Effect of Waterlogged Soil Conditions on the Production of Ethylene and on Water Relationships in Tomato Plants

The roots of tomato plants (Lycopersicon esculentum Mill., cv.Moneymaker) were exposed to low concentrations of oxygen bywaterlogging the soil or by growing the plants in nutrient solutionflushed with nitrogen gas. After 24 h, the rate of ethyleneproduction by the petioles, main stem, and shoot apex was increasedby 4–6-fold and the petioles developed epinastic curvatures.Removing the roots did not reproduce these responses. The amountsof ethylene produced by shoot tissues in response to physicalwounding was greatly increased by waterlogging the soil. The production of ethylene by roots was suppressed by the absenceof oxygen. When the roots were transferred back to an aerobicenvironment ethylene production quickly exceeded that observedin roots maintained continuously in aerobic conditions. The enhanced rate of ethylene production in the shoots occurredin the absence of increased water stress as measured with aleaf pressure chamber; leaf water potentials were increasedrather than decreased by waterlogging for 30 h or more. Thiswas associated with stomatal closure and reduced transpiration.Resistance to water flow through the plant increased as transpirationdecreased in response to waterlogging. However, at similar ratesof transpiration, resistance was normally lower in waterloggedplants than in controls.     

Regrowth and Nutrient Composition of Different Plant Organs in Grass-clover Canopies as Affected by Phosphorus and Potassium Availability

Regrowth after cutting and the distribution of nitrogen (N),phosphorus (P) and potassium (K) in different plant organs ofwhite clover and perennial ryegrass growing in pure or mixedswards were investigated under field conditions in a soil witha low-to-moderate availability of P and K. In all treatments,white clover constituted more than 70% of the above-ground biomassin the mixed swards. The petioles were the dominant pool ofdry matter throughout regrowth and contained the greatest amountsof N, P and K. Increased supply of P and K increased the growthof ryegrass, but not that of white clover in the mixed swards.The increased competition from ryegrass led to a decline inthe yield of white clover laminae as well as in the N contentper unit of dry matter in laminae, petioles and stolons. TheP content of all white clover organs also declined followingP application to the mixed swards, whereas K application increasedtheir K contents. In the pure swards of ryegrass and white clover,yields and contents of N, P and K in the dry matter were eithernot affected or increased following P and K application. Itwas concluded that commonly-used defoliation heights may remove80% or more of the nutrient and dry matter pools located inthe petioles but the remaining quantities of dry matter andnutrients in the petioles will normally exceed the correspondingquantities in the stolons. Copyright 2001 Annals of Botany Company Coexistence, competition, phosphorus, potassium, regrowth, ryegrass, white clover     

Axillary Bud Development in White Clover in Relation to Defoliation and Shading Treatments

A period of growth under shade netting in the glasshouse allowedthe cultivation of white clover stolons with an accumulationof undeveloped axillary buds similar to that often found onstolons from grass/clover swards. The subsequent capacity ofthese nodes to develop branches under different circumstanceswas investigated in three experiments. Removal of the laminaeand petioles subtending sets of four buds along a stolon reducedthe rate at which branches were initiated from the buds. Treatmentsin which petioles, or petioles plus laminae, were retained initiatedbranches more quickly. Shading the stolons reduced both therate of initiation and the percentage of buds which developed,unless both petioles and laminae were retained. There was someevidence that conditions applied to individual buds may actin the same way as the same conditions applied to sets of fourbuds and that illuminated nodes may depress the performanceof neighbouring shaded notes. Fewer buds developed at older nodes than at younger nodes duringthe summer, but during the autumn younger buds initially developedmore slowly than older buds. This suggests that buds can developat a younger nodal age in summer than in winter. When leafless stolons were cut up into component internodesbuds developed faster than on intact stolons, provided the budwas located at the end of the internode nearest the main stolongrowing point. If the bud was at the other end, branch developmentwas slower than on intact stolons. The results are discussedin relation to clover growth in sward conditions. White clover, Trifolium repens, axillary bud development, branching, growing points, defoliation, shading     

Contribution from Stolons and Roots to Estimates of the Total Amount of N2Fixed by White Clover (Trifolium repensL.)

Growth and N-accumulation rates in leaves, stolons and rootsof individual white clover plants were studied in three experimentsusing two methods. In a growth chamber experiment, the relativedifferences between tissues were found to be almost constantfor a wide range of clover plant sizes. The stolon dry matter(DM) production was 56% and the root DM production 40% of theDM production in leaves. The N yield of stolons was 30% whileN yield in roots was 34% of N yield in leaves. The effect ofN application on these relations was investigated in a glasshouseexperiment. Application of N reduced the root:shoot N ratiofrom 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e.for abovevs.below cutting-height tissues) was only reduced from0.97 to 0.80. In a field trial with two contrasting N regimes,growth and N accumulation were measured on individual cloverplants. Dinitrogen fixation was estimated by¹⁵N isotope dilutionbased on analysis of leaves-only or by including stolons. Usingleaves-only did not affect the calculation of percentage ofclover N derived from N₂fixation (% Ndfa) since the¹⁵N enrichmentwas found to be uniform in all parts of the clover. A correctionfactor of 1.7 to account for N in below cutting-height tissueis suggested when N₂fixation in white clover is estimated byharvesting the leaves only.Copyright 1997 Annals of Botany Company Leaves; N accumulation; N₂fixation; ¹⁵N isotope dilution; pastures; roots; root/shoot ratio; stolons; Trifolium repensL.; white clover     

Longevity of white clover (Trifolium repens) leaves, stolons and roots, and consequences for nitrogen dynamics under northern temperate climatic conditions

BACKGROUND AND AIMS: White clover (Trifolium repens) is, due to nitrogen (N) fixation, important to the N dynamics of several northern temperate agroecosystems. This study aimed at monitoring growth and death of major white clover plant organs to assess their potential contribution to within-season N input and risk of off-season N losses. METHODS: White clover ('Snowy') was studied in a plot and root window experiment in southeast Norway (60 degrees 42'N, 10 degrees 51'E). Leaves, stolons and roots were tagged for lifespan measurement in harvested and unharvested stands during two experimental years. The availability of soil inorganic N was measured by plant root simulator (PRS) probes. KEY RESULTS: The longevity of leaves and petioles ranged from 21 to 86 d (mean = 59 d), of main stolon sections from 111 to over 677 d (mean = 411 d) and of roots from 27 to 621 d (mean = 290 d). About 60 % of the leaves produced had turned over by the end of the growing season and another 30 % had died or disappeared by the subsequent spring. Harvesting reduced the longevity of stolons and increased plant fragmentation, but did not decrease leaf or root lifespan or increase soil N availability. From the plant organ turnover data, it was estimated that the gross N input to the soil-plant system from white clover in pure stand during two growing seasons corresponded to a 2.5-fold increase over the total N in harvestable shoots. CONCLUSIONS: The short lifespan and poor over-wintering of leaves showed their potential importance as a nitrogen source in the soil-plant ecosystem but also their potential contribution to the risk of off-season N losses.     

Phosphate Uptake and Transport by Roots and Stolons of Intact White Clover (Trifolium repens L.)

The extent to which phosphate can be absorbed directly fromthe outer medium by stolon internodes and contribute to thetotal accumulation of phosphate by intact plants of white clover(Trifolium repens L. cv. Blanca) was assessed in hydroponicexperiments in a controlled environment room. The uptake ofphosphate by intact roots or stolons was measured by sealinga segment (6-0 mm long) across a flow-cell in which ³²P-labellednutrient solution was circulated for 24 h, the rest of the rootsystem receiving unlabelled nutrient solution. The rate of uptakeof phosphate (µmol g^–1 d^–1 dry wt. basis)by roots was more than 300 times that by intact stolons. Pretreatmentof stolons by gentle abrasion to remove cuticle, so as to simulatethe condition of stolons in the field, increased the uptakeof phosphate 7-fold compared with that of intact stolons. However,the potential of stolons to contribute to the P status of whitedover in the field was calculated to be small (5%). When an incision was made through the hypodermal layer of stolons,the rate of phosphate uptake greatly increased, attaining 71%of that by root segments. This increase, which was greater athigher phosphate concentrations, indicates that the suberi.zedhypodermis constitutes a major barrier to the influx of phosphatein the stolon. After withholding phosphate for different time intervals, thesubsequent rate of phosphate uptake by roots was increased 2-3-foldafter 2 d phosphate deprivation and 3-4-fold after 6 d or 13d phosphate deprivation. A higher proportion of absorbed phosphatewas transported to shoots in phosphate-deprived plants. After1 d of uptake following restoration of the phosphate supply,the concentrations of labelled phosphate in shoots were greaterthan in control plants, although the concentrations of labelin roots was less. However, the rate of uptake of phosphateby stolons, following deprivation, was not significantly increased.These results suggest that the mechanism regulating the enhancedrate of phosphate loading into the xylem, initiated by a periodof phosphate deprivation, is specific to roots and is not inducedin stolons. The results are discussed in relation to the growth and acquisitionof phosphate by white clover in the field. Key words: Nutrient deficiency, phosphate, stolons, transport (ions), Trifolium repens     

Assessing the Geometric Structure of a White Clover (Trifolium repens L.) Canopy using3-D Digitising

A method was developed for assessing the three dimensional (3-D)geometric structure of white clover canopies. 3-D co-ordinatesof pre-defined points on leaves, petioles and stolons were measuredusing a Polhemus Fastrak electromagnetic 3-D digitiser. Digitisingprogressed downwards from the top of the canopy and plant partswere removed after they have been digitised. Leaflets were treatedas four quarter-ellipses, and petiole and stolons were treatedas cylinders. Leaf dimensions and areas calculated from 3-Dco-ordinates were within about 5% and 20% of direct measurementsmade with a ruler and a planimeter, respectively. Special softwareand freeware POV-Ray were used to reconstruct a virtual canopyfrom digitiser records and to calculate canopy characteristicssuch as leaf area index (LAI), petiole intersection area, andprofiles of leaflet areas and inclinations with height. It tookbetween 3 and 7 h to digitise 10 x 10 cm stands of clover andthe resulting information was considerably more comprehensiveand accurate than could have been obtained by the alternative‘point quadrat’ or ‘stratified clipping’methods.Copyright 2000 Annals of Botany Company White clover, Trifolium repens, geometric structure, leaf area, leaf angle, 3-D digitising     

Pressure-Induced Water and Solute Flow Through Plant Roots

Water and salt flows through detopped sunflower, tomato andred kidney bean roots under applied pressure were studied usinga pressure chamber. Values of J_v for these root systems weremeasured applying variable pressure on the root medium, andL_p calculated. The K, Na and Cl fluxes under applied pressure were comparedwith those in intact plants at the same water flow rates. Tento 100 times higher Na and Cl fluxes were observed through detoppedroots under pressure as compared to those in the unpressurized,intact plants. It is suggested that the roots under pressureare not completely analogous to intact plant roots, and thatpressure-induced flow may not be a reliable method of determiningcharacteristics of ion flow in roots in relation to water flow. Key words: Volume flow, Hydraulic conductivity, K selectivity     

Flow-induced forces on free-floating macrophytes

Free-floating macrophytes have buoyant petioles and unanchored roots; certain species are highly invasive, owing to characteristics such as high growth rates and the formation of dense floating mats that drift on wind and water currents. Water hyacinth (Eichhornia crassipes) is one example; its invasion of tropical and subtropical freshwater systems worldwide harms native ecosystems and impedes human activities. This research examines flow-induced forces and biomechanical properties of E. crassipes to better understand flow interactions and transport mechanisms. Drag forces were measured in a flume and a wind tunnel for varying approach velocities and raft configurations; from this data, drag coefficients in water (C _D^w) and air (C _D^a) were developed. Over similar Reynolds number (Re _b ) regimes, C _D^w decrease as Re _b increases while C _D ^a are invariant. For the same raft tested in air and water, water drag exceeds air drag and the value of C _D^w approaches C _D^a at high Re _b . Force–velocity relationships indicate root canopies reconfigure by streamlining in higher flow velocities while leaf canopies do not. Root canopy streamlining is further explained through biomechanical testing: we found the major vegetative structures of E. crassipes (roots, stolons, and petioles) have similar moduli of elasticity but second moments of area are three orders of magnitude smaller in roots compared to stolons or petioles, leading to significantly lower flexural rigidity in roots than in stolons or petioles. Flow interactions with the root canopy differ for an individual plant compared to a raft assemblage. Laboratory results suggest that water currents are the dominant mechanism for E. crassipes dispersal.     

Water Balance in Cucumis Plants, Measured by Nuclear Magnetic Resonance, II

Nuclear magnetic resonance (NMR) was used to investigate theeffects of changes in root temperature, of changes in the areaof root in contact with culture solution and of day/night rhythmon the water balance of a cucumber and a gherkin plant. Resultsare discussed in terms of water potential, flow rate and resistanceusing a previously presented model of water balance. As longas water uptake alone is varied, flow rate and water content(or potential) will change in the same direction. In contrast,from that model it is predicted that changes in transpirationwill affect flow rate and water content in opposite ways. Anexperimental verification of this prediction was given in theprevious paper. Results obtained by the NMR method are comparedto those determined using a dendrometer. The results demonstratethat the NMR method is a valuable tool to study plant waterbalance and that it can serve as a technique for discriminatingbetween changes in plant water balance that are due to changesin water uptake by roots and those due to changes in transpiration. Key words: Water balance model, Cucumis satious L., flow, water content, NMR, water balance measurement     

Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes

Sap flows in the xylem of plant roots in response to gradientsin water potential, either between soil and atmosphere (transpiration)or soil layers of different moisture content (termed hydraulicredistribution). The latter has the potential to influence waterbudgets and species interactions, but we lack information forall but a few plant communities. We combined heat pulse measurementsof sap flow with dye and isotope tracing techniques to gaugethe movement of xylem sap within, and exudation from, rootsof Banksia prionotes (Lindley). We demonstrated ‘ hydrauliclift’ during the dry season and provide some evidencethat extremely dry soils limit hydraulic lift. In addition wereport difficulties posed by spiralled xylem tissue in rootsfor the application of heat pulse techniques. Copyright 2000Annals of Botany Company Banksia prionotes, sap flow, hydraulic lift, heat ratio method, deuterium, stable isotopes, root architecture.     

Hydraulic conductivity and PAT determine hierarchical resource partitioning and ramet development along Fragaria stolons

Co-ordination of metabolic and physiological activity between plant parts is key to the control of growth and development. Here the movement of resources and their allocation between mother plants and daughter ramets along Fragaria stolons was quantified with respect to hierarchy. Gradients of internodal ramet leaf water potential (ψ) and stolon and ramet hydraulic conductivities (L) were measured together with apparent stolon IAA movement via the polar auxin transport pathway (PAT). These processes are linked with measurements of stolon vascular development. The pattern of tissue differentiation and lignification in sequential stele sections of stolons demonstrated the rapid acquisition of the capacity for water transport, with transpiration potentially varying systematically with stolon lignification and the acropetal decline in stolon xylem ψ. Stolon and ramet L declined acropetally, with L across older ramets being significantly lower than that of the connecting stolons. The capacity for polar IAA transport increased with stolon age; this was due to increased transport intensity in older tissues. The partitioning of dry matter was strongly hierarchical with younger ramets smaller than older ramets, while foliar concentrations of N, P, and K were greater for the younger ramets. The results show that stolon anatomy develops rapidly at the apical end, facilitating hierarchical ramet development, which is evident as a basipetal increase in L. The rapid development of transport tissue functionality enables young unrooted ramets to acquire water, in order to supply an expanding leaf area, as well as mineral ions disproportionally with respect to older ramets. This facilitates colonization and self-rooting of apical ramets. The unidirectional increase in basipetal PAT along stolons facilitates hierarchical ramet development.     

Effects of Blue Light on the Vertical Colonization of Space by White Clover and their Consequences for Dry Matter Distribution

The morphology of white clover is very sensitive to the lightenvironment, especially to the ratio of red:far-red light andto photon irradiance. However, less is known about the effectsof blue light on clover morphogenesis. Cuttings of white cloverwere grown for 56 d in two controlled chambers receiving lightwith similar photosynthetic efficiency and phytochrome photoequilibriumstate but different levels of blue light: some plants were grownunder orange light (very low blue light, 0.02 µmol m^-2s^-1)or under white light containing blue light (83 µmol m^-2s^-1).Other plants were switched from white light to orange lightorvice versa,after 30 d. The absence of blue light modifiedthe growth habit of clover and raised the laminae in the upperlayer of the canopy by increasing petiole length, and petioleangle from the horizontal, and by raising stolons above theground surface. Moreover, the absence of blue light had no effecton total leaf area and total dry weight per plant, but increasedthe leaf area and biomass of petioles of the main axis. Largerpetioles and laminae were associated with the allocation ofmore dry weight to the petiole at the same petiole thicknessbut with thinner laminae. These results indicate that a decreasein blue light is involved in the perception of, and adaptationto, shading by the plant.Copyright 1997 Annals of Botany Company Biomass allocation; blue light; growth habit; leaf area; light quality; photomorphogenesis; Trifolium repensL.; white clover     

Drought-Induced Xylem Dysfunction in Petioles,Branches, and Roots of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn

Variation in vulnerability to xylem cavitation was measured within individual organs of Populus balsamifera L. and Alnus glutinosa (L.) Gaertn. Cavitation was quantified by three different techniques: (a) measuring acoustic emissions, (b) measuring loss of hydraulic conductance while air-dehydrating a branch, and (c) measuring loss of hydraulic conductance as a function of positive air pressure injected into the xylem. All of these techniques gave similar results. In Populus, petioles were more resistant than branches, and branches were more resistant than roots. This corresponded to the pattern of vessel width: maximum vessel diameter in 1- to 2-year-old roots was 140 [mu]m, compared to 65 and 45 [mu]m in rapidly growing 1-year-old shoots and petioles, respectively. Cavitation in Populus petioles started at a threshold water potential of -1.1 MPa. The lowest leaf water potential observed was -0.9 MPa. In Alnus, there was no relationship between vessel diameter and the cavitation response of a plant organ. Although conduits were narrower in petioles than in branches, petioles were more vulnerable to cavitation. Cavitation in petioles was detected when water potential fell below -1.2 MPa. This value equaled midday leaf water potential in late June. As in Populus, roots were the most vulnerable organ. The significance of different cavitation thresholds in individual plant organs is discussed.     

The Nature and Location of Variable Hydraulic Resistance in Helianthus annuus L. (Sunflower)

Koide, R. 1985. The nature and location of variable hydraulicresistance in Helianthus annuus L. (sunflower).—J. exp.Bot. 36: 1430–1440. Hydraulic resistances for whole sunflower plants (Helianthusannuus L.) and sunflower leaves, stems, petioles and roots weremeasured. Whole plant hydraulic resistance was shown to declinewith an increase in transpiration. Leaf hydraulic resistancewas shown, with one technique employing transpiring leaves,to vary with transpiration and with another technique, employingpressure-induced flow in leaves, to be constant over a widerange of transpiration. Stem and petiole hydraulic resistanceswere constant over a wide range of exudation. Pressure-inducedflow through root systems was shown to be an inappropriate methodfor characterizing their hydraulic properties because flow mayoccur through unnatural paths. The technique employing measuredtranspiration rates and water potentials of non-growing leavesand soil is suggested to be better. The evidence presented inthis study suggests that the hydraulic resistance of the transpirationstream does vary and that the site of variability is the root Key words: Hydraulic resistance, sunflower, pressure-induced water flow     

Mobilization of nitrogen reserves during regrowth of defoliated Trifolium repens L. and identification of potential vegetative storage proteins

Although it is well established that carbon reserves contributeto shoot regrowth of leguminous forage species, little informationis available on nitrogen reserves except in Medicaqo sativaL. and Trifolium subterraneum L. In this study, reserves werelabelled with ¹⁵N to demonstrate the mobilization of endogenousnitrogen from roots and stolons to regrowing leaves and newstolons during 24 d of regrowth in white clover (Thfolium repensL.). About 55% and 70%, respectively, of the nitrogen contentsof these organs were mobilized to support the regrowth of leaves.During the first 6 d, nitrogen in regrowing leaves came mainlyfrom N reserves of organs remaining after defoliation. Afterthese first 6 d of regrowth, most of the shoot nitrogen wasderived from exogenous nitrogen taken up while the contributionof nitrogen reserves decreased. After defoliation, the buffer-solubleprotein content of roots and stolons decreased by 32% duringthe first 6 d of regrowth. To identify putative vegetative storageproteins, soluble proteins were separated using SDS-PAGE ortwo-dimensional electrophoresis. One protein of 17.3 kDa instolons and two proteins of 15 kDa in roots seemed to behaveas vegetative storage proteins. These three polypeptides, initiallyfound at high concentrations, decreased in relative abundanceto a large extent during early regrowth and then were accumulatedagain in roots and stolons once normal growth was re-established. Key words: White clover, regrowth, ¹⁵N-labelled, vegetative storage proteins, electrophoresis     

Water Retention Capacity in Root Segments Differing in the Degree of Exodermis Development

Water loss from roots back into drying soil is a problem ofpractical importance in plants growing under conditions of verylow substrate water potential, such as dry or saline areas.Root exodermis is relatively impermeable and has been suggestedto play a protective role against water loss. The relative waterretention ability was compared in root segments from exodermal(maize, onion, sunflower, Rhodes grass and sorghum) and non-exodermalspecies (Pisum sativum,Vicia fabaand wheat). Apical and basalsegments from exodermal roots, with different degrees of exodermisdevelopment, were also compared, as were segments from sorghumroots in which the exodermis thickness had been modified bysubjecting the plants to a 30 d water stress treatment. Waterretention was significantly higher in segments from exodermalroots. In each root, water loss was higher in apical than inbasal segments, regardless of the presence of exodermis. Insorghum, prolonged drought treatment increased exodermis thickeningin nodal roots, however, no differences in rates of water losswere observed in segments obtained from control and droughtedplants. Soil sheaths formed around roots of Rhodes grass growingin very dry soil with the epidermis adhering tightly to thesheath. In plants growing in the field, soil sheaths may bemore effective than the exodermis in preventing root water loss.Copyright1999 Annals of Botany Company. Root, exodermis, rhizosheaths, water loss.     

Soil Water Extraction, Measured by Computer-Assisted Tomography, in Seedling Lupinus angustifolius cv. Yandee when Healthy and Infected with Phytophthora cinnamomi

The water-mould fungus Phytophthora cinnamomi Rands causes drought-likesymptoms on many hosts, and yet the mechanisms by which infectionleads to wilting are not fully understood. This is the firststudy to describe in detail changes in soil water around theroot with infection. Computer-assisted tomography (CAT) wasused with Lupinus angustifolius L. cv. Yandee to examine drawdowns(removal of soil water) around a central root infected by P.cinnamomi in a white sand. No growth differences in roots or shoots were found betweenhealthy and diseased plants during the 8 d of the experiment.However,drawdowns failed at high levels of inoculum (8–16 /Pc-infectedmillet seeds/plant) by 8 d. Water contents in pots with uninfectedplants were in the range 0·09–0·12 cm³ watercm^–3 soil in the centre of the pot, while water contentsin pots with infected plants at 16 millet seeds applied werein the range of 0·16–0·19 cm³ water cm^–3soil in the centre of the pot. A higher transpirational demand produced lower soil water contentsnear the root but this effect was confounded with infection:disease was more pronounced with higher transpirational demand,and disease led to an increase in water content. Key words: Root disease, Phytophthora cinnamomi, water uptake, soil-root interface, computer-assisted tomography     

Patterns of 14C-labelled Assimilate Partitioning in Red and White Clover During Vegetative Growth

A quantitative analysis of the ¹⁴C-labelled assimilate suppliedby the expanded leaves on the primary shoot to growing leaves,stem, lateral shoots (branches or stolons) and roots in redand white clover was conducted during vegetative growth. Stem growth of the primary shoot was inhibited in both cloversand utilized no energy resources. The growing leaves at theprimary shoot apex of white clover imported 4 per cent of theshoot's assimilate compared with 10 per cent in red clover.At the basal end of the primary shoot, the tap root of whiteclover imported 16 per cent of the shoot's assimilate comparedwith 22 per cent in red clover. Branches in red clover and stolonsin white clover were by far the largest sinks for primary shootassimilate, importing 39 per cent and 63 per cent of the labelledassimilate, respectively. Analyses of the translocation of assimilate from individualprimary shoot leaves demonstrated that in both clovers olderleaves exported more of their assimilate to branches or stolons,whereas younger leaves exported more of their assimilate toroots, and possibly in white clover, to growing leaves at thetip of the shoot. Of the labelled assimilate exported to branchesor stolons, each primary shoot leaf exported preferentiallyto the branch or stolon in its own axil, but in addition exportedsubstantial quantities of assimilate to all other axillary shoots,particularly those arising from basal axils where the subtendingleaf had died. Trifolium repens, Trifolium pratense, red clover, white clover, assimilate partitioning, perennation     

Thermal and Water Relations of Roots of Desert Succulents

Two succulent perennials from the Sonoran Desert, Agave desertiEngelm. and Ferocactus acanthodes (Lem.) Britton and Rose, loselittle water through their roots during drought, yet respondrapidly to light rainfall. Their roots tend to be shallow, althoughabsent from the upper 20 mm or so of the soil. During 12–15d after a rainfall, new root production increased total rootlength by 47 per cent to 740 m for A. deserti and by 27 percent to 230 m for F. acanthodes; root dry weight then averagedonly 15 per cent of shoot dry weight. The annual carbon allocatedto dry weight of new roots required 11 per cent of shoot carbondioxide uptake for A. deserti and 19 per cent for F. acanthodes.Elongation of new roots was greatest near a soil temperatureof 30°C, and lethal temperature extremes (causing a 50 percent decrease in root parenchyma cells taking up stain) were56°C and -7°C. Soil temperatures annually exceeded themeasured tolerance to high temperature at depths less than 20mm, probably explaining the lack of roots in this zone. Attached roots immersed in solutions with osmotic potentialsabove -2·6 MPa could produce new lateral roots, with50 per cent of maximum elongation occurring near -1·4MPa for both species. Non-droughted roots lost water when immersedin solutions with osmotic potentials below -0·8 MPa,and root hydraulic conductance decreased markedly below about-1·2 MPa. Pressure-volume curves indicated that, fora given change in water potential, non-droughted roots lostthree to five times more water than droughted roots, non-droughtedleaves, or non-droughted stems. Hence, such roots, which couldbe produced in response to a rainfall, will lose the most tissuewater with the onset of drought, the resulting shrinkage beingaccompanied by reduced root hydraulic conductance, less contactwith drying soil, and less water loss from the plant to thesoil. Agave deserti, Ferocactus acanthodes, roots, soil, temperature, water stress, drought, Crassulacean acid metabolism, succulents