Delay of Membrane Lipid Degradation by Calcium Treatment during Cabbage Leaf Senescence

Abstract

Cabbage leaf discs (Brassica oleracea L., Capitata group) were floated adaxial side up in 0, 0.05, or 0.25 M CaCI2 solutions at 15°C for 14 d in the dark. To assess whether the delay of senescence by calcium treatment involved protection of membrane lipids, chlorophyll and protein content and the lipid composition of the membranes were determined during incubation. Chlorophyll and protein content decreased with time, in correlation with a reduction in the amount of phospholipids. The degree of unsaturation of phospholipids and free fatty acids decreased, whereas the ratio of sterol to phospholipid increased. (7) report acceleration of senescence. Although calcium treatment of fruits and vegetables has been shown repeatedly to delay senescence (7), the specific mode of action of calcium is not clear. Calcium, which plays an important role in many physiological processes, binds extracellulary to membrane PL. In this way, calcium maintains membrane integrity and controls membrane-associated functions (9). However, an increase in cytosolic calcium may also stimulate lipolytic enzyme activity and accelerate membrane deterioration (21).In the present study, we investigated whether the delay of senescence of cabbage leaf discs by calcium was related to the protection of membrane lipids from degradation during senescence. MATERIALS AND METHODS Plant Material and Environmental ConditionsCabbage (Brassica oleracea L., Capitata group) was obtained from the central warehouse of a local food distribution chain (Provigo, Quebec, QC, Canada). Leaf discs measuring 1 cm in diameter were excised from the interveinal primary leaf with a cork borer. About 25 discs were floated adaxial side up in 10 mL of 0, 0.05, or 0.25 M CaCl2 solution in 125-mL Erlenmeyer flasks. The levels of calcium were chosen after preliminary tests showed delay of Chl degradation below 0.07 M and acceleration above 0.15 M. The discs were incubated in the dark at 15 ± 10C for 14 d. Chi and Protein DeterminationTotal Chl was determined by the method of Arnon (2). Protein content was determined as described by Lowry et al. (10). Lipid Extraction and AnalysisAt the end of the incubation, the discs were fixed in boiling water for 3 min to inactivate endogenous phospholipases. Total lipids were extracted from the tissue using the procedure of Bligh and Dyer (3). The lipids in the chloroform phase were separated by TLC on 250-,Am silica gel G plates (Fisher Scientific Co., Ottawa, ON). Acetone:acetic acid:water (100:2:1, v/v) was used to separate the PL from the galacto- (12). FS were silylated directly on the silica gel (6) and assayed by GLC (HewlettPackard model 5890A, Mississauga, Canada) using cholestane as a standard. Sterol trisilyl derivatives were separated by GLC on a 25-m ULTRA 1 capillary column (HewlettPackard). The PL content was also determined by phosphorus analysis (1). An average PL mol wt of 750 was assumed for calculation of the PL content. LOX AssayLOX activity was determined spectrophotometrically at 234 nm (11). The standard assay...

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Correlations among Attributes of Senescence and Antioxidative Status of Leaf Discs during Epiphyllous Bud Differentiation in Kalanchoe pinnata Lam. (Pers.)

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Inhibition of phospholipase D by lysophosphatidylethanolamine, a lipid-derived senescence retardant

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