Stomatal responses of Populus to leaf water potential

P. J . SCHULTE,' T . M. HINCKLEY, AND R. F. STETTLER College of Forest Resources, AR-10, Universio of Washington, Seattle, WA, U.S.A. 98195

Received April 8, 1986

SCHULTE, P. J . , T. M. HINCKLEY, and R. F. STETTLER. 1987. Stomatal responses of Populus to leaf water potential. Can. J. Bot. 65: 255-260.

The leaf conductance responses of clones of Populus trichocarpa, P. deltoides, and hybrids to changing leaf water potential were examined. Unlike the other species, P. trichocarpa not acclimated to water stress showed an inability to control water loss at low leaf water potential. It is suggested that stomata of P. trichocarpa plants grown under well-watered conditions remain open in spite of a loss of turgor in the leaf. Preconditioning through water stress of developing leaves, however, leads to greater stomatal control in foliage following rewatering. Leaf conductance of stressed and rewatered P. deltoides and hybrids remained near zero, while the leaf conductance of P. trichocarpa was partially reduced.

SCHULTE, P. J . , T. M. HINCKLEY et R. F. STETTLER. 1987. Stomatal responses of Populus to leaf water potential. Can. J. Bot. 65 : 255-260.

Les rCponses de la conductance foliaire chez des clones de Populus trichocarpa, P. deltoides et de leurs hybrides au change- ment de potentiel hydrique foliaire ont CtC CtudiCes. Contrairement aux autres espbces, P. trichocarpa non-adaptC au stress hydrique montre une incapacitC au contrble des pertes d'eau i des potentiels hydriques bas. I1 est suggCrC que les stomates des plantes de P. trichocarpa croissant sous de bonnes conditions hydriques restent ouverts en dCpit d'une perte de turgescence foliaire. Un prC-conditionnement au stress hydrique des feuilles en voie de dCveloppement, cependant, entraine un meilleur contrble stomatique chez les feuilles suite i un apport d'eau. La conductance foliaire chez les plantes de P. deltoiiies et des hybrides soumises au stress hydrique puis arrosCes se maintient prbs de zCro alors que celle de P. trichocarpa est partiellement rCduite.

[Traduit par la revue]

Introduction The growth and development of trees on sites experiencing

at least occasional periods of low soil moisture or high evap- orative demand are dependent, in part, upon the ability of stomata to control water loss. However, the entry of COz into leaves requires open stomata and many plant species have evolved regulatory mechanisms which appear to balance COz uptake and water loss depending upon environmental condi- tions and plant water status (Mansfield and Davies 1985).

The species of Populus native to North America occupy a wide range of habitats. This genus is being studied for use in energy plantations developed under the short-rotation inten- sive-culture (SRIC) system of management (Weber et al. 1985). When grown under this management system, a large variation in biomass productivity has been observed both within and among populations of Populus trichocalpa and hybrids of P. trichocalpa and P. deltoides (Heilman and Stettler 1985). Since productivity is ultimately dependent upon photosynthesis and COz uptake and because of the role of stomata in gas exchange, we were interested in comparing the stomatal responses with leaf water potential of several clones of P. trichocalpa, P. deltoides, and their hybrids.

Stomata have been described as turgor-operated valves (Raschke 1975) and the aperture of a stoma is generally pro- portional to the turgor pressure inside the guard cells. Under certain conditions, other epidermal cells exert a contact pres- sure on the guard cells and thus stomatal aperture is also a function of the turgor in surrounding epidermal cells. Reduc- tions in the aperture of stomata at high leaf water potentials have been noted as a result of this phenomenon (~tzlfel t 1955; Glinka 1971 ; Edwards et al. 1976). These authors suggest that maximum stomatal opening would occur at some "optimum water deficit" such that epidermal cell turgor is zero.

'Current address: Department of Biology, University of California, Los Angeles, CA, U.S.A. 90024.

The response of stomata to dehydration of the leaf has often been studied by the measurement of leaf conductance or water loss from leaves excised from the plant and allowed to desic- cate. The stomata generally appear to close as leaf water con- tent declines, though stomata may also partially close at leaf water contents near saturation (Solirovi 1965). Based upon these considerations, our first objective was to describe the relationship between leaf conductance to water vapor (as an indicator of relative stomatal opening) and leaf water potential among Populus species and hybrids. We hypothesized that the stomata will eventually close as leaf water potential declines in a desiccating leaf. Possibly, the stomatal responses to leaf water potential are related to the drought avoidance or produc- tive abilities of individual clones or hybrids.

The object of the second portion of our study was to examine the leaf conductance and water potential relationship of foliage on plants previously exposed to a period of water stress. We hypothesized that a water stress period will modify the stomatal responses to leaf water potential. Stomatal precon- ditioning has been shown to be an important component of drought resistance (Mansfield and Davies 1985).

Materials and methods

The relationship between leaf conductance and leaf water potential was examined with leaves that were excised from a rehydrated shoot and allowed to air dry. This method will be referred to as the cut-leaf method. After removal of a shoot from the tree, the shoot was recut under water. The shoot was then rehydrated by sealing it in a plastic bag, with the cut surface under water, and allowing it to resaturate overnight. The degree of rehydration was apparent when initial water potential was measured with the pressure chamber. A leaf water potential between -0.1 and -0.2 MPa was considered near complete saturation. For purposes of examining stomatal behavior at lower water potential only, complete rehydration was not necessary. The leaves were kept at photosynthetically active radiation (PAR) levels of approximately 800 pmol m-l s-' between measurements. Light was provided by incandescent floodlamps in a water bath serving as an

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256 CAN. I . BOT. VOL. 65. 1987

infrared filter. Initially, leaf conductance measurements were paired with leaf mass determinations. When the leaf mass is coupled with leaf saturated mass and ovendry mass (24 h at 7OoC), this technique can be used to describe the relationship between leaf conductance and leaf relative water content.

Leaf conductance was estimated with a steady-state diffusion poro- meter (LiCor LI-1600). Although P. deltoides and P. trichocarpa x deltoides hybrids are amphistomatous, P. trichocarpa is hypostoma- tous (Ceulemans et al. 1984). For this reason, conductance measure- ments were restricted to the abaxial leaf surface.

Pressure-volume curves were also developed (Richter et al. 1980; Schulte and Hinckley 1985) on leaves immediately above or below the leaf used for leaf conductance measurements. This technique was used because it seemed desirable to avoid pressure-chamber measure- ments on the leaf to be used for measurement of leaf conductance. Comparisons between this technique and the use of the same leaf for both leaf conductance and water potential measurement showed, how- ever, that the general pattern of stomatal response to leaf water poten- tial was not modified by pressure-chamber measurements on the same leaf used for leaf conductance measurements. The use of pressure- volume curves enabled us to relate leaf conductance to leaf pressure potential as well as total water potential.

The plant material used included three clones of P. trichocarpa col- lected from sources in the states of Washington and Oregon as well as two sources from Chilliwack in the province of British Columbia in Canada. Also examined were two clones of P. deltoides from Illinois and four hybrids of P. trichocarpa and P. deltoides developed by Dr. Reinhard Stettler at the University of Washington and Dr. Paul Heilman at Washington State University. Cuttings obtained from these clones were planted in a greenhouse at the University of Washington. Plants were maintained in a well-watered condition by frequent watering. The study was begun when the new shoots were approximately 50 cm in length.

The effects of a period of water stress on stomatal responses to leaf water potential were observed by subjecting plants growing in the greenhouse to a gradually declining water potential. Water was with- held from the pots of trees until wilting of the foliage was noted. At this point, the wilted plants were rewatered.

As soil moisture declined in the pots, diurnal patterns of leaf con- ductance were monitored on six leaves per plant at approximately hourly intervals with a LiCor LI-1600 steady-state diffusion poro- meter. Leaf conductance measurements were made every other day. After the treated plants had regained turgor (1 -2 h), shoots were cut for resaturation under the conditions previously described for the cut- leaf method. Following resaturation, leaves were excised and the rela- tionship between leaf conductance and leaf water potential following a period of water stress was examined using the cut-leaf technique.

A field site was established near Puyallup, WA (Weber et al. 1985; Heilman and Stettler 1985). Cuttings from several sources of P. del- toides, P. trichocava, and hybrids were planted in two plots and were allowed to grow for 1 year under irrigated conditions. In the 2nd year, one plot was irrigated during the summer so as to maintain high soil moisture, and the other plot was allowed to dry. This design was then used to assess the long-term effects of water stress on plant growth and other physiological processes. For our purposes, this arrangement allowed us to compare the stomatal behavior of trees - growing under the two treatment conditions. On 2 days during the summer, the diurnal course of leaf conductance, leaf water potential, and several environmental variables (light, air temperature, relative humidity) was monitored. Leaf conductance was measured with a steady-state diffusion porometer (LiCor LI-1600) and leaf water potential was estimated with a pressure chamber (Ritchie and Hinckley 1975). Photosynthetic photon flux density (PPFD) was measured with a LiCor (LI-190SB) quantum sensor.

Results and discussion Cut-leaf experiments

In contrast to the expected results, all clones of P. tricho-

;LO "'I .o

FIG. 1. Leaf conductance responses of different clones of P. tricho- carpa, P. deltoides, and hybrids to leaf dehydration by the cut-leaf method. Each line represents a single fully expanded leaf; each data point was one leaf conductance measurement. The arrow indicates the average water potential of bulk leaf turgor loss.

carpa examined displayed little stomatal closure in response to leaf water potentials leading to loss of turgor and wilting of the leaf (Fig. 1). Similar patterns were noted for expanding leaves as well as mature leaves. The excised leaves were completely wilted (approximately 45 min) when the final leaf conductance measurement was made. In spite of wilting, leaf conductance remained high. These results are in contrast to those found in several agricultural crops by Ehlig and Gardner (1964) where the rate of water loss from detached leaves declined to near zero as leaf relative water content approached 80%. The stomatal behavior observed in this species is similar to that described by Tal (1966) with wilty mutants of tomato. The mutant varieties lose water faster than nonmutant tomato plants and the stomata do not seem to close when the leaves wilt.

The hybrids showed the usual closing response associated with leaf desiccation. Leaf conductance dropped to near zero at low water potential (Fig. 1). Similar responses were noted by Reich (1984a, 1984b) with poplar hybrids where leaf con- ductances of leaves excised in the light declined as the leaves desiccated. In many leaves, we found that leaf conductance was also low at water potentials near zero. This response is likely due to high turgor in epidermal cells surrounding the guard cells as was suggested by others who have observed this phenomenon (~ t i l f e l t 1955; Glinka 1971; Edwards et al. 1976). It should be noted that these high water potentials were only obtainable by resaturation of the shoots. Under daylight

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SCHULTE ET AL. 257

0 PP. DELTOIOES 0 P . TRICHOCRRPR A HYBRIO

m- ------*----- *----gli=:::+D::=.*------*---=-*

0600 0700 0800 0900 1 0 0 0 1 1 0 0 1200 1 3 0 0 1400 1500 T I M E OF DRY

FIG. 2. Leaf conductance of P. trichocarpa (Chilliwack, B.C., source), P. deltoides (Illinois source), and hybrids after exposure of the intact plants to water stress and wilting of the foliage (broken lines). The control plants (solid lines) were kept well watered.

conditions, actively transpiring leaves of even well-watered plants were never observed with water potentials near zero. However, if soil moisture is high in the field, foliage covered with dew in the early morning might become nearly saturated.

Populus deltoides was expected to respond quite strongly to low leaf water potential with near complete closure. However, the observed responses actually appear somewhat intermediate between the stomatal responses of P. trichocalpa and the hybrids (Fig. 1). This apparent contrast may be a result of the technique used. Leaves on trees growing in the field rarely if ever would encounter a transition from full saturation to complete loss of turgor in the course of 1 h or less. Possibly in the case of P. deltoides, certain processes leading to stomatal closure at low leaf water potential require a more gradual decline in leaf water potential to induce a response in the stomata.

Whole-plant experiments When 1-year-old sprouts were gradually water stressed in

the greenhouse, the foliage of plants that were allowed to dehydrate began to wilt on the 9th day following the last water- ing. Leaf conductance was measured hourly on the day wilting became apparent and the plants were rewatered at midafter- noon. The wilted leaves that were not killed by desiccation recovered turgor in a few hours. It was noted that while most of the wilted P. deltoides and hybrid clone leaves survived, wilted foliage of P. trichocalpa generally did not recover turgor and survive. The exceptions were younger leaves that were still expanding when the experiment was initiated.

When cuttings grown under well-watered conditions were exposed to gradually declining soil water, stomata of P. deltoides responded (as indicated by changes in leaf con- ductance) to declining leaf water potential by closing (Fig. 2). While the leaf conductance of well-watered control plants increased at sunrise and reached high values during midday, the leaf conductance of stressed foliage remained very low throughout the day. This response is somewhat opposed to what one might have expected from the results of the cut-leaf experiments described earlier. It would appear that while the stomata of P. deltoides did not show as great a response to water potential as seen in the hybrids when exposed to a rapid decrease in water potential, a gradual dehydration of

the plant over a 9-day period did result in complete closure of the stomata.

The stomata of hybrids responded in a manner almost identi- cal with those of P. deltoides. The leaf conductance of wilted leaves was low and showed no increase during the final day prior to rewatering (Fig. 2). Kelliher et al. (1980) also observed the closure of stomata in two Texas clones of P. deltoides after water stress. Radin and Ackerson (1981) noted a similar response in cotton plants when watering was discontinued; leaf conductance decreased rapidly as leaf water potential declined.

Leaves of P. trichocalpa also displayed a decrease in leaf conductance in response to gradually decreasing water poten- tial, though complete closure was not obtained (Fig. 2). The stress treatment reduced leaf conductance to approximately 50% of control plant conductances. Results from the cut-leaf experiments suggested that little or no response would occur in the present experiment, but it is apparent that the stomatal behavior of P. trichocalpa is partially modified by a period during which water stress develops gradually. Ceulemans et al. (1978) also found that low soil water ~otential decreased the leaf cbnductance of P. trichocalpa foliage but to a lesser degree than observed with hybrid clones.

On the day that plants wilted, the solar noon leaf conduc- tance of older, fully expanded leaves of P. trichocalpa (leaf number 7 or greater) showed an approximately 50% reduction in conductance, while the younger, expanding leaves showed even greater reductions in leaf conductance (Fig. 3). Solirovd (1965) examined the stomatal response of detached cabbage leaves of different ages to dehydration. Although the cabbage leaf stomata appear to have closed as the leaves desiccated, the stomata of younger leaves closed at a higher leaf relative water content than older mature leaves. Since the water content at which leaves wilted was not known, it is conceivable that differences in leaf water content at stomatal closure reflect dif- ferences in tissue properties such as osmotic potential or elasti- city and not differences in the ability of the stomata to close when the leaf wilts. Reich ( 1 9 8 4 ~ ) observed differences in the ability of the stomata of hy'brid to close following leaf excision. Although leaf conductance always declined to less than 30% of the leaf conductance prior to excision, the leaf conductance of young leaves fell more rapidly and to signifi- cantly lower values than observed with old leaves. Clearly leaf age affects the response of stomata to gradual dehydration, with stomata of older leaves being the least responsive. Our results suggest that there is some characteristic during the expansion or development of P. trichocarpa foliage that lessens the ability of a leaf to respond to a period of water stress through modification of stomatal sensitivity to leaf water potential.

After the treated plants were rewatered, leaves on rehydrated shoots were examined with the cut-leaf method. The leaf con- ductance of P. deltoides foliage indicated only a slight opening of stomata at high leaf water content, while the stomata of hybrids remain closed following rehydration of the leaf (Fig. 4). It is likely that the lack of reopening following rewatering of the plant is mediated by increased foliar abscisic acid (ABA) levels or increased partitioning of ABA into the apoplast, as suggested by Cornish and Zeevaart (1985). Beardsell and Cohen (1975) found that if excised leaves were allowed to remain wilted for 3 h (ABA levels began increasing after 2 h), recutting the leaf under water allowed the leaf to rehydrate, but the stomata did not reopen. The stomata of

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PI P. DELTOIDES 13 P . TRICHOCRRPR A HYBRlO

O ~ ~ . * ' ' ' ' ' ' I 1 3 5 7 9 11 13 15 17

LERF NUNBER

O: 4 ; ; ; l'l l'3 l'5 ;7 LERF NUn8ER

FIG. 3. Effect of leaf age on the response of two clones of P. tricho- carpa (Chilliwack, B.C., source) stomata to gradual dehydration of intact plants. These data were collected on the day the plants wilted, as represented in Fig. 2. The solid line shows the leaf conductance at solar noon of a well-watered control plant, while the broken line refers to a plant that has been allowed to dehydrate. Leaf 1 is the youngest leaf with a width of at least 2 cm.

P. trichocarpa have acquired some response to leaf water con- tent following the stress treatment (Fig. 4). Plants exposed to water stress now respond similarly to the hybrids examined by Reich (1984~) in that leaf conductance is relatively low in desiccated leaves that have been excised from the shoot. Unfortunately because of mortality of the older leaves, the cut- leaf technique was not applied to every leaf along the shoot and comparisons cannot be made between leaves of different ages in this case.

The closure of stomata with water stress is often attributed to increased ABA levels (Wright and Hiron 1969; Radin and Ackerson 1981; Cornish and Zeevaart 1984). However, the role of ABA following recovery of leaf water potential is less clear. Bulk leaf measurements of ABA do not always correlate well with the timing of stomatal recovery (Beardsell and Cohen 1975; Newville and Ferrell 1980; Ackerson 1982). It has also been suggested that the transfer of ABA between pools in the leaf (Radin and Ackerson 1982; Cornish and Zeevaart 1985) or the balance between ABA and cytokinin level (Radin et al. 1982) may be of greater significance for stomatal control than bulk leaf ABA level.

-WRTER POTENT IRL I NPR I

FIG. 4. Effect of leaf water potential on leaf conductance of P. trichocarpa, P. deltoides, and hybrids exposed to a period of water stress and rewatered. Each solid line represents a single fully expanded leaf. Measurements were made using the cut-leaf method on the same day that plants were rewatered after wilting. The arrow indicates the average water potential of bulk leaf turgor loss for P. trichocarpa.

gated plot was obviously much drier than the soil on the im- gated plot and the effect of water stress on the abscission of leaves was apparent. Many of the older leaves on shoots of the hybrids and both species of Populus had abscised. The abscis- sion of older leaves on water-stressed P. deltoides plants was also noted by Kelliher et al. (1980).

The diurnal course of leaf conductance and water potential on 10 July displays no apparent distinctions between the im- gated and nonirrigated plots (Fig. 5). It is possible that the leaf conductances of P. trichocarpa did not decline in the evening as rapidly as other clones. Greenhouse measurements have suggested that the stomata of this species do not close at night when plants are grown under well-watered conditions. The results of Pezeshki and Hinckley (1982) also indicate that stomata of P. trichocarpa growing in the field do not close completely at night; leaf conductance varied from 1 to 3 mmls under dark conditions.

On 29 August, the leaf conductance of foliage from both Populus species and the hybrid was clearly lower on the non- imgated plot as compared with the imgated plot (Fig. 6). Leaf conductances of P. trichocarpa leaves on the nonimgated plot were even lower than those of P. deltoides or hybrid clone foliage. Interestingly, leaf water potentials of foliage from hybrids and both species are not distinctly different between the irrigated and nonimgated plots and may have been lower on the imgated plot (Fig. 6). Similar results were noted by Pezeshki and Hinckley (1982) using a source of P. trichocarpa from Index, Washington. They found that when soil moisture was high and stomata were fully open, leaf water potential regularly decreased to - 1.4 MPa. After prolonged drought and stomatal closure, however. leaf water votentials never

Field experiments decreased below -0.'9 MPa. Leaf conductaices (Fig. 6) of The diurnal course of leaf conductance and water potential P. trichocarpa foliage from the nonimgated plot are compar-

was examined on 2 separate days during the summer of 1984. able with those found by Pezeshki and Hinckley (1982) during On the day of the first diurnal course (10 July) the study plots a drought period, but leaf conductances on the imgated plot were likely very similar with respect to soil water availability, are considerably higher than those found by Pezeshki and though soil water potential was not monitored. By the second Hinckley (1982) prior to a drought. It would seem probable diurnal course (29 August), however, the soil on the nonim- that the imgated conditions in our study were more favorable

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SCHULTE ET AL. 259

[I] P. OELTOIDES C) P . TRICIPXRRPR A HYBRID

0500 0700 0900 1100 1300 1500 1700 1900 2100 TIME OF DRY

0500 0700 0900 1100 1300 1500 1700 1900 2100 TIHE OF DRY

FIG. 5. Leaf conductance of field-grown poplars on 10 July 1984. The solid lines show measurements on plants in the irrigated plot, while the broken lines refer to plants on the nonirrigated plot. Measurements before 1100 were not possible because of dew on the leaves.

for minimizing water stress than predrought field conditions experienced by the clones examined by Pezeshki and Hinckley (1982). Although the soil water potential in this study was undoubtedly lower on the nonirrigated plot than on the irri- gated plot, the modified stomatal activity and abscission of foliage apparently resulted in reduced water loss. It has been suggested that stomata can be affected by root water potential independent of leaf water potential, possibly through changes in the levels of cytokinin exported by the roots (Schulze and Kiippers 1979; Turner et al. 1985; Blackman and Davies 1985). It is also possible that differences existed between plots with respect to belowground processes such as rooting depth and surface area, though this aspect was not investigated.

Conclusions The opening and closing movements of stomata are thought

to be dependent upon changes in turgor or pressure potential inside the guard cells. Therefore it might be expected that stomata will close when sufficient water is lost from the leaf to reduce guard cell turgor. Indeed this response is observed in hybrids between P. trichocalpa and P. deltoides. The observed responses of excised P. deltoides leaves may be a result of the nature of the method used in desiccating the leaf. The stomatal behavior observed in clones of P. trichocalpa is

[I] P. OELTOIDES O P. TRICHOCWWI A HYBRID

- 0500 0700 0900 1 100 1300 1500 1700 1900 2100

TIME OF DRY

0500 0700 0900 1100 1300 1500 1700 1900 2100 TIME OF DRY

FIG. 6. Leaf conductance of field-grown poplars on 29 August 1984. The solid lines show measurements on plants in the irrigated plot, while the broken lines refer to plants on the nonirrigated plot. Measurements before 1000 were not possible because of dew on the leaves.

somewhat anomalous given our current understanding of the physiology of stomatal movements. While we have not pre- sented direct evidence that guard cells in the wilted leaves had lost turgor, our results suggest that the stomata are remaining open in spite of the lack of turgor pressure in the bulk leaf and the guard cells. Further experimentation is necessary to eluci- date the mechanisms responsible for this phenomenon and to ascertain its significance for plant growth and survival, especially on dry sites.

A period of water stress appears to be capable of modifying the stomatal responses of Populus trees to leaf water potential. Immediately following such a treatment, the stomata of P. deltoides and hybrid clones remain closed in spite of favor- able water status in the leaves. Since the responses in P. deltoides differ depending upon whether the stress occurs rapidly on detached material or gradually on intact leaves, it is possible that, at least in this species, stomatal responses to leaf water potential are dependent upon changes in physiological processes that do not occur when the imposition of water stress occurs very suddenly.

A period of water stress also modified the stomatal behavior of P. trichocalpa and produced a degree of stomatal sensitivity to leaf water potential in this species. While wilted leaves did not survive unless rewatered immediately, those that did sur- vive were found to have acquired some response to leaf water

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potential. In addition, the modification of stomatal behavior appears dependent upon the leaf age and, therefore, the state of development o r expansion. The stomata of small, expand- ing leaves closed nearly completely when the leaves became wilted.

When P. trichocarpa cuttings are grown under field condi- tions, especially without imgation, occasional periods of water stress appear to lead to modification of stomatal responses to leaf water potential. The trees growing under field conditions have lower maximum conductance than greenhouse plants and are more sensitive to changes in leaf water potential. This observed effect of water stress on stomatal behavior seems to account for the differences noted earlier between our data from well-watered clones and the results of Pezeshki and Hinckley (1982). Low soil moisture appears to have affected plant pro- cesses such as stomatal function and leaf abscission such that plants growing on a drier site are capable of maintaining leaf water potentials similar to o r higher than the water potential of plants on a wetter site. Similar results were noted by Cock et al. (1985) with field-grown cassava. The modification of stomatal function is especially notable with clones of P. tricho- calpa because of the total lack of stomatal response to water potential observed when cuttings of this species are grown under well-watered conditions.

The stomatal behavior of well-watered plants of P. tricho- calpa appears to be somewhat similar to that described with wilty mutants of tomato (Tal 1966) and potato (Quame 1982); the stomata of wilted leaves remain open. This behavior is modified, however, if the leaves are exposed to water stress. The role of ABA in stomatal responses to water stress suggests that our future work should involve analysis of internal ABA levels and observation of the effects of ABA applied to devel- oping foliage.

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