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Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy
Annals of Botany 84 : 213–223, 1999 Article No. anbo.1999.0911, available online at http:\\www.idealibrary.com on
Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent : Implications for Root System Redundancy E R I C A. G R A H A M and P A R K S. N O B E L* Department of Organismic Biology, Ecology and Eolution, Uniersity of California, Los Angeles, CA 90095-1606, USA Received : 15 January 1999
Returned for revision : 6 April 1999
Accepted : 27 April 1999
A technique used for hydroponics was adapted to measure instantaneous root water uptake from the soil for a leaf succulent CAM species, Agae deserti. Comparisons were made to previously modelled water fluxes for A. deserti and to Encelia farinosa, a non-succulent C species. Net CO uptake and transpiration for A. deserti under well-watered $ # conditions occurred primarily at night whereas root water uptake was relatively constant over 24 h. Leaf thickness decreased when transpiration commenced and then increased when recharge from the stem and soil occurred, consistent with previous models. A drought of 90 d eliminated net CO uptake and transpiration and reduced the # water content of leaves by 62 %. Rewetting the entire root system for 7 d led to a full recovery of leaf water storage but only 56 % of maximal net CO uptake. Root water uptake was maximal immediately after rewetting, which # replenished root water content, and decreased to a steady rate by 14 d. When only the distal 50 % of the root system was rewetted, the time for net CO uptake and leaf water storage to recover increased, but by 30 d gas exchange and # leaf water storage were similar to 100 % rewetting. Rewetting 10 or 20 % of the root system resulted in much less water uptake ; these plants did not recover leaf water storage or gas exchange by 30 d after rewetting. A redundancy in the root system of A. deserti apparently exists for daily water uptake requirements under wet conditions but the entire root system is required for rapid recovery from drought. # 1999 Annals of Botany Company Key words : Agae deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.
INTRODUCTION Rapid water uptake after drought is advantageous to a plant, particularly in the desert where water may be available infrequently and for short durations. Yet a large root system that facilitates rapid water uptake has a large carbon requirement (Nobel, Alm and Cavelier, 1992) and represents a high-conductance pathway for water movement from the plant back to a dry soil (North and Nobel, 1991). Indeed, cost-benefit analysis has provided insight into long-term root system carbon allocation (Hunt, Zakir and Nobel, 1987). For instance, a shallow root system maximizes water uptake from light rains, as occurs for desert succulents, which also have a low root : shoot ratio (Nobel, 1988). However, because the root system can be the limiting component in the hydraulic pathway from soil to leaves (Passioura, 1988 ; Nobel and Cui, 1992), a small root system may limit short-term water uptake upon rewetting after drought and increase the time for refilling of leaf water storage and full recovery of stomatal opening. Rocky soils become heterogeneous in their water content, reflecting local trapping of water and uneven evaporation (Jury and Bellantuoni, 1976). Soil surface irregularities such as slopes and rock outcrops can channel water and thereby create moist as well as dry microhabitats in arid regions (Cannon, 1911 ; Nobel, 1991 ; Loik and Meyer, 1991). After light rains, only part of a root system may have access to * For correspondence.
0305-7364\99\080213j11 $30.00\0
moist conditions (Nobel, Miller and Graham, 1992), potentially increasing the time required for recovery from drought. Split-root experiments have examined the effects of drying part of the root system on water uptake and gas exchange (Davies and Zhang, 1991), but apparently none have examined the role of a partial root system rewetting after drought. The species used here, Agae deserti Engelm., has a low root : shoot dry mass ratio of 0n11 (Nobel, 1988) and a shallow mean rooting depth of 0n10 m (Nobel, 1997). This native of the northwestern Sonoran Desert exhibits Crassulacean acid metabolism (CAM), with characteristic nocturnal stomatal opening and tissue succulence. The water relations, gas exchange and ecophysiology of A. deserti have been extensively investigated (Nobel, 1988). Water movement from the soil into storage tissue and the transpiration stream has been modelled using electrical circuit analogues under well-watered conditions and during recovery from drought (Smith, Schulte and Nobel, 1987 ; Schulte and Nobel, 1989). By adapting a hydrophonic method for measuring root water uptake (Van Ieperen and Madery, 1994), a new technique was developed that allows the measurement of instantaneous root water uptake from the soil when the soil contains a known amount of water. Along with measurements of leaf thickness, instantaneous root water uptake measurements allow the validation of electrical-analogue models and the examination of a partial root system rewetting on water uptake after drought. In this study, three aspects of the water relations for a # 1999 Annals of Botany Company
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Graham and Nobel—Water Relations vs. Root System Properties
well-watered A. deserti are examined : root water uptake, leaf water storage, and gas exchange patterns. Water flow is also contrasted with that of a sympatric non-succulent C $ species, Encelia farinosa Gray. In addition, time courses are established for gas exchange and leaf water storage during drying of the entire root system and recovery after rewetting ; these are compared to predictions using electrical-analogue models (Smith et al., 1987). Whether the root system of A. deserti is so reduced that a rewetting of the entire root system after drought is necessary to permit timely recovery is addressed using the same water relations measurements after rewetting various fractions of the root system. MATERIALS AND METHODS Fifteen plants of Agae deserti Engelm. (Agavaceae), each having an average of eight leaves in the basal rosette, 890 g fresh mass, and about 200 g dry mass were collected from the University of California Philip L. Boyd Deep Canyon Desert Research Center at Agave Hill (33m 38h N, 116m 24h W, 820 m elevation). Three plants of Encelia farinosa A. Gray (Compositae) with an average of 180 leaves and 120 g fresh mass were also collected from the same location. Plants were maintained in a glasshouse at the University of California, Los Angeles, California, USA for about 4 months in 0n012 m$ pots with soil collected from Agave Hill. Mean day\night air temperatures were 27\15 mC and the photosynthetic photon flux (PPF, 400–700 nm), measured with an LI-191SB Line Quantum Sensor (Li-Cor, Lincoln, Nebraska, USA), was 80 % of the ambient. Plants were watered weekly with a 0n05-strength Hoagland’s solution to keep the minimum water potential of the soil above k0n5 MPa. Plants were next removed from their pots, and their root systems were carefully rinsed to remove soil. Each plant was placed at one end of a plastic rectangular tray 0n55 m long, 0n16 m wide and 0n16 m deep with its roots extending the length of the tray. Across the width of each tray, a 15 mm thick slab of grafting wax (Walter E. Clark & Son, Orange, Connecticut, USA) separated the tray into two water-tight compartments of various areas and fractions of the root system. A single port per compartment was fitted with a tube 0n30 m in length to connect to a reservoir containing water at a height approx. 2 cm above the bottom of the tray. Soil from Agave Hill was added to a depth of 0n14 m in each compartment. Plants were then transferred to Conviron E15 environmental chambers (Controlled Environments, Pembina, North Dakota, USA) with a 12 h photoperiod at a PPF of 18 mol m−# d−" on a horizontal plane at the top of the canopy and day\night air temperatures of 22\16 mC. Plants were watered weekly and were acclimated to the chambers for 4 weeks before the start of an experiment. Instantaneous net CO uptake and transpiration were # measured over 24 h periods for up to four plants simultaneously with a Li-Cor LI-6262 infrared gas analyser in the differential mode. One mid-canopy leaf of each A. deserti plant was inserted into a cylindrical transparent acrylic cuvette 50 mm in diameter whose open end was sealed to the leaf with plastic putty. For E. farinosa, approx. eight terminal leaves on a branch were inserted into a
cuvette, which was likewise sealed with plastic putty. Air flow into each cuvette was constant over 24 h, and solenoid valves periodically switched the excurrent flow of air into the gas analyser. Flow rates were measured using LFC-3 mass-flow meters (Technology Incorporated, Dayton, Ohio, USA) and adjusted so that the decrease in the mole fraction of CO in the cuvettes never exceeded 50 µmol mol−". A # Polycorder 516-24 datalogger (Omnidata International, Logan, Utah, USA) collected data every 10 min from the gas analyser, a Hygro-M1 optical dewpoint hygrometer (General Eastern Instruments, Watertown, Massachusetts, USA), and the mass flow meters. Data are expressed on a total leaf surface area basis (abaxial plus adaxial areas), which averaged 0n11 m# for the A. deserti plants used (its leaves are thick and crescent-shaped in cross-section). Leaf thickness was measured daily using dial calipers accurate to 0n05 mm or with a calibrated wire 0n2 mm in diameter inserted mid-leaf in four leaves from each plant. Instantaneous leaf thickness was measured for a leaf in the middle of the plant using an electromechanical transducer (E-Line LVDT ; Shaevitz Engineering, Pennsauken, New Jersey, USA) clamped at midleaf. Output from the transducer at a resolution of 3 µm was collected very 5 min by a microcomputer and is expressed in micrometers based on an arbitrary initial value. Water uptake by the root system during 24 h periods was determined from the change in mass of water in a reservoir connected to the port in each tray. To prevent evaporation, the soil surface was covered with plastic sheeting and aluminium foil and measurements were taken after the soil water had come into equilibrium with the water in the reservoir. Trays were placed partially submerged in a tank of water to help stabilize soil temperature and were insulated with aluminum foil. Soil temperature was measured with copper-constantan thermocouples placed in the centre of each compartment. Water uptake into a compartment was measured with the reservoir on an AE100 balance (MettlerToledo, Heightstown, New Jersey, USA) ; the balance and thermocouples were connected to a microcomputer that collected values every 5 min. For consistency with the gas exchange data, root water uptake is expressed on a total leaf area basis (abaxial plus adaxial surfaces). Soil water potential was measured using PCT-55 thermocouple soil psychrometers (Wescor, Logan, Utah, USA) placed in the centre of each compartment. Psychrometers equilibrated for 24 h in the soil before they were removed and immediately dipped into molten paraffin ; the sealed psychrometers were then placed inside an insulated chamber for 3 h to eliminate internal thermal gradients, after which a Wescor HR-33T microvoltmeter in the dewpoint mode was used to determine the water potential. Leaf water potential was measured using a TruPsi microvoltmeter (Decagon Devices, Pullman, Washington, USA) using samples taken from the middle of three leaves per plant with a cork borer 13 mm in diameter. Samples were stripped of their cuticle, immediately placed into a Decagon SC-10X thermocouple psychrometer chamber and allowed to equilibrate for approx. 3 h before measuring. Statistical significance was determined using Student’s ttest.
Graham and Nobel—Water Relations vs. Root System Properties
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Net CO2 uptake (µmol m–2 s–1)
10 A 8 6 4 2 0
Transpiration (mmol m–2 s–1)
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2
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Root water uptake (mmol m–2 s–1)
Relative leaf thickness (µm)
0 C 50
0
–50
D 3
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0 Time of day (h)
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F. 1. Instantaneous net CO uptake rates (A), transpiration rates (B), leaf thicknesses relative to an arbitrary initial value (C) and root water # uptake rates (D) for the succulent CAM species Agae deserti (—#—) and for the non-succulent C species Encelia farinosa (::=::) under $ well-watered conditions. Gas exchange and root water uptake data are based on total abaxial plus adaxial leaf surface area. Stippled bar indicates nighttime.
216
Graham and Nobel—Water Relations vs. Root System Properties Net CO2 uptake (mmol m–2 d–1)
200 A 150
100
50
0 Transpiration (mol m–2 d–1)
B 9
6
3
0 Leaf water storage (mol m–2)
C 150
100
50
Root water uptake (mol m–2 d–1)
0 D
20
10
10
0
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Time (d) F. 2. Time course for daily net CO uptake rate (A), transpiration rate (B), leaf water storage (C) and root water uptake rate (D) for A. deserti # during 28 d of soil drying. Data (expressed on a total leaf area basis for both gas exchange and root water uptake) are meansps.e. (n l 4 plants).
RESULTS Agae deserti exhibited typical CAM patterns of gas exchange, with CO uptake and transpiration occurring # primarily at night, while Encelia farinosa followed typical C $
patterns of gas exchange, where CO uptake occurs together # with photosynthesis (Fig. 1 A and B). Specifically, 64 % of the daily net CO uptake and 43 % of the transpiration # for the well-watered A. deserti occurred at night, while essentially 100 % of the CO uptake and transpiration for #
Graham and Nobel—Water Relations vs. Root System Properties
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Net CO2 uptake (mmol m–2 d–1)
200 A 150
100
50
0 Transpiration (mol m–2 d–1)
B 9
6
3
0 Leaf water storage (mol m–2)
C 150
100
50
Root water uptake (mol m–2 d–1)
0 D
20
10
0
10
20
30
Time (d) F. 3. Time course for daily net CO uptake rate (A), transpiration rate (B), leaf water storage (C) and root water uptake rate (D) for A. deserti # when the entire root system was rewetted after 90 d of drought. Data are meansps.e. (n l 4 plants).
E. farinosa occurred during the day. The daily water-use efficiency was 0n017 mol CO mol−" H O for A. deserti and # # 6n2-times lower for E. farinosa. Leaf thickness decreased for A. deserti in the early afternoon, reached a minimum at the
beginning of the night, remained relatively constant until the last third of the night and then increased throughout the morning (Fig. 1 C). Leaf thickness for E. farinosa increased in the late afternoon and through the night and then
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Graham and Nobel—Water Relations vs. Root System Properties 3 A
2
1
0
B
2
Net CO2 uptake (µmol m–2 s–1)
1
0
C
2
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0
D
2
1
0
0
2
4
6
8 Time (d)
10
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14
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F. 4. Instantaneous net CO uptake rates for A. deserti when 100 % (A) and the distal 50 % (B), 20 % (C) and 10 % (D) of the root system was # rewetted after 90 d of drought.
Graham and Nobel—Water Relations vs. Root System Properties
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0·3 A
0·2
0·1
0 B
Transpiration (mmol m–2 s–1)
0·2
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0 C
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0 D
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8 Time (d)
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F. 5. Instantaneous transpiration rates for A. deserti when 100 % (A) and the distal 50 % (B), 20 % (C) and 10 % (D) of the root system was rewetted after 90 d of drought.
decreased in the morning. The amplitude of leaf thickness variation was 2n1-times greater for A. deserti than for E. farinosa (Fig. 1 C) but constituted only about 0n10 % of the total thickness for the leaf-succulent A. deserti compared to 8n0 % for E. farinosa. The root water uptake rate for A.
deserti was relatively constant over 24 h (Fig. 1 D) and hence did not follow the pattern of transpiration. In contrast, the pattern of root water uptake for E. farinosa was similar to that of transpiration (Fig. 1 B and D). When the soil was allowed to dry for 28 d, the soil water
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Graham and Nobel—Water Relations vs. Root System Properties 4
100%
Increase in leaf thickness (mm)
3
50% 2
20%
1
0
10%
0
3
6
9
12
30
Time (d) F. 6. Increases in leaf thickness for A. deserti when 100 % (#) and the distal 50 % (=), 20 % ( ) and 10 % (W) of the root system was rewetted after 90 d of drought. Data are meansps.e. (n l 4 leaves on an individual plant).
potential decreased from k0n04p0n02 MPa (meanps.e.) at 0 d, to k1n1p0n4 MPa at 10 d, k5n8p0n9 MPa at 20 d, and to less than k10 MPa at 28 d. For A. deserti shoot water potential was k0n5p0n1 MPa at 0 d and k1n2p0n1 MPa at 28 d. Net CO uptake remained close to its initial value # throughout 14 d of drought (Fig. 2 A). At 28 d, net CO # uptake averaged 35p8 % of the maximum. Transpiration also decreased with soil drying, especially during the day, becoming significantly decreased at 14 d (P l 0n002) and averaging 22p9 % of the maximum at 28 d (Fig. 2 B). Instantaneous water-use efficiency at 0 d of drought was 0n011p0n002 mol CO mol−" H O at mid-afternoon and # # 0n026p0n009 at midnight. Daily water-use efficiency increased during soil drying, averaging 0n016p0n002 mol CO # mol−" H O at 0 d of drought and 0n025p0n003 at 28 d (P l # 0n005). Leaf water storage decreased about 24 % after 28 d soil drying (Fig. 2 C ; P l 0n001). Root water uptake decreased to zero by 7 d of soil drying (Fig. 2 D) when the soil water potential was less than the shoot water potential of k0n5p0n1 MPa. At 90 d of drought, a small daily net loss of CO # (1n4 mmol m−# d−") occurred due to respiration (Fig. 3 A). Seven days after the entire root system was rewetted (which raised the soil water potential to k0n03p0n01 MPa) average net CO uptake was 56 % of its pre-drought value, and at # 14 d of rewetting net CO uptake was not significantly # different from that before the drought (Figs 2 A and 3 A ; P l 0n4). Transpiration at 7 d of rewetting averaged 49 % of its pre-drought value and at 14 d was not significantly different from that before drought (Figs 2 B and 3 B ; P l 0n5). At 90 d of drought, leaf water content had decreased to
62 % of that before drought (Figs 2 C and 3 C ; P l 0n03). Leaf water storage increased to its pre-drought value after 7 d of rewetting, remaining constant thereafter (Figs 2 C and 3 C). Root water uptake was highest the first day after rewetting, decreasing 56 % in 14 d (Fig. 3 D) when it was not significantly different from its pre-drought value (P l 0n7). The time required for net CO uptake recovery and the # instantaneous net CO uptake patterns during recovery # depended on the fraction of the root system that was rewetted after 90 d of drought (Fig. 4). The time at which total daily net CO uptake was half of the maximum was # 8n5 d for rewetting 100 % of the root system and 11n8 d when 50 % of the root system was rewetted. At 30 d, the 20 % rewetting led to a daily net CO uptake only 9 % of # that of the 100 % and 50 % rewettings (Fig. 4). No net CO # uptake was observed at 30 d when only the distal 10 % of the root system was rewetted. Transpiration also increased when A. deserti was rewetted after 90 d of drought for all treatments except the 10 % rewetting (Fig. 5). The time at which the transpiration rate was half of the maximum was 7n0 d for the 100 % rewetting and 12n5 d for the 50 % rewetting. Total daily transpiration at 30 d was 8n2 mol m−# d−" for both the 100 and 50 % rewettings and 1n7 mol m−# d−" for the 20 % rewetting (Fig. 5). The water-use efficiency at 30 d of rewetting was 0n016 mol CO mol−" H O for the # # 100 % rewetting, 0n014 for the 50 % rewetting, and 0n009 for the 20 % rewetting. Leaf thickness increased rapidly when the entire root system was rewetted after 90 d of drought, reaching 90 % of the total increase in leaf thickness in 7n1 d (Fig. 6). The increase in leaf thickness was more gradual for the 50 %
Graham and Nobel—Water Relations vs. Root System Properties
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1·0
Root water uptake (mmol m–2 s–1)
0·8
0·6
0·4
0·2
0
1
2 Time (d)
3 29
30
F. 7. Instantaneous root water uptake rates for A. deserti when 100 % (#) and the distal 50 % (=), 20 % ( ) and 10 % (W) of the root system was rewetted after 90 d of drought. Data are on a whole plant basis and are expressed per total leaf surface area.
rewetting, 90 % recovery occurring in 27n0 d. The 20 and 10 % rewettings had much smaller total increases in leaf water storage at 30 d equalling about 30 and 10 %, respectively, that of the 100 % rewetting (Fig. 6). When the entire root system of A. deserti was rewetted after 90 d of drought, the root water uptake rate was initially high, decreasing by 71 % in 3 d (Fig. 7 ; 100 % rewetting). Similar patterns were observed when the distal 50, 20 or 10 % of the root system was rewetted. The root water uptake rate at 30 d of rewetting was similar for the 100 and 50 % rewettings, 60 % lower for the 20 % rewetting and 69 % lower for the 10 % rewetting (Fig. 7). Total water uptake over the 30 d period was 440 mol m−# for the 100 % rewetting, 404 mol m−# for the 50 % rewetting, 173 mol m−# for the 20 % rewetting and 117 mol m−# for the 10 % rewetting. DISCUSSION A technique for measuring water uptake by roots in hydroponics (Van Ieperen and Madery, 1994) was adapted so that virtually instantaneous measurements of water uptake could be made for roots in wet soil. Measurements were extremely sensitive to soil temperature, due in part to volume changes of air trapped in the soil under water saturated conditions (White and Mastalerz, 1996 ; Hershey, 1990). Nevertheless, measured root water uptake integrated over 24 h was within 1 % of total transpiration for the C $ species Encelia farinosa, and its pattern for root water uptake closely paralleled its pattern for transpiration, which occurred primarily during the daytime. The time lag (approx. 30 min) between transpiration measured by an infrared gas analyser and root water uptake measured by the adapted technique is consistent with the relatively small capacitance
for this non-succulent plant (Nobel and Jordan, 1983). This technique allows the direct measurement of root water uptake from the soil which, together with measurements of leaf water storage and transpiration, can be used to quantify previously modelled water flow in Agae deserti. The conclusion is reached that under wet soil conditions the root system of this desert succulent shows redundancy. Under well-watered conditions, A. deserti showed gas exchange patterns typical of a CAM species, exhibiting net CO uptake and transpiration primarily at night. Leaf # thickness decreased during the afternoon, as transpiration commenced ; leaf thickness increased during the last third of the night and continued to increase in the early morning after transpiration had essentially ceased. The change in leaf thickness averaged over the projected leaf surface area of a plant represented a total water flux equivalent to 41 % of total daily transpiration. Root water uptake was relatively constant over a 24 h period for A. deserti, contrary to E. farinosa but consistent with predicted rates for a hydrated plant in wet soil (Schulte and Nobel, 1989). Integrated over 24 h, root water uptake for A. deserti exceeded total daily transpiration by 8 %, indicating some water movement into storage and growth. When stomatal opening commences in the late afternoon, modelled water loss for A. deserti first occurs from the outer chlorenchyma ; lost water is replaced by that in the water storage parenchyma (Goldstein, Andrade and Nobel, 1991 ; Tissue, Yakir and Nobel, 1991). The leaf thickness of A. deserti decreased with the beginning of transpiration, indicating that water was indeed initially lost from leaf water storage. By early nighttime, leaf thickness had stopped decreasing, indicating water movement into leaf water storage was equalled by that lost through transpiration.
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Graham and Nobel—Water Relations vs. Root System Properties
Recharge of leaf water storage then began during the last third of the night, when transpiration exceeded root water uptake, and continued after transpiration had ceased in the morning. Because of the relatively constant root water uptake, this recharge must represent water moving into leaf water storage from the stem, older leaves, and young folded leaves in the centre of the rosette through a lowerresistance pathway than from the soil. Transpiration is the main driving force for water movement for A. deserti under well-watered conditions (Smith et al., 1987), yet because of leaf water storage, daily changes in its leaf water potential due to transpiration are slight ; the observed 0n10 % change in leaf thickness due to water loss from transpiration is equivalent to a decrease in water potential of only 0n024 MPa (Calkin and Nobel, 1986). The water potential difference between the shoot and the soil was about k0n5 MPa, sufficient to mask such small leaf water potential changes. Daily changes in the leaf water potential due to CAM are also small, approx. 0n03 MPa under the present conditions (Smith et al., 1987). Net CO uptake and transpiration for A. deserti continued # relatively unchanged for the first week of soil drying. As drought progressed, net CO uptake decreased more slowly # than did transpiration, reflecting a substantial decrease in daytime transpiration as the instantaneous water-use efficiency was 2n4-times lower during daytime stomatal opening than at midnight. The rate of water loss from storage in the leaves also decreased with drought length, mainly as a result of the decreasing daily duration of stomatal opening. During the first 28 d of drought, plants lost an average of 33 mol m−# water from storage ; during the last 60 d of drought, plants lost only an additional 20 mol m−#. By 7 d after rewetting following 90 d of drought, leaf water storage had increased to the pre-drought value, but the daily net CO uptake had not, probably due to the # effects of prolonged drought on photosynthetic processes (Raveh, Gersani and Nobel, 1995 ; Herppich and Peckmann, 1997). Daytime transpiration comprising one-third of total daily transpiration occurred at 7 d after rewetting, consistent with a high leaf water storage and indicative of plants kept under well-watered conditions (Nobel, 1988). Water uptake by the root system was initially rapid upon rewetting, reflecting the lower water potential of the shoot (about k1n2 MPa) and suggesting a highly conductive root system. Even after the soil water potential is below k10 MPa for many weeks, some roots close to the succulent shoot of A. deserti can have a high water status and maintain a relatively high hydraulic conductivity (North and Nobel, 1998). The hydraulic conductivity of a droughted root system also increases rapidly as the cortex is rehydrated and embolism is reduced, often within a few days after rewetting (North and Nobel, 1991). When the distal 10 % of the root system was rewetted after 90 d of drought, no net CO uptake occurred even # after 30 d of rewetting. When the distal 20 % of the root system was rewetted, net CO uptake began at 13 d and # total daily net CO uptake at 30 d was only 16 % of that # when the entire root system was rewetted. When the distal 50 % of the root system was rewetted, recovery of half-
maximal daily net CO uptake was slowed by 40 % compared # with wetting the entire root system. After an extended drought, water uptake by a distal portion of the root system is not sufficient for a rapid recovery of gas exchange and a positive carbon gain to occur. In this regard, a single rainfall event of 10 mm in the Sonoran Desert can raise the water potential of the soil at the mean rooting depth of A. deserti to k0n5 MPa, allowing water uptake by the root system (Nobel, 1988). However, 10 d later the soil water potential can decrease below that of the shoot (Young and Nobel, 1986). At 10 d after rewetting, the 10 and 20 % rewettings still had a daily net loss of carbon and the 50 % rewetting had a carbon gain equal to only one-quarter of that of the 100 % rewetting. The conclusion is thus reached that under field conditions the entire root system of A. deserti must be rewetted for maximal carbon gain to occur rapidly, indicating very little hydraulic redundancy in the root system with regard to gas exchange recovery. After a short delay, recovery of leaf water storage for the 100 % rewetting of A. deserti followed an exponential rise to a maximum within 10 d. The delay may be associated with a reduction of embolism (North and Nobel, 1991), a rehydration of the root cortical tissue (Nobel and Huang, 1992 ; North and Nobel, 1995), and stem rehydration. Rewetting the distal 50 % of the root system led to a more gradual increase in leaf thickness ; leaf thickness was 51 % of that of the 100 % rewetting at 10 d. Even after 30 d of rewetting, the recovery of leaf water storage for the 20 and 10 % rewettings was only 29 and 9 %, respectively, of the 100 % rewetting. Consistent with this result, transpiration had not recovered for the 20 and 10 % rewettings after 30 d of rewetting. Therefore, a small fraction of the root system is not sufficient for recharging leaf water storage and recovery of stomatal opening even after a month of wet soil conditions. Water uptake by the root system of A. deserti decreased within 24 h for all rewetting treatments, most likely the effect of rehydrating root cortical tissue and refilling of root capacitance. Indeed, simple exponential curves used to model capacitance (Nobel, 1991) closely described the decrease in water uptake rate for the 50, 20 and 10 % rewettings, and the mass of water initially taken up was approx. 40 % of the original fresh mass of the portion of roots rewetted. [A satisfactory (r# 0n95) simple exponential curve could not be fitted to the 100 % rewetting root water uptake data, probably because substantial root, stem and leaf capacitance refilling were taking place simultaneously.] At 30 d of rewetting, the root water uptake rate was similar for the 50 and 100 % rewettings, indicating that half of the root system may be adequate to supply all the water necessary under well-watered conditions. Indeed, split-root and partial root system wetting experiments indicate that the root systems in other species are also in excess of that required for daily transpiration needs (Tan and Buttery, 1982 ; Gallardo, Turner and Ludwig, 1994 ; Green and Clothier, 1995). The rate of water uptake for the 20 and 10 % rewettings remained low through 30 d of rewetting, possibly due to residual embolism in the proximal portions of the root system of A. deserti (North and Nobel, 1991). Even though the root : shoot dry mass ratio of 0n11 for A.
Graham and Nobel—Water Relations vs. Root System Properties deserti (Nobel, 1988) is low relative to other plants, the root system is extensive enough to allow for a rapid return to predrought leaf water status and gas exchange after rewetting. After 3 months of drought, rewetting the entire root system led to complete recovery of leaf water storage within 7 d. Rewetting only the distal half of the root system delayed water uptake and gas exchange recovery, and rewetting the distal 20 and 10 % of the root system resulted in significantly less water uptake and almost no recovery of net CO uptake # within 30 d. For a rapid recovery from drought, wetting the distal region of the root system of A. deserti is insufficient. After leaf rehydration and gas exchange recovery, water uptake by the distal half of the root system was equal to that by the entire root system and supported maximal net CO # uptake, suggesting a redundancy in the root system under well-watered conditions. In a sporadic rainfall environment such as the Sonoran Desert, however, A. deserti must rely on its entire root system for rapid recovery from drought. A C K N O W L E D G E M E N TS Financial support from the National Science Foundation grant IBN-94-19844 is gratefully acknowledged. LITERATURE CITED Calkin HW, Nobel PS. 1986. Nonsteady-state analysis of water flow and capacitance for Agae deserti. Canadian Journal of Botany 64 : 2556–2560. Cannon WA. 1911. The root habits of desert plants. Washington, DC : Carnegie Institute of Washington. Davies WJ, Zhang J. 1991. Root signals and the regulation of growth and development of plants in drying soil. Annual Reiew of Plant Physiology and Plant Molecular Biology 42 : 55–76. Gallardo M, Turner NC, Ludwig C. 1994. Water relations, gas exchange and abscisic acid content of Lupinus cosentinii leaves in response to drying different proportions of the root system. Journal of Experimental Botany 45 : 909–918. Goldstein G, Andrade JL, Nobel PS. 1991. Differences in water relations parameters for the chlorenchyma and parenchyma of Opuntia ficus-indica under wet versus dry conditions. Australian Journal of Plant Physiology 18 : 95–107. Green SR, Clothier BE. 1995. Root water uptake by kiwifruit vines following partial wetting of the root zone. Plant and Soil 173 : 317–328. Herppich WB, Peckmann K. 1997. Responses of gas exchange, photosynthesis, nocturnal acid accumulation and water relations of Aptenia cordifolia to short-term drought and rewatering. Journal of Plant Physiology 150 : 467–474. Hershey DR. 1990. Container-soil physics and plant growth. Bioscience 40 : 685–686. Hunt ER Jr, Zakir NJD, Nobel PS. 1987. Water costs and water revenues for established and rain-induced roots of Agae deserti. Functional Ecology 1 : 125–129. Jury WA, Bellantuoni B. 1976. Heat and water movement under surface rocks in a field soil : II Moisture effects. Soil Science Society of America Journal 40 : 509–513.
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Nobel PS. 1988. Enironmental biology of agaes and cacti. New York : Cambridge University Press. Nobel PS. 1991. Physicochemical and enironmental plant physiology. San Diego : Academic Press. Nobel PS. 1997. Root distribution and seasonal production in the northwestern Sonoran Desert for a C subshrub, a C bunchgrass, $ % and a CAM leaf succulent. American Journal of Botany 84 : 949–955. Nobel PS, Cui M. 1992. Hydraulic conductances of the soil, the rootsoil air gap, and the root : changes for desert succulents in drying soil. Journal of Experimental Botany 43 : 319–326. Nobel PS, Huang B. 1992. Hydraulic and structural changes for lateral roots of desert succulents in response to soil drying and rewetting. International Journal of Plant Science 153 : S163–S170. Nobel PS, Jordan PW. 1983. Transpiration stream of desert species : resistances and capacitances for a C , a C , and a CAM plant. $ % Journal of Experimental Botany 34 : 1379–1391. Nobel PS, Alm DM, Cavellier J. 1992. Growth respiration, maintenance respiration and structural-carbon costs for roots of three desert succulents. Functional Ecology 6 : 79–85. Nobel PS, Loik ME, Meyer RW. 1991. Microhabitat and diel tissue acidity changes for two sympatric cactus species differing in growth habit. Journal of Ecology 79 : 167–182. Nobel PS, Miller PM, Graham EA. 1992. Influence of rocks on soil temperature, soil water potential, and rooting patterns for desert succulents. Oecologia 92 : 90–96. North GB, Nobel PS. 1991. Changes in hydraulic conductivity and anatomy caused by drying and rewetting roots of Agae deserti (Agavaceae). American Journal of Botany 78 : 906–915. North GB, Nobel PS. 1995. Hydraulic conductivity of concentric root tissues of Agae deserti Engelm. under wet and drying conditions. New Phytologist 130 : 47–57. North GB, Nobel PS. 1998. Water uptake and structural plasticity along roots of a desert succulent during prolonged drought. Plant, Cell and Enironment 21 : 705–713. Passioura JB. 1988. Water transport in and to roots. Annual Reiew of Plant Physiology and Plant Molecular Biology 39 : 245–265. Raveh E, Gersani M, Nobel PS. 1995. CO uptake and fluorescence # responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO concentrations. Physiologia Plantarum # 93 : 505–511. Schulte PJ, Nobel PS. 1989. Responses of a CAM plant to drought and rainfall : Capacitance and osmotic pressure influences on water movement. Journal of Experimental Botany 40 : 61–70. Smith JAC, Schulte PJ, Nobel PS. 1987. Water flow and water storage in Agae deserti : osmotic implications of Crassulacean acid metabolism. Plant, Cell and Enironment 10 : 639–648. Tan CS, Buttery BR. 1982. The effects of soil moisture stress to various fractions of the root system on transpiration, photosynthesis, and internal water relations of peach seedlings. Journal of the American Society of Horticultural Science 107 : 845–849. Tissue DT, Yakir D, Nobel PS. 1991. Diel water movement between parenchyma and chlorenchyma of two desert CAM plants under dry and wet conditions. Plant, Cell and Enironment 14 : 407–413. Van Ieperen W, Madery H. 1994. A new method to measure plant water uptake and transpiration simultaneously. Journal of Experimental Botany 45 : 51–60. White JW, Mastalerz JW. 1966. Soil moisture as related to ‘ container capacity ’. Journal for the American Society for Horticultural Science 89 : 758–765. Young DR, Nobel PS. 1986. Predictions of soil-water potentials in the north-western Sonoran Desert. Journal of Ecology 74 : 143–154.
Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent : Implications for Root System Redundancy E R I C A. G R A H A M and P A R K S. N O B E L* Department of Organismic Biology, Ecology and Eolution, Uniersity of California, Los Angeles, CA 90095-1606, USA Received : 15 January 1999
Returned for revision : 6 April 1999
Accepted : 27 April 1999
A technique used for hydroponics was adapted to measure instantaneous root water uptake from the soil for a leaf succulent CAM species, Agae deserti. Comparisons were made to previously modelled water fluxes for A. deserti and to Encelia farinosa, a non-succulent C species. Net CO uptake and transpiration for A. deserti under well-watered $ # conditions occurred primarily at night whereas root water uptake was relatively constant over 24 h. Leaf thickness decreased when transpiration commenced and then increased when recharge from the stem and soil occurred, consistent with previous models. A drought of 90 d eliminated net CO uptake and transpiration and reduced the # water content of leaves by 62 %. Rewetting the entire root system for 7 d led to a full recovery of leaf water storage but only 56 % of maximal net CO uptake. Root water uptake was maximal immediately after rewetting, which # replenished root water content, and decreased to a steady rate by 14 d. When only the distal 50 % of the root system was rewetted, the time for net CO uptake and leaf water storage to recover increased, but by 30 d gas exchange and # leaf water storage were similar to 100 % rewetting. Rewetting 10 or 20 % of the root system resulted in much less water uptake ; these plants did not recover leaf water storage or gas exchange by 30 d after rewetting. A redundancy in the root system of A. deserti apparently exists for daily water uptake requirements under wet conditions but the entire root system is required for rapid recovery from drought. # 1999 Annals of Botany Company Key words : Agae deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.
INTRODUCTION Rapid water uptake after drought is advantageous to a plant, particularly in the desert where water may be available infrequently and for short durations. Yet a large root system that facilitates rapid water uptake has a large carbon requirement (Nobel, Alm and Cavelier, 1992) and represents a high-conductance pathway for water movement from the plant back to a dry soil (North and Nobel, 1991). Indeed, cost-benefit analysis has provided insight into long-term root system carbon allocation (Hunt, Zakir and Nobel, 1987). For instance, a shallow root system maximizes water uptake from light rains, as occurs for desert succulents, which also have a low root : shoot ratio (Nobel, 1988). However, because the root system can be the limiting component in the hydraulic pathway from soil to leaves (Passioura, 1988 ; Nobel and Cui, 1992), a small root system may limit short-term water uptake upon rewetting after drought and increase the time for refilling of leaf water storage and full recovery of stomatal opening. Rocky soils become heterogeneous in their water content, reflecting local trapping of water and uneven evaporation (Jury and Bellantuoni, 1976). Soil surface irregularities such as slopes and rock outcrops can channel water and thereby create moist as well as dry microhabitats in arid regions (Cannon, 1911 ; Nobel, 1991 ; Loik and Meyer, 1991). After light rains, only part of a root system may have access to * For correspondence.
0305-7364\99\080213j11 $30.00\0
moist conditions (Nobel, Miller and Graham, 1992), potentially increasing the time required for recovery from drought. Split-root experiments have examined the effects of drying part of the root system on water uptake and gas exchange (Davies and Zhang, 1991), but apparently none have examined the role of a partial root system rewetting after drought. The species used here, Agae deserti Engelm., has a low root : shoot dry mass ratio of 0n11 (Nobel, 1988) and a shallow mean rooting depth of 0n10 m (Nobel, 1997). This native of the northwestern Sonoran Desert exhibits Crassulacean acid metabolism (CAM), with characteristic nocturnal stomatal opening and tissue succulence. The water relations, gas exchange and ecophysiology of A. deserti have been extensively investigated (Nobel, 1988). Water movement from the soil into storage tissue and the transpiration stream has been modelled using electrical circuit analogues under well-watered conditions and during recovery from drought (Smith, Schulte and Nobel, 1987 ; Schulte and Nobel, 1989). By adapting a hydrophonic method for measuring root water uptake (Van Ieperen and Madery, 1994), a new technique was developed that allows the measurement of instantaneous root water uptake from the soil when the soil contains a known amount of water. Along with measurements of leaf thickness, instantaneous root water uptake measurements allow the validation of electrical-analogue models and the examination of a partial root system rewetting on water uptake after drought. In this study, three aspects of the water relations for a # 1999 Annals of Botany Company
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Graham and Nobel—Water Relations vs. Root System Properties
well-watered A. deserti are examined : root water uptake, leaf water storage, and gas exchange patterns. Water flow is also contrasted with that of a sympatric non-succulent C $ species, Encelia farinosa Gray. In addition, time courses are established for gas exchange and leaf water storage during drying of the entire root system and recovery after rewetting ; these are compared to predictions using electrical-analogue models (Smith et al., 1987). Whether the root system of A. deserti is so reduced that a rewetting of the entire root system after drought is necessary to permit timely recovery is addressed using the same water relations measurements after rewetting various fractions of the root system. MATERIALS AND METHODS Fifteen plants of Agae deserti Engelm. (Agavaceae), each having an average of eight leaves in the basal rosette, 890 g fresh mass, and about 200 g dry mass were collected from the University of California Philip L. Boyd Deep Canyon Desert Research Center at Agave Hill (33m 38h N, 116m 24h W, 820 m elevation). Three plants of Encelia farinosa A. Gray (Compositae) with an average of 180 leaves and 120 g fresh mass were also collected from the same location. Plants were maintained in a glasshouse at the University of California, Los Angeles, California, USA for about 4 months in 0n012 m$ pots with soil collected from Agave Hill. Mean day\night air temperatures were 27\15 mC and the photosynthetic photon flux (PPF, 400–700 nm), measured with an LI-191SB Line Quantum Sensor (Li-Cor, Lincoln, Nebraska, USA), was 80 % of the ambient. Plants were watered weekly with a 0n05-strength Hoagland’s solution to keep the minimum water potential of the soil above k0n5 MPa. Plants were next removed from their pots, and their root systems were carefully rinsed to remove soil. Each plant was placed at one end of a plastic rectangular tray 0n55 m long, 0n16 m wide and 0n16 m deep with its roots extending the length of the tray. Across the width of each tray, a 15 mm thick slab of grafting wax (Walter E. Clark & Son, Orange, Connecticut, USA) separated the tray into two water-tight compartments of various areas and fractions of the root system. A single port per compartment was fitted with a tube 0n30 m in length to connect to a reservoir containing water at a height approx. 2 cm above the bottom of the tray. Soil from Agave Hill was added to a depth of 0n14 m in each compartment. Plants were then transferred to Conviron E15 environmental chambers (Controlled Environments, Pembina, North Dakota, USA) with a 12 h photoperiod at a PPF of 18 mol m−# d−" on a horizontal plane at the top of the canopy and day\night air temperatures of 22\16 mC. Plants were watered weekly and were acclimated to the chambers for 4 weeks before the start of an experiment. Instantaneous net CO uptake and transpiration were # measured over 24 h periods for up to four plants simultaneously with a Li-Cor LI-6262 infrared gas analyser in the differential mode. One mid-canopy leaf of each A. deserti plant was inserted into a cylindrical transparent acrylic cuvette 50 mm in diameter whose open end was sealed to the leaf with plastic putty. For E. farinosa, approx. eight terminal leaves on a branch were inserted into a
cuvette, which was likewise sealed with plastic putty. Air flow into each cuvette was constant over 24 h, and solenoid valves periodically switched the excurrent flow of air into the gas analyser. Flow rates were measured using LFC-3 mass-flow meters (Technology Incorporated, Dayton, Ohio, USA) and adjusted so that the decrease in the mole fraction of CO in the cuvettes never exceeded 50 µmol mol−". A # Polycorder 516-24 datalogger (Omnidata International, Logan, Utah, USA) collected data every 10 min from the gas analyser, a Hygro-M1 optical dewpoint hygrometer (General Eastern Instruments, Watertown, Massachusetts, USA), and the mass flow meters. Data are expressed on a total leaf surface area basis (abaxial plus adaxial areas), which averaged 0n11 m# for the A. deserti plants used (its leaves are thick and crescent-shaped in cross-section). Leaf thickness was measured daily using dial calipers accurate to 0n05 mm or with a calibrated wire 0n2 mm in diameter inserted mid-leaf in four leaves from each plant. Instantaneous leaf thickness was measured for a leaf in the middle of the plant using an electromechanical transducer (E-Line LVDT ; Shaevitz Engineering, Pennsauken, New Jersey, USA) clamped at midleaf. Output from the transducer at a resolution of 3 µm was collected very 5 min by a microcomputer and is expressed in micrometers based on an arbitrary initial value. Water uptake by the root system during 24 h periods was determined from the change in mass of water in a reservoir connected to the port in each tray. To prevent evaporation, the soil surface was covered with plastic sheeting and aluminium foil and measurements were taken after the soil water had come into equilibrium with the water in the reservoir. Trays were placed partially submerged in a tank of water to help stabilize soil temperature and were insulated with aluminum foil. Soil temperature was measured with copper-constantan thermocouples placed in the centre of each compartment. Water uptake into a compartment was measured with the reservoir on an AE100 balance (MettlerToledo, Heightstown, New Jersey, USA) ; the balance and thermocouples were connected to a microcomputer that collected values every 5 min. For consistency with the gas exchange data, root water uptake is expressed on a total leaf area basis (abaxial plus adaxial surfaces). Soil water potential was measured using PCT-55 thermocouple soil psychrometers (Wescor, Logan, Utah, USA) placed in the centre of each compartment. Psychrometers equilibrated for 24 h in the soil before they were removed and immediately dipped into molten paraffin ; the sealed psychrometers were then placed inside an insulated chamber for 3 h to eliminate internal thermal gradients, after which a Wescor HR-33T microvoltmeter in the dewpoint mode was used to determine the water potential. Leaf water potential was measured using a TruPsi microvoltmeter (Decagon Devices, Pullman, Washington, USA) using samples taken from the middle of three leaves per plant with a cork borer 13 mm in diameter. Samples were stripped of their cuticle, immediately placed into a Decagon SC-10X thermocouple psychrometer chamber and allowed to equilibrate for approx. 3 h before measuring. Statistical significance was determined using Student’s ttest.
Graham and Nobel—Water Relations vs. Root System Properties
215
Net CO2 uptake (µmol m–2 s–1)
10 A 8 6 4 2 0
Transpiration (mmol m–2 s–1)
B 3
2
1
Root water uptake (mmol m–2 s–1)
Relative leaf thickness (µm)
0 C 50
0
–50
D 3
2
1
0 12
18
0 Time of day (h)
6
12
F. 1. Instantaneous net CO uptake rates (A), transpiration rates (B), leaf thicknesses relative to an arbitrary initial value (C) and root water # uptake rates (D) for the succulent CAM species Agae deserti (—#—) and for the non-succulent C species Encelia farinosa (::=::) under $ well-watered conditions. Gas exchange and root water uptake data are based on total abaxial plus adaxial leaf surface area. Stippled bar indicates nighttime.
216
Graham and Nobel—Water Relations vs. Root System Properties Net CO2 uptake (mmol m–2 d–1)
200 A 150
100
50
0 Transpiration (mol m–2 d–1)
B 9
6
3
0 Leaf water storage (mol m–2)
C 150
100
50
Root water uptake (mol m–2 d–1)
0 D
20
10
10
0
20
30
Time (d) F. 2. Time course for daily net CO uptake rate (A), transpiration rate (B), leaf water storage (C) and root water uptake rate (D) for A. deserti # during 28 d of soil drying. Data (expressed on a total leaf area basis for both gas exchange and root water uptake) are meansps.e. (n l 4 plants).
RESULTS Agae deserti exhibited typical CAM patterns of gas exchange, with CO uptake and transpiration occurring # primarily at night, while Encelia farinosa followed typical C $
patterns of gas exchange, where CO uptake occurs together # with photosynthesis (Fig. 1 A and B). Specifically, 64 % of the daily net CO uptake and 43 % of the transpiration # for the well-watered A. deserti occurred at night, while essentially 100 % of the CO uptake and transpiration for #
Graham and Nobel—Water Relations vs. Root System Properties
217
Net CO2 uptake (mmol m–2 d–1)
200 A 150
100
50
0 Transpiration (mol m–2 d–1)
B 9
6
3
0 Leaf water storage (mol m–2)
C 150
100
50
Root water uptake (mol m–2 d–1)
0 D
20
10
0
10
20
30
Time (d) F. 3. Time course for daily net CO uptake rate (A), transpiration rate (B), leaf water storage (C) and root water uptake rate (D) for A. deserti # when the entire root system was rewetted after 90 d of drought. Data are meansps.e. (n l 4 plants).
E. farinosa occurred during the day. The daily water-use efficiency was 0n017 mol CO mol−" H O for A. deserti and # # 6n2-times lower for E. farinosa. Leaf thickness decreased for A. deserti in the early afternoon, reached a minimum at the
beginning of the night, remained relatively constant until the last third of the night and then increased throughout the morning (Fig. 1 C). Leaf thickness for E. farinosa increased in the late afternoon and through the night and then
218
Graham and Nobel—Water Relations vs. Root System Properties 3 A
2
1
0
B
2
Net CO2 uptake (µmol m–2 s–1)
1
0
C
2
1
0
D
2
1
0
0
2
4
6
8 Time (d)
10
12
14
30
F. 4. Instantaneous net CO uptake rates for A. deserti when 100 % (A) and the distal 50 % (B), 20 % (C) and 10 % (D) of the root system was # rewetted after 90 d of drought.
Graham and Nobel—Water Relations vs. Root System Properties
219
0·3 A
0·2
0·1
0 B
Transpiration (mmol m–2 s–1)
0·2
0·1
0 C
0·2
0·1
0 D
0·2
0·1
0
2
4
6
8 Time (d)
10
12
14
30
F. 5. Instantaneous transpiration rates for A. deserti when 100 % (A) and the distal 50 % (B), 20 % (C) and 10 % (D) of the root system was rewetted after 90 d of drought.
decreased in the morning. The amplitude of leaf thickness variation was 2n1-times greater for A. deserti than for E. farinosa (Fig. 1 C) but constituted only about 0n10 % of the total thickness for the leaf-succulent A. deserti compared to 8n0 % for E. farinosa. The root water uptake rate for A.
deserti was relatively constant over 24 h (Fig. 1 D) and hence did not follow the pattern of transpiration. In contrast, the pattern of root water uptake for E. farinosa was similar to that of transpiration (Fig. 1 B and D). When the soil was allowed to dry for 28 d, the soil water
220
Graham and Nobel—Water Relations vs. Root System Properties 4
100%
Increase in leaf thickness (mm)
3
50% 2
20%
1
0
10%
0
3
6
9
12
30
Time (d) F. 6. Increases in leaf thickness for A. deserti when 100 % (#) and the distal 50 % (=), 20 % ( ) and 10 % (W) of the root system was rewetted after 90 d of drought. Data are meansps.e. (n l 4 leaves on an individual plant).
potential decreased from k0n04p0n02 MPa (meanps.e.) at 0 d, to k1n1p0n4 MPa at 10 d, k5n8p0n9 MPa at 20 d, and to less than k10 MPa at 28 d. For A. deserti shoot water potential was k0n5p0n1 MPa at 0 d and k1n2p0n1 MPa at 28 d. Net CO uptake remained close to its initial value # throughout 14 d of drought (Fig. 2 A). At 28 d, net CO # uptake averaged 35p8 % of the maximum. Transpiration also decreased with soil drying, especially during the day, becoming significantly decreased at 14 d (P l 0n002) and averaging 22p9 % of the maximum at 28 d (Fig. 2 B). Instantaneous water-use efficiency at 0 d of drought was 0n011p0n002 mol CO mol−" H O at mid-afternoon and # # 0n026p0n009 at midnight. Daily water-use efficiency increased during soil drying, averaging 0n016p0n002 mol CO # mol−" H O at 0 d of drought and 0n025p0n003 at 28 d (P l # 0n005). Leaf water storage decreased about 24 % after 28 d soil drying (Fig. 2 C ; P l 0n001). Root water uptake decreased to zero by 7 d of soil drying (Fig. 2 D) when the soil water potential was less than the shoot water potential of k0n5p0n1 MPa. At 90 d of drought, a small daily net loss of CO # (1n4 mmol m−# d−") occurred due to respiration (Fig. 3 A). Seven days after the entire root system was rewetted (which raised the soil water potential to k0n03p0n01 MPa) average net CO uptake was 56 % of its pre-drought value, and at # 14 d of rewetting net CO uptake was not significantly # different from that before the drought (Figs 2 A and 3 A ; P l 0n4). Transpiration at 7 d of rewetting averaged 49 % of its pre-drought value and at 14 d was not significantly different from that before drought (Figs 2 B and 3 B ; P l 0n5). At 90 d of drought, leaf water content had decreased to
62 % of that before drought (Figs 2 C and 3 C ; P l 0n03). Leaf water storage increased to its pre-drought value after 7 d of rewetting, remaining constant thereafter (Figs 2 C and 3 C). Root water uptake was highest the first day after rewetting, decreasing 56 % in 14 d (Fig. 3 D) when it was not significantly different from its pre-drought value (P l 0n7). The time required for net CO uptake recovery and the # instantaneous net CO uptake patterns during recovery # depended on the fraction of the root system that was rewetted after 90 d of drought (Fig. 4). The time at which total daily net CO uptake was half of the maximum was # 8n5 d for rewetting 100 % of the root system and 11n8 d when 50 % of the root system was rewetted. At 30 d, the 20 % rewetting led to a daily net CO uptake only 9 % of # that of the 100 % and 50 % rewettings (Fig. 4). No net CO # uptake was observed at 30 d when only the distal 10 % of the root system was rewetted. Transpiration also increased when A. deserti was rewetted after 90 d of drought for all treatments except the 10 % rewetting (Fig. 5). The time at which the transpiration rate was half of the maximum was 7n0 d for the 100 % rewetting and 12n5 d for the 50 % rewetting. Total daily transpiration at 30 d was 8n2 mol m−# d−" for both the 100 and 50 % rewettings and 1n7 mol m−# d−" for the 20 % rewetting (Fig. 5). The water-use efficiency at 30 d of rewetting was 0n016 mol CO mol−" H O for the # # 100 % rewetting, 0n014 for the 50 % rewetting, and 0n009 for the 20 % rewetting. Leaf thickness increased rapidly when the entire root system was rewetted after 90 d of drought, reaching 90 % of the total increase in leaf thickness in 7n1 d (Fig. 6). The increase in leaf thickness was more gradual for the 50 %
Graham and Nobel—Water Relations vs. Root System Properties
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1·0
Root water uptake (mmol m–2 s–1)
0·8
0·6
0·4
0·2
0
1
2 Time (d)
3 29
30
F. 7. Instantaneous root water uptake rates for A. deserti when 100 % (#) and the distal 50 % (=), 20 % ( ) and 10 % (W) of the root system was rewetted after 90 d of drought. Data are on a whole plant basis and are expressed per total leaf surface area.
rewetting, 90 % recovery occurring in 27n0 d. The 20 and 10 % rewettings had much smaller total increases in leaf water storage at 30 d equalling about 30 and 10 %, respectively, that of the 100 % rewetting (Fig. 6). When the entire root system of A. deserti was rewetted after 90 d of drought, the root water uptake rate was initially high, decreasing by 71 % in 3 d (Fig. 7 ; 100 % rewetting). Similar patterns were observed when the distal 50, 20 or 10 % of the root system was rewetted. The root water uptake rate at 30 d of rewetting was similar for the 100 and 50 % rewettings, 60 % lower for the 20 % rewetting and 69 % lower for the 10 % rewetting (Fig. 7). Total water uptake over the 30 d period was 440 mol m−# for the 100 % rewetting, 404 mol m−# for the 50 % rewetting, 173 mol m−# for the 20 % rewetting and 117 mol m−# for the 10 % rewetting. DISCUSSION A technique for measuring water uptake by roots in hydroponics (Van Ieperen and Madery, 1994) was adapted so that virtually instantaneous measurements of water uptake could be made for roots in wet soil. Measurements were extremely sensitive to soil temperature, due in part to volume changes of air trapped in the soil under water saturated conditions (White and Mastalerz, 1996 ; Hershey, 1990). Nevertheless, measured root water uptake integrated over 24 h was within 1 % of total transpiration for the C $ species Encelia farinosa, and its pattern for root water uptake closely paralleled its pattern for transpiration, which occurred primarily during the daytime. The time lag (approx. 30 min) between transpiration measured by an infrared gas analyser and root water uptake measured by the adapted technique is consistent with the relatively small capacitance
for this non-succulent plant (Nobel and Jordan, 1983). This technique allows the direct measurement of root water uptake from the soil which, together with measurements of leaf water storage and transpiration, can be used to quantify previously modelled water flow in Agae deserti. The conclusion is reached that under wet soil conditions the root system of this desert succulent shows redundancy. Under well-watered conditions, A. deserti showed gas exchange patterns typical of a CAM species, exhibiting net CO uptake and transpiration primarily at night. Leaf # thickness decreased during the afternoon, as transpiration commenced ; leaf thickness increased during the last third of the night and continued to increase in the early morning after transpiration had essentially ceased. The change in leaf thickness averaged over the projected leaf surface area of a plant represented a total water flux equivalent to 41 % of total daily transpiration. Root water uptake was relatively constant over a 24 h period for A. deserti, contrary to E. farinosa but consistent with predicted rates for a hydrated plant in wet soil (Schulte and Nobel, 1989). Integrated over 24 h, root water uptake for A. deserti exceeded total daily transpiration by 8 %, indicating some water movement into storage and growth. When stomatal opening commences in the late afternoon, modelled water loss for A. deserti first occurs from the outer chlorenchyma ; lost water is replaced by that in the water storage parenchyma (Goldstein, Andrade and Nobel, 1991 ; Tissue, Yakir and Nobel, 1991). The leaf thickness of A. deserti decreased with the beginning of transpiration, indicating that water was indeed initially lost from leaf water storage. By early nighttime, leaf thickness had stopped decreasing, indicating water movement into leaf water storage was equalled by that lost through transpiration.
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Graham and Nobel—Water Relations vs. Root System Properties
Recharge of leaf water storage then began during the last third of the night, when transpiration exceeded root water uptake, and continued after transpiration had ceased in the morning. Because of the relatively constant root water uptake, this recharge must represent water moving into leaf water storage from the stem, older leaves, and young folded leaves in the centre of the rosette through a lowerresistance pathway than from the soil. Transpiration is the main driving force for water movement for A. deserti under well-watered conditions (Smith et al., 1987), yet because of leaf water storage, daily changes in its leaf water potential due to transpiration are slight ; the observed 0n10 % change in leaf thickness due to water loss from transpiration is equivalent to a decrease in water potential of only 0n024 MPa (Calkin and Nobel, 1986). The water potential difference between the shoot and the soil was about k0n5 MPa, sufficient to mask such small leaf water potential changes. Daily changes in the leaf water potential due to CAM are also small, approx. 0n03 MPa under the present conditions (Smith et al., 1987). Net CO uptake and transpiration for A. deserti continued # relatively unchanged for the first week of soil drying. As drought progressed, net CO uptake decreased more slowly # than did transpiration, reflecting a substantial decrease in daytime transpiration as the instantaneous water-use efficiency was 2n4-times lower during daytime stomatal opening than at midnight. The rate of water loss from storage in the leaves also decreased with drought length, mainly as a result of the decreasing daily duration of stomatal opening. During the first 28 d of drought, plants lost an average of 33 mol m−# water from storage ; during the last 60 d of drought, plants lost only an additional 20 mol m−#. By 7 d after rewetting following 90 d of drought, leaf water storage had increased to the pre-drought value, but the daily net CO uptake had not, probably due to the # effects of prolonged drought on photosynthetic processes (Raveh, Gersani and Nobel, 1995 ; Herppich and Peckmann, 1997). Daytime transpiration comprising one-third of total daily transpiration occurred at 7 d after rewetting, consistent with a high leaf water storage and indicative of plants kept under well-watered conditions (Nobel, 1988). Water uptake by the root system was initially rapid upon rewetting, reflecting the lower water potential of the shoot (about k1n2 MPa) and suggesting a highly conductive root system. Even after the soil water potential is below k10 MPa for many weeks, some roots close to the succulent shoot of A. deserti can have a high water status and maintain a relatively high hydraulic conductivity (North and Nobel, 1998). The hydraulic conductivity of a droughted root system also increases rapidly as the cortex is rehydrated and embolism is reduced, often within a few days after rewetting (North and Nobel, 1991). When the distal 10 % of the root system was rewetted after 90 d of drought, no net CO uptake occurred even # after 30 d of rewetting. When the distal 20 % of the root system was rewetted, net CO uptake began at 13 d and # total daily net CO uptake at 30 d was only 16 % of that # when the entire root system was rewetted. When the distal 50 % of the root system was rewetted, recovery of half-
maximal daily net CO uptake was slowed by 40 % compared # with wetting the entire root system. After an extended drought, water uptake by a distal portion of the root system is not sufficient for a rapid recovery of gas exchange and a positive carbon gain to occur. In this regard, a single rainfall event of 10 mm in the Sonoran Desert can raise the water potential of the soil at the mean rooting depth of A. deserti to k0n5 MPa, allowing water uptake by the root system (Nobel, 1988). However, 10 d later the soil water potential can decrease below that of the shoot (Young and Nobel, 1986). At 10 d after rewetting, the 10 and 20 % rewettings still had a daily net loss of carbon and the 50 % rewetting had a carbon gain equal to only one-quarter of that of the 100 % rewetting. The conclusion is thus reached that under field conditions the entire root system of A. deserti must be rewetted for maximal carbon gain to occur rapidly, indicating very little hydraulic redundancy in the root system with regard to gas exchange recovery. After a short delay, recovery of leaf water storage for the 100 % rewetting of A. deserti followed an exponential rise to a maximum within 10 d. The delay may be associated with a reduction of embolism (North and Nobel, 1991), a rehydration of the root cortical tissue (Nobel and Huang, 1992 ; North and Nobel, 1995), and stem rehydration. Rewetting the distal 50 % of the root system led to a more gradual increase in leaf thickness ; leaf thickness was 51 % of that of the 100 % rewetting at 10 d. Even after 30 d of rewetting, the recovery of leaf water storage for the 20 and 10 % rewettings was only 29 and 9 %, respectively, of the 100 % rewetting. Consistent with this result, transpiration had not recovered for the 20 and 10 % rewettings after 30 d of rewetting. Therefore, a small fraction of the root system is not sufficient for recharging leaf water storage and recovery of stomatal opening even after a month of wet soil conditions. Water uptake by the root system of A. deserti decreased within 24 h for all rewetting treatments, most likely the effect of rehydrating root cortical tissue and refilling of root capacitance. Indeed, simple exponential curves used to model capacitance (Nobel, 1991) closely described the decrease in water uptake rate for the 50, 20 and 10 % rewettings, and the mass of water initially taken up was approx. 40 % of the original fresh mass of the portion of roots rewetted. [A satisfactory (r# 0n95) simple exponential curve could not be fitted to the 100 % rewetting root water uptake data, probably because substantial root, stem and leaf capacitance refilling were taking place simultaneously.] At 30 d of rewetting, the root water uptake rate was similar for the 50 and 100 % rewettings, indicating that half of the root system may be adequate to supply all the water necessary under well-watered conditions. Indeed, split-root and partial root system wetting experiments indicate that the root systems in other species are also in excess of that required for daily transpiration needs (Tan and Buttery, 1982 ; Gallardo, Turner and Ludwig, 1994 ; Green and Clothier, 1995). The rate of water uptake for the 20 and 10 % rewettings remained low through 30 d of rewetting, possibly due to residual embolism in the proximal portions of the root system of A. deserti (North and Nobel, 1991). Even though the root : shoot dry mass ratio of 0n11 for A.
Graham and Nobel—Water Relations vs. Root System Properties deserti (Nobel, 1988) is low relative to other plants, the root system is extensive enough to allow for a rapid return to predrought leaf water status and gas exchange after rewetting. After 3 months of drought, rewetting the entire root system led to complete recovery of leaf water storage within 7 d. Rewetting only the distal half of the root system delayed water uptake and gas exchange recovery, and rewetting the distal 20 and 10 % of the root system resulted in significantly less water uptake and almost no recovery of net CO uptake # within 30 d. For a rapid recovery from drought, wetting the distal region of the root system of A. deserti is insufficient. After leaf rehydration and gas exchange recovery, water uptake by the distal half of the root system was equal to that by the entire root system and supported maximal net CO # uptake, suggesting a redundancy in the root system under well-watered conditions. In a sporadic rainfall environment such as the Sonoran Desert, however, A. deserti must rely on its entire root system for rapid recovery from drought. A C K N O W L E D G E M E N TS Financial support from the National Science Foundation grant IBN-94-19844 is gratefully acknowledged. LITERATURE CITED Calkin HW, Nobel PS. 1986. Nonsteady-state analysis of water flow and capacitance for Agae deserti. Canadian Journal of Botany 64 : 2556–2560. Cannon WA. 1911. The root habits of desert plants. Washington, DC : Carnegie Institute of Washington. Davies WJ, Zhang J. 1991. Root signals and the regulation of growth and development of plants in drying soil. Annual Reiew of Plant Physiology and Plant Molecular Biology 42 : 55–76. Gallardo M, Turner NC, Ludwig C. 1994. Water relations, gas exchange and abscisic acid content of Lupinus cosentinii leaves in response to drying different proportions of the root system. Journal of Experimental Botany 45 : 909–918. Goldstein G, Andrade JL, Nobel PS. 1991. Differences in water relations parameters for the chlorenchyma and parenchyma of Opuntia ficus-indica under wet versus dry conditions. Australian Journal of Plant Physiology 18 : 95–107. Green SR, Clothier BE. 1995. Root water uptake by kiwifruit vines following partial wetting of the root zone. Plant and Soil 173 : 317–328. Herppich WB, Peckmann K. 1997. Responses of gas exchange, photosynthesis, nocturnal acid accumulation and water relations of Aptenia cordifolia to short-term drought and rewatering. Journal of Plant Physiology 150 : 467–474. Hershey DR. 1990. Container-soil physics and plant growth. Bioscience 40 : 685–686. Hunt ER Jr, Zakir NJD, Nobel PS. 1987. Water costs and water revenues for established and rain-induced roots of Agae deserti. Functional Ecology 1 : 125–129. Jury WA, Bellantuoni B. 1976. Heat and water movement under surface rocks in a field soil : II Moisture effects. Soil Science Society of America Journal 40 : 509–513.
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Nobel PS. 1988. Enironmental biology of agaes and cacti. New York : Cambridge University Press. Nobel PS. 1991. Physicochemical and enironmental plant physiology. San Diego : Academic Press. Nobel PS. 1997. Root distribution and seasonal production in the northwestern Sonoran Desert for a C subshrub, a C bunchgrass, $ % and a CAM leaf succulent. American Journal of Botany 84 : 949–955. Nobel PS, Cui M. 1992. Hydraulic conductances of the soil, the rootsoil air gap, and the root : changes for desert succulents in drying soil. Journal of Experimental Botany 43 : 319–326. Nobel PS, Huang B. 1992. Hydraulic and structural changes for lateral roots of desert succulents in response to soil drying and rewetting. International Journal of Plant Science 153 : S163–S170. Nobel PS, Jordan PW. 1983. Transpiration stream of desert species : resistances and capacitances for a C , a C , and a CAM plant. $ % Journal of Experimental Botany 34 : 1379–1391. Nobel PS, Alm DM, Cavellier J. 1992. Growth respiration, maintenance respiration and structural-carbon costs for roots of three desert succulents. Functional Ecology 6 : 79–85. Nobel PS, Loik ME, Meyer RW. 1991. Microhabitat and diel tissue acidity changes for two sympatric cactus species differing in growth habit. Journal of Ecology 79 : 167–182. Nobel PS, Miller PM, Graham EA. 1992. Influence of rocks on soil temperature, soil water potential, and rooting patterns for desert succulents. Oecologia 92 : 90–96. North GB, Nobel PS. 1991. Changes in hydraulic conductivity and anatomy caused by drying and rewetting roots of Agae deserti (Agavaceae). American Journal of Botany 78 : 906–915. North GB, Nobel PS. 1995. Hydraulic conductivity of concentric root tissues of Agae deserti Engelm. under wet and drying conditions. New Phytologist 130 : 47–57. North GB, Nobel PS. 1998. Water uptake and structural plasticity along roots of a desert succulent during prolonged drought. Plant, Cell and Enironment 21 : 705–713. Passioura JB. 1988. Water transport in and to roots. Annual Reiew of Plant Physiology and Plant Molecular Biology 39 : 245–265. Raveh E, Gersani M, Nobel PS. 1995. CO uptake and fluorescence # responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO concentrations. Physiologia Plantarum # 93 : 505–511. Schulte PJ, Nobel PS. 1989. Responses of a CAM plant to drought and rainfall : Capacitance and osmotic pressure influences on water movement. Journal of Experimental Botany 40 : 61–70. Smith JAC, Schulte PJ, Nobel PS. 1987. Water flow and water storage in Agae deserti : osmotic implications of Crassulacean acid metabolism. Plant, Cell and Enironment 10 : 639–648. Tan CS, Buttery BR. 1982. The effects of soil moisture stress to various fractions of the root system on transpiration, photosynthesis, and internal water relations of peach seedlings. Journal of the American Society of Horticultural Science 107 : 845–849. Tissue DT, Yakir D, Nobel PS. 1991. Diel water movement between parenchyma and chlorenchyma of two desert CAM plants under dry and wet conditions. Plant, Cell and Enironment 14 : 407–413. Van Ieperen W, Madery H. 1994. A new method to measure plant water uptake and transpiration simultaneously. Journal of Experimental Botany 45 : 51–60. White JW, Mastalerz JW. 1966. Soil moisture as related to ‘ container capacity ’. Journal for the American Society for Horticultural Science 89 : 758–765. Young DR, Nobel PS. 1986. Predictions of soil-water potentials in the north-western Sonoran Desert. Journal of Ecology 74 : 143–154.