PHOSPHOLIPID CATABOLISM AND SENESCENCE OF PEA FOLIAGE MEMBRANES: PARAMETERS OF Ca²⁺:CALMODULIN:PHOSPHOLIPASE A₂₋INDUCED CHANGES

Abstract

SUMMARY With progress of normal senescence of pea (Pisum sativum) foliage membranes free fatty acid: sterol ratios increase, this reflecting increment of phospholipolytic activity. The effect is simulated by exogenous application of phospholipase A 2 in the presence of Ca 2+ and calmodulin and inhibited by the calmodulin inhibitor fluphenazine. This trend is accompanied by an increase in membrane microviscosity and transition temperatures and, as evidenced by wide angle X‐ray diffraction data, indicates a change from the liquid crystalline phase to a solid gel lipid bilayer configuration. Ethylene production, associated with plant senescence, closely follows the above biophysical changes. Membranes also manifest lipoxygenase activity, increasing with age and upon application of Ca 2 , calmodulin and phospholipase A 2 the increment being inhibited by the more specific calmodulin inhibitor calmidazolium. Moreover, the polyunsatu‐rated linolenic acid enhanced phospholipase A 2 ‐induced ethylene production, whereas the saturated stearic acid was ineffective. Results are interpreted as a Ca 2+ ‐triggered and calmodulin mediated phospholipase A 2 activation whereby membrane integrity is impaired by the release of the 2‐situated polyunsaturated fatty acids which in the freed form induce lipoxygenase and specific Ca 2+ ionophoretic activity during the process of which an ethylene forming mechanism is activated.