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ABA regulated stomatal control and photosynthetic water use efficiency of potato (Solanum tuberosum L.) during progressive soil drying
Plant Science 168 (2005) 831–836 www.elsevier.com/locate/plantsci
ABA regulated stomatal control and photosynthetic water use efficiency of potato (Solanum tuberosum L.) during progressive soil drying Fulai Liua,*, Christian R. Jensena, Ali Shahanzaria, Mathias N. Andersenb, Sven-Erik Jacobsena a
The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology, Heojbakkegaard Alle´ 9, DK-2630 Taastrup, Denmark b Department of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark Received 20 July 2004; received in revised form 4 October 2004; accepted 22 October 2004 Available online 19 November 2004
Abstract Glasshouse experiments at two developmental stages (tuber initiation and tuber bulking) of potatoes (Solanum tuberosum L.) were conducted to investigate plant water relation and leaf gas exchange characteristics during progressive soil drying. Leaf relative water content (RWC), leaf water potential (Cl), root water potential (Cr), stomatal conductance (gs), photosynthesis (A), and xylem sap abscisic acid (ABA) concentration ([ABA]xylem) were determined in well-watered (WW) and drought-stressed (DS) plants. At both stages, RWC and Cl were hardly affected, significant decreases of the two parameters occurred only at severe soil water deficits; however, gs decreased much early at 2 and 1 days after imposition of stress (DAIS) at tuber initiation and tuber bulking, respectively, and coincided with decrease of Cr and increase of [ABA]xylem; while A decreased 2 days later than gs at each stage. Analyses of the pooled data of the two stages showed that gs was linearly correlated with [ABA]xylem at mild soil water deficits (i.e. Cr > 0.3 MPa); photosynthetic water use efficiency, viz. A/gs, increased linearly with decreasing gs until the latter reached 0.2 mol m 2 s 1, below this point, it decreased sharply. The results suggest that at mild soil water deficits, gs of potato is seemingly controlled by xylem-borne ABA. As a consequence of A being less sensitive than gs to soil drying, photosynthetic water use efficiency, i.e. A/gs, is increased at mild soil water deficits. # 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Abscisic acid; Drought stress; Leaf gas exchange; Photosynthetic water use efficiency; Plant water relation; Solanum tuberosum L.
1. Introduction It is well known that potatoes (Solanum tuberosum L.) are very sensitive to drought stress [1], and the tuber yield may be considerably reduced by soil moisture deficits [2]. Therefore, ensuring sufficient water supply is essential to the production of potatoes [3]. However, as the available water resource is decreasing, optimum irrigation strategies must be established to increase water use efficiency. This recalls for a deeper understanding of the physiological mechanisms of potato plants response to drought stress [4]. * Corresponding author. Tel.: +45 35283416; fax: +45 35283384. E-mail address: [email protected] (F. Liu).
Compared to many other crop plants, potatoes close their stomata at relatively low soil moisture deficits [5]. Significant decrease of stomatal conductance (gs) may occur at a very early stage of soil drying even before significant decrease in leaf water potential (Cl) can be detected [6]. The phenomenon that reduction of gs occurs before significant changes in leaf water status in droughtstressed plants has been frequently observed [7]. It is suggested that in drought-stressed plants, root-originated chemical signals can be transported to shoots and regulate shoot physiology prior to significant decrease in shoot water potential [8]. It is also established that xylem-borne ABA is one of the main molecules acting as an early non-hydraulic drought-stress signal regulating shoot physiology in many
0168-9452/$ – see front matter # 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2004.10.016
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F. Liu et al. / Plant Science 168 (2005) 831–836
crop species [7,9–12]. Surprisingly, until now this aspect had not been investigated in potatoes. It is well known that partial closure of stomata tends to increase photosynthetic water use efficiency, which can be defined as the ratio of photosynthesis (A) to gs, viz. A/gs. Jones (1992) [13] suggested that the optimum stomatal aperture may be between fully open and that given maximum water use efficiency. In this sense, it is possible to manipulate stomatal behavior by controlling soil water availability through irrigation to achieve high water use efficiency without significant diminishment on dry matter production [14]. Therefore, it is important to know the response of photosynthetic water use efficiency to progressive soil drying which may provide essential information for scheduling irrigation in the field. In the present study, we compared the sensitivity of relative water content (RWC), Cl, root water potential (Cr), xylem sap ABA concentration ([ABA]xylem), gs, and A of potato to progressive soil drying at tuber initiation and tuber bulking stages. Our objectives were to (i) illustrate the involvement of root and shoot factors in controlling gs during progressive soil drying and (ii) investigate the effects of soil water deficits on photosynthetic water use efficiency, i.e. A/gs, in this species.
the pots (DS treatment) 35 days after planting and lasted for 14 days. The rest were control plants and were WW at 95% WHC but without nutrients during the drought spell. At tuber bulking stage, irrigation was withheld from half of the plants 52 days after planting and lasted for 9 days. Plants kept WW served as controls. 2.3. gs, A, RWC, and Cl measurements After onset of drought stress, leaf gas exchange parameters (gs and A) were measured daily on the second fully expanded upper canopy leaf (one leaf per plant) from 11:00 to 13:00 h at photosynthetic light saturation (PAR > 800 mmol m 2 s 1) and ambient CO2 concentration of ca. 380 ml l 1 with a LI-6200 portable photosynthesis system (LiCor Inc., Lincoln, NE, USA). Plants were harvested every 2 days. At each harvest, Cl was measured with a pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, CA, USA) on the leaves for gas exchange measurement. RWC was determined on the same leaves for Cl measurement as follows: RWC = (f. wt d. wt)/(turgid weight d. wt). Turgid weight was determined by floating the leaf on distilled water at 20 8C under dim light for 4 h, and weighed after blotting; leaf dry weight was determined after drying the material to constant weight at 80 8C.
2. Materials and methods
2.4. Cr determination and collection of xylem sap
2.1. Plant material and growing conditions
At each harvest, xylem sap was collected by pressurizing 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 Cr was equalised. The cut surface was cleaned with pure water and dried with blotting paper. Afterwards, the pressure was increased gradually until it equalled Cl of the plant. 0.5–1.0 ml of sap was collected using a pipette from the cutting surface into an Eppendorf-vial wrapped with aluminium foil over 3–5 min in WW plants and 5– 10 min in DS plants. The sap was immediately stored at 80 8C for ABA analysis.
Pot experiments with potatoes at two developmental stages (tuber initiation and tuber bulking) were done at the experimental station of the Royal Veterinary and Agricultural University (KVL), Taastrup, Denmark. Potatoes (S. tuberosum L. cv. Folva, supplied by seed potato company Solanum Nord, Brørup, Denmark) were sown in pots (15 cm diameter and 50 cm tall) containing 1.0 kg peat (GBPindstrup Substrates no.1, pH 6.0). Before planting the seed tubers were exposed to 12–14 8C with constant dim overhead light for sprouting. Plants were grown in a glasshouse where day/night air temperature was 20/ 14 2 8C; relative humidity was 60%; photoperiod was 15 h with above 600 mmol m 2 s 1 (PAR) supplied by sunlight plus metal-halide lamps. When the plants emerged, only one shoot from the apical bud was left. Pots were randomised in the glasshouse daily. 2.2. Water supply treatment At the beginning of the experiments, pots were watered and allowed to drain freely to constant mass (about 6.4 kg); this was 100% water holding capacity (WHC). After emergence, all plants were well-watered (WW) daily to 95% WHC with nutrient solution (Pioneer NPK Macro 14–3– 23 + Mg combined with Pioneer Micro; pH 5.5; EC = 1.3). At tuber initiation stage, irrigation was withheld from half of
2.5. ABA assay [ABA]xylem was measured without further purification by an enzyme linked immunosorbent assay (ELISA) using a monoclonal antibody for ABA (AFRC MAC 252) according to Asch [15]. No cross-reaction of the antibody with other compounds in xylem sap was detected when tested according to Quarrie et al. [16]. 2.6. Data analysis and statistics Data were subjected to analysis of variance (ANOVA) procedures (SAS Institute, Inc., 1988). An exponential function was used to describe the relationship between gs
F. Liu et al. / Plant Science 168 (2005) 831–836
and [ABA]xylem [12]; while the relationship between A and gs was evaluated by a logarithmic function. The parameters of the curves were derived by the non-linear, least-squares, iteration procedure (PROC NLIN) [17].
3. Results 3.1. Plant water status, [ABA]xylem, and gas exchange during soil drying At both stages, RWC of WW plants was ca. 0.93; RWC in DS plants became significantly lower than the WW controls 10 and 6 days after imposition of stress (DAIS) at tuber initiation and tuber bulking, respectively (Fig. 1a and e). At tuber initiation, Cl of DS plants was similar to that of WW plants until 8 DAIS; thereafter, it decreased and was significantly lower than the WW controls (Fig. 1b). At tuber bulking, although Cl of DS plants was lower in relation to WW plants 2 and 3 DAIS, it became similar to that of WW
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plants 5 DAIS, thereafter, it decreased and was consistently lower than the WW controls (Fig. 1f). Cr of DS plants became significantly lower than the WW controls at 4 and 2 DAIS at tuber initiation and tuber bulking, respectively (Fig. 1c and g). At both stages, [ABA]xylem was ca. 70 pmol ml 1 in WW plants (Fig. 1d and h). Under drought stress, [ABA]xylem increased from 2 and 2 DAIS at tuber initiation and tuber bulking, respectively, and was ca. 26and 33-fold of the WW plants by the end of DS treatment (Fig. 1d and h). At tuber initiation, gs, and A of WW plants fluctuated around an average value of 0.6 mol m 2 s 1 and 20 mmol m 2 s 1, respectively (Fig. 2a and b). Under drought stress, gs and A of DS plants became significantly lower than of the WW controls 2 and 4 DAIS, respectively, and decreased significantly thereafter (Fig. 2a and b). At tuber bulking, gs and A of WW plants were slightly lower than those at tuber initiation. Significant differences between water treatments for gs and A were observed at 1 and 3 DAIS, respectively (Fig. 2c and d).
Fig. 1. Development of leaf relative water content (RWC) (a and e), leaf water potential (Cl) (b and f), root water potential (Cr) (c and g), and xylem sap ABA concentration ([ABA]xylem) (d and h) of well-watered (WW) and drought-stressed (DS) potatoes at tuber initiation and tuber bulking stages. Bars indicate the standard error of the means (S.E.M.) (n = 4).
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Fig. 2. Development of stomatal conductance (gs) (a and c), and photosynthesis (A) (b and d) of WW and DS potatoes at tuber initiating and tuber bulking stages. Bars indicate S.E.M. (n = 4).
3.2. Stomatal control during soil drying Regression analyses of the pooled data from the two stages showed that there was a clear linear relationship between [ABA]xylem and Cr (r2 = 0.91**) (Fig. 3). During the whole range of soil drying, the relationship betweengs and
[ABA]xylem could be well described by an exponential function (r2 = 0.93***). However, at mild soil water deficits when Cl was unaffected, gs decreased almost linearly from ca 0.7 to 0.15 with increasing [ABA]xylem from 50 to 400 pmol ml 1 (insert in Fig. 4). 3.3. Effect of soil drying on photosynthetic water use efficiency The relationship between gs and A was well represented by a logarithmic function (r2 = 0.89***) (Fig. 5a). Fig. 5b shows the relationship between A/gs and gs, where A/gs increased linearly with decreasing gs and reached to a peak ca. 70 mmol CO2 mol 1 H2O when gs approached to 0.2 mol m 2 s 1; when gs decreased further, A/gs decreased dramatically and closed to zero.
Fig. 3. Linear relationship between root water potential (Cr) and xylem sap ABA concentration ([ABA]xylem) in potatoes grown in drying soils. Bars indicate S.E.M. (n = 4).
4. Discussion 4.1. Stomatal control in potato during progressive soil drying
Fig. 4. Relationship between stomatal conductance (gs) and xylem sap ABA concentration ([ABA]xylem) of potatoes grown in drying soils. Insert indicates the relationship at mild soil water deficits (i.e. Cr > 0.3 MPa). Bars indicate S.E.M. (n = 4).
Our results clearly showed that gs was a sensitive process to soil drying in potatoes at two developmental stages, viz. tuber initiation and tuber bulking. Significant decrease of gs occurred much earlier than did for RWC and Cl. This result confirmed earlier findings in potatoes [6]. In the present study, we showed for the first time that [ABA]xylem of potato plants increased significantly as soil dried and was linearly correlated with Cr, indicating that like in many other crops (e.g. Lupin [18] and soybean [19]), potato roots are able to sense soil water deficits and produce ABA in root tips when Cr decrease, and the extent to which ABA accumulated in the xylem sap is dependent on Cr. In addition, our result
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potential use for scheduling irrigation for potatoes under water-limited conditions. Acknowledgements This research was partly funded by the Danish Research Council (SJVF, 23-03-0208) and by the European Commission (EU, INCO-CT-2004-509163). We thank Dr. Steven Quarrie, John Innes Centre, UK, for the antibody to ABA. Technical assistance of Britta Skov and Jørgen Hansen Thage is gratefully acknowledged.
References
Fig. 5. Photosynthesis (A) (a) and photosynthetic water use efficiency (A/gs) (b) expressed as a function of stomatal conductance (gs) of potatoes grown in drying soils. Bars indicate S.E.M. (n = 4).
clearly showed that at mild soil water deficits, i.e. Cr > 0.3 MPa, when RWC and Cl of the DS plants were similar to the WW controls, gs decreased by 75% and was linearly correlated with increasing of [ABA]xylem; while at severe soil water deficits, i.e. Cr < 0.3 MPa, the dependency of gs on [ABA]xylem became less significant. This result indicates that as in soybeans [19] root-originated xylem sap ABA might be responsible for the reduction of gs of potato during mild soil water deficits. 4.2. Improvement of photosynthetic water use efficiency at mild soil water deficits We observed that A did not decrease significantly when gs decreased from 0.7 to ca. 0.4 mol m 2 s 1; during this range A/gs increased ca. 1.7-fold. This occurred before significant decreases in RWC and Cl could be detected. Similar findings have previously been reported in soybeans [19,20,21], wheat (Tritcum aestivum) [22], cauliflower [23], and oilseed rape (Brassica napus) [24]. These studies and our results suggest that it is possible to manipulate stomatal aperture and bring about an improvement in plant water use efficiency under water-limited conditions. Actually, irrigation techniques so called regulated deficit irrigation (RDI) and partial rootzone drying (PRD) have been developed based on this kind of plant response and have shown to be able to increase plant water use efficiency [14]. Our results imply that the perspectives to apply these irrigation techniques might be promising in potato production in drought-prone regions. In conclusion, our results indicated that at mild soil water deficits, gs of potato was seemingly controlled by rootoriginated ABA. As a consequence of A being less sensitive than gs to soil water deficit, photosynthetic water use efficiency (A/gs) was improved under mild soil water deficits. The information obtained in this study is of
[1] R.A. Jefferies, Responses of potato genotypes to drought. 1. Expansion of individual leaves and osmotic adjustment, Ann. Appl. Biol. 122 (1993) 93–104. [2] G.A. Porter, G.B. Opena, W.B. Bradbury, J.C. McBurnie, J.A. Sisson, Soil management and supplemental irrigation effects on potato. I. Soil properties, tuber yield, and quality, Agron. J. 91 (1999) 416–425. [3] C. Fabeiro, F.M.D. Olalla, J.A. de Juan, Yield and size of deficit irrigated potatoes, Agric. Water Manage. 48 (2001) 255–266. [4] N.C. Turner, Further progress in crop water relations, Adv. Agron. 58 (1997) 293–338. [5] V.O. Sadras, S.P. Milroy, Soil-water thresholds for the responses of leaf expansion and gas exchange: a review, Field Crops Res. 47 (1996) 253–266. [6] R.A. Jefferies, D.K.L. Mackerron, Radiation interception and growth of irrigated and drought-stressed potato (Solanum tuberosum), Field Crops Res. 22 (1989) 101–112. [7] W.J. Davies, J. Zhang, Root signals and the regulation of growth and development of plants in drying soil, Ann. Rev. Plant Physiol. Plant Mol. Biol. 42 (1991) 55–76. [8] I.C. Dodd, W.J. Davies, The relationship between leaf growth and ABA accumulation in the grass leaf elongation zone, Plant Cell Environ. 19 (1996) 1047–1056. [9] M. Ali, C.R. Jensen, V.O. Mogensen, Early signals in field-grown wheat in response to shallow soil drying, Aust. J. Plant Physiol. 25 (1998) 871–882. [10] M.A. Bacon, S. Wilkinson, W.J. Davies, PH-regulated leaf cell expansion in droughted plants is abscisic acid dependent, Plant Physiol. 118 (1998) 157–1515. [11] A. Bahrun, C.R. Jensen, F. Asch, V.O. Mogensen, Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.), J. Exp. Bot. 53 (2002) 251–263. [12] F. Liu, C.R. Jensen, M.N. Andersen, Hydraulic and chemical signals in the control of leaf expansion and stomatal conductance in soybean exposed to drought stress, Funct. Plant Biol. 30 (2003) 65–73. [13] H.G. Jones, Plants and Microclimate: A Quantitative Approach to Environmental Plant Physiology, 2nd ed. Cambridge University Press, Cambridge, 1992. [14] W.J. Davies, S. Wilkinson, B.R. Loveys, Stomatal control by chemical signalling and the exploitation of this mechanism to increase water use efficiency in agriculture, New Phytol. 153 (2002) 449–460. [15] F. Asch, Determination of abscisic acid by indirect enzyme linked immunosorbent assay (ELISA), Technical Report, Laboratory for Agrohydrology and Bioclimatology, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Taastrup, Denmark, 2000. [16] S.A. Quarrie, P.N. Whitford, N.E. Appleford, T.L. Wang, S.K. Cook, I.E. Henson, B.R. Loveys, A monoclonal antibody to (S)-abscisic acid:
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F. Liu et al. / Plant Science 168 (2005) 831–836 its characterisation and use in a radio-immunoassay for measuring abscisic acid in crude extracts of cereal and lupin leaves, Planta 173 (1988) 330–339. SAS Institute Inc., SAS/STAT User’s Guide: Release 6.03 ed., SAS Institute, Inc., Cary, NC, 1988. I.E. Henson, C.R. Jensen, N.C. Turner, Leaf gas exchange and water relations of lupins and wheat. III. Abscisic acid and drought-induced stomatal closure, Aust. J. Plant Physiol. 16 (1989) 429–442. F. Liu, M.N. Andersen, S.-E. Jacobsen, C.R. Jensen, Stomatal control and water use efficiency of soybean (Glycine max L. Merr.) during progressive soil drying, Environ. Exp. Bot., in press. A. Olioso, T.N. Carlson, N. Brisson, Simulation of diurnal transpiration and photosynthesis of a water stressed soybean crop, Agric. For. Meteorol. 81 (1996) 41–59.
[21] R. Serraj, L.H. Allen, T.S. Sinclair, Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment, Glob. Change Biol. 5 (1999) 283–291. [22] R.A. Mitchell, V.J. Mitchell, D.W. Lawlor, Response of wheat canopy CO2 and water gas-exchange to soil water content under ambient and elevated CO2, Glob. Change Biol. 7 (2001) 599– 611. [23] M. Kochler, Analyse und Modellierung der Anpassungsreaktionen von Blumenkohl (Brassica oleracea L. botrytis) an eine limitierte Wasserversorgung, Ph.D. Thesis, Department of Horticulture, University of Hannover, 2000. [24] C.R. Jensen, V.O. Mogensen, M.N. Andersen, I.E. Henson, Gas exchange and its factorial dependency in field-grown Brassica napus L., Eur. J. Agron. 9 (1998) 53–70.
ABA regulated stomatal control and photosynthetic water use efficiency of potato (Solanum tuberosum L.) during progressive soil drying Fulai Liua,*, Christian R. Jensena, Ali Shahanzaria, Mathias N. Andersenb, Sven-Erik Jacobsena a
The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology, Heojbakkegaard Alle´ 9, DK-2630 Taastrup, Denmark b Department of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark Received 20 July 2004; received in revised form 4 October 2004; accepted 22 October 2004 Available online 19 November 2004
Abstract Glasshouse experiments at two developmental stages (tuber initiation and tuber bulking) of potatoes (Solanum tuberosum L.) were conducted to investigate plant water relation and leaf gas exchange characteristics during progressive soil drying. Leaf relative water content (RWC), leaf water potential (Cl), root water potential (Cr), stomatal conductance (gs), photosynthesis (A), and xylem sap abscisic acid (ABA) concentration ([ABA]xylem) were determined in well-watered (WW) and drought-stressed (DS) plants. At both stages, RWC and Cl were hardly affected, significant decreases of the two parameters occurred only at severe soil water deficits; however, gs decreased much early at 2 and 1 days after imposition of stress (DAIS) at tuber initiation and tuber bulking, respectively, and coincided with decrease of Cr and increase of [ABA]xylem; while A decreased 2 days later than gs at each stage. Analyses of the pooled data of the two stages showed that gs was linearly correlated with [ABA]xylem at mild soil water deficits (i.e. Cr > 0.3 MPa); photosynthetic water use efficiency, viz. A/gs, increased linearly with decreasing gs until the latter reached 0.2 mol m 2 s 1, below this point, it decreased sharply. The results suggest that at mild soil water deficits, gs of potato is seemingly controlled by xylem-borne ABA. As a consequence of A being less sensitive than gs to soil drying, photosynthetic water use efficiency, i.e. A/gs, is increased at mild soil water deficits. # 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Abscisic acid; Drought stress; Leaf gas exchange; Photosynthetic water use efficiency; Plant water relation; Solanum tuberosum L.
1. Introduction It is well known that potatoes (Solanum tuberosum L.) are very sensitive to drought stress [1], and the tuber yield may be considerably reduced by soil moisture deficits [2]. Therefore, ensuring sufficient water supply is essential to the production of potatoes [3]. However, as the available water resource is decreasing, optimum irrigation strategies must be established to increase water use efficiency. This recalls for a deeper understanding of the physiological mechanisms of potato plants response to drought stress [4]. * Corresponding author. Tel.: +45 35283416; fax: +45 35283384. E-mail address: [email protected] (F. Liu).
Compared to many other crop plants, potatoes close their stomata at relatively low soil moisture deficits [5]. Significant decrease of stomatal conductance (gs) may occur at a very early stage of soil drying even before significant decrease in leaf water potential (Cl) can be detected [6]. The phenomenon that reduction of gs occurs before significant changes in leaf water status in droughtstressed plants has been frequently observed [7]. It is suggested that in drought-stressed plants, root-originated chemical signals can be transported to shoots and regulate shoot physiology prior to significant decrease in shoot water potential [8]. It is also established that xylem-borne ABA is one of the main molecules acting as an early non-hydraulic drought-stress signal regulating shoot physiology in many
0168-9452/$ – see front matter # 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2004.10.016
832
F. Liu et al. / Plant Science 168 (2005) 831–836
crop species [7,9–12]. Surprisingly, until now this aspect had not been investigated in potatoes. It is well known that partial closure of stomata tends to increase photosynthetic water use efficiency, which can be defined as the ratio of photosynthesis (A) to gs, viz. A/gs. Jones (1992) [13] suggested that the optimum stomatal aperture may be between fully open and that given maximum water use efficiency. In this sense, it is possible to manipulate stomatal behavior by controlling soil water availability through irrigation to achieve high water use efficiency without significant diminishment on dry matter production [14]. Therefore, it is important to know the response of photosynthetic water use efficiency to progressive soil drying which may provide essential information for scheduling irrigation in the field. In the present study, we compared the sensitivity of relative water content (RWC), Cl, root water potential (Cr), xylem sap ABA concentration ([ABA]xylem), gs, and A of potato to progressive soil drying at tuber initiation and tuber bulking stages. Our objectives were to (i) illustrate the involvement of root and shoot factors in controlling gs during progressive soil drying and (ii) investigate the effects of soil water deficits on photosynthetic water use efficiency, i.e. A/gs, in this species.
the pots (DS treatment) 35 days after planting and lasted for 14 days. The rest were control plants and were WW at 95% WHC but without nutrients during the drought spell. At tuber bulking stage, irrigation was withheld from half of the plants 52 days after planting and lasted for 9 days. Plants kept WW served as controls. 2.3. gs, A, RWC, and Cl measurements After onset of drought stress, leaf gas exchange parameters (gs and A) were measured daily on the second fully expanded upper canopy leaf (one leaf per plant) from 11:00 to 13:00 h at photosynthetic light saturation (PAR > 800 mmol m 2 s 1) and ambient CO2 concentration of ca. 380 ml l 1 with a LI-6200 portable photosynthesis system (LiCor Inc., Lincoln, NE, USA). Plants were harvested every 2 days. At each harvest, Cl was measured with a pressure chamber (Soil Moisture Equipment Corp., Santa Barbara, CA, USA) on the leaves for gas exchange measurement. RWC was determined on the same leaves for Cl measurement as follows: RWC = (f. wt d. wt)/(turgid weight d. wt). Turgid weight was determined by floating the leaf on distilled water at 20 8C under dim light for 4 h, and weighed after blotting; leaf dry weight was determined after drying the material to constant weight at 80 8C.
2. Materials and methods
2.4. Cr determination and collection of xylem sap
2.1. Plant material and growing conditions
At each harvest, xylem sap was collected by pressurizing 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 Cr was equalised. The cut surface was cleaned with pure water and dried with blotting paper. Afterwards, the pressure was increased gradually until it equalled Cl of the plant. 0.5–1.0 ml of sap was collected using a pipette from the cutting surface into an Eppendorf-vial wrapped with aluminium foil over 3–5 min in WW plants and 5– 10 min in DS plants. The sap was immediately stored at 80 8C for ABA analysis.
Pot experiments with potatoes at two developmental stages (tuber initiation and tuber bulking) were done at the experimental station of the Royal Veterinary and Agricultural University (KVL), Taastrup, Denmark. Potatoes (S. tuberosum L. cv. Folva, supplied by seed potato company Solanum Nord, Brørup, Denmark) were sown in pots (15 cm diameter and 50 cm tall) containing 1.0 kg peat (GBPindstrup Substrates no.1, pH 6.0). Before planting the seed tubers were exposed to 12–14 8C with constant dim overhead light for sprouting. Plants were grown in a glasshouse where day/night air temperature was 20/ 14 2 8C; relative humidity was 60%; photoperiod was 15 h with above 600 mmol m 2 s 1 (PAR) supplied by sunlight plus metal-halide lamps. When the plants emerged, only one shoot from the apical bud was left. Pots were randomised in the glasshouse daily. 2.2. Water supply treatment At the beginning of the experiments, pots were watered and allowed to drain freely to constant mass (about 6.4 kg); this was 100% water holding capacity (WHC). After emergence, all plants were well-watered (WW) daily to 95% WHC with nutrient solution (Pioneer NPK Macro 14–3– 23 + Mg combined with Pioneer Micro; pH 5.5; EC = 1.3). At tuber initiation stage, irrigation was withheld from half of
2.5. ABA assay [ABA]xylem was measured without further purification by an enzyme linked immunosorbent assay (ELISA) using a monoclonal antibody for ABA (AFRC MAC 252) according to Asch [15]. No cross-reaction of the antibody with other compounds in xylem sap was detected when tested according to Quarrie et al. [16]. 2.6. Data analysis and statistics Data were subjected to analysis of variance (ANOVA) procedures (SAS Institute, Inc., 1988). An exponential function was used to describe the relationship between gs
F. Liu et al. / Plant Science 168 (2005) 831–836
and [ABA]xylem [12]; while the relationship between A and gs was evaluated by a logarithmic function. The parameters of the curves were derived by the non-linear, least-squares, iteration procedure (PROC NLIN) [17].
3. Results 3.1. Plant water status, [ABA]xylem, and gas exchange during soil drying At both stages, RWC of WW plants was ca. 0.93; RWC in DS plants became significantly lower than the WW controls 10 and 6 days after imposition of stress (DAIS) at tuber initiation and tuber bulking, respectively (Fig. 1a and e). At tuber initiation, Cl of DS plants was similar to that of WW plants until 8 DAIS; thereafter, it decreased and was significantly lower than the WW controls (Fig. 1b). At tuber bulking, although Cl of DS plants was lower in relation to WW plants 2 and 3 DAIS, it became similar to that of WW
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plants 5 DAIS, thereafter, it decreased and was consistently lower than the WW controls (Fig. 1f). Cr of DS plants became significantly lower than the WW controls at 4 and 2 DAIS at tuber initiation and tuber bulking, respectively (Fig. 1c and g). At both stages, [ABA]xylem was ca. 70 pmol ml 1 in WW plants (Fig. 1d and h). Under drought stress, [ABA]xylem increased from 2 and 2 DAIS at tuber initiation and tuber bulking, respectively, and was ca. 26and 33-fold of the WW plants by the end of DS treatment (Fig. 1d and h). At tuber initiation, gs, and A of WW plants fluctuated around an average value of 0.6 mol m 2 s 1 and 20 mmol m 2 s 1, respectively (Fig. 2a and b). Under drought stress, gs and A of DS plants became significantly lower than of the WW controls 2 and 4 DAIS, respectively, and decreased significantly thereafter (Fig. 2a and b). At tuber bulking, gs and A of WW plants were slightly lower than those at tuber initiation. Significant differences between water treatments for gs and A were observed at 1 and 3 DAIS, respectively (Fig. 2c and d).
Fig. 1. Development of leaf relative water content (RWC) (a and e), leaf water potential (Cl) (b and f), root water potential (Cr) (c and g), and xylem sap ABA concentration ([ABA]xylem) (d and h) of well-watered (WW) and drought-stressed (DS) potatoes at tuber initiation and tuber bulking stages. Bars indicate the standard error of the means (S.E.M.) (n = 4).
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Fig. 2. Development of stomatal conductance (gs) (a and c), and photosynthesis (A) (b and d) of WW and DS potatoes at tuber initiating and tuber bulking stages. Bars indicate S.E.M. (n = 4).
3.2. Stomatal control during soil drying Regression analyses of the pooled data from the two stages showed that there was a clear linear relationship between [ABA]xylem and Cr (r2 = 0.91**) (Fig. 3). During the whole range of soil drying, the relationship betweengs and
[ABA]xylem could be well described by an exponential function (r2 = 0.93***). However, at mild soil water deficits when Cl was unaffected, gs decreased almost linearly from ca 0.7 to 0.15 with increasing [ABA]xylem from 50 to 400 pmol ml 1 (insert in Fig. 4). 3.3. Effect of soil drying on photosynthetic water use efficiency The relationship between gs and A was well represented by a logarithmic function (r2 = 0.89***) (Fig. 5a). Fig. 5b shows the relationship between A/gs and gs, where A/gs increased linearly with decreasing gs and reached to a peak ca. 70 mmol CO2 mol 1 H2O when gs approached to 0.2 mol m 2 s 1; when gs decreased further, A/gs decreased dramatically and closed to zero.
Fig. 3. Linear relationship between root water potential (Cr) and xylem sap ABA concentration ([ABA]xylem) in potatoes grown in drying soils. Bars indicate S.E.M. (n = 4).
4. Discussion 4.1. Stomatal control in potato during progressive soil drying
Fig. 4. Relationship between stomatal conductance (gs) and xylem sap ABA concentration ([ABA]xylem) of potatoes grown in drying soils. Insert indicates the relationship at mild soil water deficits (i.e. Cr > 0.3 MPa). Bars indicate S.E.M. (n = 4).
Our results clearly showed that gs was a sensitive process to soil drying in potatoes at two developmental stages, viz. tuber initiation and tuber bulking. Significant decrease of gs occurred much earlier than did for RWC and Cl. This result confirmed earlier findings in potatoes [6]. In the present study, we showed for the first time that [ABA]xylem of potato plants increased significantly as soil dried and was linearly correlated with Cr, indicating that like in many other crops (e.g. Lupin [18] and soybean [19]), potato roots are able to sense soil water deficits and produce ABA in root tips when Cr decrease, and the extent to which ABA accumulated in the xylem sap is dependent on Cr. In addition, our result
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potential use for scheduling irrigation for potatoes under water-limited conditions. Acknowledgements This research was partly funded by the Danish Research Council (SJVF, 23-03-0208) and by the European Commission (EU, INCO-CT-2004-509163). We thank Dr. Steven Quarrie, John Innes Centre, UK, for the antibody to ABA. Technical assistance of Britta Skov and Jørgen Hansen Thage is gratefully acknowledged.
References
Fig. 5. Photosynthesis (A) (a) and photosynthetic water use efficiency (A/gs) (b) expressed as a function of stomatal conductance (gs) of potatoes grown in drying soils. Bars indicate S.E.M. (n = 4).
clearly showed that at mild soil water deficits, i.e. Cr > 0.3 MPa, when RWC and Cl of the DS plants were similar to the WW controls, gs decreased by 75% and was linearly correlated with increasing of [ABA]xylem; while at severe soil water deficits, i.e. Cr < 0.3 MPa, the dependency of gs on [ABA]xylem became less significant. This result indicates that as in soybeans [19] root-originated xylem sap ABA might be responsible for the reduction of gs of potato during mild soil water deficits. 4.2. Improvement of photosynthetic water use efficiency at mild soil water deficits We observed that A did not decrease significantly when gs decreased from 0.7 to ca. 0.4 mol m 2 s 1; during this range A/gs increased ca. 1.7-fold. This occurred before significant decreases in RWC and Cl could be detected. Similar findings have previously been reported in soybeans [19,20,21], wheat (Tritcum aestivum) [22], cauliflower [23], and oilseed rape (Brassica napus) [24]. These studies and our results suggest that it is possible to manipulate stomatal aperture and bring about an improvement in plant water use efficiency under water-limited conditions. Actually, irrigation techniques so called regulated deficit irrigation (RDI) and partial rootzone drying (PRD) have been developed based on this kind of plant response and have shown to be able to increase plant water use efficiency [14]. Our results imply that the perspectives to apply these irrigation techniques might be promising in potato production in drought-prone regions. In conclusion, our results indicated that at mild soil water deficits, gs of potato was seemingly controlled by rootoriginated ABA. As a consequence of A being less sensitive than gs to soil water deficit, photosynthetic water use efficiency (A/gs) was improved under mild soil water deficits. The information obtained in this study is of
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