ENVIRONMENTAL STRESSES AFFECT TOMATO MICROSOMAL MEMBRANE-FUNCTION DIFFERENTLY THAN NATURAL RIPENING AND SENESCENCE

On the premise that environmental stresses may induce changes in microsomal membrane function characteristic of accelerated senescence, the effects of physical damage (PD), chilling injury (CI), heat shock (HS) and controlled atmosphere (CA) on tomato fruit (Lycopersicon esculentum cv. Trust) were compared to changes occurring during normal ripening and senescence. A physiological pattern for ripening and senescence of tomato fruit was established based on weight loss patterns, colour development, ion leakage, lipid fatty acid profiles of microsomal membrane lipids, microsomal K+ stimulated ATPase activity and electrophoretically separated protein patterns of microsomal membranes. Control fruit displayed increases in redness, weight loss, ion leakage and saturation index (SI) of membrane lipids, as well as the appearance of membrane-associated polygalacturonase (polygalacturonase isozyme II - PGII) and its beta-subunit and decreased microsomal membrane K+ stimulated ATPase activity during a three-week storage period. Relative to controls, PD and CI fruit displayed increased ion leakage and a decreased rate of red colour development, increased ATPase activity and SI. Membrane-associated PGII appeared sooner in PD fruit, but its appearance was delayed in CA and CI fruit, relative to controls. Tomatoes exposed to HS and CA treatments, however, displayed ripening and senescence physiological symptoms similar to control fruit. Thus, environmental stresses induced membrane changes that were expressed differently than seen during senescence.