Physiologia Plantarum 133: 354–362. 2008 Copyright ª Physiologia Plantarum 2008, ISSN 0031-9317

Light-induced transpiration alters cell water relations in figleaf ( ficifolia) seedlings exposed to low root temperatures Seong Hee Leea, Janusz J. Zwiazeka and Gap Chae Chungb,* aDepartment of Renewable Resources, 4-42 Earth Sciences Building, University of Alberta, Alberta, Canada T6G 2E3 bDivision of Biotechnology, Agricultural Plant Stress Research Center, College of Agriculture and Life Sciences, Chonnam National University, Gwangju 500-757, Korea

Correspondence Water relation parameters including elastic modulus (e), half-times of water *Corresponding author, w exchange (T 1/2), hydraulic conductivity and turgor pressure (P) were e-mail: [email protected] measured in individual root cortical and cotyledon midrib cells in intact figleaf gourd (Cucurbita ficifolia) seedlings, using a cell pressure probe. Received 4 December 2007; revised 7 January 2008 Transpiration rates (E) of cotyledons were also measured using a steady-state porometer. The seedlings were exposed to low ambient (approximately 22 21 doi: 10.1111/j.1399-3054.2008.01082.x 10 mmol m s ) or high supplemental irradiance (approximately 300 mmol m22 s21 PPF density) at low (8C) or warm (22C) root temperatures. When exposed to low irradiance, all the water relation parameters of cortical cells remained similar at both root temperatures. The exposure of cotyledons to supplemental light at warm root temperatures, however, resulted in a two- to w three-fold increase in T 1/2 values accompanied with the reduced hydraulic conductivity in both root cortical (Lp) and cotyledon midrib cells (Lpc). Low root temperature (LRT) further reduced Lpc and E, whether it was measured under low or high irradiance levels. The reductions of Lp as the result of respective light and LRT treatments were prevented by the application of 1 mM ABA. Midrib cells required higher concentrations of ABA (2 mM) in order to

prevent the reduction in Lpc. When the exposure of cotyledons to light was accompanied by LRT, however, ABA proved ineffective in reversing the inhibition of Lp. LRT combined with high irradiance triggered a drastic 10-fold reduction in water permeability of cortical and midrib cells and increased e and w T 1/2 values. Measurement of E indicated that the increased water demand by the transpiring was fulfilled by an increase in the apoplastic pathway as principal water flow route. The importance of water transport regulation by transpiration affecting the hydraulic conductivity of the roots is discussed.

Introduction uptake, being detrimental to growth of cucumber plants Figleaf gourd is frequently used as a rootstock for a (Ahn et al. 1999a). Following the initial study (Ahn et al. chilling-sensitive cucumber plant in areas in which low 1999b), a series of studies showed that the hydraulic root temperatures (LRT) interrupt the process of water conductivity of cortical cells (Lp) of figleaf gourd roots

Abbreviations – AQP, aquaporin; e, elastic modulus; E, transpiration rate; Lp, hydraulic conductivity of root cortical cells; Lpc, w hydraulic conductivity of cotyledon midrib cells; P, turgor pressure; LRT, low root temperature; T 1/2, half-times of pressure relaxation.

354 Physiol. Plant. 133, 2008 did not change significantly in response to LRT (8C), with reduced AQP expression, the osmotic water per- unlike that of the chilling-sensitive cucumber root sys- meability of isolated protoplasts was reduced to 17- to tem (Lee and Chung 2005, Lee et al. 2004, Lee et al. 30-fold (Martre et al. 2002). This indicates that when the 2005a, 2005b). In those studies, we measured Lp of corti- cell-to-cell water flux constitutes a significant proportion cal cells using excised roots (Lee et al. 2005a, 2005b). For of the total root water flux in Arabidopsis plants, it can be the evaluation of Lp, half-times of water exchange of regulated by the activity of AQP or by the abundance of w individual cells (T 1/21/Lp) were used as a direct AQPs in cell membranes. measure of changes in hydraulic conductivity because In the present study, we hypothesized that the presence the cell elastic modulus (e) and turgor pressure (P) did not of transpiration may induce the alteration of cellular change significantly when temperature was reduced (Lee water relations in the cotyledon midrib and root cortex in et al. 2005b). intact, not excised, figleaf gourd plants and also modify However, it has been known that transpiration strongly cell responses to LRT. We investigated the changes in cell affects P in cortical cells, a process that cannot be ob- P and e because the elastic properties of cells, which are served with excised roots. In wheat and maize, for exam- the functions of turgor pressure, are as significant as the ple, radial P gradients along the cortical cells were not water permeability of the cell membrane (Steudle et al. found under the conditions of 100% relative humid- 1977). Because our previous study showed that ABA was ity, following excision, and in plants without mature an effective modulator of AQP gating (Lee et al. 2005a), leaves (Rygol et al. 1993). It is particularly interesting to we also tested this assumption again with intact figleaf note that the common features of each of these appear to gourd plants, in the presence of both LRT and high be a lack of transpiration flow across the root cortex. The irradiance. presence of such P gradients is a sharp contrast to the previous works on same species (Zhu and Steudle 1991), Materials and methods possibly because of the artificial conditions that are associated with the use of excised segments of root. Palta Plant material et al. (1987) also noticed that light-induced transpiration clearly influenced both individual cell P and the overall of the figleaf gourd (Cucurbita ficifolia Bouche´) dimensions of the taproot of beet. The question, were germinated for 2–3 days at 25C in the dark on then, arises as to the influence of stomatal conductance tap water-soaked filter paper. After germination, the - on the overall water relations in the root system. In lings were transferred to 5-l containers (eight seedlings addition, strong light intensifies the effect of LRT with the per container) with aerated 1/5 strength solu- necrotic lesions and injuries appearing on the leaves tion (Cooper 1975). The containers were positioned in (Allen and Ort 2001, Jun et al. 2001), indicating that a growth chamber (12-h photoperiod, temperature: 25/ the sensitivity or tolerance of a plant to low temperature 22C, PPF density: approximately 300 mmol m22 s21). may not be determined if excised roots are used as The figleaf gourd seedlings utilized in the experiments experimental materials. Changes in hydraulic conductiv- were 6–8 days old and possessed fully expanded ity of root and cells occurring in accordance with cotyledons. transpiration demand may provide an additional mech- anism for the modulation of water relations. Neverthe- Light and low root temperature treatments less, it remains unclear as to the manner in which root and leaf hydraulic conductivities are coordinated and linked The figleaf gourd plants were exposed to supplemental with transpiration. light (approximately 300 mmol m22 s21) or ambient light It has been proposed that at high rates of transpiration, (approximately 10 mmol m22 s21 PPF density) during the apoplast constitutes the principal water flow route in LRT treatment. LRT (8C) treatment was applied with roots, and the hydraulic resistance of roots is low, thus a circulating water bath (Haake, Berlin, Germany). For facilitating rapid water uptake (Steudle and Peterson the cell pressure probe measurements, cold solution was 1998). This bulk flow is driven by hydrostatic gradients pumped through a heat exchanger in order to adjust the established by transpiration. In the roots, the vascular temperature to the desired values using a thermostat elements of the xylem function as ducts, which collect cooler. The solution temperature was controlled with water and transfer it rapidly to the shoots. On the contrary, a thermocouple, which was positioned proximally to the at low rates of transpiration, such as occur at night or roots. External medium was circulated to minimize under stress conditions, osmotic flow through aquaporins unstirred layers outside the root. The fixed root system (AQPs) may constitute the predominant pathway (Steudle was covered with paper towel to prevent the infiltration and Peterson 1998). In transformed Arabidopsis plants of light to root. The roots of intact figleaf gourd plants

Physiol. Plant. 133, 2008 355 were punctured with a microcapillary tube at low (8C) or illumination of the medium, the solution was positioned warm (22C) temperatures, which were controlled by the far away from the light exposure. Thereafter, pressure w external thermostat. At the same time, the cotyledons relaxations were conducted in order to measure T 1/2 in were exposed to either supplemental light using a lamp both root and midrib cells. (MT 400 DL/BH; Eye, Iwasaki, Japan; and Connector PDc6, 400W, Switzerland) or laboratory ambient light. In Data analysis order to avoid the excessive heating of the leaf because of light, the lamp was positioned over overhead, and mea- The data were analyzed using paired and unpaired t-test surements of leaf temperatures with a porometer (LI-1600; to determine the effects of light and LRT. Results were LI-COR Inc., NE) confirmed that the additional light did considered statistically significantly different at P 0.05. not induce a significant increase in leaf temperature. Results Cell pressure probe and transpiration measurements Cell elasticity A cell pressure probe was employed to determine the The e of root cortical cells exposed to ambient and sup- w T 1/2, P and e of individual cortical cells in the primary plemental irradiance measured 5.1 0.3 and 5.8 roots and midrib cells of the cotyledons in the intact 0.3 MPa (mean SE,n¼ 14, paired t-test, P ¼ 0.23). figleaf gourd plants (Azaizeh et al. 1992). The probe was Similar to the root cortical cells, a small increase in e of the filled with silicone oil (type AS4; Wacker, Mu¨nchen, cotyledon midrib cells from 6.5 0.6 to 7.6 0.5 MPa Germany). The tip of a glass capillary tube attached to the (n ¼ 10) in response to light was observed (paired t-test, probe ranged from 7 to 10 mm. Each of the intact plants P ¼ 0.17). In another set of measurements, when the was fixed to a metal sledge with magnets. The nutrient cotyledons were exposed to high irradiance and, at the solution, as used for the hydroponic culturing of the same time, root temperature was reduced from 22Cto seedlings, flowed along each of the roots. When the root 8C, there was a strong increase in e of the cotyledon cortical or cotyledon midrib cells of intact plants were midrib cells from 7.9 0.4 to 10.1 0.5 MPa (n ¼ 7, punctured, different light and temperature treatments paired t-test, P ¼ 0.08). Similarly, the increase in e of root were applied and measurements were performed before cortical cells from 6.3 0.4 to 9.1 0.9 MPa (n ¼ 8, and after the treatment. When a cell was punctured, the paired t-test, P ¼ 0.01) was quite large. cell sap formed a meniscus with the oil. The meniscus was then gently pushed to a position close to the surface of the Effect of irradiance level and root temperature on root and midrib in order to restore the original cell the water relations of root cortical cells volume. P became steady within a few minutes and then the hydraulic parameters of the cell were evaluated Under low irradiance (approximately 10 mmol m22 s21)

(Azaizeh et al. 1992). In order to measure Lp and Lpc, and warm temperature conditions (22C), the P of cortical w hydrostatic pressure relaxations were generated by the cell and T 1/2 of the intact figleaf gourd roots were probe and recorded on a computer, and Lp and Lpc measured between 0.3 and 0.4 MPa and 1.1 and 1.3 s, calculated according to Azaizeh et al. (1992). For the respectively (Fig. 1A). When illuminated at high irradi- measurements, 7–14 cells (one cell per plant, n ¼ 7–14) ance (approximately 300 mmol m22 s21), P was reduced were used. by 0.05–0.1 MPa and Lp was reduced by two- to three- The transpiration rates of the cotyledons (E) were fold as compared with low irradiance (Fig. 1A, Table 1). measured using a steady-state porometer (LI-1600; LI- However, LRT alone did not increase P (data not shown). w COR Inc.) as described previously (Wan et al. 2004a). Both P and T 1/2 almost recovered to original values following a return to low irradiance within 10 min (Fig. 1A). Under low irradiance conditions, Tw did Application of ABA 1/2 not change regardless of whether the root temperature w One millimolar ABA stock solution was prepared by was low or warm and hence ABA did not affect T 1/2,as dissolving (1/2)-cis, trans-ABA (Sigma Chemicals, shown in Table 1 and Fig. 2A. The application of high Deisenhofen, Germany) in methanol and stored at 220C. irradiance alone at 22C to intact seedlings resulted in The working solution was made by diluting this stock a reduction in Lp by more than 50% (Table 1). Further- solution. When the cells had achieved steady half-times, more, LRT combined with high irradiance resulted in 1or2 mM ABA was added to the circulating root medium a drastic reduction by 10-fold in Lp (Table 1) and w before or following the treatments. In order to avoid increased T 1/2 (Fig. 2B). Interestingly, the application

356 Physiol. Plant. 133, 2008 Table 1. Effects of irradiance levels (low irradiance: approximately A Root (22°C) 10 mmol m22 s21 and high irradiance: approximately 300 mmol m22 s21) 0.45 and root temperatures (low: 8C and warm: 22C) on the hydraulic Low irradiance conductivity of root cortical cells (Lp) and cotyledon midrib cells (Lp )of High irradiance c 0.40 intact figleaf gourd plants. One or 2 mM ABA was applied to the root cortical and cotyledon midrib cells, respectively, before or after LRT Low irradiance treatment. For the measurements, 7–14 cells were punctured (one cell (MPa) 0.35 per plant). Means SE are shown (n ¼ 7–14). Different letters in each P w T = 1.3 s column indicate significant differences (unpaired t-test, P 0.05). 1/2 T w = 1.2 s 1/2 0.30 Lp Lpc 26 21 21 26 21 21 w Irradiance Treatment (10 ms MPa ) (10 ms MPa ) T = 3.4 s 1/2 0.25 Low 22C 1.10 0.15a 1.03 0.20e 0103040 8C 0.82 0.15a 0.30 0.01f 22C 1 ABA 0.90 0.10a 1.01 0.24e B Cotyledon (22°C) a e 0.60 Low irradiance 8C 1 ABA 1.00 0.16 1.01 0.17 ABA 1 8C 0.90 0.13a — T w = 1.6 s 1/2 High 22C 0.40 0.03c 0.30 0.01f 0.45 d h High irradiance 8C 0.08 0.01 0.12 0.00 22C 1 ABA 0.73 0.17b 0.56 0.07g 8C 1 ABA 0.07 0.00d 0.25 0.01f

(MPa) 0.30 a Low irradiance ABA 1 8C 1.19 0.18 — P T w = 1.5 s 1/2 0.15 w T w = 3.2-3.9 s treatment proved ineffective in reducing T 1/2 (Fig. 3A). 1/2 w However, 2 mM ABA reduced T 1/2 in the midrib cells of 0.00 0102030 LRT-treated plants (Fig. 3B) to the levels measured in the Time (min) cotyledon midrib cells of plants exposed to warm (22C) root temperatures. The high irradiance treatment resulted

Fig. 1. Typical responses of turgor pressure (P) and half-times for water in a reduction in Lpc in the midrib cells by three- to four- w exchange (T 1/2) of individual root cortical (A) and cotyledon midrib (B) fold as compared with what was observed under low cells of intact figleaf gourd seedlings exposed to different irradiances irradiance conditions and Lpc decreased by an additional 22 21 (approximately 10 and 300 mmol m s ) as described in the Materials factor of three times in the presence of LRT (Table 1). The and methods. treatment of the roots with 2 mM ABA resulted in a partial

reversal of the effects of high irradiance and LRT on Lpc of ABA prior to LRT treatment prevented the reduction in w Lp and restored its value to those seen under low and T 1/2 (Fig. 3C, Table 1). irradiance conditions with warm temperature treatment (Table 1). Effect of irradiance level and root temperature on the transpiration rates Effect of irradiance level and root temperature on the water relations of cotyledon midrib cells Exposure to high irradiance caused an approximately two-fold increase in transpiration rates (E) at both root When measured under low irradiance conditions, the P of temperatures (22 and 8C) (Fig. 4). At both high and low cotyledon midrib cells varied between 0.3 and 0.5 MPa irradiance levels, LRT strongly reduced E. Application (Fig. 1B). The range of values was somewhat greater than of ABA after LRT had no effect on E. At warm root w that observed in the root cortical cells. However, T 1/2 temperature, the application of 2 mM ABA reduced E by was similar to the values observed in root cortical cells, approximately two-fold when exposed to high irradiance and measured between 1 and 2 s in the presence of low but no effect at low irradiance. However, application of irradiance (Fig. 1B). P was reduced by approximately ABA prior to LRT increased E at low irradiance. w 0.1–0.2 MPa and T 1/2 increased by two- to four-fold upon the application of high irradiance (Fig. 1B). Tw 1/2 Discussion and P recovered to near original values following a return to low irradiance (Fig. 1B). Under low irradiance con- The insensitivity of figleaf to LRT was previously ditions, LRT induced a reduction of Lpc by a factor of 3 shown with excised root systems, based on the identical w (Table 1). The application of ABA at 1 mM after LRT T 1/2 values when the root temperature was lowered to

Physiol. Plant. 133, 2008 357 A A Low irradiance (8°C) Root 1.0 Low irradiance (8°C) Cotyledon 0.6

0.8 + 1 µM ABA 0.5 1 µM ABA 0.6 (MPa)

P (MPa) w w

P T = 3.5 s T = 4.1 s 0.4 1/2 1/2 0.4 w T w = 4.0 s w T = 1.3 s 1/2 T = 3.0 s 1/2 T w = 1.3 s 1/2 T w = 1.2 s 1/2 1/2 0.2 0.3 0.0 012 78910 01236 81012 B B Low irradiance (8°C) Cotyledon High irradiance (8°C) Root 0.8 0.6

2 µM ABA 0.6 0.4 1 µM ABA T w = 1.0 s T w = 3.5 s 1/2

(MPa) 1/2

P (MPa) 0.4

w w P T = 3.7 s 0.2 T w = 15 s 1/2 T = 1.1 s 1/2 T w = 14 s 1/2 1/2 T w = 14 s 1/2 0.2

0.0

0125 6 01238101214 Time (min) C 0.6 High irradiance (8°C) Cotyledon Fig. 2. Typical results of the effect of low root temperature (8C) and irradiance levels (low irradiance (A): approximately 10 mmol m22 s21 and high irradiance (B): approximately 300 mmol m22 s21) on the turgor 2 µM ABA w pressure (P) and half-times of water exchange of root cortical cells (T 1/2) 0.4 of intact figleaf gourd seedlings. ABA was applied during the low tem- perature treatment. w P (MPa) w T = 4.1 s T = 7.0 s 1/2 1/2 w T w = 3.1 s T = 5.9 s 1/2 0.2 1/2 8C compared with 20C (Lee et al. 2005a, 2005b) and similar results were obtained in the present study when the intact plants were exposed to LRT under low irra- diance levels. The most interesting finding in this study is 0123 78910 that combined treatment with LRT and high irradiance Time (min) induced drastic increases in e, resulting in low Lp and Lp c Fig. 3. Typical results of the effects of low root temperature (8C) and values, thereby indicating that the intact figleaf gourd irradiance levels (low irradiance (A and B): approximately 10 mmol m22 plants evidence different water relations compared with s21 and high irradiance (C): approximately 300 mmol m22 s21) on the those observed in the excised root systems. It was turgor pressure (P) and half-times of water exchange of cotyledon midrib w w suggested that the insensitivity of Lp and T 1/2 to LRT as cells (T 1/2) of intact figleaf gourd seedlings. One (A) or 2 mM (B and C) well as to ABA could represent the LRT-resistant char- was applied during the low temperature treatment. acteristics of figleaf gourd root systems (Lee et al. 2005b). The P of the root cortical and cotyledon midrib cells in potential (Cayley et al. 2000, Steudle 2001). Increased w intact plants declined upon exposure to high irradiance T 1/2 in both cortical and midrib cells infers an increased w accompanied by increase in T 1/2. This reduction could hydraulic resistance in the cell-to-cell pathway and that be reversed by a return to low irradiance as shown in tension caused by high transpiration, may partially cause Fig. 1. A decrease in P may occur in transpiring plants, a switch from cell-to-cell to hydraulic apoplastic trans- either because of osmoregulatory changes or by cohe- port. This has been demonstrated by Morillon and sion/tension, thereby resulting in a reduction in water Chrispeels (2001) with Arabidopsis, in that a negative

358 Physiol. Plant. 133, 2008 e w 15 high Lp. As partially determines the T 1/2, one should Low irradiance w expect that T 1/2 would decrease. However, in the High irradiance w c present study, there was an increase in T 1/2, which 12

) resulted in a decrease in Lp during high-light-combined

–1 w s LRT. It is likely that increased T 1/2 and e may induce –2 9 higher resistance of water flow as a survival strategy a against light-combined LRT.Because Lp and e are strongly a a a a a 6 a w

(mmol m dependent on P in some algae, it might increase the T 1/2

E b b with decreasing the P (Steudle et al. 1982). 3 The maintenance of the balance of the soil–plant–air continuum requires a responsive and elaborate water flux

0 control system. There is growing evidence that AQPs 22°C8°C 22°C + 8°C + 2 µM ABA perform a key function in the regulation of the hydraulic 2 µM ABA 2 µM ABA + 8°C conductance of leaves and roots, and that together with Treatment stomatal conductance, these processes are utilized to regulate water flux in plants (Clarkson et al. 2000, Fig. 4. Effects of root temperature (22 and 8C) and irradiance levels Cochard et al. 2004, Javot and Maurel 2002, Martre (low irradiance: 10 mmol m22 s21 and high irradiance: 300 mmol m22 s21) on the transpiration rates (E) in cotyledons of figleaf gourd seedlings. et al. 2002, Steudle 2001, Tyree et al. 2005, Wan et al. ABA was applied before or after low temperature treatment. Means SE 1999). It is believed that in rapidly transpiring plants, are shown (n ¼ 6–10 plants). Different letters indicate significant hydrostatic forces predominate over osmotic forces, and differences (unpaired t-test, P 0.05). the apoplastic path plays a predominant role in this process (Steudle and Peterson 1998). By way of contrast, correlation between high transpiration rate and AQP (PIP) when the stomata are closed, osmotic forces predomi- abundance exists. This suggests that high water flux nate, and water movement occurs largely through the associated with the transpiration stream follows domi- cell-to-cell pathway (Steudle and Peterson 1998). In an nantly an apoplastic pathway without the involvement of earlier study, we showed that the ratio of hydrostatic and AQPs. In contrast, Cochard et al. (2007) concluded that osmotic Lp in the figleaf gourd root system was ap- leaf hydraulic conductance increased in response to light proximately 3:4, using a pressure chamber (Lee et al. without stomatal opening and water flow across walnut 2005a). Our present results appear to bolster the notion leaves preferentially followed a cell-to-cell pathway that with increasing transpiration-driven hydrostatic closely linked to membrane AQPs. The transcriptional forces, cell-to-cell transport may become less important regulation of AQPs by light eventually led to activation of in figleaf gourd roots and cotyledons. a cell-to-cell route and consequently higher leaf conduc- Our present results evidenced a profound increase in w tance. The ability of plants to maximize leaf gas exchange T 1/2 (decrease in Lp) of root cortical and midrib cells while buffering leaf water status, through the control of of intact plants, thereby suggesting that, unlike in the AQP abundance, was proposed to show the ‘revised excised roots, water was transported principally through composite water transport model’ (Cochard et al. 2007). the apoplastic pathway during transpiration. Therefore, Whether time required to synthesize new AQPs under the increased root pressure observed in the excised high irradiance or other environmental conditions makes roots (Lee et al. 2005a) is not likely to be present in the such subtle differences with regard to the water transport root system of an intact plant exposed to light. Second, the w pathway deserves further investigation. T 1/2 value in the root cortical cells of the excised figleaf The fact that the presence of gradients of P along the gourd was affected less by LRT than were the excised root cortical cells was abolished by stopping transpira- cucumber roots (Lee et al. 2005a, 2005b), which have tion or excising the roots indicates a direct transpiration been demonstrated to be highly sensitive to LRT. How- effect on root cell P (Meuser and Frensch 1998). Tyerman ever, contrary to the previous results (Lee et al. 2005a, et al. (1989) had reported that the relation between e and 2005b), LRT combined with light induced a 10-fold

P was not altered significantly by different NaCl concen- decrease in the Lp and Lpc (Table 1) values of the intact trations but e tended to be small for roots grown at high figleaf gourd plants, thereby indicating an increased salinity because these roots had lower P. However, in the resistance in the cell-to-cell pathway, and therefore, the present study, there was a clear tendency for e to increase contribution of this pathway to total water flow may be in response to light combined with LRT, although P diminished. The apoplastic conductance can be inferred declined. The higher e means a low extensibility and from changes in the cell Lp and transpiration rates. Cell w a rigid cell wall and is associated with lower T 1/2 and hydraulic conductivity manifested a pattern similar to

Physiol. Plant. 133, 2008 359 that observed in the entire root system (Lee et al. 2005a). conductivity of cucumber roots (Lee et al. 2005b). We Dramatic increases in osmotic hydraulic conductivity, also demonstrated that ABA treatment caused increases in when the excised root system was returned to higher Lp and Lpc in plants exposed to supplemental light, as temperature from LRT, suggested an increase in AQP- compared with the plants that were not pretreated with mediated transport (Lee et al. 2005a). However, it is ABA. Hence, it appears plausible that ABA may, either interesting that the overall axial water flow did not re- directly or indirectly, stabilize AQPs, thereby enhancing turn to the initial value in intact figleaf gourd when light- the resistance of roots to unfavorable losses of water combined LRT was changed to the higher temperature permeability occurring during stress conditions (Lee et al. with light (data not shown). It suggests that light- 2005b). combined LRT may breakdown the AQPs in cell mem- In conclusion, we showed in the present study that brane of intact plants, unlike that of excised root system. figleaf gourd plants responded to light with higher

Because the responses of water flow were quite different in transpiration rates and reductions in Lp and Lpc, thereby excised and intact plants, it could be assumed that indicating that the increased water demand by the regulation of AQPs between root and shoot might link transpiring plants was fulfilled by an increase in the during transpiration. However, this response needs to be apoplastic pathway. When the exposure of leaves to light further studied and confirmed in whole plant and cell level. was accompanied by LRT, ABA proved ineffective in Although Christmann et al. (2007) have demonstrated reversing the inhibition of Lp, thereby suggesting that that ABA is synthesized upon osmotic stress primarily in different mechanisms may be involved in the deactivation w the leaves, not in the roots, the role of ABA as a long- of AQP activity. The drastic increase in e and T 1/2 in the distance stress signal generated in the roots has been case of combined LRT and high irradiance treatment firmly established (Sauter et al. 2001, 2002). ABA pro- administered to the intact plant provides insight into the duced in the roots during drought stress is transported question as to whether water flow through an excised in the transpiration stream to the shoots where ABA root system is representative of that occurring in in- improves growth by increasing water status and cell tact transpiring plants (Cosgrove 1997, Neil Emery and turgor through transpirational control (Thompson et al. Salon 2002). 2007). Hose et al. (2000) reported that ABA augmented the water permeability of root cell membranes and the Acknowledgements – The excellent technical support of osmotic hydraulic conductivity of whole roots by in- Burkhard Stumpf (Department of Plant Ecology, University fluencing the water channels. ABA also regulates the of Bayreuth, Germany) is acknowledged. This work was sup- AQP gene expression as reported in different plant ported by grants from the Korea Science and Engineering Foun- species including Arabidopsis (Jang et al. 2004), rice dation (KOSEF) to the Agricultural Plant Stress Research Center (Liu et al. 1994), Brassica napus (Gao et al. 1999) and (APSRC, R11-2001-09201004-0) of Chonnam National Uni- radish (Suga et al. 2002). Although there remains no clear versity, Korea, and from the Natural Sciences and Engineering understanding of the mechanisms of ABA action on Research Council of Canada (NSERC). International Joint Research between Korea and Germany, supported by the AQPs, it has been suggested that ABA binds to AQPs and respective governments is also appreciated. reduces the activation energy for AQPopening (Wan et al. 2004b). The present results demonstrate that ABA could partially reverse the reduction in cell hydraulic conduc- tivity caused by high irradiance and LRT in roots and References cotyledons, and this is probably ascribable to its effect on Ahn SJ, Im YJ, Chung GC, Cho BH, Suh SR (1999a) AQPs. Interestingly, unlike those of root cortical cells, Physiological responses of grafted cucumber leaves and cotyledon midrib cells exposed to supplemental light rootstock roots affected by low root temperature. Sci and/or LRTrequired higher concentrations of ABA (2 mM) Hortic 81: 397–408 to partly reverse the inhibition of cell hydraulic conduc- Ahn SJ, Im YJ, Chung GC, Cho BH (1999b) Inducible tivity. The uptake and transport dynamics of ABA in our expression of plasma membrane H1-ATPase in the roots study may differ between roots and cotyledons. There- of figleaf gourd plant under chilling root temperature. fore, it is difficult to speculate regarding the physiological Physiol Plant 106: 35–40 significance of the differences in ABA concentrations Allen DJ, Ort DR (2001) Impacts of chilling temperatures required to generate responses in root and cotyledon on photosynthesis in warm-climate plants. Trends Plant cells. The application of ABA prior to administration of Sci 6: 36–40 LRT treatment prevented a reduction in cell water per- Azaizeh H, Gunse B, Steudle E (1992) Effects of NaCl and meability, and ABA supplied following the onset of chil- CaCl2 on water transport across root cells of maize ling exerted an ameliorative effect on the cell hydraulic (Zea mays L.) seedlings. Plant Physiol 99: 886–894

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