Root hydraulic conductance and aquaporin abundance respond rapidly to partial root-zone drying events in a riparian Melaleuca species

Authors

  • Elizabeth H. McLean,

    1. Ecosystems Research Group, School of Plant Biology M090, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
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  • Martha Ludwig,

    1. School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
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  • Pauline F. Grierson

    1. Ecosystems Research Group, School of Plant Biology M090, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
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Author for correspondence:
Elizabeth H. McLean
Tel: +61 8 64887914
Email: elizmc@graduate.uwa.edu.au

Summary

  • Drying a portion of a root system (partial root-zone drying (PRD)) can induce partial stomatal closure, but this response is not always observed. We hypothesized that some of the variation in PRD response reflects adaptations to the native environment, where plants subjected to frequent PRD events may display a greater degree of root-level compensation.
  • Here, we examined PRD responses of Melaleuca argentea, a tree native to intermittent waterways in which PRD events are common. Seedlings were grown with part of their root system in soil and part in an aquatic compartment, mimicking conditions often observed in the field.
  • The aquatic roots initially provided two-thirds of total water uptake, but draining the aquatic compartment had no effect on stomatal conductance, so long as soil moisture remained c. 80% of field capacity. Water uptake from the soil compartment increased threefold within 24 h, with a corresponding transient threefold increase in root hydraulic conductance (Lp), an increase in plasma membrane intrinsic protein 1 (PIP1) aquaporins at 24 h, and a decrease in PIP2 aquaporins by 48 h.
  • Our results demonstrate that PRD can induce rapid changes in Lp and aquaporin expression in roots, which may play a role in short-term water uptake adjustments, particularly in species adapted to heterogeneous water availability.

Introduction

Partial root-zone drying (PRD) occurs when a part of the root system dries out, while water remains available to other portions of the root system. Plant responses to PRD have been studied primarily in agricultural settings, in attempts to reduce irrigation while maintaining crop yields and quality (Davies et al., 2002; Kang & Zhang, 2004). However, heterogeneity of water availability around root systems is a far more general phenomenon, occurring widely in natural environments. River banks provide an extreme example of a heterogeneous environment, although most if not all plants may experience some degree of heterogeneity of water supply throughout their lifespan. Riparian trees can be subject to natural cycles of drying and re-wetting of portions of their root systems, especially in arid and semi-arid environments where stream flows are often intermittent or strongly seasonal (Dettinger & Diaz, 2000). This heterogeneous water availability around root systems, both spatially and temporally, can result in frequent PRD events.

PRD often results in partial stomatal closure, leading to increased water use efficiency. In some cases, stomatal closure appears to be caused by a chemical signal from the drying roots, and not simply water deficit, as the stomata re-open when the dried root portion is severed (Gowing et al., 1990; Liu et al., 2001). Increased abscisic acid (ABA) content of the dried roots and shoot xylem sap and increased xylem sap pH have been observed during stomatal responses to PRD, and are considered likely signalling mechanisms (Davies et al., 2002; Dodd et al., 2008). However, the extent of the stomatal response to PRD varies considerably. For example, the stomatal conductance (gs) of Citrus aurantium subject to PRD was c. 70% that of fully watered control plants (Zekri & Parsons, 1990), while in another study the gs of Acer pseudoplatanus saplings under PRD declined to c. 30% of that of controls (Khalil & Grace, 1993). In other experiments using Lupinus cosentinii, Quercus robur, Betula pendula, Olea europaea, Phaseolus vulgaris and Persea americana, no stomatal response to PRD was observed; instead gs and rates of water use consistent with pre-PRD conditions were maintained (Gallardo et al., 1994; Fort et al., 1997, 1998; Centritto et al., 2005; Wakrim et al., 2005; Neuhaus et al., 2007). While differences in PRD response can be partly explained by the variable conditions used within each study, some of the variation may also reflect species differences (e.g. Croker et al., 1998). We hypothesize that differences among species in PRD response reflect adaptations to environmental heterogeneity or homogeneity. If this hypothesis is correct, we would expect gs and water use to be unaffected by PRD in species commonly subjected to natural PRD events, including some riparian trees, provided that adequate water is available to other parts of the root system. If a plant is able to maintain water uptake via the remaining hydrated zones, the best adaptive strategy may be to continue water use, and therefore photosynthesis and growth, at a constant rate.

The mechanisms by which plants maintain water uptake via only a portion of the root system are unclear. Even when total water use is reduced in response to PRD, the uptake rates of the wet roots can increase, thus compensating partly for reduced uptake from the dried parts of the root system (Lawlor, 1973; Zekri & Parsons, 1990; Kang et al., 2003; Mingo et al., 2004). The increased flow rates in the wet root portion during PRD may result solely from changes in hydrostatic gradients, or the roots might also adjust their hydraulic conductance (Lp) according to demand. Kang et al. (2003) found that pear (Pyrus communis) tree roots may have increased Lp in response to PRD, although this result was inferred from sap flux data and not measured directly. However, there are many examples of roots rapidly adjusting Lp to match demand under other circumstances. Partial root excision in wheat (Triticum aestivum) plants resulted in increased Lp of the remaining roots (Vysotskaya et al., 2004). Lp is known to fluctuate diurnally, in synchrony with transpirational demand (Henzler et al., 1999; Beaudette et al., 2007; Vandeleur et al., 2009). Shading can also induce a reduction in fine root Lp (McElrone et al., 2007). If plants do increase Lp under PRD, then the adjustment must be rapid, and localized to the watered root portion.

Root water uptake occurs via parallel apoplastic, symplastic and transcellular pathways (Steudle, 1994). The relative conductance of these pathways depends heavily on the extent of hydrophobic apoplastic barriers and the nature of the gradients involved (i.e. hydrostatic or osmotic), but the apoplast is generally considered the path of least resistance (Steudle & Peterson, 1998; Steudle, 2000). Rapid adjustment in apparent Lp can either result directly from changes in the gradients driving water flow, by changing the contribution of each flow pathway (Steudle, 2000), or through changes in the permeability of the root tissues themselves. Although anatomical changes can affect apoplastic Lp over longer timescales (Enstone et al., 2003), the only known means of rapid permeability adjustment is via transcellular Lp, through changes in the quantity or activity of root cell plasma membrane intrinsic protein (PIP) aquaporins. There is growing evidence that short-term changes in Lp may be generated primarily through aquaporins (Maurel et al., 2010). Transient increases in root Lp during early drought may be caused by increased PIP permeability (Hose et al., 2000), reduction of root Lp under anoxia can occur through proton gating of PIPs by cytosolic acidification (Tournaire-Roux et al., 2003), and the reduction and subsequent recovery of root Lp during chilling is associated with changes in expression and phosphorylation states of PIPs (Aroca et al., 2005). If rapid root Lp adjustments do occur in the wet root portion during PRD, aquaporins may be involved.

The riparian tree Melaleuca argentea is widespread across northern Australia, in locations where permanent shallow ground water is accessible. Following flood events, M. argentea commonly grows extensive aquatic root mats within temporary pools that form in the creek beds. Sap flux studies have shown that these aquatic roots can provide the majority of the water used by the tree, especially when adjacent bank soils are relatively dry (Graham, 2001). However, as the pools dry out, the aquatic roots die back and water uptake must occur via different parts of the root system. Given the natural frequency of drying events, we hypothesized that M. argentea would not respond to PRD with stomatal closure but would maintain water uptake by compensating with other parts of the root system. We also hypothesized that such a compensatory effect would involve rapid increases in root hydraulic conductance, which may be mediated by changes in aquaporin content or permeability. We tested these hypotheses with seedlings of M. argentea in a split-root system, mimicking the conditions frequently observed in the field with a portion of the roots in soil, and a portion in an aquatic pool. This design allowed us to test the effects of draining the aquatic pool on the shoot, and on the remaining hydrated part of the root system.

Materials and Methods

Plant material and growth conditions

Seedlings of Melaleuca argentea W. Fitzg. were collected from an intermittent creek in the semi-arid Pilbara region of northwestern Australia (22.9114°S, 119.2139°E). Seedlings were planted into pots of river sand with slow-release fertilizer (OsmocoteR Native Gardens; Scotts, Baulkam Hills, Australia), and grown for several months before transplanting into split-root pots for PRD experiments. Genetically identical plants were generated for Expt 3 (see next section) by taking stem segment cuttings from a single seedling. Cuttings were grown for 9 months before transplanting into split-root pots. Seedlings of c. 1 m height and 1–2 cm basal stem diameter were used in all experiments. Experiments took place between October 2008 and May 2010, in a glasshouse at the University of Western Australia (31.9828°S, 115.7997°E).

Experimental design

The PRD responses of M. argentea were tested in three experiments. Expt 1 assessed field-collected seedlings for stomatal responses, tissue water status and root growth responses to PRD over a 14-d period. Water stress treatments were also conducted as comparisons to verify the sensitivity of our measurements. Four treatments were included, with five to six replicate plants in each; a PRD treatment, PRD with restricted water (PRD-RW), complete root drying (CRD) and two fully watered control treatments. Expt 2 examined survival and recovery of the dried root portion after 28 d of PRD-RW treatment, to determine whether rapid root death may explain a lack of signalling from the drying roots. The control treatment consisted of six replicates, and the PRD-RW eight replicates, four of which were re-wetted at 28 d for the recovery treatment. Finally, Expt 3 tested for rapid responses in root Lp and aquaporin expression. Genetically identical plants derived from cuttings were used to reduce variability among replicates. Plants were treated with PRD for either 24 or 48 h, or remained fully watered as controls. Four replicate plants of each treatment were measured for root Lp, and a second set of four replicates harvested for aquaporin analysis. Treatments were applied across a 3-wk period to allow measurements and sample collection to be conducted on all plants between 11:00 and 14:00 h. Conditions within the glasshouse for each experiment are shown in Table 1. Details of the treatments for all experiments are described at the end of this section.

Table 1.   Glasshouse conditions during the partial root-zone drying experiments on Melaleuca argentea
 Expt 1Expt 2Expt 3
  1. Values give the total range of daily observations during the experimental periods.

  2. See the Materials and Methods section for descriptions of experiments.

  3. nd, not determined; PAR, photosynthetically active radiation.

Maximum temperature (°C)28–4227–3624–29
Minimum temperature (°C)12–2118–248–13
Maximum relative humidity (%)45–71nd57–67
Minimum relative humidity (%)26–49nd28–44
Maximum PAR (μmol m−2 s−1)690–14511000–1600800–1332

Split-root pots consisting of two 15-l compartments were used for Expts 1 and 2; for Expt 3, two 7-l compartments were used, as the plants were required to be transportable. For all experiments, one compartment of each pot was fully sealed and filled with a diluted modified Hoagland’s nutrient solution (Hoagland & Arnon, 1950) to form the aquatic compartment, and the second compartment had drainage holes in the base and was filled with river sand to form the soil compartment. The moisture-release properties of the river sand as determined in a pressure chamber are shown in Supporting Information Table S1. Approximately one-third of the root system was placed in the aquatic compartment, with the remaining two-thirds including the taproot in the soil compartment. Soil compartments were watered with quarter-strength modified Hoagland’s solution. The dilution of the Hoagland’s solution in the aquatic compartment was calculated from the field capacity of the river sand (c. 200 ml l−1), so that the quantity of nutrients per unit volume was equal between the two compartments. The aquatic compartments were replenished daily, and the solution changed every 7–10 d. Soil surfaces were covered with a 1-cm layer of white plastic beads, and aquatic compartments covered tightly with white plastic to minimize evaporation and light penetration. Plants were allowed to establish in the split-root pots for 3–5 wk before the start of treatment, by which time there was copious new root growth in both compartments.

Plants were allocated randomly across treatments, and treatments were begun at midday, by draining the aquatic compartments. The soil compartments of PRD plants were watered twice daily with nutrient solution. For the PRD-RW treatment, the soil compartment was watered only once per day, which allowed the water content to fall below 80% (v/v) of field capacity. Melaleuca argentea is a riparian species that has the highest rates of growth and transpiration under semi-saturated conditions; 80% of field capacity of river sand would thus restrict water for this species. For the CRD treatment, the aquatic compartment was drained, and watering of the soil compartment ceased. Two groups of controls were used, with soil compartments watered either once per day as controls for PRD-RW, or twice per day as controls for PRD. The moisture content of the soil compartments during Expt 1 is shown in Fig. S1. The aquatic compartments of control plants remained filled and maintained as already described.

Stomatal conductance and tissue water status

Stomatal conductance (gs) was measured on the abaxial surface of young, mature, exposed leaves with a leaf porometer (Decagon Devices, Pullman, WA, USA). At each sampling, measurements were taken on three leaves per plant and then averaged to give a single value for each plant. Leaf water potential (Ψl) was measured with a Scholander-type pressure chamber (PMS Instruments, Albany, OR, USA). A single mature leaf from the upper half of the plant was cut and the balancing pressure determined immediately. Measurements of gs and Ψl were conducted in all experiments to verify consistency of plant response to PRD.

Leaf and fine-root (< 2 mm diameter) water contents were determined from 0.2–1-g samples collected mid-morning. Roots were washed clean of sand and surface dried thoroughly on paper towel. Samples were weighed immediately upon harvesting, and then frozen in liquid nitrogen and freeze-dried. Samples were re-weighed and original moisture contents calculated, before processing of the samples for other analyses.

Plant water use

Plant water uptake was measured as the change in pot weight over the time intervals between scheduled watering. Details of the method used to determine uptake from each compartment are provided in Methods S1. Water uptake is expressed relative to pretreatment values, to normalize for variation in plant size. Water uptake measurements were conducted in all experiments to verify consistency of plant response.

Root hydraulic conductance

Hydraulic conductance of roots (denoted L prior to normalization against root surface area) in both split-root pot compartments was measured with a high-pressure flow meter (HPFM; Dynamax Inc., Houston, TX, USA), using the ‘transient’ method (Tyree et al., 1995). All measurements of L were conducted between 11:00 and 14:00 h. Portions of roots of diameter up to 10 mm were excavated in each soil compartment, the selected root was cut under water, and the cut end was attached to an HPFM coupling with a watertight seal. Transient pressure ramps were applied immediately. Water flow into the root and applied pressure were logged every 3 s while applied pressure was increased at a rate of 3–7 kPa s−1, from 0 to 550 kPa. At least three successive ramps were applied to each root. L was calculated as the slope of the linear region (between 200 and 550 kPa for most measurements) of the relationship between flow and applied pressure. Successive ramps applied to the same root gave similar values of L, and no consistent differences among samples in the direction of change between successive ramps were observed. The temperature of the laboratory during measurement periods remained between 15.5 and 16.5°C.

The roots measured for L were separated from the rest of the root system and their fine-root portions (< 2 mm diameter) dried at 60°C. The values of root L were normalized against fine root dry mass as a proxy for root surface area, to yield values of Lp. The correlation between fine-root dry mass and surface area was established by scanning samples of fine roots and analysing with WinRhizo software (Regent Instruments Inc, Quebec City, Canada), before drying (Fig. S2).

Xylem sap osmolarity

Xylem sap was collected between 12:00 and 13:00 h on sampling days to assess the osmotic gradients from soil solution to xylem sap. Branchlets c. 20 cm in length were cut from the seedlings and inserted into a Scholander-type pressure chamber. Bark was removed from the end 5 mm of the stump, to avoid contamination with phloem contents. The stem was then pressurized to no more than 0.4 MPa above the balancing pressure using a single pressurization, to avoid dilution of the sap with cellular water (Berger et al., 1994). The first droplet of xylem fluid was discarded and the cut surface blotted clean to remove cellular contents of wounded cells. A few drops of fluid were then allowed to flow into a vial, which was sealed and frozen at −80°C until analysis. The osmolarity of 20-μl samples was determined by freezing point depression with a Fiske 210 micro-osmometer (Advanced Instruments Inc., Norwood, MA, USA).

Root growth

Photographs of roots in the soil compartments of the split-root pots were taken through windows cut into the pot walls, to assess growth rates of the hydrated root portion. Windows were kept sealed between measurement time-points to prevent roots from being exposed to light. Each photograph was taken of the same region, at the mid-point of each pot (50 cm depth), and cropped to a final image for analysis of 10–20 cm2. All fine roots visible in the images were traced in Adobe Illustrator (Adobe Systems Inc., San Jose, CA, USA), and the tracings analysed in WinRhizo (Regent Instruments Inc., Quebec City, Canada) to obtain root length measurements.

Protein extraction and immunoblotting

Treatments were staggered so that all samples for protein analysis could be harvested on the same day, between 12:00 and 13:00 h. Frozen fine-root tissue was ground under liquid nitrogen and mixed with protein extraction buffer (50 mM HEPES-KOH, pH 7.1, 1 mM EDTA, 1 mM dithiothreitol, 5 mM MgCl2, 1 mM PMSF and 1% polyvinylpolypyrrolidone (w/v)) using 200 μl of buffer per 100 mg of tissue powder. Samples were further homogenized in the buffer, and then sodium dodecyl sulphate (SDS) was added to 2% (w/v). The samples were heated to 95°C for 5 min and clarified by centrifugation at 12 000 g. The total protein concentration of the samples was determined using a bicinchoninic acid quantification kit (Pierce, Rockford, IL, USA), with bovine serum albumin as a standard, according to the manufacturer’s instructions.

The specificity of primary antibody binding was tested by immunoblot analysis (Fig. S4). Protein samples were separated on 12% polyacrylamide gels (SDS-PAGE) with molecular mass standards (GE Healthcare; http://www.gelifesciences.com) and then transferred to Hybond C nitrocellulose membrane (GE Healthcare; http://www.gelifesciences.com) by semi-dry electroblotting (Bio-Rad; http://www.bio-rad.com) for 1.5 h at 0.54 mA cm−2. Membranes were blocked for 1 h in 5% (w/v) skimmed milk powder in TBS-T (Tris-buffered saline, pH 7.9, containing 0.1% (v/v) Tween 20), and then incubated overnight at 4°C with either anti-PIP1 (AS09487; Agrisera, Vännäs, Sweden) or anti-PIP2 (AS09491) at 1 : 1000 dilution in blocking buffer. Membranes were then washed in TBS-T, incubated with anti-rabbit IgG conjugated to horseradish peroxidase (GE Healthcare; http://www.gelifesciences.com) at 1 : 3000 dilution in TBS-T for 1 h, and washed again in TBS-T with a final wash in TBS. The signal was detected with chemiluminescence substrate (ECL; Pierce, Rockford, IL, USA) according to the manufacturer’s instructions, and imaged with a LAS3000 LumiImager (Fujifilm Corp., Tokyo, Japan).

Immunoassays for quantification were performed as dot-blots (Heinicke et al., 1992). Samples were diluted to a uniform total protein concentration, and 2 μl of each applied to a grid on Hybond C nitrocellulose membrane (GE Healthcare; http://www.gelifesciences.com). One sample was used to create a standard curve, against which the relative signal intensity of other samples was measured. Antibody binding and detection were performed as described for the immunoblots. The dot-blots were imaged with dark and flat frame corrections (Gassmann et al., 2009), with several images taken across a range of exposure times. Densitometry was performed with ImageJ software (Abramoff et al., 2004). Three replicate dot-blots were quantified with each primary antibody, and the relative signal intensities averaged for each sample.

Statistical analysis

Differences between control and treatment groups were analysed by repeated measures ANOVA for time series data (gs, Ψl, tissue water content, water uptake and root growth), and by single-factor ANOVA at selected time-points. Differences among treatments in Lp and PIP expression were analysed by single-factor ANOVA with Tukey’s HSD. Analysis was performed in Microsoft Excel 2003 and sas version 9.1 (SAS Institute Inc., Cary, NC, USA). Differences are reported as significant where < 0.05.

Results

Stomatal conductance and leaf water status were unaffected by PRD

The gs of the PRD plants did not differ significantly from that of the controls at any measurement point during 14 d of treatment (Fig. 1a,c). However, gs began to decrease after c. 24 h in the restricted water supply treatments, in which either less frequent watering allowed soil moisture content to fall below 80% of field capacity each day (PRD-RW), or watering of the soil ceased completely (CRD) (Fig. 1b). After the initial decrease, gs of the PRD-RW plants recovered to control values at c. day 7, while gs of the CRD plants continued to decrease (Fig. 1d).

Figure 1.

Stomatal conductance (gs) of Melaleuca argentea seedlings under partial root-zone drying (PRD) and water stress treatments. (a) gs of control and PRD plants during the first 48 h of treatment. The PRD treatment was applied at midday (time zero, indicated by the arrow), by draining the aquatic portion of the root system. Black bars on the x-axis indicate dark periods. Values are mean ± SE of five plants. (b) gs of control plants, PRD plants with restricted water supply to the wet roots (PRD-RW) and completely droughted plants (CRD) during the first 48 h of treatment. Values are mean ± SE of six plants. (c) Midday gs of the same plants shown in (a), over 14 d of treatment. (d) Midday gs of the same plants shown in (b), over 14 d of treatment.

Even with restricted water supply to the wet root portion (PRD-RW), the Ψl of PRD plants did not differ from that of the controls at any measurement point, with midday Ψl remaining between −0.7 and −1.5 MPa, and predawn Ψl between −0.05 and −0.5 MPa (Fig. 2a,b; only PRD-RW and control treatments shown). Predawn Ψl of the droughted (CRD) plants was significantly lower than that of controls by day 7 (Fig. 2a), at which point soil water content in the CRD treatment had fallen to 38 ± 3% (v/v) of field capacity. However, midday Ψl of the CRD plants was lower than that of controls from 24 h of treatment (Fig. 2b). Leaf water content was also unaffected by PRD-RW, but decreased slightly in the CRD plants and was lower than that in controls by day 14 (Fig. 2c).

Figure 2.

Leaf water status of Melaleuca argentea plants under partial root-zone drying (PRD) with restricted water supply, and drought treatments. (a) Predawn leaf water potential (Ψl) of control plants (closed circles), PRD plants with restricted water supply to the wet roots (PRD-RW; open circles) and completely droughted plants (CRD; squares). (b) Midday Ψl of control, PRD-RW and CRD plants. (c) Leaf water content (LWC) of control, PRD-RW and CRD plants. Values are mean ± SE; = 6 plants for Ψl and = 5 plants for leaf water content. Where error bars are not visible they are smaller than the symbols.

Survival of the dried root portion

The water content of fine roots in the hydrated zone remained the same as the that of control in PRD-RW plants, but was lower than that of the control in CRD plants at day 14 (Fig. 3a). The drying aquatic roots rapidly lost water content relative to the control in both the PRD-RW and CRD treatments (Fig. 3b). However, at least some of the dry roots were able to survive for as long as 4 wk because, when re-wetted after 28 d of PRD-RW, aquatic root water uptake resumed within 24 h, and returned to control values within days (Fig. S3). Water uptake by roots in the soil compartment concurrently began to decline, also returning to control values.

Figure 3.

Fine-root water content in Melaleuca argentea seedlings during partial root-zone drying (PRD). Seedlings had roots divided between an aquatic compartment and a soil compartment. The PRD treatment was applied by draining the aquatic compartment, while the soil compartment remained well watered. (a) Moisture content of the soil roots of control (closed circles), PRD (open circles) and completely droughted (CRD; squares) plants. (b) Moisture content of roots in the aquatic compartment of control, PRD and CRD plants. All values are mean ± SE of four plants. Where error bars are not visible they are smaller than the symbols.

Water uptake and Lp of the hydrated roots increased during PRD

In untreated (control) plants 8 wk after transplanting, the dry mass of fine roots in the aquatic compartment was 32%, and in the soil compartment 67% (SE 2%; = 4) of the total fine-root biomass. However, just before the start of treatments, water uptake via the aquatic roots was 58% (SE 5%; = 17) of the total plant water use. Therefore, draining the aquatic compartments to induce PRD dried one-third of the root system, but amounted to removing nearly two-thirds of the plants’ water supply.

Total water use by PRD plants did not differ significantly from controls at any measurement point during 14 d of treatment (Fig. 4a). During the first 5 h after the aquatic compartments were drained, there was a significant decline in the total water uptake by PRD-RW plants relative to the control, but by 24 h total water uptake by PRD-RW plants had increased and did not differ significantly from that of controls for the remainder of the experiment (Fig. 4b). Water uptake by the CRD plants also declined during the first 5 h of treatment, and continued to fall thereafter (Fig. 4b).

Figure 4.

Water uptake during partial root-zone drying (PRD). (a) Change in the total water uptake by control and PRD Melaleuca argentea plants, relative to pretreatment water uptake. Values are mean ± SE of five plants. (b) Change in total plant water uptake relative to pretreatment water uptake, by control plants, PRD plants with restricted water supply to the remaining wet part of the root system (PRD-RW), and completely droughted plants (CRD). Values are mean ± SE of six plants. (c) Water uptake from the hydrated compartment by control (closed circles) and PRD (open circles) plants, relative to pretreatment uptake, during the first 48 h of treatment. The PRD treatment was applied at midday (at time zero), and water uptake measured as the change in pot mass during 4–5-h time intervals. Values are plotted at the mid-points of the measurement intervals, and the start and end-points of the intervals are indicated by vertical dashed lines. The black bars on the x-axis indicate dark periods. Values are mean ± SE of four plants.

Water uptake by roots in the hydrated soil compartment of the PRD plants did not differ from that of control plants during the first 5 h of treatment. However, by 24 h, uptake by the hydrated root portion of the PRD plants had increased. By day 2, uptake was on average more than threefold greater than the control plants’ uptake from that compartment (Fig. 4c). As the roots in the soil compartment of control plants were on average responsible for 42% of the total plant water uptake, the trebling of their uptake rate under PRD enabled the total water uptake of the PRD plants to remain at control values.

Midday Lp did not differ significantly between roots growing in the aquatic and soil compartments in plants that did not experience drying (control treatment; Fig. 5). After 24 h of partial drying, the Lp of the wet soil roots was on average more than threefold that of the control, the same magnitude of change as seen in the rates of water uptake by those roots. However, after 48 h of partial drying, Lp of the wet soil roots had returned to control values. The Lp of the dried aquatic roots of PRD plants was not significantly affected by the treatment (Fig. 5). Xylem sap osmolarity was also unchanged by the PRD treatment (Table 2), so the radial osmotic gradient across the roots (between the external root medium and the xylem) is unlikely to have differed between treatments.

Figure 5.

Hydraulic conductance (Lp) of the watered root portion during partial root-zone drying (PRD). Melaleuca argentea seedlings had roots divided between two pot compartments, an aquatic and a soil compartment. The PRD treatment was applied by draining the aquatic compartment, while the soil compartment remained well watered. Aquatic roots, open bars; soil roots, stippled bars. Values are mean ± SE of four plants. Different letters indicate a significant difference (< 0.05).

Table 2.   Osmolarity (mOsmol kg−1) of xylem fluid collected at midday from stems of partial root-zone dried (PRD) and control Melaleuca argentea plants
TreatmentTime after treatment
24 h48 h96 h
  1. Values are mean ± SE of five plants.

PRD4.8 ± 0.54.6 ± 0.43.4 ± 0.3
Control4.6 ± 0.84.4 ± 0.55.0 ± 0.9

Growth of fine roots in the hydrated root portion

The rate of root length increase in the wet soil compartments as measured at transparent pot windows is shown in Fig. 6. The difference between control and PRD root growth was not significant; however, there was an overall trend in the data. Root growth rate in the wet compartments of PRD plants was slightly higher during the first 2 d of treatment, and then returned to the growth rate seen in control plants.

Figure 6.

Fine-root growth rate in the watered root portion during partial root-zone drying (PRD). Root growth rates were measured in control (closed circles) and PRD (open circles) Melaleuca argentea plants, at windows in the pot walls. Values are mean ± SE of four plants.

Abundance of PIP1 and PIP2 aquaporins in the hydrated roots was altered by PRD

The relative abundance of aquaporin proteins in fine-root samples was determined by immunoassays using antibodies generated against conserved regions of Arabidopsis thaliana PIP1 or PIP2 sequences. Binding of the antibodies to M. argentea total protein samples was highly specific (Fig. S4). After 24 h of PRD, there was a small but significant increase in the total PIP1 pool in the hydrated root portion, compared with the control treatment (Fig. 7a). By 48 h of partial drying, PIP1 content in the hydrated roots was more variable, and was not significantly different from either the control or the 24-h value. Total PIP2 content did not differ from that of the control at 24 h of PRD treatment, but at 48 h of PRD PIP2 levels had declined relative to 24 h (Fig. 7b).

Figure 7.

Abundance of plasma membrane intrinsic protein (PIP) aquaporins in the watered root portion during partial root-zone drying (PRD) in Melaleuca argentea. The relative abundances of (a) PIP1 and (b) PIP2 aquaporin subfamily proteins in untreated (control) plants and plants subjected to 24 and 48 h of PRD were determined. Values are mean ± SE of four plants. Different letters indicate a significant difference (< 0.05) within each panel.

Discussion

Adaptive responses to heterogeneous water supply

Melaleuca argentea responded to PRD with a rapid increase in water uptake by the remaining hydrated roots, which allowed leaf function and water status to remain unchanged. This manner of responding to PRD events probably represents an adaptation to the highly variable environments in which M. argentea is found. We used a well-watered, sandy substrate in this study to minimize the influence of changes in soil conductance that occurs with soil drying, particularly in clays (Nobel & Cui, 1992). Our experimental soil conditions are also consistent with those in which M. argentea naturally occurs. However, the lower gs observed when water supply was restricted to the wet root portion of M. argentea clearly shows how the wet roots were unable to compensate below a certain threshold of soil water availability. Many agricultural PRD experiments use deficit rates of irrigation (below the evapotranspiration rate of untreated plants) and have in fact measured ‘drought’ response, making it difficult to determine to what extent the plants are responding to hydraulic or nonhydraulic signals. It is likely that more species than currently recognized would in fact fully compensate for PRD events if soil conductance and water content around the wet root portion were not limiting.

In cases where leaf responses to PRD are not observed, the plant is clearly displaying root-level compensatory adjustments. Drying a portion of the roots of M. argentea seedlings induced an increased Lp in the remaining hydrated roots within 24 h, which may have mediated the rapid increase in the rate of water uptake by those roots. An increase in fine-root growth in the wet soil compartment may have provided longer term compensation. Observations of root growth in other species during PRD have yielded variable findings; however, increased wet-zone root growth under PRD was observed in Quercus robur saplings and Prunus persica trees, along with little or no gs response (Fort et al., 1997; Goldhammer et al., 2002; Abrisqueta et al., 2008). PRD also weakly stimulated root growth in Citrus aurantium, corresponding with a mild stomatal response (Zekri & Parsons, 1990; Mingo et al., 2004). Increased root Lp and fine-root growth in wet zones may be important means by which species adapted to soil moisture heterogeneity compensate for partial drying events.

Production of a signal for stomatal closure by the dried roots (probably ABA, or changes in xylem sap pH that then induce changes in ABA concentrations or compartmental distribution within leaf cells) appears to be the primary PRD response in some plants. Observations of stomatal re-opening after severing of dried roots demonstrated that in those cases, the wet roots were capable of compensating, but were prevented from doing so through signalling of stomatal closure (Gowing et al., 1990; Croker et al., 1998; Liu et al., 2001). An increase in ABA is frequently, although not always, detectable in the drying roots, xylem sap and leaves during stomatal responses to PRD (e.g. Neales et al., 1989; Liu et al., 2001). By contrast, in PRD studies where no stomatal response was detected, ABA content did not change, suggesting that in some cases the signal is simply not produced by the drying roots (Gallardo et al., 1994; Fort et al., 1997, 1998; Wakrim et al., 2005). Consequently, while we did not measure ABA, we argue that it is likely that, in M. argentea seedlings under PRD, the dry roots did not generate any changes in ABA. Species in which the absence of stomatal PRD responses has been observed still produce an ABA efflux from roots under drought, so do not lack the ability to produce or respond to ABA (Gallardo et al., 1994; Fort et al., 1997, 1998). Rather, it seems that, in some species, the extent of root ABA signalling is dependent on the integrated soil moisture across the whole root system, or on interactions with shoot responses to water stress. The response of a species to soil moisture heterogeneity probably has implications for the way in which it responds to water stress.

The variations in PRD response that have been observed among plants may in part be attributable to adaptations to soil moisture heterogeneity. The numerous crop species with strong leaf-level PRD responses may have adapted to relatively homogeneous soil conditions during their domestication. In homogeneous soil environments, localized soil drying might indicate imminent drought, making root signalling to conserve soil water a useful adaptation. Stomatal conductance and growth of cultivated lettuce (Lactuca sativa) declined when upper soil layers were dried, while a wild lettuce (L. serriola) native to spatially heterogeneous soil habitats did not show any leaf-level responses to the treatment (Gallardo et al., 1996). It might also be expected that large plants would encounter greater heterogeneity simply as a result of the scale of their root systems, and indeed many of the species reported to respond weakly or not at all to PRD are trees. Croker et al. (1998) observed a weak, negative correlation between PRD stomatal response and drought sensitivity in six temperate tree species. Drier environments tend to be more heterogeneous in a range of soil properties (Jackson & Caldwell, 1993; North & Nobel, 2000), so adaptation to soil moisture heterogeneity might account for the weaker PRD stomatal responses in some drought-tolerant species. Contrary to our hypothesis, Salix dasyclados, the only other riparian tree examined under PRD to date, showed a 30–40% reduction in gs during PRD (Liu et al., 2001). However, S. dasyclados is native to regions where waterways are much more consistent than in northern Australia, the region where M. argentea occurs. More data are needed on the PRD responses of species with well-described habitats before any firm conclusions can be made as to the adaptive nature of the responses; however, our study with M. argentea provides a clear example.

Adjustments in root hydraulic conductance during PRD

Our results show that increased water uptake by the hydrated roots during PRD, rather than simply being a passive process, may involve an active change in the Lp of the hydrated roots. Adjustments of root Lp could occur in any species that compensates fully or partly for partial drying events via the hydrated roots. In our study, the hydrated root portion was required to triple its previous water uptake in order to fully compensate for the partial drying. Notably, the measured Lp of these roots was also approximately three times that of the control, suggesting a causal relationship between the increase in water uptake and the increase in Lp. However, the Lp of the hydrated roots then returned to control values after 48 h, while the water uptake rate remained high. Increased fine-root growth during the first few days of PRD may have contributed enough to water uptake capacity to allow the decline in Lp. Root growth in the soil compartment had certainly occurred by 48 h, and although not significantly greater in the PRD plants than in the controls, our measurements were made via transparent windows which do not always accurately reflect total root growth (Glinski et al., 1993). Water use from the hydrated compartment increased within hours of partial drying, but total water uptake was not identical to control values until day 4, suggesting some gradual continuing adjustment, which would be consistent with the growth of new roots.

Changes in aquaporin content were detected in the hydrated root portion, which corresponded with the measured root Lp. PIP1 abundance increased after 24 h of PRD, followed by a decline in PIP2s and possibly also PIP1s by 48 h. The magnitude of change in PIP levels was small, but could still be sufficient to account for the observed changes in Lp. Individual PIP isoforms vary in their location, conductance to water and response to a given treatment, suggesting that each may have a slightly different biological function (Jang et al., 2004; Hachez et al., 2006; Galmés et al., 2007). As the PIP antibodies used in this study will have detected many isoforms, the signal of change from one or a few isoforms will be diluted within the total pool. The identity, location within the root, and the proportion of proteins in ungated states in our study are all unknown, and await further examination. Nonetheless, the small change in total PIP protein levels observed in this study could be physiologically significant, and the correspondence with Lp suggests that changes in PIP levels could be involved in root responses to PRD.

PIP2s generally have high water permeability, while PIP1s often have little or no measurable permeability (Chaumont et al., 2000; Kaldenhoff et al., 2008). However, interaction between PIP1 and PIP2 isoforms in the formation of heterotetramers can dramatically increase total permeability (Fetter et al., 2004; Mahdieh et al., 2008; Vandeleur et al., 2009). A correlation between root Lp and PIP1 levels with no change in PIP2 was observed in bean (Phaseolus vulgaris) and grapevine (Vitis vinifera). ABA application to the roots of bean plants increased both Lp and the PIP1 protein pool (Aroca et al., 2006). In grapevine, VvPIP1;1 is proposed to regulate water transport to match demand diurnally and under water stress via its interaction with VvPIP2;1, which did not change in transcript levels (Vandeleur et al., 2009). A similar mechanism may operate in M. argentea during the response to PRD, with PIP1 and PIP2 isoforms interacting to produce the observed effects on Lp. The difference between the PIP1 and PIP2 subfamilies in the direction and timing of their response hints at the complexity that may exist at the molecular level in root responses to PRD.

In our study, stomata of M. argentea did not close during PRD and there was increased water uptake from the wet roots. Lp and aquaporin adjustments in the wet roots under PRD raise the question of how these responses are signalled. Although ABA can up-regulate Lp and PIP abundance (Mariaux et al., 1998; Hose et al., 2000; Aroca et al., 2006), it is unlikely to be the signalling mechanism in this case as ABA concentrations appear not to change in wet roots under PRD, whether or not stomatal responses occur (e.g. Zhang et al., 1987; Fort et al., 1997, 1998). PIP abundance and gating can be regulated by several other stimuli, including Ca+, pH and pressure pulses (Wan et al., 2004; Luu & Maurel, 2005). Direct hydraulic ‘signalling’ may be the dominant mechanism of controlling water use, particularly in woody plants (Brodribb, 2009). Consequently, the hydraulic pressure effects of shoot demand may act directly to modulate aquaporin levels and Lp in the hydrated roots of M. argentea.

The results presented here indicate a possible role for aquaporins in root water uptake under a sudden increase in transpirational demand. The widely accepted composite transport model (CTM) predicts that, under the strong hydrostatic gradients produced by transpiration, water flow should be largely apoplastic, with aquaporin-mediated flow playing a lesser role (Steudle, 2000). Under the CTM, observations of rapid adjustment of root Lp to match transpirational demand are explained by changes in the apoplastic contribution as a direct result of gradient changes (Steudle & Frensch, 1996; Steudle & Peterson, 1998). However, as Vandeleur et al. (2009) argue, gradient-induced changes in flow pathways cannot account for differences in Lp as measured under consistent gradients and conditions. Transient pressure ramps by HPFM apply the same hydrostatic pressures in each measurement, and the osmotic gradient appears consistent as xylem sap osmolarity and applied nutrient solutions did not differ between PRD and control treatments. HPFM measurements can themselves affect the osmotic gradients within the root, as water is injected into the root xylem, potentially polarizing solutes (Knipfer et al., 2007). However, the consistent Lp observed between successive pressure ramps in our experiment indicates that the degree of polarization caused by measurements was likely to be small, and similar among treatments. If the driving forces are the same, the remaining possibility to explain a difference in Lp is a change in the permeability of the root tissues themselves. The rapid but not immediate response in the rate of root water uptake in our study also argues against an entirely passive mode of compensation, but rather is consistent with a process requiring several hours for its induction. Adjustments in aquaporin quantity or permeability are the only currently known mechanisms by which rapid and reversible changes in tissue permeability can occur (Maurel et al., 2010), and may explain our observations; however, we cannot entirely rule out other explanations.

There are several other lines of evidence suggesting that aquaporins may at times play a larger role in root water uptake than is usually supposed. In some plants, purely apoplastic root radial flow appears not to be possible, requiring water to enter cell-to-cell pathways (Knipfer & Fricke, 2010). Numerous studies using aquaporin inhibitors point to a substantial contribution of aquaporins to total root Lp in a wide range of species and conditions (Javot & Maurel, 2002). Transgenic approaches have also revealed significant aquaporin contributions, with one A. thaliana isoform even appearing to increase hydrostatic root Lp, but not osmotic Lp (Martre et al., 2002; Siefritz et al., 2002; Javot et al., 2003; Postaire et al., 2010). The levels of aquaporin transcripts in roots also peak at midday, corresponding with the time of highest transpiration (Henzler et al., 1999; Beaudette et al., 2007; Vandeleur et al., 2009). Whether aquaporins are in general important when water uptake rates are high is not yet clear, but evidence is growing that they play a significant role.

Conclusions

Plant responses to PRD events vary enormously, as recorded in the literature; some of the variation may be attributable to species adaptations to soil moisture heterogeneity or homogeneity. Species appear to fall onto a continuum in PRD response, from those responding primarily at the leaf level, to those responding mainly via root-level compensation. Some degree of compensatory increase in water uptake from the wet zone during PRD occurs in many species, and our results with M. argentea show that this may involve short-term increases in root Lp, possibly via changes in aquaporin levels. Our findings form part a growing body of evidence that aquaporin-mediated water flow may be significant when root water uptake rates are high.

Acknowledgements

Funding for this project was provided by the Australian Research Council in collaboration with Rio Tinto Iron Pty Ltd (LP0776626). The assistance of the Ecosystems Research Group in glasshouse and laboratory work is gratefully acknowledged. We also thank three anonymous reviewers for their useful comments on an earlier version of the manuscript.

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