The Pigeonpea-Rhizobium Symbiosis as Affected by High Root Temperture: Effect on Nodule Formation

The effect of elevated temperature on root hair formation, adsorptionof rhizobia, and nodulation of pigeonpea was studied. Nodulationwas adversely affected at both 28°C and 37°C, and theeffect was more pronounced during the first 3 d of nodule formation.Temperatures above 32°C resulted in the reduction or evencomplete absence of root hairs. The root hairs formed at elevatedtemperatures were spheroid and stunted in growth. The numberof loosely and firmly adsorbed cells of Bradyrhizobium spp.(Cajanus) strain CC1021 on pigeonpea roots were reduced to 49%and 38%, respectively, at 37°C. Key words: Pigeonpea, Bradyrhizobium spp. (Cajanus), high root temperature, root hairs, adsorption     

The Effect of Moisture Stress on Infection of Trifolium subterraneum L. by Rhizobium trifolii Dang

Moisture stress and method of inoculation greatly affected thenumber and distribution of infected root hairs and nodules ofyoung seedlings of Trifolium subterraneum. A reduction of soilmoisture from 5·5 to 3·5% (–0·36to –3·6 x 10⁵ Pa) significantly decreased the numberof infection threads and completely inhibited nodulation, althoughthe number of rhizobia in the rhizosphere was unaffected. Atlow soil moisture levels the root hairs were abnormally shortand swollen. Infection and nodulation were little affected between5·5 and 9·5% moisture (–0·36 to –0·089x 10⁵ Pa). Distribution of infected root hairs depended on the initialplacement of the inoculum; with the inoculum mixed evenly throughthe soil, infection threads occurred at discrete foci alongthe root. With seedlings inoculated at planting, infection threadswere restricted to the top 1–2 cm of root, even at thehighest soil moisture tested. Watering increased the number of infections in plants grownat 3·5% moisture; nodules were formed at a rate equivalentto non-stressed plants. Watering also enabled movement of theseedling-borne inocula; new infections were formed along theroot surface bearing mature root hairs.     

The Influence of the Concentration of Gaseous Ammonia on its Uptake by the Leaves of Italian Ryegrass, with and without an Adequate Supply of Nitrogen to the Roots

Whitehead, D. C. and Lockyer, D. R. 1986. The influence of theconcentration of gaseous ammonia on its uptake by the leavesof Italian ryegrass, with and without an adequate supply ofnitrogen to the roots.—J. exp. Bot. 38: 818–827. Plants of Italian ryegrass (Lolium multiflorum Lam.) were grownin pots of soil with two rates of ¹⁵N-labclled nitrate, oneproviding adequate, and the other less than adequate, N formaximum growth. After 25 d in a controlled environment cabinet,the plants were transferred to chambers and exposed for 33 dto NH₃in the air at one of nine concentrations ranging from14 to 709 µg NH₃ m^–3. Increasing the concentrationof NH₃ in the air increased the dry weight of the shoots ofplants grown at the lower but not the higher rate of nitrate.The content of total N in the plant shoots (% dry weight) waslinearly related to NH₃ concentration; at 709 µg NH₃ andin both sets of plants it was more than double the content at14 µg NH₃ m^–3. Calculations, based on ¹⁵N enrichment,indicated that the amount of N taken up from the NH₃ per unitleaf area increased linearly with increasing concentration ofNH₃ in the air uptake (µg dm^–2 h^–1) = 0.1009xat the lower rate of nitrate and 0-0829x at the higher rateof nitrate, where x is the concentration of NH₃ in the air expressedas µg NH₃m^–3. The proportion of the total plant N that was derived from theNH₃ ranged from 4?0% at a concentration of 14 µg NH₃ m^–3with the higher rate of nitrate addition to 77?5% at a concentrationof 709 µg m^–3 with the lower rate of nitrate addition.The proportions of the total N in the water-insoluble proteinof the leaf tissue that were derived from nitrate and gaseousNH₃ were similar to the proportions in the whole leaf material. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum     

Studies on the Physiology of Nodule Formation: With two Figures in the Text: A Reappraisal of the Effect of Preplanting

Nutman (Arm. Bot. 21, 321, 1957) found that preplanting agarslopes with Trifolium pratense L. or Medicago sativa L. advancedthe time when second lots of plants of these species grown onthe same slopes initiated nodules, and depressed the total numberof nodules formed. He attributed these effects to root secretionswhich at low concentration hasten initial nodulation but athigher concentrations inhibit nodule formation. Further workhas now shown that initial nodulation is stimulated becausethe preplant removes traces of nitrate from the medium. Theamount of nitrate in the tap water used to prepare the medium(6?5 p.p.m. N) also increases the number of nodules formed onthe control plants, and this effect explains to a considerableextent the depression of nodule numbers by preplanting. Initial nodulation was delayed by small amounts of nitrate andnitrite but not by other forms of combined nitrogen (ammonium,asparagine, and urea). All forms of combined nitrogen testedincreased the number of nodules formed over a period of 8 weekswhen supplied at an initial concentration of 20 p.p.m. N.     

The Uptake of Gaseous Ammonia by the Leaves of Italian Ryegrass

Lockyer, D. R. and Whitehead, D. C. 1986. The uptake of gaseousammonia by the leaves of Italian ryegrass.—J. exp. Bot.37: 919–927. Plants of Italian ryegrass (Lolium multiflorum Lam.) grown insoil with two rates of added ¹⁵N-labelled nitrate were exposed,in chambers, for 40 d to NH₃ in the air at concentrations of16, 118 and 520 µg m^–3. At the highest concentrationof NH₃, this source provided 47?3% of the total nitrogen inplants grown with the lower rate of nitrate addition (100mgN kg^–1 dry soil) and 35?2% with the higher rate (200mgN kg^–1 dry soil) At the intermediate concentration ofNH₃, the contributions to total plant N were 19?6% and 10?8%,respectively, at low and high nitrate while, at the lowest concentrationof NH₃, they were 5?1% and 32%. Most of the N derived from theNH₃ remained in the leaves, but some was transported to theroots. The amount of N derived from the NH³ that was presentin the leaves was not reduced by washing the leaves in waterat pH 5?0 before harvesting, indicating that the N was assimilatedby the plant and not adsorbed superficially. Rates of uptakeof NH₃ per unit leaf area ranged from 1?7 µg dm^–2h^–1 at a concentration of 16 µg m^–3 to 29?0µg dm^–2 h^–1 at a concentration of 520 µgm^–3 and with the lower rate of nitrate addition. Increasingthe supply of nitrate to the roots slightly reduced the rateof uptake of NH₃ per unit leaf area. Uptake of N from the higherrate of nitrate was reduced at the highest concentration ofNH₃ in the air. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum     

Rhizobium-stimulated Callose Formation in Clover Root Hairs and its Relation to Infection

Callose was detected in the cell walls of the tips of growingroot hairs of Trifolium species and the non-legume Phleum pratenseusing u.v. fluorescence of fresh material stained with 0·005%aniline blue. Inoculation of the roots with Rhizobium trifolii,R. leguminosarum, R. meliloti, and R. japonicum, or additionof 10^–7 and 10^–8 M indole-3-acetic acid (IAA) increasedtip callose formation. Most tip callose was formed at 12 °C, and amounts declinedprogressively at 18, 24, and 30 °C, with very little formedat 36 °C. Tip calloso usually became less and disappearedin individual root hairs as they aged. Callose which appeared prominently in the host cell walls atthe points of initiation of infection threads did not usuallydisappear as the hairs matured. There was little or no extensionof callose along the infection thread and none in the threadtip or in the cell nucleus. Presumptive regions of callose hadsimilar structure and electron density as root hair wall materialand were sometimes related to arrays of vesicles in the hostcytoplasm. The external surface of the hair wall bore smallpegs or papillae (0·1–0·2 µm) continuouswith the outer layer of the wall and possibly associated withattachment of bacteria. Bacteria were usually umboriate at thepoint of attachment and their polyphosphate granules were muchlarger near the root hair than at the distal end.     

Effects of Root Temperature and Form of Nitrogen Nutrition on Nitrate Reductase Activity in Oilseed Rape (Brassica napus L.)

The effect of root temperature and form of inorganic nitrogensupply on in vitro nitrate reductase activity (NRA) was studiedin oilseed rape (Brassica napus L. cv. bien venu). Plants weregrown initially in flowing nutrient solution containing 10 µMNH₄NO₃ and then supplied with either nitrate or ammonium for15 d at root temperatures of 3, 7, 11 or 17 °C. Shoot temperatureregime was similar for all plants; 20/15 °C, day/night.Root NRA was highest when roots were grown at 3 and 7 °C.In laminae and petioles NRA was highest when roots were 11 or17 °C. The plants supplied with ammonium had much lowerlevels of NRA in roots after 5 d than the plants supplied onlywith nitrate. NRA in the laminae of plants supplied with ammoniumwas low relative to that in plants supplied with nitrate onlywhen root temperature was 11 or 17 °C. Values of the apparent activation energy (E_a) of NR, calculatedfrom the Arrhenius equation, in laminae and petioles were differentfrom roots suggesting difference in enzyme conformation. Evidencethat the temperature at which roots were growing affected E_awas equivocal. Oilseed rape, Brassica napus L., activation energy, ammonium, Arrhenius equation, nitrate, root temperature, nitrate reductase     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 2. A Comparison of Monocultures and Mixed Swards

White clover (Trifolium repens L.) and Perennial ryegrass (Loliumperenne L.) plants were grown, in Perlite, in simulated swardsas either monocultures or mixtures of equal plant numbers. Theywere supplied with a nutrient solution either high (220 µgg^–1) or low (40 µg g^–1) in ¹⁵N-labelled nitrateand grown to ceiling yield at either high (20°C day/15°Cnight) or low (10°C day/8°C night) temperature. Temperature had little effect on the maximum rates of grosscanopy photosynthesis which were similar in High-N grass andHigh-N and Low-N clover monocultures. However these maxima werereached more slowly in clover than grass, and more slowly atlow rather than high temperature. Nitrogen supply increasedphotosynthesis in grass but not in clover. Clover had higherN contents than grass in all four treatments, although in anygiven treatment its N content was lower, and contribution ofN₂-fixation relative to nitrate uptake higher, in mixture thanin monoculture. Conversely, grass had higher N contents in mixturethan monoculture, because more nitrate was available per plantand not because of transfer of biologically fixed N from clover. Under Low-N, clover outyielded grass in mixture, particularlyat high temperature. The grass plants in the Low-N mixtureshad higher N contents and higher SLA, LAR and shoot: root ratiosthan those in monoculture. It is proposed that competition forlight is the cause of the low relative yield and negative aggressivityof grass in these swards. Under High-N, grass outyielded cloverin monoculture and mixture, at both temperatures but particularlyat low temperature when grass had a high aggressivity. Nitrogenand yield component analyses shed no light on clover's apparentlylow competitive ability and evidence is drawn from the previouspaper to demonstrate that grass grew faster than clover onlyas spaced individuals during non-com petitive growth. The relativemerits of measures of competitive ability based on final harvestdata and physiological data taken over a growth period are discussed. Trifolium repens L., white clover, Lolium perenne, perennial ryegrass, competition, temperature, nitrogen     

Flower Induction by Nitrogen Deficiency in Lemna paucicostata 6746

Lemna paucicostata 6746, a short-day plant, produced flowerbuds even under continuous light when cultured in nitrogen-deficientmodified Hoagland medium with 1% sucrose for 3 days or morefollowed by culture on nitrogen-rich medium (either nitrateor ammonium). Flowering was also induced by culture on mediumcontaining 20–100 µM nitrate as the sole nitrogensource for 10 days or more, but not on medium with a low ammoniumconcentration. However, if plants cultured on medium containing5–20 µM ammonium as the sole nitrogen source for10 days were grown in a nitrogen-rich medium for a further 4days, they produced flower buds. Thus, nitrogen deficiency caninduce day length-independent flowering in Lemna paucicoslata6746, but nitrogen is required for the manifestation of flowering. (Received January 31, 1986; Accepted April 24, 1986)     

Xylem fluxes of fixed N through nodules of the legume Acacia littorea and haustoria of an associated N-dependent root hemiparasite Olax phyllanthi

Nodulated 1-1.5-year-old plants of Acacia littorea grown inminus nitrogen culture were each partnered with a single seedlingof the root hemiparasite Olax phyllanthi. Partitioning of fixedN between plant organs of the host and parasite was studiedfor the period 4–8 months after introducing the parasite.N fluxes through nodules of Acacia and xylem-tapping haustoriaof Olax were compared using measured xylem flows of fixed Nand anatomical information for the two organs. N₂ fixation duringthe study interval (635 µg N g FW nodules^–1 d^–1)corresponded to a xylem loading flux of 0.20 µg N mm^–2d^–1 across the secretory membranes of the pencycle parenchymaof the nodule vascular strands. A much higher flux of N (4891µg mm^–2 d^–1) exited through xylem at the junctionof nodule and root. The corresponding flux of N from host xylemacross absorptive membranes of the endophyte parenchyma of Olaxhaustorium was 1.15 µg N mm^–1 d^–1, six timesthe loading flux in nodules. The exit flux from haustorium toparasite rootlet was 20.0 pg N mm^–1 d^–1, 200-foldless than that passing through xylem elements of the nodule.Fluxes of individual amino compounds in xylem of nodule andhaustorium were assessed on a molar and N basis. N flux valuesare related to data for transpiration and partitioning of Cand N of the association recorded in a companion paper. Key words: Olax phyllanthi, host-parasite relationships, N flux, Acacia, N₂ fixation     

Effect of Root Temperature on the Infection Processes and Nodulation in Lotus and Stylosanthes

Infection threads were observed abundantly in the root hairsof Lotus corniculatus L., but very rarely in L. hispidus, Desf.,in response to infection by Rhizobium strains 3001 and 3002.Numbers of infections differed between species and strains andwere also affected by temperature. In L. corniculatus all thenodules originated from infection threads, but in L. hispidusmost nodules appeared to originate by direct bacterial penetrationthrough the epidermis, and infected root hairs were very rarelyseen. Both species of Lotus were tolerant to cold temperatures,the minimum temperature for nodulation being 10 ?C. The optimumtemperature for nodulation of L. corniculatus was 20 ?C with3001 and between 27 and 30 ?C with 3002, a few nodules beingformed with both strains at 35 ?C. L. hispidus formed more nodulesthan L. corniculatus and the optimum temperature for both thestrains was between 25 and 27 ?C. No infection threads were seen in root hairs or nodules of Stylosanthesguyanensis (Aubl.) S. W. and S. humilis H.B.K. infected withRhizobium strain CB1552, and all the nodules were formed inthe axils of lateral roots. Optimum temperature for nodulationin S. guyanensis and S. humilis was around 27 ?C; nodulationwas completely inhibited at 15 ?C and very few nodules wereformed at 35 ?C. Both in Lotus and Stylosanthes the transfer of plants from suboptimalto optimal and supraoptimal temperatures increased nodulation.Delayed inoculation and excision of root tips increased nodulation.     

Root temperature and root-hair infection ofTrifolium subterraneum L. cv. Cranmore

Summary The infection of root hairs and nodulation ofT. subterraneum L. cv. Cranmore byRhizobium trifolii strain TA1 was extremely sensitive to root temperature. Within the range 11°–19°C infection was progressively delayed as root temperature decreased but at 7°C no root hairs were infected by 40 days. At 7°C lateral root formation was stimulated in the root zone where nodules formed at 19°C.     

The Effects of Nitrogen Supply on the Absorption and Distribution of Copper in Red Clover (Trifolium pratense L.) Grown in Flowing Solution Culture with a Low, Maintained Concentration of Copper

The absorption and distribution of Cu in red clover (Trifoliumpratense L.) were measured in plants grown in flowing solutionculture with Cu maintained throughout at 0.5 µg 1^–1and N supplied either as nitrate or through symbiotic fixation.Although there was a decrease in Cu absorption, both with time,and with a depleted nitrate supply, it increased to its formerrate when nitrate was adjusted to 10 mg N 1^–1 after aperiod of depletion. Differences in absorption between plantsdependent upon fixation and those supplied with nitrate wererelated to the slower initial growth of the plants fixing N.Considerable proportions (> 30 per cent) of the absorbedCu were retained by the roots. At the final harvest, and withthe exception of plants grown with nitrate adjusted to 0.1 mgN 1^–1 after a period of depletion, the proportion of theCu retained was related to the concentration of N in the roots.The different N treatments produced differences in Cu concentrationin the shoots, and the effects were greater in the youngestfully expanded leaves than in older leaves. Trifolium pratense L., red clover, absorption, copper, flowing solution culture, nitrogen     

Effect of copper excess in environment on soybean root viability and morphology

The effects of increase copper concentrations in medium (10–150 μM CuSO₄) on growth and viability of the roots of two-week-old soybean seedlings (Glycine max L., cv. Dorintsa) were studied. Copper excess suppressed biomass accumulation and linear plant growth; copper affected root growth much stronger than shoot growth. The presence of 10 μM CuSO₄ in medium suppressed accumulation of plant biomass by 40% and the root length by 70%; in the presence of 25 μM CuSO₄, these indices were equal to 80 and 90%, respectively. In the presence of 50 μM CuSO₄, roots ceased to grow but biomass and shoot length still increased slightly. 150 μM CuSO₄ was lethal for plants. The earliest sign of excessive copper toxicity was the accumulation of MDA, indicating activation of membrane lipid peroxidation. A significant increase in MDA content was observed at plant incubation in medium with 10 μM CuSO₄ for 1 h; in this case, the content of copper in the roots increased from 36 ±1.8 (in control) to 48 ± 2.4 μg/g dry wt. The number of dead cells (permeable for the dye Evans Blue) was doubled in the presence of 200 μg/g dry wt within the root; this occurred in 72 h of growth in medium with 10 μM CuSO₄, in 6 h at 25 μM CuSO₄, in 3 h at 50 μM CuSO₄, and 1 h at 150 μM CuSO₄. Toxicity of copper excess was manifested stronger in dividing and elongation cells of the root apex (root meristem and the zone of elongation) than in more basal root regions. Copper excess resulted in the formation of breaks in the surface cell layers of the root tips and affect root morphology. When plant grew in medium with 10 μM CuSO₄, a distance of lateral root formation zone from the root tip decreased markedly, and spherical swellings were formed on the tips of lateral roots. The higher copper concentrations (50 and 150 μM) suppressed completely the development of lateral roots.     

Histology of the Morphogenic Response in Thin Cell Layer Explants from Vegetative Tobacco Plants

The morphogenic response of thin cell layers (TCLs) from vegetativetobacco (Nicotiana tabacum L.) plants can be directed very preciselyby varying the concentrations of benzyladenine (BA) and -naphthaleneacetic acid (NAA) in the culture medium. Medium containing 1·6µM BA and 0·5 µM NAA was optimal for shootformation, concentrations of 0·5 µM BA and 1·6µM NAA were optimal for the induction of shoots and rootson the same explant, whereas concentrations of NAA higher than16 µM resulted in callus proliferation only. Polarityin the distribution of the shoot buds was observed, i.e. a switchfrom basal to apical shoot formation occurred with increasingNAA concentrations, suggesting basipetal transport of NAA. Histologicalexamination of TCLs on shoot induction medium revealed thatfirst cell divisions occurred within 2 d in cortical cells whichwere directly in contact with the medium along the longitudinalcut surface, and after 2 d in subepidermal cells along the lateraledges of the explants. Individual lateral buds originated fromone subepidermal and one or more epidermal cells, while apicalbuds originated from single subepidermal or cortical cells locateddirectly at the apical end of the explant. After culture ofTCLs for 2-3 d on root/shoot induction medium cells in the regeneration-competentsubepidermis elongated, while on callus induction medium subepidermalcells elongated and dedifferentiated. The regeneration systemas described in this study will be used to identify cells competentfor regeneration as well as for transformation.Copyright 1994,1999 Academic Press Nicotiana tabacum L., tobacco, thin cell layer explants, cell competence, shoot development, polarity     

Growth and swainsonine production of Swainsona galegifolia (Andr.) R. Br. untransformed and transformed root cultures

Untransformed and transformed root cultures of Swainsona galegifollawere established for swainsonine production. Transformed rootsgrew faster and produced higher swainsonine levels (62.3 µgg^–1 DW) than untransformed roots (23.6 ,µg g^–1DW) or roots of intact plants (8.7 µg g^–1 DW). Transformationof a number of plant genotypes using A. rhizogenes strain LBA9402 showed that plant genotype Influences swainsonine levelin transformed roots but that a wide range of swainsonine levelscan be induced by separate transformation events in the samegenotype. Enhancement of swainsonine production was attemptedby treatment with sugars and induction of polyploid roots. Key words: Agrobacterium rhizogenes, root cultures, Swainsona galegifolia, swainsonine     

Reactivation of Cytoplasmic Streaming in a Tonoplast-Permeabilized Cell Model of Acetabularia

To find whether cytoplasmic streaming in Acetabularia is controlledby Ca²⁺, a tonoplast-permeabilized cell model was prepared usinga vacuolar perfusion technique. The cytoplasmic streaming remainedalmost normal after perfusion with EGTA medium (10 mM EGTA,40 mM PIPES, 5mM MgCl₂ and 800 mM sorbitol, pH 6.9), but stoppedwithin 10 min when saponin medium (EGTA medium plus 50 µg/mlsaponin, 50 µg/ml hexokinase and 5 mM glucose) was perfused.This model system was reactivated with a solution containing0.5 mM ATP and different concentrations of Ca²⁺ (reactivationmedium). With the reactivation medium at pCa 6–5, theresumed streaming lasted for about 10 min before the cytoplasmaggregated. At pCa 4–3, the streaming was observed onlyfor a few minutes because the cytoplasm aggregated quickly.At pCa 7, no reactivated movement was observed. Reactivationwas not induced in an ATP- or Mg²⁺-deficient medium even inthe presence of an adequate concentration of Ca²⁺, and was inhibitedby 50 µg/ml cytochalasin B or 1 mM N-ethylmaleimide. We concluded from these observations that the cytoplasmic streamingin Acetabularia is very likely to be driven by the actomyosinsystem in the presence of Mg-ATP and Ca²⁺ at pCa 6–5. (Received October 31, 1984; Accepted April 1, 1985)     

The Effect of Nutrient Solution Composition on the Length of Root Hairs of Wheat (Triticum aestivum L.)

The length of root hairs of wheat (Triticum aestivum L.) growingin Long Ashton nitrate nutrient solution was affected mainlyby pH and the concentrations of calcium and nitrate. Root hairlength was decreased by high pH (> 7.0) and by low calciumconcentrations but was increased by nitrate. Buffering at pH6.5 was found to affect the relationship between nutrient solutionstrength and the length of root hairs. The interactions betweencalcium, nitrate and buffering were determined. Phosphate concentrationdid not affect the length of root hairs of wheat, although lowconcentrations have been reported to stimulate root hair growthin other plants. The mechanisms by which pH, calcium and nitratemay affect root hair length are discussed. Key words: Root hairs, Triticum aestivum, Wheat, Nutrient solutions     

Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

The abundance and biomass of the large heterotrophic dinoflagellateNoctiluca scintillans, together with the changes in its potentialprey items, were monitored in the Seto Inland Sea, Japan, duringsummer 1997 (17 July-11 August). Growth and grazing rates ofNscintillans fed natural plankton populations were also measuredeight and seven times, respectively, during the survey period.The abundance and biomass of N scintillans averaged over thewater column (19 m) were in the range 1–345 cells 1^–1(temporalaverage = 93 cell1^–1) and 0.1–49.6 µg C l^–1(temporalaverage = 13.8 µg C l^–1; three times higher thanthat of calanoid copepods during the same period). Noctilucascintillans populations followed the changes in phytoplankton:N.scintillans biomass was increasing during the period of diatomblooms and was at a plateau or decreasing during periods oflow chlorophyll a. The growth rates of N.scintillans (µ)were also consistent with the wax and wane of the N.scintillanspopulation: N.scintillans showed highest growth rates duringdiatom blooms. A simple relationship between µ and chlorophylla concentration was established, and the production of N.scintillanswas estimated using this relationship and the measured biomass.The estimated production averaged over the water column wasin the range >0.1–5.2 µg C l^–1 day^–1(temporalaverage = 1.4 µg C l^–1 day^–1; 64% of the productionof calanoid copepods during the same period). Diatom clearancerates by N.scintillans were in the range 0.10–0.35 mlcell^–1 day^–1, and the phytoplankton population clearanceby N.scintillans was >12% day^–1. Thus, although thefeeding pressure of N.scintillans on phytoplankton standingstock was low, N.scintillans was an important member of themesozooplank-ton in terms of biomass and production in the SetoInland Sea during summer.     

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