Membrane damage as a result of dehydration was studied in Lotus corniculatus L. cv. Carrol seeds which had been pregerminated for 0, 12, and 24 hours prior to dehydration. During reimbibition, desiccation-tolerant (0-and 12-hour) seeds leaked relatively low quantities of all solutes (total electrolytes, potassium, phosphate, sugar, amino acid, and protein). Desiccation-sensitive (24-hour) seeds leaked higher levels, but evidence of selective permeability remained. Membrane damage was not manifested as a complete removal of the diffusion barrier, although its permeability properties were dramatically altered. Consequently, the plasmalemma was not ruptured or torn by the dehydration treatment, but a more subtle structural alteration occurred.The possibility that seed membranes form a hexagonal rather than a lamellar phase at moisture contents below 20% was investigated by x-ray diffraction. Phospholipids were extracted from desiccation-tolerant (0-hour) and desiccation-sensitive (24-hour) seeds and hydrated to 5, 10, 20, and 40% water. This phospholipid-water system was examined using lowand wide-angle x-ray diffraction and was found to be exclusively lamellar, even at 5% water. Consequently, membrane damage and the leakage of cytoplasmic solutes from seeds cannot be explained by the formation of a hexagonal phase by membrane phospholipids.lapse to form large discontinuities in the lipid bilayer. This possibility is supported by the observation that the transition from desiccation-tolerant to desiccation-sensitive occurs coincidently with the initiation of radicle elongation (9). Since elongation is facilitated by the uptake of water into the vacuole (5), it is conceivable that the rapid loss of this water collapses the cell and, in so doing, breaks the continuity of the plasmalemma and/or the tonoplast. However, this model does not explain why dehydration damage is induced only when seeds are dehydrated below 20% water (9). An alternative model is that dehydration induces an alteration in the hydrophobic-hydrophilic interaction within the membrane so that the structure of the cellular membranes below 20% hydration is different than that above 20% hydration. Water uptake by desiccation-tolerant seeds reinstates the original structure of the cellular membranes, whereas the membranes of desiccation-sensitive seeds are unable to reform completely. A structural rearrangement of membrane phospholipids by dehydration has been proposed by Simon (13), who suggested that membrane lipids in seeds form a hexagonal phase below 20% moisture. However, his proposal has not been experimentally verified using seed phospholipids.This study was intended to investigate these two models of induced damage. The results indicate that dehydration induces a reorganization of the cellular membranes in dehydration-sensitive seeds; however, this reorganization does not involve a lipid phase change.Dehydration treatments of seeds have also been shown to influence seed viability adversely and to increase electrolyte leakage (1, 5, 9). S...
Smooth microsomal membranes were isolated from axes of soybean (Glycine max L. Merr.) seeds at the dehydration-tolerant (6 hours of imbibition) and dehydration-susceptible (36 hours of imbibition) stages of development and were exposed to free
Axes of soybean seeds are tolerant to dehydration at 6 hours of imbibition, but susceptible to dehydration injury if dried at 36 hours of imbibition. Smooth microsomal membranes were isolated from axes imbibed for 6 hours (dehydration tolerant state) and 36 hours (dehydration susceptible state) before and after dehydration treatment. The phase properties and the lipid composition of the membrane fraction were investigated. Wide angle x-ray diffraction patterns of microsomal membranes from axes imbibed for 6 or 36 hours indicated a liquid-crystalline to gel phase transition at approximately 7C. Membranes from axes dehydrated at 6 or 36 hours of imbibition and rehydrated for 2 hours exhibited a phase transition at 7°C and 47°C, respectively. Changes in fatty acid saturation did not account for the changes in phase properties. However, the increased phase transition temperature of the membranes from dehydration injured axes was associated with an increase in free fatty acid:phospholipid molar ratio and a decrease in phospholipid:sterol ratio. These results suggests that dehydration prompted a deesterification of the linkage between glycerol and fatty acid side chains of the phospholipid molecules in the membrane. The resultant increase in free fatty acid content in the membrane is thought to alter the fluidity and phase properties of the membrane and contribute to dehydrtion injury.Injury to cellular membrane systems is a common phenomenon ofenvironmental stresses (9,11,16,25). Although increased leakage of cytoplasmic solutes after exposure to stress has been generally accepted as indicative of membrane lesions (11,16,22), the exact mechanism of membrane injury has not clearly been defined. Therefore, precise characterization of the nature of membrane injury is required to understand the mechanism of stress tolerance in plant tissues.Seeds are generally tolerant to dehydration at maturity and during the early stages of germination but lose dehydration tolerance as germination proceeds (7,22). Therefore, the germinating seed can be used as a system to study dehydration tolerance (7,22). We have previously shown that soybean seeds can be imbibed for 6 h and dehydrated to 10% moisture without loss of viability and vigor. However, if soybean seeds were imbibed for 36 h, tolerance of this dehydration treatment is lost (22). Although the loss of dehydration tolerance coincides with cell elongation, cell enlargement does not appear to be related to the loss of dehydration tolerance (22). Furthermore, dehydration injury in soybean seeds occurred in the axis, not in the cotyledons, and only when dehydrated below 20% moisture (22). ' Supported by the Natural Sciences and Engineering Research Council of Canada.Ultrastructural studies on dehydration-injured seeds (4,5) and freeze fracture studies of the phycobiont Trebouxia after drying (20), demonstrated alterations in membrane structure induced by dehydration. Recent experimental evidence obtained using low angle x-ray diffraction (13) did not confirm Simons' (26) ...
Small-angle x-ray diffraction studies were performed on gel phase-oriented bilayers of dipalmitoylphosphatidylcholine (DPPC) and DPPC containing 40 mol% of either palmitic acid (PA) or palmitic acid brominated at the 2-position (BPA). Oriented samples were prepared using a method developed by us, which is as simple as powder sample preparations while offering all the advantages of oriented samples made by traditional methods. Phases were determined using swelling experiments with structure factors plotted in reciprocal space, creating a relatively smooth curve as the amount of water between the bilayers was changed. Continuous Fourier transforms were also calculated to further test the consistency of the phase assignments. The diffraction data were used to calculate absolute electron density profiles for different bilayers to a resolution of 5-6 A. Analysis indicates the following: (a) The electron density profiles for the three preparations are virtually identical in the hydrocarbon chain region. (b) There is a decrease in the electron density of the glycerol backbone-headgroup region and d-space in DPPC-PA compared to DPPC. (c) The bromine of fatty-acid brominated at the 2-position is in the vicinity of the glycerol backbone. (d) The bilayer thickness of DPPC containing either brominated or unbrominated fatty acid remains relatively constant with increased levels of hydration, unlike DPPC bilayers.
X-ray diffraction has been applied in measuring the helical pitch of the gramicidin channel in oriented bilayers of dilauroylphosphatidylcholine (DLPC) and dimyristoylphosphatidylcholine (DMPC) at a polypeptide concentration of 9.1 mol %. The diffraction data show the helical pitch of gramicidin to be 4.7 +/- 0.2 A in both gel and liquid-crystalline phase bilayers, with and without monovalent cations. In addition, the width of the reflection due to the pitch of the helical gramicidin channel is consistent with a five turn helix.
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