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Effects of deficit irrigation (DI) and partial root drying (PRD) on gas exchange, biomass partitioning, and water use efficiency in potato
Scientia Horticulturae 109 (2006) 113–117 www.elsevier.com/locate/scihorti
Effects of deficit irrigation (DI) and partial root drying (PRD) on gas exchange, biomass partitioning, and water use efficiency in potato Fulai Liu a,*, Ali Shahnazari a, Mathias N. Andersen b, Sven-Erik Jacobsen a, Christian R. Jensen a a
The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology, Højbakkegaard Alle´ 13, DK-2630 Taastrup, Denmark b Department of Agroecology, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark Received 9 September 2005; received in revised form 16 March 2006; accepted 3 April 2006
Abstract The effect of ‘partial root drying’ (PRD) compared with other irrigation strategies, viz. full irrigation (FI) and deficit irrigation (DI), on morphological and physiological characteristics of potato (Solanum tuberosum L. cv. Folva) were investigated at tuber initiation stage. Potatoes were grown in pots with roots split equally between two soil columns. In FI, the whole root system was irrigated at 100% of evapotranspiration (ETfull); in DI 50% of ETfull was irrigated to the whole root system; in PRD 50% of ETfull was irrigated to one soil column while the other was allowed to dry, and irrigation was shifted when 80–85% of plant available water in the dry side had been used. Midday leaf water potential was similar for all treatments at 9 days after onset of treatments (DAT), while it was significantly lower in PRD than in FI and DI plants at 21 DAT. Photosynthesis, stomatal conductance, and transpiration were generally greater in FI plants. Compared to FI, both DI and PRD significantly decreased leaf area and biomass. DI decreased biomass allocation to leaves and stems while increasing it into roots and tubers. PRD increased biomass allocation to roots. ET under PRD was ca. 80% of DI and FI until six DAT, thereafter ET of DI and PRD was similar and was significantly less than that of FI. Water use of DI and PRD plants was 37% less than that of FI. Water use efficiency (WUE) and transpiration efficiency were similar for PRD and FI plants, and were significantly less than those of DI plants. Conclusively, given the same amount (50% of ETfull) of irrigation, PRD has no advantages compared to DI in terms of biomass production and WUE in potato at tuber initiation stage. # 2006 Elsevier B.V. All rights reserved. Keywords: Plant growth; Partial root drying; Solanum tuberosum L.; Stomatal conductance; WUE
1. Introduction Potato (Solanum tuberosum L.) is one of the most important crops in the world (Fabeiro et al., 2001). Due to its sparse and shallow root system, potato is very sensitive to water stress and tuber yield may be considerably reduced by soil water deficits (Porter et al., 1999). Therefore, irrigation is always needed for production of high yielding crops (Fabeiro et al., 2001). However, the increasing worldwide shortage of water resources requires optimization of irrigation management in order to improve water use efficiency (WUE). Deficit irrigation (DI) and partial root drying (PRD) are water-saving irrigation strategies (Kang and Zhang, 2004). DI irrigates the entire root zone with an amount of water less than
* Corresponding author. Tel.: +45 35283416; fax: +45 35282175. E-mail address: [email protected] (F. Liu). 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.04.004
the potential evapotranspiration (ETfull) and the minor stress that develops has minimal effects on the yield (English et al., 1990). DI has proved successfully with a number of crops; however it has been difficult to manage in potatoes (Lynch et al., 1995). PRD involves alternate watering to each side of the plant root system, by which it allows the plant to explore rootsourced ABA signaling to regulate plant growth and water use thereby increasing WUE (Dry et al., 2000). PRD has been found to be promising in several crops (Kang and Zhang, 2004). However, until now PRD has not been studied in potatoes. It is suggested that plants under PRD performed better than under DI when the same amount of water was applied. Davies and Hartung (2004) proposed that PRD could stimulate root growth and maintain a constant ABA signaling to regulate shoot physiology; whereas plants under DI, some of the roots in dry soils for long period may die and signaling may diminish and shoot water deficits may occur. Based on this, it is plausible to suggest that PRD maybe promising for potatoes. Therefore, the
114
F. Liu et al. / Scientia Horticulturae 109 (2006) 113–117
objective of this study was to test whether PRD could maintain biomass production and improve plant WUE as compared to full-irrigation (FI) and DI. 2. Materials and methods 2.1. Plant material and growing conditions Potato tubers (S. tuberosum L. cv. Folva) were planted in pots (25.2 cm diameter and 40 cm tall) filled with 24 kg sandy soil with a bulk density of 1.27 g dry weight cm3. Soil water contents (u, vol.%) at full water holding capacity and permanent wilting point were 18% and 3.0%, respectively. The pots were separated diametrically with plastic sheets into equally sized compartments. A piece of plastic was removed from the middle of the sheet where the seed tubers were planted. Fertilizers were applied on the top soil (0–20 cm) at rates of 150 N, 30 P, 220 K, 30 Mg, and 100 S kg/ha. TDR (time domain reflectometer, TRASE, Soil Moisture Equipment Corp., USA) probes (20 cm in length) were installed in the middle of each soil column to monitor the average u in the compartments. Plants were grown in a glasshouse where day/ night air temperature was 20/14 2 8C; photoperiod was 15 h with above 600 mmol m2 s1 photosynthetic active radiation (PAR) supplied by sunlight plus metal-halide lamps. 2.2. Irrigation treatments
Fig. 1. Vapor pressure deficit (VPD) and photosynthetic active radiation (PAR) in the glasshouse during daily gas exchange measurements. DAT denotes days after onset of treatment.
tion efficiency (TE) was calculated as the ratio between A and T, i.e. A/E. 2.4. Evapotranspiration, biomass production, plant leaf area, and WUE Evapotranspiration was calculated based on the TDR measurements. For FI plants, ET (L) at day i was calculated as: ETfull;i ¼ 9:23 ½ð17:5% u1;iþ1 Þ þ ð17:5% u2;iþ1 Þ
(1)
for DI and PRD plants, ET (L) at day i was calculated as: Pots were well watered at u of 17.5 vol.% during the first 4 weeks after planting. At 29 days after planting plants (when the plants had an average leaf area of 1400 cm2 plant1, and leaf, stem, and root biomass of 5.58, 4.80, and 3.32 g plant1, respectively), were subjected to three irrigation treatments: (1) full irrigation (FI) in which both soil compartments were watered daily at 9:00 h to 17.5 vol.% to compensate the full evapotranspiration water loss during the previous day; (2) deficit irrigation in which 50% of ETfull was given daily evenly to both soil compartments; (3) partial root drying in which one compartment was watered daily with 50% of ETfull while the other was allowed to dry; when the available soil water of the dry soil had decreased to 15–20% the irrigation was shifted between the two compartments. Irrigation treatments lasted 22 days and each treatment had eight plants. 2.3. Leaf water potential and gas exchange Midday leaf water potential was measured with a pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, CA, USA) on the second fully expanded upper canopy leaves from 11:00 to 13:00 h at 9 and 21 days after onset of treatments (DAT). Photosynthetic rate (A), stomatal conductance (gs), and transpiration (T) were measured daily on the second fully expanded upper canopy leaflets from 11:00 to 13:00 h at ambient CO2 concentration of ca 380 ml l1 with a LI-6200 portable photosynthesis system (LiCor Inc., Lincoln, NE, USA). The daily values of photosynthetic active radiation and vapour pressure deficit (VPD) are shown in Fig. 1. Transpira-
ETi ¼ 9:23 ½ðu1;i u1;iþ1 Þ þ ðu2;i u2;iþ1 Þ þ 0:5 ETfull;i (2) where 9.23 is the volume of each soil compartment (L), u1,i and u2,i are the soil water content (vol.%) at day i of compartments 1 and 2, respectively, before irrigation. Plant water use during the treatment period was calculated as the sum of the daily ET. At final harvest, plant leaf area was measured with a leaf area meter (model 3050A; Li-Cor Lincoln, NE, USA). Root material was obtained by washing off the soil. Biomass of leaves, stems, roots, and tubers were determined after oven drying for 48 h at 80 8C. WUE was calculated as the plant total biomass production divided by the plant water use during the treatment period. 2.5. Data analysis and statistics Data were subjected to analysis of variance (ANOVA) procedures (SAS Institute, 6.03 ed, 1988). Appropriate standard errors of the means (S.E.) and L.S.D.s at P = 0.05 were calculated. Tukey’s Studentised Range (H.S.D.) test was applied to compare growth parameters of plants, which had experienced different irrigation treatments. 3. Results and discussion 3.1. Soil and plant water status Changes of u in FI, DI, and PRD pots are shown in Fig. 2. The daily mean u of FI plants was about 16%, while in DI u
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Fig. 3. Biomass partitioning among leaf, stem, root, and tuber of potato plants grown under FI, DI, and PRD treatments. Values that are significantly different at P < 0.05 have no letters in common (H.S.D. test).
while PRD plants had a significant lower leaf water potential (0.85 MPa), and which might have been due to severe water stress in the dry side of the treatment. 3.2. Leaf area, biomass production, and partitioning
Fig. 2. Daily soil volumetric water content (u) in two parts of the pots under three irrigation treatments, FI, DI, and PRD before (the lower values) and after irrigation (the upper values). Bars indicate the standard error of the means (S.E.) (n = 8).
decreased steadily over time to an average value of 6.0% at 10 DAT and remained constant thereafter (Fig. 2A). u for PRD depended on wetting and drying cycles (Fig. 2B). In the first cycle u of wetting side was kept above 11% while u of the drying side decreased to an average value of 6.0% at 10 DAT. In later cycles, a mean u of 7–10 and 4–6% was recorded for the wetting and drying side, respectively (Fig. 2B). Failed to maintain a high u of the wet side in PRD irrigating with 50% of ETfull had been observed in several crops (e.g. Kirda et al., 2004; Wakrim et al., 2005). It has been always observed that plants extract much more water from the wet side leading to a steadily declining of u in that side. Despite of declining in u in DI and PRD pots, the midday leaf water potential of those plants was largely maintained at a similar level to FI (0.53 MPa) on 9 DAT. However, on 21 DAT only DI plants had a similar leaf water potential to that of FI (0.73 MPa);
At final harvest, leaf area of PRD and DI was significantly less than that of FI (Table 1). This is in agreement with former observations that leaf area expansion is significantly inhibited in potatoes under soil water deficits (Jefferies and Mackerron, 1989). We found that DI and PRD decreased plant biomass to a similar extent as compared to FI (Table 1); even through the pattern of biomass partitioning among different plant organs was different. In relation to FI, DI decreased biomass allocation to leaves and stems while increasing it into roots and tubers; whereas PRD increased biomass allocation to roots (Fig. 3). Promoting root growth under PRD has been reported in maize (Kang et al., 1998), grapevine (Dry et al., 2000), and tomato (Mingo et al., 2004), and this has been considered as an advantage of PRD irrigation. The proportion of tuber dry weight was significantly higher under DI than under FI and PRD (Fig. 3). We supposed that tuber initiation and bulking might have been restricted to a larger extent by a severe soil water deficit in the dry soil column under PRD than that by a less severe soil water deficit in both soil columns under DI (Fig. 2). 3.3. Gas exchange In general, DI and PRD treatments decreased A, gs, and T although except for several days at which these parameters were
Table 1 Effects of irrigation treatments on plant leaf area, biomass production, water use (cumulative ET), and water use efficiency (WUE) in potato Treatment
Leaf area (cm2 plant1)
Biomass production (g plant1)
Cumulative ET (L plant1)
WUE (g L1)
FI DI PRD
6081a 3513b 3728b
84.07a 57.27b 52.87b
16.77a 10.61b 10.62b
5.01b 5.40a 4.98b
Means with same letters (a and b) within columns are not significantly different using HSD at P < 0.05. FI: full irrigation; DI: deficit irrigation; PRD: partial root drying.
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similar for all the treatments (Fig. 4). Similar effect of DI and PRD on gas exchange has been shown in maize (Kirda et al., 2005). We also noticed that gs and T might be partly regulated by VPD, being that the lower VPD during the day the smaller
Fig. 5. Time course of evapotranspiration (ET) of potato plants under FI, DI, and PRD treatments. Data points are means (n = 8). Vertical bars indicate L.S.D.s (P = 0.05) and asterisks denote significant difference among the three treatments at P < 0.05.
differences of gs and T between treatments (Figs. 1 and 4B, C). Compared to FI plants, reduction in gs occurred when the leaf water potential was not affected in both DI and PRD treatments indicating that root-sourced chemical (probably ABA) signaling may be responsible for gs reductions (Liu et al., 2005). 3.4. Plant water use and water use efficiency ET in PRD was significantly less than in DI and FI at three to five DAT, thereafter ET in DI and PRD was similar and was significantly less than that in FI (Fig. 5). As T of individual leaves was similar between DI and PRD plants, the lower ET in PRD was probably due to reduced soil evaporation during the 1st days after onset of treatments. This is in accordance with the fact that PRD only wetted half of the soil surface, which may reduce soil evaporation. Despite of this, the total plant water use during the experimental period was similar in DI and PRD, and was 37% less than in FI (Table 1). Compared with FI plants, WUE and TE were increased by DI and not by PRD treatment (Table 1; Fig. 4D). This result does not support the hypothesis that WUE would have been improved under PRD irrigation (Kang and Zhang, 2004). The reasons for this are not clear, and merit further studies. 4. Concluding remarks
Fig. 4. Changes in photosynthesis (A) (A), stomatal conductance (gs) (B), transpiration (T) (C), and transpiration efficiency (TE) (D) of potato leaves under FI, DI, and PRD treatments. Data points are means (n = 8). Vertical bars indicate L.S.D.s (P = 0.05) and asterisks denote significant difference among the three treatments at P < 0.05.
Under the conditions of the present study we were able to conclude that, given the same amount (50% of ETfull) of irrigation, PRD has no advantages compared to DI in terms of biomass production and WUE in potato at tuber initiation stage. It seems that PRD treatment defined in this study is not feasible for potato during tuber initiation stages. We are aware of that to achieve the effect of PRD on stomatal conductance and plant water use without diminishing biomass production, several important issues must be addressed in future studies: (1) how much water should be irrigated to the wet soil column? (2) When should irrigation be shifted from wet to dry side? (3) At
F. Liu et al. / Scientia Horticulturae 109 (2006) 113–117
which growth stages should PRD be applied? These studies may contribute the successful application of PRD irrigation in potato as well as in other crops. Acknowledgements Fulai Liu thanks the Danish Research Council (SJVF, 23-030208) for the financial support of 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). References Davies, W.J., Hartung, W., 2004. Has extrapolation from biochemistry to crop functioning worked to sustain plant production under water scarcity? In: Proceeding of the 4th International Crop Science Congress, 26 September– 1 October 2004, Brisbane, Australia. Dry, P.R., Loveys, B.R., During, H., 2000. Partial drying of the rootzone of grape. 2. Changes in the patterns of root development. Vitis 39, 9–12. English, M.J., Musick, J.T., Murty, V.V.N., 1990. Deficit irrigation. In: Hoffman, G.J., Howell, T.A., Solomon, K.H. (Eds.), Management of Farm Irrigation System. American Society of Agricultural Engineers, St. Joseph, pp. 631–663. Fabeiro, C., Martı´n de Santa Olalla, F., de Juan, J.A., 2001. Yield and size of deficit irrigated potatoes. Agric. Water Manage. 48, 255–266. Jefferies, R.A., Mackerron, D.K.L., 1989. Radiation interception and growth of irrigated and drought-stressed potato (Solanum tuberosum). Field Crops Res. 22, 101–112.
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Kang, S.Z., Zhang, J.H., 2004. Controlled alternate partial root-zone irrigation: its physiological consequences and impact on water use efficiency. J. Exp. Bot. 55, 2437–2446. Kang, S.Z., Liang, Z.S., Hu, W., Zhang, J.H., 1998. Water use efficiency of controlled alternate irrigation on root-divided maize plants. Agric. Water Manag. 38, 69–76. Kirda, C., Cetin, M., Dasgan, Y., Topcu, S., Kaman, H., Ekici, B., Derici, M.R., Ozguven, A.I., 2004. Yield response of greenhouse grown tomato to partial root drying and conventional deficit irrigation. Agric. Water Manag. 69, 191–201. Kirda, C., Topcu, S., Kaman, H., Ulger, A.C., Yazici, A., Cetin, M., Derici, M.R., 2005. Grain yield and N-fertilizer recovery of maize under deficit irrigation. Field Crops Res. 93, 132–141. Liu, F., Jensen, C.R., Shahnazari, A., Andersen, M.N., Jacobsen, S.E., 2005. ABA regulated stomatal control and photosynthetic water use efficiency of potato (Solanum tuberosum L.) during progressive soil drying. Plant Sci. 168, 831–836. Lynch, D.R., Foroud, N., Kozub, G.C., Farries, B.C., 1995. The effect of moisture stress at three growth stages on the yield components of yield and processing quality of eight potato cultivars. Am. Potato J. 72, 375–386. Mingo, D.M., Theobald, J.C., Bacon, M.A., Davies, W.J., Dodd, I.C., 2004. Biomass allocation in tomato (Lycopersicon esculentum) plants grown under partial rootzone drying: enhancement of root growth. Funct. Plant Biol. 31, 971–978. Porter, G.A., Opena, G.B., Bradbury, W.B., McBurnie, J.C., Sisson, J.A., 1999. Soil management and supplemental irrigation effects on potato: I. Soil properties, tuber yield, and quality. Agron. J. 91, 416–425. SAS Institute Inc, 1988. SAS/STAT User’s Guide: Release 6.03 ed. SAS Inst., Inc., Cary, NC. Wakrim, R., Wahbi, S., Tahi, H., Aganchich, B., Serraj, R., 2005. Comparative effects of partial root drying (PRD) and regulated deficit irrigation (RDI) on water relations and water use efficiency in common bean (Phaseolus vulgaris L.). Agric. Ecosys. Environ. 106, 275–287.
Effects of deficit irrigation (DI) and partial root drying (PRD) on gas exchange, biomass partitioning, and water use efficiency in potato Fulai Liu a,*, Ali Shahnazari a, Mathias N. Andersen b, Sven-Erik Jacobsen a, Christian R. Jensen a a
The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology, Højbakkegaard Alle´ 13, DK-2630 Taastrup, Denmark b Department of Agroecology, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark Received 9 September 2005; received in revised form 16 March 2006; accepted 3 April 2006
Abstract The effect of ‘partial root drying’ (PRD) compared with other irrigation strategies, viz. full irrigation (FI) and deficit irrigation (DI), on morphological and physiological characteristics of potato (Solanum tuberosum L. cv. Folva) were investigated at tuber initiation stage. Potatoes were grown in pots with roots split equally between two soil columns. In FI, the whole root system was irrigated at 100% of evapotranspiration (ETfull); in DI 50% of ETfull was irrigated to the whole root system; in PRD 50% of ETfull was irrigated to one soil column while the other was allowed to dry, and irrigation was shifted when 80–85% of plant available water in the dry side had been used. Midday leaf water potential was similar for all treatments at 9 days after onset of treatments (DAT), while it was significantly lower in PRD than in FI and DI plants at 21 DAT. Photosynthesis, stomatal conductance, and transpiration were generally greater in FI plants. Compared to FI, both DI and PRD significantly decreased leaf area and biomass. DI decreased biomass allocation to leaves and stems while increasing it into roots and tubers. PRD increased biomass allocation to roots. ET under PRD was ca. 80% of DI and FI until six DAT, thereafter ET of DI and PRD was similar and was significantly less than that of FI. Water use of DI and PRD plants was 37% less than that of FI. Water use efficiency (WUE) and transpiration efficiency were similar for PRD and FI plants, and were significantly less than those of DI plants. Conclusively, given the same amount (50% of ETfull) of irrigation, PRD has no advantages compared to DI in terms of biomass production and WUE in potato at tuber initiation stage. # 2006 Elsevier B.V. All rights reserved. Keywords: Plant growth; Partial root drying; Solanum tuberosum L.; Stomatal conductance; WUE
1. Introduction Potato (Solanum tuberosum L.) is one of the most important crops in the world (Fabeiro et al., 2001). Due to its sparse and shallow root system, potato is very sensitive to water stress and tuber yield may be considerably reduced by soil water deficits (Porter et al., 1999). Therefore, irrigation is always needed for production of high yielding crops (Fabeiro et al., 2001). However, the increasing worldwide shortage of water resources requires optimization of irrigation management in order to improve water use efficiency (WUE). Deficit irrigation (DI) and partial root drying (PRD) are water-saving irrigation strategies (Kang and Zhang, 2004). DI irrigates the entire root zone with an amount of water less than
* Corresponding author. Tel.: +45 35283416; fax: +45 35282175. E-mail address: [email protected] (F. Liu). 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.04.004
the potential evapotranspiration (ETfull) and the minor stress that develops has minimal effects on the yield (English et al., 1990). DI has proved successfully with a number of crops; however it has been difficult to manage in potatoes (Lynch et al., 1995). PRD involves alternate watering to each side of the plant root system, by which it allows the plant to explore rootsourced ABA signaling to regulate plant growth and water use thereby increasing WUE (Dry et al., 2000). PRD has been found to be promising in several crops (Kang and Zhang, 2004). However, until now PRD has not been studied in potatoes. It is suggested that plants under PRD performed better than under DI when the same amount of water was applied. Davies and Hartung (2004) proposed that PRD could stimulate root growth and maintain a constant ABA signaling to regulate shoot physiology; whereas plants under DI, some of the roots in dry soils for long period may die and signaling may diminish and shoot water deficits may occur. Based on this, it is plausible to suggest that PRD maybe promising for potatoes. Therefore, the
114
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objective of this study was to test whether PRD could maintain biomass production and improve plant WUE as compared to full-irrigation (FI) and DI. 2. Materials and methods 2.1. Plant material and growing conditions Potato tubers (S. tuberosum L. cv. Folva) were planted in pots (25.2 cm diameter and 40 cm tall) filled with 24 kg sandy soil with a bulk density of 1.27 g dry weight cm3. Soil water contents (u, vol.%) at full water holding capacity and permanent wilting point were 18% and 3.0%, respectively. The pots were separated diametrically with plastic sheets into equally sized compartments. A piece of plastic was removed from the middle of the sheet where the seed tubers were planted. Fertilizers were applied on the top soil (0–20 cm) at rates of 150 N, 30 P, 220 K, 30 Mg, and 100 S kg/ha. TDR (time domain reflectometer, TRASE, Soil Moisture Equipment Corp., USA) probes (20 cm in length) were installed in the middle of each soil column to monitor the average u in the compartments. Plants were grown in a glasshouse where day/ night air temperature was 20/14 2 8C; photoperiod was 15 h with above 600 mmol m2 s1 photosynthetic active radiation (PAR) supplied by sunlight plus metal-halide lamps. 2.2. Irrigation treatments
Fig. 1. Vapor pressure deficit (VPD) and photosynthetic active radiation (PAR) in the glasshouse during daily gas exchange measurements. DAT denotes days after onset of treatment.
tion efficiency (TE) was calculated as the ratio between A and T, i.e. A/E. 2.4. Evapotranspiration, biomass production, plant leaf area, and WUE Evapotranspiration was calculated based on the TDR measurements. For FI plants, ET (L) at day i was calculated as: ETfull;i ¼ 9:23 ½ð17:5% u1;iþ1 Þ þ ð17:5% u2;iþ1 Þ
(1)
for DI and PRD plants, ET (L) at day i was calculated as: Pots were well watered at u of 17.5 vol.% during the first 4 weeks after planting. At 29 days after planting plants (when the plants had an average leaf area of 1400 cm2 plant1, and leaf, stem, and root biomass of 5.58, 4.80, and 3.32 g plant1, respectively), were subjected to three irrigation treatments: (1) full irrigation (FI) in which both soil compartments were watered daily at 9:00 h to 17.5 vol.% to compensate the full evapotranspiration water loss during the previous day; (2) deficit irrigation in which 50% of ETfull was given daily evenly to both soil compartments; (3) partial root drying in which one compartment was watered daily with 50% of ETfull while the other was allowed to dry; when the available soil water of the dry soil had decreased to 15–20% the irrigation was shifted between the two compartments. Irrigation treatments lasted 22 days and each treatment had eight plants. 2.3. Leaf water potential and gas exchange Midday leaf water potential was measured with a pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, CA, USA) on the second fully expanded upper canopy leaves from 11:00 to 13:00 h at 9 and 21 days after onset of treatments (DAT). Photosynthetic rate (A), stomatal conductance (gs), and transpiration (T) were measured daily on the second fully expanded upper canopy leaflets from 11:00 to 13:00 h at ambient CO2 concentration of ca 380 ml l1 with a LI-6200 portable photosynthesis system (LiCor Inc., Lincoln, NE, USA). The daily values of photosynthetic active radiation and vapour pressure deficit (VPD) are shown in Fig. 1. Transpira-
ETi ¼ 9:23 ½ðu1;i u1;iþ1 Þ þ ðu2;i u2;iþ1 Þ þ 0:5 ETfull;i (2) where 9.23 is the volume of each soil compartment (L), u1,i and u2,i are the soil water content (vol.%) at day i of compartments 1 and 2, respectively, before irrigation. Plant water use during the treatment period was calculated as the sum of the daily ET. At final harvest, plant leaf area was measured with a leaf area meter (model 3050A; Li-Cor Lincoln, NE, USA). Root material was obtained by washing off the soil. Biomass of leaves, stems, roots, and tubers were determined after oven drying for 48 h at 80 8C. WUE was calculated as the plant total biomass production divided by the plant water use during the treatment period. 2.5. Data analysis and statistics Data were subjected to analysis of variance (ANOVA) procedures (SAS Institute, 6.03 ed, 1988). Appropriate standard errors of the means (S.E.) and L.S.D.s at P = 0.05 were calculated. Tukey’s Studentised Range (H.S.D.) test was applied to compare growth parameters of plants, which had experienced different irrigation treatments. 3. Results and discussion 3.1. Soil and plant water status Changes of u in FI, DI, and PRD pots are shown in Fig. 2. The daily mean u of FI plants was about 16%, while in DI u
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115
Fig. 3. Biomass partitioning among leaf, stem, root, and tuber of potato plants grown under FI, DI, and PRD treatments. Values that are significantly different at P < 0.05 have no letters in common (H.S.D. test).
while PRD plants had a significant lower leaf water potential (0.85 MPa), and which might have been due to severe water stress in the dry side of the treatment. 3.2. Leaf area, biomass production, and partitioning
Fig. 2. Daily soil volumetric water content (u) in two parts of the pots under three irrigation treatments, FI, DI, and PRD before (the lower values) and after irrigation (the upper values). Bars indicate the standard error of the means (S.E.) (n = 8).
decreased steadily over time to an average value of 6.0% at 10 DAT and remained constant thereafter (Fig. 2A). u for PRD depended on wetting and drying cycles (Fig. 2B). In the first cycle u of wetting side was kept above 11% while u of the drying side decreased to an average value of 6.0% at 10 DAT. In later cycles, a mean u of 7–10 and 4–6% was recorded for the wetting and drying side, respectively (Fig. 2B). Failed to maintain a high u of the wet side in PRD irrigating with 50% of ETfull had been observed in several crops (e.g. Kirda et al., 2004; Wakrim et al., 2005). It has been always observed that plants extract much more water from the wet side leading to a steadily declining of u in that side. Despite of declining in u in DI and PRD pots, the midday leaf water potential of those plants was largely maintained at a similar level to FI (0.53 MPa) on 9 DAT. However, on 21 DAT only DI plants had a similar leaf water potential to that of FI (0.73 MPa);
At final harvest, leaf area of PRD and DI was significantly less than that of FI (Table 1). This is in agreement with former observations that leaf area expansion is significantly inhibited in potatoes under soil water deficits (Jefferies and Mackerron, 1989). We found that DI and PRD decreased plant biomass to a similar extent as compared to FI (Table 1); even through the pattern of biomass partitioning among different plant organs was different. In relation to FI, DI decreased biomass allocation to leaves and stems while increasing it into roots and tubers; whereas PRD increased biomass allocation to roots (Fig. 3). Promoting root growth under PRD has been reported in maize (Kang et al., 1998), grapevine (Dry et al., 2000), and tomato (Mingo et al., 2004), and this has been considered as an advantage of PRD irrigation. The proportion of tuber dry weight was significantly higher under DI than under FI and PRD (Fig. 3). We supposed that tuber initiation and bulking might have been restricted to a larger extent by a severe soil water deficit in the dry soil column under PRD than that by a less severe soil water deficit in both soil columns under DI (Fig. 2). 3.3. Gas exchange In general, DI and PRD treatments decreased A, gs, and T although except for several days at which these parameters were
Table 1 Effects of irrigation treatments on plant leaf area, biomass production, water use (cumulative ET), and water use efficiency (WUE) in potato Treatment
Leaf area (cm2 plant1)
Biomass production (g plant1)
Cumulative ET (L plant1)
WUE (g L1)
FI DI PRD
6081a 3513b 3728b
84.07a 57.27b 52.87b
16.77a 10.61b 10.62b
5.01b 5.40a 4.98b
Means with same letters (a and b) within columns are not significantly different using HSD at P < 0.05. FI: full irrigation; DI: deficit irrigation; PRD: partial root drying.
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similar for all the treatments (Fig. 4). Similar effect of DI and PRD on gas exchange has been shown in maize (Kirda et al., 2005). We also noticed that gs and T might be partly regulated by VPD, being that the lower VPD during the day the smaller
Fig. 5. Time course of evapotranspiration (ET) of potato plants under FI, DI, and PRD treatments. Data points are means (n = 8). Vertical bars indicate L.S.D.s (P = 0.05) and asterisks denote significant difference among the three treatments at P < 0.05.
differences of gs and T between treatments (Figs. 1 and 4B, C). Compared to FI plants, reduction in gs occurred when the leaf water potential was not affected in both DI and PRD treatments indicating that root-sourced chemical (probably ABA) signaling may be responsible for gs reductions (Liu et al., 2005). 3.4. Plant water use and water use efficiency ET in PRD was significantly less than in DI and FI at three to five DAT, thereafter ET in DI and PRD was similar and was significantly less than that in FI (Fig. 5). As T of individual leaves was similar between DI and PRD plants, the lower ET in PRD was probably due to reduced soil evaporation during the 1st days after onset of treatments. This is in accordance with the fact that PRD only wetted half of the soil surface, which may reduce soil evaporation. Despite of this, the total plant water use during the experimental period was similar in DI and PRD, and was 37% less than in FI (Table 1). Compared with FI plants, WUE and TE were increased by DI and not by PRD treatment (Table 1; Fig. 4D). This result does not support the hypothesis that WUE would have been improved under PRD irrigation (Kang and Zhang, 2004). The reasons for this are not clear, and merit further studies. 4. Concluding remarks
Fig. 4. Changes in photosynthesis (A) (A), stomatal conductance (gs) (B), transpiration (T) (C), and transpiration efficiency (TE) (D) of potato leaves under FI, DI, and PRD treatments. Data points are means (n = 8). Vertical bars indicate L.S.D.s (P = 0.05) and asterisks denote significant difference among the three treatments at P < 0.05.
Under the conditions of the present study we were able to conclude that, given the same amount (50% of ETfull) of irrigation, PRD has no advantages compared to DI in terms of biomass production and WUE in potato at tuber initiation stage. It seems that PRD treatment defined in this study is not feasible for potato during tuber initiation stages. We are aware of that to achieve the effect of PRD on stomatal conductance and plant water use without diminishing biomass production, several important issues must be addressed in future studies: (1) how much water should be irrigated to the wet soil column? (2) When should irrigation be shifted from wet to dry side? (3) At
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which growth stages should PRD be applied? These studies may contribute the successful application of PRD irrigation in potato as well as in other crops. Acknowledgements Fulai Liu thanks the Danish Research Council (SJVF, 23-030208) for the financial support of 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). References Davies, W.J., Hartung, W., 2004. Has extrapolation from biochemistry to crop functioning worked to sustain plant production under water scarcity? In: Proceeding of the 4th International Crop Science Congress, 26 September– 1 October 2004, Brisbane, Australia. Dry, P.R., Loveys, B.R., During, H., 2000. Partial drying of the rootzone of grape. 2. Changes in the patterns of root development. Vitis 39, 9–12. English, M.J., Musick, J.T., Murty, V.V.N., 1990. Deficit irrigation. In: Hoffman, G.J., Howell, T.A., Solomon, K.H. (Eds.), Management of Farm Irrigation System. American Society of Agricultural Engineers, St. Joseph, pp. 631–663. Fabeiro, C., Martı´n de Santa Olalla, F., de Juan, J.A., 2001. Yield and size of deficit irrigated potatoes. Agric. Water Manage. 48, 255–266. Jefferies, R.A., Mackerron, D.K.L., 1989. Radiation interception and growth of irrigated and drought-stressed potato (Solanum tuberosum). Field Crops Res. 22, 101–112.
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