Water transport in sunflower root systems: effects of ABA, Ca2+ status and HgCl2

Journal of Experimental Botany, Vol. 50, No. 339, pp. 1607–1612, October 1999 Water transport in sunflower root systems: effects of ABA, Ca2+status and HgCl2 Jose ´M. Quintero1, Jose ´M. Fournier2and Manuel Benlloch2,3 1Department of Ciencias Agroforestales, E.U.I.T.A., Ctra. Utrera, km. 1, E-41013, Sevilla, Spain 2Department of Agronomia, E.T.S.I.A.M., Apdo. 3048, E-14080, Co ´rdoba, Spain Received 5 July 1999; Accepted 8 July 1999 Abstract Peterson, 1998). When osmotic gradients are present, the water flow is negligible in the apoplast (Quintero et al., Excised 20-d-old sunflower roots (Helianthus annuus 1998; Steudle and Peterson, 1998) and transport occurs L. cv. Sun-Gro 380) with different Ca2+status were mainly from cell to cell (Steudle and Frensch, 1989; used to study the effects of root Ca2+status and Steudle, 1994). In recent years, it has been shown that abscisic acid (ABA) on the exudation rate (Jv), the membrane-integral proteins, or aquaporins, act as waterhydraulic conductivity of the root (Lpr), the flux of specific channels (Chrispeels and Maurel, 1994; Maggio exuded Ca2+(JCa), and the gradient of osmotic pres- and Joly, 1995). The presence of aquaporins in roots sure between the xylem and the external medium. Jv (Yamamoto et al., 1991; Niemietz and Tyerman, 1997) and Lprincreased in direct proportion to the Ca2+ suggests that radial flow of water could be mainly transstatus of the root. Addition of ABA (4 mM) at the onset cellular (Chrispeels and Maurel, 1994; Maggio and Joly, of exudation in the external medium made Jv and Lpr1995) and may be controlled by the opening or closing rise, and this effect also increased with the Ca2+ of these water channels (Chrispeels and Maurel, 1994; status. The effects of HgCl2and its interaction with Steudle and Henzler, 1995). ABA on water transport in the root were also studied. The existence of a regulatory mechanism which controls Addition of HgCl2(1 mM) 2 h after the onset of exudaradial transport of water in the root has yet to be tion in the external medium quickly inhibited Jv, indedetermined (Quintero et al., 1998). It is known that roots pendently of the presence of ABA in the root medium. offer the greatest resistance to water flow (Weatherley, The results recorded here point to the involvement of 1982), and that hydraulic conductivity of the root (Lpr) ABA and Ca2+in the regulation of root water flow, may be affected by diverse forms of abiotic stress, such as well as the existence of aquaporins in the cell as salinity (Munns and Passioura, 1984; Joly, 1989), membranes of sunflower roots. anaerobiosis (Zhang and Tyerman, 1991), drought (North and Nobel, 1991) or nutritional stress (Radin and Key words: ABA, aquaporins, calcium, exudation rate, Matthews, 1989; Radin, 1990; Quintero et al., 1998). Helianthus annuus, hydraulic conductivity, sunflower. Among the endogenous factors related to water flow in plants, the role of abscisic acid (ABA) has received most attention. The role of ABA in the regulation of root Introduction water flow has yet to be firmly established, as experimental Radial transport of water in the root occurs simultanresults have often been contradictory (Glinka, 1980; eously via two parallel routes: cell to cell (including Fiscus, 1981; Markhart, 1984; BassiriRad and Radin, symplastic and transcellular pathways) and apoplast 1992). In isolated sunflower roots, ABA appears to (Steudle and Frensch, 1996; Steudle and Peterson, 1998). promote exudation (Glinka, 1980; Fournier et al., 1987; The predominance of one route over the other depends Quintero et al., 1998), due to its effect on both the on species, extent of root development and whether water hydraulic conductivity of the root (Glinka, 1980; Quintero absorption is determined by a hydrostatic or an osmotic et al., 1998) and the release of ions into the xylem vessels (Glinka, 1980). In contrast, in bean root systems, usinggradient (Steudle and Frensch, 1996; Steudle and 3To whom correspondence should be addressed. Fax: +349 57 218569. E-mail: [email protected] © Oxford University Press 1999 Downloaded from https://academic.oup.com/jxb/article-abstract/50/339/1607/485182 by UNIVERSIDAD DE SEVILLA user on 11 April 2018 1608 Quintero et al. 2.5 mM CaCl2(control plants) or without calcium (low-Ca2+ a pressure chamber to increase water flux through the plants). The basic composition of this new solution was: 2.5 mM roots, no short-term effect of ABA was found on their KNO3, 2.5 mM NaNO3, 0.5 mM KH2PO4, 1.0 mM MgSO4, hydraulic conductivity; and in the long-term, a decrease 12.5 mMH 3BO3, 1.0 mM MnSO4, 1.0 mM ZnSO4, 0.25 mM was even observed (Fiscus, 1981). CuSO4, 0.2 mM(NH 4)6Mo7O24, and 10 mM Fe-ethylenediamine- di-o-hydroxyphenylacetic acid. In the experiments with HgCl2, More is known about the role of ABA in regulating the plants were continuously kept in standard nutrient solution. water flow in the shoot. ABA is synthesized in roots in In all cases, NaOH was used to adjust the pH of the nutrient response to water stress conditions, producing various solution to 5.5. In all assays, four plants were grown for each effects in the shoot. One such effect is the opening of treatment. outward K+channels, the loss of turgor and closing of The plants were grown in the growth chamber for 15 d. The nutrient solutions were continuously aerated using an air pump. stomata (Davies and Zhang, 1991; Chandler and The volume was adjusted daily to 790 ml. The nutrient solutions Robertson, 1994). It has recently been reported that ABA were renewed at day 7 and, in experiments with HgCl2, also induces an increase in the concentration of cytosolic Ca2+ the day before the exudation assays. in guard cells (Gilroy et al., 1991; McAinsh et al., 1990, 1992), preceding the closing of stomata (McAinsh et al., Exudate collection 1990). The concentration of cytosolic Ca2+in guard cells The exudation experiments were performed with 20-d-old plants is essential for controlling the opening of the stomata and were started 30 min after switching on the lights of the (MacRobbie, 1997; McAinsh et al., 1997), either through growth chamber. In the same flasks in which the plants had its effect on various plasma membrane K+channels grown, the nutrient solution was changed for a new solution (Schroeder and Hagiwara, 1989) and the tonoplast (Ward according to the type of assay. Abscisic acid (4 mM, mixed isomers, Sigma) and HgCl2(1 mM) were added to this new and Schroeder, 1994) of guard cells, or on the ATPase of solution: ABA at the onset of exudation and HgCl22 h after H+in the plasma membrane of these cells ( Kinoshita the onset of exudation. Immediately after the change of solution, et al., 1995). the plants were detopped 1 cm above the transition zone, and Less is known about the effect of Ca2+on water pieces of tightly fitting latex tubing were affixed to the cut transport in the root system than in the shoot. It has stumps. The exudate from the xylem vessels was collected in test tubes during 3, 4 or 6 h according to the type of assay; been reported that salinity decreases hydraulic conducduring this period, the external medium of the root was tivity in maize roots, and that the addition of extra continuously aerated using an air pump, and kept at 25 °C. The calcium to the salinized media causes ameliorative effects volume of exudate collected was determined by measuring the on hydraulic conductivity (Azaizeh and Steudle, 1991; difference of weight of the test tube before and after the Azaizeh et al., 1992). collection period. The exudates were individually frozen and stored at −20 °C. Afterwards, the roots were individually The object of the present work was mainly to study washed for 5 min in 150 ml of a cold 5 mM CaSO4solution the effect of ABA and Ca2+status on the water flow in (5°C) to allow exchange of the cell walls contents. Finally, the sunflower root systems. The aim was to know whether roots were also weighed, frozen and stored at −20 °C. ABA and Ca2+are involved in regulating water transport across the root. Other analyses The osmotic pressure of the exudates and of the external medium was determined by means of a thermocouple psychro- Materials and methods meter (Decagon Devices, Inc., Pullman, WA, USA). The values of hydraulic conductivity of the roots (Lpr) were calculated Plant material and growth conditions using: Sunflower seeds (Helianthus annuus L. cv. Sun-Gro 380, Eurosemillas S.A., Co´rdoba, Spain) were surface-sterilized in Jv=ssrLpr(px–po)(1) 0.5% (v/v) sodium hypochlorite for 1 min, and germinated in the dark for 4 d at 28 °C in perlite moistened with 5 mM CaCl2.where Jv is the exudation rate; pxand poare the osmotic On the fourth day, the seedlings were put in a plant growth pressures of the exudate and the external medium, respectively; chamber with a relative humidity between 60% and 80%, and ssr is the overall reflection coefficient of the root, the value a day/night temperature of 22/18 °C, a photoperiod of 14 h of which is assumed to be unity (BassiriRad et al., 1991). of light and a photosynthetic photon flux density of K+and Ca2+were determined by atomic absorption 350 mmol m−2s−1(fluorescent tubes, Sylvania cool-white spectrophotometry (Perkin Elmer 1100 B), either directly in the VHO). The next day, the 5-d-old seedlings were transferred exudate or after extraction from the roots or shoots with a 10% individually to glass flasks wrapped in aluminium foil. The acetic acid solution (Benlloch et al., 1989). flasks contained 790 ml of a standard nutrient solution with the following composition: 2.5 mM KCl, 2.5 mM Ca(NO3)2,Experimental design and statistical analysis 1.0 mM MgSO4, 0.25 mM Ca(H2PO4)2, 12.5 mMH 3BO3, 1.0 mM MnSO4, 1.0 mM ZnSO4, 0.25 mM CuSO4, 0.2 mM A random experimental design was utilized in all cases. Linear regressions were established between the Ca2+status of the(NH4)6Mo7O24, and 10 mM Fe-ethylenediamine-di-o-hydroxy- phenylacetic acid. root and every one of the hydraulic parameters (exudation rate, hydraulic conductivity and osmotic pressure gradient), and alsoIn order to obtain plants with different Ca2+levels, the plants were transferred (without rinsing their roots) 24 h before with the Ca2+flux in the xylem sap. In all cases, the analysis of linear regression was significant (P<0.01).the exudation assay to a new nutrient solution with either Downloaded from https://academic.oup.com/jxb/article-abstract/50/339/1607/485182 by UNIVERSIDAD DE SEVILLA user on 11 April 2018 Xylem exudation in roots 1609 Fig. 1. Effect of the Ca2+status of the root and of ABA in the root medium on the exudation rate (A), the hydraulic conductivity of the root (B), the Ca2+flux of the xylem sap (C), and the osmotic pressure gradient between the xylem sap and the medium (D). Roots were exuding for 6 h in a solution containing CaSO4(0.1 mM), glucose (10 mM) and with (&) or without (%) ABA (4 mM). In all cases, the analysis of linear regression was significant (P<0.01). Results hydraulic parameters and with the Ca2+flux in the xylem sap; results showed that as the Ca2+level increased, the Although calcium starvation for 24 h neither modified exudation rate (Jv) also rose (Fig. 1A). Similarly, the plant growth (data not shown) nor altered the nutritive hydraulic conductivity of the root (Lpr) and the flux of K+status (Table 1), it reduced internal Ca2+content, exuded Ca2+(JCa) rose in direct proportion to Ca2+ which was more marked in the root (Table 1). The Ca2+ concentration in the root (Fig. 1B,C). In contrast, the level in the root affected the exudation process of isolated osmotic pressure gradient, formed between the xylem and roots exhibiting root pressure (Table 2). In roots with the external medium, decreased in direct proportion to low calcium levels, the exudation rate was inhibited (Jv)Ca2+concentration (Fig. 1D). and the stimulating effect of ABA on exudation disap- Addition of ABA (4 mM) at the onset of exudation peared (Table 2). produced various effects in the root (Fig. 1). The action The Ca2+status of the root was correlated with various Table 2. Effect of the Ca2+status of the root and of ABA in the Table 1. Shoot and root K+and Ca2+levels (expressed in mmol root medium on the exudation rate (Jv) (expressed in mlg−1 g−1FW) FW h−1) The plants were grown for 14 d in standard nutrient solution, and for 1 d in other solution with either Ca2+(control plants) or without Ca2+Roots were exuding for 6 h in a solution containing CaSO4(0.1 mM), glucose (10 mM) and with (+) or without (−) ABA (4 mM). Values(low-Ca2+plants) (see Materials and methods). Values are means of four plants ±SE of the mean. are means of four roots ±SE of the mean. Ca2+Treatment Exudation rate (Jv)Treatment K+Ca2+ Shoot Root Shoot Root Control −ABA 33.1±3.5 +ABA 47.6±5.3 Low-Ca2+−ABA 25.3±1.8Control 148.7±1.4 118.4±2.0 33.6±0.9 2.4±0.1 Low-Ca2+151.1±2.4 115.1±2.1 27.6±1.3 1.4±0.1 +ABA 26.8±2.0 Downloaded from https://academic.oup.com/jxb/article-abstract/50/339/1607/485182 by UNIVERSIDAD DE SEVILLA user on 11 April 2018 1610 Quintero et al. In other experiments, the long-term effect of HgCl2 was studied. Results showed that 2 h after its addition to the external medium of the roots, HgCl2(1 mM) significantly inhibited the exudation rate (Jv) and the hydraulic conductivity of the root (Lpr) (Table 3). Discussion Our results suggest that Ca2+and ABA are involved in the regulation of water flow across the root. The extent to which ABA enhanced water flow across the root depended on the concentration of Ca2+in the root: higher concentrations of Ca2+provided greater sensitivity to ABA (Table 2; Fig. 1A). The effect of Ca2+and ABA on water flow in the root can be explained only by their promotive effect on root hydraulic conductivity and not Fig. 2. Effect of HgCl2and of ABA in the root medium on the by changes in the osmotic pressure gradient between the exudation rate (Jv). Roots were obtained from plants grown in standard xylem and the surrounding medium (Fig. 1). The possibilnutrient solution for 15 d, and they were exuding for 3 h in fresh ity that the effect of Ca2+starvation on water flow in the standard nutrient solution plus glucose (10 mM). ABA (4 mM) was added to the root medium at the onset of exudation and HgCl2(1 mM) root could be produced by injurious effects on root 2 h later. Control (#), ABA ($), HgCl2(6), ABA plus HgCl2(+). structure, have to be discarded. In these experiments, Values are means of four roots ±SE. Ca2+starvation was not severe for different reasons: (1) when the plants were transferred to nutrient solutionof ABA on the exudation rate (Jv) and the hydraulic conductivity of the root (Lpr) also depended on the without Ca2+, the plant roots were not rinsed, retaining the apoplastic calcium; (2) the duration of starvation wasinternal Ca2+concentration: as that Ca2+concentration increased, so did the effect of ABA, with the result that only 24 h. Furthermore, the assays with detached root systems were always performed with Ca2+in the sur-its action was greater in roots with higher Ca2+concentrations. Significant differences were recorded with respect rounding medium (CaSO40.1 mM). In addition, Ca2+ starvation neither modified plant growth nor altered theto controls for concentrations above 2 mmol g−1fr. wt. (Fig. 1A, B). On the other hand, ABA had no significant nutritive K+status (Table 1), which indicates that the plants were not affected by serious disorders.effect on either the flux of exuded Ca2+(JCa) or the osmotic pressure gradient between the xylem and the The hydraulic conductivity of the root (Lpr) was calculated using equation 1. The value of ssr is controversial.external medium (Fig. 1C, D). In other experiments, the short-term effect (1 h) of Several authors have assumed that ssr=1 (Fiscus, 1981; BassiriRad et al., 1991; Quintero et al., 1998). However,HgCl2on water transport in roots was studied with and without ABA in the external medium (Fig. 2). In the other authors have reported that ssr values are substantially smaller than unity (Steudle and Frensch, 1996;presence of ABA (4 mM), the exudation rate (Jv) was greater than in the control roots (Fig. 2). The addition Steudle and Peterson, 1998). In bean root systems, Fiscus determined ssr=0.99, and ABA did not affect this valueof HgCl2(1 mM) to the external medium of the root 2 h after the onset of exudation had a rapid and dramatic (Fiscus, 1981). In excised maize roots, ssr values between 0.64 and 0.73 have been calculated (Azaizeh and Steudle,effect on the exudation rate (Jv) both in control and in ABA-treated roots. These results showed that the inhibit- 1991), and these values were not significantly different in roots with different calcium status. These findings indicateory effect of HgCl2on the exudation rate (Jv) was similar in the presence and in the absence of ABA (Fig. 2). that ssr does not change under the different experimental Table 3. Effect of the HgCl2in the root medium on the exudation rate (Jv), the hydraulic conductivity of the root (Lpr), and the osmotic pressure gradient between the xylem sap and the medium Roots were obtained from plants grown in standard nutrient solution for 15 d, and they were exuding for 4 h in fresh solution plus glucose (10 mM). HgCl2(1 mM) was added 2 h after the onset of exudation. Values correspond at 2 h after treatment and are means of four roots ±SE of the mean. Treatment Jv LprOsmotic pressure gradient (mlg−1FW h−1)(mlg−1FW h−1MPa−1) (MPa) Control 51.9±8.4 756.6±99.2 0.047±0.009 HgCl230.7±2.0 592.9±8.6 0.053±0.005 Downloaded from https://academic.oup.com/jxb/article-abstract/50/339/1607/485182 by UNIVERSIDAD DE SEVILLA user on 11 April 2018 Xylem exudation in roots 1611 conditions used in this work. It has been assumed here et al., 1999). Furthermore, in this work, at the end of the short-term experiments with Hg2+, roots looked healthythat ssr=1 in order to calculate Lpr, but even if ssr were smaller than unity, it would not affect the conclusions and their K+status was similar in both root types (124.8±3.3 and 129.3±1.6 in treated and control roots,about the effect of different treatments on Lpr. However, it is possible that the values of Lprcould be respectively). In long-term experiments, the results were similar. Therefore, it is considered that the Hg2+concen-underestimated. Curiously, findings elsewhere have reported that ABA tration and exposure durations used here only seemed to affect water transport through roots.and Ca2+are involved in regulating stomatal opening and closing. Under stress conditions, ABA inhibits the These results suggest that aquaporins were present in sunflower root cell membranes and were involved intranspiration across the stomata ( Kearns and Assmann, 1993); this action is preceded by an increase in cytosolic regulating water flow in the root when, as in this experiment, root pressure produced water flow in the radicleCa2+concentration in occlusive cells (McAinsh et al., 1990). In the present study, ABA and Ca2+promoted system (Quintero et al., 1998). The inhibitory effect of HgCl2was similar in both the presence and the absencewater flow across the root, seemingly contrasting with its effect on the transpiration across the stomata: in the root, of ABA (Fig. 2), suggesting that ABA and aquaporins are independently involved in the regulation of water flowhigh levels of Ca2+enhanced water flow; in the shoot, high Ca2+contents in occlusive cells contributed to in the root. stomatal closure. The apparently contradictory effects in the root and in the shoot have also been described for K+: stomatal closure, and the subsequent inhibition of Acknowledgement the transpiration across the stomata, was accompanied This work was supported by the Ministerio de Educacio ´ny by low levels of K+in occlusive cells (Kearns and Cultura (Spain) under grant number PB95–0976. Assmann, 1993); in the root, however, K+starvation promoted water flow (Quintero et al., 1998). The role of ABA and Ca2+in the regulation of water flow in the shoot (McAinsh et al., 1997) and in the root References (described here), and of K+in both plant organs (Kearns Azaizeh H, Gunse B, Steudle E. 1992. Effects of NaCl and and Assmann, 1993; Quintero et al., 1998), suggest that CaCl2on water transport across root cells of maize (Zea the same mechanism is involved in the action of ABA, mays L.) seedlings. Plant Physiology 99, 886–894. Azaizeh H, Steudle E. 1991. Effects of salinity on water Ca2+and K+in the root and in the shoot; although they transport of excised maize (Zea mays L.) roots. Plant manifest themselves differently, both effects pertain to the Physiology 97, 1136–1145. same physiological function at the whole-plant level: that BassiriRad H, Radin JW. 1992. Temperature-dependent water is, keeping the plant properly hydrated in situations and ion transport properties of barley and sorghum roots. of stress. II. Effects of abscisic acid. Plant Physiology 99, 34–37. BassiriRad H, Radin JW, Matsuda K. 1991. Temperature- It has recently been suggested that aquaporins are dependent water and ion transport properties of barley and involved in water transport over both long and short sorghum roots. I. Relationship to leaf growth. Plant distances (Chaumont et al., 1998). There is also evidence Physiology 97, 426–432. indicating that the mercurial sulphydryl reagents inhibit Benlloch M, Moreno I, Rodrı´guez-Navarro A. 1989. Two modes aquaporins and reduce root hydraulic conductivity of rubidium uptake in sunflower plants. Plant Physiology 90, 939–942. (Maggio and Joly, 1995; Carvajal et al., 1996, 1999). The Carvajal M, Cooke DT, Clarkson DT. 1996. Responses of wheat results reported here support this theory: the treatment plants to nutrients deprivation may involve the regulation of with 1 mM HgCl2swiftly inhibited water flow across the water-channel function. Planta 199, 372–381. root (Fig. 2). The effect of HgCl2on water transport may Carvajal M, Martı ´nez V, Alcaraz CF. 1999. Physiological be explained by its inhibitory effect on root hydraulic function of water channels as affected by salinity in roots of paprika pepper. Physiologia Plantarum 105, 95–101. conductivity and not by changes in the osmotic pressure Chandler PM, Robertson M. 1994. Gene expression regulated gradient between the xylem and the surrounding medium by abscisic acid and its relation to stress tolerance. Annual (Table 3). It has been suggested that the regulation of Review of Plant Physiology and Plant Molecular Biology sulphydryl groups by a plasma membrane reductase 45, 113–141. system can alter the transport of osmotically important Chaumont F, Barrieu F, Herman EM, Chrispeels MJ. 1998. Characterization of a maize tonoplast aquaporin expressed cations, specially K+, across the root cell plasma memin zones of cell division and elongation. Plant Physiology brane (Welch et al., 1993). However, the treatment with 117, 1143–1152. 1mMHg2+used in the present work, did not alter the Chrispeels MJ, Maurel C. 1994. Aquaporins: The molecular flux of K+into the xylem (data not shown). Others basis of facilitated water movement through living plant authors have obtained similar results even with higher cells? Plant Physiology 105, 9–13. Davies W, Zhang J. 1991. Root signals and the regulation of Hg2+concentrations (Maggio and Joly, 1995; Carvajal Downloaded from https://academic.oup.com/jxb/article-abstract/50/339/1607/485182 by UNIVERSIDAD DE SEVILLA user on 11 April 2018 1612 Quintero et al. growth and the development of plants in drying soil. Annual North GB, Nobel PS. 1991. Changes in hydraulic conductivity and anatomy caused by drying and rewetting roots of AgaveReview of Plant Physiology and Plant Molecular Biology 42, 55–76. deserti (Agavaceae). American Journal of Botany 78, 906–915. Quintero JM, Fournier JM, Ramos J, Benlloch M. 1998. K+Fiscus EL. 1981. Effects of abscisic acid on the hydraulic conductance of and the total ion transport through Phaseolus status and ABA affect both exudation rate and hydraulic conductivity in sunflower roots. Physiologia Plantarum 102,root systems. Plant Physiology 68, 169–174. Fournier JM, Benlloch M, De La Guardia MD. 1987. Effect of 279–284. Radin JW. 1990. Responses of transpiration and hydraulicabscisic acid on exudation of sunflower roots as affected by nutrient status, glucose level and aeration. Physiologia conductance to root temperature in nitrogen- and phosphorus-deficient cotton seedlings. Plant Physiology 92,Plantarum 69, 675–679. Gilroy S, Fricker MD, Read ND, Trewavas AJ. 1991. Role of 855–857. Radin JW, Matthews MA. 1989. Water transport properties ofcalcium in signal transduction of Commelina guard cells. The Plant Cell 3, 333–344. cortical cells in roots of nitrogen- and phosphorus-deficient cotton seedlings. Plant Physiology 89, 264–268.Glinka Z. 1980. Abscisic acid promotes both volume flow and ion release to the xylem in sunflower roots. Plant Physiology Schroeder JI, Hagiwara S. 1989. Cytosolic calcium regulates ion channels in the plasma membrane of Vicia faba guard65, 537–540. Joly RJ. 1989. Effects of sodium chloride on the hydraulic cells. Nature 338, 427–430. Steudle E. 1994. Water transport across roots. Plant and Soilconductivity of soybean root systems. Plant Physiology 91, 1262–1265. 167, 79–90. Steudle E, Frensch J. 1989. Osmotic responses of maize roots:Kearns EV, Assmann SM. 1993. The guard cellenvironment connection. Plant Physiology 102, 711–715. water and solute relations. Planta 177, 281–295. Steudle E, Frensch J. 1996. Water transport in plants: role ofKinoshita T, Nishimura M, Shimazaki K. 1995. Cytosolic concentration of Ca2+regulates the plasma membrane H+- the apoplast. Plant and Soil 187, 67–79. Steudle E, Henzler T. 1995. Water channels in plants: do basicATPase in guard cells of fava bean. The Plant Cell 7, 1333–1342. concepts of water transport change? Journal of Experimental Botany 46, 1067–1076.MacRobbie EAC. 1997. Signalling in guard cells and regulation of ion channel activity. Journal of Experimental Botany Steudle E, Peterson CA. 1998. How does water get through roots? Journal of Experimental Botany 49, 775–788.48, 515–528. Maggio A, Joly RJ. 1995. Effects of mercuric chloride on the Ward JM, Schroeder JI. 1994. Calcium-activated K+channels and calcium-induced calcium release by slow vacuolar ionhydraulic conductivity of tomato root systems. Plant Physiology 109, 331–335. channels in guard cell vacuoles implicated in the control of stomatal closure. The Plant Cell 6, 669–683.Markhart AH. 1984. Amelioration of chilling-induced water stress by abscisic acid-induced changes in root hydraulic Weatherley PE. 1982. Water uptake and flow in roots. In: Lange OL, Nobel PS, Osmond CB, Ziegler H, eds.conductance. Plant Physiology 74, 81–83. McAinsh MR, Brownlee C, Hetherington AM. 1990. Abscisic Encyclopedia of plant physiology, Vol. 12B. Berlin: Springer Verlag, 79–109.acid-induced elevation of guard cell cytosolic Ca2+precedes stomatal closure. Nature 343, 186–188. Welch RM, Norvell WA, Schaefer SC, ShaffJE, Kochian LE. 1993. Induction of iron (III) and copper (II) reduction inMcAinsh MR, Brownlee C, Hetherington AM. 1992. Visualizing changes in cytosolic-free Ca2+during the response of stomatal pea (Pisum sativum L.) roots by Fe and Cu status. Does the root-cell plasmalemma Fe(III)-chelate reductase perform aguard cells to abscisic acid. The Plant Cell 4, 1113–1122. McAinsh MR, Brownlee C, Hetherington AM. 1997. Calcium general role in regulation cation uptake? Planta 190, 555–561. Yamamoto YT, Taylor CG, Acedo GH, Cheng CL, Conklingions as second messengers in guard cell signal transduction. Physiologia Plantarum 100, 16–29. MA. 1991. Characterization of cis-acting sequences regulating root-specific gene expression in tobacco. The Plant CellMunns R, Passioura JB. 1984. Hydraulic resistances of plants. III. Effects of NaCl in barley and lupin. Australian Journal 3, 371–382. Zhang WH, Tyerman SD. 1991. Effect of low O2concentrationof Plant Physiology 11, 351–359. Niemietz CM, Tyerman SD. 1997. Characterization of water and azide on hydraulic conductivity and osmotic volume of the cortical cells of wheat roots. Australian Journal of Plantchannels in wheat root membrane vesicles. Plant Physiology 115, 561–567. Physiology 18, 603–613. Downloaded from https://academic.oup.com/jxb/article-abstract/50/339/1607/485182 by UNIVERSIDAD DE SEVILLA user on 11 April 2018