Plant and Soil 267: 319–328, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
319
Growth responses of seminal roots of wheat seedlings to a reduction in the water potential of vermiculite Mohammad Akmal1,2 & Tadashi Hirasawa1,3 1 Faculty
of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183-8509, Japan. Agronomy, NWFP Agricultural University, Peshawar, Pakistan. 3 Corresponding author∗
2 Department of
Received 7 December 2003. Accepted in revised form 9 April 2004
Key words: growth, seminal root, solute accumulation, turgor, water potential, wheat
Abstract We examined the elongation rate, water status and solute accumulation in the seminal roots of wheat seedlings (Triticum aestivum L.) that were growing in vermiculite with a water potential (w ) ranging from −0.03 to −1.10 MPa. The elongation rate of the primary seminal root was similar to that of the first pair of seminal roots but that of the second pair of seminal roots was lower at all values of w tested. The elongation rate was highest in vermiculite with a w of −0.03 MPa but did not decrease significantly until the w was reduced to −0.15 MPa. Further reductions in w reduced the elongation rate markedly. The w of mature tissues was always similar to that of vermiculite. The osmotic potential (o ) decreased to the same extent as the decrease in w . Thus, the turgor pressure (p ) remained unchanged even in vermiculite with a low w . In elongating tissues, w and o were far lower than they were in mature tissues and, thus, reductions in turgor were not significant. Even when the w of vermiculite changed, there were no consistent changes in terms of a difference in w between elongating plus mature tissues and vermiculite. There were also no consistent changes in levels of osmotica, calculated using the van’t Hoff’s law, in the elongating tissues but the levels in mature tissues increased in vermiculite with a low w . Our results suggest that (1) reductions in root elongation in vermiculite with a low w were caused by reductions in the extensibility and/or increases in the yield threshold of cell walls and by reductions in the hydraulic conductivity of the tissues; and (2) a seminal root regulates its growth to keep turgor pressure unchanged. Abbreviations: o – osmotic potential; p – turgor potential; w – water potential.
Introduction The responses of plants to a reduction in w are reflected most conspicuously by changes in growth rate (Hsiao et al., 1973), but, under drought conditions, growth of roots is not suppressed, as compared with that of shoots (Blum et al., 1983), and, in fact, the elongation of roots can actually be promoted by drought (Hida et al., 1995; Hirasawa et al., 1994, 1998; Sharp and Davies, 1979). Increases in root growth might clearly be advantageous to plants in drying soil and might be important for the establishment of seedlings, which are so vulnerable to drying of ∗ FAX No: +81-42-360-8830. E-mail:
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the surface layers of soil. Many researchers have suggested that the depth and density of the root system are very important for the avoidance of water stress not only under conditions of severe depletion of soil moisture (Angus et al., 1983; Inanaga et al., 1996; Proffitt et al., 1985; Yoshida and Hasegawa, 1982), but also when soil is only moderately moist (Hida et al., 1995; Hirasawa et al., 1994). It is important to clarify the mechanisms responsible for the growth responses of roots to fluctuations in soil moisture. Roots under drought stress have a substantial capacity for osmotic adjustment in elongating regions (Greacen and Oh, 1972; Serraj and Sinclair, 2002; Sharp and Davies, 1979; Westgate and Boyer, 1985), which helps to maintain turgor under drought conditions. How-
320 ever, details of the maintenance of turgor, as a result of osmotic adjustments, in the control of root growth have not been considered in any detail (Kramer and Boyer, 1995; Pritchard et al., 1991). If we are to understand the significance of osmotic adjustments in plant growth, we need to examine the actual status of osmoticum accumulation not only in growing regions but also in mature tissues as a whole. Wheat, one of the most important crops, worldwide, is grown in both dry and humid regions. The growth of wheat plants is strongly influenced by the availability of soil moisture during the growing season (Pandey et al., 2001). The development of the root system has a significant effect on the performance of a wheat plant in moisture-deficient soil (Proffitt et al., 1985) and even in humid soil (Nakamura et al, 2003). The growth of wheat roots is strongly influenced by soil moisture (Mian et al., 1993; Morita and Okuda, 1994; Nakamura et al., 2003). Pritchard et al. (1991) found that reductions in root growth under moderate osmotic stress were due primarily to wall rheology and not to cell turgor. Once osmotic adjustment has been completed, cell turgor remained constant throughout the expansion region (Pritchard et al., 1991). However, the information available about the growth response of wheat roots to soil moisture conditions remains fairly limited. We do not understand the mechanisms that control increases in root growth upon moderate decreases in soil moisture. Moreover, the actual responses of the various roots to reductions in soil w and the osmotic adjustments in various regions of the root remain to be investigated. The present study of young wheat seedlings that were growing in vermiculite was performed to investigate the growth responses of seminal roots to reductions in the w of the vermiculite and to identify the factors responsible for these responses.
Materials and methods Materials and treatments Wheat (Triticum aestivum L.) cultivar Bandowase was used in the experiment. This cultivar is classified as I to II in the degree of winter habit and was commonly grown in Kanto area in Japan several years ago. Seeds were washed in a 0.25% solution of NaOCl for 10 min, rinsed with de-ionized water and placed on wet filter paper in a glass Petri dish. The seeds were allowed to germinate at 25 ◦ C in darkness for approximately 30 h.
Individual seedlings with a radicle of about 5 mm in length were used for measurements. The vermiculite from Fukushima, Japan was sieved through 2-mm mesh. Then de-ionized water containing 10−4 M CaCl2 was added to the vermiculite to produce a w that ranged from −0.03 MPa to −1.10 MPa on the basis of a pre-determined relationship: vw = 0.062 − 10.314/WC, where vw is w of vermiculite (MPa) and WC is water content (%) on gravimetric basis. After addition of de-ionized water, the vermiculite was allowed to stand in sealed plastic containers at 25 ◦ C for about 70 h. The containers were shaken regularly at intervals of several hours in order to distribute substrate moisture homogeneously. The vermiculite was placed in glass boxes (130 mm × 120 mm × 30 mm) and seedlings were positioned against the glass surface in each box. They were separated from the vermiculite by a sheet of black polyester fibers (BDK; Yunichika, Tokyo). This forced the seedlings to grow at the place between the black sheet and a glass plate. The boxes were inclined at an angle of 50◦ from the horizontal. This experimental system facilitated measurements of root length, prevented the roots from disappearing into the substrate and also prevented the vermiculite from sticking to the roots. For the measurement of the elongation rate of seminal roots, each glass box contained only one plant. Two and five plants were grown in each glass box for the profile of the elongation rate along a root and for the measurements of water and osmotic potentials, respectively. The open top surface of the glass box was covered with thin plastic film to prevent loss of moisture by evaporation during measurements. The boxes containing vermiculite with different water potentials were arranged at random in a temperature-controlled growth chamber. Plants were grown at 25 ◦ C in darkness through day and night in the chamber except when markings were made on the exterior glass surface of the boxes or during photography. After each measurement, the water content of the vermiculite in the box was determined and w was estimated. The coefficient of variance in the w of vermiculite was about 3% among the replicates and there were no consistent changes in water content of vermiculite during measurements. Growth measurements Rate of root elongation Transplanted seedlings were allowed to grow for 3 days in boxes filled with vermiculite. The positions
321 of root apices were marked periodically on the glass surface under dim green light using pens of various colors to indicate the time scale. Photographs were taken with a digital camera (COOLPIX 990; Nikon, Tokyo) at the end of experiment and rates of root elongation were analyzed from computer images with appropriate software (Sigma Scan; Jandel Scientific Software, San Rafael, CA). Profile of elongation rate To identify the mature and elongating regions of individual seminal roots in boxes one day after transplanting, the glass wall against which roots were growing was carefully removed in a moisture-saturated chamber, in order to expose roots with the minimum loss of water. Roots were marked gently with a marker (YMSCR1-BK; Zebra, Tokyo) at approximately 1 mm intervals from the apex. The glass wall of the box was then replaced. Then photographs were taken at 1-h intervals with a digital camera (COOLPIX 990) under room light. The displacement of marks from the root apex was determined with the computer software (Sigma Scan). Relative elongation rate along a root was calculated using the fitting proposed by Morris and Silk (1992) and elongation and mature regions of roots were identified. We performed preliminary experiments to compare root growth rates upon exposure of roots to normal room light during marking and photography with root growth rate in the absence of exposure to light. We also compared growth of roots with and without marking root surfaces. Light and marking of roots had no effect on root growth. Measurements of water and osmotic potentials We measured w and the osmotic potential (o ) of tissues by an isopiestic technique with a thermocouple pschrometer (Boyer and Knipling, 1965; Boyer, 1995). Seedlings with a radicle of about 5 mm in length were allowed to grow for about 45 h in vermiculite at 25 ◦ C. In the moisture-saturated box, we separated whole roots into mature and elongating regions with a razor blade. We included a small amount of mature tissue (equals to 25% of the elongating tissue) in the latter segments (Boyer, 1995; Hirasawa et al., 1997). We collected 15 uniform roots in the case of plants grown in vermiculite of a w of −0.03 MPa and of a w of −0.15 MPa, respectively, and 30 roots each in the case of plants grown in vermiculite of a w of −0.55 MPa and of −1.10 MPa. We placed tissues on the bottom of the psychrometer chamber,
which had been coated with melted and re-solidified petroleum jelly. Measurements were taken from 3 to 5 h after tissues had been placed in the chamber. We confirmed that the water potential of the elongating tissue did not change for a few hours after equilibration which reached about 3 to 4 h after tissues had been placed in the psychrometer chamber. Measurements were corrected for heat of respiration, even though the effect was very small (Boyer, 1995). To determine o , we used a procedure for sample collection similar to that used for measurements of root w with the exception that mature tissues were not included for measurements of elongating regions. Both elongating and mature segments were collected in separate airtight glass bottles. They were frozen immediately in liquid nitrogen and stored in a deep freezer at −80 ◦ C. For measurements of o , tissues were thawed at room temperature (25 ◦ C) for 30 min. The p of each root segment was determined by subtraction of respective values of o from w . All manipulations and handling for gathering of root samples for measurements were performed inside the moisture-saturated chamber and required about 20 min at most. To examine the effects of water loss on w during this period, we measured w for mature root tissue immediately after excision and for the same tissue after it had been left in the chamber for 20 min after excision, using a dew point psychrometer (HR 33T; Wescor, Logan, UT). There was no significant difference between the two measurements. Estimation of the accumulation of solutes in roots The lengths of elongating and mature segments, after measurements of o , were recorded using a ruler and diameters were measured under a microscope (AFX II A; Nikon, Tokyo) at a magnification of ×100. The volume of each root was calculated as if it was a cylinder. Averages of five to eight measurements were taken as diameter at different positions on elongating and on mature segments. The diameter of a root did not change significantly by freezing and thawing. The volume of the root apex, as a cup, was also estimated and included in the volume of elongating segments. The solute concentration in roots was calculated as follows: o = −RTCS i, where R is 8.32 × 10−6 MPa m3 mol−1 K−1 ; T is 298 K; and CS is solute concentration (mol l−1 ). The activity coefficient of the solute (i) was assumed to be 1.0 (Meyer and Boyer, 1981). The amount of osmotica in root segments was
322
Figure 1. Time courses of increases in length of wheat seminal roots after transplanting to vermiculite with various water potentials (w ). Bars represent standard deviations (SD) (n = 5). There were five different seminal roots (root 1 to root 5) emerging at different times from the same plant. Each replicate consisted of one glass box where one plant was grown. Where no bar is visible, it is covered by the symbol. Solid circles, open circles, solid triangles and open triangles represent roots growing in vermiculite with a w of −0.03, −0.15, −0.55 and −1.10 MPa, respectively.
estimated as the product of the segment volume and the concentration of the osmotica.
Results Increases in root length and elongation rates Figure 1 shows the growth of seminal roots in vermiculite at values of w that ranged from −0.03 MPa to −1.10 MPa. The primary seminal root, referred to here as root 1, emerged first. A couple of hours after the emergence of root 1, the first pair of seminal roots, referred to as roots 2 and 3, respectively, appeared. The second pair of seminal roots, referred to as roots 4 and 5, respectively, emerged at a later time depended on the availability of moisture to the seedlings. The 6th seminal root did not appeared in the cultivar used in the experiment. In vermiculite with higher values of w , the second pair of seminal roots appeared within one day after transplanting. However, they did not increase in length for the duration of the experiment (72 h) in the vermiculite with a w of −1.10 MPa. The increase in length of root 1 was linear with respect to time and it was followed by increases in length of all other roots of seedlings during meas-
urements in vermiculite with different values of w , with the exception of the second pair of roots in vermiculite with a w of −1.10 MPa. In other words the growth response of the first three seminal roots to w , after their emergence, appeared to be stable during all measurements down to the w of −1.10 MPa. We estimated the average elongation rate of each seminal root from the slope of the graphs of root length versus time (Table 1). The elongation rate tended to be higher for root 1 and lower for roots 4 and 5 in vermiculite with high and low values of w . The differences in rates of elongation among the first three roots did not increase significantly in vermiculite with the lower values of w . For each seminal root, the elongation rate was lower in vermiculite with lower values of w , namely −0.55 MPa and −1.10 MPa (Figure 1 and Table 1). However, the growth response to a moderate reduction in the w of vermiculite was not clear. Therefore, we investigated the effects on root elongation of a moderate reduction in w in further detail (Figure 2). The elongation rate at −0.03 MPa was observed to be approximately 1.16 mm h−1 for root 1, and the rate was sensitive to a reduction in water potential from a high w (−0.03 MPa) to a moderately low w
323 Table 1. Elongation rates (mm h−1 ) of seminal roots in vermiculite with different water potentials (w ). The rates ware estimated from slopes of graphs of root length against time. Each value is a mean of the results from 5 replicates with the standard deviation in parentheses. Root 4 and 5 did not emerge at the vermiculite w of −1.10 Mpa Vermiculite w (MPa)
Root 1
Root 2
Root 3
Root 4
Root 5
Sum of 5 roots
−0.03
1.19a (0.08)
1.10ab (0.09)
1.08ab (0.08)
0.72bc (0.45)
0.62c (0.35)
4.71 (0.91)
−0.15
1.03a (0.06)
0.98ab (0.08)
0.74bc (0.12)
0.86abd (0.11)
0.61cd (0.33)
4.23 (0.45)
−0.55
0.46a (0.09)
0.44a (0.08)
0.29ab (0.08)
0.31ab (0.26)
0.14b (0.18)
1.65 (0.29)
−1.10
0.30a (0.05)
0.20a (0.11)
0.25a (0.08)
–
–
0.74 (0.19)
Means on a single line followed by the same letter are not significantly different by LSD test (P < 0.05).
Figure 2. Elongation rates of seminal roots in vermiculite at various water potentials (w ). The elongation rate is given as the average rate, from 24 h to 72 h after the seedling was transplanted, and each elongation rate was approximately constant. Bars represent SD (n = 5). Each replicate consisted of one glass box where one plant was grown. Where no bar is visible, it is covered by the symbol. Means followed by the same letter are not significantly different by LSD test (P < 0.05).
324 Table 2. Water potentials (w ), osmotic potentials (o ) and turgor pressures (p ) of elongating and mature tissues of a root (root 1) grown in vermiculite with different water potentials. Numbers in parentheses are standard deviations (n = 4). Differences between water potentials of vermiculite and those of elongating root tissue (w ) are shown in the last column Vermiculite w (MPa)
w
Elongating segment o p (MPa)
w
Mature segment o p (MPa)
w (MPa)
−0.03
−0.52a (0.08)
−1.48a (0.12)
0.96a (0.19)
−0.08a (0.01)
−0.83a (0.03)
0.75a (0.03)
0.49a (0.08)
−0.15
−0.66 b (0.03)
−1.63a (0.18)
0.97a (0.17)
−0.14a (0.01)
−0.90a (0.05)
0.76a (0.05)
0.51a (0.03)
−0.55
−1.12c (0.04)
−2.07b (0.15)
0.95a (0.18)
−0.60b (0.08)
−1.39b (0.03)
0.80a (0.09)
0.57a (0.04)
−1.10
−1.60d (0.07)
−2.68c (0.08)
1.08a (0.09)
−1.21c (0.11)
−2.02c (0.03)
0.81a (0.09)
0.50a (0.07)
Means in a single column followed by the same letter are not significantly different by LSD test (P < 0.05).
(−0.30 MPa). A significantly reduced rate of elongation of approximately 1 mm h−1 was observed in vermiculite with a w of −0.09 MPa to −0.25 MPa. Thereafter, further reductions in water potential reduced the rate of root elongation markedly. We observed similar results for roots 2 and 3. However, the elongation rate of roots 4 and 5 was low even at a w of −0.03 MPa and it was affected by a reduction in w to −0.55 MPa. The sum of the elongation rates of all seminal roots decreased slowly with reductions in the w of vermiculite to −0.30 MPa and decreased markedly at values of w below −0.55 MPa. Profile of the elongation rate in a single root Figure 3 illustrates relative elongation rate of roots in vermiculite with various values of w . The profile of the elongation rate of a seminal root was dome-shaped. As the w of vermiculite decreased, the relative elongation rate decreased and the elongating region of the root also became shorter. The expansion of tissue was restricted to the region that was approximately 5 mm from the root apex at values of w of −0.03 MPa and −0.15 MPa, approximately 4 mm from the root apex at −0.55 MPa, and approximately 3 mm from the root apex at −1.10 MPa. Water and osmotic potentials of root tissues The water and osmotic potentials and the turgor pressure of the elongating and mature tissues were determined for roots (root 1) growing in vermiculite with
Figure 3. Profiles of relative elongation rate (RER) of roots growing in vermiculite with different water potentials (root 1). Each value is a mean of the measurements for five to six seedlings. Solid circles, open circles, solid triangles and open triangles represent roots growing in vermiculite with a w of −0.03, −0.15, −0.55 and −1.10 MPa, respectively.
different values of w (Table 2). The w of mature tissues was always close to that of the vermiculite in which seedlings were growing. When the w of the vermiculite was reduced, we observed a similar reduction in o in the mature tissues. As a result, p remained high and there was no significant reduction in the p of mature tissues even when the w of the vermiculite was reduced. The w of elongating tissues was far lower than that of mature tissues. Reductions in the w of the vermiculite also reduced the o of the elongating tissues significantly. As a result, the
325 Table 3. Sizes and levels of osmotica for elongating and mature tissues of roots (root 1) and for entire roots grown in vermiculite with different water potentials. Numbers in parentheses are standard deviations (n = 5) Vermiculite w (MPa)
Length (mm)
Elongating Diameter Osmotica (mm) (µ mol)
Length (mm)
Mature Diameter (mm)
Osmotica (µ mol)
Osmotica in entire root (µ mol)
−0.03
5.2a (0.07)
0.61a (0.05)
0.90a (0.19)
48.5a (0.70)
0.63a (0.02)
4.96c (0.40)
5.86b (0.48)
−0.15
5.1a (0.16)
0.55b (0.03)
0.81a (0.15)
43.4b (1.23)
0.65a (0.05)
5.16bc (0.72)
5.96b (0.73)
−0.55
4.1b (0.05)
0.52b (0.04)
0.73a (0.08)
31.2c (3.84)
0.68a (0.08)
6.46a (1.14)
7.19a (1.17)
−1.10
3.3c (0.30)
0.55b (0.06)
0.83a (0.16)
18.6d (1.98)
0.71a (0.06)
6.02ab (0.64)
6.85ab (0.62)
Means within a single column followed by the same letter are not significantly different by LSD test (P < 0.05). ‘Osmotica in entire root’ is the sum of the levels in the elongating and mature tissues.
turgor pressure of elongating tissues also remained high even in vermiculite with a lower w . There was also no clear difference in w between the vermiculite and the elongating tissues even when the w of the vermiculite was reduced. Accumulation of osmotica The diameter of the elongating regions of roots was significantly reduced (P < 0.05) in the vermiculite with a w of −0.15 MPa (Table 3). Further reductions in the w of the vermiculite had no statistically significant effect on root diameter. Nevertheless, the root diameter of mature regions tended to increase in seedlings grown in vermiculite with a w of −0.55 MPa and −1.10 MPa, as compared with those in vermiculite with a higher w although the difference was not significant. The level of osmotica in the entire elongating and mature tissues of a root were compared. No significant differences in the case of elongating root tissues were found. The level of osmotica in the case of mature root tissues was larger in seedlings grown in vermiculite with a w of −0.55 and −1.10 MPa. The levels in the entire roots also tended to be larger for the seedlings in vermiculite with the lower values of w
Discussion The elongation rate was higher for the first three roots (roots 1, 2 and 3) than for roots 4 and 5, which emerged later. This feature of the elongation rate was
fundamentally unchanged as the w of the vermiculite was reduced, although roots 4 and 5 did not appear in vermiculite with a w of −1.10 MPa (Table 1). We can conclude that the elongation of the various seminal roots is synchronized at their emergence, irrespective of the moisture conditions. When the w of the vermiculite was reduced, the rate of elongation of the seminal roots of wheat plants also fell and it fell markedly when the w was below −0.55 MPa (Figure 2). The w of vermiculite at which the root elongation rate fell to the half of the original rate was between −0.4 MPa and −0.5 MPa. The growth response of roots seems less sensitive to a reduction in the w of vermiculite in wheat than in maize. In maize, the growth rate decreased markedly as the w of vermiculite was reduced and the w of vermiculite at which the root elongation rate fell by half was estimated to be from −0.2 MPa to −0.3 MPa (Sharp et al., 1988). There was no increase in the elongation rate of any of the seminal roots of wheat seedlings under moderate water stress (Figure 2). This result differs from that reported by Sharp and Davies (1979) in maize in a soil at the fourth leaf stage. But, even in maize, any increases in root elongation rate were not observed in the seedlings a few days after germination (Sharp et al., 1988). The factors responsible for the difference in the growth response and for the enhancement of the root elongation rate remain to be identified. A moderate reduction in the w of vermiculite reduced the average diameter of roots in the elongating region (Table 3) and a similar effect of the w of vermiculite
326 on root diameter was observed in maize (Sharp et al., 1988). However, further reductions in w did not reduce the diameter any further in wheat. Indeed, the root diameter in the mature region tended to increase in vermiculite with a lower w (Table 3). The length of the elongating region, measured from the root apex, was 5 mm in vermiculite with a high w as it was in hydroponically grown wheat roots (Pritchard et al., 1991) and pea roots (Hirasawa et al., 1997) but the distance was far longer in maize (Sharp et al., 1988). The maximum local rate of growth of a wheat root (approximately 0.9 h−1 ; Figure 3) was larger than that of maize roots (Sharp et al., 1998), thus, the shorter length of the elongating region might result in the lower elongation rate observed in wheat roots. The length of the elongating region decreased with reductions in the w of vermiculite (Figure 3). As observed in maize (Sharp et al., 1988) and as previously noted in wheat (Pritchard et al., 1991), the apical portion of the elongating region was unaffected by reductions in the w of the vermiculite and the elongation rate decreased in basal part of the elongating region in vermiculite with a low w . Thus, it appears that the decrease in the elongation rate of a root that occurs for a few days after a seedling has been transplanted to vermiculite with a low w is the result of a reduction in the elongation rate in the basal portion of the elongating region and also to a reduction in the length of the elongating region. The decrease in the elongation rate of a root can be analyzed by comparing cell wall properties and hydraulic properties in relation to cell growth. The growth rate of cells depends on the extensibility of the cell wall, the turgor pressure and the yield threshold (Green et al., 1971; Lockhart, 1965). We detected no differences in turgor pressure (Table 2). Thus it appears that a decrease in cell wall extensibility and/or an increase in yield threshold were responsible for the decrease in elongation rate. A reduction in cell wall extensibility was observed in soybean hypocotyls in vermiculite with a low w (Nonami and Boyer, 1990a, b) and changes in yield threshold were also observed in maize roots treated with an osmoticum (Frensch and Hsiao, 1995). Changes in the actual values of the extensibility and yield threshold remain to be investigated. With respect to hydraulic properties, we know that the hydraulic conductance and w of both the mature tissues and the growing cells influence the growth rate of cells (Lockhart, 1965; Boyer et al., 1985). We found no differences in w between vermiculite and
elongating tissues and between mature and elongating tissues among treatments. These results indicate that there were no differences in the driving force of water for growth among the treatments (Nonami et al., 1997). We do not know whether water was transported to the elongating tissues directly from the vermiculite or from the mature tissues. In the former case, the hydraulic conductance in the elongating cells might be responsible for the difference in water transport. In the later case, the hydraulic conductance from mature cells to the elongation cells might be responsible for this difference (Miyamoto et al., 2002). Osmotic adjustment in roots has been well characterized (Morgan, 1984; Serraj and Sinclair, 2002). Roots accumulate solutes and maintain a high turgor pressure even when w falls. It is possible that a significant osmotic adjustment in the growth zone could maintain water uptake from a rooting medium with a low w (Fricke and Peters, 2002), and/or divert water from other plant organs to growth zone of the root, which would allow the sustained growth of roots in dry soil (Fricke and Peters, 2002; Rodriguez et al., 1997; Serraj and Sinclair, 2002). Osmotic adjustment in roots that allows the continued or even increased development of roots into deeper, wet soil would give plants access to a larger water reservoir (Serraj and Sinclair, 2002). Wheat roots were able to maintain high turgor pressure in both mature and elongating tissues even in vermiculite with a low w of −1.10 MPa because the wheat roots were able to accumulate solutes in the elongating and mature regions with reductions in the w of vermiculite. A similar phenomenon was noted in the elongation regions of wheat roots, in which turgor was constant over the entire region even though rates of elongation were quite different among elongation regions (Tomos and Pritchard, 1994). By contrast, in maize, turgor pressure decreased significantly in vermiculite with a low w (Spollen and Sharp, 1991). Growth response of roots to the reduction in w of vermiculite was less sensitive in wheat than maize, as discussed above. Roots with a high capacity for osmotic adjustment might be able to suppress a reduction in elongation rate in wheat as compared to maize (Sharp et al., 1988). The levels of osmotica did not differ significantly among treatments in elongating tissues of wheat roots although osmoticum accumulation rather decreased in maize roots grown in vermiculite with lower w (Sharp et al., 1990). We might be able to think that the elongation of wheat roots is regulated carefully to reflect the availability of osmotica. However, the
327 levels of osmotica rather increased in mature tissues in vermiculite with a w of −0.55 and −1.10 MPa. This could result from the increase of root diameter in mature region, which occurred after a completion of elongation. Based on this evidence, it might be more plausible that a wheat seminal root might regulate its growth to keep turgor pressure unchanged under water stress. The nature of the accumulated solutes; the time courses of accumulation of each solute, of changes in water potential and of changes in turgor pressure; and the effects of turgor pressure on the properties of cell wall and hydraulic properties of membranes in elongating cells all remain to be investigated.
Acknowledgements The authors thank Yumi Shimazaki for her assistance in the analysis of root elongation. This research was supported in part by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (nos. 13556005 and 14656006) and by a Grantin-Aid (Bio Cosmos Program) from the Ministry of Agriculture, Forestry and Fisheries, Japan. M. Akmal was supported during his stay at Tokyo University of Agriculture and Technology by the Ministry of Science and Technology, Government of Pakistan.
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