Blackwell Science, LtdOxford, UKPCEPlant, Cell and Environment0016-8025Blackwell Science Ltd 2005? 2005 284492499 Original Article Hydraulic acclimation in cotton Y. Li et al.
Plant, Cell and Environment (2005) 28, 492–499
Long-term hydraulic acclimation to soil texture and radiation load in cotton YAN LI1, HAO XU1,2 & SHABTAI COHEN3 1
Fukang Station of Desert Ecology & Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, 40–3 South Beijing Road, Xinjiang 830011, P.R. CHINA, 2Graduate School, Chinese Academy of Sciences, 19A, Yu-Quan Road, Beijing 100039, P.R. China and 3Institute of Soil, Water and Environmental Sciences, ARO Volcani Center, PO Box 6, Bet Dagan 50250, Israel
ABSTRACT The concept of root contact hypothesizes that the absorbing roots grown in sandy soil are only partially effective in water uptake. Co-ordination of water supply and demand in the plant requires that the capacity for water uptake from the soil should correspond to an operational rate of water loss from the leaves. To examine how the plant hydraulic system responds to variations in soil texture or evaporative demand through long-term acclimation, an experiment was carried on cotton plants (Gossypium herbaceum L.), where three grades of soil texture and three grades of evaporative demand were applied for the whole life cycle of the plants. Plants were harvested 50 and 90 d (fully grown) after sowing and root length and leaf area measured. At 90 d hydraulic conductance was measured as the ratio of sap flow (measured with sap flow sensors or gravimetrically) and water potential. Results showed that for plants grown at the same evaporative demand, those in sandy soil, where rootspecific hydraulic conductance was low, developed more absorbing roots than those grown in heavy-textured soil, where root specific conductance was high. This resulted in the same leaf specific hydraulic conductance (1.8 ¥ 10-4 kg s-1 Mpa-1 m-2) for all three soils. For plants grown in the same sandy soil, those subjected to strong evaporative demand developed more absorbing roots and higher leafspecific hydraulic conductance than those grown under mild evaporative demand. It is concluded that when soil texture or atmospheric evaporative demand varies, plants co-ordinate their capacities for liquid phase and vapour phase water transport through long-term acclimation of the hydraulic system, or plastic morphological adaptation of the root/leaf ratio. Key-words: evaporative demand; hydraulic conductance; leaf area; root length; root-to-shoot ratio; soil texture.
INTRODUCTION Current thinking is that at high evaporative demand plants operate within hydraulic limits dictated by the maximum Correspondence: Dr Yan Li. E-mail:
[email protected]
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axial water gradient which can be applied to the xylem without causing cavitation and xylem dysfunction (Sperry 2000). Transpiring plants need to extract water from the soil in order to compensate for water loss from leaves, and to maintain a favourable plant water status and normal growth. While the potential rate of water loss from a plant depends on its shoot dimension and the atmospheric condition it is subjected to, the capacity for water extraction from soil is determined by the root system and the physical conditions in the surrounding soil. Accumulated evidence supports the premise that plants have the capability to coordinate vapour phase (from leaf to atmosphere) and liquid phase (from soil to leaf) water transport through short-term regulation and long-term acclimation (Sperry 2000; Meinzer 2002; Mencuccini 2003). Much work has focused on short-term regulation through the adjustment of stomata opening (e.g. Meinzer & Grantz 1990a, b; Fuchs & Livingston 1996; Sperry 2000; Hubbard et al. 2001; Sperry et al. 2002). However, looking at the life cycle of annual plants or trees, structural acclimation is almost certainly the dominant process by which the hydraulic system responds to a range of external and endogenous stimuli and to changes in the intensity and direction of these stimuli over time, while short-term regulation should just be small fluctuations centred around the development or acclimation of the hydraulic system (Mencuccini 2003). For instance, Hacke et al. (2000) compared the root/leaf ratios of genetically similar loblolly pine (Pinus taeda L.) grown in sandy and loam soil, and found that the trees grown in sandy soil had a nearly six-fold higher root/leaf ratio than trees grown in loam soil. Despite the importance of structural acclimation in modifying plant hydraulic systems (Mencuccini 2003), this topic has largely been neglected, both in terms of original empirical research (but see Magnani, Mencuccini & Grace 2000) and theoretical analyses. Another theory that inspired the current study is the concept of root contact. In a study on root water uptake, Herkelrath, Miller & Gardner (1977) proposed that in sandy soil, absorbing roots might be only partially effective in taking up water due to incomplete contact between the root surface and soil particles. The root contact theory is based on the assumption that part of the root surface exposed to air-filled large pores in sandy soil, is ineffective © 2005 Blackwell Publishing Ltd
Hydraulic acclimation in cotton 493 in water uptake. This concept has subsequently been invoked frequently (e.g. Bristow, Campbell & Calissendorff 1984; Jensen et al. 1993; Sperry et al. 1998) and, in some cases, the physical discontinuity between the root surface and the soil particles was experimentally confirmed (Kooistra et al. 1992; North & Nobel 1997). However, several implications of the theory have yet to be tested. Namely, if the roots in sandy soil are only partially effective in water uptake, plants grown in sandy soil should develop more absorbing roots than those grown in heavier textured soils, in order to co-ordinate vapour and liquid phase water transport. In addition, the hydraulic conductance of a unit root for water uptake from sandy soil should be lower than that in fine textured soils. On the other hand, it might be expected that when soil conditions are the same, those grown under strong atmospheric evaporative demand should develop more absorbing roots than those under mild evaporative demand, following the same need to co-ordinate water transport capacity. In the current study, cotton plants were grown in different soil textures and evaporative demands for the whole life cycle. Long-term acclimation, or plastic adaptation, of the plant hydraulic system was examined in term of variations in root leaf-1 ratio and root and leaf specific hydraulic conductance.
are given in Fig. 1. As the particle diameter of the sandy soil was 0–500 mm and that of the clay was 0–50 mm, the particle size distribution for the mixture of these two (volumetric ratio of 1 : 1, hereafter referred to as mixed) can be easily inferred (Fig. 1). Three levels of atmospheric evaporative demand were created by placing the plants in the open field, under a shade net that reduced global radiation by 60% and in the hall of Fukang Station Laboratory Building (about 85% global radiation reduction compared with the open field). Pan (E601, Chinese standard evaporation pan) evaporation under the net and in the hall was 67 and 28% of that in the open field, respectively.
Plant materials and measurement of root/leaf ratio Ninety plastic pots (8 L capacity) were filled with sandy, clay or mixed soils; 30 pots for each soil texture. In addition, a further 30 pots were filled with sandy soil to grow plants under reduced evaporative demand. Cotton seeds were sown on 15 April 2002, and after emergence only one plant was left to grow in each pot. Thirty pots for each soil texture were placed in the open field. In addition, 15 sandy-soilfilled pots were placed under the net and 15 on the floor of the Laboratory Building hall, respectively. Prior to sowing and after filling the pots with soil, all pots were watered and drained continuously for 10 d (for pots filled with clay or mixed soil) or 5 d (for pots filled with sandy soil), in order to wash out the salt or nutrients. All potted plants were kept well-watered and treated with pesticide to avoid physiological stress during the whole growing period. Flower buds were removed on emergence to allow only vegetative growth. Fertilizers were given by dissolving them in the irrigation water. For the clay or mixed soil, fertigation was carried out once a week. For the sandy soil with low nutrient retention capacity, it was carried out twice a week with half of the amount given to the clay or mixed soil. Drainage from each pot was collected separately and returned to the pot during the next irrigation to avoid nutrient loss and to keep the nutrition at the same level for the different types of soil.
MATERIAL AND METHOD Creation of grades in soil texture and evaporative demand The experiment was carried out during the cotton-growing season of 2002 at Fukang Station of Desert Ecology, Chinese Academy of Sciences, which is located at the southern periphery of the Gubantonggut Desert, in the hinterland of the Eurasian continent (87∞56¢ E, 44∞17¢ N, 475 m a.s.l.). Three grades of soil texture were created by taking local fine-textured soil, nearby desert sand and the mixture of these two with volume ratio 1 : 1. The particle size distributions of the desert sand (hereafter referred to as sandy) and the local fine-textured soil (hereafter referred to as clay)
Percentage by weight (%)
20
Sandy soil from desert (sandy)
Local heavy texture soil (clay)
(b)
(a) 15
10
5
0 0
100
200
300
400
500 0
10
20
30
40
Particle diameter of the soils (mm) © 2005 Blackwell Publishing Ltd, Plant, Cell and Environment, 28, 492–499
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Figure 1. Particle size distribution of the two soils contrasting in soil texture: (a) sandy soil; (b) clay soil. Data were obtained from the particle size measurement by a laser diffraction system (Sympatec GmbH, System-Partikel-Technik, Clausthal-Zellerfeld, Germany).
494 Y. Li et al.
Total water conductance of individual plants grown in the different soil textures and evaporative demands was quantified after 90 d, when outdoor plants had reached full size. Sap flow of the plants was measured on the base of the plant stem (and water flux was considered equal to plant water uptake from the soil) with a compensation heat pulse system calibrated for cotton plants (Cohen et al. 1988). As the plants grown in the hall were too small for the heat pulse system, water loss from these plants was quantified by weight loss with balances of ± 0.05 g resolution. Soil evaporation was minimized by sealing the pots with plastic sheets and adhesive tape. Drainage from the pot bottom was avoided by carefully scheduling irrigation so that drainage would stop close to the time of measurement, thus ensuring a sufficient water supply for the plants. Leaf water potential was measured simultaneously with three HR-33T dew-point microvolt meters each equipped with 10 C-52 sample chambers and a PS-10 switching box (Wescor, Inc. Logan, UT, USA). Three round leaf samples were taken from the youngest mature leaves of each plant with a onehole paper punch for leaf water potential determination. As the heat pulse system was equipped with 10 sensors and the three dew-point microvolt-meters were equipped with 30 leaf chambers, measurements of sap flow and leaf water potential could feasibly be done on 10 plants simultaneously. However, for the leaf samples to reach equilibrium in the chamber more than 1 h was usually required. To ensure a 1 h interval for leaf water potential measurement, leaf samples were loaded into chambers before samples from the previous hour were read and removed. Thus for three leaf samples taken from each plant every hour, six leaf chambers were occupied. Hence with 30 chambers measurements could only be done on five plants simultaneously. The diurnal course of sap flow and leaf water potential was measured on 10 plants (on two days in succession) for each grade of soil texture and evaporative demand. After these measurements, leaf area and root length of each plant were determined as described in the preceding paragraph.
Root/leaf ratio Figure 2 presents the root length to leaf area ratio for young and mature plants [50 d (b) and approximately 100 d (a) after emergence] grown in the open field. Large and statistically significant differences (T-test at a = 0.05) were found in these ratios for the different soils, with the ratio for clay soil (approximately 8–10 cm-1) being approximately half of that for sandy soil (approximately 17–19 cm-1), and the mixed soil resulting in intermediate ratios. The ratios decreased slightly from young to large plants, but this change was statistically significant only in the clay soil. As all plants were grown under the same high evaporative demand, but were well irrigated and otherwise kept free of any physiological stress during growth, soil texture differences (Fig. 1) must have been the only cause for the differences in root/leaf ratio; namely at the same evaporative demand plants grown in the sandy soil developed more roots than those grown in the heavy soils. Figure 3 presents the root/leaf ratio of the plants grown in the sandy soil under different atmospheric evaporative demands. The highest values, about 19 cm-1, were obtained in the open field, where evaporative demand was highest, and the lowest, approximately 9 cm-1, was obtained indoors at low evaporative demand. Since the growth medium was the same sandy soil, the differences in root/ leaf ratio must have been related to evaporative demand; namely under high evaporative demand plants developed more roots.
(b) Young plants
-2
Determination of plant hydraulic conductance
RESULTS
Root/leaf ratio (cm cm )
Fifty days after emergence, half of the open field plants (15 pots for each texture) were sacrificed to measure total leaf area and root length of each plant. All leaves were cut from the branches and stem, placed in a plastic bag and stored in a 4 ∞C cold room after measuring fresh weight. The whole root system was carefully washed out of the soil, and roots from each pot were sealed in a plastic bag and stored in the cold room. Leaf area and root length were then measured with CI-400 CIAS (Computer Imaging Analysis Software; CID Co., Logan, UT, USA) after scanning and saving the images of the fully stretched leaves and roots with a computer scanner. Large roots (diameter > 1 mm), which were considered as water conducting rather than water absorbing, were not included in the total root length calculation. In fact, the overall length of large roots was negligible relative to total plant root length.
A 20
B
C
(a)
16
Full-sized plants
12 8 4
A
B
0
C
The soils Figure 2. Root-to-leaf ratios of the cotton plants grown in different soil textures: (a) full-sized plants; (b) young plants. Total length of roots of a plant was presented in centimetres and corresponding leaf area in cm2. Vertical bars indicate two standard errors of the mean (n = 15). Columns with different letters at the bottom are significantly different (t-test at a = 0.05).
© 2005 Blackwell Publishing Ltd, Plant, Cell and Environment, 28, 492–499
Hydraulic acclimation in cotton 495 be obtained by plotting measured leaf water potential (a negative pressure) against the flow rate of liquid water through a plant (Cohen, Fuchs & Cohen 1983; Li et al. 2002). Figure 4 presents examples of the linear relationship obtained between water flow rate and leaf water potential (LWP) for one plant from each grade of soil texture (Fig. 4a–c) and each grade of evaporative demand (Fig. 4d– f). The slope of the linear relationship is considered to be the hydraulic conductance (Gallardo et al. 1996). Hydraulic conductance of 50 cotton plants was thus determined, 10 plants for each grade of soil texture or evaporative demand, where the outdoor sandy soil treatment was used in both series. Figure 5 shows the relationships between hydraulic conductance and leaf area or total root length, for plants grown outdoors in the three soil types (Fig. 5a & b) and for plants grown in sandy soil under three grades of evaporative demand (Fig. 5c & d). Figure 5a shows that the relationship between plant hydraulic conductance and leaf area was the same, regardless of differences in soil texture and root/leaf ratios. The slope, that is the leaf specific hydraulic conductance of the plants (Kl = 0.66 L h-1 MPa-1 m-2, or 1.8 ¥ 10-4 kg s-1 MPa-1 m-2), expresses the ability of the hydraulic system to supply water to the leaves (Tyree & Ewers 1991). Thus, Fig. 5a shows that the plants grown
-2
Root/leaf ratio (cm cm )
20
16
12
8
4
0
A
A
B
In field
Under net
In house
Figure 3. Root-to-leaf ratios of the cotton plants grown under different evaporative demands. Vertical bars indicate two standard errors of the mean (n = 15). Columns with different letter at the bottom are significantly different (t-test at a = 0.05).
Hydraulic conductance of plants Hydraulic conductance of a plant is the change in flow rate of liquid water through the plant per change in water potential driving the flow. For intact plants, the value of the hydraulic conductance (or its inverse, the resistance) may
0.10
(c)
(f)
Mixed soil
In house
0.08 Y = -0.1307X- 0.0556
Y = -0.0226X- 0.0099
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R = 0.714
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Leaf water potential (MPa) © 2005 Blackwell Publishing Ltd, Plant, Cell and Environment, 28, 492–499
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Figure 4. Determining hydraulic conductance of the plant from leaf water potential and corresponding water flow rate in the plant. Plant hydraulic conductance was obtained as the slope of the least squares linear regression. One example is given for each treatment (a, b, c, d, e, f).
496 Y. Li et al.
-1
-1
Hydraulic conductance of plant (L h MPa )
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Clay Sandy Mixed
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Total root length of plant (m)
at the same atmospheric evaporative demand developed identical Kl, despite the contrasting soil textures. In Fig. 5b, total root length of the plants was plotted against corresponding hydraulic conductance, and significantly different slopes were obtained for plants grown in the sandy and clay soil, respectively. Root specific hydraulic conductance (Kr), which expresses the water supplying ability of unit length of roots + root-to-soil interface (Gallardo et al. 1996), was 4.8 ¥ 10-4 and 9.7 ¥ 10-4 L h-1 MPa-1 m-1 for sandy and clay soils, respectively; that is, Kr in clay soil was twice that in the sandy soil (Fig. 5b). The value of Kr for mixed soil, not presented in Fig. 5b for clarity, was intermediate, 0.66 ¥ 10-4 L h-1 MPa-1 m-1. Hence, Fig. 5b demonstrates that Kr varied with soil texture, and in particular, for sandy soil Kr was low. Figures 5c and d show the relationships between the hydraulic conductance of the plants and corresponding total root length (Fig. 5d) and total leaf area (Fig. 5c), for cotton plants grown in sandy soil in the open field (in field), under the net (under net) or in the building hall (in house). Figure 5d demonstrates that these plants developed identical Kr (4.7 ¥ 10-4 L h-1 MPa-1 m-1), regardless of the differences in atmospheric evaporative demand. However, in the open field Kl was approximately twice (Figs 5c and 0.72 L h-1 MPa-1 m-2) that in the house (0.37 L h -1MPa-1 m-2). Kl for plants grown under the net, not presented for clarity, was intermediate (0.58 L h -1MPa-1 m-2). Thus, Fig. 5c and d show that Kl varied with atmospheric evaporative demand, and plants grown under reduced evaporative demand developed low Kl (Fig. 5c). However, for the same growth medium Kr of the plants was constant (Fig. 5d). Figure 6 summarizes the variation of Kr with soil texture (Fig. 6a) and the variation of Kl with evaporative demand (Fig. 6b). Table 1 presents the maximum transpiration rate
Figure 5. Total leaf area or total root length of the individual plants plotted against the corresponding plant hydraulic conductance, for different soil textures (a, b) and evaporative demands (c, d).
(per leaf area) and minimum leaf water potential measured during the determination of plant hydraulic conductance.
DISCUSSION Determining hydraulic conductance of plants from fluxpotential relationships (Fig. 4) assumes a steady state. In the early morning or late afternoon hours when changes in plant water storage (i.e. capacitance) are relatively large, steady state can not be assumed. However, since sap-flow was measured near the stem base of the plants, it measured water uptake from the soil-root system and not evaporation from the leaves, so the influence of the capacitance of the shoot system on the results was probably minor, resulting in the highly linear relationships shown in Fig. 4. Similar highly linear relationships were found by Moreshet et al. (1996) for non-stressed cotton in a loamy clay soil using the same methods. In Fig. 4 the large intercepts on the water potential axis indicate significant, non-zero water potential for zero flow (y0). Non-zero values of the same magnitude (approximately 0.5 MPa) have been reported for y0 and discussed elsewhere, with no clear relationship to soil texture or plant species (Cohen et al. 1983; Lascano & van Bavel 1984; Cohen, Moreshet & Fuchs 1987; Moreshet et al. 1996; Li et al. 2002). Cohen et al. (1983) and Li et al. (2002) proposed that y0 represents the water potential at the soil– root interface; namely the water potential that the plant senses in well-watered soil. Li et al. (2002) proposed that immediately after irrigation or at predawn, the soil surrounding the roots is as wet as in the bulk soil. Shortly after water uptake begins, a dry layer develops at the soil–root interface and by the time water uptake rate is large enough to be detected the water potential at the root surface is y0, and subsequently water potential at the interface remains
© 2005 Blackwell Publishing Ltd, Plant, Cell and Environment, 28, 492–499
Hydraulic acclimation in cotton 497
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1.0x10
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(a)
Figure 6. Root or leaf specific conductance (Kr or Kl) for plants grown in different soil textures (a) or different atmospheric evaporative demands (b). Vertical bars indicate two standard errors of the mean (n = 10). Columns with the same letter at the bottom are significantly different (t-test at a = 0.05).
constant throughout the day due to the large water capacity of the well-watered bulk soil. For a dry soil with low capacitance, y0 may vary significantly during the day and the fluxpotential relationship is not linear (Moreshet et al. 1996; Li et al. 2002). Following this line of thinking, the hydraulic conductance derived as in Fig. 4 was in fact the hydraulic conductance of plant + interface. For a well-watered soil, bulk soil water potential can be safely assumed as to be zero since it is at least an order of magnitude smaller than leaf water potential or y0, and thus y0 is the drop in water potential from the bulk soil to the soil–root interface. The current study provides experimental evidence that root contact is directly related to root (+ interface) specific hydraulic conductance (Kr), as proposed originally by Her-
kelrath et al. (1977). Figures 5b and 6a show that in sandy soil Kr was only approximately half that of plants grown in the clay soil when the only difference in growing conditions was soil texture. Hence, the difference in Kr of the plants probably resulted from the soil texture and not from a change in root structure. In the sandy soil absorbing roots are partially exposed to large air-filled soil pores, which create a partial physical discontinuity at the soil–root interface for water movement from soil to roots (Herkelrath et al. 1977). That partial discontinuity makes the root surface only partially effective in water uptake, and in response, plants developed high root/leaf ratios (Fig. 2; Hacke et al. 2000) in order to compensate for the low unit root water uptake capacity. It should be noted that the physical discontinuity at soil–root interface observed by Kooistra et al. (1992) and North & Nobel (1997) did not unequivocally validate the hypothesis proposed by Herkelrath et al. (1977), as the latter proposed the concept of root contact for sandy soil in general, whereas the physical discontinuity was observed when soil was either excessively loose (Kooistra et al. 1992) or extremely dry (North & Nobel 1997). The current study validates the root contact concept for well-watered, compacted sandy soil. The Kr values obtained in the current study (Fig. 6a) are of the same order of magnitude as reported by Gallardo et al. (1996) for two annual crops; and the Kl values (Fig. 6b) obtained here are of the same order of magnitude as reported by Tsuda & Tyree (2000) for several annual crops and Cohen & Naor (2002) for apple trees (Malus domestica Borkh.). Cotton displays anisohydric behaviour when exposed to water stress (Tardieu & Simonneau 1998), since mid-day LWP in these conditions can be more than 0.5 MPa lower than after irrigation (Moreshet et al. 1996). However, the well-irrigated cotton plants in this experiment behaved isohydrically; namely, at high evaporative demand mid-day LWP in the different treatments and on different days did not vary significantly (Table 1). The value of this quasiconstant mid-day leaf water potential is a compromise between the hydrological environment to which a plant is adapted and the increasing cost of plant function as water potential declines; and is highly correlated with hydraulic constraints (Tyree & Sperry 1988; Sperry 2000). It is therefore interesting to consider what would happen if root-toshoot ratios were to remain constant in the different soil
Table 1. Minimum leaf water potential (LWP) and maximum transpiration rate (T, on a leaf area basis) for each soil texture and evaporative demand class Treatment Soil texture
Minimum LWP (MPa) Maximum T (L h-1 m-2)
Evaporative demand
Sandy
Mixed
Clay
In field
Under net
In house
-1.21 ± 0.04c 0.52 ± 0.02c
-1.17 ± 0.03c 0.49 ± 0.04c
-1.26 ± 0.05c 0.55 ± 0.03c
-1.24 ± 0.03c 0.48 ± 0.02c
-1.08 ± 0.05b 0.39 ± 0.04b
-0.81 ± 0.03a 0.14 ± 0.01a
Note: Data are the average (± SE) of 10 plants measured on two successive days. Data for different treatments were not measured on the same day, but were measured on 10 successive clear days. Data in the same row marked with different superscript letters are significantly different (t-test at a = 0.05). © 2005 Blackwell Publishing Ltd, Plant, Cell and Environment, 28, 492–499
498 Y. Li et al. textures. Lower Kr in the sandy soil would lead to reduced leaf specific hydraulic conductance (Kl), and LWP, leaf conductance, transpiration and photosynthesis would be reduced. The observed increased root-to-shoot ratios in the sandy and mixed soils (a two-fold variation), which completely compensated for the reduced Kr, allowed the upper part of the plant, and especially the leaves, to operate identically in the different soils (Table 1). Examination of leafspecific transpiration and LWP reported for Pinus taeda under non water stress conditions (Hacke et al. 2000) indicates that there too Kl was similar in soils of different texture. For Pinus taeda this acclimation required a six-fold variation in root-to-shoot ratios. However, when Pinus taeda was grown with fertilization shoot size increased, resulting in lower Kl and leaf conductance (Ewers, Oren & Sperry 2000). The latter was probably accompanied by lower root-to-shoot ratios, as indicated by the reported reduced specific hydraulic conductance. The response of root-to-shoot ratios to fertilization highlights the importance of maintenance of the same total fertilizer availability in the different treatments in our study (see Methods), even though the different soils retained different amounts of nutrient. The gradual reduction in root-to-shoot ratios observed here with decreasing evaporative demand is probably caused by the common response of shade-grown plants to develop lower root-to-shoot ratios. Decreases in root-toshoot ratio in response to shade have also been reported for grasses (Samarakoon, Wilson & Shelton 1990), wheat (Mitchell et al. 1996), citrus (Raveh et al. 2003), several weeds (Begna et al. 2002) and two shade plants (Raveh, Nerd & Mizrahi 1998). Begna et al. (2002) studied light response of several species, where leaves were fed carbohydrates in order to compensate for differences in photosynthesis. In that case, where light response was assumed to have been decoupled from photosynthate accumulation, root-to-shoot ratios still increased at low light, indicating a direct morphological response to reduced light. Even so, this response is appropriate to the limiting factor, light for photosynthesis, and it could be interpreted as an acclimation to increase light capture when water uptake capacity is ample. However, if low Kl plants were to be transferred to higher evaporative demand conditions, their leaf conductance and photosynthesis would be severely reduced, at least until they could develop more extensive roots. What is missing in our experiment is a series of treatments in which evaporative demand varies independently of light, and light is maintained at photosynthetically saturating levels. As at low evaporative demand there was no compensation for the reduced Kr of sandy soil relative to the clay soil, we assume that the increased root-to-shoot ratio obtained when Kr was low and evaporative demand high was a response to a stimulus that occurred at high evaporative demand. What was that stimulus? Assuming that initially the plants had the same size root system, and similar midday LWP, lower Kr would result in lower water potential in the roots. So the increased root-to-shoot ratios (Fig. 5b) would correspond to lower root water potentials. This is not
surprising, since roots are less inhibited by low water potentials than shoots (Hsiao & Xu 2000) and can maintain normal growth at low water potential when shoot growth is inhibited (Wu et al. 1994). Our results indicate that the response of plants subjected to different soil texture and evaporative demand is adapted to the limiting factor encountered during plant development, namely water transport or light capture; the former through development of enough roots to maintain constant Kl and the latter through developing more leaves per root in an effort to capture more light. The plant’s maintenance of constant Kl at high evaporative demand by adjustment of root size is further evidence for the hydraulic limitations to plant development, which is rapidly becoming axiomatic to our understanding of plant water relations and water use. Assuming that at high evaporative demand LWP is limited to a value close to that for the onset of cavitation in the stem and root xylem (Tyree & Sperry 1988), the observed constraints mean that maximum leaf-specific transpiration is also constant. This has specific implications for predicting water use, namely the maximum transpiration rate in well-watered conditions in different soils can be predicted from plant leaf area, using constants for LWP and Kl. In conclusion, this study demonstrates that the long-term acclimation of the plant hydraulic system or the plasticity of plant morphology plays a major role in determining plant water economy, and probably their carbon economy as well (Meinzer 2003).
ACKNOWLEDGMENTS This work was financially supported by a Knowledge Innovation Project from Chinese Academy of Sciences (CAS) (Grant KZCX3-SW-326) and a grant to Y.L. from National Natural Science Foundation of China (No. 40471048). The authors thank all staffs at Fukang Station of Desert Ecology for their excellent support in conducting this experiment, special thanks to Mr Zhong-Dong Lan and Mr Xue-Can Liu for technical help.
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