Apparent induction of key enzymes of the glyoxylic acid cycle in senescent barley leaves

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Abstract
The activities of two key enzymes of the glyoxylic-acid cycle, isocitrate lyase and malate synthase, can barely be detected in mature, presenescent primary leaves of barley (Hordeum vulgare L.) but are apparently induced in senescent leaf tissue. Upon incubation of leaf segments in permanent darkness, the activities appear and increase dramatically up to the sixth day and thereafter decline. The glyoxylic-acid cycle may thus be functional during foliar senescence. The main period of galactolipid loss is characterized by RQ values as low as 0.63, indicating that long-chain fatty acids produced from thylakoidal acyl-lipids may be utilized for gluconeogenesis involving corresponding glyoxisomal metabolic pathways. Foliar senescence may be characterized by a peroxisomeglyoxysome transition analogous to the glyoxisome-peroxisome transition in greening cotyledons of fat-storing seeds.