Lipid mobilization in Citrus cotyledons during germination

Abstract

In the present work, lipid mobilization in C. limon (L.) Burm. f. cotyledons and the axial control of this process were studied. Two lipases were found in extracts from cotyledons: one with optimal activity at pH 5.0 (acid lipase) and another with a pH optimum between 7.5-8.0 (alkaline lipase). Both lipase activities showed a parallel pattern of evolution, increasing to a peak by the 16th day of germination. Most acid lipase activity was recovered in the fat layer obtained from crude extracts, and the alkaline lipase was located mainly in the glyoxysomes. The rate of lipid breakdown was lower in excised cotyledons when compared with intact ones. However, substantial hydrolysis of lipids occurs in the absence of the axis. In excised cotyledons, both acid and alkaline lipase activities were markedly reduced, indicating that the presence of the axis was necessary for maximum enzyme formation. Also, development of isocitrate lyase activity was partially inhibited by removal of the axis. The inhibition of isocitrate lyase activity observed when cotyledons were detached from the embryo cannot be explained by the source-sink model, as glucose and sucrose levels were lower in excised cotyledons than in intact ones. Incubation of excised cotyledons in gibberellic acid (10 -5 M) or kinetin (10 -4 M) solutions was effective in maintaining the development of lipase activities at similar levels than in intact cotyledons. Gibberellic acid (10 -5 M) produced a stimulation of isocitrate lyase activity in excised cotyledons, but not reaching the activity level developed by intact cotyledons. The data presented indicate that lipolysis in Citrus cotyledons seems to be only partially dependent on the axis. It is concluded that enzyme regulation in Citrus cotyledons cannot be entirely explained by hormonal control or source-sink relationships.