Abaxial and Adaxial Stomatal Density, Stomatal Conductances and Water Status of Bean Primary Leaves as Affected by Paclobutrazol

The plant growth retardant, paclobutrazol at 8.5 or 17.0 μM concentrations effectively inhibited the stem elongation and primary leaf expansion of bean seedlings. Although the retardant reduced the relative water content in well-watered plants, the water and pressure potentials remained high in the primary leaves. K⁺, Na⁺, Mg²⁺ and Ca²⁺ contents in the primary leaves of the paclobutrazol-treated plants were not significantly different from those in the control. The stomatal density increased on both surfaces but the length of guard cells was not reduced significantly on the adaxial epidermes of the paclobutrazol-treated primary leaves. The inhibitory effect of paclobutrazol on the abaxial stomatal conductances became more pronounced with time during the light period but the adaxial surfaces displayed similar or slightly higher conductances than those of the control. The transpiration rate on a unit area basis did not change significantly or increased in the treated leaves thus the reduced water loss of paclobutrazol-treated plants was due to the reduced leaf area. Stomatal conductances of the adaxial surfaces responded more intensively to exogenous abscisic acid and the total leaf conductance decreased faster with increasing ABA concentration in the control than in the paclobutrazol-treated leaves. Paclobutrazol, an effective inhibitor of phytosterol biosynthesis, not only amplified the stomatal differentiation but increased the differences between the adaxial and abaxial stomatal conductances of the primary leaves.     

Osmotic relations of the drought-tolerant shrub Artemisia tridentata in response to water stress

Turgor maintenance, solute content and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above ?2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to ?5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.     

Growth Responses and Allocation of Assimilates of Rice Seedlings by Paclobutrazol and Gibberellin Treatment

Paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)methyl-4,4-dimethyl-2-(1h-1,2,4-trizol-1-yl)penten-3-ol] effectively decreased vegetative growth of rice (Oryza sativa L.) seedlings and increased the chlorophyll content. The number of veins in a leaf, the calculated number of stomata per leaf, and the length of guard cells were not altered by the paclobutrazol treatment, suggesting an effect on cell elongation. The allocation pattern of carbohydrates was changed by either gibberellin (GA) or paclobutrazol treatment. GA₃ induced more shoot growth and less accumulation of starch than the control and paclobutrazol-treated seedlings. Photosynthetic ability was not affected by either paclobutrazol or GA₃ treatment. Paclobutrazol-treated plants allocated a smaller amount of photosynthates for vegetative shoot growth and stored more as starch in the crowns than the control and GA₃-treated plants. The same starch degrading activity in the crown tissue of paclobutrazol-treated seedlings as in control plants suggests that the accumulated starch is utilized in a normal activity for growth including leaf emergence, tiller formation, and root production, resulting in improved seedling quality. Received May 30, 1996; accepted December 10, 1996     

Graphical evaluation and partitioning of turgor responses to drought in leaves of durum wheat

The relationship between relative water content (R) and turgor potential (_p) may be derived from pressure-volume (PV) curves and analyzed in various ways. Fifty PV curves were measured with the pressure chamber on leaves of durum wheat (Triticum durum L.). The plots of _p versus R were highly variable and could not be adequately described by a single mathematical function. The area below the curve was therefore determined by means of an area meter. This procedure gave the integral of turgor from full saturation to the turgor-loss point. Responses to drought treatment could thus be quantified and partitioned into effects of osmotic adjustment and elastic adjustment. These two adjustment responses, which are probably of different metabolic origin, together improve turgor maintenance in durum wheat considerably.Abbreviations and symbols PV pressure-volume - R relative water content - T_i turgor integral between full saturation and turgor-loss point - _p turgor (pressure) potential     

Root growth and water relations of oak and birch seedlings

Summary First year seedlings of English oak (Quercus Cobur) and silver birch (Betula pendula) were subjected to pressure-volume analysis to investigate the water potential components and cell wall properties of single leaves. It was hoped that this rapid-drying technique would differentiate between reductions in plant solute potential resulting from dehydration and the effects of solute accumulation.Comparison of results from these experiments with those of slow drying treatments (over a number of days) with plants growing in tubes of soil, indicated that some solute accumulation may have occurred in drying oak leaves. High leaf turgor and leaf conductance were maintained for a significant period of the drying cycle. Roots of well-watered oak plants extended deep into the soil profile, and possibly as a result of solute regulation and therefore turgor maintenance, root growth of unwatered plants was greater than that of their well-watered counterparts. This was particularly the case deep in the profile. As a result of deep root penetration, water deep in the soil core was used by oak plants to maintain plant turgor, and quite low soil water potentials were recorded in the lower soil segments.Root growth of well-watered birch seedlings was prolific but roots of both well-watered and unwatered plants were restricted to the upper part of the profile. Root growth of unwatered plants was reduced despite the existence of high soil water potentials deep in the profile. Shallow rooting birch seedlings were unable to use this water.Pressure-volume analysis indicated that significant reductions of water potential, which are required for water uptake from drying soil, would occur in oak with only a small reduction in plant water content compared to the situation in birch. This was a result of the low solute potential in oak leaves combined with a high modulus of elasticity of cell walls. Deep rooting of oak seedlings, combined with these characteristics, which will be particularly important when soil deep in the profile begins to dry, mean that this species may be comparatively successful when growing on dry sites.     

Controlling plant growth via the gibberellin biosynthesis system – I. Growth parameter alterations in apple seedlings

'York Imperial' apple seedlings ( Malus domestica Borkh.) were continuously supplied via the roots with paclobutrazol [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol)], a triazole GA biosynthesis inhibitor, at 0.68 μ M in a nutrient solution. In comparison to controls, seedlings treated with paclobutrazol for 66 days showed a 91% reduction in shoot length, a 66% reduction in leaf area but only a 17% reduction in leaf number. This effect could be reversed by GA₃ applied to the foliage at 71.4 μ M 0, 19 or 35 days after paclobutrazol was initially supplied and leaf area values for paclobutrazol-treated seedlings given both treatments did not differ significantly from controls. Plots of growth data indicate linearity of shoot longitudinal growth of GA₃-treated seedlings. Leaf area increase was non-linear after GA₃ treatment up to approximately 30 days, when the rate dropped. On a per shoot basis, leaf weight closely followed leaf area but on a per unit area basis, paclobutrazol-treated leaves were heavier than controls; GA₃ applications temporarily reversed this trend.     

Seasonal, diurnal and rehydration-induced variation of pressure-volume relationships in Pseudotsuga menziesii

Seasonal and diurnal variation and rehydration effects of pressure-volume parameters in Pseudotsuga menziesii (Mirb.) Franco from a plantation in central Pennsylvania, USA, were evaluated during May-September, 1989. Predawn elastic modulus was lowest in overwintering and newly expanded shoots in May and June, respectively, whereas predawn osmotic potentials at full and zero turgor were lowest in May and in early September, following an August drought. Seasonal variation in predawn relative water content at zero turgor was highly correlated with increases and decreases in elastic modulus and osmotic potential. Diurnal osmotic adjustment resulted in nearly constant turgor pressure, despite decreases in bulk shoot water potential. Elastic modulus decreased diurnally on 1 August and increased on 3 September. Decreases in osmotic potential and/or elastic modulus on 24 June and 1 August lowered the relative water content at zero turgor. Plateaus in pressure-volume data caused by excess apoplastic water, were present in 67% of naturally rehydrated shoots and in all of the shoots artificially rehydrated for 3, 6, 12 and 24 h, and they increased in volume with rehydration time. Plateaus represented 80–95% of the excess apoplastic water lost during pressure-volume analysis. Correcting for plateaus via linear regression had no significant effect on osmotic potential at full turgor; however, uncorrected elastic modulus and relative water content at zero turgor were often significantly lower than the plateau-corrected values, particularly in artificially rehydrated shoots. Plateau-corrected osmotic potential at full turgor and osmotic potential at zero turgor were significantly higher in most artificially rehydrated shoots than in those naturally rehydrated as the result of loss of symplastic solutes. Corrected elastic modulus decreased following 12 and 24 h of rehydration and corrected relative water content at zero turgor increased in as little as 3 h of rehydration. These results indicate that seasonal and diurnal patterns of tissue-water parameters in Pseudotsuga menziesii vary with plant phenology and drought conditions, and that the length of rehydration period is an important consideration for pressure-volume studies.     

Cold hardiness and water relations parameters in Rhododendron cv. Catawbiense Boursault subjected to drought episodes

We determined whether increase in cold hardiness of Rhododendron cv. Catawbiense Boursault induced by water stress was correlated with changes in tissue water relations. Water content of the growing medium was either maintained near field capacity for the duration of the study or plants were subjected to drought episodes at different times between 15 July and 19 February. Watering during a drought episode was delayed until soil water content decreased below 0.4 m³ m⁻³ then watering was resumed at a level to maintain soil water content between 0.3 and 0.4 m³ m⁻³. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves. Water relations parameters were determined using pressure-volume analysis. Exposure to drought episodes increased cold hardiness during the cold acclimation stage in late summer and fall but not during the winter. When water-stressed plants were re-watered to field capacity, the previous gain in cold hardiness gradually disappeared. Water relations parameters correlating with seasonal changes of cold hardiness included dry matter content (r =−0.67). apoplastic water content (r =−0.60), and water potential at the turgor loss point (r = 0.40). Changes of cold hardiness in water-stressed plants in reference to well-watered plants were correlated with changes of all water relations parameters, except for osmotic potential at full turgor (r = 0.13). It is proposed that water stress reduced the hydration of cell walls, thereby increasing their rigidity. Increased rigidity of cell walls could result in a development of greater negative turgor pressures at subfreezing temperatures and therefore increased resistance to freeze dehydration.     

Bound Water in Durum Wheat under Drought Stress

To study drought stress effects on bound water, adsorption isotherms and pressure-volume curves were constructed for two durum wheat (Triticum durum Desf.) cultivars: Capeiti 8 (drought tolerant) and Creso (drought sensitive). Plants were grown under well-watered and water-stressed conditions in a controlled environment. Differential enthalpy (ΔH) was calculated through van't Hoff analysis of adsorption isotherms at 5 and 20°C, which allowed us to determine the strength of water binding. ΔH reached the most negative values at approximately 0.06 gram H₂O/gram dry weight and then increased rapidly for well-watered plants (until 0.10 gram H₂O/gram dry weight) or more slowly for drought-stressed plants (until 0.15-0.20 gram H₂O/gram dry weight). Bound water values from pressure-volume curves were greater for water-stressed (0.17 gram H₂O/gram dry weight) than for well-watered plants (0.09 gram H₂O/gram dry weight). They may be estimates of leaf moisture content where ΔH reaches the less negative values and hence some free water appears. With respect to the well-watered plants, tightly bound water tended to be less bound during drought, and more free water was observed in cv Creso compared to cv Capeiti 8 at moisture contents >0.10 gram H₂O/gram dry weight.     

Diel Patterns of Water Potential Components for the Crassulacean Acid Metabolism Plant Opuntia ficus-indica when Well-Watered or Droughted

Under well-watered conditions, chlorenchyma acidity in cladodes of Opuntia ficus-indica increased substantially at night, fully accounting for the 0.26-megapascal nocturnal increase in osmotic pressure in the outer 2 millimeters. Osmotic pressure in the inner part of the chlorenchyma and in the water-storage parenchyma did not change significantly over 24-hour periods. Three months of drought decreased nocturnal acid accumulation by 73% and essentially abolished transpiration; also, 27% of the chlorenchyma water and 61% of the parenchyma water was lost during such drought, but the average tissue osmotic pressure was little affected. Turgor pressure was maintained in the chlorenchyma after 3 months of drought, although it decreased sevenfold in the water-storage parenchyma compared with the well-watered condition. Moreover, the nocturnal increases in turgor pressure of about 0.08 megapascal in the outer part of the chlorenchyma was also unchanged by such drought. The water potential magnitudes favored water movement from the parenchyma to the chlorenchyma at the end of the night and in the reverse direction during the late afternoon. Experiments with tritiated water support this pattern of water movement, which is also in agreement with predictions based on electric-circuit analog models for Crassulacean acid metabolism plants.     

Paclobutrazol- and Hardening-Induced Thermotolerance of Wheat: Are Heat Shock Proteins Involved?

The present report describes the effects of paclobutrazol andheat hardening treatments on the protein synthesis patternsin imbibing and germinating wheat seedlings (Triticum aestivumL. cv Frederick) during heat stress. A heat hardening treatmentgiven during the imbibition period induced the transient expressionof 118, 90, 70 and 18 kDa heat shock proteins (HSPs). However,the hardening and paclobutrazol treatments did not enhance thethermotolerance of imbibed seeds or etiolated seedlings. Bycontrast, the hardening and paclobutrazol treatments enhancedthe thermotolerance of light-grown seedlings. While, both hardenedand unhardened control seedlings synthesized several HSPs duringa high temperature stress period, these proteins were not synthesizedby the paclobutrazol-treated, light-grown seedlings. Thus, HSPsynthesis during heat shock may have been a manifestation ofstress perception by the seedlings and may not have mediatedthe thermotolerance induced by the triazole treatments. Sincedifferential thermotolerance was only apparent in light-grownseedlings, it is suggested that chloroplasts may be requiredfor the expression of paclobutrazol- and hardening-induced thermoprotection.Additional evidence indicating that chloroplasts are an importantsite of injury during high temperature stress was obtained fromchlorophyll fluorescence measurements. (Received July 11, 1994; Accepted October 26, 1994)     

Seasonal variation in the tissue water relations of Picea glauca

Seasonal variation in water relations of 3-yearold white spruce (Picea glauca (Moench) Voss) shoots, monitored with pressure-volume curves over 28 months, was closely related to shoot phenology and was sensitive to environmental fluctuations during both summer growth and winter dormancy. Turgor maintenance capacity was lowest during rapid shoot elongation from late May to early July; this was indicated by the lowest total turgor pressures, the highest (least negative) osmotic potentials at full turgor and the turgor loss point, the smallest differences between osmotic potentials at full turgor and the turgor loss point, the highest relative water contents at turgor loss and a linear decline in cell elasticity with decreasing turgor pressure. This suggests that the high susceptibility of white spruce seedlings to growth check after transplanting is largely attributable to the poor turgor maintenance capacity of this species in early summer.     

Genotypic variation in tissue water relations of leaves and roots of black walnut (Juglans nigra) seedlings

Genetic variation in the drought response of leaf and root tissue water relations of seedlings of eight sources of black walnut ( Juglans nigra L.) was investigated using the pressure-volume technique. Tissue water relations were characterized at three stages of a drying cycle during which well-watered plants were allowed to desiccate and then were reirrigated.
Sources varied both in the capacity for and degree of leaf and root osmotic adjustment, and in the mechanism by which it was achieved. A decrease in osmotic potential at the turgor loss point (ψ_πp) of 0.4 MPa was attributable to increased leaf tissue elasticity in seedlings of four sources, while seedlings of an Ontario source exhibited a 0.7–0.8 MPa decline in ψ_πp as a result of both increased solute content and increased leaf tissue elasticity. Seedlings of a New York source showed no detectable osmotic adjustment.
In roots, decreased ψ_πp (osmotic potential at full hydration) and ψ_πp were observed under drought. Sources that exhibited significant leaf osmotic adjustment also generally showed a similar response in roots. Tissue elasticity and ψ_πp of roots were higher than those of shoots, whereas ψ_πp of the two organs was similar for most sources. Because of greater elasticity, roots exhibited a more gradual decline in turgor and total water potential than did leaves as tissue relative water content decreased.     

Leaf water characteristics and drought acclimation in sunflower genotypes

The responses of leaf water parameters to drought were examined using three sunflower (Helianthus annuus L.) genotypes. Osmotic potential at full water saturation (π¹⁰⁰), apoplastic water fraction (AWF) and bulk elastic modulus (BEM) were determined by pressure-volume curve analysis on well watered or on water-stressed plants (−1.0 MPa Ψ₁ < −1.5 MPa) previously drought-pretreated or not. The drought-pretreated plants were subjected to a 7-day drought period (predawn leaf water potential reached −0.9 MPa) followed by 8 days of rewatering. In well watered plants, all genotypes in response to drought acclimation displayed a significantly decreased π¹⁰⁰ associated with a decrease in the leaf water potential at the turgor-loss point (decrease in Ψ_tlp was between 0.15 and 0.21 MPa, depending on the genotype). In two genotypes, drought acclimation affected the partitioning of water between the apoplastic and symplastic fractions without any effect on the total amount of water in the leaves. As a third genotype displayed no modification of AWF and BEM after drought acclimation, the decreased π¹⁰⁰ was only due to the net accumulation of solutes and was consistent with the adjustment of the photochemical efficiency observed previously in this genotype in response to drought acclimation. In water-stressed plants, the osmotic adjustment (OA) can increase further beyond that observed in response to the drought pretreatment. However, the maintenance of photosynthetic rate and stomatal conductance at low leaf water potentials not only depends on the extent of osmotic adjustment, but also on the interaction between OA and AWF or BEM. Adaptative responses of leaf water parameters to drought are thus quite contrasted in sunflower genotypes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of drought stress and selenium supply on growth and physiological characteristics of wheat seedlings

The paper studied the effects of drought stress, selenium (Se) supply and their combination on growth and physiological characteristics of wheat (Triticum aestivum L. cv Shijiazhuang NO. 8) seedlings. The experimental design included two water treatments (well-watered, 75% of maximum field capacity; drought stress, 30% of maximum field capacity) and two Se levels (0; 0.5 mg/kg) to determine whether Se can modify the negative impacts of drought stress on seedling growth and physiological traits. Drought stress caused a marked decline in growth parameters and soluble protein content, whereas it induced an increase in root activity, proline content and the activities of peroxidase (POD) and catalase (CAT) of leaf tissue. On the other hand, Se supply induced an increase in biomass accumulation only under well-watered condition. Under drought stress, Se supply increased free proline content, root activity and the activities of POD and CAT in leaf tissue, but did not significantly affect on growth parameters. These results implied that drought stress brought harmful effects on wheat seedlings, and that Se supply was favorable for biomass accumulation of wheat seedlings under well-watered condition. However, it did not significantly affect on biomass accumulation under drought stress, although it increased root activity and activities of some antioxidant index in experimental periods.     

Cytokinin-producing, plant growth-promoting rhizobacteria that confer resistance to drought stress in Platycladus orientalis container seedlings

One of the proposed mechanisms through which plant growth-promoting rhizobacteria (PGPR) enhance plant growth is the production of plant growth regulators, especially cytokinin. However, little information is available regarding cytokinin-producing PGPR inoculation on growth and water stress consistence of forest container seedlings under drought condition. This study determined the effects of Bacillus subtilis on hormone concentration, drought resistance, and plant growth under water-stressed conditions. Although no significant difference was observed under well-watered conditions, leaves of inoculated Platycladus orientalis (oriental thuja) seedlings under drought stress had higher relative water content and leaf water potential compared with those of noninoculated ones. Regardless of water supply levels, the root exudates, namely sugars, amino acids and organic acids, significantly increased because of B. subtilis inoculation. Water stress reduced shoot cytokinins by 39.14 %. However, inoculation decreased this deficit to only 10.22 %. The elevated levels of cytokinins in P. orientalis shoot were associated with higher concentration of abscisic acid (ABA). Stomatal conductance was significantly increased by B. subtilis inoculation in well-watered seedlings. However, the promoting effect of cytokinins on stomatal conductance was hampered, possibly by the combined action of elevated cytokinins and ABA. B. subtilis inoculation increased the shoot dry weight of well-watered and drought seedlings by 34.85 and 19.23 %, as well as the root by 15.445 and 13.99 %, respectively. Consequently, the root/shoot ratio significantly decreased, indicative of the greater benefits of PGPR on shoot growth than root. Thus, inoculation of cytokinin-producing PGPR in container seedlings can alleviate the drought stress and interfere with the suppression of shoot growth, showing a real potential to perform as a drought stress inhibitor in arid environments.     

Effects of nitrogen limitation on water relations of jack pine (Pinus banksiana Lamb.) seedlings

The water relations of shoots of young jack pine (Pinus banksiana Lamb.) seedlings were examined 6 and 15 weeks after the initiation of four different dynamic nitrogen (N) treatments using a pressure-volume analysis. The N treatments produced a wide range of needle N concentrations from 12 to 32 mg g^?1 dry mass and a 10-fold difference in total dry mass at 15 weeks. Osmotic potential at full turgor did not change over the range of needle N concentrations observed. Osmotic potential at turgor-loss point, however, declined as N concentrations decreased, indicating an increased ability of N-deficient jack pine plants to maintain turgor. The increase could be attributed largely to an increase in cell wall elasticity, suggesting that elasticity changes may be a common, significant adaptation of plants to environmental stresses. Dry mass per unit saturated water almost doubled as needle N level dropped from 32 to 12 mg g^?1 and was inversely correlated to the bulk modulus of elasticity. This suggests that cell wall elasticity is determined more by the nature of its cross-linking matrix than by the total amount of cell wall material present. Developmental change was evident in the response of some water relation variables to N limitation.     

Tolerance of Mycorrhiza infected pistachio (Pistacia vera L.) seedling to drought stress under glasshouse conditions

The influence of Glomus etunicatum colonization on plant growth and drought tolerance of 3-month-old Pistacia vera seedlings in potted culture was studied in two different water treatments. The arbuscular mycorrhiza (AM) inoculation and plant growth (including plant shoot and root weight, leaf area, and total chlorophyll) were higher for well-watered than for water-stressed plants. The growth of AM-treated seedlings was higher than non-AM-treatment regardless of water status. P, K, Zn and Cu contents in AM-treated shoots were greater than those in non-AM shoots under well-watered conditions and drought stress. N and Ca content were higher under drought stress, while AM symbiosis did not affect the Mg content. The contents of soluble sugars, proteins, flavonoid and proline were higher in mycorrhizal than non-mycorrhizal-treated plants under the whole water regime. AM colonization increased the activities of peroxidase enzyme in treatments, but did not affect the catalase activity in shoots and roots under well-watered conditions and drought stress. We conclude that AM colonization improved the drought tolerance of P. vera seedlings by increasing the accumulation of osmotic adjustment compounds, nutritional and antioxidant enzyme activity. It appears that AM formation enhanced the drought tolerance of pistachio plants, which increased host biomass and plant growth.     

Effect of paclobutrazol on membrane lipids in apple seedlings

The effect of paclobutrazol [( 2RS, 3RS )-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2, 4-triazol-1-yl) pentan-3-ol] on the fatty acid composition of polar lipids and on the sterol content in apple ( Malus domestica Borkh. cv. York Imperial) seedlings was determined. Polar lipids isolated from leaves, stems and roots included mono- and digalactosyldiglycerides and the phospholipids phosphatidylinositol, phosphatidylcholine, phosphatidylglycerol and phosphatidylethanolamine. The predominant fatty acids in membrane polar lipids were palmitic (C16:0), linolnic (C18:2) and linolenic (C18:3). The predominant sterol, both free and esterified, was β-sitosterol. There were no significant alterations in the fatty-acid composition of glyco- and phospholipids from paclobutrazol-treated apple seedlings. In contrast, a significant decrease in the content of β-sitosterol and campesterol occurred in treated tissues. The decline in sterol content continued with increasing duration of paclobutrazol treatment, and was most pronounced in the root tissue.     

Cell wall changes in white spruce (Picea glauca) needles subjected to repeated drought stress

White spruce [ Picea glauca (Moench) Voss] seedlings were preconditioned by subjecting them to 3 cycles of a mild drought stress. After 1 week of stress relief their water status, soluble carbohydrate content and cell wall composition in newly formed needles were examined and compared with those in control seedlings. Both preconditioned and control seedlings were subsequently subjected to a severe drought stress and again analyzed. Preconditioning treatment both before and during subsequent stress exposure lowered osmotic potentials at full hydration, and after the loss of turgor, decreased lignin content and increased hemicellulose content of the cell walls. Severe drought had similar but more drastic effects on seedling water relations, sugar accumulation and cell wall hemicellulose content; it also decreased cell wall pectin levels. The decrease in pectin levels was accompanied by a loss of galactose and glucose from pectic substances. Little change in cellulose content was observed as a result of preconditioning and severe drought.