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Crop Science 40:131-137 (2000)
© 2000 Crop Science Society of America

CROP PHYSIOLOGY & METABOLISM

Response of Cassava to Water Deficit

Leaf Area Growth and Abscisic Acid

Alfredo A.C. Alvesa and Tim L. Setterb

a EMBRAPA, Cassava and Fruit Crops Unit, Caixa Postal 007, 44.380-000 Cruz das Almas, Bahia, Brazil
b Dep. of Soil, Crop and Atmospheric Sciences, Cornell University, 519 Bradfield Hall, Ithaca, NY 14853 USA

tls1{at}cornell.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Cassava (Manihot esculenta Crantz) responds to decreases in water status by pronounced stomatal closure and decreased leaf area growth. Many water deficit responses are thought to be regulated by abscisic acid (ABA). To evaluate the extent to which ABA accumulated in a temporal pattern related to water deficit and leaf area growth, five cassava genotypes were grown in greenhouse conditions and subjected to water deficit and recovery treatments during the vegetative-growth stage. Young and mature leaves were sampled for analysis of area growth and ABA. Under water deficit, leaves from all genotypes rapidly accumulated large amounts of ABA in both mature and young leaves. Correspondingly, young leaves halted leaf expansion growth and transpiration rate decreased. Young leaves accumulated more ABA than mature leaves in both the control and stressed treatments. The high ABA levels under water deficit were completely reversed to control levels after 1 d of rewatering. This rapid return to control ABA levels corresponded with a rapid recovery of leaf area growth rates. We postulate that the rapid reduction in leaf area growth and stomatal closure observed in our study may be due to cassava's ability to rapidly synthesize and accumulate ABA at an early phase of a water deficit episode.

Abbreviations: ABA, abscisic acid • DAP, days after planting • HBS, Hepes buffered saline • TBST, Tris-buffered saline-Tween detergent


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
CASSAVA is one of the most important staple foods in the human diet in the tropics, and is ranked as the sixth most important source of calories in the human diet worldwide (Food and Agriculture Organization of the United Nations, 1996). Although it is grown in a wide range of climates (between 30°N and 30°S latitude), from drought-prone to well-watered regions, it is commonly cultivated in areas receiving <800 mm rainfall per year with a dry season of {approx}4 to 6 mo, where tolerance to water deficit is an important attribute (El-Sharkawy, 1993).

Studies have indicated that when water is available, cassava maintains a high stomatal conductance and can keep internal CO2 concentration high, but when water becomes limiting, it closes stomata in response to even small decreases in soil water potential (El-Sharkawy and Cock, 1984). The rapid closure of cassava stomata and the resulting decline in transpiration lessens the decrease in leaf water potential and soil water depletion, thus protecting leaf tissues from turgor loss and desiccation (El-Sharkawy and Cock, 1984; Palta, 1984; Cock et al., 1985). Such behavior in response to early stages of soil water depletion has been described as isohydric, a behavior shared by cowpea [Vigna unguiculata (L.) Walp], maize (Zea mays L.), and several other crop plants (Tardieu and Simonneau, 1998). In addition, leaf area growth is decreased in response to water stress and is rapidly reversed following the release from stress (Connor et al., 1981; Palta, 1984; El-Sharkawy and Cock, 1987). This response limits the development of plant transpirational surface area during water deficit and keeps sink demand well balanced with plant assimilatory capacity.

A regulatory system that could potentially contribute to cassava's sensitivity to water deficit is the one involving the stress hormone ABA. In addition to closing stomates, ABA promotes characteristic developmental changes that can help plants cope with water deficit, including restriction of shoot growth (Creelman et al., 1990) and leaf area expansion (Van Volkenburgh and Davies, 1983; Lecoeur et al., 1995), and stimulation of root extension (Sharp et al., 1993, 1994). Moreover, studies have indicated that for certain plant responses, sensitivity to water deficit is correlated with changes in ABA concentrations (Trejo et al., 1995; Borel et al., 1997) and genotypic responsiveness to ABA (Blum and Sinmena, 1995; Cellier et al., 1998).

Information has not yet been reported about ABA contents in cassava. Considering the evidence that in cassava stomatal conductance and leaf area growth are decreased during water stress episodes, and that ABA has a role in stomatal closure and leaf area growth reduction, we sought to determine the temporal pattern of ABA accumulation during onset of stress and ABA decline during recovery in relation to concomitant changes in transpiration and leaf area growth. The objectives of this work were (a) to determine the temporal patterns of ABA accumulation in mature leaves and in immature expanding leaves during water deficit and after release from stress and (b) to determine the extent to which the stress and recovery response of leaf area growth is associated with the temporal pattern of ABA accumulation.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Plant Material and Growth Conditions
Five genotypes from the Cassava Germplasm Bank of the Brazilian Agricultural Research Corporation (Embrapa Cassava and Fruit Crops Dept., Bahia, Brazil) were used in this study. The genotypes and the ecosystems from which they came are listed in Table 1 .


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  		Table 1 Regions of origin and climatic ecosystems for the cassava genotypes used

 
Cuttings with three to four nodes were vertically planted in 3-L pots with peatmoss–vermiculite–perlite (1:1:1, v/v/v) supplemented with 5 g of Peters 15-16-17 (W.R. Grace and Co., Fogelsville, PA), 2 g of pulverized limestone, 6 g of FeSO4, 0.3 g of trace elements (Peters FTE 555), and 1 g of wetting agent (AquaGro G, Aquatrols, Pennsauken, NJ). The plants were grown in a greenhouse at average day/night temperatures of 24:19°C, with occasional exposure to temperatures of 33°C (max) and 17°C (min), and ambient solar illumination of 38 mol photosynthetically active radiation m-2 d-1 from 4 June to 3 Aug. 1995. All pots were maintained at field capacity with nutrient solution containing 1.08 g of Peters 15-16-17 per liter of solution until 45 d after planting (DAP), when the treatments were applied.

Treatments and Statistical Design
After 45 d (referred to as Day 0), when the plants had a height of 90 ± 24 cm (mean ± SD) and 19 ± 4 fully expanded leaves, the plants were randomly assigned to two treatment groups of 15 plants containing three plants (replicates) of each genotype. In one group, the pots were maintained at field capacity by applying nutrient solution with an automatic drip irrigation system at 2-h intervals; the other group was subjected to water stress by withholding water from Day 0 to 6 followed by rewatering on Day 6, and daily irrigation to field capacity until Day 15. Sampling was performed from Day 0 to 15. The degree of water deficit was estimated by weighing the pots (plus plant) at 0, 3, and 6 d after withholding water. After 3 and 6 d without watering, the soil water content decreased to 47 ± 3% and 40 ± 3% of free-drained soil capacity, respectively. This water deficit resulted in the wilting of some lower leaves. Water deficit was estimated by the rate of transpiration, which was calculated daily in equivalent neighboring plants of each genotype. Transpiration was calculated as the mass of water lost by each pot and plant system per day minus the mass of evaporation from the soil surface, which was estimated on control pots without plants and with soil water contents spanning the experimental range.

Plants were assigned to treatments and sampling dates in a completely random design. Analyses of variance were conducted to compare genotypes under water stress treatments at each sampling date.

Leaf Area Analysis
On each plant, leaves were identified on Day 0 (45 DAP) according to their node position with respect to the oldest folded leaf (F1 leaf). Those leaves younger than F1 (folded) were designated F2, F3, and etc., and those older (unfolded) were designated UF1, UF2, etc.

On Day 0 (45 DAP), the F1 leaf was tagged. The central lobe length of this tagged leaf blade, as well as that of the F2, F3, UF1, UF2, and UF3 leaves, was measured with a ruler. The leaf area (A) was estimated using the relationship: A = aLb, where L (cm) is central lobe length and a and b are coefficients determined according to the shape of the central lobe. Several cassava leaf development studies have indicated that equations of this form accurately describe the allometric relationship for cassava leaves (Connor and Cock, 1981; Yao et al., 1988). The genotypes in our study differ in their central lobe shape. To determine relationships between leaf area and leaf dimensions for different leaf shapes, we measured leaf area with an integrator (LI-3000, LI-COR, Lincoln, NE) on leaves with the following shapes: linear, hastate, lanceolate, oblong, and obovate. For each leaf shape, 220 leaves in different developmental stages were collected from plants and the following were measured: central lobe length (L), central lobe width (W), total number of lobes (N), and leaf area (A). We found satisfactory prediction of A using L alone, according to the following equations obtained for different shapes: (a) linear: 0.2142L2.2038 (r = 0.92); (b) hastate: 0.2689 L2.2225 (r = 0.94); (c) lanceolate: 0.7551L1.8964 (r = 0.92); (d) oblong: 0.9441L1.8985 (r = 0.95); and (e) obovate: 1.6507L1.7806 (r = 0.91). The L/W ratios for each shape were: linear = 9 to 13; hastate = 6 to 8; lanceolate = 5 to 6; oblong = 4; and obovate = 3. The appropriate equation was used in accordance with the central lobe shapes, as follows: oblong genotypes (BGM 252, BGM 911, and BGM 1013), lanceolate genotype (BGM 116), and hastate genotype (BGM 1148). During our study, genotype BGM 911 progressively developed leaf tip necrosis and distortion of shape, rendering unreliable the estimation of its leaf area from measurements of central lobe length. Thus, the leaf areas presented here exclude data for BGM 911.

Growth measurements were taken at 0, 3, and 6 d after withholding water (Days 0, 3, and 6), and at 3, 6, and 9 d after rewatering (Days 9, 12, and 15). For each 3-d interval, the leaf area and rate of leaf area growth was determined based on the increment of leaf area at each 3-d interval and calculated in square centimeters per day.

Abscisic Acid Extraction and Analysis
Mature and expanding leaves were sampled for ABA analysis at 3 and 6 d after withholding water (Days 3 and 6), and at 1, 3, and 6 d after rewatering (Days 7, 9, and 15). For mature leaves, six leaf disks (diam. = 0.46 cm) were sampled from the last fully expanded leaf. Due to their smaller size, for expanding leaves, three disks (diam. = 0.46 cm) were sampled from the two youngest unfolded (expanding) leaves (UF1 and UF2) to form a composite sample. One disk was from UF1, which had expanded to {approx}12% of its fully expanded leaf area, and two disks were from UF2 , which had expanded to {approx}30% of its fully expanded leaf area. Leaf disks from equivalent neighboring plants were sampled to determine the fresh and dry weight of leaf samples.

For ABA extraction, leaf disks were homogenized in 500 µL (six disks of mature leaves) or 250 µL (three disks of expanding leaves) of 760 mL L-1 ethanol, incubated 24 h at 4°C, and centrifuged 5 min at 4000 g. Supernatants (160 µL for mature and 80 µL for expanding leaves) were vacuum-dried at <30°C, resuspended in 160 µL of 200 mL L-1 methanol and applied onto C18 chromatography columns in 1-mL syringe barrels containing 0.6 g of 40-µm particle size packing material (bonded phase-octadecyl silane, J.T. Baker Chemicals, Phillipsburg, NJ). The ABA-containing fraction was eluted in 500 µL of solvent (500 mL L-1 methanol, 10 mL L-1 acetic acid), vacuum-dried, and redissolved in 80 and 40 µL (mature and expanding leaves, respectively) of Hepes buffered saline (HBS) solution (50 mM N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid], 1 mM MgCl2, 10 mM NaCl, 0.2 g L-1 NaN3, pH 7.5). Aliquots from this solution were assayed for ABA by indirect enzyme linked immunosorbant assay (ELISA) as described by Walker-Simmons (1987) and Ober et al. (1991), with slight modification. Briefly, plates (Corning High Binding 25802, Corning, NY) were coated overnight with 1.4 µg of ABA-4'-BSA conjugate in 200 µL of 50 mM NaHCO3 buffer, pH 9.6, containing 0.2 g L-1 NaN3 at 4°C. Plates were washed six times with Tris-buffered saline-Tween detergent (TBST) solution [50 mM Tris-hydroxymethyl amino methane, pH 7.5 containing 1 mM MgCl2, 10 mM NaCl, 0.1 g L-1 NaN3, and 1 g L-1 Tween-20 (P-7949, Sigma Chemical Co., St. Louis, MO)]. Samples were then incubated with primary antibody with the following in each well: 90 µL of HBS, 10 µL of plant sample, and 100 µL of HBS containing 0.1 µg of anti-ABA monoclonal antibody (clone 15-I-C5, Mertens et al., [1983]; currently available from Agdia Inc., Elkhart, IN). Concurrently, a triplicate set of (+)ABA standards (Sigma Chemical Co.) containing a series from 2 to 0.01 pmol per well served as a calibration curve. The antibody was added last to all wells on a plate. After incubation overnight at 4°C, plates were washed four times with TBST and 200 µL of secondary antibody solution containing 10 nL of anti-mouse IgG-alkaline phosphatase conjugate (A-3562, Sigma Chemical Co.) in HBS was added. After incubation overnight at 4°C, plates were washed four times with TBST and 0.2 µg p-nitrophenyl phosphate was added in 200 µL of buffer containing 0.9 M diethanolamine, pH 9.8, and 3 µM MgCl2. Plates were incubated for {approx}1 h at 24°C, and absorbance at 405 nm was read with a plate reader (model 750, Cambridge Technology, Watertown, MA). (+)ABA content was determined by calculations based on (+)ABA calibration standards.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Transpiration Rate
Whole-plant transpiration was measured to indicate the timing of water deficit onset (Fig. 1) . In the first 2 d of withholding irrigation water, transpiration rate remained high, indicating that stomatal conductance had not yet decreased. Between Day 2 and Day 3, as soil water was further depleted, transpiration rate abruptly decreased so that by Days 3 and 6 of water deficit, transpiration rate was only 36 and 14% of respective control plants. Other studies of cassava in this growth environment indicated that such decreases in transpiration are due to stomatal closure (Alves, 1998).



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  		Fig. 1 Transpiration rate in cassava plants during 6 d of water deficit. Each point is the average of five genotypes (one plant per genotype) and bars represent standard error of the mean

 
Leaf Area Growth
Water deficit substantially reduced leaf area growth in the four monitored cassava genotypes. Analysis of variance of the leaf area growth and ABA data indicated that although there were significant (P <= 0.05) effects of water deficit duration, genotypes and genotype x water deficit effects were not significant (Table 2) . Hence, the data presented below are averaged for the four genotypes. After 3 and 6 d of withholding irrigation, the area of the F1 leaf was only 27 and 11% of respective control leaf areas (Fig. 2A) , corresponding to a fivefold and 21-fold decrease in leaf area growth rate, respectively (Fig. 2B). Given that soil moisture reserves were present until Day 2, when transpiration data indicated the plants first experienced water deficit (Fig. 1), the data indicate that leaf area growth declined abruptly and to near zero after onset of plant water stress.


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  		Table 2 Analysis of variance for leaf area growth rate in the first folded leaf (F1 leaf) in cassava genotypes with water deficit treatments of 3 and 6 d. Analysis includes genotypes BGM 116, 252, 1148, and 1013, but excludes 911 (see Materials and Methods for details). Genotypes and water deficit treatments are the fixed main effects

 


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  		Fig. 2 (A) Individual leaf area growth and (B) leaf area growth rate in the first folded leaf (F1 leaf) estimated at 3-d intervals in cassava plants under water stress (6 d of water deficit followed by 9 d of rewatering) and control. Average of four genotypes with three replicates. Bars represent standard error of the mean (n = 12). Rate of leaf area growth was calculated based on the increment of leaf area at each 3-d interval; the values are plotted at the last day the data were collected

 
However, after rewatering, growth of the F1 leaf area increased rapidly. During the first 3 d of growth following rewatering, leaf area growth rate in the stress treatment increased to {approx}60% of the control and it remained at a substantial level between Day 9 and Day 12, while the controls declined, approaching zero at Day 15 (Fig. 2B). This suggests that although water stress decreased total leaf area, growth was delayed, so at Day 12 and 15 (6 and 9 d after rewatering), growth rates of the F1 leaves in stressed plants exceeded those in controls.

The degree of leaf area reduction was influenced by leaf age when water deficit started. After 6 d of water deficit, leaf area was reduced (in relation to control) from 49% in the oldest unfolded leaf (UF3 leaf) to 95% in the newest folded leaf (F3 leaf) (Table 3) . Also in younger leaves (F1–F3), the recovery of leaf area growth after rewatering was delayed, reaching substantial rates at 6 d after rewatering (data not shown). The extent of recovery was such that at the final sampling, leaf area was reduced from 33 to 50% in the sampled leaves (Table 3).


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  		Table 3 Leaf area in cassava leaves at various developmental stages in response to 6 d of water deficit (Day 6), and 9 d after rewatering (Day 15). Average of four genotypes with three replicates ± standard error of the mean

 
Leaf Abscisic Acid Accumulation
Three and six days after withholding irrigation, all cassava genotypes accumulated large amounts of ABA in both expanding (Fig. 3A) and mature leaves (Fig. 3B). These high ABA levels were almost completely reversed with respect to control levels after 1 d of rewatering (Fig. 3). In relation to controls, expanding leaves accumulated from 6- to 27-fold higher levels of ABA after 6 d of withholding irrigation, while mature leaves accumulated from 11- to 30-fold (Table 4) .



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  		Fig. 3 Abscisic acid (ABA) concentration in (A) expanding and (B) mature cassava leaves in controls and after 3 and 6 d of water deficit (Days 3 and 6) followed by periods of 1, 3, and 6 d of rewatering (Days 7, 9, and 12). Average of five genotypes with three replicates. Bars represent standard error of the mean (n = 15)

 

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  		Table 4 Abscisic acid concentrations in expanding (UF1 and UF2) and mature leaves of five cassava genotypes in control and water stress treatment (6 d of water deficit). Also shown is the ratio of abscisic acid concentration in stress/control (S/C). Average of three replicates

 
Although they were much thinner and had little development of chloroplasts, the sites of ABA synthesis (Popova-Losanka, 1998), expanding leaves accumulated about twofold higher ABA levels on a leaf area basis than mature leaves under either control or water stress conditions (Days 3 and 6) (Fig. 4) .



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  		Fig. 4 Abscisic acid (ABA) concentration in expanding (UF1 and UF2) and mature cassava leaves in control and water deficit treatments (3 and 6 d of stress [DAS]). Average of five genotypes with three replicates. Bars represent standard error of the mean (n = 15)

 
A substantial portion of variation in ABA concentration was explained by genotypes. At Day 6 of the water deficit treatment, genotypes differed (P <= 0.05) in ABA content per square centimeter in mature leaves (Table 4). In well-watered controls, genotypes differed in ABA content in both expanding and mature leaves, thus creating genotypic differences in the stress/control ratio of ABA contents. Nevertheless, in expanding leaves of the stress treatment, genotypes did not differ in ABA content per unit of leaf area.


   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Leaf Area Growth
The current study was designed to detect daily changes in leaf area growth rate in response to water deficit, and recovery in the same monitored leaves after rewatering. Our results indicated that cassava leaf area growth responds rapidly to both onset of plant water deficit, and recovery during rewatering (Fig. 2). In previous studies of cassava, the declines in leaf expansion rate during onset of water deficit were more gradual than those found in our study (Connor and Cock, 1981; Palta, 1984; Yao et al., 1988). This result can be explained by the more gradual decline in soil water content in the previous studies that were conducted in large pots (Palta, 1984) and deep soil (Connor and Cock, 1981; Yao, 1988) conditions. Thus, although field conditions generally involve gradual soil dry-down, our study, which used a pot system that allows rapid imposition of water stress, demonstrated that cassava is able to rapidly decrease its leaf area growth in response to water deficit.

As in previous studies (Connor and Cock, 1981; Palta, 1984), this study demonstrated that the cassava leaves in which leaf expansion is inhibited during water deficit rapidly regain their leaf expansion rate during recovery, such that expansion is temporarily shifted to the recovery phase. This behavior might contribute to cassava's adaptation to environments of periodic drought followed by renewed rainfall. By delaying leaf area growth until water is available, development of transpirational surface is restrained and development of sink demand by growing leaves may be kept in balance with supply of photoassimilate.

Leaf Abscisic Acid Accumulation
Abscisic acid increased rapidly in both photosynthetically mature source leaves and in immature sink leaves of water-stressed plants (Fig. 3 and Table 4). Although such ABA increases have been frequently documented in mature leaves of numerous species, few studies have documented ABA levels in immature expanding leaves (Trewavas and Jones, 1991; Hartung and Davies, 1994).

Abscisic acid determinations for cassava have not been previously reported. There are some ABA studies in castorbean (Ricinus communis L.) and moleplant (Euphorbia lathyris L.), which, as cassava, are in the family Euphorbiaceae. In castorbean, water stress induced a ninefold increase of ABA concentration in mature leaves (Zeevaart, 1977) and up to an 11-fold ABA increase in xylem sap (Jokhan et al., 1996). In E. lathyrus, water stress increased the ABA concentration 10-fold in expanding leaves and fivefold in mature leaves (Sivakumaran and Hall, 1978).

In all stressed and control treatments, the ABA concentration in immature expanding leaves was higher (~about twofold) than that in mature leaves (Fig. 4). Very similar outcomes have been reported in other Euphorbiaceae species: R. communis (Zeevaart and Boyer, 1984; Jeschke et al., 1997) and E. lathyrus (Sivakumaran and Hall, 1978), and in other species (Cornish and Zeevaart, 1984; Cowan and Railton, 1987). Although immature leaves do not have large numbers of chloroplasts, the postulated site of ABA synthesis (Popova-Losanka, 1998), there are several possible explanations for the high level of ABA accumulation in them. First, water stress stimulates the rates of both biosynthesis and degradation of ABA, and there is evidence that ABA catabolism into biologically inactive products increases with leaf age (Cornish and Radin, 1990). So, the difference between expanding and mature leaves may be a consequence of the lower capacity of expanding leaves to catabolize ABA. Second, the difference between expanding and mature leaves might be due to differences in ABA transport. In R. communis, as well as in many other species, it has been shown that ABA is transported in the phloem (Zeevaart, 1977) and that radiolabelled ABA is rapidly exported from mature leaves at a velocity expected for phloem transport (Zeevaart and Boyer, 1984). Thus ABA export from mature leaves may deliver a considerable flux of ABA to young sink leaves and produce the ABA profiles observed in the stressed leaves.

Although the genotypes used in this study were not selected on the basis of their differences in sensitivity to drought, their regions of origin are distinctly different climatic ecosystems that vary in relation to drought pressure (Table 1). Genotypes differed in ABA concentration during water deficit in mature leaves and during well-watered conditions in expanding leaves (Table 4). The magnitude of ABA increase during stress relative to control (S/C ratio) varied from fivefold (genotype BGM 252) to 27-fold (genotype BGM 1148) in expanding leaves and from 11-fold (genotype BGM 252) to 30-fold (genotype BGM 1148) in mature leaves (Table 4). However, BGM 252 and BGM 1148 are both from ecosystems with long dry seasons, and their ABA accumulation behavior brackets other genotypes from more humid regions (Table 1). Thus, considering the climatic ecosystems from which the genotypes originated, there were no apparent relationships between genotypic adaptation to dry seasons and tendency for ABA accumulation.

The genotypes in this study also did not differ in their leaf area growth responses to water deficit (Table 2). Thus, the growth and ABA responses observed in this study defined general characteristics of cassava across a broad range of genotypes. It is possible that in field conditions, where soil water is more gradually depleted, genotypes would express differences in their ABA accumulation and other stress responses. For example, researchers have reported genotypic differences in the timecourse of leaf area growth following recovery from long-term drought (Connor and Cock, 1981), the extent to which leaf extension growth is inhibited in response to mild, incipient water deficit (Palta, 1984), and ability to retain leaves (versus abscission) during drought (Connor and Cock, 1981).

Relationship of Abscisic Acid Concentration to Leaf Area Growth Rate and Transpiration Rate
In our study, the extent to which leaf area growth was inhibited corresponded with the extent of ABA accumulation across water deficit and recovery cycles, and in leaves of all development stages. A role for ABA in inhibiting leaf area growth has been supported by studies involving external application of ABA by feeding roots with various concentrations of ABA (Zhang and Davies, 1990) or stem injection of ABA (Tardieu et al., 1993). These studies have demonstrated that ABA inhibits leaf area growth with a concentration dependence similar to the relationships observed for ABA accumulation in plants subjected to drying soils. Furthermore, studies of an ABA-deficient mutant of barley (Hordeum vulgare L.) indicated that inhibition of leaf elongation rate involves an ABA-dependent mechanism that responds to xylem sap pH (Bacon et al., 1998).

The results found in this study indicate that the reduction of leaf area growth rate (Fig. 2) and ABA accumulation (Fig. 3) occurred before Day 3. Correspondingly, transpiration rate was sustained until Day 2 and then it abruptly decreased from Day 2 to Day 3 (Fig. 1). Studies have indicated that in cassava, stomatal closure is an early event in the response to water-limited conditions, usually well in advance of a detectable decrease in leaf water potential (El-Sharkawy and Cock, 1984; Palta, 1984; El-Sharkawy et al., 1992). Moreover, the extent and rapidity of control by stomata is so great that several authors have suggested that in species with such high responsiveness to water deficit, leaf water potential is not a good index of the stress experienced, but rather it is an indicator of the success of the stomatal response in maintaining stable water status (Tardieu and Simonneau, 1998). This behavior contrasts with many other species, which respond to water deficit episodes by maintaining partially open stomates, and correspondingly maintaining rates of photosynthesis while allowing tissue water potential to progressively decline (Ismail and Davies, 1997; Tardieu and Simonneau, 1998). Such behavior is often coupled with osmotic adjustment so that tissue turgor and growth are maintained as the stress develops. Although the ability of the latter species to tolerate low water potentials may be adaptive for plant survival in extreme low rainfall ecosystems, Blum (1996) has suggested that for highly productive crops there might be a yield penalty associated with genotypes that employ osmotic adjustment and other specialized drought survival responses. Thus, cassava may have a drought response appropriate for environments with cycles of intermittent or seasonal drought, followed by recovery of water status, when the bulk of growth occurs. Cassava's ability to rapidly accumulate ABA and to halt leaf area growth during dry cycles and to rapidly recover after rewatering, as demonstrated in the current work, is consistent with high productivity in such environments.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Contribution from the Dep. of Soil, Crop and Atmospheric Sciences, Cornell University, Ithaca, NY 14853. Supported, in part, by the Brazilian Agricultural Research Organization (EMBRAPA), Ministry of Agriculture and Food Supply.

Received for publication February 2, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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