Variation in hydraulic architecture and gas-exchange in two desert sub-shrubs, Hymenoclea salsola (T. & G.) and Ambrosia dumosa (Payne)

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1 Oecologia (2000) 125:1 10 Springer-Verlag 2000 Jonathan P. Comstock Variation in hydraulic architecture and gas-exchange in two desert sub-shrubs, Hymenoclea salsola (T. & G.) and Ambrosia dumosa (Payne) Received: 13 December 1999 / Accepted: 31 March 2000 Abstract Adjustment of hydraulic architecture in response to environmental conditions was studied in two warm-desert sub-shrubs, Hymenoclea salsola and Ambrosia dumosa, both at the level of genetic adaptation along a climatic gradient and plastic response to immediate growth conditions. Individuals of both species originating from southern populations developed higher leafspecific hydraulic conductance in the common greenhouse than individuals from northern populations. Hydraulic conductance was higher in plants grown at high temperature, but did not vary as a function of growth relative humidity. Hydraulic conductance was not correlated within species with individual variation in vessel diameter, cavitation vulnerability, or root:shoot ratio, but was strongly, negatively correlated with the fraction of total plant biomass allocated to leaves. For both species, stomatal conductance (g s ) at high leaf-to-air vapor pressure difference (ν) was tightly correlated with variability in hydraulic conductance, as was the sensitivity of stomatal closure to increasing ν. Experimentally increasing shoot water potential by soil pressurization, under conditions where high ν had already caused stomatal closure, led to substantial stomatal reopening in both species, but recovery was significantly higher in H. salsola. Hydraulic conductance was higher in H. salsola than A. dumosa. H.salsola also differed from A. dumosa by being a representative of a highly specialised group of desert shrubs which use the twigs as a major photosynthetic organ. The southern population of H. salsola produced far fewer leaves and relied much more heavily on twig photosynthesis than the northern population. At the wholeplant level, increased reliance on twig photosynthesis was associated with higher leaf-specific hydraulic conductance, but equivalent whole-plant photosynthesis on either a dry weight (µmol CO 2 g 1 ) or nitrogen basis (µmol CO 2 g 1 )). This suggests that twig photosynthesis might be one way of increasing hydraulic conductance J. Comstock ( ) Boyce Thompson Institute, Tower Road, Ithaca, NY 14853, USA jpc8@cornell.edu Tel.: , per unit photosynthetic canopy by increasing allocation to an organ which simultaneously performs photosynthetic, support, and transport functions. Key words Climatic ecotypes Hydraulic limitation Hydraulic signaling Plant morphology Allocation Introduction Relationships between leaf-specific hydraulic conductance (LSC) and transpiration (E) Hydraulic architecture is increasingly studied with respect to the limitations placed by within-plant water transport on plant productivity. Stomata play a wellunderstood key role as the control point for integrating the often conflicting needs to capture CO 2 from the atmosphere for photosynthesis and growth, and to limit water loss from plant tissues to avoid dehydration (Cowan 1977). A substantial body of empirical data and theory has been produced over the past few decades on the degree of limitation actually imposed on photosynthesis and growth by stomatal closure (Farquhar and Sharkey 1982), and how stomata should behave in response to environmental variability to maximize carbon gain over a range of different time-scales (Aphalo and Jarvis 1993; Ball et al. 1987; Cowan and Farquhar 1977; Leuning 1995; Monteith 1995). In recent years, this area has seen considerable development in two related areas: (1) the potential role of hormones, especially those borne in the transpiration stream itself, to make stomata responsive to root:shoot communications (Dodd et al. 1996; Schurr and Schulze 1996; Tardieu 1996; Whitehead 1998), and (2) the role of hydraulic signals which could be transduced either in the root as a function of root and soil water status, or at the photosynthetic organ where, at the end of the transpiration stream, plant water potentials will be lowest (Comstock and Mencuccini 1998; Fuchs and Livingston 1996; Saliendra et al. 1995).

2 2 Transduction of a water-status signal at the photosynthetic organ could be especially important if the hydraulic conductance of the plant, and the consequent water potential gradients between root and shoot, are variable and potentially limiting to plant function. Considerable evidence exists that hydraulic conductance is indeed partially limiting. Strong correlations have been observed between stomatal and hydraulic conductances in both crop (Meinzer et al. 1990; Mencuccini and Comstock 1999; Sober 1997; Sohan et al. 1999; Sperry and Pockman 1993) and wild plant species (Bond and Kavanagh 1999; Irvine et al. 1998; Meinzer et al. 1999; Ryan and Yoder 1997). Several studies have also reported that short-term manipulations of shoot water status can be directly linked to changes in stomatal aperture (see previous paragraph). Relationship to climate The expected relationship between plant hydraulic conductance and productivity should be very sensitive to climatic factors that affect photosynthetic water-use efficiency. In this context, the intermountain west of North America presents some excellent experimental gradients. Due, in part, to the presence of north-south-running mountain chains bounding it on both the east and west, a continuous belt of aridland ecosystems runs from northern Mexico to southern Canada. Although mostly classified as arid or semi-arid, these ecosystems vary greatly in mean annual temperature and the seasonality of precipitation, both of which strongly affect the leaf-to-air vapor pressure difference (ν) driving water-loss during the growing season. Comstock and Ehleringer (1992) reported that some warm-desert species occupying the southern half of this gradient experience a more than 2-fold difference in growing-season ν among different populations, and, for Hymenoclea salsola, showed a correlation between an index of growing-season ν and interpopulation variation in both tissue-level intrinsic water use efficiency (i.e., intracellular [CO 2 ] as indicated by carbon isotope discrimination), and also canopy architecture (i.e., relative contribution of twigs to the photosynthetic canopy). The geographic variation in these traits suggests that population-level adaptation has occurred in H. salsola to adjust intrinsic plant factors in a manner that may compensate for climatic factors influencing plant water use and status. Direct measurements of hydraulic conductance, however, were not previously made on plants from this gradient. New studies were therefore undertaken to test (1) the hypothesis that plants operating under much higher ν would develop greater hydraulic conductance per unit of photosynthetic area supported, (2) that this variation is important in determining stomatal conductance (g s ), (3) whether both genetic adaptation and plastic response to growth environment were important, and (4) what aspects of plant anatomy and/or allocation among organs would be most important in determining variation in hydraulic conductance. Methods Plant material Seed of H. salsola (T. & G.) and Ambrosia dumosa (Payne), both subshrubs of the Mojave and Sonoran deserts of western North America, was collected from natural populations and grown in the greenhouse at the Boyce Thompson Institute for Plant Research in Ithaca, NY (300 m). In the wild, H. salsola generally occurs in deep sandy or gravelly soils, especially intermittent stream beds (desert washes), while A. dumosa is broadly distributed on slopes and flats, often with thin soil and minimal water-holding capacity. For both species, seed collection sites were chosen from both the southern and northern extremes of the natural range. The northern collection site was dominated by Larrea scrub and Joshua tree woodland just south of the Beaverdam mountains at 945 m elevation and N latitude. The southern seed-collection site was located in similar topography in the Organ Pipe National Monument on the Arizona-Mexico border at 512 m elevation and N latitude. These sites had strongly contrasting conditions during the growing season, driven largely by the seasonality of precipitation (Mencuccini and Comstock 1997). The northern site had a strong unimodal precipitation pattern with maximums in the winter months, and most plant growth in the spring as temperatures warmed. The southern site had milder winters and a bimodal precipitation pattern. In the south, both spring and summer growing periods could regularly support activity by the study species, and the growing season was warmer. Cultural conditions The plants were grown in 30-dm 3 pots in a soil mix of 3:1:1 fritted clay (Turface):silica sand:pasteurized topsoil, and were watered daily with nutrient solution containing 55:18:55 ppm N:P:K from Peter s Excel. Photoperiod from combined artificial (an alternating bank of 1000-W high-pressure Na vapor, 1000-W Super Metal Halide, and 150-Watt incandescent floodlights) and natural lighting was 12 h with a total irradiance ( nm) of 44 mol m 2 day 1 in all treatments. All treatments were set up in well-ventilated greenhouses with internal fans for stirring foliage. CO 2 concentrations were monitored continuously by a single infra-red gas-analyser (IRGA) (Horiba, model PIR-2000, Irvine, Calif., USA) which cycled continuously between air sampling lines from each of the three greenhouses and an outside reference line. Mean CO 2 was 375/390 µmol mol 1 (day/night). Although variations in mean daily {CO 2 } of up to 15 µmol mol 1 were seen as a function of different weather patterns, all the greenhouse bays and outside air had the same daily mean values±1.0 µmol mol 1. Plants from both populations were grown under three contrasting conditions of temperature and humidity to test whether temperature itself or ν during growth had a greater effect on the plastic development of hydraulic conductance. These growth treatments included a hot environment (33/20 C day/night) at low humidity, 26% daytime relative humidity (RH), a similar hot environment at high humidity (67% RH), and a cool environment (23/20 C day/night) at low RH (37%), referred to as hot-dry (h), hot-humid (hh) and cool-dry (c), respectively. The hh and c had the same ν. The species had photosynthetic temperature optima very near 29 C (Comstock and Ehleringer 1988), and so photosynthetic capacities at 23 and 33 C were expected to be very similar. Cuvette measurements were spread out over several months and successive plantings where made to reduce variation in age at time of measurement. Seedling cohorts were started in the late summer through fall at monthly intervals. Seed was sown directly into the 30-l pots and there was no transplanting. Measurement occurred in late winter and spring from February till May. At the time of measurement, all plants were 5 6 months old, and the main stems had extensive secondary growth. The growth periods were timed to occur in winter, because summers in Ithaca were moderately humid, and the h and especially c environmental con-

3 3 ditions were attainable in the greenhouse only in the winter season when ambient humidity was low. Gas exchange Water flow rates through the plant, as well as stomatal responses to environmental influences and leaf water potential, were measured using steady-state gas-exchange techniques. Gas exchange was measured in a whole-plant cuvette system described in Comstock and Mencuccini (1998). Gas-exchange calculations were made following von Caemmerer and Farquhar (1981) and stomatal ratios treated as described in Comstock and Ehleringer (1993). Gas-exchange of all plants was measured under a single set of cuvette conditions regardless of growth temperature. Ambient CO 2 in the cuvette was 360±5 µmol mol 1, leaf temperature was 30±1 C, and irradiance was 1.8 mmol m 2 s 1 ( nm). Hydraulic conductance Leaf-specific hydraulic conductance (LSC, mmol m 2 s 1 MPa 1 ) was calculated as the slope of the relationship between shoot water potential and transpiration (E, mmol m 2 s 1 ) (Passioura 1988). It is important to note that the term leaf-specific is somewhat misleading for H. salsola, and is kept here only for consistency with other studies and to avoid coining new terminology. Not leaf area per se, but total photosynthetic surface area including both leaf and green (i.e., fully photosynthetic) twig area was used in this calculation. A. dumosa lacked photosynthetic twigs, and only leaf area was used to calculate LSC. There was also an apparent offset in pressure observed at zero transpiration which could not be fully explained by bulk soil water potential. This offset was seen to shift slightly on a diurnal basis, becoming more negative in the afternoon than the morning (Comstock and Mencuccini 1998). Data were corrected for intercept drift prior to calculation of LSC by regression. Pressurization of soil compartment Soil pressurization was used both to measure the total waterpotential difference through the plant for calculation of LSC, and to observe stomatal responses to changes in leaf water potential. The root system was enclosed in a pressure-chamber with a split lid allowing the intact stem to leave the pressure chamber and enter a shoot gas-exchange cuvette at ambient air pressure. This permitted experiments in which the shoot water potential was manipulated directly by pressurization of the soil compartment. The theory of how soil pressurization affects shoot water potential has been discussed in several previous papers (e.g., Comstock and Mencuccini 1998; Passioura 1980). The expected water potential of the foliage is: Ψleaf = Ψsoil E/ LSC + P (1) where E is transpiration (mmol m 2 s 1 ), and P is soil compartment pressure (MPa). Water potential gradients throughout the crown Total differences in water potential between the soil and shoot were measured primarily using a balance-point method. The soil compartment was pressurized as needed until a cut twig in the canopy just began to exude xylem sap. At this time, the pressure reading on the soil compartment was considered equal to the total difference needed to support the current transpiration rate. Balance points were measured on small twigs which had had their tips trimmed back a few cm and directed out through the cuvette wall via small ports drilled for this purpose. Sap exudation was determined visually with a hand lens. Balance points were measured at several canopy positions: (1) a twig from the plant caudex that was stripped of leaves and covered externally with grease to eliminate any water loss, which was taken to be a probe of the waterpotential relations at the root-shoot transition at soil level; (2) terminal leafy twigs in lower, middle and upper crown positions; and (3) the terminal twig of the original main leader of the plant. Immediately after the balance-point was determined for the last time (see measurement sequence below) the cuvette lid was opened and leaf samples taken for Scholander pressure chamber determination. The leaves were taken from twigs currently being held at 0 MPa xylem water potential by the balance technique, loosely wrapped during transfer in a damp paper towel, and sealed within 60 s into a Scholander chamber also lined with damp towels to minimize continued water loss during measurement. The Scholander readings therefore reflected water potential differences from the twig xylem to the associated transpiring leaf tissues. Measurement sequence All plants were subjected to a consistent set of cuvette measurements from which both LSC and the sensitivity of stomata to leaf water potential were determined. Plants were first allowed to equilibrate under high light and reach a maximum level of gas exchange with a low leaf-to-air humidity difference (ν) of 10 mmol mol 1. The soil compartment was first at normal ambient pressure. The humidity was reduced in several steps until ν reached 35 mmol mol 1. Sufficient time was given (about 30 min) for plants to reach new steady-state gas-exchange values at each humidity. Finally, three additional points were taken while pressurizing the soil compartment as needed until a cut twig in the canopy just began to exude xylem sap. This balancing pressure (P, MPa) was measured first at ν=35 mmol mol 1, but only on the greased twig from the caudex; ν was then lowered to 10 mmol mol 1 for measurement of all crown positions described above, and, finally, the caudex probe was measured again at 35 mmol mol 1. Natural abundance of stable isotopes The abundance ratio, R, of the stable isotopes of carbon, 13 C: 12 C, was measured as a long-term index of intracellular CO 2 concentrations (c i ) during growth. Leaf samples were initially dried and ground to 40mesh. The data were expressed relative to the PDB standard as: ( Rsample Rstandard)* 1000 δ sample =, R (2) standard The analyses were performed at the SIRFER facility at the University of Utah, and at CoBSIL, the Cornell and Boyce Thompson Stable Isotope Laboratory in Ithaca, New York. Reported isotope data are presented as discrimination (, ) between leaf and air carbon pools (Farquhar et al. 1989). is theoretically related to photosynthetic carbon uptake as: = a+ b a c i ( ) (3) ca where a and b are the isotopic discrimination constants associated with diffusion and net carboxylation, respectively, and c a and c i are the ambient and intracellular CO 2 concentrations, respectively. was calculated from isotopic data on air and plant tissues as (Farquhar et al. 1989): δair δplant = (4) δplant δ air was not directly measured, but was assumed to be equal to the global mean value of 8.3 when CO 2 was equal to 361 µmol mol 1. Variation in δ air for different plant cohorts was estimated using the measured greenhouse CO 2 concentration and assuming that isotopic ratio of the air conformed to a standard Keeling plot with intercept at 27. Ambient CO 2 in the greenhouse was very

4 4 constant throughout the winter, varying only 2 or 3 µmol mol 1 among months and less than 1 µmol mol 1 among the three wellventilated greenhouses. Although plants were 5 months old at measurement, due to geometric growth rates most tissue sampled for was produced during the last month and only that period was averaged for estimating δ air. This made the δ air correction vary less that 0.1 among treatments or greenhouses. Harvest data After the gas-exchange measurements, each plant was subjected to a total biomass harvest to permit an evaluation of how LSC and stomatal behavior were related to the distribution of standing biomass. Measurements included basal diameter of the main stem (mesuredwith callipers), total length of stems and green twigs, stem surface area, and mean diameter of different age classes measured as total projected area divided by length, dry weight broken down into categories of fine root, coarse root, tap root, main stem, other woody stems, green leaf-bearing twigs, and leaves. All tissues were dried under forced convection at 60 C until weight loss ceased. This was as little as 2 days for most leaf samples, but up to several days for stems larger than 1 cm in diameter. Projected area of leaves and stems was measured with a leaf area meter (model LI-3200, LI-COR Instruments, Lincoln, Neb., USA) calibrated with a paper comb (Comstock and Ehleringer 1990). Fine roots (less than 1 mm) were harvested by washing the soil in an elutriation chamber with water and air jets stirring the heavier material from below. Roots were captured on screens in the effluent. Vessel radius was measured on cross-sections of each main stem using a light microscope, drawing tube, and digital input board. Results Hydraulic architecture and variation in LSC Fig. 1 Illustration of balance-point data. The balance-point is defined as the gas-phase pressure needed in the root chamber to bring a wet meniscus to the surface of a cut twig-tip in the canopy. Each plant in the study was subjected to similar measurements, and the hydraulic conductance was calculated from the slopes of the indicated relationships LSC was calculated as the slope of a linear relationship between E and water potential difference (Comstock and Mencuccini 1998; Passioura 1988). A typical dataset used in this calculation is shown for one individual of A. dumosa in Fig. 1. The caudex probe allowed for a twopoint regression estimating both slope and intercept. A correction for intercept drift was calculated from a second high ν balance point (not shown). The non-zero intercept was attributed to below-ground portions of the pathway and taken to be constant for all crown positions (Comstock and Mencuccini 1998; Reiger and Litvin 1999; Stirzaker and Passioura 1996). Measurement of all twigs at high ν was not undertaken, both because of the time required to measure so many balance points, and also because this avoided extreme overpressurization of the lower canopy while reaching balance points for the upper canopy positions at high E. The measured slope varied substantially with canopy position, being greatest for the defoliated probe of the caudex water potential and least for the terminal leader. This decrease in LSC with plant height was as expected from simple pathlength considerations. Somewhat unexpected was the large difference between average upper crown twigs and the leader (Fig. 1). These positions had similar pathlength and differed only in age and the ontogenetic pattern of crown branching. The original leaders were the oldest tissues in the upper crown, and were no longer actively growing. For subsequent analyses relating LSC to other physiological and harvest data, LSC root refers to the slope of the caudex twig, and LSC crown refers to an average value of the three crown heights (dotted lines in Fig. 1) excluding the leader. LSC leader was excluded from LSC crown because (1) it was measured in only a limited number of cases, (2) it was not representative of any large fraction of the total crown, and (3) it was highly erratic, sometimes being similar to other upper canopy sites and sometimes far lower. Above-ground resistance from the caudex to the leader averaged 86% greater than an average upper canopy twig position in A. dumosa, and 153% greater in H. salsola. Leaf water potentials were measured on leaves, while the twigs were simultaneously being held at their balance points soil pressurization. Mean Ψ leaf under these conditions averaged 0.47 and 0.48 MPa for A. dumosa and H. salsola, respectively. Because leaf water potentials were only measured on about half of the plants, calculated LSC crown values compared with other parameters do not include this part of the pathway. Variation in gas-exchange and relationship to LSC Both desert species showed a strong stomatal response to decreasing ambient humidity, such that transpiration rates at higher ν (Comstock and Mencuccini 1998) were limited by stomatal closure (Figs. 2,3,4). For plants grown in the h environment, the two species showed very similar, linear relationships between g s and LSC, which included both northern and southern populations

5 5 Fig. 2 Relationship of stomatal conductance (g s ) to LSC crown when transpiration was low due to low leaf-to-air vapor pressure difference (ν). Although the overall regression was weakly significant at P<0.05, neither species nor geographic races were significantly different in mean g s. Symbol codes: plant species: A.d Ambrosia dumosa, H.s. Hymenoclea salsola; seed sources: N northern, S southern; growing conditions: h hot (33 C, 26% RH), hh hot humid (33 C, 67% RH), and c cool (23 C, 37% RH). All plants were measured at 30 C leaf temperature. Each point represents one plant of both species. When transpiration was low because of low ν, g s was high and only weakly related to LSC crown (Fig. 2). As ν increased, however, plants with high LSC crown proved less sensitive to high transpiration rates and maintained high g s, while plants with low LSC crown showed strong stomatal closure (Fig. 3). As a consequence, there was a very strong relationship between g s and LSC crown at high ν (Fig. 4). Although all populations appeared to follow the same fundamental relationship between g s and LSC, they were distinguished by different mean values of LSC crown (Figs. 2, 3, 4, Table 1). The slope species, A.dumosa, had lower LSC crown than the wash species, H. salsola. For both species, populations originating from the south had higher LSC crown than the populations originating from the north of the species range. The significant differences in LSC crown among species and populations were mirrored by differences in mean g s at high ν, but not at low ν; g s at low ν was not significantly different among species or geographic origins. During pressurization of the soil compartment for balance points, water potentials in the shoot were raised. In response to this manipulation, although ν remained at Fig. 3 Relationship between stomatal sensitivity to low humidity and LSC crown. Sensitivity is defined as the percent reduction in g s after humidity in the measurement cuvette had been dropped and ν increased from 10 to 35 mbar bar 1. Symbols as in Fig. 2. Each point represents one plant 35 mmol mol 1, g s showed substantial (though not complete) recovery towards its original high value at ν=10 mmol mol 1. This relative recovery was greater in H. salsola than in A.dumosa, but did not differ with geographic origin (Table 1). A strong correlation was found between measured on bulk leaf tissue and the c i value measured at ν of 35 mmol mol 1 (Fig. 5). However, only for A. dumosa was significantly correlated with LSC. In H. salsola, although plants were healthy and growing vigorously, net photosynthetic rates (A) were somewhat lower than expected, and more variable. This variability in A apparently decoupled the expected link between LSC, gs, and in H. salsola. Differences in between northern and southern populations, seen in previous common garden studies (Comstock and Ehleringer 1992), were not seen in this dataset. Plastic response to growth environment When the northern population of H. salsola was grown at contrasting humidities but the same high temperature, there was no change in LSC, or g s measured under cuvette conditions (cuvette measurement involved a g s vs. ν response at the same, intermediate temperature for all growth treatments) (Figs. 2,3,4, Table 2). Humid grown

6 6 Fig. 4 Strong dependency of g s on leaf-specific hydraulic conductance (LSC crown ) when stomatal opening was associated with high transpiration rates due to high ν. Plants were all light-saturated and near the leaf temperature optimum of 30 C. Symbols as in Fig. 2. Each point represents one plant Fig. 5 Correlation of carbon isotope discrimination of leaves ( ) with intracellular CO 2 concentration (c i ) measured in the cuvette. This shows a consistency of stomatal behavior during growth and during response to instantaneous cuvette measurements at high ν (35 mbar bar 1 ). Symbols as in Fig. 2. Each point represents one plant plants had higher, however, indicating that in response to contrasting growth environment conditions they may have maintained higher g s in the greenhouse. Despite contrasting ν, this difference in g s would have made E during growth more similar than expected, and may explain the similar development of LSC crown. In contrast, plants grown at lower temperature had significantly lower LSC crown, lower g s under the uniform cuvette conditions, but a similar to plants grown at higher temperature but with similar low RH. This suggests that g s under growth conditions followed variation in RH treatments, but development of LSC crown was, in contrast, more related to growth temperature than RH. served the dual function of transport and organ of photosynthesis, had a higher allocation to twigs. This was most notable in the southern population, for which twigs were, in fact, the most important photosynthetic surface. Percent allocation to leaves was higher in A. dumosa than H. Salsola, and higher in northern than southern populations of both species (Table 1). Percent allocation to leaves was the only harvest variable to be strongly correlated with LSC crown (Fig. 7), which decreased as allocation to leaves increased. LSC crown was highest in the southern population of H. salsola, where twigs contributed over half of the photosynthetic surface and the high allocation to twig tissues served a dual function. Variation in biomass allocation patterns Harvest data on standing biomass at the date of measurement shows several interesting contrasts (Fig. 6). No difference was seen between species or populations in relative allocation to roots (Table 1). Consistent with their preferred microhabitats and growth habits, H. salsola had a higher allocation to the taproot, while A. dumosa had higher allocation to woody branches. The most dramatic difference, and one with a significant species environment interaction in Table 1 was the allocation to leaf-bearing twigs. H. salsola, in which twigs Gas exchange and plant architecture The differences in allocation resulted in strong differences in total photosynthetic surface area per gram of plant among both species and populations (Table 1). However, the high LSC crown of plants with low total area promoted greater stomatal opening, and higher photosynthesis per unit area. Neither photosynthesis per unit biomass nor per unit nitrogen varied among species or populations (Table 1). No correlation was found between LSC crown and xylem vessel diameter. The plants in this study were also

7 7 Table 1 Comparison of gas-exchange and harvest data across species and geographic origin. All plants were grown in a common greenhouse in Ithaca, New York. Mean values and the re-sults of a two-way factorial ANOVA are given for each measure. Sample sizes were 5 plants for the northern seed sources and 6 plants for the southern seed sources for each species (g s stomatal conductance, LSC leaf-specific hydraulic conductance, PS photosynthetic) Variable Mean values Error mean Species Geographi- Species square cal origin site Hymenoclea Ambrosia (site) salsola dumosa North South North South Leaf 13 C discrimination, Net photosynthesis, µmol m 2 s Net photosynthesis, nmol g 2 s Net photosynthesis, µmol g N 1 s g s at ν=35 mmol mol 1, mol m 2 s g s at ν=10 mmol mol 1, mol m 2 s Decrease in g s (as ν increases from to 35 mmol mol 1 ), % g s (recovery with soil press.), % LSC crown (soil:twig), mmol m 2 s 1 MPa LSC stem (caudex:twig), mmol m 2 s 1 MPa LSC root (soil:caudex), mmol m 2 s 1 MPa Leaf area: mass ratio, cm 2 g Twig area: mass ratio, cm 2 g Twig diameter, mm Twig length: mass ratio, m g PS area per total mass, m 2 kg Fine root, % of total biomass Coarse root, % of total biomass Tap root, % of total biomass Main stem, % of total biomass Woody branch, % of total biomass Leaf, % of total biomass PS tissue, % of total biomass Table 2 Comparison of northern seed source of H. salsola grown under different environmental conditions (hh day temperature 33 C, relative humidity RH 67%; h day temperature 33 C, RH 26%; c day temperature 23 C, RH 37%). Mean values for growth conditions with different superscripts are significantly different from each other (Tukey LSD); n=5 plants per treatment Variable Error mean square P Means hh h c Leaf 13 C discrimination, a 23.0 b 22.5 b g s at ν=35 mmol mol 1, mol m 2 s a a b LSC crown (soil to twig-tip), mmol m 2 s 1 MPa a 24.6 a 18.7 b measured individually for cavitation vulnerability. Although strong differences between species and small differences between populations were found (Mencuccini and Comstock 1997), no correlation was found within species between vulnerability to cavitation and LSC. Discussion Variation in LSC crown and effects on gas-exchange These results supported the hypothesis that genetic selection along a climatic gradient would help match LSC crown with the evaporative demand of the environment (Table 1). Similar results were found in Pinus sylvestris (Berninger et al. 1995; Mencuccini and Grace 1995) where LSC crown increased along a climatic gradient of increased ν because of increased allocation to branchwood. This trend permits the maintenance of high g s throughout the climatic gradient, but altered allocation patterns could still lead to reduced total leaf area and lower growth rates. Increased allocation to non-productive branch tissues in P. sylvestris could have reduced intrinsic growth rate. In the desert sub-shrubs, LSC crown was very strongly correlated with percent allocation to leaves (Fig. 7). This shift in allocation was most extreme for the southern populations of H. salsola. This did not result in a reduction in whole-canopy gas-exchange rate per unit canopy biomass, however, because the twig itself became the primary organ of photosynthesis in these plants (Table 1). Comstock and Ehleringer (1988) made a de-

8 8 Fig. 6 Distribution of standing biomass following whole-plant harvest at 5 months of age. Particularly prominent is the very high allocation to photosynthetic twigs characterizing the specialized crown morphology of the southern H. salsola. Each bar represents the mean of 5 plants (+SE) tailed comparison of photosynthetic behavior of leaves and twigs in H. salsola, and found that individual twigs have much lower photosynthetic rates (compared to leaves on the same respective plants) if expressed as uptake per unit biomass or nitrogen content of the photosynthesizing organ. This was similar to the findings for other species (Nilsen 1992; Osmond et al. 1987). The data measured here at the whole-plant level, however, show that this apparent low efficiency is indeed, as was suggested, apparently offset by the multiple functions being performed by twigs (i.e., support, transport, and photosynthesis), and that twig canopies can be just as efficient as leaf canopies at the whole-plant level of integration (Fig. 7, Table 1). Plastic responses of LSC crown to temperature and evaporative demand were also observed in this study, consistent with the assertion that high transpiration rates during growth promoted increased development of hydraulic capacity (Fig. 4, Table 2). Other reports in the literature support the notion that this plastic response is mediated through transpiration rate and water-potential gradient, since LSC crown was reduced when transpiration was low due to stomatal closure under elevated CO 2 (Bunce and Ziska 1998; Heath et al. 1997) even though temperature and ν were held constant. Stomatal conductance (g s )was highly correlated with LSC crown (Figs. 3, 4) and very sensitive to direct manipulation of leaf water potential (Table 1). A large number of studies in the literature reveal a strong correlation between g s and LSC crown, while LSC crown varies due to causes as variable as crown architecture (Hubbard et al. 1999; Mencuccini and Grace 1996; Ryan and Yoder 1997), drought (Irvine et al. 1998), salinity stress (Loustau et al. 1995; Sohan et al. 1999), ozone stress (Grantz and Yang 1996), or site irradiance (Maherali et al. 1997). The stomatal reopening response to soil pressurization and consequent elevation of leaf water potential agrees with previous reports (Comstock and Mencuccini 1998; Fuchs and Livingston 1996; Saliendra et al. 1995) and indicates that at least some part of this correlation is due to direct feedback control between leaf water potential and g s. Hydraulic architecture Fig. 7 Dependence of leaf-specific hydraulic conductance (LSC crown ) on biomass allocation. Plants with the highest allocation to leaves have the lowest hydraulic support per unit photosynthetic area. Although LSC as used in this paper includes all photosynthetic surfaces and not just leaves per se, leaves in the context of %allocation refers strictly to leaves and not to photosynthetic twigs, which also serve a transport function. Symbols as in Fig. 2. Each point represents one plant Consistent, perhaps, with an herbaceous evolutionary past and current growth habits intermediate between large herbaceous perennials and woody shrubs, rather steep water potential differences were observed in the cm of shoot height for both study species, and yet total belowground hydraulic resistance was still considerably greater than total shoot axial resistance (Table 1). Gradients of this magnitude per unit height would probably not be sustainable in plants of large stature. A very large, abrupt water potential difference was observed between the twigs and leaves. It is not possible to tell from this dataset whether this gradient was primarily in the petioles, leaf veins, or in symplastic portions of the pathway associated with movement from veins to evaporative sites. All of these have been identified as sites of unusually high hydraulic resistance in past studies (Tyree et al. 1993; Yang and Tyree 1993; Zimmermann 1983). In general, stomatal behavior was much better correlated

9 with total resistance rather than the resistance of any specific subportion of the pathway, but a limited dataset on the subpathway from twigs to leaves prevented its inclusion in the final estimate of whole-plant LSC. Lack of apical dominance Studies of trees with strong apical dominance have revealed a higher hydraulic conductance to the dominant apex than subdominant or suppressed apices (Zimmermann 1983). Replacement of a lost tree leader is associated with reestablishment of a very high LSC crown for the leader tissue (Spicer and Gartner 1998). In contrast, both of the species in this study were desert subshrubs with no prolonged apical dominance, limited branch lifespans, and frequent canopy renewal via basal suckering. At 5 or 6 months of age, the original apex of the first vigorous shoot was still present on most of the study plants. In a very few cases it had already died, and in a few others, it was visibly senescent while vigorous growth at the whole-crown level continued at other loci. For most plants, including all the plants where the balance pressure of the leader was measured, visible signs of senescence were not apparent and the original leader was identifiable only by tracing the sometimes complex branching pattern. Nonetheless, hydraulic conductance to these canopy regions was consistently much lower than the average for the whole crown (e.g., Fig. 1). Since, regardless of the outward health of the original leader tissue, active growth had in all cases shifted to major sidebranches from lower stems and caudex, this observation is consistent with the supposition that high hydraulic conductance is preferentially maintained at strongly growing meristems, and that a loss of it may be an early sign of senescence. 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