always greater in lucerne than in sainfoin. This was largelya matter of differences between the species in LAI, althoughat higher LAIs the more erect structure of lucerne leads toa better utilization of photosynthetically active radiation. Onobrychis vicifolia Scop, sainfoin, Medicago sativa L., lucerne, photosynthesis, water relations, temperature, canopy structure     

Flag Leaf Photosynthesis of Triticum aestivum and Related Diploid and Tetraploid Species

Rates of net photosynthesis of the flag leaves of 15 genotypesof wheat and related species were measured throughout theirlife, using intact leaves on plants grown in the field. At thestage when rates were maximal, they were in general highestfor the diploid species, intermediate for the tetraploidspeciesand lowest for Triticum aestivum (means of 38, 32 and 28 mgCO₂ dm^–2 h^–1 respectively). Rates were stronglynegatively correlated with leaf area, leaf width and the meanplan area per mesophyll cell and positvely correlated with stomatalfrequency and number of veins per mm of leaf width. The differencesamong species in these attributes were mainly related to ploidylevel. It was not possible to determine the relative importanceof each anatomical feature, though the changes in stomatal frequencyhad only slight effects on stomatal conductance and the observeddifferences in rates of photosynthesis were much greater thanwould be expected from those in stomatal conductance alone. There was genetic variation in rates of light dependent oxygenevolution of isolated protoplasts and intact chloroplasts butno difference attributable to ploidy. The mean rate, 91 µmolO₂ mg^–1 chlorophyll h^–1, equivalent to 3.9 mg CO₂mg^-1chlorophyll h^-1 was considerably less than the rate of photosynthesisin comparable intact leaves, which was 7.2 mg CO₂ mg^–1chlorophyll h^–1. The total above-ground dry matter yields were least for thewild diploids T. urartu and T. thauodar and the wild tetraploidT. dicoccoides, but the other wild diploids produced as muchdry matter as the hexaploids. The prospects of exploiting differences in photosynthetic ratein the breeding of higher yielding varieties are discussed. Triticum aestivum L., wheat, Aegilops spp, photosynthesis, stomatal conductance, stomatal frequency, polyploidy     

A Sensitive Auxanometer for Field Use

The essential characteristics of an auxanometer designed tomeasure the extension rates of grass leaves in the field areoutlined. The construction, using a linear-variable differentialtransformer (LVDT) to measure displacement, is described. Laboratorytests showed a linear relation between output and armature displacement,which was independent of changes in temperature between 1 and24 °C. Field tests on winter wheat leaves showed good agreementbetween auxanometer and rule measurements of leaf extensionmade over time periods of 24, 12, and 2 h. The tests and ananalysis of the errors, including the thermal expansion of theauxanometer stand, show that this auxanometer can reliably andaccurately measure leaf extension rates of between 0·1and 10 mm h^–1 in wind speeds up to 4·5 m s^–1.     

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     

A Model for C3 Leaves Describing the Dependence of Net Photosynthesis on Irradiance: II. APPLICATION TO THE ANALYSIS OF FLAG LEAF PHOTOSYNTHESIS

Measurements of the photosynthesis-light response of flag leavesin a winter wheat crop were made during the period from maximumelongation until complete senescence. Immediately followingleaf elongation, the maximum rates of photosynthesis and thevalues of efficiency at low light were in the range 2.8–3.6g CO₂ m^–2 h^–1 and 8–11 µg CO₂ J^–1respectively. The shape of the photosynthesis-light responseremained constant throughout and was close to a ‘Blackman’type response rather than a rectangular hyperbola. The resultswere analysed, therefore, using a more recent model which isa non-rectangular hyperbola. Stomatal and internal resistanceswere equally important in limiting the maximum rate of photosynthesis.     

Water Potential, Growth, and Polyribosome Content of the Stressed Wheat Apex

The effect of drought on the growth, ribosomal content, andwater potential of the immature floral apex of wheat plantswas studied under controlled environment conditions. Duringdrought the water potential of the apex (measured with a thermocouplepsychrometer) decreased at approximately the same rate as thatof expanded leaves. Elongation and differen tiation of the floralapex ceased at approximately –12 x 10⁵ Pa and the polyribosomalcontent decreased from 50% of the total ribosomal populationto less than 10%. At this water potential also, elongation ofexpanding leaves was severely inhibited. With continued drought the water potential of the apex continueddecreasing. The exposed leaves died at a water potential ofabout –35 x 10⁵ Pa but the apex was still alive at a waterpotential of –60 x 10⁵ Pa and after rewatering it eventuallyresumed growth.     

A Differentiator for use with Electronic Auxanometers

An electronic differentiator was designed to display directly,in units of mm h^–1, the extension rate of leaves measuredwith displacement transducers. The differentiator calculatesthe mean extension rate occurring over predetermined time intervalsof 0·25 or 1·0 h and discriminates rates of 0·01mm hr^–1. It reduces considerably the work needed to determinemean extension rates from chart records of transducer outputvoltage and improves the accuracy of calculated extension rates.     

Effects of Anoxia on Wheat Seedlings: I. INTERACTION BETWEEN ANOXIA AND OTHER ENVIRONMENTAL FACTORS

Anoxia was imposed on 4–6-d-old, intact wheat seedlings,after the roots had first been exposed for 1 d to O₂ concentrationsbetween 0·016 and 0·06 mol m^–3. Apices ofthe main axis of the seminal roots were considered to have toleratedanoxia if elongation occurred after return from anoxia to air,hereafter called ‘retention of elongation potential’.During anoxia, elongation potential was retained longer in rootsof intact seedlings than in 0–5 mm excised root tips suppliedwith 50 mol m^–3 glucose. In intact seedlings, elongation potential was retained longerat 15°C than at 25°C, and at pH 50 and 60 than at pH40. These differences between treatments were maintained inthe presence of exogenous glucose, and glucose supply prolongedthe retention of elongation potential in all anoxic treatments. Elongation potential was retained much longer at very low 02concentrations (0006 to 00l mol m^–3) than under anoxia;this was established at pH 40. Anoxia inhibited the transport of sugars from the shoots and/orendosperm to the root by 79-97%, as assessed from experimentswith roots of intact plants exposed to anoxia at pH 60 and 15°C. Overall, the results demonstrate: (i) that the occurrence ofadverse effects of anoxia during waterlogging in the field mayinteract with other environmental factors and (ii) that thereare pronounced difficulties integrating data on tolerance toanoxia obtained in different laboratories. Key words: Anoxia, wheat seedlings, pH, temperature     

Carbon Partitioning in Mature Leaves of Pepper: Effects of Daylength

Grange, R. 1. 1985. Carbon partitioning in mature leaves ofpepper: effects of daylength.—J. exp. Bot. 36: 1749–1759. The partitioning of recently fixed carbon has been examinedin mature pepper leaves grown in 6, 10 or 14 h photoperiodsat different irradiances chosen to give similar radiation integralsand in a 6 h photoperiod at the lowest of these irradiances.The partitioning of carbon into export, starch, sugars and respirationwas followed over the photopenod and the subsequent night ina mature leaf. The maximum export rate during the day (approximately 18 µgC cm^–2 leaf h^–1) was not significantly differentamong the treatments. Net photosynthesis rate was directly relatedto irradiance; the proportion of net photosynthesis exportedduring the day was 33% in 6-h days and 57% in 14-h days. Leafstarch accumulation (as a proportion of net photosynthesis rate)increased slightly when plants were grown in 6-h days. The remobilization of starch and sugars at night allowed exportrates to remain similar over 24 h when plants were grown in10-h or 14-h photoperiods. Leaves grown in 6-h days showed nosignificant changes in export rate during the first few hoursof night but exhausted their starch reserves during the nightand export rates declined. Sucrose and hexose levels decreased at the onset of darkness,but did not fall below 40 µg cm^–2 in plants grownin 10-h or 14-h photoperiods; when this level was reached after3–4 h of darkness, starch breakdown began. In leaves grownin both 6-h treatments, sucrose levels fell below 40 µgcm^–2 when starch reserves were depleted during the nightand the export rate decreased concurrently. The results are discussed in relation to the control of exportand starch metabolism in the leaf. Key words: Pepper, partitioning, daylength     

Carbon Dioxide Exchange of Spring-wheat in Relation to Age and Photon-flux Density at Different Growth Temperatures

CO₂-exchange rates (CER) of the sixth and the flag leaves oftwo spring-wheat varieties, Kolibri and Famos, were comparedusing an open-circuit infrared gas analysing system. Measurementswere repeated every two weeks starting when leaf blades werefully expanded. Single plants were grown in a controlled environmenthaving a photopuiod of 15 h and a day/night temperature of 24/19°C(H), 18/13 °C (M), and 12/7 °C (L) respectively untilapprox. 2 weeks after anthesis and at 18/13 °C until maturity.The photosynthetic photon-flux density (PPFD) at the top ofthe plants was 500 µE m^–2 sec^–1. During themeasurements PPFD was gradually reduced from 2000 to 0 µEm^–2 sec^–1 whereas the temperature was maintainedat the respctive growth-temperatures during the light period.The CER of the sixth leaf declined fairly similarly for bothvarieties, except for Kolibri where a faster decline was observedduring the first two weeks after full leaf expansion. The CERof the flag leaf declined more slowly than that of the sixthleaf. With the flag leaf of Famos, the decline was nearly linear,whereas with Kolibri it was very slow during the first few weeksbut rapid as the leaves further senesced. This pattern becamemore pronounced as the growth temperature decreased. The declinein relation to leaf age was much smaller at low PPFD than athigh PPFD during the same period. At full leaf expansion Kolibrireached higher maximum CER than Famos except at H. As the PPFDwas reduced the difference became smaller and at very low PPFDsuch as 50 µE m^–2 sec^–1 was reversed for thesixth leaf. Under optimum growth conditions maximum values ofCER were greater than 50mg CO₂ dm^–2h^–1 and PPFDfor light saturation was close to 2000 µE m^–2 sec^–1.A comparison between the actual CER and a fitted curve widelyused, PN=(a+b/l)^–1–DR, showed that the goodnessof fit strongly depends on cultivar, treatment and leaf ageas well as on the number and the level of PPFD from which datafor calculations are taken. Triticum aestivum, L., wheat, photosynthesis, photon-flux density, light response, carbon, dioxide exchange     

Leaf Extension in Zea mays: III. FIELD MEASUREMENTS OF LEAF EXTENSION IN RESPONSE TO TEMPERATURE AND LEAF WATER POTENTIAL

Leaf extension rates of young maize plants were measured inthe field. Large diurnal fluctuations of air temperature fromabout 5—35°C caused extension rates to vary from 0·4mm h^–1– 3·6 mm h^–1. When the temperatureof the shoot apical meristem was kept at 30–34°C,extension rates remained constant, despite diurnal fluctuationsof air temperature. Leaf water potentials () above –8 or –9 bars hadlittle apparent effect on extension rate. It was concluded that gradients within leaves could account for this.     

Kinetics of Leaf Growth in Lolium temulentum at Optimal and Chilling Temperatures

Lolium temulentum plants were grown at 20 °C, under an 8-hdaylength, in a controlled-environment chamber, and the kineticsof leaf expansion were observed by measuring the movement ofan optical grid attached to the fourth leaf. The leaf emerged23–24 d after sowing and was fully expanded 9–10d later. Extension rate was maximal between the second and fifthdays after emergence and declined markedly thereafter. Duringthe rapid growth phase the rate of elongation exhibited a distinctdiurnal rhythm, fluctuating between 1.9 to 2.3 mm h^–1in the light period, and 1.3 to 1.7 mm h^–1 in the dark.A circadian oscillation with a period of about 27 h was observedin leaves elongating in continuous darkness. When plants weretransferred to 5 °C soon after emergence of the fourth leafthere was an immediate reduction in rate of growth to about22 per cent of the rate at 20 °C: the Q₁₀ for the mean elongationrate in the range 20–5 °C was 3.7. When plants weretransferred from 20 to 2 °C at fourth leaf emergence, meanextension rate declined to less than 5 per cent, correspondingto a Q₁₀ in the range 5–2 °C of more than 300. Furthermore,growth at 2 °C was confined almost entirely to the darkphase of the photoperiod cycle. The responsive tissue was shownto be a small area of expanding leafless than 1.5 cm above theshoot apex and the possible mechanisms underlying low temperatureeffects in this region are discussed. Lolium temulentum L., leaf growth, auxanometer, low temperature, diurnal rhythm     

High Net Assimilation Rates of Sunflower Plants in an Arid Climate

Net assimilation rates of sunflower plants (Heliantkus annuus),grown widely spaced with soil nutrients and water non-limiting,reached 2.0 g dm^–3 wk^–1 in clear weather at midsummerin an arid climate. These rates exceed all previously recordedand are roughly double those hitherto taken to be maximal insunflower. They suggest that maximum rates of photosysnthesisin the most active leaves were 50–65 mg CO₂ dm_–2h_–1. These high rates are a response to the high levels of radiationin the arid climate. They imply that (given non-limiting soil)plants can attain higher productivity in the arid climate thanin any other.     

The Effect of Current Photosynthesis on the Origin of Translocates in Old Tomato Leaves

The rate of carbon transport from an old tomato leaf (54 days),grown at 80 W m^–2, was measured under light flux densitiesbetween 7 and 90 W m^–2. Under low light, the rate of carbontransport over a 6 h period was about 1 mg C dm^–2 h^–1,well in excess of the concurrent photosynthetic rate. The lossfrom these leaves of ¹⁴C-leaf assimilate which was fixed beforethe experimental period amounted to 62 per cent of the totalinitial uptake and was higher than that from leaves with higherconcurrent photosynthetic rates. The higher loss of ¹⁴C fromleaves with low photosynthetic rates was due to a greater contributionof ¹⁴C from the starch and residue fractions. The rate of transportappeared to be determined by the concentration of the labilesucrose, not the total sucrose concentration. In comparisonwith young fully-expanded tomato leaves (Ho, 1976) the sizeof the labile sucrose pool appeared to decrease with age. Thephotosynthesistranslocation coefficient was low (k₁k₂=0•21)for an old tomato leaf. Based on these results a scheme of carbonpartitioning in relation to translocation is proposed. Criteriafor assessing the efficiency of translocation in leaves arediscussed.     

Drought effects on cellular and spatial parameters of leaf growth in tall fescue

The effect of drought and recovery on cellular and spatial parametersof the growth process in tall fescue leaves was studied in twoexperiments. In both experiments plants grown on vermiculiteand maintained in a controlled environment were submitted toa 7 d drought period generated by withholding water. Droughtwas followed by a 3 d recovery period in experiment II. As leafelongation rate (LER) decreased during developing drought boththe growth zone length (initially 40 mm) and the maximum relativeelemental growth rate (initially 0.09 mm mm^–1 h^–1during the dark period of diurnal cycles) within the growthzone declined. But the growth zone still exhibited a lengthof approximately 15 mm when LER approached 0 under severe drought(–2.0 MPa predawn leaf water potential). The growth potentialof the basal 15-mm-long portion of the leaf was conserved duringthe period when drought effected the complete arrest of leafelongation. A (retrospective) analysis of the position-timerelationships of epidermal cells identified on leaf replicas(experiment II) indicated that the cell flux out of the growthzone responded very sensitively to drought. Before drought theflux was maximum at approximately 3.2 cells (cell file h)^–1during the dark period. Flux decreased to 0 when leaf elongationstopped. Flux also varied diurnally both under well-wateredand droughted conditions. In well-watered conditions it wasabout 30% less during the light than the dark period. Cell elongationwas also sensitive to drought. Under well-watered conditionsepidermal cell elongation stopped when cells attained a lengthof approximately 480 µm. During developing drought cellsstopped elongating at progressively shorter lengths. When LERhad decreased to almost nil, cells stopped elongating at a lengthof approximately 250 µn. When drought was relieved followinga 2 d complete arrest of leaf elongation then cells shorterthan 250 µm were able to resume expansion. Following rewateringcell flux out of the growth zone increased rapidly to and abovethe pre-drought level, but there was only a slow increase overtime in the length at which cell elongation stopped. About 2d elapsed until the leaf growth zone produced cells of similarlength as before drought (i.e. approximately 480 µm). Key words: Epidermal cell length, cell flux, (leaf) growth zone, leaf elongation rate, relative elemental growth rate, position-time relationships (path line, growth trajectory), drought, water deficit     

An analysis of relative elemental growth rate, epidermal cell size and xyloglucan endotransglycosylase activity through the growing zone of ageing maize leaves

The effect of development on leaf elongation rate (LER) andthe distribution of relative elemental growth rate (REGR), epidermalcell length, and xyloglucan endotransglycosylase (XET) activitythrough the growing zone of the third leaf of maize was investigated.As the leaf aged and leaf elongation slowed, the length of thegrowing zone (initially 35 mm) and the maximal REGR (initially0.09 mm mm^–1 h^–1) declined. The decline in REGRwas not uniform through the growth profile. Leaf ageing sawa maintenance of REGR towards the base of the leaf. Epidermalcell size was not constant at a given position in the growingzone, but was seen to increase as the leaf aged. There was apeak of XET activity close to the base of the growing zone.The peak of XET activity preceded the zone of maximum REGR.XET activity declined as leaves aged and their elongation rateslowed. When leaf elongation was complete a distinct peak ofXET activity remained close to the base of the leaf. Key words: Leaf elongation rate (LER), relative elemental growth rate (REGR), xyloglucan endotransglycosylase (XET)     

Diel variability in nitrogenous nutrient uptake by phytoplankton in the Chesapeake Bay plume

Diel patterns in the uptake of nitrogenous nutrients were observedin the coastal plume of the Chesapeake Bay system, but the specificpatterns varied with season. During the winter months, ratesof NH₄⁺ and urea uptake were significantly higher at night thanduring the day, and rates of NO₃^– uptake were higher duringthe day. During the summer, rates of NH₄⁺ and urea uptake weresignificantly higher at night only during half the studies conducted;during the remaining studies, there was either no significantdifference or rates of uptake of NH₄⁺ were higher during theday. Rates of NO₃^– uptake during the summer months werealso higher during the day than at night. Seasonal differenceswere also apparent in the time of day at which maximum observeduptake rates of each nitrogen nutrient occurred. During thewinter-spring months, maximum observed rates of NO₃^– uptakeoccurred between first light and noon, whereas during the summermonths, maximum observed uptake rates of NO₃^– occurredboth morning and afternoon, and consistently 9–16 h afterthe maximum observed peak in the uptake of reduced nitrogen.We interpret these findings in terms of seasonal shifts in nitrogennutritional status of the assemblages, as well as species-specificdifferences in the effect of a given stimulus (e.g. a nitrogenpulse at the mouth of the Bay) to entrain an uptake response,and we suggest that the extent of this variability must be understoodbefore generalizations about the use of f-ratios as characteristicsof specific populations or water masses can be drawn.     

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     

Roles of Ethylene and Indol-3yl-acetic Acid in Petiole Epinasty in Helianthus annuus and the Modifying Influence of Gibberellic Acid

The effect of 12 h exposure to ethylene upon epinastic curvatureand elongation of a 5-cm segment in the attached petiole ofHelianthus annuus has been investigated in either normal orGA₂-treated plants. Curvature of segments occurred rapidly inthe first. 6 h during exposure of normal plants to either 1.0or 40.0 parts/10⁶ ethylene, and continued slowly from 6 to 12h. After the ethylene treatment, recovery from the induced curvaturewas completed in 12 h. In 0.2 parts/10⁶ ethylene, recovery fromthe epinastic curvature began during the second half of thetreatment period. Pre-treatment of plants with 60 µg GA₃,did not change the epinastic response to 40.0 parts/10⁶ ethylene.In 10.0 parts/10⁶ ethylene, recovery commenced towards the endof the treatment period, while in 1.0 parts/10⁶ the onset ofepinasty was delayed by about 6 h. In 0.2 parts/10⁶ ethylenethe epinastic response was slight. Ethylene accelerated elongation in the upper half of the petiolesegment. This response was completed within 12 h in all concentrationsand in both normal and GA₃-treated plants. The mean elongationrate in the lower half was depressed from 4.6 to 1.0 mm 24h^–1in 40.0 parts/10⁶ but immediately afterwards it rose to 14.2mm 24 h^–1. A similar response occurred in 1.0 parts/10⁶.In contrast, the elongation of the lower half of the petiolesegment was stimulated by 0.2 parts/10⁶ ethylene. GA₂-treatedplants showed an initial depression of elongation in the lowerhalf in 10.0 or 40.0 parts/10⁶ ethylene, but in the second partof the treatment period the elongation rate recovered to thatof the control segments. Both 0.2 and 1.0 parts/10⁶ ethylenestimulated elongation growth in the lower half of segments inGA₂-treated plants. Removal of the leaf lamina inhibited segment elongation, butdid not affect the growth response of the upper half to 40.0parts/10⁶ ethylene. In contrast the lower half of the segmentno longer showed the usual growth responses to 40.0 parts/10⁶ethylene, although these were partially retained when 10µgof IAA was applied to the cut end of the petiole.     

RNA Synthesis in Phaseolus Chloroplasts: I. RIBONUCLEIC ACID SYNTHESIS IN CHLOROPLAST PREPARATIONS FROM PHASEOLUS VULGARIS L. LEAVES AND SOLUBILIZATION OF THE RNA POLYMERASE

Chloroplast preparations from the young primary leaves of Phaseolusvulgaris L. cv. Canadian Wonder carry out the DNA-dependentincorporation of UTP into RNA at rates between 8 and 14 pmolUTP µg^–1 chlorophyll h^–1. It is estimatedthat 90% of the activity was localized in the chloroplasts.The incorporation proceeded for between 20 and 30 min at 35°C. The maximum rates of RNA synthesis were attained atpH 8.3, in the presence of 15 mM MgCl₂. Chloroplasts were alsoactive, to a lesser extent, with 1.5 mM MnCl₂. The simultaneouspresence of MnCl₂ and MgCl₂ resulted in inhibition of activity.Nuclear material prepared from young P. vulgaris leaves incorporatedUTP at a rate of about 12 pmol UTP µg^–1 DNA h^–1.On a chloroplast (Tritonsoluble) DNA basis chloroplast activitywas over 40-fold that of nuclei. Methods of solubilizing chloroplastRNA polymerase were explored. Yields of over 75% were achieved,but methods suitable for one species were not always successfulwhen applied to another. The highest yields of the P. vulgarisenzyme were obtained using EDTA and KCl. All methods resultedin solubilization of DNA. RNA synthesis by the soluble P. vulgarisenzyme proceeded for more than 40 min at 35 °C.