Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy

A technique used for hydroponics was adapted to measure instantaneousroot water uptake from the soil for a leaf succulent CAM species,Agave deserti. Comparisons were made to previously modelledwater fluxes for A. deserti and to Encelia farinosa, a non-succulentC₃species. Net CO₂uptake and transpiration forA. deserti underwell-watered conditions occurred primarily at night whereasroot water uptake was relatively constant over 24 h. Leaf thicknessdecreased when transpiration commenced and then increased whenrecharge from the stem and soil occurred, consistent with previousmodels. A drought of 90 d eliminated net CO₂uptake and transpirationand reduced the water content of leaves by 62%. Rewetting theentire root system for 7 d led to a full recovery of leaf waterstorage but only 56% of maximal net CO₂uptake. Root water uptakewas maximal immediately after rewetting, which replenished rootwater content, and decreased to a steady rate by 14 d. Whenonly the distal 50% of the root system was rewetted, the timefor net CO₂uptake and leaf water storage to recover increased,but by 30 d gas exchange and leaf water storage were similarto 100% rewetting. Rewetting 10 or 20% of the root system resultedin much less water uptake; these plants did not recover leafwater storage or gas exchange by 30 d after rewetting. A redundancyin the root system of A. deserti apparently exists for dailywater uptake requirements under wet conditions but the entireroot system is required for rapid recovery from drought.Copyright1999 Annals of Botany Company Agave deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.     

A Two-dimensional Model for Water Uptake by Desert Succulents: Implications of Root Distribution

Water uptake by Agave deserti and Ferocatus acanthodes was predictedusing a two-dimensional simulation model in which the soil arounda plant was divided into a series of layers and concentric cylindricalshells. Root lengths in 0.05 m thick soil layers were determinedfor both species in the field, where mean root depths were only0.11 m for A. deserti and 0.10 m for F. acanthodes. For a yearwith average precipitation (159 mm), 42 per cent of the annualprecipitation could be taken up by A. deserti and 25 per centby F. acanthodes. Predicted water uptake by both species wasgreater from the upper soil layers (above 0.15 m) for averageand dry years, but was greater from the deeper layers for awet year. The actual root distribution for both species ledto more water uptake than when all of the roots were in a singlelayer. The large number of days per year when the soil temperaturesexceeded 57 °C (the temperature for 50 per cent inhibitionof uptake of a vital stain by root cells) may exclude rootsfrom the 0.00–0.05 m soil layer, even though water uptakewhen all roots were located there was predicted to be maximal.Therefore, the observed root distribution of A. deserti andF. acanthodes may be limited near the soil surface by high temperaturesand at maximum depths by water availability for all but wetyears. Agave deserti, Ferocactus acanthodes, desert succulents, root system, root distribution, soil temperature, water uptake     

A Finite-element Model of Radial and Axial Conductivities for Individual Roots: Development and Validation for Two Desert Succulents

A morphologically explicit numerical model for analysing wateruptake by individual roots was developed based on a conductornetwork, with specific conductors representing axial or radialconductivities for discrete root segments. Hydraulic conductivity(L_p; m s^–1 MPa^–1) was measured for roots of Agavedeserti Engelm. and Opuntia ficus-indica (L.) Miller by applyinga partial vacuum to the proximal ends of excised roots in solution.L_p was also measured for 40- to 80-mm segments along a root,followed by measurements of axial conductivity and calculationof radial conductivity. Predicted values of L_p for entire rootsbased on two to ten segments per root averaged 1.04±0.07(mean±s.e. mean for n = 3) of the measured L_p for A.deserti and 1.06±0.10 for O. ficus-indica. The modelalso closely predicted the drop in water potential along theroot xylem (^xylem); when a tension of 50 kPa was applied tothe proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica,the measured ^xylem to midroot averaged 30 kPa compared witha predicted decrease of 36 kPa. Such steep gradients in ^xylemsuggest that the driving force for water movement from the soilto young distal roots may be relatively small. The model, whichagreed with an analytical solution for a simple hypotheticalsituation, can quantify situations without analytical solutions,such as when root and soil properties vary arbitrarily alonga root. Agave deserti, electrical circuit analog, hydraulic conductivity, Opuntia ficus-indica, water potential     

Rectifier-like Activities of Roots of Two Desert Succulents

Axial and radial water flows for roots in response to appliedhydrostatic pressure drops, water loss from roots after variousperiods of drying, and development of new roots after rewettingdroughted plants were examined for two sympatric desert succulents.Agave deserti Engelm. and Ferocactus acanthodes (Lemaire) Brittonand Rose. For a 40 kPa hydrostatic pressure drop applied to20 mm long root pieces, radial water flows from the epidermisto the root xylem were 2- to 5-fold greater at the tip thanat midlength and were much less than axial flows along the xylem.Upon drying detached roots in air at 20 °C and a water vapoursaturation deficit of 1.2 kPa (50% relative humidity), radialwater flow decreased more than 10-fold in 3–6 h, and couldrecover to the original level 6 h after rewetting. The rateof water loss from attached roots of plants dried in air at20 °C and a 1.2 kPa saturation deficit decreased about 200-foldin 72 h, which would greatly limit water loss from the plantto a drying soil. At 96 h after rewetting roots of A. desertithat had been exposed to air at 20 °C and a 1.2 kPa saturationdeficit for 120 h, rehydration of existing roots and developmentof new roots contributed about equally to water uptake by thewhole plant. In summary, roots of these desert succulents canreadily take up water from a wet soil but do not lose much waterto a dry soil, thus effectively acting like rectifiers withrespect to plant-soil water movement. Key words: Agave, Cactus, Drought, Root, Water flow, Xylem     

Influences of Water Status, Temperature, and Root Age on Daily Patterns of Root Respiration for Two Cactus Species

Daily patterns of root respiration measured as CO₂, efflux werestudied at various soil water potentials, temperatures, androot ages for individual, attached roots of the barrel cactusFerocactus acanthodes and the platyopuntia Opuntia ficus-indica.The daily patterns of root respiration for both establishedroots and rain roots followed the daily patterns of root temperature.Root respiration increased when root temperature was raisedfrom 5 °C to 50 °C for F. acanthodes and from 5 °Cto 55 °C for O. ficus-indica; at 60 °C root respirationdecreased 50° from the maximum for F. acanthodes and decreased25° for O. ficus-indica. Root respiration per unit d. wtdecreased with root age for both species, especially for rainroots. Root respiration rates for rain roots were reduced tozero at a soil water potential (     

Root Respiration for Agave deserti: Influence of Temperature, Water Status and Root Age on Daily Patterns

Root respiration, measured as CO₂ efflux, was studied for asucculent perennial from the Sonoran Desert, Agave deserti,with a new technique using individual, attached roots. The dailypatterns of root respiration closely followed the daily patternsof root temperature for both established roots and rain roots,with higher rates during the day when root temperature averaged27?C and lower rates at night when root temperature averaged17?C. When root temperature was raised from 5?C to 40?C, rootrespiration increased about 7-fold; from 45 ?C to 55 ?C, rootrespiration decreased about 2-fold, except for old establishedroots. Root respiration per unit dry weight for both root typesdecreased with age, the initial decrease being greater for rainroots than for established roots. Root respiration rates forrain roots were reduced to zero at a soil water potential (_soil)of –0.9 MPa and did not recover upon rewatering. Upondrying, root respiration rates for established roots were maintainedat about 12% of maximum, even when _soil fell to –1.6 MPa,and fully recovered 1.5 d after rewatering the soil. Such responsesof rain and established roots must be taken into account whenassessing the carbon costs for the root system. Key words: Agave deserti, CO₂ exchange, root respiration, temperature, soil water potential     

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     

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     

Phloem Exudate Collected via Scale Insect Stylets for the CAM Species Opuntia ficus-indica under Current and Doubled CO2 Concentrations

A method was devised for collecting phloem sap from the CAMspecies Opuntia ficus-indica using severed stylets of a scaleinsect (Dactylopius opuntiae), for which exudation could continuefor up to 5 d. For both basal (planted) cladodes and first-orderdaughter cladodes, the concentrations of sucrose and total aminoacids in the phloem exudate were virtually constant over 24-hperiods whereas the chlorenchyma osmolality had sizeable increasesduring the night under both current and doubled atmosphericCO₂ concentrations. Sucrose, total amino acids, and potassiumaccounted for 56, 21, and 9%, respectively, of the osmolalityof the phloem exudate, which was about 350 mOsm at the two CO₂concentrations; valine, isoleucine, leucine, tyrosine, glutamine,and lysine accounted for about 70% of the total amino acids.Doubling the CO₂ concentration led to approx. 5% more sucrose,560% more mannose and 17% less amino acids in the phloem exudateand also significantly increased mannose, starch and glucomannanin the chlorenchyma. Atmospheric CO₂ concentrations thus affectedvarious solute properties in the phloem and the chlorenchymaof O. ficus-indica.Copyright 1995, 1999 Academic Press Dactylopius opuntiae, Opuntia ficus-indica, cladode, CO₂ concentrations, Crassulacean acid metabolism, phloem exudate     

Hydraulic Conductances of the Soil, the Root-Soil Air Gap, and the Root: Changes for Desert Succulents in Drying Soil

Water movement to and from a root depends on the soil hydraulicconductivity coefficient (L^soil), the distance across any root-soilair gap, and the hydraulic conductivity coefficient of the root(L^P). After analytical equations for the effective conductanceof each part of the pathway are developed, the influences ofsoil drying on the soil water potential and L^soil are describedduring a 30 d period for a loamy sand in the field. The influencesof soil drying on L^P for three desert succulents, Agave deserti,Ferocactus acanthodes, and Opuntia ficus-indica, are also describedfor a 30 d period. To quantify the effects of soil drying onthe development of a root-soil air gap, diameters of 6-week-oldroots of the three species were determined at constant watervapour potentials of –1.0 MPa and –10 MPa as wellas with the water vapour potential decreasing at the same rateas soil drying during a 30 d period. The shrinkage observedfor roots initially 2·0 mm in diameter averaged 19% duringthe 30d period. The predominant limiting factor for water movementwas L^P of the root for the first 7 d of soil drying, the root-soilair gap for the next 13 d, and L^soil thereafter. Compared withthe ease of water uptake from a wet soil, the decrease in conductancesduring soil drying, especially the decrease in L^soil causedthe overall conductance to decrease by 3 x 10³-fold during the30 d period for the three species considered, so relativelylittle water was lost to the dry soil. Such rectifier-like behaviourof water movement in the soil-root system resulted primarilyfrom changes in L^soil and, presumably, is a general phenomenonamong plants, preventing water loss during drought but facilitatingwater uptake after rainfall. Key words: Agave deserti, Ferocactus acanthodes, Opuntia ficus-indica, rectification, soil water potential, water movement     

Allometric Root/Shoot Relationships and Predicted Water Uptake for Desert Succulents

Root morphology, shoot morphology, and water uptake for Agavedeserti and Ferocactus acanthodes of various sizes were studiedusing allometric relationships (y = ax^b) and a previously developedwater uptake model. Shoot surface area increased with shootvolume with an exponent b of 0.75 for both species. Root lengthand the ground area explored by the roots increased with shootsurface area with b's of 0.72 for A. deserti and 0.92 for F.acanthodes. Various sized individuals had about the same ratioof root length to explored ground area, with higher values occurringfor A. deserti. Predicted water uptake averaged over the exploredground area was approximately constant over a 10⁴-fold rangein shoot surface area, suggesting that shoot size confers nointraspecific competitive advantage for water uptake. For theroot lengths per explored ground area observed in the field,water uptake was predicted to be 85 per cent of maximal; wateruptake could be increased by the production of more rain roots.When differences in shoot volume were accounted for by allometry,small plants had relatively less shoot surface area and relativelymore root length per shoot volume than did large plants, whichmay be important for the water relations of seedling establishment. Agave deserti, Ferocactus acanthodes, allometry, desert succulents, root distribution, root length, seedling growth, seedling establishment, shoot surface area, shoot volume, water uptake     

Root Hydraulic Conductivity of Two Cactus Species in Relation to Root Age, Temperature, and Soil Water Status

The effects of root age, temperature, and soil water statuson root hydraulic conductivity (L_P) were investigated for twocactus species, Ferocactus acanthodes and Opuntia ficus-indica.The volumetric flux density of water was measured for excisedroot segments, either using negative hydrostatic pressures appliedto the proximal end or using reverse flow of water from theroot to the soil. For both species, L_P at 20 ?C increased withroot age, average values reaching a maximum of 3.9 ? 10^–7m s^–1 MPa^–1 for F. acanthodes and 5.2 ? 10^–7m s^–1 MPa^–1 for O.ficus-indica at 11 to 17 weeksof age; L_P subsequently declined with increasing root age forboth species. L_P was maximal at a temperature of about 10 ?Cfor the youngest roots (1–3 weeks), this optimum shiftingto 40 ?C for 8-week-old roots of both species. For older roots(up to 1.5-years-old), L_P increased with temperature from 0?C to 50 ?C, with a Q₁₀ of 1.3 between 20 ?C and 30 ?C. At asoil water potential (_soil) of –0.016 MPa, root L_P wasindependent of the direction of water flow for both species.Depending on root age, L_P declined 45- to 500-fold for F. acanthodesand 90- to 800-fold for O.ficus-indica as _soil was reduced from–0.016 to –1.06 MPa, consistent with a rectifier-likebehaviour with respect to water movement between soil and roots.Incorporation of such responses into water uptake models shouldlead to a better understanding of root function. Key words: Ferocactus acanthodes, Opuntia ficus-indica, water potential, tension, reverse flow     

Influences of root distribution and growth on predicted water uptake and interspecific competition

Summary Root distribution and growth measured in the field were incorporated into a water uptake model for the CAM succulent Agave deserti and its nurse plant Hilaria rigida, a common desert bunchgrass. Agave deserti responds to the infrequent rainfalls of the Sonoran Desert by extending its existing established roots and by producing new roots. Most of such root growth was completed within one month after soil rewetting, total root length of A. deserti increasing by 84% for a seedling and by 58% for a mediumsized plant in the summer. Root growth in the winter with its lower soil temperatures was approximately half as much as in the summer. For a 15-year period, predicted annual root growth of A. deserti varied more than 18-fold because of annual variations in rainfall amount and pattern as well as seasonal variation in soil temperature. Predicted annual water uptake varied 47-fold over the same period. The nurse plant, which is crucial for establishment of A. deserti seedlings, reduced seedling water uptake by 38% during an average rainfall year. Lowering the location of the root system of a medium-sized A. deserti by 0.24 m reduced its simulated annual water uptake by about 25%, reflecting the importance of shallow roots for this desert succulent. Lowering the root system of a medium-sized H. rigida by 0.28 m increased the simulated annual water uptake of an associated A. deserti seedling by 17%, further indicating the influence of root overlap on competition for water.     

Frequencies, Microclimate and Root Properties for Three Codominant Perennials in the Northwestern Sonoran Desert on North-vs.South-facing Slopes

At a site in the northwestern Sonoran Desert the percent groundcover for the C₃subshrubEncelia farinosawas eight-times higheron more arid 20° south-facing slopes than on 20° north-facingslopes at 820 m elevation, and was six-times higher on north-facingslopes at a 300-m-lower elevation, also the more arid condition.The ground cover of the C₄bunchgrassPleuraphis rigidadecreasedover 50% from 20° north-facing slopes to the more arid conditionsof a 36° north-facing slope, a 20° south-facing slopeand a 20° north-facing slope at a 300-m-lower elevation.The CAM leaf succulentAgave desertialso had greater ground coverfor the 20° north-facing slopes at 820 m compared with 520m. For these three codominants that averaged 58% of the totalground cover, the key for the relative frequency ofE. farinosawasapparently its greater root growth on the warmer slopes duringthe winter. The key for the other two species was most likelysoil water availability, especially during the seedling stageforA. deserti. The wetter soil conditions on 20° north-facingslopes at 820 m apparently led to individual plants ofP. rigidathatwere twice as large as on south-facing slopes. Thus root propertiesmay exert the primary influence on relative plant frequencyin this desert ecosystem for which soil temperature and wateravailability are crucial.Copyright 1997 Annals of Botany Company Agave deserti; Encelia farinosa; Pleuraphis rigida; rooting patterns; soil temperature; Sonoran Desert; water availability     

Water Influx Characteristics and Hydraulic Conductivity for Roots of Agave deserti Engelm.

Various plant and environmental factors influence the hydraulicproperties for roots, which were examined using negative hydrostaticpressures applied to the proximal ends of individual excisedroots of a common succulent perennial from the Sonoran Desert,Agave deserti Engelm. The root hydraulic conductivity, L^p, increasedsubstantially with temperature, the approximately 4-fold increasefrom 0.5°C to 40°C representing a Q¹⁰ of 1.45. Suchvariations in L_p with temperature must be taken into accountwhen modelling water uptake, as soil temperatures in the rootzone of such a shallow-rooted species vary substantially bothdaily and seasonally. At 20°C, L_p was 2.3 x 10^–7 ms^{macron}1MPa^{macron}1for 3-week-old roots, decreasing to abouthalf this value at 10 weeks and then becoming approximatelyhalved again at 6 months. For a given root age, L_p for rainroots that are induced by watering as lateral branches on theestablished roots (which arise from the stem base) was aboutthe same as L_p for established roots. Hence, the conventionalbelief that rain roots have a higher L_p than do establishedroots is more a reflection of root age, as the rain roots tendto be shed following drought and thus on average are much youngerthan are established roots. Unlike previous measurements onroot respiration, lowering the gas-phase oxygen concentrationfrom 21% to 0% or raising the carbon dioxide concentration from0.1% to 2% had no detectable effect on L_p for rain roots andestablished roots. L_p for rain roots and established roots wasdecreased by an average of 11% and 35% by lowering the soilwater potential from wet conditions (_soil=0 kPa) to {macron}40kPa and {macron}80 kPa, respectively. Such decreases in L_p mayreflect reduced water contact between soil particles and theroot surface and should be taken into account when predictingwater uptake by A. deserti. Key words: Gas phase, rain roots, root age, soil, temperature, water potential     

Radial Hydraulic Conductivity of Individual Root Tissues of Opuntia ficus-indica (L.) Miller as Soil Moisture Varies

The constraints on water uptake imposed by individual root tissueswere examined forOpuntia ficus-indicaunder wet, drying, andrewetted soil conditions. Root hydraulic conductivity (L_P) andaxial conductance (K_h) were measured for intact root segmentsfrom the distal region with an endodermis and from midroot witha periderm;L_Pwas then measured for each segment with successivetissues removed by dissection. Radial conductivity (L_R) wascalculated fromL_PandK_hfor the intact segment and for the individualtissues by considering the tissue conductivities in series.Under wet conditions,L_Rfor intact distal root segments was lowestfor the cortex; at midroot, where cortical cells are dead,L_Rforthe cortex was higher and no single tissue was the predominantlimiter ofL_R.L_Rfor the endodermis and the periderm were similarunder wet conditions. During 30d of soil drying,L_Rfor the distalcortex increased almost three-fold due to the death of corticalcells, whereasL_Rfor the midroot cortex was unaffected;L_Rforthe endodermis and the periderm decreased by 40 and 90%, respectively,during drying. For both root regions under wet conditions, thevascular cylinder had the highestL_R, which decreased by 50–70%during 30d of soil drying. After 3d of rewetting, new lateralroots emerged, increasingL_Rfor the tissues outside the vascularcylinder as well as increasing uptake of an apoplastic tracerinto the xylem of both the roots and the shoot. The averageL_Rforintact root segments was similar under wet and rewetted conditions,but the conductivity of the tissues outside the vascular cylinderincreased after rewetting, as did the contribution of the apoplasticpathway to water uptake. Opuntia ficus-indica; prickly pear; root hydraulic conductivity; endodermis; periderm; apoplast; lateral root emergence     

Heterogeneity in Water Availability Alters Cellular Development and Hydraulic Conductivity along Roots of a Desert Succulent

Plants of the desert succulent Agave deserti were grown in partitionedcontainers to determine whether heterogeneity in soil moistureleads to differences in cellular development and hydraulic conductivityalong individual roots. Roots from containers with a dry distalcompartment (furthest from the shoot), a wet middle compartment,and a dry proximal compartment had distal regions (includingthe root tips) that were more suberized and lignified in theendodermis and adjacent cell layers than were root regions fromthe wet middle compartment. Proximal root regions about 40 mmfrom the succulent shoot base were also relatively unsuberized,suggesting that both external and internal supplies of waterdelayed tissue maturation. Root segments from wet middle compartmentsand from dry proximal compartments had higher hydraulic conductivitythan did the more suberized root segments from dry distal compartments.Unlike distal root segments from wet compartments, segmentsfrom dry compartments suffered no decrease in hydraulic conductivityafter immersion in mercuric chloride, suggesting that aquaporinactivity diminished for roots during drought. The possible closureof water channels could help limit root water loss to a dryingsoil. The delayed development of suberized cell layers may allowroot regions to maximize water uptake from wet soil patches(such as under rocks), and the relatively immature, absorptiveroot region near the base of the shoot may help A. deserti capturewater from a briefly wetted surface soil. Copyright 2000 Annalsof Botany Company Agave deserti, root plasticity, water uptake, aquaporins, suberization, endodermis, divided pots.     

Carbon Translocation Between Parents and Ramets of a Desert Perennial

Agave deserti, a semelparous, Crassulacean acid metabolism perennialoccurring in the northwestern Sonoran Desert, propagates primarilyvegetatively by ramets produced on rhizomes that extend lessthan 10 cm from the base of a parent plant. Carbon translocationfrom parents to ramets, measured after exposing leaves to ¹⁴CO₂,was essentially complete in 7 d, with parents exporting 3·3%of their assimilated carbon to ramets. Shading ramets belowlight compensation for 6 weeks more than doubled the amountof carbon exported from the parent to shaded ramets, comparedwith unshaded ramets. The total amount of carbon imported bya ramet from its parent was independent of the mass of the ramet.Although the net movement of carbon is expected to be towardsthe ramets, parents also received carbon from labelled ramets,indicating bidirectional translocation. The physiological integrationof parents and ramets allows ramets to draw upon the reservesof the parent for up to 14 years, a longer period than for mostother reported clonal species, thereby facilitating ramet growthand establishment in a resource-limited environment. Agave deserti Engelm., clonal, physiological integration, translocation, ¹⁴CO₂     

Long-term effects of a doubled atmospheric CO2 concentration on the CAM species Agave deserti

To examine the effects of a doubled atmospheric CO₂ concentrationand other aspects of global climate change on a common CAM speciesnative to the Sonoran Desert, Agave deserti was grown under370 and 750 µmol CO₂ mol^–1 air and gas exchangewas measured under various environmental conditions. Doublingthe CO₂ concentration increased daily net CO₂ uptake by 49%throughout the 17 months and decreased daily transpiration by24%, leading to a 110% increase in water-use efficiency. Underthe doubled CO₂ concentration, the activity of ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco) was 11% lower, phosphoenolpyruvatecarboxylase was 34% lower, and the activated:total ratio forRubisco was 25% greater than under the current CO₂ concentration.Less leaf epicuticular wax occurred on plants under the doubledCO₂ concentration, which decreased the reflectance of photosyntheticphoton flux (PPF); the chlorophyll content per unit leaf areawas also less. The enhancement of daily net CO₂ uptake by doublingthe CO₂ concentration increased when the PPF was decreased below25 mol m^–2 d^–1 when water was withheld, and whenday/night temperatures were below 17/12 C. More leaves, eachwith a greater surface area, were produced per plant under thedoubled CO₂ concentration. The combination of increased totalleaf surface area and increased daily net CO₂ uptake led toan 88% stimulation of dry mass accumulation under the doubledCO₂ concentration. A rising atmospheric CO₂ concentration, togetherwith accompanying changes in temperature, precipitation, andPPF, should increase growth and productivity of native populationsof A. deserti. Key words: Crassulacean acid metabolism, gas exchange, global climate change, Sonoran Desert     

Hydraulic Conductivity and Anatomy for Lateral Roots of Agave deserti During Root Growth and Drought-induced Abscission

Hydraulic conductivity (L_p), radial conductivity (L_R), axialconductance (K_h), and related anatomical characteristics forlateral roots of Agave deserti were investigated during rootgrowth and drought-induced abscission. The elongation rate oflateral roots averaged 5 mm d^–1 under wet conditions andwas reduced 95% by 17 d of drought (