Unstressed Seedlings Research Articles

Comparative responses of three fuel wood yielding plants to PEG-induced water stress at seedling stage

Three species of fast growing fuel wood yielding plants locally available (Acacia holosericea, Bauhinia variegata and Cassia siamea) were characterized in respect of their responses to water stress. Seedlings (25 days) of these species, exposed to two levels of water stress (−0.5 and −1.0 MPa) induced by PEG-6000 for 24 h, were analysed for relative water content (RWC) and the contents of chlorophyll, protein, soluble sugars and proline in leaves along with activities of catalase, peroxidase and superoxide dismutase (SOD). RWC was lower in stressed compared to the unstressed seedlings. However, stress-induced decline in RWC was lowest in B. variegata. Chlorophyll and protein contents declined with increasing levels of water stress, decline being least in B. variegata. Soluble sugar and proline contents increased under water stress particularly in B. variegata. The enzyme activity of catalase (EC-1.11.1.6), peroxidase (EC-1.11.1.7) and SOD (EC-1.15.1.1) decreased with increased levels of water stress. Such decline in the activity of these enzymes was least in B. variegata. Apparently, B. variegata is potentially the species most tolerant to water stress among these three fuel wood-yielding plants.

Osmotic adjustment in roots and leaves of two sorghum genotypes under NaCl stress

Seedlings of two sorghum genotypes [Sorghum bicolor (L.) Moench], one salt tolerant (CSF 20) and the other salt sensitive (CSF 18) were grown in nutrient solution containing 0, 50 and 100 mmol.L-1 NaCl for seven days and the osmotic potential (Ys) and the contribution of organic and inorganic solutes to the Ys were determined in the leaves and roots. Salinity reduced the Ys of the cellular sap of leaves and roots in both genotypes, mainly in the salt sensitive one. The higher decrease in the Ys in the salt sensitive genotype was mostly due to higher accumulation of Na+ and Cl- that probably exceeded the amount needed for the osmotic adjustment. Among the inorganic solutes, K+ contributed the most to the Ys in control unstressed seedlings, but its contribution decreased as salt stress increased, especially in the salt sensitive genotype. Soluble carbohydrates and amino acids were the organic solutes that contributed the most to the leaf and root Ys, respectively. No statistically significant difference in these organic solute contributions to the leaf Ys between genotypes was observed. Their contributions to the root Ys, however, were higher in the salt tolerant genotype, especially at higher NaCl concentration. Proline contribution to leaf and root Ys was quite small in both genotypes and its accumulation was not related to salt tolerance. Our results suggest that the salt tolerant genotype was able to maintain a more adequate osmotic pool in the leaves and roots under salt stress than the salt sensitive genotype.

Isolation, characterization and chromosomal location of a novel zinc-finger protein gene that is down-regulated by salt stress

mRNA transcripts from salt-stressed and unstressed rice seedlings were detected by differential display to show alterations in abundance. One transcript, designated OsZFP1, was found to be significantly down-regulated by salt stress. OsZFP1 encodes a protein of 145 amino acids with three putative Cys2/Cys2-type zinc-finger domains. A homology search of GenBank databases showed that OsZFP1 is homologous to the rat and human ZIS(zinc-finger, splicing) proteins and the human nucleopore complex protein Nup358 in the zinc- finger domains. Genomic Southern analysis indicated that the OsZFP1 gene was present as a single-copy sequence in the rice genome. Restriction fragment length polymorphism mapping assigned the OsZFP1 gene to the distal position of chromosome 6. A RT-PCR assay showed that the OsZFP1 transcripts were more abundant in rice shoots than in the roots. Decreases in the level of OsZFP1 transcripts were detected in the shoots after 6 h of salt stress and in the roots after 3 h of salt stress. In addition, the expression of OsZFP1 in rice shoots was significantly repressed by exogenous application of abscisic acid. The results suggest that OsZFP1 represents a novel type of zinc-finger protein gene in plants and that it is implicated in the responses of rice plants to salt stress.

Gas Exchange of Spring Barley and Wheat Grown under Mild Water Shortage

During mild water stress (decrease of full water capacity from 60 to 35 %) net photosynthetic rate (PN) of four spring barley and wheat genotypes was about twice lower than that for unstressed plants and was mainly limited by non-stomatal factors. Availability of CO2 from intercellular spaces did not change significantly when stomatal conductance (gs) decreased from 0.25-0.35 to 0.15-0.20 mol(H2O) m-2 s-1. There may be two main processes leading to similar intercellular CO2 concentration (ci) in stressed and unstressed seedlings despite of twice lower PN under mild water stress: (a) lower diffusion of CO2 through stomata represented by lower gs, (b) lower consumption of CO2 by photosynthetic apparatus of stressed plants. Last factor is partially pronounced by lower response of PN to ci observed for stressed than for control plants.

Water Stress and the Development of Cankers by Diplodia mutila on Quercus robur

AbstractSome experiments were conducted (in duplicate) to determine the effects of the osmotic potential on the growth of Diplodia mutila, and the effects of the pre‐dawn water potential on the development of cankers by D. mutila on Quercus robur. Mycelium of D. mutila was grown on liquid medium enriched with various amounts of polyethylene glycol so as to give osmotic potential values of ‐0.35, ‐1.18, ‐1.75, ‐3.00 and ‐3.98 MPa. At the two lowest MPa values (‐3.00 and ‐3.98 MPa) D. mutila growth was slower in relation to the fresh weight and the dry weight of the mycelium. To determine the pre‐dawn water potential 30 3‐year‐old Q. robur seedlings were inoculated with D. mutila mycelium. Water‐stressed seedlings had lower pre‐dawn water potential values (MPa ‐2.92) and longer cankers than the unstressed seedlings (MPa ‐0.79). It is concluded that Q. robur, a highly hygrophilous oak, suffers from water stress to such an extent that its structural and chemical defences are overcome and the seedlings are seriously damaged. A similar situation is observed in nature, where frequent and prolonged droughts may induce states of water stress that account for the spread and the incidence of D. mutila in Italy.

Water stress inhibits hydraulic conductance and leaf growth in rice seedlings but not the transport of water via mercury-sensitive water channels in the root

The mechanisms by which moderate water stress (adding polyethylene glycol 6000 to the root medium) induces a sustained inhibition of growth in emerging first leaves of intact rice (Oryza sativa) seedlings was investigated under growth-chamber conditions. Early (24 h) inhibition of leaf growth was not related to changes in root size or in osmotic potential gradients and cell wall-yielding characteristics in the leaf-expansion zone of stressed seedlings. However, reductions in root-to-leaf hydraulic conductance (L) were measured in two rice cultivars after 4 or 24 h at various levels of water stress, and these reductions correlated well with the inhibition of leaf growth. We assayed L by a psychrometric method and, in intact seedlings, by a novel osmotic-jump method. The addition of 0.5 mM HgCl2 to the root medium to inhibit water transport through Hg-sensitive water channels in the roots did not inhibit leaf growth in unstressed seedlings. However, both leaf growth and L were additionally reduced (by 49% and 43%, respectively) within minutes of adding HgCl2 to roots of water-stressed seedlings. Water stress therefore appeared to increase the transport of water via Hg-sensitive water channels. Other mechanisms were apparently involved in inhibiting overall L and leaf growth.

Interactive effects of elevated [CO2] and drought on cherry (Prunus avium) seedlings II. Photosynthetic capacity and water relations

Cherry seedlings (Prunus avium) were grown from seed for two growing seasons in three ambient [CO2] (∼350 μmol mol−1) and three elevated [CO2] (ambient+∼350 μmol mol−1) open‐top chambers, and in three outside blocks. A drying cycle was imposed in both the growing seasons to half the seedlings: days 69–115 in the first growing season, and in the second growing season days 212–251 on the same seedlings which had already experienced drought. Stomatal conductance was significantly reduced in elevated [CO2]‐grown, unstressed seedlings in both the first and second growing seasons, but was not caused by a decrease in stomatal density. Droughted seedlings showed little or no reduction in stomatal conductance in response to elevated [CO2]. However, stomatal conductance was highly correlated with soil water status. Photosynthetic rate increased significantly in response to elevated [CO2] in both water regimes, leading to improvement in instantaneous transpiration efficiency over the whole duration of the experiment, but there was no relationship between instantaneous transpiration efficiency and long‐term water use efficiency. The Amax was strongly reduced in the second growing season, but unaffected by [CO2] treatment. Although photosynthetic rate was not down‐regulated, Rubisco activity was decreased by elevated [CO2], possibly because of the increased leaf carbon: nitrogen ratio which had occurred by the ends of the two growing seasons. Elevated [CO2] did not improve plant water relations (for example, bulk leaf – water potential, osmotic potentials at full and zero turgor, relative water content at zero turgor, bulk modulus of elasticity of the cell) and thus did not increase water‐stress tolerance of cherry seedlings.

Characterization of a pine multigene family containing elicitor-responsive stilbene synthase genes.

Young pine seedlings respond to environmental stress by induced synthesis of pinosylvin, a stilbene phytoalexin. Heartwood of pine trees is characterized by a high content of pinosylvin. The formation of pinosylvin from cinnamoyl-CoA and three molecules malonyl-CoA catalysed by pinosylvin synthase is typical of the genus Pinus. Its enzyme activity not detectable in unstressed seedlings is substantially increased upon application of stimuli like UV-light or infection with the phytopathogenic fungus Botrytis cinerea. A genomic DNA library was screened with pinosylvin synthase cDNA pSP-54 as a probe. Ten clones were isolated and grouped into five subclasses according to the size of their introns. After subcloning into plasmid T7T3, four different members of the five gene subclasses were characterized by sequencing. Emphasis was put on isolating various promoters and analyzing and comparing their responsiveness. The amino acid sequences deduced from genes PST-1, PST-2, PST-3 and PST-5 shared an overall identity of more than 95%. In gene PST-5, the putative translation start site ATG was replaced by CTG. While promoter regions near the TATAA box were almost identical PST-1, PST-2 and PST-3, further upstream sequences differed substantially. Differences in promoter strength were analysed both in transgenic tobacco plants and by transient expression in tobacco protoplasts. Constructs used contained the bacterial beta-glucuronidase under the control of the promoters of pine genes PST-1, PST-2 and PST-3. Upon treatment with UV light or fungal elicitor, the promoter of PST-1 showed highest responsiveness and led to tissue-specific expression in vascular bundles. The data suggest that in pine the gene product of PST-1 is responsible for both the stress response in seedlings and pinosylvin formation in the heartwood.

Changes in proline level in response to osmotic stress and exogenous amino acids in Raphanus sativus L. seedlings

The changes in endogenous proline levels of Raphanus sativus L. seedlings was monitored in presence of exogenous amino acids in normal and osmotically stressed seedlings. In unstressed seedlings, proline uptake was detected only at higher (1 mM) concentration of applied L-proline. however, proline uptake was promoted at all (1 µM to 1000 µM) concentrations of applied L-proline under osmotic stress conditions. Amongst other exogenous amino acids, L-leucine, L-glutamic acid, L-alanine, and L-histidine enhanced endogenous levels of proline, while exogenous hydorxyproline and γ-amino butyric acid reduced it.

Changes in root hydraulic conductance (KR) of Olea oleaster seedlings following drought stress and irrigation

Quasi‐steady‐state measurements of root hydraulic conductance (KR) of Olea oleaster Hoffmgg. et Link potted seedlings were performed using a pressure chamber with the aim of: (a) measuring the impact of different water‐stress levels on a KR; (b) measuring the kinetics of KR recovery several days after soil rewetting; (c) relating changes in KR to changes in root anatomy and morphology. Increasing water‐stress was applied in terms of ratio of leaf water potential (ΨL) measured at midday to that at zero turgor (ΨTLP), i.e. ΨL/ΨTLP=0·5, 1·0, 1·2, 1·6; KR was measured initially and at 24, 48, 72, 96 h after irrigation.Values of KR in seedlings stressed to ΨL/ΨTLP=1·2 increased for 48 h after irrigation from 0·23 to 0·97×10−5 kg s−1 m−2 MPa−1 i.e. from 16% to 66% of that measured in unstressed seedlings. A marked shift of the x‐axis intercept of the straight line relating flow to pressure (zero flow at non‐zero pressure) was recorded initially after irrigation and persisted up to 48 h. Recovery of KR occurred within 24 h after irrigation in seedlings at ΨL/ΨTLP=0·5 and 48 h later in those at ΨL/ΨTLP=1·0.Severe drought stress (ΨL/ΨTLP=1·6) caused anatomical changes to roots which formed a two‐layered exodermis with thicker suberized walls and a three‐ to four‐layered endodermis with completely suberized tangential walls. Recovery of KR in these roots required resumed growth of root tips and emergence of new lateral roots.

Lathyrus Dehydrin : A Drought Inducible cDNA Clone — Isolation and Characterization

Dehydrin like transcripts (drought inducible mRNA) accumulated in drought stressed shoots and roots of Lathyrus sativus. ABA also induced the expression of dehydrin in unstressed seedlings but to a lower extent. A cDNA expression library was prepared in lambda ZAP vector from mRNA of drought stressed shoots of Lathyrus sativus cv P-24 and screened with pea dehydrin cDNA probe. One positive clone (∼ 500 bp) was purified and characterised. The partial nucleotide sequence of the clone Lsd, contained a possible open reading frame which would encode a polypeptide with region of significant amino acid sequence similarity to dehydrin from other plant species. The genomic counterpart of Lsd cDNA was found to be present within a fragment of ∼ 3.4 and ∼ 3.2 kb in parent as well as in the somaclones of Lathyrus sativus, by using different restriction enzymes.

Differences in Plant and Soil Water Relations in and Around a Forest Gap in West Africa during the Dry Season may Influence Seedling Establishment and Survival

Summary 1 Soil and plant-water relations were studied in a canopy opening and surrounding forest shade in a moist tropical forest in Ghana using seedlings of two tree species with different regeneration strategies, the pioneer Terminalia superba and the nonpioneer light demander Entandrophragma utile. 2 During two consecutive dry seasons, soil matric potential varied from -30 kPa at the end of the rainy season, to below -2.5 MPa in the middle of the dry season. During part of the first dry season soil matric potential was higher in the gap than in the surrounding forest. In the second dry season no differences were observed. 3 Leaf water potential varied from about -750 kPa at dawn for unstressed seedlings of both species to <-2.5 MPa when seedlings were permanently wilted. For both species, the maximum stomatal conductance of unstressed seedlings varied from 300 to 450 mmol m-2 s-' for those in the gap centre to 150 mmol m-2 s-' for those under forest shade. Maximum conductance decreased to between 10 and 30 mmol m-2 s-1 in drought-stressed seedlings during the middle of the dry season. The two species responded similarly to a drying soil profile but the decrease in leaf water potential and stomatal conductance was more rapid under forest canopy than in the centre of the gap. 4 Only 7% of the seedlings of Terminalia superba and 4% of Entandrophragma utile survived the dry season under forest shade. In contrast, 85% and 78%, respectively, survived of those growing in the centre of the forest gap. At all positions, seedlings of T. superba had larger relative height growth rates during the wet season than E. utile (29.6, 21.4 and 8.4% month-' compared with 5.2, 20.0 and 1.6% month-', for gap centre, margin and forest shade, respectively). 5 The observed changes in leaf water potential and stomatal conductance indicated that despite the reduced irradiance, shaded seedlings experienced more drought stress during the dry season than seedlings growing in the forest gap. The reasons for this and possible consequences for trees which differ in leaf phenology and regeneration strategy are discussed.

Chlorophyll Fluorescence as a Measure of Cold Hardiness and Freezing Stress in 1 + 1 Douglas-Fir Seedlings

Abstract Two-year-old seedlings of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) from two seed lots with different levels of predicted cold hardiness were grown in separate nurseries. Seedlings were lifted every 4 wk from October 1991 through March 1992 and exposed to controlled freezing temperatures. Fluorescence emissions of the seedlings were measured before and 1 and 3 days after each freezing test. Destructive morphological assessments were made 7 days after freezing. Unstressed seedlings were measured during 6 additional test periods through September 1992. Chlorophyll fluorescence had a significant linear relationship to needle freezing damage and seedling survival, but there was no significant linear relationship between fluorescence of control seedlings and frost hardiness. The LT50 (lethal temperature for 50% of the seedlings) fluorescence measurements, however, tended to be lower before and higher after the seedlings were hardened off. These results indicate that chlorophyll fluorescence is capable of detecting nonvisible damage to seedlings. Continued research may show this to be a valuable tool for rapidly identifying damaged seedlings in nurseries and hence for aiding vital management decisions. For. Sci. 41(3):564-575.

Pea dehydrins: identification, characterisation and expression.

An antiserum raised against dehydrin from maize (Zea mays) recognised several polypeptides in extracts of pea (Pisum sativum) cotyledons. A cDNA expression library was prepared from mRNA of developing cotyledons, screened with the antiserum and positive clones were purified and characterised. The nucleotide sequence of one such clone, pPsB12, contained an open reading frame which would encode a polypeptide with regions of significant amino acid sequence similarity to dehydrins from other plant species. The deduced amino acid sequence of the pea dehydrin encoded by B12 is 197 amino acids in length, has a high glycine content (25.9%), lacks tryptophan and is highly hydrophilic. The polypeptide has an estimated molecular mass of 20.4 kDa and pI = 6.4. An in vitro synthesised product from the clone comigrates with one of the in vivo proteins recognised by the antiserum. A comparison of the pea dehydrin sequence with sequences from other species revealed conserved amino acid regions: an N-terminal DEYGNP and a lysine-rich block (KIKEKLPG), both of which are present in two copies. Unexpectedly, pea dehydrin lacks a stretch of serine residues which is conserved in other dehydrins. B12 mRNA and dehydrin proteins accumulated in dehydration-stressed seedlings, associated with elevated levels of endogenous abscisic acid (ABA). Applied ABA induced expression of dehydrins in unstressed seedlings. Dehydrin expression was rapidly reversed when seedlings were removed from the stress or from treatment with ABA and placed in water. During pea cotyledon development, dehydrin mRNA and proteins accumulated in mid to late embryogenesis. Dehydrin proteins were some of the most actively synthesised at about the time of maximum fresh weight and represent about 2% of protein in mature cotyledons.

Drought tolerance in faster‐ and slower‐growing black spruce (Picea mariana) progenies: II. Osmotic adjustment and changes of soluble carbohydrates and amino acids under osmotic stress

The possibility was considered that osmotic adjustment, the ability to accumulate solutes in response to water stress, may contribute to growth rate differences among closely‐related genotypes of trees. Progeny variation in osmotic adjustment and turgor regulation was investigated by comparing changes in osmotic and pressure potentials, soluble carbohydrates, and amino acids in osmotically stressed seedlings in 4 full‐sib progenies of black spruce [Picea mariana (Mill.) B. S. P.] that differed in growth rate under drought. Osmotic stress was induced by a stepwise increase in the concentration of polyethylene glycol (PEG)‐3350 from 10 (w/v) to 18 and 25%, which provided osmotic potentials in solution culture of ‐0.4, ‐1.0 and ‐2.0 MPa each for 3 days. All 4 progenies maintained a positive cell turgor even at 25% PEG, due to a significant decline in osmotic potential. Although total amino acids, principally proline, increased, ca 60% of the decrease in osmotic potential was attributable to soluble carbohydrates and glucose was the major osmoregulating solute. There was little progeny variation in any of measured parameters in unstressed seedlings. Compared to two slower‐growing progenies, the two progenies capable of more vigorous growth under drought in the field accumulated more soluble carbohydrates (mainly glucose and fructose), developed lower osmotic potential and maintained higher turgor pressure when osmotically‐stressed in solution culture. The ability to adjust osmotically and maintain turgor under drought stress could thus be a useful criterion for the early selection of faster‐growing, drought‐tolerant genotypes.

The Level of Abscisic Acid in the Root Tip of Runner Bean Seedlings: Implications for the ABA-Uptake Carrier

Summary High performance liquid chromatography and gas chromatography using an electron-capture detector have been combined in a method for the determination of the content of free abscisic acid in small amounts ( Phaseolus coccineus L. cv. ‹Prizewinner› root-tip tissue. The identification of abscisic acid was verified by mass spectrography and the method gave estimated overall recoveries of abscisic acid from the tissue consistently greater than 60%. A stereological analysis of transmission electron micrographs of thin sections was used to determine the relative volumes of cellular compartments at either end of that region of the root tip in which carrier-mediated abscisic acid transport has been shown (Astle, M. C. and P. H. RUBERY (1980) Planta, 150 ,312–320). Using the two sets of data and published estimates of the pH of cytoplasmic compartments, the concentration of abscisic acid in the major compartments was calculated. The results define (for unstressed seedlings as used by Astle and Rubery) the range of concentration of abscisic acid occurring in the cytoplasm and cell wall compartments in the region of the root in which the carrier was detected. The values for abscisic acid concentration in the cell wall compartment are so low relative to the K m of the transport carrier that it is not reasonable to suppose that the presence of the carrier will affect the distribution of abscisic acid between compartments to any appreciable extent under normal conditions.

Effects of abscisic acid and its acetylenic alcohol on dormancy, root development and transpiration in three conifer species

Abscisic acid (ABA) and a synthetic analog, the 2‐cis acetylenic alcohol, were compared to evaluate their effectiveness in conditioning seedlings of Douglas‐fir [Pseudotsuga menziesii (Mirb.) Franco], Engelmann spruce (Picea engelmannii Parry) and lodgepole pine (Pinus contorta Dougl.). Following preconditioning with ABA and the analog, seedlings were water stressed with the osmoticum polyethylene glycol (PEG) 3350. The effects of the growth regulators on transpiration, net photosynthesis, their ratio, called water use efficiency, and cell water relations parameters were then compared in stressed and unstressed plants. The antitranspirant action of these compounds varied depending on the species, the growth regulator, and the level of stress. ABA promoted transpiration in unstressed seedlings for all 3 species seven days after application. The analog was superior to ABA as an antitranspirant in osmotically‐stressed lodgepole pine and Engelmann spruce, but neither compound was effective in Douglas‐fir. For Douglas‐fir and Engelmann spruce, net photosynthesis remained consistently higher in ABA‐treated plants during the two levels of osmotic stress, relative to control and analog treatments. Neither compound had any effect on root development or cell water relations. ABA, and to a lesser extent its analog, hastened terminal bud formation in seedlings exposed to short days and low temperatures.

ECTOMYCORRHIZAE AND RATE OF WATER DEFICIT DEVELOPMENT INFLUENCE PHOTOSYNTHESIS IN PINUS TAEDA L. SEEDLINGS

Pinus taeda L. seedlings inoculated with the ectomycorrhizal fungus, Pisolithus tinctorius, were grown in a glasshouse for eight months, and then subjected to rapidly developing cyclic water deficits, or to a single slowly developing water deficit. Water deficits developed at a rate of -0.16 MPa per day (predawn total water potential) for five cyclic water deficits, and at -0.04 MPa per day for the slow water deficit. In unstressed seedlings, carbon exchange rates (CER) did not differ between noninoculated and inoculated seedlings. During slow water deficit development, CER steadily declined. During rapid water deficit development, CER remained unchanged, then declined rapidly when water potentials fell below -1.3 MPa. Inoculated seedlings had higher CER when water potential was lower than -1.5 MPa.

ECTOMYCORRHIZAE AND RATE OF WATER DEFICIT DEVELOPMENT INFLUENCE PHOTOSYNTHESIS IN PINUS TAEDA L. SEEDLINGS

Pinus taeda L. seedlings inoculated with the ectomycorrhizal fungus, Pisolithus tinctorius, were grown in a glasshouse for eight months, and then subjected to rapidly developing cyclic water deficits, or to a single slowly developing water deficit. Water deficits developed at a rate of -0.16 MPa per day (predawn total water potential) for five cyclic water deficits, and at -0.04 MPa per day for the slow water deficit. In unstressed seedlings, carbon exchange rates (CER) did not differ between noninoculated and inoculated seedlings. During slow water deficit development, CER steadily declined. During rapid water deficit development, CER remained unchanged, then declined rapidly when water potentials fell below -1.3 MPa. Inoculated seedlings had higher CER when water potential was lower than -1.5 MPa.

Lipids of soybean and sunflower seedlings grown under drought conditions

Abstract Soybean and sunflower seedlings were grown under field irrigation and water deficit conditions. One set of plants was submitted to water depletion 12 days after sowing and all seedlings were harvested 20 days after sowing. In comparison to controls, the decrease in the soil water potential to - 1.5 MPa produced in both of the stressed species a reduction in leaf water potential with a consequent increase in the water saturation deficit, which was higher in sunflower. Only sunflower suffered a reduction in height, fr. wt, dry wt and in the lipid extract and lipid phosphorus content. Phospholipids decreased by more than 50% in stressed sunflower seedlings while they remained constant in soybean. Diacylglycerol and triacylglycerol concentrations increased in both species under water stress. The free sterol/phospholipid molar ratio was higher in the stressed seedlings of both species. The glycolipid content increased in soybean but was reduced in sunflower plants, with the monogalactosyldiacilglycerol being present in the largest amounts. The degree of unsaturation of the fatty acids of the various lipid classes in general did not change in response to water stress, with the exception of the diacylglycerols where it decreased. In all the lipid classes examined, unsaturation was higher than 60% in both stressed and unstressed seedlings. The free sterols presented a contrasting behaviour, showing an increase in stressed soybean and a reduction in stressed sunflower. Under water deficit conditions there was no change in the ‘more planar’ to ‘less planar’ sterol ratio.