7
f Plant Physiol. Vol. 140. pp. 754-760(1992) Introduction Physiological Responses of Loblolly Pine (Pinus taeda L.) Seedlings to Drought Stress: Osmotic Adjustment and Tissue Elasticity c. E. MEIERt, R.J. NEWTON h ,]. D. PURYEAR 2 , and S. SEN 2 1 Southern Forest Experiment Station, United States Forest Service, 2500 Shreveport Highway, Pineville, LA 71360 2 Department of Forest Science, Texas Agricultural Experiment Station, Texas A&M University System, College Station, TX 77843-2135, USA Received October 30, 1991 . Accepted May 30, 1992 Summary The physiological responses of osmotic adjustment and turgor maintenance may be critical for seedling survival under drought stress. The objectives were to determine: 1) if osmotic adjustment in shoots and changes in shoot tissue elasticity could be induced by cyclical gradual development of water deficits in seedlings, 2) if osmotic adjustment occurred in roots, and 3) to characterize important solutes contribut- ing to osmotic adjustment. Cyclical drought stress treatments were applied to 5- to 6-month-old Pinus taeda L. seedlings from two half-sib families in a controlled-environment chamber by withholding irriga- tion until predawn xylem potentials reached desired levels. Predawn xylem water potentials did not fall below -0.6MPa in the irrigated, nonstressed seedlings, while in the stressed treatment, seedlings were ir- rigated to saturation only when predawn xylem potentials reached -1.5 to - 2.0 MPa. Pressure-volume curve analysis and solute analyses after five or more stress cycles confirmed significant osmotic adjust- ment. Osmotic adjustment through proline and monosaccharide (fructose, glucose) accumulation in fas- cicles was observed. Shoot tissue elasticity was reduced by drought stress. Osmotic adjustment was also demonstrated in both suberized and nonsuberized root portions of 16-month-old seedlings after only one drought cycle. No differences between the two families in cell wall elasticity capacities were observed. Pressure-volume curve analyses showed no significant differences in osmotic adjustment between the two families. Key words: Pinus taeda, elasticity, monosaccharides, osmotic adjustment, pressure-volume analysis, proline, psychrometry. Abbreviations: Em .. = bulk modulus of elasticity; MPa = Megapascals; w. - osmotic potential; w. o = osmotic potential at full saturation; w. p = osmotic potential at turgor loss point; W p - pressure potential; PPF - photosynthetic photon flux; P-V - pressure-volume; SWF - symplastic water fraction; w = water potential. The maintenance of cell turgor as soil moisture and/or plant water potential (v) decline is critical for normal cell function and plant survival. Stomatal behaviour, photosyn- thesis and cell growth and division are highly sensitive to turgor potential (v p ) (Turner and Jones, 1980; Bradford and Hsiao, 1982). A number of physiological mechanisms inter- act to maintain V p' The most attention has been given to the mechanism of osmotic adjustment, because osmotic po- tential (w.) holds a fundamental position as a water potential component (e.g. Hinckley et al ., 1980; Meinzer et aI., 1986; " To whom correspondence should be sent. © ]992 by Gustav Fischer Verlag, Stuttgart

Physiological Responses of Loblolly Pine (Pinus taeda L.) Seedlings to Drought Stress: Osmotic Adjustment and Tissue Elasticity

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f Plant Physiol. Vol. 140. pp. 754-760(1992)

Introduction

Physiological Responses of Loblolly Pine (Pinus taeda L.) Seedlings to Drought Stress: Osmotic Adjustment and Tissue Elasticity

c. E. MEIERt, R.J. NEWTONh ,]. D. PURYEAR2

, and S. SEN2

1 Southern Forest Experiment Station, United States Forest Service, 2500 Shreveport Highway, Pineville, LA 71360

2 Department of Forest Science, Texas Agricultural Experiment Station, Texas A&M University System, College Station, TX 77843-2135, USA

Received October 30, 1991 . Accepted May 30, 1992

Summary

The physiological responses of osmotic adjustment and turgor maintenance may be critical for seedling survival under drought stress. The objectives were to determine: 1) if osmotic adjustment in shoots and changes in shoot tissue elasticity could be induced by cyclical gradual development of water deficits in seedlings, 2) if osmotic adjustment occurred in roots, and 3) to characterize important solutes contribut­ing to osmotic adjustment. Cyclical drought stress treatments were applied to 5- to 6-month-old Pinus taeda L. seedlings from two half-sib families in a controlled-environment chamber by withholding irriga­tion until predawn xylem potentials reached desired levels. Predawn xylem water potentials did not fall below -0.6MPa in the irrigated, nonstressed seedlings, while in the stressed treatment, seedlings were ir­rigated to saturation only when predawn xylem potentials reached -1.5 to - 2.0 MPa. Pressure-volume curve analysis and solute analyses after five or more stress cycles confirmed significant osmotic adjust­ment. Osmotic adjustment through proline and monosaccharide (fructose, glucose) accumulation in fas­cicles was observed. Shoot tissue elasticity was reduced by drought stress. Osmotic adjustment was also demonstrated in both suberized and nonsuberized root portions of 16-month-old seedlings after only one drought cycle. No differences between the two families in cell wall elasticity capacities were observed. Pressure-volume curve analyses showed no significant differences in osmotic adjustment between the two families.

Key words: Pinus taeda, elasticity, monosaccharides, osmotic adjustment, pressure-volume analysis, proline, psychrometry.

Abbreviations: Em .. = bulk modulus of elasticity; MPa = Megapascals; w. - osmotic potential; w.o = osmotic potential at full saturation; w.p = osmotic potential at turgor loss point; W p - pressure potential; PPF - photosynthetic photon flux; P-V - pressure-volume; SWF - symplastic water fraction; w = water potential.

The maintenance of cell turgor as soil moisture and/or plant water potential (v) decline is critical for normal cell function and plant survival. Stomatal behaviour, photosyn-

thesis and cell growth and division are highly sensitive to turgor potential (vp) (Turner and Jones, 1980; Bradford and Hsiao, 1982). A number of physiological mechanisms inter­act to maintain V p' The most attention has been given to the mechanism of osmotic adjustment, because osmotic po­tential (w.) holds a fundamental position as a water potential component (e.g. Hinckley et al., 1980; Meinzer et aI., 1986; " To whom correspondence should be sent.

© ]992 by Gustav Fischer Verlag, Stuttgart

Sobrado, 1986; Joly and Zaerr, 1987). Osmotic adjustment is a decrease in the symplastic osmotic potential that results from solute accumulation within the symplasm (Hinckley et al., 1980; Parker and Pallardy, 1985).

Osmotic adjustment has been shown to be a mechanism for turgor maintenance at moderate 'Ir. It occurs in soybean hypocotyls (Meyer and Boyer, 1972, 1981), sorghum leaves Gones and Turner, 1978), oak (Parker and Pallardy, 1988), and pine (Nguyen and Lamant, 1989) roots, and in a variety of other plants and plant parts (Morgan, 1984; Hinckley et al., 1983). Although turgor appeared to be maintained in osmotically stressed Pinus taeda L. callus (Newton et al., 1989 a), there was no osmotic adjustment observed (Newton et al., 1989b). The accumulating solutes have been generally identified to be sugars, organic acids, amino acids, and potas­sium ions Gones et al., 1981; Meyer and Boyer, 1972; Newton et al., 1989 b). In particular, the amino acid proline also has been shown to accumulate in a variety of water­stressed tissues (Aspinall and Paleg, 1981; Bhaskaran et al., 1985), including pine (Newton et al., 1986, 1987).

While osmotic adjustment appears to be a common pro­cess in many woody plants (Meinzer et al., 1986; Sobrado, 1986; Abrams, 1988), it has attracted our attention because of the need to increase early drought tolerance of outplanted tree seedlings. If seedling osmotic adjustment can be enhanced by preconditioning with drought stress, then survival of transplants perhaps may be increased (Hennessey and Dougherty, 1984; Parker and Pallardy, 1985; Seiler and Johnson, 1985). Pinus taeda is the most important commer­cial pine species in Texas and the southeastern United States. In Texas and the Western Gulf, drought stress is the single greatest cause of seedling mortality (Karr et al., 1984). It has been demonstrated in Pinus taeda that preconditioning, drought-stress treatments can induce osmotic adjustment (Hennessey and Dougherty, 1984; Seiler and Johnson, 1985; Bongarten and Teskey, 1986; Meier and Newton, 1987), but preconditioning drought cycles triggered no osmotic adjust­ment in Picea rubens (Seiler and Cazell, 1990). Genotypic variation in osmotic adjustment was observed in P. taeda (Bongarten and Teskey, 1986) whereas it was not observed in Juglans nigra (Parker and Pallardy, 1985).

Drought stress has been shown to alter the elastic proper­ties of tissues. In Microseris laciniata (Castro-Jimenez et al., 1989), Juglans nigra (Parker and Pallardy, 1985), Pinus taeda (Emadian, 1988), Pseudotsuga menziesii Goly and Zaerr, 1987), and Malus domesticata (Davies and Lakso, 1979), drought stress increased tissue elasticity. However, drought stress decreased elasticity in Acer saccharum (Three et al., 1978), Sorghum bicolor Gones and Turner, 1978), and Hor· deum vulgare (Melkonian et al., 1982). Less elastic tissues tend to be associated with low turgor as tissue water content decreases (Davies and Lakso, 1979; Parker and Pallardy, 1985), and increased tissue elasticity has been proposed as a mechanism of turgor maintenance (Weatherly, 1970; Pavlik, 1984; Jones et al., 1981). Bowman and Roberts (1985) suggest that decreases in tissue elasticity over the long term result in lower turgor and thus lower tissue water potential, a condi­tion that could enhance water uptake from dry soils.

The objectives of this study were to determine: 1) if shoot osmotic adjustment and changes in shoot tissue elasticity

Osmotic adjustment in pine seedlings 755

could be induced by cyclical gradual development of water deficits, 2) if osmotic adjustment occurred in roots, and 3) to characterize important solutes contributing to osmotic ad­justment in two families of Pinus taeda seedlings.

Materials and Methods

Plant materials

The experimental design of all experiments was 2 families x 2 water stress treatments. The 2 families were the same for each ex­periment but the water stress treatments were different for each ex­periment. Seedlings were grown from seed collected in orchards of two half-sib families of Pinus taeda L. One family (GR1-8), provid­ed by the Texas Forest Service, originated from the western edge of the species natural range in Texas where the rainfall is 1020 mm (Baker and Langdon, 1990) and lack of adequate growing season pre­cipitation limits the westward extension of the P. taeda range (Fo­wells, 1965). The other family (8-76), provided by Weyerhaeuser Co., originated from the North Carolina Coastal Plain where preci­pitation is 1500 mm (Baker and Langdon, 1990).

Growth conditions, drought stress treatments, water status measurements and solute analyses

In Experiment 1 the effect of drought stress on bulk shoot water relations parameters (it ~o, it ~P' Em .. ) and proline levels was investi­gated. Plants were grown in Rootrainers (Spencer-Lemeaire In­dustries, Ltd., Edmonton, Alberta, Canada) filled with fritted clay (vol. = 1 L). This medium was selected because it retains large amounts of water relatively uniformly adsorbed over the -0.1 to -1.1 MPa range of soil water potentials and has excellent aeration characteristics (Van Bavel et al., 1978). Seedlings were irrigated daily for the first five or six months after germination. For five days of each week, they received 85 mL of fertilizer solution developed for pine seedlings (Miller, 1982). For the other two days per week, all containers were irrigated past saturation with distilled water to limit accumulation of salts. Seedlings were grown in a controlled-environ­ment chamber during the entire experiment. Temperature and rela­tive humidity were maintained at 25 ± 1 °C and 87 ± 5 %, respec­tively). Day length was 16 h with 640 ~mol m-2

S-1 of photo­synthetic photon flux (PPF) at plant height. Seedlings were rotated within the chamber to assure a uniform growth environment.

Five months after germination, seedlings were randomly assigned to nonstressed or stressed treatments. [Seedling height, root collar diameter, and shoot and root dry mass were measured at this point and again following the drought stress treatments. These biomass data are reported elsewhere (Meier and Newton, 1987)].

At the beginning of the stress cycling phase, fertilizer was with­held from both stressed and nonstressed seedlings. The nonstressed seedlings received 90 mL of distilled water daily. In the stressed treatment, water was withheld from seedlings until predawn fascicle water potentials reached -1.5 MPa (Fig. 1 a). Fascicle 'Ir was de­termined using the pressure chamber technique (Ritchie and Hinck­ley, 1975) on medium-age, fully mature fascicles. At this point, all seedlings were irrigated with 90 mL distilled water to saturation for two days, then irrigation was again withheld to begin the next stress cycle. During this drought stress phase, predawn fascicle 'Ir was de­termined daily. At the end of the fifth stress cycle, ten seedlings per family per treatment were removed from the controlled environ­ment chamber for rehydration. The intact seedling root systems were immersed in distilled water, and seedling shoots were covered with plastic bags. Contact between the foliage and bag was min­imized. These forty seedlings were then allowed to rehydrate at 3 ± 0.5 °C for 36 to 48 h. Bulk shoot water relations parameters

756 C. E. MEIER, R. J. NEWTON, J. D. PURYEAR, and S. SEN

...J c(

i= z w ~ o 0-

a:: w ~ ~

...J c(

i= z w !5 0-

a:: w ~ ~

Or------------------------------(~a~)

-1

-4

-4

- NONSTRESSED 8-76 - - .... - - STRESSED 8-76 ~ NONSTRESSED GR1·8 - - .. - - STRESSED GR1·8

-5~~~~~~~~~~~~~~~~~

o 20 40 60 80 100 120 140 160 180

DAYS Fig. 1: Predawn fascicle water potential of loblolly pine seedlings from two families (GRI-8, 8-76) under nonstressed (irrigated daily) and stressed (irrigation withheld) treatments. (a) Experiment 1 with all treatments irrigated to saturation when stressed seedlings reached approximately -1.5 MPa. (b) Experiment 2 with all treat­ments irrigated to saturation when stressed seedlings reached ap­proximately - 2.0 MPa.

(Y .. o, Y .. p; Em .. ) were characterized from pressure-volume (P-V) curves (Tyree and Hammel, 1972) for hydrated shoots. Osmotic po­tentials at full saturation ("t' .. 0) and the turgor loss point ("t' .. p) were estimated using methods described by Cheung et al. (1975). Maxi­mum bulk modulus of elasticity (emu) was derived following the method of Bowman and Roberts (1985).

The remaining potted seedlings were then subjected to additional stress cycles. Proline levels were determined during the eighth cycle in non-rehydrated fascicles collected at predawn using high-pressure liquid chromatography according to procedures described pre­viously (Bhaskaran et al., 1985; Newton et al., 1986, 1987), as mod­ified from Tapuhi et al. (1981). Fascicles from three seedlings per treatment per family were lyophilized and individually analyzed.

In Experiment 2, the effect of water deficits on sugar levels in the fascicles was investigated. The seedlings were also grown in Root­rainer containers of fritted clay under identical controlled environ­ment conditions as in Experiment 1. The irrigation regime was also identical during the first six months after germination. Six-month­old seedlings were subjected to either six or seven drought cycles in which irrigation was withheld until predawn fascicle water po­tentials reached - 2.0 MPa for all but the final drought cycle and "t' was allowed to decrease below - 2.5 MPa in the final drought cycle (Fig. 1 b). During the drought cycles, predawn fascicle "t' was mon-

itored every second day on five seedlings per family per treatment (two fascicles per seedling). During the final cycle, fascicles from three seedlings per family per treatment were periodically sampled and lyophilized for individual sugar analyses. Sugar (sucrose, fructose, glucose) levels in these non-rehydrated fascicles were de­termined by gas-liquid chromatography according to procedures de­scribed previously (Newton et aI., 1986), which were modified from Sweeleyet al. (1963).

The effect of water deficits on osmotic adjustment in roots was investigated in Experiment 3. Seedlings were grown in Deepots O. M. McConkey Co., Sumner, WA, vol. = 660mL) filled with fritted clay for approximately one year under the same controlled environ­ment and irrigation conditions as were the first two experiments. One-year-old seedlings were transplanted into Treepot containers (Stuewe and Sons, Inc., Corvallis, OR, vol. = 8 L) filled with fritted clay, insuring that root systems were well-separated to minimize root binding. In order to allow roots to infiltrate this enlarged fritted clay volume, seedlings were grown in a greenhouse under similar cultural conditions for two months, then they were re-acclimated to growth chamber conditions for one additional month. Some of these seedlings were then subjected to one, 25-day gradual drought cycle in which irrigation was withheld until predawn fascicle "t' reached -1.54 MP A. The other seedlings were irrigated frequently and maintained a mean predawn fascicle "t' of -0.46 MPa. Predawn fascicle "t' was determined using a pressure chamber every second day on one mature fascicle from five seedlings per family per treat­ment. At the end of the cycle, the seedlings were rehydrated as pre­viously described. After rehydration the if was measured on a lat­eral root of at least 3 mm diameter using a pressure chamber. Roots were divided into suberized and nonsuberized portions in order to focus more specifically on these separate, potential regions of osmotic adjustment. The "t' .. of these root portions was determined psychrometrically (Wescor, Inc., Logan, UT, models HR33T, C52) using the expressed sap method of Turner et al. (1980) as modified by Newton et al. (1989 a, 1989 b). Turgor potential was calculated as: Yp = if-if ...

Statistical Analyses: Effects of treatment, family, and treatment x family interaction were analyzed with analysis of variance.

Results

With both families treated in Experiment 1, osmotic po­tentials at both full saturation (ir .. 0) and at zero turgor (ir .. p) were significantly lower in the cyclically-stressed seedlings than in nonstressed seedlings (Table 1). ir .. o and ir .. p values

Table 1: Shoot osmotic potential at full saturation ("t' .. o), osmotic potential at turgor loss point ("t' .-p), bulk modulus of elasticity (emax)

and symplastic water fraction (SWF) in rehydrated Pinus taeda seed­lings in response to the cyclic drought stress of Experiment 1.*

Family Treatment "t' .. o "t'.-p em", SWF (MPa) (MPa) (MPa)

8-76 nonstressed -1.3Sb•• -1.95' 10.6' .36"

stressed -1.60C -2.18b 17.3b .32' GR1-S nonstressed -1.2S' -1.S9' 12.0' .36'

stressed -1.40b -2.09b 21.2b .27' Family x Treatment Interaction N.S.*** N.S. N.S. N.S.

* Exp. 1. Seedlings rehydrated after treatment with five drought stress cycles; data obtained from pressure-volume analyses.

** Means followed by the same letter within a column are not signifi­cantly different at the 0.05 level.

*** Not significant at the 0.05 level.

Osmotic adjustment in pine seedlings 757

Table 2: Free proline changes in fascicles of loblolly pine seedlings in response to the cyclic drought stress of Experiment 1.*

8-76 GRl-8

if Proline if Proline Day* Treatment No. of Days (MPa) g kg-! (dry mass)** (MPa) g kg-! (dry mass)**

83 nonstressed 3 -0.60 0.02HO.01 -0.46 0.035±0.03 stressed 3 -0.55 0.052±0.01 -0.51 0.057±0.01

90 nonstressed 7 -0.83 0.036±0.03 -0.68 0.019±0.005 stressed 7 -1.13 0.089±0.08 -0.86 0.037 ±0.009

93 nonstressed 10 -0.81 0.02HO.01 -0.66 0.015 ±0.006 stressed 10 -1.87 0.146±0.13 -1.44 0.041±0.02

95 nonsressed 12 -0.84 0.016±0.01 -0.63 0.021 ±0.008 stressed 12 -2.08 O.050±0.04 -1.89 0.099±0.098

* Exp. 1. Seedlings subjected to seven previous drought stress cyles (total 80 d) and sampled during the eighth cycle. ** Mean of three samples from three individual seedlings.

were not significantly different between the two families (Table 1). The 'IT TO value is the more direct indicator of osmotic adjustment because it shows the amount of solutes per unit of symplastic water at the highest possible volume of symplastic water (at full turgor). In contrast, the 'IT Tp value is heavily influenced by the degree of cell wall elasticity (Cheung et aI., 1975). Compared with nonstressed seedlings, drought-stressed seedlings had a larger value for Em"" indicat­ing a decrease in tissue elasticity (Table 1). Family 8-76 ap­peared to have more elastic tissues than GR1-8 in both non­stressed and stressed seedlings, but the differences were not significant. There were no significant differences in SWF due to family or water deficit.

During the eighth drought cycle, proline levels per kilo­gram tissue dry mass were determined in fascicles over 12 days (Table 2). Compared to the nonstressed seedlings, pro­line levels were 2 to 6 times greater in fascicles of stressed seedlings throughout the 12 day period. At the beginning of the stress cycle (on day 83), proline levels were higher in stressed seedlings than in nonstressed seedlings of both families. This occurred even though the water potentials of the stressed and nonstressed seedlings were nearly equal at the beginning of the stress cycle due to re-irrigation. There­fore, the increased early proline level must have resulted from accumulation during the previous drought stress cycles. Furthermore, this accumulation was apparent even though the seedlings had been rehydrated and were cur­rently under no drought stress. Therefore, proline appears to function as an accumulating osmoticum in stressed fascicles of loblolly pine seedlings. Consistent differences in proline levels between the two families were not observed.

To determine if drought stress in these seedlings was ac­companied by an accumulation of foliar sugars, sugar levels (sucrose, fructose, and glucose) in the fascicles of stressed and nonstressed seedlings were measured during the final drought cycle of Experiment 2. At the beginning of the final cycle, monosaccharides (glucose and fructose) were signifi­cantly higher in the fascicles of stressed seedlings in both families (Figs. 2 b, 3 b). In a pattern similar to the observed proline accumulation, these data indicated that the previous five drought stress cycles resulted in sugar-solute accumula­tion and osmotic adjustment. Throughout the final cycle, stressed seedlings of family GR1-8 maintained their accu­mulation of monosaccharides, which were significantly

m w c Ei:UJ cdl :J:E 0>-o. c('C

m." o~ zs 0 :::E

6.-----------------------.----,

0

200

150

100

50

0 110

--tr- NONSTRESSED 8-76 ........ STRESSED 8-76

............................

f\\ '.

LAST IRRIGATION

+ 120 130 140

DAYS

(8)

(b)

150

Fig. 2: Mean fascicle sugar levels of 8-76 seedlings in Experiment 2 under nonstressed (irrigated daily) and stressed (irrigation withheld for six previous drought stress cycles) treatments. (a) sucrose; (b) glucose plus fructose. Vertical bars represent standard deviation.

higher than those of nonstressed seedlings (Fig. 3 b). Drought stress in fascicles of GR1-8 seedlings was asso­ciated with sugar accumulation, and thus osmotic adjust­ment. After the first few days of the stress cycle, fascicles of stressed 8-76 seedlings did not maintain this capacity for sugar accumulation, and the stressed seedlings showed no dif-

758 C. E. MEIER, R. J. NEWTON, J. D. PURYEAR, and S. SEN

6~--------------------------.

4

0

200

en 150 ... c~ -(I) a: (I) c(as :z::E <.>~ <.>"" c(

100 en"" 001!!: zS 0 ::::E 50

?30

--0- NONSTRESSED GR1-8 •••••••• STRESSED GR1-8

~. t······· ..................................

LAST IRRIGATION

.. 140 150 160

DAYS

(a)

(b)

170

Fig. 3: Mean fascicle sugar levels of GRl-8 seedlings in Expe~im~nt 2 under nonstressed (irrigated daily) and stressed (Irrigation withheld for five previous drought stress cycles) treatments. (a) sucrose; (b) glucose plus fructose. Vertical bars represent standard deviation.

ferences in sugar accumulation when compared with the nonstressed controls (Figs. 2 a, 2 b).

In Experiment 3, osmotic adjustment occurred in both suberized and nonsuberized portions of roots. A significant reduction in if r of about 0.2 MPa was observed in roots of family 8-76 and a reduction of 0.2 to 0.3 MPa occurred in GR1-8 roots due to drought stress (Table 3). Because the roots had been rehydrated, these reductions in if r indicate significant osmotic adjustment. A similar degree of adjust­ment was found in both the older, suberized root portions and the younger nonsuberized tips, which had grown after seedling transplant and during the drought phase. These data are the first report of osmotic adjustment in roots of Pinus taeda. No significant differences in root osmotic adjustment between the two families were observed.

Discussion

Evaluation of P. taeda tissue water relations by both psych­rometric and pressure-volume methods have indicated signif-

Table 3: Root water (if), osmotic (if r ), and turgor (ifp) potentials in rehydrated Pinus taeda seedlings in response to drought stress in Ex­periment 3. * Family Treatment Suberized Nonsuberized

it itr itp itr itp (MPa) (MPa) (MPa) (MPa) (MPa)

8·76 nonstressed _O.l1a** -0.61a 0.50' -0.59a 0.48a

stressed -0.14a -0.83b 0.69b -0.84b 0.70b

GR1·8 nonstressed _0.13' -0.66a 0.53a -0.56a 0.43a

stressed _O.13a -0.90b 0.77b -0.88b 0.75b

Family x Treatment Interaction N.S.··· N.S. N.S. N.S. N.S.

• Exp. 3. Seedlings rehydrated after one stress cycle; it measured with a pressure bomb; it r measured psychrometrically; it p estimated from the equation: it - it·it r

•• Jeans followed by the same letter within a column are not significantly dif· ferent at the 0.05 level.

••• Not significant at the 0.05 level.

icant osmotic adjustment due to drought stress. The decrease in if ro of stressed seedling fascicles (Table 1) allowed for maintenance of turgor at lower fascicle water potentials. The if ro values are similar in magnitude to those previously re­ported (Hennessey and Dougherty, 1984), and magnitudes of adjustment, especially of the 8-76 family, are similar. Seiler and Johnson (1985) reported that fascicle if r values of re­cently re-watered loblolly pine seedlings were 0.45 MPa lower in cyclically-stressed seedlings than in nonstressed seedlings, indicating a stronger osmotic adjustment response than observed here. In comparing levels of stress with those imposed by other workers, it should be noted that the cyclic stress imposed by Seiler and Johnson (1985) approx­imated the if in this study, but the cyclic stress which Hen­nessey and Dougherty (1984) imposed reached only a min­imum if value of - 0.75 MPa. The rate of development of water deficit is also a critical factor in determining the amount of osmotic adjustment Oones and Rawson, 1979; Turner et al., 1987; Turner and Jones, 1980).

Foliar increases in both free proline and sugars due to cyclic drought stress coupled with the decreases in both if ro

and if rp (Table 1) strongly support the conclusion that osmotic adjustment had occurred. After seven drought cycles, proline levels in stressed seedlings were consistently higher than their nonstressed counterparts (Table 2). Both families accumulated hexose sugars as a result of five drought stress cycles (Figs. 2 b, 3 b). This capacity for osmotic adjust­ment with sugar accumulation appeared to be maintained if the fascicle if was maintained above -1.5 to -1.8 MPa. When the fascicle if approached - 2.5 MPa after the fifteenth day of stress (Fig. 1 b), these accumulated sugar levels de­clined (Figs. 2 b, 3 b) in family 8-76, which experienced a more rapid decrease in fascicle if (Fig. 1 b). This decline may be related to a greater loss in needle turgor with 8-76 seed­lings (Pers. observ.). This faster if decline in the 8-76 seed­lings was probably due to greater leaf areas, as indicated by their higher shoot dry mass (Meier and Newton, 1987).

Increased tissue elasticity has been proposed as an addi­tional mechanism for decreasing if rp and providing the ca­pacity to maintain growth (Weatherly, 1970; Hinckley et aI., 1980; Pavlik, 1984). However, in drought stressed seedlings of both these Pinus taeda families, osmotic adjustment was accompanied by decreases in tissue elasticity. The effects of

drought stress on tissue elasticity are not clear, with increases reported in some species and decreases observed in others. These different observations in drought stressed plants may be due to the differences in the relative water content of the apoplast Ooly and Haerr, 1987).

These two Pinus taeda families displayed similar drought responses regarding root and foliar osmotic adjustment, shoot tissue elasticity, and SWF. Shoot tissues of both families became less elastic following cyclical drought stress, while SWF was not significantly affected. Osmotic adjust­ment was demonstrated in the roots of both families after only one drought stress treatment. Both families showed similar foliar osmotic adjustment responses to the cyclical drought stress.

Acknowledgements

This work was partially supported by the T AES ERA Program. Technical Article No. 25793, Texas Agricultural Experiment Sta­tion, College Station, TX 77843 . Appreciation is extended to E. McGee for preparation of the manuscript and to P. Micks and D. Hrabal for technical assistance.

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