Journal of Experimental Botany, Vol. 51, No. 350, pp. 1595-1616,
September 2000
© 2000 Oxford University Press
Sensitivity of growth of roots versus leaves to water stress: biophysical analysis and relation to water transport
Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA
Received 12 December 1999; Accepted 3 July 2000
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Abstract |
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 Top Abstract IntroductionContrasting growth between roots...Hydraulic isolation of root...Water transport through the...Field observations of {Psi}...Osmotic adjustment in relation...Concluding discussionAppendixReferences |
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Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (
) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re-establishment of gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield-threshold turgor. These adjustments permit roots to resume growth under low . In contrast, in leaves under reductions in of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade-off between high xylem conductance and strong osmotic adjustment. Key words: Expansive growth, yield threshold, hydraulic isolation of growth zone, leaky cable model of root water uptake.
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Introduction |
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TopAbstractIntroductionContrasting growth between roots...Hydraulic isolation of root...Water transport through the...Field observations of {Psi}...Osmotic adjustment in relation...Concluding discussionAppendixReferences |
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A plant transports a huge amount of water—in the range of 200 to 1000 times the dry mass of its body over its life time. This is the result of having to keep the interior of its leaves open to the atmosphere for the adequate absorption and assimilation of carbon dioxide, with the inevitable consequence of water vapour escaping from the leaves. Water transport is closely intertwined with the myriad of plant processes, including photosynthesis, translocation, mineral nutrition, hormonal regulation, and numerous molecular and genetic facets. This paper will discuss a few selected aspects of water transport, mostly in relation to growth by cell expansion. The focus will be on the growth of roots relative to that of leaves in response to water stress, viewed mainly from a biophysical perspective, and the implications in the field in terms of adaptation to water-limited environments. Also discussed is the link between osmotic adjustment and water transport in terms of hydraulic conductance of the plant. As made obvious by several other papers in this volume (Munns et al., 2000
; Steudle, 2000; Tardieu et al., 2000), the literature on water transport and on growth as affected by water deficit is extensive and sometimes contradicting. The intent here is not to discuss the field as a whole, but to pull together selected ideas and results, much of them from our own group, some quite old and some new and not yet published, to provide an overview from a personal vantage point. One important facet not considered here is water transport as related to xylem embolism or cavitation under high tension (Milburn and Johnson, 1966; Boyer, 1971), an area of much research interest (Sperry et al., 1996; McCully et al., 1998; Tyree et al., 1999). |
Contrasting growth between roots and leaves under water stress—a biophysical perspective |
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TopAbstractIntroductionContrasting growth between roots...Hydraulic isolation of root...Water transport through the...Field observations of {Psi}...Osmotic adjustment in relation...Concluding discussionAppendixReferences |
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Leaf growth defines the canopy size of a plant for capturing sunlight and carrying out photosynthesis to gain carbon and energy. Root growth defines the extent to which a plant explores soil for water and mineral nutrients. Growth of the two organs, however, are in competition for assimilates produced by the leaves and for minerals and water taken up by roots. Yet growth of roots and leaves are co-ordinated and their sizes relative to each other vary dynamically in response to environmental conditions, in a way that tends to optimize the utilization of assimilates and other resources (Wilson, 1988
). How the co-ordination is achieved remains unclear, in spite of recent progress made in the understanding of expansive growth. Discussed in this section are some of the recent advances in the study of more physical aspects of growth of roots and leaves. In the following sections the characteristics of water transport to the growth zone of roots and leaves are analysed, and a combination of transport and growth-related processes and features is proposed as the mechanism co-ordinating the growth of the two organs under conditions of soil moisture depletion.
Equations of Lockhart
Cells and organs expand and contract with changes in water content. To separate out elastic changes in size not associated with growth, it is necessary to define expansive growth as the irreversible enlargement of cells or organs. The enlargement refers particularly to the dimensions of the cell wall. The more physical aspects of expansive growth is usually examined in terms of the equations of Lockhart (Lockhart, 1965). The first Lockhart equation relates the relative rate of irreversible increase in volume of a cell to its turgor pressure (
p):
 | (1) |
p-Y), which is termed growth effective turgor. The loosening ability of the cell wall is reflected in both m and Y, which are measures of rheology of the cell wall (plastic properties). Because the chemical and structural properties of the growing cell wall are intimately linked to wall metabolism and growth regulators (Lockhart, 1965; Nakahori et al., 1991), m and Y are also (Bradford and Hsiao, 1982). Volumetric extensibility m is a three-dimensional expression of plastic extensibility of the wall and incorporates the effects of cell geometry as well (Lockhart, 1965). Equation 1 emphasizes the fact that p must be above the threshold value of Y for the cell to grow. In addition to the implicit link of m and Y to metabolism, also implicit in equation (1) is the role of water potential () and solute (osmotic) potential (s) in determining p (=-s). It is a mistake to view m and Y in the equation as constants, as often done. They have long been recognized to change with changes in cell water status (Green, 1968; Acevedo et al., 1971; Green and Cummins, 1974). These changes provide additional means for the plant to adjust growth of its organs to cope with water stress (Hsiao et al., 1976, 1998).
With sustained growth, water must be transported continuously into the cell since most of the expansion in volume is due to added water. The common equation for water transport is to relate the water flow to the conductance of the path, C, and the difference in driving the transport. Lockhart combined the transport equation with equation 1 to obtain
 | (2) |
° is of the medium surrounding the growing cell or of the source of water, and s is solute potential within the cell (Lockhart, 1965). It is seen in equation 2 that m has the same units as C. In the situation where C>>m, the denominator of the fraction on the right side of equation 2 becomes C and cancels the C in the numerator. Consequently equation 2 reverts to equation 1. An example of where the simpler equation 1 instead of the more complicated equation 2 is adequate to describe growth is the case of cells in maize roots bathed in a flowing aqueous solution (Frensch and Hsiao, 1995). There C was assessed to be large relative to m. On the other hand, studies have pointed to conductance as a major factor limiting growth of cells in the stem (hypocotyl) of soybean seedlings with roots in water-deficient vermiculite (Nonami and Boyer, 1990). In that case, the low conductance of the radial path between the xylem and the growing cells was attributed to a layer of small cells about 200 µm thick separating the ring of xylem vessels from the cortical and epidermal cells (Nonami et al., 1997).
In this paper, for simplicity, growth at the cellular scale will be discussed mostly in terms of the parameters in equation 1. The role of conductance and gradient will be assessed when water transport over longer distances to the growth zone is considered.
Effects of water stress on expansive growth and dynamic responses
Leaf growth has long been known to be very sensitive to inhibition by water stress (Boyer, 1968) whereas root growth is more resistant (Westgate and Boyer, 1985). This difference in sensitivity is illustrated for maize in Fig. 1. Elongation rate of the fifth leaf of maize was maximal when of the growth zone tissue was the highest (-0.75 MPa). Any reduction in growth zone reduced the elongation rate, to the extent that elongation stopped when was reduced to -1.1 MPa (Fig. 1A), a total reduction in growth zone of only 0.3 MPa. For maize roots growing in vermiculite, elongation was also reduced by small reductions in medium (Fig. 1B). Further reductions in , however, had less effect, and elongation continued at more than one-third of the maximum rate even when medium was reduced to -1.9 MPa, 0.4 MPa below the permanent wilting point of -1.5 MPa. of the root growth zone must have been still lower, since a gradient is necessary for the root to absorb water from the medium to maintain growth.
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To gain an insight into the mechanism underlying the ability of roots to grow under such low medium
, it is illuminating to examine the dynamic changes when water stress is imposed suddenly. In an early study, Acevedo et al. showed that when of the solution bathing roots of a maize seedling was suddenly reduce from zero to -0.2 MPa, leaf growth stopped momentarily, then slowly resumed after some adjustment, reaching a steady rate about only one-third of the original (Acevedo et al., 1971). After the root medium was raised suddenly back to 0 MPa, there was a burst of rapid leaf elongation, lasting for about 1.5 h, followed by a return to the original rate. With a more marked reduction in root medium , to -0.3 MPa, leaf growth stopped for more than 2 h before resuming at a very slow rate. Kuzmanoff and Evans found that growth of lentil roots went through similar rate adjustments upon a stepwise reduction or increase in medium (Kuzmanoff and Evans, 1981). Growth recovery under reduced , however, was much stronger in those roots. Even after a 0.7 MPa stepping down of medium , lentil roots recovered to grow essentially at the full rate after 50 min of adjustment. How was this adjustment achieved in terms of the parameters in Lockhart equation? Osmotic adjustment and the maintenance of partial or full turgor under water stress was shown to play a role in early studies (Greacen and Oh, 1972; Meyers and Boyer, 1972; Hsiao et al., 1976). More details came from studies using the pressure microprobe to monitor changes in cell turgor pressure while growth was also monitored during stepwise changes in medium water status. Hsiao and Jing found that when the medium for the primary root of a germinating maize seedling was suddenly reduced from zero to -0.42 MPa by changing to an osmotic solution, turgor of cells in the root growth zone was reduced nearly proportionally, as quickly as could be measured (within a couple of minutes), and growth stopped (Hsiao and Jing, 1987). Growth restarted and turgor began to increase, however, in a few minutes. Initially the growth rate and turgor increased rapidly, then slowed with time. Growth recovered fully in 45 min in spite of the low . Turgor, on the other hand, recovered only to a level still 0.15 MPa below the value before the imposition of water stress. The results are consistent with the concept of yield threshold turgor and the need for turgor to rise above this threshold for growth to resume. The initial rapid turgor rise was the result of rapid osmotic adjustment. Also the fact that growth under the stress had recovered fully while turgor recovery was only partial showed that Y and probably m in the Lockhart equation must have adjusted to achieve the original growth rate. Other authors (Green, 1968; Green and Cummins, 1974) had emphasized the tendency of expansive growth to be self-stabilizing as conditions varied by adjusting the loosening of the cell wall. It is desirable to separate out the contribution of any adjustment in Y from that in m in the maintenance of growth under water stress.
Differentiating changes in Y from changes in m and other methodological problems
Analysing growth in terms of the parameters in Lockhart's equations, especially at the whole organ level, can involve a number of uncertainties, particularly difficult is how to distinguish changes in m from changes in Y when growth is altered by changes in conditions. This difficulty is further aggravated by the loose use of the term ‘extensibility’. Lockhart originally termed m ‘gross extensibility’, to differentiate it from the physically better defined extensibility used in the one-dimensional stress–strain relationship, on which his equations were based. For decades, extensibility has also been used to denote the relative or absolute uniaxial extension of pieces of excised tissue per unit of uniaxial force, applied externally to the tissue, without consideration of a force threshold. Clearly, that is distinct from the Lockhart m, because Y is not accounted for and the force is acting in one dimension, instead of in three dimensions as exerted by turgor pressure. Extensibility has also been used in the literature in a general and non-quantitative sense to denote the ability of the cell wall to extend. To avoid confusion, in this paper volumetric extensibility, denoted by the coefficient m, is used only in the sense defined by the Lockhart equation, and the ability of the cell wall to extend in a general sense is referred to as wall loosening ability.
Separating out changes in Y from those in m is not simple. It is pertinent to summarize here the various methods used. In early studies, Y and m were evaluated from approximately linear plots of growth rate versus p. Growth rate and p were varied by varying tissue water status and p was calculated as the difference between measured and s of the tissue. Under the assumption that Y and m remained the same for different growth rates, the slope of the plot was taken as m, and the intercept with the x-axis, as Y. Obviously, if Y or m changed as the result of water status differences, the method would not yield definitive results (Frensch and Hsiao, 1994). Later, a stress or turgor relaxation technique (Cosgrove, 1985) was used to evaluate Y and m. Growing organ or tissue was monitored for its expansion after it was excised and deprived of a water source. The cell wall continued to relax (irreversibly) under high turgor and the organ continued to expand as long as its p was greater than Y. Turgor declined with time because of wall relaxation and expansion stopped when turgor dropped to the level where p=Y. Y then was obtained by measuring p at that point (Cosgrove, 1985) or by calculating p from measured and s. Once Y was obtained, m was calculated from Y and the growth rate. A variation of this technique is the pressure block method (Cosgrove, 1987). Expansion of the excised tissue was monitored inside a pressure chamber and pressure was applied and raised in the chamber until expansion just stopped. At that point the applied pressure is a measure of how much p exceeds Y, or the growth effective turgor (p-Y). By measuring p, Y was then calculated. Underlying these methods was the assumption that Y did not change during the stress relaxation or the time it took to block expansion by pressure. Should Y change with time to some lower limit, then these techniques determine the lower limit of Y, not the Y at the moment of tissue excision or at the start of the pressure blocking. The underestimation of Y in turn would lead to calculated m being too low. Green showed decades ago that Y was reduced rather quickly under low p in Nitella (Green, 1968). The pressure block data (Cosgrove, 1987), obtained on higher plants, in fact suggested that Y might have decreased during the determination. The initial stoppage of expansion by pressure did not last and the pressure had to be increased several times before expansion was prevented for good.
The tendency of walls of growing cells to continue to relax when p>Y causes measured by thermocouple psychrometry on excised tissue to be too low because the required equilibration of several hours provides ample time for wall relaxation. Consequently, the turgor calculated would be lower than that at the time of excision and would closely reflect the lower limit value of Y. Some of the early results indicating that Y did not change with water status were based on calculations using measured in psychrometers and the conclusion is questionable. The problem of wall relaxation could be minimized if tissue is measured at a low temperature (e.g. 5 °C) using a method that requires only a short time for equilibration. The Shardakov dye method (Slavik, 1974) has been used for that purpose (Hsiao and Jing, 1987).
To keep the time of measurements short and thus minimizing changes in Y and m, Okamoto et al. developed the pressure jump method for the study of the growth of excised hypocotyl segments of Vigna (Okamoto et al., 1989). Pressure is increased stepwise by a small amount (e.g. 0.02 MPa) in the xylem and maintained for only a very short time (e.g. 3 min) while elongation of the segment is continuously monitored. Extensibility m is calculated as the ratio of increase in relative growth rate to the increase in p, which is approximated by the applied pressure if hydraulic conductance of the cells (C) is large relative to m. The underlying assumption is that the small and very brief increase in pressure does not alter m and Y. By subjecting the growing segment intermittently to several pressure steps of different magnitudes, Y can be determined by linear extrapolation. Another way to determine Y is by tracking cell p with a pressure microprobe and determining at what p growth just stops as p falls, and at what p growth resumes as p increases (Frensch and Hsiao, 1994). This will be described in more detail in the next section.
With rapid perturbations in turgor and growth measured over a short time interval, there is a concern that changes in rates of measured growth may be confounded by elastic changes in the cell or organ volume or length. Proseus et al. have shown that elastic changes of growing Chara internodal cells, determined at a cold temperature that eliminated growth, may be subtracted from the total length change to obtain changes in the true growth rate at a warm temperature (Proseus et al., 1999). The pressure jump technique (Okamoto et al., 1989) appears to be free from the problem of elastic changes because p is altered only by small amounts and growth rates are based on steady-state measurements.
Dynamic adjustments in yield threshold and volumetric extensibility
Recent evidence indicates that as water stress develops, Y and m may change within minutes in the direction that aids in the maintenance of growth. Skilful use of the pressure microprobe have enabled Frensch and Hsiao to determine instantaneous Y of maize roots during the transitional period from the time of stress imposition to the recovery in growth under the same stress (Frensch and Hsiao, 1994). An example of the data is shown in Fig. 2. When the nutrient solution bathing the root was switched suddenly to one containing mannitol at s=-0.29 MPa, turgor decreased instantly and growth stopped (Fig. 2A). Within 5 min or so, turgor began to recover at a fast rate via osmotic adjustment and growth started again slowly after 10 min (Fig. 2B). The rate of turgor increase had slowed by then but growth rate continued to increase with the further small increases in turgor. After 25 min, growth recovered to a steady rate that was about two-thirds of that before stress and turgor reached a value that was 0.12 MPa lower than before stress (Fig. 2A). By recording growth without interruption and monitoring p continuously in the same cell (curves of solid lines, Fig. 2B) when medium was stepped down or up, it was possible to determine the turgor at which growth stopped (left large circle, Fig. 2B) or resumed (right large circle, Fig. 2B). By definition these p values are the values of yield threshold Y. In Fig. 2B it is seen that the first Y (Y1), where growth stopped, was more than 0.1 MPa higher than the second Y (Y2), where growth resumed. That is, Y decreased substantially during the initial 10 min of stress, and this decrease in Y enable growth to start again at a lower p. Frensch and Hsiao determined the first and second Y by exposing roots to reductions of different magnitudes (Frensch and Hsiao, 1995). When these Y values are plotted against the time it took to reach them after the downstep in (Fig. 3), it is seen that the reduction in Y began early, a few minutes after the imposition of water stress. It appears that less than 20 min was required to achieved the maximal reduction in Y. The data also show that the larger the downstep in , the greater was the reduction in Y when growth resumed (Frensch and Hsiao, 1995). There is a limit to the reduction, however, and the data suggest that the minimal Y reachable was around 0.35 MPa (under a reduction in medium of 0.6 MPa), in agreement with the range of Y measured by the turgor relaxation (Cosgrove, 1985; Matyssek et al., 1988) or pressure block (Cosgrove, 1987) technique on other crop species. For maize roots, Y did not decline when medium was lowered by 0.1 MPa and growth recovered quickly and fully, effected only by osmotic adjustment (Frensch and Hsiao, 1995). Only with further lowering in medium was Y reduced. The problem of elastic changes confounding the Y determined for maize roots appears to be minimal, as Frensch and Hsiao elaborated on earlier (Frensch and Hsiao, 1995).
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) and withdrawal (
) of -0.29 MPa mannitol and the determination of yield threshold turgor Y. (A) Time course of 5 min mean elongation rate (•--•) measured with an LVDT and of cell turgor (
) represents
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Other evidence of quick change in Y came from the data on turgor and growth after
of the medium was raised suddenly to the original value. As shown in Fig. 2A
, there was a burst of growth and a jump in turgor as soon as medium was stepped up to the original level. A few minutes after the step the very high growth rate declined, to a rate slightly lower than the original after another 10 min. In contrast, turgor remained nearly 0.1 MPa higher than the original. Similar results were obtained by Hsiao and Jing (Hsiao and Jing, 1987). The slower growth at a higher turgor after the growth burst is indicative of raised values of Y. The possibility of a greatly reduced m accounting for all of the reduction in growth per unit of turgor without the elevation of Y seems unlikely.
Comparing leaf and root growth under water stress in terms of Lockhart's equation
Detailed time-courses of changes in turgor and growth rate upon stepwise changes in water status for leaves are very few. For the situation of water stress developing slowly over days, growth zone turgor was reported to be completely maintained by osmotic adjustment initially (Michelena and Boyer, 1982; Van Volkenburgh and Boyer, 1985) but the growth rate slowed. Eventually growth dropped to zero as water stress became more severe. Turgor, however, was nearly as high as before the onset of stress. Compared to leaves of recently watered plants, stressed leaves acidified their growth zone apoplast more slowly and had a higher surface pH (Van Volkenburgh and Boyer, 1985). Wall extension, measured in vitro with an extensometer on methanol-boiled leaf segments, was lower for the water stress leaves compared to the control. Turgor was calculated as the difference between and s measured by thermocouple psychrometry, raising the question of whether the reported constant p might have been an artefact of wall relaxation during the measurements. However, Hsiao and Jing also found that growth of leaves was reduced by mild water stress in spite of turgor maintenance in a field study on sorghum (Hsiao and Jing, 1987). They measured of the growth zone with the Shardkov dye method at 5 °C using an equilibration time of less than 10 min. The cold temperature and short measurement time should have essentially eliminated wall relaxation. The slower growth in spite of the full maintenance of turgor was due at least partly to a shortening of the leaf growth zone under water stress (Walker and Hsiao, 1993). Growth zone is also shortened in roots under water stress (Sharp et al., 1989; Spollen and Sharp, 1991).
As for effects of stepwise changes in medium on growth and turgor in leaves, it was already mentioned that the early data (Acevedo et al., 1971) showed the kinetic of growth responses of maize leaves to be similar to that of roots, but recovery in growth after the down-step was much weaker. In that study, turgor was not measured. Later Hsiao and Jing, also using the Shardkov method at cold temperature, investigated changes in maize leaf growth and turgor at 15 min intervals upon a stepping down in medium of 0.25 MPa (Hsiao and Jing, 1987). Growth stopped for about 15 min, then began to recover slowly as p began to recover through osmotic adjustment. This is in contrast to the recovery in turgor and growth within minutes after the 0.29 MPa down-step for roots (Fig. 2). After 75 min, turgor had recovered to the level prior to stress imposition, but growth of the leaf remained partly inhibited. These results indicate that under water stress, wall loosening ability in the leaves was reduced (an increase in Y or a reduction in m), in contrast to the enhanced wall loosening ability in roots as evinced by the reduction in Y (Fig. 3).
Leaves of the Gramineae are often chosen for studies because their growth is predominantly one-dimensional with clear gradients in the longitudinal direction. On the other hand, their growth zone is wrapped in older leaves and difficult to reach with a pressure microprobe. It is not surprising that the initial studies of turgor and growth using a pressure microprobe were on leaves of dicots (Shackel et al., 1987, on grape leaves). A more detailed study with the microprobe on leaf growth in relation to water stress was on another dicot, Begonia argenteo-guttata L. (Serpe and Matthews, 1992). Turgor was measured in the epidermal cells near the central midvein. With a down-step in root medium of 0.2 or 0.3 MPa, growth stopped and turgor dropped. Growth remained zero for 25 min or more and then recovered gradually to reach a slower but steady rate. Turgor, however, did not recover measurably and remained reduced over the 2 h stress period. Hence, there was no osmotic adjustment in Begonia leaves and the resumption in growth under water stress was the result of an enhanced loosening ability of the cell wall.
A similar study was carried out on maize leaves with somewhat similar results (A Thomas and TC Hsiao, unpublished results). A window was carefully cut in the coleoptile sheathing the first leaf of seedlings to expose the growth zone of the leaf for the measurement of turgor with the pressure microprobe. Upon a 0.4 MPa down-step of root medium , growth was stopped within minutes and remained virtually zero for approximately 1 h, before resuming at a very slow rate. Turgor in the growth zone of the leaf declined by nearly 0.3 MPa over a 30 min period, then increased very slowly thereafter.
Overall, it may be said that there is a sharp contrast in the responses to reductions in tissue between the root and the leaf. The root adjusts osmotically and its turgor recovers quickly but only partially under water stress. With the quick lowering of Y and possibly increases in m, root elongation can recover fully under mild water stress, at reduced turgor. Root growth is maintained partially even down to the permanent wilting point and beyond. The leaf osmotically adjusts either slowly or not at all (Serpe and Matthews, 1992). The loosening ability of its cell wall is either reduced (increase in Y as reported by Hsiao and Jing, 1987) or at least not markedly enhanced under water stress. Consequently, leaf growth is much more inhibited by a given reduction in medium or tissue compared to the root.
The discussion on the contrasts between roots and leaves in their growth responses to water stress has emphasized the more biophysical aspects and changes. Obviously these are underlain by physiological and biochemical changes. Of particular relevance is the role of ABA in altering the growth responses of roots versus leaves. Under water stress ABA increases both in leaves and roots (reviewed by Hsiao, 1973) and more ABA is transported from roots to leaves (Zhang and Davies, 1990; Davies and Zhang, 1991). Convincing evidence was obtained by Sharp and coworkers (Spollen et al., 1993; Sharp et al., 1994) in long-term experiments indicating that ABA maintains root growth while inhibiting shoot growth in maize at low . Inhibition of ABA synthesis by the chemical fluridone depressed root elongation and promoted shoot elongation of etiolated maize seedlings at low , but had little effect on root or shoot elongation at high (Saab et al., 1990). A maize mutant deficient in ABA maintained better shoot growth but suffered more inhibition of root growth at low compared to the wild type. Under water stress ABA concentration was the highest at the apical 3 mm of root where the relative elongation rate was fully maintained (Saab et al., 1992). Exogenous ABA at the right concentration overcomes the effect of ABA synthesis inhibitor on the growth of root and shoot at low , and at low modified the growth of ABA-deficient mutant to resemble that of the wild type (Sharp et al., 1994). Interestingly, recent evidence (Sharp et al., 2000) indicates that ABA at the normal endogenous level is also needed in maintaining good shoot and leaf growth in ABA-deficient mutants of tomato. Without adequate ABA the production of excessive ethylene apparently reduced shoot growth. Taken together, these results provide strong evidence that ABA plays a central role in orchestrating the differential long-term growth responses to water stress of root and shoot. What is unclear is whether ABA is similarly involved in the rapid changes in the growth parameters of the Lockhart equation in the two organs. Those changes were very rapid, taking place within minutes or a fraction of an hour after a down-step in . The increase in tissue ABA effected by water stress and the increased transport of ABA from root to shoot may take considerably longer (Hsiao and Bradford, 1983), and cannot be easily invoked to explain the early responses.
Some recent exciting developments in molecular biology are very pertinent to expansive growth. Evidence is accumulating implicating expansin proteins in the effects of water stress on expansive growth. As discussed in the paper by Wu and Cosgrove in this volume, expansins promote wall loosening and acid induced growth (Wu and Cosgrove, 2000). Both the amount of expansins in the cell wall and the responsiveness of the wall to expansins were altered by water stress in a way that facilitates root growth at low . As yet there appears to be no study on the effects of water stress on expansins in leaves. It would be important to know whether at low expansins undergo changes in direction in leaves opposite to that in roots, and whether the level of expansins may change fast enough to account for the fast changes in wall loosening ability, such as that indicated by the rapid reduction in Y in Fig. 3.
The other exciting development is the recent isolation of two types of proteins from the growth zone of Vigna hypocotyls, one apparently functioning specifically to change the yield threshold Y, and the other, the extensibility m (Okamoto-Nakazato et al., 2000). So far these proteins have only been tested on killed and reconstituted Vigna growth tissue and the effects of water stress have yet to be determined.
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Hydraulic isolation of root apex, water transport, and root growth in drying soil |
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TopAbstractIntroductionContrasting growth between roots...Hydraulic isolation of root...Water transport through the...Field observations of {Psi}...Osmotic adjustment in relation...Concluding discussionAppendixReferences |
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The particular anatomical and hydraulic feature of the root apex must also be considered when examining the mechanism for the preferential growth of roots over leaves under water stress. In maize roots, the xylem cells differentiate very slowly and do not mature until they are displaced a substantial distance from the growth zone (Wang et al., 1991
; McCulley, 1995). Consequently, the apical region of the root is hydraulically largely isolated from the more basal part (Frensch and Steudle, 1989; Frensch and Hsiao, 1993) with developed xylem vessels, which supply water to the shoot. This can be seen in the half-time of pressure relaxation of the xylem cells or elements. As shown in Fig. 4, the half-time was much longer for xylem elements located in the apical 35 mm, a reflection of the high resistance to water movement in this region. As the root grows, the apical growing region moves continuously into new soil volume containing yet-to-be-used water. Therefore, of the growth zone should remain high and remote from the influence of tension in the xylem generated by leaf transpiration. To examine this effect in quantitative terms requires model simulation because the root acts as a leaky conduit and in the stele and water uptake at any point along the root are the result of complex interactions among the radial and axial conductances, external , and at the basal end of the root (Landsberg and Fowkes, 1978). As mentioned earlier, the gradient necessary for water uptake in the growth zone to maintain growth has been modelled with spatial details (Molz and Boyer, 1978; Silk and Wagner, 1980), but only for non-transpiring conditions. Water uptake along a root to supply water to the shoot has also been modelled with spatial details (Landsberg and Fowkes, 1978; Frensch and Steudle, 1987), but without considering the growth zone and growth-induced water uptake. To define the role of hydraulic isolation of the growth zone in maintaining root growth under drying conditions, it is necessary to simulate growth zone under transpiring conditions. For the simulation, radial and axial hydraulic conductances must be known or estimated.
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Radial and axial hydraulic conductances of maize roots
Frensch and Steudle measured axial (Rx) and radial (Rr) hydraulic resistance of maize primary roots along the length between 20 and 130 mm from the apex and found Rr to be essentially constant over this length, with an average value of 4x106 MPa s m-1 (Frensch and Steudle, 1989
). Using a different technique based on pressure kinetics of single cells at different depths within the root cylinder measured with a pressure microprobe, radial hydraulic conductivity (m2 s-1 MPa-1) of the growing region of maize root has been determined by Frensch and Hsiao (Frensch and Hsiao, 1995). Their value corresponded to an Rr of 2.5x106 MPa s m-1. Rx was found to remain essentially constant between 70 and 130 mm, but to increase markedly from 60 mm toward the apex, to the extent that Rx at 20 mm was almost three orders of magnitude higher than that at 60 mm (Frensch and Steudle, 1989). There is no direct measurement to indicate that Rx would continue to increase from 20 mm toward the apex. Anatomical data (Frensch and Steudle, 1989), however, suggest that this is the case. They found only two mature early metaxylem elements per cross-section at 20 mm, but none at 10 mm. Early metaxylem vessels of maize roots, with a diameter more than four times that of protoxylem vessels (Frensch and Steudle, 1989), should be much more conductive. This points to the likelihood of still higher Rx at 10 mm.
Simulation of single root water uptake and growth zone water potential
The model of Landberg and Fowkes (Landberg and Fowkes, 1978) as used by Frensch and Steudle (Frensch and Steudle, 1989) can be traced to the leaky cable theory dealing with electrical flow in nerve fibres (Taylor, 1963). Starting with that model, a sink term is added to account for water uptake and transport in the growth zone. That made it possible to calculate the gradients and water fluxes along both the non-growing part and the growth zone of a root from known hydraulic resistances and external . The basic equations are those for radial water uptake and axial water transport. For radial water flux Jr (flow per unit root surface area per unit time, m3 m-2 s-1),
 | (3) |
denote the beginning and the end of the radial path, s for surface of the root and x for the xylem or centre of the root. Rr (MPa s m-1) is the radial total resistance from the root surface to the xylem or centre of the root per unit root surface area. The inverse of Rr is radial conductance, which is distinct from radial conductivity.
For total axial flow Qx (m3 s-1) at any particular point along the length of a root,
 | (4) |
gradient along the length of the root may be derived:
 | (5) |
, R, and S must be used since they may vary with location along the root. Where Rx remains constant along the root, the middle term on the right side drops out of equation 5. For the mature part of the root where there is no growth, the sink term (SRx) is set to zero.
The spatial pattern of local water gain due to growth (water deposition rate) has been measured by a number of investigators. The local rates for maize root (Sharp et al., 1990) were fitted with a second degree polynomial and used as input for the sink term in equation 5. The Appendix provides more details on the derivation of equation 5 and its use in simulation, including the model inputs and the techniques used to solve equation 5 to obtain the changes in xylem gradient along the root. With changes in x gradient along the root calculated using equation 5 and x at the basal end of the root and soil along the root as boundary conditions, the gradients in x were calculated. Radial and axial flux of water were then computed using equations 3 and 4.
Simulated patterns of root and water uptake
The simulation was run for several situations. The first case is where soil is at field capacity (approximated by s=-0.03 MPa) all along the root, a likely situation after a good rain or irrigation. The root was assumed to be 120 mm long with x=-0.25 MPa at 120 mm. The results (Fig. 5A) show that there is a steep x gradient along the root, with x rising to nearly of the soil as the growth zone is approached (Fig. 5A, upper). The steep axial gradient in exists not only because of the high axial resistance in the region where xylem is not yet developed, but also because of the leaky cable behaviour of the root. The low at the basal end of the root is quickly dissipated along the root by fast radial water uptake (Jr, Fig. 5A, lower) driven by the large 
between the xylem and soil. This, together with the high axial resistance near the apex, enabled the growing apex to maintain its near to that of the surrounding soil. The difference in between the soil and the centre of the root in the growth zone at the apex is the result of growth induced depression in (Molz and Boyer, 1978; Silk and Wagner, 1980).
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It should be noted that in Fig. 5
the radial transport (Jr) is in terms of flux density at a particular point along the length of the root whereas the axial flow is cumulative and represents the sum of all upstream radial fluxes, minus the water used for growth from the apex up to the point in question along the root. It is informative to examine the flux and flow in the growth and adjacent region, enlarged in the inset of the lower figures. Where axial flow (Qx) is positive, the net flow along the root is directed toward the base of the root. Where axial flow is negative, the net flow is in the reverse direction, directed toward the zone of high growth rate. In the inset of Fig. 5A, it is seen that axial flow was directed toward the growth zone up to 15 mm from the apex, although the growth zone ended at 10 mm in the simulation.
With the much faster water uptake (Jr) at the basal portion of the root in Fig. 5A, the soil surrounding the older part of the root will dry faster, leading in time to a gradient in soil (and hence s) along the root. The shape of this gradient is not obvious and depends on hydraulic properties of the soil and the root, as well as transpiration rate and root elongation rate, and the duration of soil water depletion. Opting for simplicity, a simulation was run for the situation where s decreased linearly from -0.03 MPa at the apex to -0.21 MPa at 120 mm, but x remained at -0.25 MPa at 120 mm as in Fig. 5A. The result (Fig. 5B) shows interesting contrasts with Fig. 5A. As might be expected, the total uptake (Qx at 120 mm) is reduced by almost 50%, as a consequence of the reduction in soil at the basal end. Radial uptake was much reduced in the basal portion, but was maintained almost the same in the portion younger than 30 mm, in spite of the fact that the overall soil for the younger portion is substantially lower than that simulated in Fig. 5A. This occurred because the tension in the xylem (as negative x) was better transmitted to the younger portion (Fig. 5B, upper) compared to the situation depicted in Fig. 5A, creating the radial gradient needed to maintain the uptake rates of the younger part of the root. Note that x was set to be the same at 120 mm in both situations (Fig. 5A, B). In the uniformly wet soil situation (Fig. 5A), however, more tension was dissipated by the high radial uptake rates in the basal portion, so less tension was transmitted to the xylem in the younger portion. For a situation of drying soil (Fig. 5B), it could be said that the cable acted as if it partly sealed itself in the drier part of the soil. That is, the root is really a ‘reversibly leaky cable’. Because the profiles of for the growth zone (0–10 mm) are very similar for the two situations (Fig. 5A versus B), the growth rate as a function of location along the root was assumed to be unchanged and the same sink function (last term of equation 5) was used in the simulation of the two situations.
The most important result overall in comparing the two simulations is that x of the growth zone was barely reduced in the case of the drying soil (Fig. 5B) in comparison with the case of wet soil (Fig. 5A), showing the advantage of the slow maturation of xylem vessels close to the growth zone.
A simulation was also run for the case of a longer root, 180 mm, in a soil at field capacity, and under the same boundary conditions as in Fig. 5A. The results (Fig. 5C) show that the longer root increased the total water uptake only by a small amount (8.6%) and the radial uptake was confined even more to the basal portion. This may be said to be the result of more dissipation of xylem tension along the longer root length before the younger part was reached. Consequently, of the growth zone and the adjacent newly mature zone was very slightly higher and the axial flow was directed toward the growth zone for a slightly longer distance (changed from 15 mm to 17.2 mm, insets of Fig. 5A and C) basal to the growth zone.
The simulated results in Fig. 5 involved a number of simplifications. Most important was the fact that the impact of soil hydraulic conductivity was omitted by assuming that at the root surface was the same as of the soil. To be more realistic, the simulation should include the segment of water transport path from the bulk soil to the root surface, which is jointly determined by the gradient, geometric of the soil–root interface, and hydraulic conductivity of the soil. Because hydraulic conductivity of the soil decreases approximately exponentially with decreases in soil water content (Hillel, 1982), the effect of soil water depletion at the basal end of the root in shifting water uptake to the more apical region would be substantially more pronounced than that seen by comparing Fig. 5A with 5B. Because the model does not yet take soil hydraulic conductivity into account, no attempt is made to simulate a drought situation when soil could fall to much below -0.21 MPa and soil hydraulic conductivity becomes the major factor limiting water uptake.
Relevance of simulation results to field behaviour of roots
So far the discussion concerns laboratory studies and simulations and can be considered largely theoretical. How are these concepts manifested in the field? One manifestation is that for plants relying on soil-stored water without rain or irrigation, roots grow more into new and yet unexplored soil where there is moisture. Figure 6 compares the profile of root distribution at the end of the season of a maize crop that was not irrigated and growing virtually only on water stored in the soil at planting with one that was regularly irrigated. Roots of the well-irrigated treatment proliferated mostly in the upper 0.5 m of the soil. Below 1.0 m root length density dropped to less than 1 cm root cm-3 soil. Without irrigation, but starting with a soil profile at field capacity, roots of the unirrigated treatment depleted the soil water deeper and deeper over the season, and roots proliferated much more in the lower depth layers, where the root length densities were more than twice those of the well-irrigated treatment (Fig. 6). As the simulations (Fig. 5B) made clear, hydraulic isolation of the growth zone of the root coupled with the leaky-cable characteristics of the mature portion of the root ensures that root growth has the first call on the water contained in the newly intercepted soil volume, largely in disregard of the demand for water from the shoot manifested as increased xylem tension. The maintenance of root growth as water stress develops and the shifting of growth to the deeper soil layers where there is remaining water (Fig. 6) are also aided by two other factors. One is the enhancement in cell wall loosening and rapid osmotic adjustment which sustains root growth in the face of declining , as discussed in an earlier section. The other is the fact that leaf growth is inhibited by even very mild water stress while leaf photosynthesis continues unabated (Boyer, 1970; Acevedo et al., 1971; Bradford and Hsiao, 1982). This reduction in the strength of sink for assimilates above-ground should make more assimilates available for root growth. As the outcome, not only is root growth favoured relative to shoot growth in order to explore the soil more thoroughly for water, the distribution of roots is also shifted to the wetter soil to make more effective use of each mm of root length for water uptake and transport to the top.
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Another field observation that may be explained in terms of the fundamental understanding is the pattern of water extraction of an annual crop from a drying soil profile and after a heavy rain or irrigation. When there is no rain and the crop grows on water stored in the soil, the extraction front moves downward with the growth of roots and the depletion of water in the depth where the roots have resided for some duration (Fig. 7
). The rapid deepening of the roots in the drying soil was presumably facilitated by the high at the growing tips maintained by their hydraulic isolation and continuous movement into the wet and yet to be exploited soil. At the same time, the leaky cable characteristics of the roots also plays a critical role. The ‘cable’ is leaky as long as there is substantial water uptake along its length, which dissipates the gradient initiated at the proximal end and minimizes uptake at the distal end (Fig. 5A). As the water depletes in the upper part of the soil profile, however, uptake at the proximal part of roots becomes minimal and the gradient is transmitted more fully to the distal end, enabling more effective water removal from the deeper part of the soil. As mentioned earlier, the root is really a ‘reversibly leaky cable’ and acts as if it seals itself in the parts where there is little or no water to be taken up. This notion applies not only to single long roots, but in principle also to root systems with branches. Hence, the removal of water from the deeper layers of a depleting soil profile does not require a huge drop in at the proximal end of the root system. For the crop depicted in Fig. 8, around 70 DAP (days after planting) when the maximum water extraction rate occurred at the depth of 1.8 m and very little water was extracted from the dry soil above the depth of 1.0 m, leaf of the crop was only 0.2–0.3 MPa lower than that for the well-watered control (Fereres et al., 1978) and transpiration was nearly as high, as can be deduced from the sum of extraction rates for all depth layers in Fig. 7