Journal of Experimental Botany, Vol. 51, No. 350, pp. 1627-1634,
September 2000
© 2000 Oxford University Press
Hormonal changes induced by partial rootzone drying of irrigated grapevine
1 Horticulture Research Unit, CSIRO Plant Industry, PO Box 350, Glen Osmond, SA 5064, Australia
2 Department of Horticulture, Viticulture and Oenology, University of Adelaide, Australia
Received 15 September 1999; Accepted 3 May 2000
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Abstract |
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Partial rootzone drying (PRD) is a new irrigation technique which improves the water use efficiency (by up to 50%) of wine grape production without significant crop reduction. The technique was developed on the basis of knowledge of the mechanisms controlling transpiration and requires that approximately half of the root system is always maintained in a dry or drying state while the remainder of the root system is irrigated. The wetted and dried sides of the root system are alternated on a 10–14 d cycle. Abscisic acid (ABA) concentration in the drying roots increases 10-fold, but ABA concentration in leaves of grapevines under PRD only increased by 60% compared with a fully irrigated control. Stomatal conductance of vines under PRD irrigation was significantly reduced when compared with vines receiving water to the entire root system. Grapevines from which water was withheld from the entire root system, on the other hand, show a similar reduction in stomatal conductance, but leaf ABA increased 5-fold compared with the fully irrigated control. PRD results in increased xylem sap ABA concentration and increased xylem sap pH, both of which are likely to result in a reduction in stomatal conductance. In addition, there was a reduction in zeatin and zeatin-riboside concentrations in roots, shoot tips and buds of 60, 50 and 70%, respectively, and this may contribute to the reduction in shoot growth and intensified apical dominance of vines under PRD irrigation. There is a nocturnal net flux of water from wetter roots to the roots in dry soil and this may assist in the distribution of chemical signals necessary to sustain the PRD effect. It was concluded that a major effect of PRD is the production of chemical signals in drying roots that are transported to the leaves where they bring about a reduction in stomatal conductance.
Key words: Grapevine, hormonal changes, chemical signals, partial rootzone drying.
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Introduction |
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The production of wine grapes is frequently dependent on irrigation and this is particularly true in southern Australia where the climate is often characterized by high potential evaporation and low rainfall during the growing season. This dependence on irrigation and the realization that water resources can no longer sustain continued development (Scales, 1995
) requires that new vineyard development will be become increasingly dependent on improvements in the efficiency of water use. The consequences of supplying water, through irrigation or from rainfall, at a level which is less than optimal will result in the activation of various water deficit response mechanisms in the crop. The plant function most likely to be influenced by water deficit is stomatal conductance and partial stomatal closure can lead to a decrease in transpiration and, possibly, an increase in transpiration efficiency, that is the dry matter produced per unit of water transpired (Bergmann et al., 1982; Davies et al., 1978; Düring et al., 1997; Turner, 1997). The deliberate withholding of irrigation water may, therefore, be a legitimate management strategy to manipulate crop water use and this is embodied in the technique known as regulated deficit irrigation (RDI), where irrigation input is removed or reduced for specific periods during the crop cycle (Alegre et al., 1999; Boland et al., 1993; Goodwin and Macrae, 1990). RDI may also result in changes to specific characteristics such as the size of grape berries (McCarthy, 1997). This is important because the flavour compounds which determine wine quality are located principally in the berry skin and an increase in skin to flesh ratio may improve fruit quality and value. However, for RDI, the required water deficit may be difficult to determine and requires careful monitoring of soil moisture. This is especially true in the case of wine grapes where the deficit must be applied for a relatively short period between fruit set and veraison (McCarthy, 1997). The possibility of stimulating some of these water deficit responses in a more controlled and sustained way with a view to improving water use efficiency has been investigated. This has resulted in the development of an irrigation technique which has been called partial rootzone drying (PRD) (Dry et al., 1996; Loveys et al., 2000). Implementation of PRD requires that an irrigation system is established such that the rootzone can be simultaneously exposed to both wetted and drying soil. The main effects of PRD in grapevine are that water use efficiency is increased, vegetative vigour is reduced while crop yield and berry size are not significantly reduced. The reduction in canopy density can result in better light penetration to the bunch zone and a consequent improvement in grape quality (Dry et al., 1996). The technique is now undergoing extensive commercial trials.
The idea of using PRD as a tool to manipulate water deficit responses in this way had its origin in the observation that root-derived abscisic acid was important in determining grapevine stomatal conductance (Loveys, 1984) and the later demonstration (Gowing et al., 1990) that split-root plants could be used to show that many of the effects of water stress could be explained in terms of the transport of chemical signals from root to shoot without changes in water relations. It was argued (Loveys, 1991) that it should be possible to manipulate vegetative development if, through management of irrigation, both wet and dry root zones could be maintained. The necessary chemical signals would be derived from the dry roots and water supplied from the wet roots would prevent the development of severe water deficits. Experiments with potted and field-grown grapevines showed that both shoot growth and transpiration could be significantly reduced by PRD (Dry et al., 1996; Dry and Loveys, 1999; Loveys et al., 1998). One of the important features of PRD is that the wetted side of the vine is alternated on a 10–14 d cycle. This was found to be necessary because the effects of partial drying could not be sustained for long periods if one part of the root system remained permanently in dry soil while the remaining part was permanently irrigated (Dry et al. 1996). This has been attributed to the transient nature of ABA accumulation in roots in dry soil (Loveys et al., 2000)
In this paper further evidence is presented on the nature of the chemical signals generated during PRD and some of the other responses which result from uneven root drying are described.
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Materials and methods |
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Split-root potted vines
Dormant Vitis vinifera cuttings (cv. Sultana ) 400–500 mm long and 15–20 mm in diameter were selected in winter. Each cutting was sawn longitudinally for 150–200 mm from the base towards the tip with a bandsaw and rooted in a heat-bed (25 °C) inside a cool-room (2 °C). Rooted cuttings were planted such that each half of the cutting base was in a different 12 l plastic pot containing potting mix consisting of sand and composted pine bark (1/2, v/v) together with ferrous sulphate, 0.6 g l-1 and a slow release fertilizer, Osmocote-plus, 1 g l-1. The plants were grown in a temperature-controlled greenhouse with natural light for several months, then transferred to a shade-house. In the winter, prior to experimental use the following summer, the plants were cut back to one- or two-node spurs. Split-root vines (cv. Chardonnay) were produced in the same way but were grown outdoors for two years prior to use in two 75 l pots containing a sand/composted pine bark soil mixture. In experiments with potted vines there were four replicates of each treatment.
Split-root field vines
Cuttings (cv. Cabernet Sauvignon) were prepared as above. After one season's growth in 12 l pots the cuttings were planted into the field. The soil had been prepared by burying a plastic membrane vertically to a depth of 1.5 m. The divided root system was arranged so that half was on either side of the plastic membrane. Soil moisture on either side of the membrane was monitored with capacitance sensors (Sentek Environmental Solutions, Adelaide) at depths of 100, 200, 300, 400, 500, 600, 700, and 1000 mm. The vines were planted in two adjacent rows, each with four, three-vine panels. Treatments were replicated four times and the centre vine of each panel was measured.
Irrigation
In pot experiments each pot was irrigated with a single drip emitter, with two or three irrigations per day to maintain soil water close to field capacity. In field experiments water was applied with 2 l h-1 drip emitters, two per vine, positioned 300 mm on either side of the vine trunk, Water volumes applied to the field vines were monitored with flow meters placed in each irrigation line.
Moisture release curve
A moisture release curve for the pine bark potting mix was prepared according to a method described previously (Graecen, 1989).
Stomatal conductance and leaf water potential (
L)
Stomatal conductance of leaves was determined using a portable porometer (Delta-T AP4, Delta-T Devices, Cambridge, UK). The terminal part of the main leaf lobe was placed into the cup on the head unit which was positioned normal to the sun. Measurements were conducted during cloudless periods on exposed leaves between 10.00 h and 14.00 h. The device was calibrated before use on every occasion using the supplied calibration plate; if the measurement period was longer than 1.5 h, the device was recalibrated. L was measured with a pressure bomb on single leaves from each replicate plant at each sample interval.
Movement of deuterium-enriched water
Water enriched with the stable isotope deuterium was applied to one side of split-root 2-year-old potted Vitis vinifera cv. Cabernet Sauvignon vines. The vines had two shoots with 7–8 leaves per shoot. The volume of each pot was 4.5 l. Deuterium-enriched water (250 ml, 
2H=964.5
) was applied to one side of the treated vines whilst the control vines received 250 ml of tap water (2H=-18). The irrigated pots were then covered with foil to prevent evaporation of the water. Roots from the pot not receiving the labelled water were harvested on days 1, 4, 7, 10, and 12 of two vines per treatment. Soil moisture was measured by time domain reflectometry (TDR) using a Trase® instrument in one nonirrigated pot. Roots were collected prior to sunrise, when stomata were still closed. During root sorting, the roots were kept in plastic bags to prevent water evaporation. Immediately after weighing the roots were transferred to 250 ml round-bottom flasks, immersed in kerosene and sealed with plastic corks. An azeotropic distillation was used to extract the water. Analyses for 2H were performed by reducing 25 µl of sample to H2 over uranium at 800 °C and further analysed using a PD2 (Europa Scientific Ltd.) GEO 20–20 stable isotope gas ratio mass spectrometer. Isotopic concentrations were expressed as delta values (2H) in per mill () relative to V-SMOW (Vienna Standard Mean Ocean Water). A positive value of x indicates the sample is isotopically ‘heavier’ relative to the standard whilst a negative value indicates a depletion in the heavy isotope relative to the standard.
Benzyladenine treatment
Chardonnay split-root vines growing in 75 l pots were irrigated on one side only (PRD) or on both sides (control). The side of the PRD vines which was irrigated was switched every 14 d. There were four control vines, four PRD vines sprayed with water and four PRD vines sprayed with benzyladenine (BA, 20 mg l-1, Abbott Australasia). The BA sprays were applied at approximately 6 d intervals over a 36 d period.
Xylem sap ABA determination
Xylem sap was expressed from grapevine canes by inserting them into a pressure bomb and holding the applied pressure close to the tissue water potential for about 20 s (Loveys, 1984). The expressed liquid was stored on ice in pre-weighed sample tubes before freezing at -40 °C. Sap from 3 or 4 canes was bulked for each sample. One sample was collected at each sample time from each of the four replicate vines for the control and PRD treatment. The sap samples were thawed and an internal standard of [2H6]ABA (20 ng) was added before they were dried, methylated with ethereal diazomethane, dried and redissolved in methanol for analysis by gc/ms.
Abscisic acid and cytokinin determinations
Roots from 3-year-old field-grown Vitis vinifera cv. Cabernet Sauvignon split-root vines were used to determine the relationship between soil drying and concentration of ABA and some cytokinins in roots. The treatments were: (a) one side allowed to dry while the other side was irrigated every third day (PRD) and (b) both sides irrigated every third day (‘control’). The total amount of water applied to these vines was the same for both treatments since the drip emitters on the PRD vines had twice the delivery rate as those applied to the control vines. Soil cores were collected from both sides of control vines and separately from the dried and wet side of PRD treated vines using a 700 mm steel tube (diameter 55 mm). The soil samples were stored on ice during transport. The roots were sorted from the soil cores in the laboratory under low light conditions and stored at -40 °C until required.
Tissue extraction
To the frozen root sample (0.3–1.5 g) 2 ml of cold, modified Bieleski fixative 1 (Bf1) (60 : 20 : 15 : 5 by vol.; CH3OH : H2O : CHCl3 : HCOOH) (Bieleski, 1964; Emery et al., 1998a) was added and ground into a slurry. The internal standard mixture, containing 90 ng [2H6]ABA and 25 ng of some cytokinins namely [2H5] Z, [2H5][9R] Z was added (all standards: Apex Organics Devon, UK). Additional Bf1 solution was added to bring the volume up to a solvent to sample ratio of 10 : 1, v : v. Samples were thoroughly vortexed, sonicated for 1 min and centrifuged for 10 min at 4 °C and 5000 g. The pellet was re-extracted twice using Bieleski fixative 2 (60 : 35 : 5 by vol., CH3OH : H2O : HCOOH (Emery et al., 1998a)), vortexed, sonicated and centrifuged (10 min, at 4 °C and 5000 g) saving the supernatant. The combined supernatants were dried in a rotary film evaporator to 1 ml at 38 °C. Flasks were rinsed with 4 ml 0.1 M HOAc. After a freeze–thaw cycle (-20 °C) the samples were centrifuged at 5000 g and 4 °C for 10 min.
To separate ABA from cytokinins cation exchange columns (Alltech SCX) were used and preconditioned with 15 ml 0.1 M HOAc. The sample was loaded on the column and washed with 15 ml 1 M HOAc while retaining eluates from the load and wash step for ABA analysis. Cytokinins (containing glucosides and ribosides) were then eluted using 20 ml 2N NH4OH; each fraction was then dried separately in a rotary film evaporator. The sample from the ion exchange step was dissolved in 10% acetonitrile and purified by HPLC on a C18 column using a solvent gradient of acetonitrile and TEAB (Horgan and Kramers, 1979). Retention times of authentic standards were determined and fractions containing zeatin and zeatin riboside were collected and recombined prior to permethylation.
Leaf tissue (approximately 150 mg fresh weight) was extracted in boiling water. An internal standard of [2H6]ABA (50 ng) was added immediately after cooling the water extract. Further purification and quantitative analysis was as described previously (Loveys and van Dijk, 1988).
Gas chromatography
The cytokinin samples were permethylated as described previously (Horgan and Scott, 1987) and further steps of the method were based on a modified procedure (Emery et al., 1998b). To avoid any risk of oxidizing the reagents all steps were carried out under argon. DMSA (methyl sulphinyl carbanion, 0.08 M) was generated using 5 ml DMSO (dimethylsulphoxide, Sigma) and 100 mg freshly weighed potassium tert-butoxide (Sigma) which were stirred thoroughly for 10 min at room temperature. Then the DMSA was centrifuged for 6 min at 15 °C and 2000 g. To Reacti-vials® containing the dried samples 50 µl DMSA were added and stirred. Then 10 µl methyl iodide was added under argon. After 30 min at room temperature the reaction was quenched using 40 µl deionized water. The cytokinins were immediately partitioned three times using 100 µl CHCl3, combined in microfuge tubes and dried under a stream of argon.
The dried sample was redissolved in 150 µl dichloromethane and centrifuged (10 min 8000 g). Finally the extract was made up in 5 µl dichloromethane and 2 µl injected onto the GC-MS column. The GC analysis for permethylated cytokinins was carried out using a Hewlett Packard GC System (HP 6890 Series) with a DB 5MS column (J&W Scientific). The GC was operating in a splitless mode. The oven temperature ramp was 60 °C at the beginning, followed by a fast ramp of 20 °C min-1 to 200 °C and a slow ramp of 5 °C min-1 to 300 °C. This temperature was held for 10 min.
The GC-MS was used in selective ion monitoring (SIM) mode.
The dried samples containing ABA were methylated with fresh ethereal diazomethane. The sample was redissolved in methanol and 1 µl injected onto the GC-MS column. The GC-MS analysis for derivatized ABA was performed using the same instrument and column described above. The GC-MS was operating in a splitless mode. The initial oven temperature was 40 °C, followed by a ramp of 12 °C min-1 to 240 °C.
The ion pairs monitored using SIM mode were 194/190 and 166/162.
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Results |
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When water was withheld from one pot of Sultana vines which had their root systems divided between two pots (wet/dry treatment), stomatal conductance fell to about 20% of that of well-watered controls over the course of about 7 d (Fig. 1a
). Stomatal conductance of vines from which water was withheld from both pots (dry/dry treatment) fell much more rapidly and stomatal closure was essentially complete after 5 d. At this time these plants were rewatered and conductance recovered over the following 20 d. The unirrigated pots of the wet/dry treatment were rewatered on day 14 and stomatal conductance recovered slowly. The first measurements of leaf ABA content were made on day 4 of the experiment and by that time the ABA concentration in the dry/dry treatment had risen 5-fold compared with a fully irrigated control and the wet/dry treatment (Fig. 1b). By the following morning the ABA concentration had risen further. These plants were rewatered at 11.00 h that day and by 16.00 h the leaf ABA content had fallen significantly (P<0.001) and 2 d later the ABA was the same as the well watered control. By contrast, the leaf ABA of the wet/dry treatment increased little when compared to the well-watered control, and it was only on day 8, when stomatal conductance reached its minimum, that the increase in ABA content was significant (P=0.015). Leaf water potential (L) was measured at 10.00 h on day 5. The L of the dry/dry treatment was significantly lower than either the wet/dry or the fully watered treatments (Table 1). At 17.00 h, 6 h after the dry/dry treatment was rewatered, L of this treatment had risen significantly (P<0.0001). There was no difference in L between the wet/dry and fully irrigated treatments at either sample time (P>0.2).
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) Both pots dried, (•) one pot dried. Conductance values which were significantly (P<0.05, ANOVA) less than that of a fully irrigated control are shown with an asterisk. (b) Leaf ABA content, nmol g-1 fresh weight. Standard error bars (n=4) are shown when bigger than the symbol. (
) fully irrigated. Arrows indicate time of rewatering.
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Thus, a highly significant reduction in stomatal conductance occurred in the dry/dry treatment and this was associated with a 5-fold increase in bulk-leaf ABA. By contrast, bulk-leaf ABA changed little in the wet/dry treatment, but stomatal conductance was reduced by up to 80%. One interpretation of this is that in the case of dry/dry treatment bulk-leaf ABA increases as a consequence of both in situ synthesis, stimulated by reduced leaf water potential, and import from the roots, but the small increase in leaf ABA in the wet/dry treatment was due to import from the roots alone. In a number of experiments it has been shown that ABA increases by between 10-fold and 40-fold in drying grape roots (Loveys et al., 2000
). Further evidence to support the idea that changes in leaf ABA are the result of transport from roots was obtained by measuring the ABA content of xylem sap in field-grown Cabernet Sauvignon vines under PRD irrigation. The ABA concentration of xylem sap was significantly (P<0.05) higher in the PRD vines in the samples taken around midday (Fig. 2a). Mean stomatal conductance during this period was significantly (P<0.05) lower in the PRD vines than in the well-watered controls (274 and 354 mmol m-2 s-1, respectively). Consideration of ambient relative humidity and temperature allowed an estimate of leaf transpiration and these data combined with the measured sap ABA content suggested that the flux of ABA to the leaves was 34% higher in the PRD than in the control vines between 11.00 h and midday.
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The pH of the xylem sap of field-grown Cabernet Sauvignon vines was measured throughout the day and was found to be, on average, 0.24 units higher in the PRD treatment (Fig. 2b
). There was a significant (P<0.05) rise in sap pH in both control and PRD vines when the samples taken at 08.00 h were compared with samples taken at 20.00 h.
The ABA in the PRD vines may represent a positive root-sourced signal, but the overall effect of partial root drying may depend also on additional changes in other chemical signals. Since cytokinins are known to influence stomatal opening (Bradford, 1983) and are principally root-derived (Letham and Palni, 1983; Beck and Wagner, 1994), concentrations of these compounds were measured in grapevines undergoing PRD. The effects of exogenously applied cytokinin on the PRD response were also determined.
The effect of PRD on the stomatal conductance of potted Chardonnay vines could be completely reversed by the foliar application (20 mg l-1) of benzyladenine (BA) (Fig. 3). The effect was greatest 2–3 d after the leaves were sprayed and the effect declined over the following 4–5 d. A second treatment with BA renewed the effect.
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Both zeatin and zeatin riboside were reduced by the PRD treatment in roots, shoot tips and the three axillary buds immediately subtending the shoot tip of field-grown Cabernet Sauvignon vines (Table 2
). The reduction in cytokinin and the increase in ABA meant that the ABA/cytokinin ratio changed substantially during an irrigation cycle. Figure 4 shows this ratio in the roots of field-grown Cabernet Sauvignon vines in the currently irrigated and currently drying roots as well as the uniformly irrigated control vines. The ratio was high in the currently wet side on day 1 because this represents the end of the previous cycle and is equivalent to the dry side sample at day 10.
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) Irrigated side of PRD vines, (
) drying side of PRD vines, (•) uniformly irrigated vines.One of the requirements of the current hypothesis to explain the effects of root drying on stomatal behaviour is that ABA is moved out of roots in drying soil into the transpiration stream. This may be facilitated if these roots were partially rehydrated by nocturnal movement of water from regions of high water potential. This imported water and its dissolved solutes may then subsequently be transpired. Figure 5b
shows that such water redistribution does take place. Water enriched in deuterium was supplied to one root system of a split-root vine. As the other root system dried, water derived from roots in the drying soil became enriched in deuterium (by up to 120) showing that water movement had occurred from the root system in the wet soil with a high abundance of 2H to roots in the dry soil. There was no appreciable change in isotope ratio in water from the dry roots when tap water was supplied to the other root system. The enrichment did not occur until the volumetric soil water had fallen to between 5% and 6% (Fig. 5a). A moisture release curve for this soil suggested that this represented a soil matric potential slightly lower than -100 kPa, although it was difficult to be precise because the slope of the relationship between volumetric soil water and matric potential changes very rapidly in this region.
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Discussion |
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There are very large changes in the ABA content of grapevine roots which are in contact with drying soil (Loveys et al., 2000
), but these do not necessarily translate to an equivalent change in leaf ABA. In a pot experiment where a comparison was made between partial and complete rootzone drying, partial drying resulted in an 80% reduction in stomatal conductance but leaf ABA increased by only 60% compared with that of fully irrigated plants, whereas when a similar reduction in stomatal conductance occurred in response to drying the entire root system, foliar ABA increased 5-fold. In the case of the partial rootzone drying, changes in foliar ABA may be due to import from the transpiration stream, whereas in situ synthesis may also have contributed to the larger increases in bulk-leaf ABA of the fully droughted plants since it has been shown that excised grape leaves rapidly accumulate ABA as their water potential falls (Loveys and Kriedemenn, 1973; Loveys and Düring, 1984). The excess ABA was not responsible for the incomplete recovery of stomatal function following rewatering, as leaf ABA fell by 50% within a few hours of rewatering and for the remainder of the experiment it was actually lower than that of fully irrigated plants at a time when stomatal conductance was still significantly reduced.
The flux of ABA from root to shoot was increased only slightly (c. 30%) in field-grown Cabernet Sauvignon vines subject to PRD, but it has been shown previously (Loveys, 1984) that significant changes in stomatal conductance are associated with relatively small changes in bulk leaf ABA. The amount of ABA accumulated in leaves may not be so important in determining stomatal aperture as Jia and Zhang have shown that it is the concentration of ABA in the xylem that is the most important determinant of stomatal function (Jia and Zhang, 1999). However, the presence of ABA in the xylem appears to be necessary for the stomatal response to changes in evaporative demand. It has been shown previously for Riesling grape leaves (Loveys, 1991) that when evaporative demand was increased by changing the leaf to air vapour pressure gradient from 1.2 to 3.2 kPa, after a short period of acclimation, transpiration did not change significantly if 0.36 µM (+)ABA was present in the transpired solution. On the other hand, if the transpired solution did not contain ABA then transpiration rate increased significantly in response to the elevated evaporative demand. The elevated concentration of ABA in the xylem of PRD vines may therefore contribute to their greater transpirartion efficiency, allowing them to respond to changes in ambient evaporative demand more effectively. The increase in xylem sap pH associated with the PRD treatment may also have contributed to the regulation of stomatal conductance. The transpiration of Commelina communis leaves was reduced as the pH of the xylem sap increased, and this effect required the presence of ABA in the transpired solution (Wilkinson and Davies, 1997).
The increase in ABA in the drying roots, combined with the availability of water drawn from the wetter roots, may have an impact on root growth as it has been shown that grapevines subject to PRD irrigation show increased root development in the deeper soil layers when compared with fully irrigated controls (Dry et al., 2000) and it is known that ABA can maintain root growth under conditions of low soil moisture which result in an inhibition of shoot growth (Saab et al., 1990; Sharp, 1996). This effect on root growth may be augmented by the reduction in cytokinin concentration observed in the roots of grapevines and in the very large differences in ABA to cytokinin ratios that occur in roots during PRD cycles, since it is known that root growth is inhibited by increased endogenous cytokinin (Auer, 1996; Medford et al., 1989). The movement of water within plants in response to water potential gradients has been described variously as ‘downward siphoning’ (Smith et al., 1997, 1999), ‘hydraulic lift’ (Caldwell and Richards, 1989; Dawson, 1993), ‘inverse hydraulic lift’ (Schulz et al., 1998), and ‘hydraulic redistibution’ (Burgess et al., 1998).
Root-derived cytokinin may also impact on changes in stomatal conductance resulting from PRD treatment since it is possible to reverse the effects of drying roots completely by exogenous application of benzyladenine. The effect was transient, however, requiring repeated applications to sustain the reversal. Following several weeks of repeated BA applications it was noted that the development of lateral shoots on the canes was enhanced, compared with the unsprayed vines. Similar differences have been noted when comparing fully irrigated and PRD vines (Dry et al., 1996; Dry and Loveys, 1999) and it may be reasonable to speculate that this reduced shoot development in PRD vines is due to a reduction in cytokinin availability from the roots. The lowering of zeatin and zeatin riboside concentrations in shoot tips and axillary buds by the treatment provides further support for this conclusion.
It is hoped that investigations such as those described here will allow a better understanding of the physiological basis for the irrigation technique called partial rootzone drying and in so doing enable informed recommendations to be made for the commercial implementation of the technique. For example, the authors are currently investigating whether the technique may be appropriate for other woody horticultural crops and it may be possible to differentiate grapevine, citrus and pome fruit on the basis of differing abilities of these crops to sustain the production of root-derived chemical signals under conditions where part of the root system remains dry for extended periods. This attribute may be due to the intensity of hydraulic redistribution and it will impact directly on the most appropriate irrigation period for each side of the tree. Such a strategy is likely to be more rewarding and cost effective than an empirical approach.
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Acknowledgments |
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We thank the Grape and Wine Research and Development Corporation and the Land and Water Resources Research and Development Corporation for financial support, Sue Maffei for expert technical assistance
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Notes |
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3 To whom correspondence should be addressed. Fax: +61 8 83038601, Email: brian.loveys{at}pi.csiro.au
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References |
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