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To determine how transmembrane osmotic gradients perturb the structure and dynamics of biological membranes, we examined the effects of medium dilution on the structures of osmolyte-loaded lipid vesicles. Our preparations were characterized by dynamic light scattering (DLS) and nuclear magnetic resonance (NMR) spectroscopies. Populations of Escherichia coli phosphatidylethanolamine (PE) or dioleoylphosphatidylglycerol (DOPG) vesicles prepared by the pH jump technique were variable and polymodal in size distribution. Complex and variable structural changes occurred when PE vesicles were diluted with hypotonic buffer. Such vesicles could not be used as model systems for the analysis of membrane mechanical properties. NaCl-loaded, DOPG vesicles prepared by extrusion through 100 nm (diameter) pores were reproducible and monomodal in size distribution and unilamellar, whereas those prepared by extrusion through 200-, 400-, or 600-nm pores were variable and polymodal in size distribution and/or multilamellar. Time and pressure regimes associated with osmotic lysis of extruded vesicles were defined by monitoring release of carboxyfluorescein, a self-quenching fluorescent dye. Corresponding effects of medium dilution on vesicle structure were assessed by DLS spectroscopy. These experiments and the accompanying analysis (Hallett, F.R., J. Marsh, B.G. Nickel, and J.M. Wood. 1993. Biophys. J. 64:000-000) revealed conditions under which vesicles are expected to reside in a consistently strained state.

Chilling injury (CI) is a physiological defect of plants and their products that results in reduced quality and loss of product utilization following exposure to low but nonfreezing temperatures. To design more effective control strategies and maximize shelf‐life, it is necessary to develop an understanding of the biochemical mechanism(s) responsible for the initiation of CI. Despite considerable efforts in this field of study, there is no general agreement on the cause or nature of CI, or even the primary event(s) triggering low temperature damage. The first unified theory to explain CI was founded on low temperature induced membrane lipid phase transitions leading to a loss of membrane integrity and physiological dysfunction. This was modified to account for the observation that the level of certain high melting phospholipids appears to be related to the chill sensitivity of many plant tissues. Membranes and changes in their physical characteristics are further implicated as having a role in CI by the discovery that chilling stress evokes an elaborate membrane retailoring response that leads to increased fluidity at reduced temperatures. Others have postulated that CI results from the direct effect of reduced temperatures on enzymes or the indirect effect of membrane perturbations on intrinsic enzymes. The redistribution of cellular calcium has most recently been advanced as the primary transducer of CI. The weight of this theory rests on the role of calcium as a secondary messenger for many cellular functions. In this review it is also speculated that lipid peroxidation may have a role in the development of irreversible injury during low temperature stress. Its effect would be similar to the senescent processes of free radical damage to tissue and progressive membrane rigidification.

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ABSRACTThe various quality aspects of chilling injury (Ct) serve as the focus of this review in which symptoms, occurrence and its alleviation are discussed. CI is a term used to describe the physiological damage that occurs in many plants and plant products, particularly those of tropical and subtropical origin, as a result of their exposure to low but nonfreezing temperatures. The substantial economic consequences of CZ have provided the impetus for studying/developing effective means of alleviating symptoms which manifest this disorder. A diversity in plant responses to low temperature stress exists, including alterations in ethylene biosynthesis, increased respiration rates, cessation of protoplasmic streaming, increased solute leakage, and uncoupling of oxidative phosphorylation. These various responses ultimately give rise to an array of visual symptoms (e.g., surface pitting, water rot, poor color development, general loss of structural integrity) which can render severe losses in product quality borh pre-and postharvest. A number of different methods are available by which to alleviate symptom development, including manipulation of storage conditions (e.g. , temperature cycling, hypobaric and modijed atmosphere storage), exogenous chemical treatments (e. g. , application of phospholipids, antioxidants, calcium) and genetic modijcation of chill sensitive species. These are discussed with respect to their effectiveness and possible control mechanisms.

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Flat-plate compression, constant area compression, and puncture tests were examined for their sensitivity in differentiating the firmness of previously chilled (6C, 85% RH, 15 days) and nonchilled mature-green tomato (Lycopersicon esculentum Mill cv. Caruso) fruit during 10 days of ripening at 22C. Firmness, as measured by each of the three methods, progressively decreased (P < 0.001) with ripening. Previously chilled tomatoes were initially softer (P < 0.01) than nonchilled tomatoes, as measured by puncture of whole fruit and constant area compression of pericarp tissue sections, but not by flat-plate compression of whole fruit. Flat-plate compression was therefore found to be a relatively insensitive method by which to measure differences in tomato firmness that are characteristic of slightly chilling-injured fruit.

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