JXB Advance Access originally published online on September 18, 2006
Journal of Experimental Botany 2006 57(14):3727-3735; doi:10.1093/jxb/erl131
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RESEARCH PAPER |
Physiological responses of potato (Solanum tuberosum L.) to partial root-zone drying: ABA signalling, leaf gas exchange, and water use efficiency
1The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Crop Science, Højbakkegaard Allé 13, DK-2630 Taastrup, Denmark
2Department of Agroecology, Danish Institute of Agricultural Sciences, Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
* To whom correspondence should be addressed. E-mail: fl{at}kvl.dk
Received 4 May 2006; Accepted 20 July 2006
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Abstract |
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The physiological responses of potato (Solanum tuberosum L. cv. Folva) to partial root-zone drying (PRD) were investigated in potted plants in a greenhouse (GH) and in plants grown in the field under an automatic rain-out-shelter. In the GH, irrigation was applied daily to the whole root system (FI), or to one-half of the root system while the other half was dried, for 9 d. In the field, the plants were drip irrigated either to the whole root system near field capacity (FI) or using 70% water of FI to one side of the roots, and shifted to the other side every 5–10 d (PRD). PRD plants had a similar midday leaf water potential to that of FI, whereas in the GH their root water potential (
r) was significantly lowered after 5 d. Stomatal conductance (gs) was more sensitive to PRD than photosynthesis (A) particularly in the field, leading to greater intrinsic water use efficiency (WUE) (i.e. A/gs) in PRD than in FI plants on several days. In PRD, the xylem sap abscisic acid concentration ([ABA]xylem) increased exponentially with decreasing r; and the relative [ABA]xylem (PRD/FI) increased exponentially as the fraction of transpirable soil water (FTSW) in the drying side decreased. In the field, the leaf area index was slightly less in PRD than in FI treatment, while tuber biomass was similar for the two treatments. Compared with FI, PRD treatment saved 30% water and increased crop water use efficiency (WUE) by 59%. Restrictions on leaf area expansion and gs by PRD-induced ABA signals might have contributed to reduced water use and increased WUE. Key words: Abscisic acid, leaf area, partial root-zone drying, potato, stomatal conductance, water use efficiency
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Introduction |
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Potato (Solanum tuberosum L.) is one of the most important crops in the world in terms of its use in human food and the starch industry (Fabeiro et al., 2001). Due to its sparse and shallow root system, potato is very sensitive to drought stress (Jefferies, 1993), and tuber yield may be considerably reduced by soil moisture deficits (Porter et al., 1999). Therefore, irrigation is always needed for the production of high-yielding crops (Fabeiro et al., 2001). However, the increasing worldwide shortage of water resources requires the optimization of irrigation management in order to improve water use efficiency (WUE).
Partial root-zone drying (PRD) is a new water-saving irrigation strategy presently being investigated in many countries (Kang and Zhang, 2004). PRD involves irrigating only part of the root zone, leaving the other part to dry to a predetermined level before the next irrigation. PRD allows the induction of the abscisic acid (ABA)-based root-to-shoot chemical signalling system to regulate growth and water use and thereby increase WUE (Davies et al., 2002). PRD has been shown to be successful in grapevines (Stoll et al., 2000) and in other fruit trees (Kang et al., 2002), and is claimed to be promising also for field crops (Kang et al., 1998, 2000; Kirda et al., 2005) and vegetables (Kirda et al., 2004; Dorji et al. 2005; Zegbe et al., 2006). Most recently, a study by Saeed et al. (2005) showed that PRD could also modify shoot growth and increase WUE in potatoes. However, the physiological basis for improving WUE in potatoes under PRD remains unknown.
It is well known that the PRD technique was developed based on the hypotheses that roots in the dry soil column sense soil drying and induce ABA-mediated signal transduction pathways to reduce leaf expansion and stomatal conductance; simultaneously, the wet soil column can provide sufficient water to the plants and maintain a high water status of the shoot (Davies and Zhang, 1991; Kang and Zhang, 2004). ABA-induced partial stomatal closure and reduced leaf area have been considered to be the main physiological causes for saving water in plants under PRD treatment (Davies et al., 2002). Our recent studies in potatoes imposing progressive soil drying have shown that root-sourced ABA reduces stomatal conductance at mild soil water deficits and results in an increased intrinsic WUE (Liu et al., 2005). Based on the aforementioned studies, it is plausible to suggest that ABA signalling and its subsequent effects on stomatal conductance and leaf expansion may play a central role in improving WUE in PRD-treated potatoes.
In the present study, experiments in a greenhouse (GH) and in the field under an automatic rain-out-shelter were done to explore the physiological basis for improving WUE in potato under PRD irrigation. The aim of the GH experiment was to test the basic hypothesis of PRD that drying part of the root system could induce ABA signalling in the absence of changing leaf water status. Thus, the plants with a split-root system were subjected to soil drying in only one half of the root system while the other half was fully irrigated, and the irrigation was not shifted between sides. However, alternating irrigation between different parts of the root system is essential for sustaining a continuous ABA-based chemical signal to regulate shoot physiology (Davies and Hartung, 2004), therefore, in the field study during the long period of PRD treatment, the irrigation was shifted from one side of the plants to the other every 5–10 d in order to achieve a long-term effect of PRD on ABA signalling, leaf gas exchange, and WUE in potatoes.
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Plant material and growing conditions
In the GH, potato tubers (S. tuberosum L. cv. Folva, supplied by seed potato company Solanum Nord, Brørup, Denmark) were planted in pots (17 cm diameter and 50 cm deep). The pots were filled with 13.3 kg of sandy soil to 47.5 cm height with a bulk density of 1.27 g dry weight cm–3. The soil had a volumetric soil water content (
) of 17.5% at full pot-holding capacity (PHC) and 4.0% at permanent wilting point (PWP). The pots were evenly separated into two compartments with plastic sheets such that water exchange between the two compartments was prevented. A piece of plastic [width (5 cm)xheight (14 cm)] was removed from the middle of the sheet where the seed tubers were planted. Before planting, the seed potatoes were exposed to 12–14 °C with constant dim overhead light for sprouting. During the germination phase, only one sprout was allowed to emerge. The roots from this sprout were evenly distributed between the two separated compartments. TDR (time domain reflectometer; TRASE, Soil Moisture Equipment Corp., USA) probes (33 cm in length) were installed in the middle of each soil column to monitor the average soil water content in the compartments. The climate conditions in the GH were: 20/14±2 °C day/night air temperature, 15 h photoperiod and >500 µmol m–2 s–1 photosynthetic active radiation supplied by sunlight plus metal-halide lamps. After emergence, all plants were well watered daily to 95% PHC with nutrient solution (Pioneer NPK Macro 14-3-23 + Mg combined with Pioneer Micro; pH=5.5; EC=1.3).
The field experiment was carried out in 2005 in South Jutland, Denmark at the Government research station, Jyndevad (latitude 54°9' N, longitude 9°13' E). To avoid rain, the plants were grown under an automatic rain-out-shelter. The soil is coarse textured meltwater sand. The soil has a of 16.1% at field capacity (FC) and a of 6.3% at PWP. Fertilizers were applied at rates of 150 N, 30 P, 220 K, 30 Mg, 200 S kg ha–1 8 cm below and 12.5 cm next to the row where the seed tubers were later placed. Potato seed tubers (cv. Folva) were treated with Monceren against fungal diseases and pre-heated at 14–16 °C until 1 mm sprouts had emerged. The tubers were planted on 12 April in rows (75 cm between rows and 25 cm between seeds). Seed tubers were ridged with 20 cm of soil in prepared furrows.
Irrigation treatments
In the GH, plants were kept well watered at a of 17.5% during the first 5 weeks after planting. Thereafter, plants were subjected to two irrigation treatments: (i) full irrigation (FI) in which both soil compartments were watered daily at 09.00 h to 17.5% to compensate the full water loss during the previous day; and (ii) partial root drying (PRD) in which half of the root system was watered to 17.5% while the other was allowed to dry until the of the dry soil had decreased to 7%. The treatments lasted for 9 d. In the field, the plots (eight plots, each of 5.25x4 m) were encircled by a protection area of 1 m width. The field trial was a complete randomized design with two subsurface drip irrigation treatments (FI and PRD) and with four replicates from tuber bulking to tuber maturing stages (i.e. 21 June–22 August). The FI treatment was fully irrigated and kept close to FC. The PRD treatment received 70% of the irrigation water volume of FI at each irrigation event and the irrigation was shifted between the two sides of the plants (below denoted as the right and left side) every 5–10 d. The irrigation water use during the treatment period was 201 mm for FI plots and 140 mm for PRD plots. The drip lines used (NETAFIM, Tel Aviv, Israel) had a distance of 25 cm and 50 cm between two emitters in the FI and the PRD treatments, respectively. For the FI treatment, one drip line was placed in the ridge 10 cm below the ridge top; each emitter was close to the seed potato with 5.33 emitters m–2. For the PRD treatment, two drip lines were placed in parallel in the top of the ridge positioned similarly to the FI treatment except that each emitter was placed midway between two seed potatoes, i.e. 12.5 cm from each seed potato. Before each irrigation event, was measured by TDR probes (77 cm in length). Three TDR groups were installed vertically in the top of the ridge 12.5 cm left, 12.5 cm right, and close to the plants. Daily average in the root zone (60 cm in depth determined from the flat soil surface when there was no ridges) of FI and PRD plants was calculated as the TDR-measured before each irrigation event plus half of the increment by irrigation water input.
Gas exchange, plant water potentials, xylem sap collection, and ABA determinations
After the onset of treatments, in both the GH and the field, net photosynthetic rate (A) and stomatal conductance (gs) were measured daily on the second fully expanded upper canopy leaflets (three replicates in the GH and 12 replicates in the field) from 11.00 h to 13.00 h at an ambient CO2 concentration of 
380 µl l–1 with a LI-6200 portable photosynthesis system (Li-Cor Inc., Lincoln, NE, USA). Leaf water potential (l) was measured after gas exchange measurements with a pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, CA, USA) on the same leaves from 12.00 h to 14.00 h on eight out of nine occasions and 20 out of 57 occasions in the GH and in the field, respectively. The daily values of vapour pressure deficit (VPD) during gas exchange measurements in both experiments are shown in Fig. 1.
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In the GH, plants were harvested every 2 d. At each harvest, xylem sap was collected by pressurizing the roots of the potted plant in a Scholander-type pressure chamber. The entire pot was sealed into the pressure chamber and the shoot was detopped at 15–20 cm from the stem base. With the stem stump protruding outside the chamber, pressure was applied until the root water potential (
r) was equalized. The cut surface was cleaned with pure water and dried with blotting paper. It is known that the concentration of ABA in the sap could be affected by the balancing pressure applied (Jokhan et al., 1996). Thus, the pressure was increased gradually until it equalled l of the plant, in order to obtain a sap flow rate similar to the transpiration rate of the plant. However, as the hydraulic resistance of the shoot was removed, the flow rate could be higher than the actual transpiration rate, and thus the ABA concentration in the sap might have been underestimated. A 0.5–1.0 ml aliquot of sap was collected using a pipette from the cutting surface into an Eppendorf vial wrapped with aluminium foil over 3–5 min in FI plants and 5–7 min in PRD plants. The sap was immediately stored at –80 °C for ABA analysis. In the field, the xylem sap was collected using root pressure according to Bahrun et al. (2002). On days 15, 29, 35, 43, 50, and 56 after the onset of treatment (DAT) (these days are close to the ends of the drying cycles for one side of the roots), three plants per plot were detopped 3–5 cm above the soil surface at 22.00 h, the cutting surface was immediately wiped with absorbent tissue to remove contaminants, then silicon tubing was fitted to the stems. The tubes were sealed with parafilm and wrapped in aluminium foil to prevent evaporation and protect against dust and insects. Over 2–3 h, 2–3 ml of sap were collected. The sap was stored at –80 °C for ABA analysis.
The concentration of ABA in the xylem ([ABA]xylem) was analysed without further purification by an enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody for ABA (AFRC MAC 252) according to Asch (2000). No cross-reaction of the antibody with other compounds in the xylem sap was detected when tested according to Quarrie et al. (1988).
Leaf area index, plant water use, tuber yield, and WUE
These parameters were determined only in the field experiment. Plant water use during the experimental period was calculated based on the amount of irrigation and TDR soil moisture measurements. At the beginning and during the treatments (1, 15, 30, 48, and 57 DAT), plant leaf area was measured with a leaf area meter (model 3050A; Li-Cor Inc.) and leaf area index was calculated as leaf area per unit ground area. At final harvests (57 DAT), tubers were collected, and the tuber biomass was determined after oven-drying for 48 h at 80 °C. WUE was calculated as the increment of tuber biomass divided by the plant water use during the treatment period.
Data analysis and statistics
To facilitate data comparison between the two experiments, in the PRD drying side in both experiments was converted into the fraction of transpirable soil water (FTSW), which was calculated by using Equation 1 (Sinclair and Ludlow, 1986):
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r. Data were subjected to analysis of variance (ANOVA) procedures (SAS Institute Inc., 1999–2001). Appropriate standard errors of the means (SE) and least significant differences (LSDs) at P=0.05 were calculated. Tukey's studentized range (HSD) test was applied to detect significant effect of treatment. |
Results |
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Soil water dynamics
Changes of
in FI- and PRD-treated plants during the experimental periods are shown in Fig. 2. In the GH at the onset of PRD, of the drying side declined quickly during the first 4 d and became slower thereafter, and was about 7% by the end of the treatment. Although the of the PRD wet side was generally lower than that of the FI pots (which maintained an average above 16%), there was no statistical difference between them. In the field, FI plants were always irrigated at 1–1.5% less than FC (=16.1%) (Fig. 2b); before each shift of irrigation, of the PRD wet side reached that of the FI except for the periods of 20–28 and 35–44 DAT. The differences of between the PRD dry and wet sides were significant during the first three shifts and subsequently were smaller (Fig. 2b).
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Time-courses of shoot physiology and [ABA]xylem
In the GH,
l was basically the same for FI and PRD plants on eight out of nine occasions during the treatment period (Fig. 3a); while in the field, l of PRD plants was significantly lower than that of FI plants on five out of 20 occasions (Fig. 3e). Figure 3b and f show the day-to-day changes of A under FI and PRD treatments. In the GH, only at 5 DAT was A of PRD plants significantly lower than that of FI plants. In the field, A of PRD plants was significantly lower than that of FI plants on six out of 24 occasions, while on other days during the treatment period it was similar to that of FI plants. In the GH, gs of PRD plants was similar to that of FI plants on seven out of nine occasions; only on two occasions (6–7 DAT) was it significantly less than that of FI plants (Fig. 3c). In the field, gs of PRD plants was significantly less than that of FI plants on 10 out of 24 occasions during the treatment period (Fig. 3g). Intrinsic WUE was significantly higher in PRD plants as compared with FI on two out of nine and five out of 24 occasions in the GH and in the field, respectively. On other days, it was similar for the two treatments except for 1–2 cases (Fig. 3d, h). Figure 4 shows the relationship between the intrinsic WUE (A/gs) and gs, and there was a clear tendency of increasing A/gs with decreasing gs.
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In the GH,
r of PRD-treated plants decreased steadily and became significant lower than that of FI plants at 5 DAT (Fig. 5a). [ABA]xylem was 60 pmol ml–1 in FI plants (Fig. 5b). Under PRD, [ABA]xylem increased from 5 DAT, and was 9-fold that of the FI plants by the end of treatment (Fig. 5b). There was a clear exponential relationship between [ABA]xylem and r (r2=0.99, P <0.001) (Fig. 6). In the field, [ABA]xylem of the FI plants was 200 pmol ml–1, which was significantly lower than that of PRD plants on the first three out of six occasions (i.e. at 15, 29, and 35 DAT) (Fig. 7). When the [ABA]xylem in the PRD plants was expressed relative to that of FI, a common relationship between relative [ABA]xylem and FTSW of the two experiments was observed, being that relative [ABA]xylem increased exponentially (r2=0.98, P <0.001) with decreasing FTSW (Fig. 8).
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Leaf area index, biomass production, plant water use, and WUE
These parameters were determined only in the field experiment. During the PRD treatment period, the leaf area index increased from about 3.1 to 4.2, and then decreased to 3.2 at the last harvest. The leaf area index of the PRD treatment tended to be lower than that of the FI treatment until the fourth harvest, while it was slightly higher than that of the FI treatment in the last harvest (Fig. 9). Tuber biomass was recorded in the last harvest, and no significant differences were found between the treatments (Table 1). Plant water use of PRD plants during the experimental period was 30% less than that of FI plants (Table 1). WUE, calculated based on tuber biomass production and plant water use during the experimental period, was 58.7% greater in PRD than in FI plants (Table 1).
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Discussion |
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In the GH,
of the wet side of the PRD plants was kept close to PHC, and l of the plants was similar to that of FI plants (Figs 2a, 3a). This is in line with the basic assumption of PRD that l could be largely maintained (Sobeih et al., 2004). However, a decrease of l in the PRD plants was found in the field, indicating that soil water stress around the root system was severe enough to cause a reduction in l (Figs 2b, 3e). It is notable that a low l was often observed during the days when the wet side of the root system had a low , and which was not significantly higher than that of the drying side (Fig. 2b). Therefore, keeping a high of the wet side in PRD may be essential to maintain a high l of the plants. This is supported by recent findings of Zegbe et al. (2004, 2006), who demonstrated that the amount of water given to the wet side was important in maintaining a high l in PRD-treated processing tomatoes. In the present study, in order to maintain of the PRD wet side close to that of FI, in the GH 83% water used for FI was applied to the PRD plants; while in the field, 70% of water used for FI was applied to the PRD plants and of the wet side was significantly lower than that of the FI treatment on many days during the treatment. A similar pattern of soil water dynamics under PRD irrigation has been reported by other authors (Kirda et al., 2004, 2005). The causes for this are not clear; therefore, in future studies with PRD irrigation, more attention should be paid to soil physical properties and soil water distribution in the root zone. Moreover, Mingo et al. (2004) showed that of the PRD wet side could be maintained as high as in the FI when the same amount of water of FI was applied, and l was similar for PRD and FI plants. However, Sobeih et al. (2004) observed that, with half the amount of irrigation, the of the PRD wet side was still kept close to that of FI and the plants could maintain the same l as that of FI plants. The reasons for this discrepancy are not known. Putative genotypic differences in the mechanisms of controlling plant water use and leaf water status (e.g. isohydric versus anisohydric behaviour) and differences in experimental conditions may be involved.
In the GH, although l was largely maintained, r of the plants decreased steadily with decreasing of the drying side in PRD treatment (Fig. 5a). A decrease of predawn l (which may have a similar value to r) while maintaining midday l has also been observed in PRD-treated cotton (Gossypium hirsutum) plants (Tang et al., 2005). These results indicate that root hydraulic status had been affected in PRD-treated plants. By decreasing r, the plants may be able to maintain a substantial water potential gradient between the roots and the soil, so that the rates of water uptake might be largely sustained in response to partial soil drying. Besides, for both experiments, it is noted that, compared with l, gs seemed to be more sensitive to PRD. This was particularly clear in the field experiment, where gs was significantly lowered in 10 out of 24 instances during the PRD treatment period (Fig. 3g). It is proposed that lowered l may have acted as a hydraulic signal partly closing stomata; however, chemical signals, mainly ABA, produced in the root tips of the PRD plants are most likely to have caused stomatal closure on days with similar l in the two treatments. This seems to be true as in both experiments an increase of [ABA]xylem in PRD plants was observed, even though on only three out of six sampling occasions in the field was the increase significant (Figs 5b, 7). In agreement with this, in a previous study with the same cultivar of potato, we found that when r >–0.3 MPa, gs was largely controlled by [ABA]xylem (Liu et al., 2005). Furthermore, it is shown here for the first time that [ABA]xylem increased exponentially with decreasing r in PRD plants in the GH (Fig. 6). A linear relationship between the two parameters for potatoes grown under progressive soil drying was reported previously (Liu et al., 2005). Such kinds of relationships are important in developing mechanistic models to predict ABA production and its influence on gs in plants under different irrigation regimes. However, it should be borne in mind that the relationship between r and [ABA]xylem was obtained based on the data from the GH experiment in which no alternation of irrigation was done in the PRD treatment. Therefore, it may not be applicable in the field study where irrigation was shifted several times between the two sides of the root system. Nevertheless, according to this relationship, it is considered that the increase of (Fig. 2b) and thus r in the latter stages of the PRD treatments in the field may explain the convergence of [ABA]xylem between the two treatments.
It is noteworthy that [ABA]xylem of FI plants in the field was much higher than that observed in the GH. It is suggested that the method for collecting the xylem sap may be critical. In the field, the sap was collected using natural root pressure from 22.00 h to 24.00 h in the absence of transpiration, the flow rate of the sap was very slow, and the sap may therefore be concentrated, resulting in a relative high [ABA]xylem; whereas in the GH, the sap was collected by pressurizing the roots at a pressure equivalent to l during the day, the flow rate of the sap was much higher than that during night under natural root pressure, so that the sap might have been diluted, leading to a relative low [ABA]xylem. However, when [ABA]xylem of PRD plants was expressed relative to that of FI plants and of the PRD drying side was converted into FTSW, there was a common exponential relationship between the two parameters (Fig. 8). This relationship clearly indicates that ABA production from the root tips under PRD treatment was directly related to the soil water availability in the drying side. Accordingly, the time of alternating of irrigation between the two sides may be defined as when the FTSW of the drying side had decreased to a certain level that may induce a sustainable ABA signal. However, one should be aware that the roots may extract a substantial amount of water from the PRD wet side as shown in the GH, and ABA produced by the dry roots might have been diluted. Therefore, it was considered that [ABA]xylem detected from the base of the stem may be associated much more closely with the integrated soil water deficits in the whole root zone rather than with the soil water status in part of the root system alone.
Moreover, gs is controlled not only by the soil drying-induced ABA signalling, but climate conditions, plant growth stage, and genotypic factors are also involved. In the present study, gs might have been partly regulated by VPD, particularly in the GH. It was clear that the lower the VPD during the day, the smaller the differences of gs were between the treatments (Figs 1, 3c, g). Similar results have been reported by Maurel et al. (2004) in PRD-treated chestnut (Castanea sativa) saplings.
It is obvious that, in both experiments, A was less sensitive to PRD treatment than gs (Fig. 3b, c, f, g). As a consequence of this, intrinsic WUE (i.e. A/gs) was increased under PRD. This was the case as shown in Fig. 3d and h, where intrinsic WUE was significantly higher in PRD plants as compared with FI on two out of nine and five out of 24 occasions in the GH and in the field, respectively. It is well known that, due to the non-linearity relationship between A and gs, partial stomatal closure would lead to an increase in intrinsic WUE (Jones, 1992). We have previously shown a linear increase of intrinsic WUE with decreasing gs in potatoes exposed to progressive soil drying (Liu et al., 2005). In the present study, data from both experiments showed a clear trend of increasing intrinsic WUE when gs decreased (Fig. 4), confirming earlier findings with the same cultivar (Liu et al., 2005). Based on these results, it is believed that PRD-induced ABA signalling reduces gs, resulting in an increase of intrinsic WUE, which may ultimately lead to an increase in crop WUE.
A large body of evidence has shown that reduction in leaf size is the first morphological response to soil water deficits in potatoes (Jefferies and MacKerron, 1989). In the present study, mild soil water deficits developed in the pot and in the field under PRD treatments (Fig. 2a, b), and the leaf area index was slightly less in the PRD treatment during most of the season (Fig. 9). Similarly, Saeed et al. (2005) reported that PRD treatment significantly reduced plant leaf area in potato. As l was maintained on most days during the treatment, it was speculated that soil drying signals including ABA under PRD irrigation might partly account for the reduced leaf area index. A similar explanation for the restriction of leaf expansion in PRD-treated plants has been proposed in grapevine (Stoll et al., 2000), cotton (Tang et al., 2005), and other crops (Kang and Zhang, 2004). A smaller green leaf area will not only reduce crop transpiration, but also reduce crop photosynthetic rate due to less light interception. However, in potatoes, a leaf area index of 3 is sufficient for the interception of 90–95% of the incoming radiation, which can maintain the maximum growth rate (Khurana and McLaren, 1982). In the present study in the field, the average leaf area index during the PRD treatment was 3.3 (Fig. 9), which may significantly reduce crop transpiration but can still maintain crop photosynthesis and growth rate. This may partly explain why PRD treatment could maintain tuber yield (Table 1) even though the irrigation water was reduced by 30% (Table 1). As a result of yield maintenance and reduced plant water use, crop WUE of PRD treatment was 59% higher than that of FI treatment (Table 1).
Collectively, the present results suggest that restrictions on stomatal opening and leaf expansion by PRD-induced ABA signals might have contributed to the reduced water use and improved WUE in potatoes. However, beside these effects, other physiological modifications caused by PRD such as an enhanced root system and an increased ability to take up soil nutrients (e.g. nitrogen) might also play a role in improving WUE (Kirda et al., 2005). Despite this, it can be concluded that PRD is a promising water-saving irrigation strategy for potato production in areas with limited water resources. However, to optimize the PRD irrigation strategy for the crop, more issues should be studied in the future: (i) how can [ABA]xylem be predicted with a partial decrease of in the root zone; (ii) is there a causal relationship between [ABA]xylem and gs under PRD; and (iii) how does PRD affect root growth and soil nutrient availability?
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Acknowledgements |
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FL thanks the Danish Research Council (SJVF, 23-03-0208) for the financial support for his postdoctoral research. This study was also partly supported by the European Commission (WATERWEB, EU, INCO-CT-2004-509163 and SAFIR, EU, FOOD-CT-2005-023168).
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Abbreviations |
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A, net photosynthetic rate; ABA, abscisic acid; DAT, days after treatment; FC, field capacity; FI, full irrigation; FTSW, fraction of transpirable soil water; GH, greenhouse; gs, stomatal conductance; PHC, pot-holding capacity; PRD, partial root-zone drying; PWP, permanent wilting point; TDR, time domain reflectometer; VPD, vapour pressure deficit; WUE, water use efficiency..
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References |
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