Seasonal changes in the water balance of Douglas-fir (Pseudotsuga menziesii) seedlings growg under different light intensities

ALLAN P. DREW School of Forestq rrtld Woorl Prodrrcts, Michigcltl Techtlologicrrl Utliuersity, Ho~rglltotr, MI, U.S.A. 49931

A N D

WILLIAM K. FERRELL School ofForestry, Oregot1 Sttrte Ut~iuersity, Corutrllis, OR, U.S.A. 97331

Received September 5, 1978

DREW, A. P., and W. K. FERRELL. 1979. Seasonal changes in the water balance of Douglas-fir (Psel~rlots~rgcr t71etlziesii) seedlings grown under different light intensities. Can. J . Bot. 57: 666-674.

The water relations of germinant seedlings of Douglas-fir (P.se~rrlots~rgcr tnetrziesii (Mirb.) Franco) grown outdoors under 9, 44, and 100% full light were analyzed during the summer, autumn, and winter seasons. Empirical regression models, based on the relationship plant water potential = f(soil water potential, leaf conductance), were determined for selected light treatment -season combinations. Plant water potential is higher during active summer shoot growth than after elongation growth has ceased in the autumn owing to earlier stomata1 closure in response to soil drought in summer compared with a more abrupt closure at a lower plant water potential in the autumn.

For all light treatments, leaf conductance declined in the winter independent of plant water potential and simultaneously with the onset of subfreezing air temperatures. During the first winter, water potential of seedlings was higher than during the previous summer or autumn over a wide range of equivalent soil water potentials.

Seedlings grown under low light intensity are less drought resistant and have lower plant water potential than those grown under full light regardless of soil moisture status.

DREW, A. P., et W. K. FERRELL. 1979. Seasonal changes in the water balance of Douglas-fir (Pse~ctlots~cgtr metlziesii) seedlings grown under different light intensities. Can. J . Bot. 57: 666674.

Lesauteurs ont analyse, pendant I'ete, I'automne et I'hiver, les relations hydriques de plantules du Pseutlotsugcr tnetlziesii (Mirb.) F~xnco cultivees B I'exterieur en presence de 9.44et 100%de la pleine lumitre. Ils ont determine, pour certaines combinaisons de la saison et de la lumiere, des modtles de regression empiriques bases sur la relation suivante: potentiel hydrique = f(potentie1 hydvique du sol, conductance foliaire). Le potentiel hydrique de la plante est plus eleve pendant la ptriode de croissance estivale de la tige que pendant la periode oh I'elongation a cesse B I'automne, parce que les stomates se ferment plus t6t en reaction B la sicheresse du sol pendant I'ete, comparativement B une fermeture plus instantanee h un potentiel hydrique plus bas chez la plante h I'automne.

A tous les traitements B la lumiere, la conductance foliaire diminue en hiver, indipendamment du potentiel hydrique de la plante et simultanement B I'apparition de tempiratures de I'air sous le point de congelation. Pendant le premier hiver, pour une gamme de potentiels hydriques du sol equivalents, le potentiel hydrique des plantules a i t6 plus eleve que pendant I'Cte ou I'automne precedents.

Les plantules cultivies sous une faible intensite lumineuse rksistent moins B la secheresse et ont un potentiel hydrique plus bas que celles cultivees sous la pleine lumiere, quel que soit I'Ctat de l'humiditi du sol.

[Traduit par le journal]

Introduction Douglas-fir (Pseudots~lgn menziesii (Mirb.)

Franco) is distributed in western North America along the Pacific Coast and Rocky Mountains where the climate is often mild and humid but with dry summers. As an apparent consequence of a warm, dry growing season, the species has evolved a degree of drought resistance not found in many temperate-zone conifers. On xeric sites, such as

those occurring in the Rocky Mountains or south- ern Oregon, ecotypes exist which are more drought resistant than coastal or more mesic sources (Pharis and Ferrell 1966; Ferrell and Woodard 1 966).

The general term 'drought resistance' may be separated into components of drought avoidance and drought tolerance (Levitt 1956). Douglas-fir seedlings avoid drought by developing longer root

0008-4026/79/060666-09$01 .OO/O 01979 National Research Council of Canada/Comseil national de recherches du Canada

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DREW AND FERRELL 667

systems (Lavender and Overton 1972), by early spring root growth (Heiner and Lavender 1972), by having lower transpiration rates (Zavitkovski and Ferrell 1968, 1970), or by becoming preconditioned to drought stress (Unterschuetz et rrl. 1974). The latter probably involves elements of drought toler- ance as well as avoidance.

Drought tolerance, or the ability of a plant to withstand severe tissue dehydration, has been shown to be greater in more xeric inland sources of Douglas-fir than in coastal sources (Pharis and Fer- rell 1966).

Drew and Ferrell (1977) have shown that Douglas-fir seedlings possess marked ability to ac- climate to different conditions of light and temper- ature by altering the balance between shoot and root growth. Under moderate environmental con- ditions, seedlings grown under full sun allocate proportionately greater amounts of dry matter to root growth than to shoot growth and have thicker needles. As light intensity during growth di- minishes, the proportion of photosynthate allo- cated to root growth also diminishes, shoot de- velopment is favored, and needles are thinner with more exposed area.

Since these changes under high light intensity favor enhanced drought avoidance, they should be reflected in the water balance or economy of water use by the plant. A better developed root system with a more xerophytic leaf anatomy should also favor more efficient water u~ take . better conserva- tion of internal moisture, and generally higher plant water potentials, important under drought stress. The work reported herein tests this hypothesis by examining the water balance of seedlings accli- mated to varying light intensities.

The seasonal perspective is also important. First-year Douglas-fir seedlings go through stages of phenology based on timing of shoot and root development and advent of dormancy and winter hardening. These may be quantitatively assessed and distinct phenological periods or 'seasons' may be established and used as a basis for sampling and ultimate assessment of the water balance and drought resistance ability of Douglas-fir.

Methods Douglas-fir seedlings of a North Santiam, Oregon, seed

source were grown outdoors under three intensities of light, i.e., 100, 44, and 9% light, following procedures described by Drew and Ferrell (1977). Starting o n August 24, 1971, seedlings ap- proximately 4 months old were removed from their outdoor treatments and brought indoors where measurements of transpi- ration and plant and soil water potential were made. Growth analysiscarried out on these same seedlings has been previously reported (Drew and Ferrell 1977). Additional water relations

data were also collected during the summer of 1972 on 4- month-old seedlings to supplement the 1971 data.

Results are compared on a seasonal basis: summer, autumn, and winter. At the time of summer measurements, seedlings were in a state of active shoot growth, and in autumn. seedlings had ceased shoot elongation and set a terminal bud. The transi- tion from autumn to winter was reflected in a change in t~xnsp i - ~ x t i o n rate. Seedlings in their first season of growth maintained shoot growth activity into mid o r late September when bud set defined commencement of the 'autumn' period. In 1971. 'winter' began in late October; in 1972, it was late September.

During the last 4 months of 1972, transpiration of foul 5- month-old seedlings which had ceased terminal growth were monitored under conditions of high moisture availability. These fourplants!from the 100% light treatment) were brought into the labolxtory the night preceding their day of measurement and the pots were placed in 1 c m of water. Equiliblxtion by capillary rise ensured a constant soil moisture content on each biweekly sampling date . Changes in transpiration rate over time were taken to represent the climatic influence upon seedling water relations.

Another study in the summer of 1972 clarified certain diurnal changes in water potential for plants growing under the three light treatments. Here, xylem water potential was measured with a pressure bomb (Scholander et (11. 1965). Leaf resistance was measured simultaneously by a diffusion porometer (Turner and Parlange 1970).

Transpilxtion of seedlings was measured in a somewhat modified open system (Bierhuizen and Slatyer 1964; Un- terschuetz et (11. 1974). A base-line relative humidity of 48.5% (19.I0C) was achieved by bubbling air through tempelxture- controlled water baths. As the air entered a water-jacketed. Plexiglas cuvette, it was warmed slightly to 22.0°C and then passed by a wire-wound LiCl hygrometer which was encased in a watertight chamber maintained at 19.I0C. T h e hygrometer output was recorded and, by means of a calibration curve, converted to absolute humidity. T h e difference between this reading and the base-line humidity was transpiratory loss. This loss was related to flow rate and expressed on the basis of leaf surface area to give milligrams H,O per square centimetre per hour. Transpiration rates were divided by the calculated vapor pressure gradient for each seedling to give leaf conductance in centimetres per second.

Cuvette wind speed was kept constant at 50 cm s-I , sufficient to minimize boundary-layer resistance. Wind was supplied to the seedling by means of a vertical riser of polyethylene tubing with a slit down o n e side.

Light intensity during measurement of transpiration was pro- vided by incandescent bulbs with infrared filters and was about 36% of full sunlight between 400 and 700 nm, with a peak at 635 nm. This intensity was well above saturating light for photo- synthesis in Douglas-fir seedlings (Krueger and Ferrell 1965).

In addition to controls over the shoot environment during measurement of transpiration. soil and root temperature were controlled with a small water bath kept at 19.1°C. Soil tempera- ture at time of measurement (after 1 o r 2 h equilibration) was 21.4"C.

Immediately following measurement of transpiration rate, the plant was removed from the cuvette and severed at ground level and xylem water potential was measured with a pressure bomb.

Leaf surface area was measured in an optical planimeter (Davis) et (11. 1966) and adjusted to account for two needle surfacis . No corrections were made for needle curvature.

Finally, a soil sample was taken from the root zone of each pot for glxvimetric assessment of soil moisture content, which was then converted to soil water potential with a pressure-

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668 CAN. J . BOT. VOL. 57, 1979

membrane apparatus desorption curve that expressed soil mat- ric potential (bars) as a function of soil water content. Pots were well watered except just prior to measurement when they were allowed to dry to subjectively determined levels of water con- tent to span a wide range of Y,,,,,. Soil matric potential is a good measure of total soil water potential when osmotic potential is negligible, as is likely the case with the clay forest soil used here. Salter and Williams (1963) describe this method and its use over a range of tension between -0.33 and - 15 bars. In the present study, the range was extended by linear extrapolation to -20 bars using a log-log graph. Plants whose soil had a matric poten- tial less than -20 bars were excluded from the study.

Water relations analysis was based on the relationship Y,,,,,,, = f(Ysoil,Lc), where Y,,, ,,,, and Y,,,I are plant and soil water potentials, and L, is leaf conductance. All data were converted to base ten logarithms and then fit to a multiple-regression model of the form

for different light-season treatment combinations. In the model, Y = xylem water potential in bars, XI = the soil water potential in bars, XZ = the calculated leaf conductance in centimetres per second based on measured transpiration rate, B,, = the appro- priate constants, and e = residual error.

The stepwise multiple-regression program generated water balance models for a number of tr-eatment-season combina- tions. These took the form of 2 x I factorials, the comparisons being made of summer versus autumn over all treatments, and

light versus 100% light for the autumn period. Analyses of variance were computed for all models, except that no statistical analysis was applied to winter data since these data represented a changing state physiologically and not as constant a physiological situation as seemed to be true in summer and autumn.

Results Water relations data for autumn and winter

periods were collected only from plants grown during 1971. Summer data, however, consisted of a combination of that collected during 1971 and 1972 (Table 1). As seedling growth and dry matter dis- tribution varied between the 2 years (Drew and Ferrell 1977), it was necessary to determine whether or not summer water relations behavior also varied with year.

A multiple regression model, Yplant = f(YsOil,Lc), was thus constructed for all possible summer treatment-year comparisons. When tested, no significant variation owing to treatment and only limited variation owing to year was present. Thus, the data from summers of both years were com- bined to give a sample size of 44 (Fig. 1).

For seedlings in a state of active summer shoot growth, drying of the soil results in a slow initial response in Yplant but later a more rapid change as YsOil declines to -20 bars. Leaf conductance de- clines rapidly, beginning above a Y,,,, of - 10 bars; later the decline was much slower.

During the autumn, after shoot growth had ceased and terminal buds had formed, a different response was observed (Fig. 2). Most notable are

TABLE 1. Sample sizes for water relations analysis. All data on 1st-year seedlings were collected during 1971

except where otherwise indicated

Light treatment

Season 100% 44% 9%

Summer 11 5 4 8* 8* 8*

Autumn 12 18 18 Winter 16 15 17

the lower levels of water potential in seedlings at medium and high values of leaf conductance, i.e., above a YsOil of -5 bars. These averaged 3 bars lower than in summer, and leaf conductance was also higher. During soil drying, Y,,lan, initially de- creased rapidly from above - 13 bars and then ta- pered off between - 16 and - 17 bars as leaf con- ductance declined, stomatal control becoming evi- dent (Fig. 3).

During the summer, Yplant decreased gradually between - 10 and - 15 bars, while during the au- tumn, the decline was rapid between - 10 and - 17 bars, the differences presumably reflecting differ- ential stomatal control. Then, at low leaf conduc- tances where the curves intersect and beyond, Yplant continues to decrease at a faster rate for summer than for autumn seedlings. Thus, at low leaf conductances and low YsOil, it is evident that Y,,lant in the summer at any value of LC below about 0.04 cm s-' is lower than during the autumn (Figs. 132).

Figure 2 is based on a combination of all three light treatments to give a sample size of 48 seed- lings. This was justified on the assumption that similar seasonal changes should be reflected in each treatment even though the light treatments were found to have different effects upon water rela- tions.

Highly significant differences (1% level) were found between summer and autumn models. This was accounted for by the following factors, each statistically significant: (1) inequality of intercepts, i.e., BOsummer + Bo ,u,umll; (2) differences in effect of YsOil upon Yplant; and (3) a different YSoil x LC in- teractive effect upon Yplant.

To clarify and lend support to the laboratory studies, diurnal changes in leaf conductance and plant water potential were monitored during some of the hottest days of the summer of 1972. On one of these days, July 29, air temperature reached 36°C. A trend of decreasing water potential and increas- ing leaf conductance was apparent from sunrise to midmorning, then an increase in water potential

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DREW AND FERRELL

I I I I 0 - 5 -10 -15 - 2 0

SOlL WATER POTENTIAL(bars)

FIG. 1. Trivariate linear expression representing summer behavior of Douglas-fir seedlings for all treatments based on the relationship, 'Y,,,,,, = f('Y,,,,L,). The equation for the expression is log 'Y,,,,,, = 0.828 - 0.259 log 'Y,,,, - 0.391 log LC + O.O18(l0g 'Y,,,,,)' - 0.217(10g LC)' - 0.418(10g 'Y,,,I X log LC).

FALL

XYLEM WATER POTENTIAL(borr1

0 0 - 5 -10 -15 - 2 0

SOlL WATER POTENTIAL(bars)

FIG. 2. Trivariate linear expression representing autumn behavior of Douglas-fir seedlings for all treatments based on the relationship 'Y,,,,, = f('Y,,,iI,L,). The equation for the expression is log 'Y,,,,,, = 1.290 + 0.054 log 'Y,,,,, + 0.253 log LC + O.O87(Iog 'Y,,i1)2 + O.O36(10g LC)' - 0. I lO(l0g 'Ymil x log L,).

was accompanied by declining leaf conductance. The minimum Ypl,, measured was around - 14 bars, usually it was not less than - 12 bars. Appar- ently, stomatal closure in midmorning reduced water loss and prevented plant water potential from declining further. This inferred stomatal response observed in the 'field' was similar to that found in seedlings measured in the laboratory, where

stomatal closure between - 10 and - 15 bars re- duced leaf conductance most markedly.

Autumn water relations behavior (Figs. 4,5) was tested by regression, the water relations of plants grown at 100% light differing significantly (1% level) from those grown under 9% light. The plants grown at 44% light were not significantly different from those grown at the other two treatments.

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670 CAN. J. BOT.

100% LIGHT

U) A 44% LIGHT

L 0

9% LIGHT 9 - J -25 5 I- z W I- X -20

11: W

G 3

rn

- 15 A

5 A

J I * X

-10 0

0 A

A A

-5 - 0 .05 .I0 .I5 .20 .25 .30

LEAF CONDUCTANCE(C~.S-I)

FIG. 3. V,,l,m as a function of leaf conductance of Douglas-fir seedlings grown under three light treatments measured during autumn and summer.

0 100% LIGHT 9% LIGHT

0 0 -5 -10 -15 -20

SOIL WATER POTENTIAL(bars)

FIG. 4. The autumn response in VXsl,, as a function of VSoil for 6-month-old Douglas-fir seedlings grown at 100% and 9% light.

Seedlings grown under low light had a signi- ficantly lower (5% level) Y',lantfor any given level of YSoi, than those grown under full light conditions (Fig. 4).

Similarly, for any level of leaf conductance, plants grown at 9% light had a lower Y',,,,, than

VOL. 57. 1979

0 .05 .I0 .I5 .20 .25

LEAF CONDUCTANCE(C~.S- '1

FIG. 5 . The autumn response in LV,,.l,,, a s a function of leaf conductance for 6-month-old Douglas-fir seedlings grown at 100% and 9% light.

those grown at 100% light (Fig. 5). This pattern in the water balance characteristic of seedlings in the autumn was radically altered as air temperature dropped in September (1972) and October (1971) with the onset of winter.

For the sample of four plants grown under 100% light, as temperatures decreased, transpiration rate at 22°C declined between September 14 to De- cember 21, 1972 (Fig. 6). This decline was most pronounced before October 30 and after November 22. In the interim time, minimum outside air tem- peratures were around 5"C, and there was little change in transpiration rate measured at 22°C. The pronounced drop in transpiration on September 28 was probably related to a frost the previous night.

In the winter, leaf conductance dropped notice- ably at levels of Y',Ian, where during the autumn it had been high (Fig. 7). For plantsgrown at 9% light, leaf conductances higher than - 15 bars Y',lant in the winter were mostly less than 0.05 cm s-I, whereas in the autumn, they had been above 0.15 cm ss l . Thus, a fivefold drop in leaf conductance took place after October 24, 1971. Seedlings from other light treatments showed a similar response.

Since transpiration rates fell gradually over time after the first frost, the plants measured sub-

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DREW A N D FERRELL

0-0 TRANSPIRATION RATE a t 22'C ''O rg 0--C AIR TEMPERATURE

SEPT. OC T. NOV. DEC 1972

FIG. 6. The decline in tlxnspilxtion rate of Douglas-fir seed- lings with low air temperature during the last 4 months of 1972. Air temperature is shown as the mean of daily minima over the preceding period. The arrow denotes the first frost of the year.

FALL

WINTER

A

U) L

0 9 - J 9

LEAF CONDUCTANCE(C~.S-~)

FIG. 7. Autumn and winter response of xylem water potential as a function of leaf conductance for Douglas-fir seedlings grown at 9% light. Seedling ages were6and 8 monthsold, respectively.

sequently were in somewhat of a state of flux. Even so, certain general interpretations of treatment variations are allowable. When the 9 and 100% light treatments are compared, seedlings of the former are obviously at a lower Yplant than the latter over a

I 0 100% LIGHT S% LIGHT

-35 O -I

I I I I 0 -5 -10 - 5 -20

SOIL WATER POTENTIAL(bars)

FIG. 8. Winter response of YXyl,, as a function of 'P,,,, I for 8-month-old Douglas-fir seedlings grown at 100% and 9% light.

wide range of YsOil (Fig. 8). The plants grown at 44% light were intermediate.

The pronounced decline in leaf conductance in the winter is also reflected in YPIant (Figs. 4, 8). Seedlings grown at lom light are under less mois- ture stress in the winter than in the fall over a wide range of YsOil. However, seedlings from all light treatments growing in soil at field capacity tended to be at a slightly lower YPIant when measured at field capacity in the winter compared with in the autumn.

Discussion S~lmmer 017d A~ltutnn Behauior

The water balance of young Douglas-fir seedlings shows marked differences between summer and autumn. During active shoot growth, seedlings close their stomata more gradually under decreas- ing Yplant than after bud set in the autumn as judged by slopes of declining leaf conductance. Stomata1 closure also occurs sooner in the summer with re- spect to plant water potential than it does in the autumn (Fig. 3). As a result, trivariate linear ex- pressions for the two seasons, based on the model Yplant = f(YsOil,L,), exhibit marked differences in response to soil drought (Figs. 1 , 2). This amounts to about 3 bars advantage in YPlant at low to moder- ate soil drought for rapidly elongating seedlings.

In the autumn (Fig. 2), stomata appear to be more open at YSOi1 of -5 bars and higher than during

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672 C A N . J . BOT. VOL. 57, 1979

summer (Fig. 1). This allows for more rapid desic- cation and a lower YPlant. However, under severe soil drought, Y,,,,,, of autumn seedlings is higher than that of summer seedlings. This could be re- lated to greater cuticular water loss of young needle tissue or to incomplete stomatal closure.

During the period of active needle and stem elon- gation, it would appear to be to the plant's advan- tage to maintain a high Y,,,, and so a high turgor pressure. This would ensure that metabolic proces- ses associated with photosynthesis, respiration, and growth and differentiation of new tissues can proceed efficiently. After meristematic activity subsides and elongation and differentiation of the new shoot has largely ceased, the requirement of a high Y for growth functions may not be as critical. These apparent differences in stomatal functioning may be adaptive in that early closure of stomata in the summer ensures a high Y,,,,, thus favoring growth processes. At the same time, autumn clo- sure is gradual enough that the plant can still main- tain a certain amount of C 0 2 exchange with the ambient air, critical in the case of 4-month-old seedlings with little stored food. However, it should be kept in mind that because of mesophyll resistance to CO, diffusion, a component not in- cluded in the diffusion pathway of H,O out of aleaf, closure of stomata will have a greater effect upon increasing leaf resistance to water loss than to CO, diffusion. Partial closure of stomata may thus con- serve moisture but not eliminate CO, uptake. The summer response illustrates a depth of drought- avoidance ability in young seedlings extending be- yond the simple ability to close stomata ~ ~ n d e r stress and may be an adaptation to the dry summer environment.

In the autumn, the lower the light under which plants are grown, the lower the Y,,,,, for any given level of leaf conductance (Fig. 5). The plants grown under low light had lower Ypla,, than plants grown under 100% light at equivalent Y,,,, (Fig. 4). This is most likely a consequence of less root and greater shoot development in the former (Drew and Ferrell 1977). It should be emphasized, however, that in the following year, 1972, 1st-year seedlings showed the opposite trend in shoot-root balance, i.e., lower light intensity during growth resulted in better root development. Had these plants been analyzed for changes in water balance, the plants grown at low light might very well have had a more favorable water economy than the plants grown in full sun. Full light growing conditions, therefore, should not be assumed to always maximize internal water con- servation through optimizing shoot-root balance.

For 2-year-old plants grown under full light,

stomatal closure occurred at Y,,,,, lower than - 20 bars (Drew, unpublished data). Closure between - 19 and -22 bars has been reported in 2-year-old Douglas-fir (Lopushinsky 1969) and at -20 bass in 1- to 3-m-tall Douglas-fir (Running 1976). Leaf con- ductance in 40-m-tall trees declines at -25 bars and at - 17 bars in 30-cm seedlings (Running 1976). First-year-old seedlings, being more susceptible to desiccation by virtue of a shallow root system, close their stomata at a higher Y than older, more deeply rooted, less drought-susceptible plants. Other features of stomatal functioning and water relations shown here for 1st-year seedlings may be similarly modified for older plants or trees.

Witljer Behnuior Low nighttime temperatures in late autumn

decreased transpiration (Fig. 6), especially at the time of first subfreezing temperatures. Van den Driessche (19690, 1969b) and Parker (1955) have shown a strong correlation between hardiness changes and low temperatures, especially first frosts, in several conifers. Simultaneously with in- creased hardiness in spruce and pine, transpiration rates have declined (Christersson 1972), and in Pit~rrs 17igt.0, the period of maximum hardiness coincided with the period of lowest transpiration (Parkel- 1963). Parker (1961, 1963) also noted that low transpiration rates were not related to lowered leaf water content in a number of coniferous species, and Timmis (1973) showed that cold-hardy Douglas-fir tissue was at a higher moisture content than nonhardy tissue over a wide range of needle Y. In Xcrt~jhirrtn, Drake and Raschke (1974) note that chilling of greenhouse-grown plants caused a re- duction in stomatal conductance.

The present study supports the contention that decreased leaf conductance in winter is not related to changes in Y of leaf tissue but to adirect effect of stomatal closure. At plant water potentials of - 10 to - 15 bars, stomatal conductance may be only 20% of what it was in the autumn even though soil moisture is near field capacity (Fig. 7).

Michael (1966) noted a high resistance to desic- cation in detached twigs of Douglas-fir and other conifers in the winter and attributed it to stomatal closure. Thus, the hardening process in Douglas-fir seedlings apparently confers drought resistance as well as frost resistance. Indeed, the effects of drought and frost upon leaf tissue are very similar in that as in drought, protoplasm is dehydrated by movement of water out of cells to sites of intercel- lular ice formation. Siminovitch and Briggs (1953) found a linear relation between resistance of Robinia bark cells to desiccation and resistance to

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DREW AND FERRELL 673

freezing, while Chen et al. (1977) in Cornlrs stolonifera and Timmis and Tanaka (1976) in Pselrdotslrga menziesii showed that water stress

ar mess. would induce frost h d' Closure of stomata due to low temperatures is

correlated with greater xylem water potential over a wide range of YsOil (Figs. 4, 8). This trend was exponential and resulted in higher Y between - 15 and -25 bars of YPl,,,, the same region of Y where stomatal closure in autumn occurred (Fig. 5). This - 15 to -25 bar region may be critical to avoidance of frost damage just as during the autumn it is a critical stress level with regard to drought. Timmis (1973) concluded that tissue water in frost-hardy Douglas-fir needles is at a lower Y owing to the combination of osmotic and matric forces. These forces may account for the somewhat reduced Ypl,,, at very high YSOll as well as at low YsOll, whereas at intermediate levels of YsO,l, the effect of stomatal closure becomes the controlling factor. Just how higher YxYl,, in winter is related to in- creased cold resistance in 1st-year seedlings is not clear.

Alden (1971) reports that in both normal and potassium-deficient Douglas-fir trees, the potas- sium content declined over winter increasing again in spring. As potassium appears intimately as- sociated with guard cell functioning in a number of species (Willmer and Pallas 1973), winter closure of stomates in Douglas-fir could be related to reduced potassium levels in needles at this time.

Numerous studies have shown that abscisic acid (ABA) causes stomatal closure. Webber (1974) measured seasonal levels of ABA in Douglas-fir and found its concentration followed the dormancy cycle, i.e., high in winter and decreasing just prior to spring budbreak. Raschke et (11. (1976) also found that chilling ofxclnthilrm increased leaf ABA levels and was associated with heightened stomatal sensitivity to CO, (Drake and Raschke 1974) and ultimate closure. Blake and Ferrell (1977) noted ABA increases in drought-stressed Douglas-fir seedlings associated with stomatal closure. High levels of ABA in winter may thus bring about stomatal closure. Kinetic analysis of increases in ABA relative to decreases in potassium ion in leaves would be most useful, as would studies on the effect of exogenous K+ on stomatal behavior in conifer leaves in the winter.

Acknowledgments This work was carried out with the financial as-

sistance of a Weyerhaeuser fellowship and McIn- tire-Stennis funds for which the authors are ex-

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