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.     

The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

Spring wheat (Triticum aestivum cv. Warimba) plants were grownin a controlled environment (20°C) in two photoperiods (8or 16 h). In the first instance, plants were maintained in eachof the photoperiods from germination onwards at the same irradiance(375 µE m^–2 s^–1). In the second case, allplants were grown in a long photoperiod until 4 days after double-ridgeinitiation when half the plants were transferred to a shortphotoperiod with double the irradiance (16 h photoperiod at225 or 8 h at 475 µE ^–2 s^–1). The rates of growth and development of the apices were promotedby the longer photoperiod in both experiments. Shoot dry weightgain was proportional to the total light energy received perday whereas the dry weight of the shoot apex increased withincreasing photoperiod even when the total daily irradiancewas constant. The principal soluble carbohydrate present in the shoot apexwas sucrose, although low concentrations of glucose and fructosewere found in the apices of long photoperiod plants late indevelopment. Sucrose concentration was invariably greater inthe slow-growing apices of short photoperiod plants, but roseto approach this level in the long photoperiod plants when theterminal spikelet had been initiated. Triticum aestivum, wheat, apex, spikelet initiation, photoperiod, flower initiation     

The Pattern of Respiration Rate in the Vegetative Barley Plant

In two experiments with young barley plants, respiration rate,carbohydrate content and growth rate of the whole plant weremeasured. When 18-day-old plants were darkened the rate of respirationand the levels of soluble carbohydrate fell in parallel overthe following 30 h. When the dark respiration rate of plantswas followed from 7 to 24 days respiration rate and solublecarbohydrate levels did not change together, nor did the respirationrate (R) follow the empirical relationship with photosynthesis(P) and d. wt (W) R = aW + bP, suggested by McCree. Hordeum distichum L. (Lam), barley, respiration, carbohydrate content     

The Respiration of Mature Field Bean (Vicia faba L.) Leaves During Prolonged Darkness

Dark respiration in attached and detached mature leaves of thefield bean (Vicia faba L.) was studied whilst leaves experiencedup to 60 h of darkness. The results showed: (1) the initialrespiration rate to vary according to the irradiance duringthe previous photoperiod; (2) the dark respiration rate (perunit area) of attached leaves to be essentially constant duringa normal 12 h night although there was a rapid loss in leafd. wt during this time; (3) after 12 h, the respiration rateof attached leaves decayed to an asymptotic value at about 36h; (4) the respiration rate of leaves detached at the end ofthe photoperiod and maintained in the dark on deionised water,decayed only after 36 h of darkness; (5) there was no differencebetween the respiration rate of attached and detached leavesduring the normal 12 h night. It is concluded that the dark respiration of attached fieldbean leaves is intially related to the synthesis and translocationof sucrose in addition to maintenance. After about 36 h, whenthe rate of CO₂ efflux is more or less steady, the CO₂ effluxreflects the intensity of maintenance processes only. The maintenancerespiration rate (determined after 60 h in the dark) rangedfrom 062 to 151 mg CO₂ (g d. wt)^–1 h^–1 but wasrelatively unaffected by several applied treatments. Vicia faba L., field bean, respiration, maintenance, nitrate, non-structural carbohydrate, export     

Comparisons of the Respiratory Effluxes of Nodules and Roots in Six Temperate Legumes

The specific respiration rates of nodulated root systems, ofnodules and of roots were determined during active nitrogenfixation in soya bean, navy bean, pea, lucerne, red clover andwhite clover, by measurements on whole plants before and afterthe removal of nodule populations. Similar measurements weremade on comparable populations of the six legumes, lacking nodulesbut receiving abundant nitrate-nitrogen, to determine the specificrespiration of their roots. All plants were grown in a controlled-environmentclimate which fostered rapid growth. The specific respiration rates of nodulated root systems ofthe three grain and three forage legumes during a 7–14-dayperiod of vegetative growth varied between 10 and 17 mg CO₂g^–1 (dry weight) h^–1. This mean value consistedof two components: a specific root respiration rate of 6–9mg CO₂ g^–1 h^–1 and a specific nodule respirationrate of 22–46 mg CO₂ g^–1 h^–1. Nodule respirationaccounted for 42–70 per cent of nodulated root respiration;nodule weight accounted for 12–40 per cent of nodulatedroot weight. The specific respiration rates of roots lackingnodules and utilizing nitrate nitrogen were generally 20–30per cent greater than the equivalent rates of roots from nodulatedplants. The measured respiratory effluxes are discussed in thecontext of nitrogen nitrogen fixation, nitrate assimilation. Glycine max, Phaseolus vulgaris, Pisum sativum, Medicago sativa, Trifolium pratense, Trifolium repens, soya bean, navy bean, pea, lucerne, red clover, white clover, nodule respiration, root respiration, fixation, nitrate assimilation     

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     

Responses of N2 Fixation-linked Respiration to Host-plant Energy Status in White Clover Acclimated to a Controlled Environment

Ryle, G. J. A., Powell, C. E. and Gordon, A. J. 1988. Responsesof N₂ fixation-linked respiration to host-plant energy statusin white clover acclimated to a controlled environment.—J.exp. Bot. 39: 879–887. Single plants of white clover, acclimated to a controlled environmentand dependent for nitrogen on N₂ fixation in their root nodules,were darkened, defoliated or exposed to enhanced CO₂ levelsto establish the quantitative relationships between the photosynthesisof the host plant and the N₂ fixation metabolism of root nodules. The nodule respiration associated with N₂ fixation (FLR) declinedrapidly to 10–15% of its normal rate following plant darkeningearly in the photoperiod. Darkening at progressively later intervalsduring the photoperiod demonstrated a positive, apparently linearrelationship between duration of illumination and total FLRduring the photoperiod and the following night period. Completeor partial defoliation reduced FLR according to the leaf arearemoved: again, there was a strong positive correlation betweencurrent rate of photosynthesis, whether of defoliated or undefoliatedplants and the FLR of root nodules. Doubling the current rateof photosynthesis, by enhancing CO₂ levels around the shoots,promoted FLR within 1–2 h when plants were stressed bylack of light. However, enhanced CO₂ levels increased FLR onlyslowly over a period of several hours in plants entrained tothe normal growing conditions. It is concluded that, in these plants acclimated to a uniformand favourable controlled environment, the supply and utilizationof photosynthetic assimilate in N₂ fixation was finely balancedand quantitatively linked during a single diurnal period andthat nodule functioning was not depressed by lack of energysubstrate. Key words: White clover, N₂ fixation, photosynthesis.     

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     

Respiratory Efflux of Carbon Dioxide from Mature and Meristematic Tissue of Uniculm Barley During Eighty Hours of Continuous Darkness

Young plants of uniculm barley were grown singly in pots ina growth room at 23/21 °C, and an irradiance of 655 µEm^–2s^–1 during each 12 h photoperiod. At the fifth leaf stage,they were subjected to 80 h of continuous darkness during whichthe rates of CO₂ efflux of vegetative shoot meristems, and maturefully expanded leaves, were separately monitored. Respiratoryefflux from the meristematic tissue was initially high, 12–15mg CO₂ g^–1 h^–1, equivalent to a daily loss in weightof 20–25 per cent. It remained high, or even rose slightly,during what would have been the normal dark period, but thenfell sharply. Even so, it was still three times that of themature tissue at the end of the experimental period. The rateof CO₂ efflux of the mature tissue began low, and fell evenfurther during the first 12 h of darkness. It then levelledoff at a rate of 2·0–2·5 mg CO2 g^–1h^–1, equivalent to a daily loss in weight of about 3 percent. It is suggested that the rate of ‘mature tissue’respiration, established after 12–24 h of darkness, mightbe a useful selection criterion to employ in attempts to increasethe total dry matter yield of the grass crop by breeding. Hordeum vulgare L., barley, respiration, synthetic respiration, maintenance respiration, meristem, mature tissue respiration, simulated sward     

Temperature and Time-course of the Carbon Balance of Vegetative Tomato Plants During Prolonged Darkness: Examination of a Method of Estimating Maintenance Respiration

In order to examine the suitability of estimating maintenancerespiration in prolonged darkness, the variation of structuraldry matter (SDM) was calculated on vegetative tomato plantsduring 48 h of darkness. For that purpose, the time-coursesof respiration rate and carbohydrate content were recorded inshoots and roots at temperatures of 10, 15, 20, and 25 °C Two exponential declines of respiration rate, separated by ashort resumption, were observed in shoots and roots, differentcarbohydrate pools might be involved. Respiration rate was alwayshigher in roots than in shoots: the part played by energy costsof mineral absorption has to be investigated. After 14 h ofdarkness, a fall in respiration rate was associated with a progressiveexhaustion of sucrose and starch - which was quicker at highertemperatures - and a decrease in shoot to root carbon translccation.After 24 h of darkness, respiration stabilized at all temperatures.However, structural growth persisted throughout the dark periodat 10 °C, stopped after about 14 h darkness at. 15 and 20°C, and became negative beyond 24 h at 25 °C The hypothesis of maintenance of SDM after a period of darknesscan thus be invalidated. The simple observation of the time-courseof respiration rate does not allow complete inferences to bemade concerning biomass maintenance Lycopersicon esculentum Mill., tomato, respiration, maintenance respiration, carbohydrate reserves, translocation, structural dry matter, temperature     

Influence of Carbohydrates on Photosynthesis in Single, Rooted Soybean Leaves Used as a Source-Sink Model

The single rooted leaf of soybean (Glycine max L. Merr.) wasused to study source-sink relationships in photosynthesis. Whenthe leaves were kept under a regime of 10 h light (410–480µmol photons m^–2, 400–700 nm)–14 h dark,they did not expand, the increase in leaf dry weight almoststopped, and photosynthetic activity remained at a high andconstant level for 8 d while the dry weight of the roots increasedat a constant rate throughout the period. Thus, under this conditionthe leaf and the root system served as the only source and sinkorgans, respectively. When leaves grown for 7 d under this conditionwere placed under continuous light to alter the source/sinkbalance in photosynthate, the root dry weight increased at aconstant rate equal to that found under the 10 h light–14h dark condition. The leaf dry weight markedly increased andby day 5 of continuous light had increased 1.6-fold, mainlyas a consequence of accumulation of starch and sucrose, whichwere not translocated for root growth. The continuous lightcaused an abrupt decrease in the photosynthetic activity (40%of initial value by day 5). However, the activity recoveredalmost completely after a 32-h transfer to darkness. Significantnegative correlations existed between photosynthetic activityand the sucrose and starch contents in the rooted leaves placedunder continuous light. When the plants were treated with variouslight conditions, there was no significant difference (p<0.01)among the regression line slopes for photosynthetic activityon the sucrose content, but there was some deviation among thosefor the photosynthetic activity on the starch content. Thisresult suggests that sucrose accumulated in the leaf has a moredirect influence on photosynthetic activity when the source/sinkbalance was altered. (Received September 9, 1985; Accepted February 21, 1986)     

Changes in the Root System of Wheat Seedlings Following Root Anaerobiosis I. Anatomy and Respiration in Triticum aestivum L.

Anatomical changes in roots of wheat seedlings (Triticum aestivumL. cv. Hatri) following oxygen deficiency in the rooting mediumwere investigated. The response of the plant to stress was testedat a very early developmental stage when the first adventitiousroots had just emerged. In order to analyze the adaptation ofdifferent roots, respiration rates of the roots 1–3 and4–n were compared with the respiration rates of the totalroot system. Oxygen deficiency was induced either by flushingnutrient solution with nitrogen or flooding of sand. In contrast to plants grown in well aerated media, both stressvariants led to a significant increase of the intercellularspace of the root cortex in seminal and first adventitious roots.Radial cell enlargement of cortical cells near the root tip,cell wall thickenings in flooded sand cultures and an increasein phloroglucinol-stainable substances were found to be furtherindicators of low oxygen supply. The roots 4–n which were promoted in growth under hypoxiashowed higher respiration rates; hence the total root respirationwas not restricted. Triticum aestivum L. cv. Hatri, wheat, roots, anatomy, anaerobiosis, stress, root respiration, intercellular space     

The Inhibition of Growth in Higher Plants by a Homologous Series of Guanidines and its Reversal by Spermine

The effects of guazatine, synthalins A and B and a homologousseries of aliphatic monoguanidines on the growth of cress, barleyand oat seedlings, and apple cell suspension cultures have beenstudied. In the homologous series of aliphatic monoguanidines[NH₂C(=NH)NH(CH₂)_x–1CH₃] greatest inhibition was foundwith x = 8-10 for cress, barley and oats and x= 10–14for apple cells. Spermine partially reversed the inhibitionin the light for cress and barley but in the dark no reversalwas found. Technical guazatine inhibited growth to a greaterextent than pure guazatine, and was comparable in toxicity tosynthalin B in cress, barley and oats. Reversal by spermineof inhibition due to guazatine and synthalin B was greater inthe light than in the dark in these plants. Calcium ions didnot reverse the toxicity of guazatine, synthalin B or dodine.Reversal of the inhibition by guazatine, synthalin B and dodineof the growth of the apple cells was considerably greater withspermine than with spermidine. Lepidium sativumcress, Hordeum vulgarebarley, Avena sativaoat, Malus sylvestrisapple, guanidines, guazatine, synthalins, dodine, spermine, spermidine     

Respiration and Phloem Translocation in the Roots of Chickpea (Cicer arietinum)

Root growth in chickpea (Cicer arietinum) has been studied fromthe early vegetative phase to the reproductive stage in orderto elucidate its growth and maintenance respiration and to quantifythe translocation of assimilates from shoot to root. A carbonbalance has been drawn for this purpose using the growth andrespiration data. The increase in the sieve tube cross-sectionalarea was also followed simultaneously. Plants growing in a nutrient culture medium were studied todetermine the relative growth rate (RGR) 5–60 d aftergermination. RGR declined from 113 to 41 mg d^–1 g^–1during the measurement period. Simultaneous with the RGR analysis,respiration rate was also measured using an oxygen electrode.The respiration rate declined as the plants aged and a drasticreduction was recorded following anthesis. The relationshipbetween RGR and respiration rate was used to extrapolate themaintenance respiration (m) and growth respiration (1/Y_EG).The respiration quotient (r.q.) of the roots was 1.2 and theQ₁₀ in the range 20–25 °C was 2·2. A carbon balance for the roots was constructed by subtractingthe carbon lost during respiration from that gained during growth.The roots were found to respire no less than 80% of the carbontranslocated. The increase in the cross-sectional area composed of sieve tubeswas measured near the root-shoot junction as the plants grew.Chickpea has storied sieve plates which simplifies these measurements.Their cross-sectional area increased during growth mainly becauseof an increase in sieve tube number. The diameter of individualsieve tubes remained constant. Specific mass transfer (SMT) values for seive tubes into theroots have been computed during various stages of growth. SMTvalues were relatively constant before anthesis (approx. 6·5g h^–1 cm^–2), but decreased following anthesis. Wedid not evaluate possible retranslocation from roots: any suchretranslocation would have the effect of increasing our SMTvalues. Chickpea, Cicer arietinum, legume, root, respiration, phloem, translocation, carbon balance, specific mass transfer, sieve-tube dimensions     

The Relationship between Sugar and Amino Acid Export to the Phloem in Young Wheat Plants

The relationship between amino acid and sugar export to thephloem was studied in young wheat plants (Triticum aestivumL. ‘Pro-INTA, Isla Verde’) using the EDTA-phloemcollection technique. Plants grown with a 16 h photoperiod showeda rapid decrease in the concentration of sugars and amino acidsin the phloem exudate from the beginning of the dark period.When plants grown with a 16 h photoperiod were kept in the darkfor longer than 8 h the free amino acid content in leaves andexudate (on a dry weight basis) increased continually throughoutthe 72 h of darkness. During the first 24 h of darkness thesugars in the phloem exudate decreased to 30% of the initialvalue, and returned to the control level when plants were returnedto light. When plants grown under low light intensity for 10d were transferred to high light intensity, they showed an increasein leaf sugar content (dry weight basis) after 3 d but therewere no differences in leaf free amino acid content (dry weightbasis) compared to low-light plants. The sugar concentrationin the phloem exudate was increased by higher light intensities,but there was no difference in the amino acid concentrationof the phloem exudate, and thus the amino acid:sugar ratio inthe phloem decreased in the high-light plants. The present resultssuggest that amino acids can be exported to the phloem independentlyof the export of sugars. Copyright 1999 Annals of Botany Company Sugar exudation, amino acid transport, nitrogen, phloem, transport, wheat, Triticum aestivum L.     

A Study of the Relationship between Root and Shoot Metabolism

The influence of light and dark on the respiration rate, sugarlevel, and rubidium uptake of the roots of young sunflower andbarley plants has been investigated. The rate of root respirationwas found to be closely correlated with the rate of carbohydratetransport from the shoots. This enabled the energy requiredfor root growth to be calculated. A period of darkness causeda reduction in rubidium uptake and transport which appearedto be due to the reduced rate of root metabolism in the dark.The experiments of some earlier workers on the effect of transpirationon rubidium uptake by young sunflower and barley plants wererepeated. The results indicated that they were studying themetabolic effect of light and dark on salt uptake as well asthe effect of transpiration.     

Diurnal Functioning of the Legume Root Nodule

Diurnal changes in plant and nodule performance were studiedin 28–9 d plants of Pisum sativum L. in two environments,both with a 12 h (27 000 lx):12 h::light:dark cycle, but one(A) with a fluctuating temperature-humidity regime (photoperiod18 ?C, 60 per cent relative humidity:night 12 ?C, 85 per cent),the other (B) with constant temperature (18 ?C) and humidity(75 per cent). Fixation rate (C₂H₂ reduction), respiratory output of the nodulatedroot, and nodule sugar level increased throughout the photoperiod,whereas nodule soluble nitrogen level declined steadily. Reversalof these trends in the night period led, at its end, to minimain fixation rate, sugar level and respiration, but a maximumin soluble nitrogen. The A environment produced the greaterday:night fluctuations in transpiration and nodule soluble nitrogen,but B, with its higher night temperature, induced the more pronounceddecrease in fixation at night. Slightly less nitrogen was fixed during the photoperiod thanduring the night in the A environment, yet since some fixationproducts were retained in the nodules at night and not releaseduntil the next photoperiod, the day: night difference in nitrogenexport from nodules was 1.8:1. The photoperiod of A was alsoa time of higher nodule respiration and replenishment of nodulesugar and starch, so that the nodules' requirement for translocatedcarbohydrate was more than twice that at night. Humidity decrease in the photoperiod (of A) elicited higherrates of transpiration and a more rapid than normal emptyingof soluble nitrogen from the nodules: elevation of humidityhad the opposite effects. Shoot removal (A-grown plants) causednodule sugar levels to fall rapidly below those normally encounteredin intact plants.     

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremelyhigh above-ground dry-weight productivity of 40 Mg ha^–1yr^–1. To help understand the below-ground capabilitiesthat support the high above-ground productivity of these Crassulaceanacid metabolism plants, roots were studied in the laboratoryand in plantations near Mexico City. For approximately 15-year-oldplants, the lateral spread of roots from the plant base averaged1.3 m and the maximal root depth was 0.8 m, both considerablygreater than for desert succulents of the same age. Root andshoot growth occurred all year, although the increase in shootgrowth at the beginning of the wet season preceded the increasein growth of main roots. New lateral roots branching from themain roots were more common at the beginning of the wet season,which favoured water uptake with a minimal biomass investment,whereas growth of new main roots occurred later in the growingseason. The root: shoot dry weight ratio was extremely low,less than 0.07 for 6-year-old plants of both species, and decreasedwith plant age. The elongation rates of main roots and lateralroots were 10 to 17 mm d^–1, higher than for various desertsucculents but similar to elongation rates for roots of highlyproductive C₃ and C₄ agronomic species. The respiration rateof attached main roots was 32 µmol CO₂ evolved kg^–1dry weight s^–1 at 4 weeks of age, that of lateral rootswas about 70% higher, and both rates decreased with root age.Such respiration rates are 4- to 5-fold higher than for Agavedeserti, but similar to rates for C₃ and C₄ agronomic species.The root hydraulic conductivity had a maximal value of 3 x 10^–7ms^–1 MPa^–1 at 4 weeks of age, similar to A. deserti.The radial hydraulic conductivity from the root surface to thexylem decreased and the axial conductivity along the xylem increasedwith root age, again similar to A. deserti. Thus, although rootsof A. mapisaga and A. salmiana had hydraulic properties perunit length similar to those of a desert agave, their highergrowth rates, their higher respiration rates, and the greatersoil volume explored by their roots than for various desertsucculents apparently helped support their high above-groundbiomass productivity Key words: Crassulacean acid metabolism, productivity, root elongation rate, root system, water uptake     

Diurnal Production and Utilization of Photosynthate in Nodulated White Clover

Gordon, A. J., Mitchell, D. F., Ryle, G. J. A. and Powell, C.E. 1987. Diurnal production and utilization of photosynthatein nodulated white clover.—J. exp. Bot. 38: 84–98. A steady-state ¹⁴C-labelling technique was used to examine thediurnal carbon fixation, storage and export characteristicsof white clover leaves. Approximately 70% of fixed carbon wasexported to other organs during the photoperiod. The remainingcarbon was stored mainly as starch (80% at the end of the photoperiod)with smaller amounts of sucrose, hexoses and charged compounds.Carbon export from the leaf at night was provided by remobilizationof starch. During the photoperiod it was estimated that c.60% of carbonexported from the leaf was directed towards the nodulated root;45% to nodules and 15% to roots. The 40% directed towards theshoot was supplemented by a further 11% of carbon (in the formof amides) re-exported from the nodules. During the photoperiod, all organs of the plant accumulatedcarbohydrate which was available for use during darkness, inconjunction with a diminished supply of exported carbon fromleaves. Nodules exhibited a striking pattern of carbohydratestorage and depletion. The levels of sucrose and starch in thenodules at the end of the photoperiod were sufficient to maintainN₂ fixation for 8–9 h of the 12 h dark period. We proposethat continued import from leaves provided the additional sucrosenecessary to support undiminished nodule function throughoutthe entire dark period. Key words: White clover, photosynthate, starch, carbohydrate, nodules, N₂ fixation     

Effects of Photoperiod on Germination of Seeds of Talinum triangulare (Jacq.) Willd

Seed germination in Talinum triangulare as affected by photoperiod,with or without previous incubation in the dark in water at25 or 4 °C, was studied. The time course and quantity ofseed germination in photoperiods of 1 h and above were similarwith or without dark pretreatment, but the time to half maximumgermination was reduced from 12 days in non-dark pretreatedseeds to 4 days in seeds given 20 days in the dark at 25°C.A photoperiod of 0·25 h gave a lower rate and total germinationthan photoperiods of 1 h and above. Un-pretreated seeds required17 cycles of 24 h photoperiod for maximum germination as comparedwith 7 or less cycles if the seeds received more than 10 daysdark pretreatment at 25 °C. Both the rate and total germinationin light increased as the length of dark pretreatment at 25°C was increased from zero to 30 days. Incubation of theseeds in water in the dark at 4 °C for 5 to 30 days priorto illumination at 21 °C, reduced both the rate and quantityof seed germination in light as compared with those similarlyincubated in the dark at 25 °C. However, previous incubationin the dark for 30 days at 4 °C partially substituted forthe light requirement. The possible mechanism of breakage ofseed dormancy in Talinumis discussed in relation to these andother findings. Talinum triangulare (Jacq.), Willd, light, photoperiod, seed germination