Drought-induced hydraulic limitations constrain leaf gas exchange recovery after precipitation pulses in the C3 woody legume, Prosopis velutina
The hypothesis that drought intensity constrains the recovery of photosynthesis from drought was tested in the C3 woody legume Prosopis velutina, and the mechanisms underlying this constraint examined.
Hydraulic status and gas exchange were measured the day before a 39 mm precipitation pulse, and up to 7 d afterwards. The experiment was conducted under rainout shelters, established on contrasting soil textures and with different vegetation cover at the Santa Rita Experimental Range in southeastern Arizona, USA.
Rates of photosynthesis and stomatal conductance after re-watering, as well as the number of days necessary for photosynthesis to recover after re-watering, were negatively correlated with predawn water potential, a measure of drought intensity (R2 = 0.83, 0.64 and 0.92, respectively).
Photosynthetic recovery was incomplete when the vascular capacity for water transport had been severely impaired (percentage loss of hydraulic conductance > 80%) during the drought, which largely increased stomatal limitations. However, changes in biochemical capacity or in mesophyll conductance did not explain the observed pattern of photosynthesis recovery. Although the control that hydraulic limitations impose on photosynthesis recovery had been previously inferred, the first empirical test of this concept is reported here.
Precipitation is one of the most important factors controlling primary productivity in terrestrial ecosystems, and increases in importance as mean annual precipitation decreases (Huxman et al., 2004a). Accordingly, ecosystems in areas with low mean annual precipitation are predicted to be most susceptible to anticipated changes in rainfall associated with climate warming (de Dios et al., 2007). The rate of photosynthetic carbon assimilation (A), a key process related to primary productivity, varies widely over the growing season in arid and semiarid ecosystems, and often responds significantly to changes in resource supply associated with pulsed inputs of growing season precipitation (Sala & Lauenroth, 1982; Williams & Ehleringer, 2000; Huxman et al., 2004b). Differences in growing conditions, such as soil texture, vegetation cover and atmospheric humidity, along with the high spatial and temporal variance of precipitation in arid and semiarid regions, produce a highly heterogeneous mosaic of water availability which may change dramatically even over very short timescales of hours and days (Reynolds et al., 2004). This variation in water availability results in substantial variation of the photosynthetic gas exchange response following precipitation inputs during the growing season in these environments (Huxman et al., 2004b; Reynolds et al., 2004; Ignace et al., 2007; Patrick et al., 2007; Resco et al., 2008).
Temporal up-scaling of leaf photosynthetic fluxes in these ecosystems is problematic because of the different time lags observed for different components of leaf gas exchange regulation (Tuzet et al., 2003). In response to a precipitation pulse, stomatal conductance (gs), A and plant water potential may be temporarily decoupled from each other, although the underlying mechanism has not yet been elucidated (Yan et al., 2000; Tuzet et al., 2003; Resco et al., 2008).
These uncertainties arise partly from our incomplete understanding of how gas exchange recovers from drought after new pulses of precipitation. In recent years, a general model of drought effects on photosynthesis limitations has been proposed, based on the interplay between biochemical, stomatal and mesophyll limitations (Flexas et al., 2006). Stomatal limitations to photosynthesis are regarded as the prevailing limiting factor except when: plants are operating in the asymptotic part of the A/gs relationship; and gs drops below 0.05–0.1 mol m−2 s−1. Biochemical limitations are generally thought to take over after these thresholds. However, decreases in mesophyll conductance (gm), the conductance of CO2 from the substomatal cavity to the site of carboxylation, has been increasingly reported as another dynamic factor limiting photosynthesis (Flexas et al., 2008).
Although photosynthesis responses to drought are relatively well understood, there is a surprising paucity of studies on photosynthesis recovery from drought (Flexas et al., 2006). A and gs typically increase in response to a biologically significant precipitation pulse, until they reach a ‘peak’ value, and then return to values comparable to those before the irrigation. Peak response rates of A and gs (Ap and gp, respectively) following a precipitation pulse, as well as the time lag (τ) necessary to reach that peak value, are often reported to depend upon the intensity of water stress antecedent to the pulse (Huxman et al., 2004b). However, the mechanisms underlying this relationship are currently being debated. The Flexas et al. (2006) model of photosynthesis responses to drought predicts that incomplete recovery of photosynthesis occurs when, as a result of previous drought, biochemical capacity needs to be restored. When gs drops below 0.05 mol m−2 s−1, the concentration of antioxidant compounds in photosynthetic tissues increases while the carboxylation capacity is impaired (Flexas et al., 2006). Hence, incomplete photosynthesis recovery may occur when the plant needs to repair its carboxylation capacity after drought-mediated oxidative stress. Alternatively, stomatal limitations may prevail if the plant has suffered a large percentage loss of hydraulic conductance (PLC) during the drought (Wheeler & Holbrook, 2007). Stomata close when vascular supply diminishes, and incomplete photosynthesis recovery is to be expected when the plant has experienced a large PLC, because xylem refilling under tension is problematic (Clearwater & Goldstein, 2005; Lovisolo et al., 2008). Finally, Galmes et al. (2007) observed that incomplete recovery of gm after re-watering was also important in limiting photosynthesis recovery, because a large resistance to CO2 diffusion from the substomatal cavity to the chloroplast diminishes the substrate for carboxylation, and gm is not driven solely by physical diffusion but seems to be controlled by plasma membrane intrinsic proteins (aquaporins) and/or by the activity of carbonic anhydrase (Bernacchi et al., 2002), which, in turn, are highly sensitive to drought (Kaldenhoff et al., 2008).
In this study, we monitored the dynamics of A and gs in the C3 woody legume Prosopis velutina Woot (mesquite) over a period spanning the day before and up to 7 d after a large precipitation pulse. Mesquite was chosen for this study because of its historic encroachment into grasslands in southwestern North America and its documented impacts on community structure and ecosystem processes (Scholes & Archer, 1997; McClaran et al., 2003; Williams et al., 2006; Yepez et al., 2007; Knapp et al., 2008). Photosynthesis recovery was assessed after two months of imposed drought and at the peak of the summer rainy season on clay loam and sandy loam soils, on different aged seedlings of P. velutina, as well as on plants growing on bare ground or with interspecific competition from perennial C4 grasses. This variety of growing environments allowed us to evaluate drought recovery under contrasting amounts of antecedent water stress. For expediency, ‘antecedent’ will be used throughout the text to indicate values measured the day before the irrigation input (abbreviated as ‘D−1’), whereas ‘peak’ denotes the day on which the highest value of A in response to the irrigation pulse was recorded (abbreviated as ‘p’), unless otherwise noted.
The first goal of this study was to quantify the importance of antecedent conditions on constraining the recovery of Ap, gp and τA (the number of days between the precipitation pulse and Ap after re-watering (Ogle & Reynolds, 2004)). The second goal was to test whether the constraint exerted by antecedent conditions on drought recovery is imposed by biochemical, stomatal or mesophyll limitations in P. velutina. The third goal was to test the generality of this relationship in other desert species for which data were available from the literature.
Materials and Methods
Field measurements were conducted under experimental rainout shelters installed on sandy loam and clay loam soils at the Santa Rita Experimental Range in southeastern Arizona, USA (English et al., 2005). The rainout shelters and the experimental plots they covered were established in 2002 as part of a global change experiment to investigate mesquite seedling establishment under different seasonal precipitation regimes (50% increase or decrease of the long-term average in summer or winter precipitation), soil textures (sandy loam and clay loam) and grass cover (bare ground, native or invasive C4 grass). A cohort of 30 seeds was planted yearly on each plot and, because of highly variable treatment-induced patterns of seedling establishment, we were unable to assess seedling physiology across all treatment combinations. Two separate seedling cohorts (1 and 4 yr of age for the present study) were available for intensive physiological measurements. Adequate numbers of 1- and 4-yr-old seedlings were available in grass-free, bare plots on each of the two soil surfaces, but in plots dominated by the native C4 grass Heteropogon contortus (L.) Beauv only 1-yr-old seedlings were available for measurements and only on the sandy loam surface (Table 1). A detailed description of the demographic patterns is provided by Resco (2008).
|Code||Species||Data source||Ecosystem||Soil texture||Ground cover||Age||Measurement|
|Pv1||Prosopis velutina||This study||Sonoran||Sandy loam||Heteropogon contortus stand||1 yr||After drought|
|Pv2||Prosopis velutina||This study||Sonoran||Sandy loam||Bare ground||1 yr||After drought|
|Pv3||Prosopis velutina||This study||Sonoran||Sandy loam||Bare ground||4 yr||After drought|
|Pv4||Prosopis velutina||This study||Sonoran||Sandy loam||H. contortus stand||1 yr||Rainy season|
|Pv5||Prosopis velutina||This study||Sonoran||Sandy loam||Bare ground||1 yr||Rainy season|
|Pv6||Prosopis velutina||This study||Sonoran||Sandy loam||Bare ground||4 yr||Rainy season|
|Pv7||Prosopis velutina||This study||Sonoran||Clay loam||Bare ground||1 yr||After drought|
|Pv8||Prosopis velutina||This study||Sonoran||Clay loam||Bare ground||4 yr||After drought|
|At||Artermisia tridentata||Loik (2007)||Great Basin||NA||NA||Adult||After drought|
|Pt||Prusia tridentata||Loik (2007)||Great Basin||NA||NA||Adult||After drought|
|Ps||Pascopyrum smithii||Schomp (2007)||Mixed Prairie||NA||Native vegetation||Adult||Rainy season|
The rainout shelters excluded natural precipitation (mean annual precipitation of 394 mm at the sandy loam site and 430 mm at the clay loam site (Fravolini et al., 2005)) from each plot. On June 10, 2006, after 2 months of imposed drought, we applied a 39 mm irrigation pulse to all of the experimental plots on the sandy loam and clay loam surfaces. Another 39 mm pulse was applied to the plots at the sandy loam site on August 1, 2006, at the peak of the summer rainy season, during a period of frequent experimental irrigation. The experiment was conducted on these highly contrasting soil textures, vegetation covers and different parts of the year to ensure large differences in the antecedent stress (Fig. 2, Table 1).
Photosynthetic gas exchange measurements
Spot gas exchange measurements (n = 3–5) were performed at 07:30 h the day before the irrigation and 1, 3, 5 and 7 d afterwards with a portable photosynthesis system (LI-6400, Li-Cor Inc., Lincoln, NE, USA). Environmental conditions inside the leaf chamber were set to match early morning conditions. Light intensity, block temperature and CO2 concentration were 700 µmol m−2 s−1, 30°C and 400 ppm, respectively. Leaf-to-air vapor pressure deficit (D) was maintained in the range 1.5–3.5 kPa, depending on environmental conditions. Leaves were marked and the same leaf was measured each day of the pulse period.
To quantify the importance of antecedent water stress for constraining the pulse response, we measured predawn water potential (Ψpd), an integrated indicator of water availability in the rhizosphere when nocturnal transpiration is negligible, the day before the irrigation pulse. To test whether this constraint originated from limitations in leaf biochemical capacity, CO2 diffusion through the mesophyll, or through the stomata, we estimated maximal leaf carboxylation capacity (Vcmax) and mesophyll conductance (gm), and measured gs the day before the watering. A possible role for hydraulic limitations as driver of the pulse response was evaluated by estimating PLC at midday, through a previously established relationship between xylem tension and PLC (see later). Ψpd, Vcmax, gm and gs were also monitored 1, 3, 5 and 7 d after the precipitation pulse, to characterize differences in drought recovery across treatments.
Ψpd and midday water potential (Ψmd) measurements were taken on additional seedlings not used for gas exchange within each experimental plot. They were determined on leaves of 1-yr-old P. velutina seedlings using Peltier thermocouple psychrometers (PST-55 Wescor Inc., Logan, UT, USA), and with a Scholander-type pressure chamber (PMS Instruments, Corvallis, OR, USA) on the 4-yr-old seedlings (n = 3). No statistical difference in estimated Ψ from these two techniques was found in an independent test (Resco et al., 2008). A slight modification to the original design of the PST-55 was necessary to measure leaves in these soil psychrometers. We enclosed leaves inside an isolated custom-built chamber, and immersed the chamber in a water bath at 25°C until equilibration occurred. Further details are provided by Resco et al. (2008) and by the manufacturer's website (http://www.wescor.com).
Response curves relating net photosynthetic rate (A) to variation in the leaf intercellular (Ci) and chloroplast (Cc) concentration of CO2 were developed the day before the watering between 06:00 and 10:00 h, following Long & Bernacchi (2003) at saturating light (1500 µmol m−2 s−1) using the same leaves on which spot gas exchange measurements had been made. Leaves were allowed to acclimate to chamber conditions at a CO2 concentration (Ca) of 400 ppm, after which gas exchange parameters were recorded. Gas exchange rates were then determined sequentially as Ca was reduced to 300, 200, 100 and 50 ppm, and then as Ca was returned to 400 ppm and then sequentially at 600, 800, 1000, 1200, 1400, 1600, 1800 and 2000 ppm. Each curve was developed within 30–40 min. Vcmax was estimated from the Farquhar et al. (1980) photosynthesis model, following the assumptions and model-fitting approach of Sharkey et al. (2007).
Mesophyll conductance to CO2 transfer (gm) was estimated with the ‘variable J’ method on the same leaves used to develop A/Ci curves. This method compares electron transport rate (J) measured from chlorophyll fluorescence (Genty et al., 1989) with J estimated with the Farquhar et al. (1980) model. The difference between both estimates is assumed to be related to gm; see Harley et al. (1992) and Long & Bernacchi (2003) for a detailed description of the method. Four parameters are required to obtain J from chlorophyll fluorescence: (i) the photochemical efficiency of photosystem II (ΦPSII), which was determined using a portable pulse-modulated fluorometer (FMS2, Hansatech Instruments, King's Lynn, UK) immediately after development of each A/Ci curve, and after acclimating each leaf for 10 min at the irradiance value at which the previous A/Ci curve had been measured (Maxwell & Johnson, 2000; Loik & Holl, 2001); (ii) leaf absorptance, which was assumed to be 0.86 for Prosopis (Asner et al., 1998); (iii) irradiance (same as in the A/Ci curve); and (iv) the fraction of absorbed irradiance that reaches PSII, which was assumed to be 0.5 for C3 plants (Ögren & Evans, 1993). Warren (2006) provides a critical analysis on the limitations of using this approach to estimate gm.
Vulnerability to cavitation
Direct estimates of PLC in the limited population of Prosopis seedlings were not possible to attain, because of the destructive nature of the measurement. Hence, we had to estimate PLC from vulnerability to cavitation (VC) curves, which relate PLC to xylem tension (Px), using the dehydration method (Cochard et al., 1992), where each point of the curve is from an individual plant. Eleven 1-yr-old plants growing on the clay loam site were harvested after the measurements in June, and 16 1-yr-old seedlings from the sandy loam site were harvested after experiment termination in August. Although the growth of new vascular tissue between June and August is possible, P. velutina is a ring-porous species, and the new growth of small-diameter vessels will have a minor impact on the VC curve, because water flow scales with the fourth power of conduit diameter (Tyree & Ewers, 1991). Moreover, the VC curves reported here are remarkably similar to those reported for stems of adult P. velutina trees growing nearby (Hultine et al., 2006), which indicates that the different collection times likely had a negligible effect on the VC curve. For instance, plants growing at the sandy loam site lost 50% of conductance at c.−2.05 MPa in Hultine et al. (2006), and at −2.15 MPa in this study; and 75% of conductance at c. −4.10 MPa in Hultine et al. (2006), and at −3.85 MPa in this study (Fig. 1). Indeed, we also used the VC curve obtained by Hultine et al. (2006) to estimate PLC in the 4-yr-old seedlings in Fig. 6, but observed no significant difference in the relationship reported between PLC with the drought recovery parameters (not shown).
Plants were harvested under water, at least 10 cm below the root collar, and transported to the laboratory in wet paper towels inside zip bags. In the laboratory, they were allowed to air dry to different stages, from 1 up to 10 d (−0.1 to −14 MPa). The whole plant was kept inside a zip bag the night before measurement, to allow for equilibration of spatial gradients in water potential. Before the conductivity measurement, a segment centered on the root collar was cut under water. Hydraulic conductivity was measured in this segment as the ratio between the flow of deionized water (measured by XYL’EM, Bronkhorst, France (Cruiziat et al., 2002)) and the gravity-induced pressure gradient (10 kPa). Maximum conductivity was estimated after flushing the segment at high pressure (100 kPa, refer to the XYL’EM manual for further details (http://www.bronkhorst.fr/fr/produits/xylem_emboliemetre)). Xylem tension was measured with a Scholander type pressure chamber (PMS Instruments, Corvallis, OR, USA) in the above-ground part of the plant and VC curves were fitted through a Weibull function (Sperry et al., 1998, Fig. 1). PLC was then estimated by substituting Ψmd for Px in the equations given in Fig. 1. Ψmd measures the minimum water potential throughout the day, and we thus estimated the maximum PLC that might occur.
Water potential in the leaves is necessarily more negative than in the stem for water to flow. Because Ψmd was measured in leaves and not in the stem, we may have consistently overestimated stem PLC. However, this study was performed in rather short mesquite plants (< 40 cm), and spatial gradients in Ψ from the stem to the leaf will likely be in the order of a few tenths of a kPa (Tyree & Ewers, 1991), a negligible error for the accuracy necessary in this study (Fig. 1).
To quantify the importance of antecedent water stress (Ψpd.D−1) in constraining the pulse response in mesquite and in other desert species, we conducted data searches on Web of Science (http://portal.isiknowledge.com/) for studies that were performed on C3 plants; that reported the pulse response of spot gas exchange at leaf level for at least the day before the watering and up to the day when Ap and gp occurred; and that maintained comparable light intensities inside the photosynthesis chamber. Unfortunately, only one study (Gillespie & Loik, 2004; Loik, 2007) matched our criteria. Loik (2007) reported results for two shrubs from the southwestern Great Basin desert −Purshia tridentata (Rosaceae) Pürsh and Artemisia tridentata (Asteraceae) Nutt. We also incorporated the results from an unpublished thesis (Schomp, 2007), where a pulse response study was conducted in mixed-grass prairie in southeastern Wyoming as a component of the Prairie Heating and CO2 Enrichment (PHACE) experiment (http://www.phace.us/). Schomp (2007) reported results for the C3 perennial grass Pascopyrum smithii (Poaceae) (Rybd.) A. Love (Table 1).
We examine the relationship between Ψpd.D−1 and Ap, gp and τA through least-squares fitting. Because the different species included in this analysis had different photosynthetic capacities (Table 1), we normalized Ap as percentage photosynthesis recovery:
% recovery = 100 (Âp,s / Âmax,s)(Eqn 1)
where Âp,s is the average (n = 3–5) of the value of Ap for species s; and Âmax,s is the maximum of the mean assimilation rates under optimum conditions (during the peak of the rainy season) for species s: 22 µmol m−2 s−1 for mesquite (this study), 18 µmol m−2 s−1 for A. tridentata (Gillespie & Loik, 2004), 15 µmol m−2 s−1 for P. tridentata (Gillespie & Loik, 2004) and 20 µmol m−2 s−1 for P. smithii (Schomp, 2007; V. Resco, unpublished).
Then, we evaluate in mesquite whether the relationship between drought recovery and antecedent stress is mediated by antecedent stomatal limitations (gs.D−1), antecedent Vcmax (Vcmax.D−1) or antecedent mesophyll conductance (gm.D−1), by examining whether these parameters correlate with Ap. To understand why higher-stressed plants did not attain the same Ap as lower-stressed plants, we tested for differences in gs, Vcmax and gm across treatments when Ap is reached (gp, Vcmax.p, and gm.p, respectively), through analysis of variance. Finally, because gs could be influenced by both PLC and leaf-to-air vapor pressure deficit (D), we partitioned the effects of these two through stepwise regression following the model selection criteria proposed in Crawley (2007). Nonlinear curve fits were just chosen to establish empirical relationships, but without exploring the potentially relevant biological information stored in the parameters. We used R 2.5.0 (R Development Core Team, Vienna, Austria) in all of our statistical analyses.
Constraints on drought recovery imposed by antecedent drought stress
Because this study was performed on plants growing on different sites and under different plant covers, the underlying assumption is that a comparable degree of hydration across treatments was attained after the application of the 39 mm irrigation pulses, such that variance in Ap does not merely reflect differences in post-irrigation Ψpd. Indeed, Fig. 2c shows that there is no significant relationship between peak Ψpd and antecedent Ψpd (P = 0.63). Moreover, the relationship between peak Ψpd and Ap was not significant (P = 0.56, Fig. 2b), indicating that the results from this experiment are not an artifact originating from post-irrigation differences in plant water status.
Antecedent water stress, as indicated by Ψpd.D−1, explained 83, 92 and 64% of the variance in Ap, τA and gp, respectively (2, 3), suggesting that antecedent conditions exert substantial control on drought recovery.
No differences in antecedent values of Vcmax and gm across treatments were observed (Table 2, Fig. 4). However, we did observe differences in antecedent gs (Table 2, Fig. 4), such that gs.D−1 was lower in the seedlings growing at the sandy loam site in June than in the other treatments (Fig. 4). The control of antecedent stress on drought recovery does not seem to be mediated by nonstomatal limitations, as no significant relationship (P > 0.3) was observed between Vcmax.D−1 or gm.D−1 with Ap (Fig. 5b,c) in P. velutina seedlings. However, the degree of antecedent gs was significantly correlated (P < 0.01, R2 = 0.53) with photosynthesis recovery from drought (Fig. 5a).
Stomatal limitations in our system may result from either high D and/or PLC. Stomatal conductance responds rather rapidly to variations in D, and no direct mechanistic link is to be expected between gas exchange after re-watering and the D that occurred during drought. However, the effects of increasing PLC after a prolonged period of drought may last for some time, even after re-watering, if complete recovery of hydraulic capacity is not achieved. Indeed, PLCD−1 was significantly correlated with Ap (R2 = 0.83, Fig. 6c), τA (R2 = 0.97, Fig. 6b) and gp (R2 = 0.82, Fig. 6a), suggesting that limited hydraulic conductivity caused the observed differences in the dynamics of photosynthesis recovery from drought.
Antecedent water stress also imposed an important constraint on Ap and gp in A. tridentata, P. tridentata and P. smithii, and no interspecific differences were apparent, since these values fell within the 95% confidence interval for the Prosopis data (2, 3). However, τA was up to 4 d shorter in these species than in P. velutina (Fig. 3a).
Effects of post-irrigation conditions on photosynthetic recovery
We only observed significant differences in gs across treatments after the application of the 39 mm precipitation pulses, but not in peak values of Vcmax and of gm (Table 2). The effect of peak stomatal limitations on Ap could be the result of the previously reported differences in antecedent PLC, but also of differences in peak D (Dp) across seasons, as Dp was significantly lower in June than in August (P < 0.01). To partition the effect of PLCD−1 from that of Dp on stomatal conductance, we performed a stepwise regression where we compared a model with PLCD−1 as the only independent variable (Fig. 6b) with another regression model where both Dp and PLCD−1 were independent variables. Dp and PLCD−1were not significantly correlated with each other (P > 0.05). The inclusion of Dp did not significantly improve the performance of the model (P = 0.13), and we thus concluded that the effect of different D on gp across seasons was overridden by that of PLCD−1.
Photosynthesis limitations and recovery from drought
Our results partially support the generality of a recently proposed conceptual model on the changes in photosynthetic limitations with drought (Flexas et al., 2006), which predicts that stomatal limitations prevail except under severe stress (defined as gs < 0.05 mol m−2 s−1), when biochemical limitation starts to operate. We failed to observe any statistical difference in Vcmax or in gm across treatments before the irrigation pulse (Table 2), although antecedent stomatal conductance varied from 0.06 to 0.33 mol m−2 s−1.
A prediction from this photosynthesis model is that recovery from drought will be complete and immediate when no reductions in biochemical capacity are experienced, because a relaxation in gs is often assumed after the water input (Flexas et al., 2006; Galmes et al., 2007). Stomatal aperture is thought to be regulated by changes in epidermis and guard cell water potential, which, in turn, are affected by D as well as by xylem hydraulic conductance (Brodribb et al., 2003; Buckley, 2005), amongst others. Whereas an increase in relative humidity usually accompanies a large precipitation pulse, and this would alleviate atmospheric stress, our data indicate that photosynthetic gas exchange recovery may be neither complete nor fast when a large proportion of the hydraulic capacity has been lost as a result of drought. For P. velutina, photosynthesis limitation in response to a pulse remains at around or below 10–20%, and the time lag necessary to reach this peak assimilation value is between 1 and 3 d, when the percentage loss of hydraulic conductance antecedent to the pulse is < 80%. However, an incomplete recovery of photosynthesis from drought was observed when antecedent PLC was above 80% (Fig. 6).
Previous studies had suggested thresholds for incomplete drought recovery in terms of a minimum stomatal conductance (Flexas et al., 2006; Galmes et al., 2007). However, we have shown that proposing this threshold as a function of gs, instead of PLC, may be misleading: complete and timely photosynthetic recovery is to be expected if low gs.D−1 is the result of large antecedent D (alone or in combination with a low Ψpd, which does not lead to substantial cavitation), but incomplete drought recovery will follow when large PLCD−1 is motivating the low antecedent stomatal conductance.
With the goal of predicting the time lag necessary to reach peak gas exchange, one must take into account the relationship between water infiltration and soil texture. A regression model was able to explain 95% of the variation in τA as a function of PLCD−1 for the ‘sandy loam seedlings’, whereas the ‘clay loam seedlings’ fell out of this regression line (Fig. 6a). This is likely because of the longer time required for water infiltration and for water potential to change in finer-textured soils (Hillel, 2004), the observed correlation between the time necessary to reach Ap with the time necessary to reach peak Ψpd (P < 0.05, R2 = 0.63), and differences in rooting depth across soil textures (Resco, 2008).
The accumulation of abscisic acid (ABA) signaling stomatal closure has previously been reported as another mechanism affecting photosynthetic gas exchange recovery from drought (Davies & Zhang, 1991). However, high ABA concentrations are not likely to persist for > 3 d after water stress is relieved (Davies & Zhang, 1991). An ABA-mediated response would not explain either the long τA or the low Ap observed 5–7 d after watering in the seedlings with the highest PLCD−1 (Fig. 6). Our results support the findings of Fuchs & Livingston (1996), who suggested that woody plants rely more on hydraulic signals, whereas ABA regulation is probably more common in herbaceous plants.
Our results also support a rapidly increasing body of literature showing the dependence of photosynthetic gas exchange on plant hydraulic properties (Brodribb & Feild, 2000; Sperry, 2000; Maherali et al., 2006; Sack & Holbrook, 2006; Brodribb et al., 2007), although previous studies focused mainly on interspecific comparisons between different parameters related to maximum photosynthetic and hydraulic capacities. Moreover, considering the reported paucity of studies on photosynthetic drought recovery (Flexas et al., 2006; Galmes et al., 2007), this may be the first report linking biochemical, mesophyll and stomatal limitation with plant hydraulic architecture. Indeed we provide, to the best of our knowledge, the first study demonstrating that hydraulic limitations determine peak rates of A, gs and τA associated with the short-term responses to precipitation pulses.
Do antecedent drought-induced hydraulic limitations constrain photosynthetic recovery across species?
We observed that the degree of antecedent water stress explains up to 87% of the variance in the pulse response in P. velutina. We failed to find significant differences in the relationship between Ap and gp with Ψpd.D−1 (2, 3) as a function of species identity, as values for A. tridentata, P. tridentata and P. smithii fell within the 95% confidence interval. This result may seem surprising at first, as these species are likely to have different VC curves. However, when antecedent drought stress is so severe that Ψpd drops to the values reported here (–3.8 to –4.6 MPa), PLC values of all four species are likely to converge at rather large values. Indeed, photosynthesis recovery of these three species was around or above 40%, which, according to our model for Prosopis, would imply a PLCD−1 of 80% or higher. Published VC curves for some of these species, although growing at different sites, support this possibility (Kolb & Sperry, 1999).
Our estimate of percentage photosynthesis recovery is very sensitive to the maximum photosynthetic rates for a given species (Âmax,s, Eqn 1). We conducted a sensitivity analysis which indicated that values for the recovery of photosynthesis in A. tridentata, P. tridentata and P. smithii fell within the 95% confidence interval in 2, 3when the error in Amax was up to 15%.
Galmes et al. (2007) and Flexas et al. (2006) observed an incomplete photosynthetic recovery from drought when antecedent gs dropped below 0.15 mol m−2 s−1 across a range of phylogenetically and functionally diverse plant species. However, they did not observe any reductions in the leaf biochemical capacity until gs was smaller than 0.05 mol m−2 s−1, and stomatal limitations seemed to prevail after re-watering. Hence, it could be hypothesized that hydraulic limitations developed during drought, as PLCD−1 rises when gs.D−1 drops below 0.15 mol m−2 s−1, are a widespread mechanism limiting photosynthetic recovery, at least, until biochemical limitations arise. Unfortunately, the paucity of studies in the literature on this important topic prevent us from developing any further synthetic advancements. It is an important research need to elucidate the link between stem hydraulics and photosynthesis recovery from drought.
Understanding photosynthesis responses to precipitation may prove extremely useful for the temporal up-scaling of leaf fluxes. Ignace et al. (2007) showed that in two C4 grasses, the cumulative carbon gain following precipitation pulses was, under some environmental conditions, highly predictable based upon Ψpd.D−1. Although predicting cumulative carbon gain was beyond the scope of this study, our results suggest that by understanding how hydraulics constrain photosynthesis, we may develop simpler quantitative models of leaf-level rates of gas exchange than current approaches (Tuzet et al., 2003; Schymanski et al., 2008), which would require less data input and parameterization, and without compromising accuracy. Moreover, understanding plant responses to pulses may prove key to develop water-saving land management techniques, and to mitigate effects of anticipated global warming.
In this study, we observed that drought-induced hydraulic limitations strongly constrain photosynthetic recovery after re-watering in P. velutina. PLCD−1 < 80% seems to be the threshold value after which photosynthesis recovery is incomplete, and this threshold roughly corresponds to gs.D−1 < 0.15 mol m−2 s−1. Contrary to recent hypotheses, incomplete photosynthesis recovery may occur even without reductions in leaf biochemical capacity. There is a great need for further tests on the mechanisms constraining drought recovery.
Funding was provided by NSF (DEB-0417228) to DGW. N. Pierce, A. Eilts and G. A. Barron-Gafford provided much assistance during the field measurements. This work benefited from discussions with L. Patrick, D. Ignace, J. M. Moreno, C. Fenoll, F. Valladares and I. Aranda. Comments by David Ackerly, Kiona Ogle, Michelle Holbrook and two anonymous reviewers on previous versions of this manuscript were instrumental in improving its quality. We remain deeply indebted to all of them, and to all other collaborators (too many to list here) who have helped over the years in establishing and maintaining the rainout shelters.
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