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     

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     

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     

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 (     

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     

Changes in Structure and Hydraulic Conductivity for Root Junctions of Desert Succulents as Soil Water Status Varies

Variations in hydraulic conductivity (L_P) and the underlying anatomical and morphological changes were investigated for main root-lateral root junctions of Agave deserti and Ferocactus acanthodes under wet, dry, and rewetted soil conditions. During 21 d of drying, L_P and radial conductivity (L_R) increased threefold to fivefold at junctions of both species. The increase in L_R was accompanied by the formation of an apoplastic pathway for radial water movement from the surface of the junction to the stele for A. deserti and by the rupture of periderm by emerging primordia of secondary lateral roots for F. acanthodes. During 7 d of rewetting, L_R decreased for junctions of A. deserti, as apoplastic water movement was not apparent, but L_R was unchanged for F. acanthodes. Axial conductance (K_h) decreased during drying for both species, largely because of embolism related to the degradation of unlignified cell wall areas in tracheary elements at the root junction. The resulting apertures in the cell walls of such elements would admit air bubbles at pressure differences of only 0.12-0.19 MPa. Rewetting restored K_h for both species, but not completely, due to blockage of xylem elements by tyloses. About 40% of the primary lateral roots of the monocotyledon A. deserti abscised during 21 d of drying. For the dicotyledon F. acanthodes, which can form new conduits in its secondary xylem, only 10% of the primary lateral roots abscised during 21 d of drying, consistent with the much greater frequency of lateral roots that persist during drought in the field compared with the case for the sympatric A. deserti.     

Mucilage in Cacti: Its Apoplastic Capacitance, Associated Solutes, and Influence on Tissue 5

Mucilage content in the stems of four sympatric cactus speciesvaried from none for Ferocactus acanthodes, 19% by dry weightfor Opuntia basilaris, 26% for Opuntia acanthocarpa, and 35%for Echinocereus engelmannii. Although the mucilage differedchemically among the species (the arabinose content ranged from17% to 51% of the sugar monomers), its relative capacitance(change in relative water content per unit change in water potential)remained about 15 Mpa^–1. The relative capacitance of thewater-storage parenchyma averaged 1·04 Mpa^–1 andwas consistent with the mucilage content, being lowest for F.acanthodes and highest for E. engelmannii. Mucilage isolatedfrom hydrated tissue was accompanied by solutes with an osmoticpressure of about 0·2 MPa. Such associated solutes influencethe water-release characteristics of mucilage and hence itsrole as an apoplastic capacitor. In particular, extracellularsolutes can facilitate the release of appreciable mucilage-boundwater to the cells at tissue water potentials occurring duringthe initial phases of drought. Key words: Echinocereus engelmannii, Ferocactus acanthodes, Opuntia acanthocarpa, Opuntia basilaris, water potential isotherms     

MAIN ROOT-LATERAL ROOT JUNCTIONS OF TWO DESERT SUCCULENTS: CHANGES IN AXIAL AND RADIAL COMPONENTS OF HYDRAULIC CONDUCTIVITY DURING DRYING

The influence of junctions between main roots and lateral roots on water flow was investigated for the desert succulents Agave deserti and Ferocactus acanthodes during 21 d of drying in soil. Under wet conditions, the junctions did not restrict xylem water flow from lateral roots to main roots, consistent with predictions of axial conductance based on vessel diameters. Embolism caused by drying reduced such axial conductance more at the junctions than in adjoining root regions. Connective tracheary elements at the junctions were abundantly pitted and had large areas of unlignified primary wall, apparently making them more susceptible to embolism than vessels or tracheids elsewhere in the roots. Unlike the decrease in axial conductance, the overall hydraulic conductivity of the junction increased during drying because of an increase in the conductivity of the radial pathway. Despite such increases, main roots may not lose substantial amounts of water to a dry soil during drought, initially because embolism at the junctions can limit xylem flow and later because soil hydraulic conductivity decreases. Moreover, the increased root hydraulic conductivity and a potentially rapid recovery from embolism by connective tracheary elements may favor water uptake near the junctions upon soil rewetting.     

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 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     

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.     

Responses of a CAM Plant to Drought and Rainfall: Capacitance and Osmotic Pressure Influences on Water Movement

Daily and seasonal patterns in water flow and water potentialwere investigated for the Crassulacean acid metabolism succulentAgave deserti during an extended summer drought and for a periodfollowing rainfall. Field measurements of transpiration andof osmotic pressure changes over selected 24 h periods wereused as input variables for a computer model of water flow thatwas based on an electrical circuit analog of the whole plant.Parameters such as root resistance and tissue capacitance werealso varied to reflect the effects of changing plant or soilwater status. The model predicted internal water flow and waterpotential during the drought cycle and was used to assess therole of tissue osmotic properties in water uptake from the soiland in internal water redistribution. For plants under wet soil conditions, 55% of the night-timetranspiration was derived from water storage, this storage beingrecharged during the day. As drought progressed, transpirationand the nocturnal increase in osmotic pressure declined, althoughthe osmotic pressure itself increased. The difference in osmoticpressure between the water storage tissue and the chlorenchymacaused a net flow of water into the chlorenchyma after 3 weeksof drought, thereby increasing chlorenchyma turgor pressure.Simulations also indicated that a large increase in root resistancemust occur to prevent substantial water loss from the plantto the dry soil. After rainfall, recharge of plant water storagewas complete within one week, although full recovery in theamplitude of daily osmotic pressure variations took longer. Key words: Agave deserti, transpiration, water potential, water storage     

Influence of rocks on soil temperature,soil water potential,and rooting patterns for desert succulents

Summary At a site in the Sonoran Desert, subterranean rocks and exposed boulders affected soil water potential as well as root morphology and distribution. For Agave deserti, the number of lateral roots per unit length of main root was 11 times higher under rocks and six times higher alongside rocks than in rock-free regions. Total root length per unit soil volume for Echinocereus engelmannii averaged 3-fold higher within 1 cm of boulders than 5 cm away, where the soil was drier. The total length of lateral roots per unit length of main root for Ferocactus acanthodes was 4.2 m m^–1 under rocks but only 0.8 m m^–1 in rock-free regions. The number of lateral roots per unit length of main root for Opuntia acanthocarpa was 7-fold higher alongside rocks than in rock-free regions and even higher under rocks. For transplanted and watered A. deserti, the number of new main roots produced per 1–2 month interval averaged 13 for five plants on the north side of boulders, 8 on the south side, 11 for five plants with half of their roots under rocks, 2 for those with half of their roots over rocks, and 3 for the control plants without rocks. Laboratory experiments showed that the soil water potential under rocks for 10 and 30 mm waterings stayed above –0.5 MPa for 13 and 19 d longer, respectively, than for regions away from rocks. The shortwave absorptance of granitic rocks from the field site was 0.82, the thermal conductivity coefficient was 1.50 W m^–1 °C^–1, and the volumetric heat capacity was 1.75 MJ m^–3 °C^–1. Field measurements indicated that 5-cm-thick buried rocks decreased the diel variation in soil temperatures on their undersurface by only 0.4° C compared with soil. Thus, the primary influence of rocks at the field site on root proliferation and branching for the four species was apparently caused by influences on soil water content.     

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.     

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     

Soil O2 and CO2 Effects on Apparent Cell Viability for Roots of Desert Succulents

Roots of desert succulents occupy the upper layers of porous,well-aerated soils. However, roots of Agave deserti, Ferocactusacanthodes, and Opuntia ficus-indica all tolerated many daysof soil anoxia; 0% O₂ in the soil gas phase for 30 d reducedthe fraction of cells taking up the vital stain neutral red,an average of only 18% for the cortex and 6% for parenchymacells within the stele of perennial established roots. Ephemeralrain roots, induced by watering as branches on the establishedroots, were more susceptible to 0% O₂ in the soil gas phase;19 d abolished stain uptake for cortical cells and 32 d forstelar parenchyma cells. Soil CO₂ levels above the 0.1% observedin the root zone in the field rapidly reduced uptake of neutralred; the fraction of cortical cells taking up the stain decreased30% in 10 h at 0.5% CO₂ and was abolished in 9 h at 2% and 7h at 10% CO₂ averaged for the three species. Rain roots weresomewhat more susceptible than established roots to elevatedsoil CO₂ levels, and stelar parenchyma cells were much lesssusceptible than were cortical cells. When uptake of the vitalstain was abolished by elevated soil CO₂, no anatomical evidenceof cellular damage was observed. For A. deserti exposed to 2%CO₂, the pH of macerated root tissue decreased about 0.35 pHunit over 10 h; CO₂ apparently entered the cells, lowered theintracellular and/or cell wall pH, and prevented the accumulationof neutral red. Elevated soil CO₂ also inhibits root respirationfor the three desert succulents considered. Hence, the restrictionof such species to porous soils may reflect the relatively rapidinhibiting effects of elevated soil CO₂ levels rather than arequirement for high soil O₂ levels, consistent with the observationthat desert soils tend to have low gas-phase CO₂ levels near0.1% compared with 1% or more in the root zone of non-desertspecies. Key words: Agave deserti, Ferocactus acanthodes, neutral red, Opuntia ficus-indica, pH     

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     

TEMPERATURE,WATER AVAILABILITY,AND NUTRIENT LEVELS AT VARIOUS SOIL DEPTHS-CONSEQUENCES FOR SHALLOW-ROOTED DESERT SUCCULENTS,INCLUDING NURSE PLANT EFFECTS

Soil conditions were evaluated over the rooting depths for Agave deserti and Ferocactus acanthodes from the northwestern Sonoran Desert. These succulents have mean root depths of only 10 cm when adults and an even shallower distribution when seedlings, which often occur in association with the nurse plant Hilaria rigida, which also has shallow roots. Maximum soil temperatures in the 2 cm beneath bare ground were predicted to exceed 65 C, which is lethal to the roots of A. deserti and F. acanthodes, whereas H. rigida reduced the maximum surface temperatures by over 10 C, providing a microhabitat suitable for seedling establishment. Water Availability was defined as the soil-to-plant drop in water potential, for periods when the plants could take up water, integrated over time. Below 4 cm under bare ground, simulated Water Availability increased slightly with depth (to 35 cm) for a wet year, was fairly constant for an average year, and decreased for a dry year, indicating that the shallow rooting habit is more advantageous in drier years. Water uptake by H. rigida substantially reduced Water Availability for seedlings associated with this nurse plant. On the other hand, a 66–90% higher soil nitrogen level occurred under H. rigida, possibly representing its harvesting of this macronutrient from a wide ground area. Phosphorus was slightly less abundant in the soil under H. rigida compared with under bare ground, the potassium level was substantially higher, and the sodium level was substantially lower. All four elements varied greatly with depth, N and K decreasing and P and Na increasing. Based on the known growth responses of A. deserti and F. acanthodes to these four elements, growth was predicted to be higher for plants in soil from the shallower layers, most of the differences being due to nitrogen.     

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.     

Extreme temperatures and thermal tolerances for seedlings of desert succulents

Summary Extreme temperatures near the soil surface, which can reach 70°C at the main study site in the northwestern Sonoran Desert, markedly affect seedling survival. Computer simulations indicated that for the rather spherical barrel cactus Ferocactus acanthodes (Lem.) Britt. & Rose the maximum surface temperature decreased 8°C and the minimum temperature increased 3°C as the seedling height was increased from 1 mm up to 50 mm. Simulated changes in shortwave and longwave irradiation alone showed that shading could decrease the maximum temperature by about 5°C for the common desert agave, Agave deserti Engelm., and raise the minimum 1°C. Actual field measurements on seedlings of both species, where shading would affect local air temperatures and wind speeds in addition to irradiation, indicated that shading decreased the average maximum surface temperature by 11°C in the summer and raised the minimum temperature by 3°C in winter.Seedlings grown at day/iight air temperatures of 30°C/20°C tolerated low temperatures of about -7°C and high temperatures of about 56°C, as measured by the temperature where stain uptake by chlorenchyma cells was reduced 50%. Seedling tolerance to high temperatures increased slightly with age, and F. acanthodes was more tolerant than A. deserti. Even taking the acclimation of high temperature tolerance into account (2.7°C increase per 10°C increase in temperature), seedlings of A. deserti would not be expected to withstand the high temperatures at exposed sites, consistent with previous observations that these seedlings occur only in protected microhabitats. Based primarily on greater high temperature acclimation (4.3°C per 10°C), seedlings of F. acanthodes have a greater high temperature tolerance and can just barely survive in exposed sites. Wide ranges in photoperiod had little effect on the thermal sensitivities of either species. When drought increased the chlorenchyma osmotic pressure from about 0.5 MPa to 1.3 MPa, seedlings of both species became about 2°C less tolerant of high temperatures, which would be nonadaptive in a desert environment, and 2°C more tolerant of low temperatures, which also occurs for other species.In conclusion, seedlings of A. deserti and F. acanthodes could tolerate tissue temperatures over 60°C when acclimated to high temperatures and below -8°C when acclimated to low temperatures. However, the extreme environment adjacent to desert soil requires sheltered microhabitats to protect the plants from high temperature damage and also to protect them from low temperature damage at their upper elevational limits.