Photosynthetic carbon metabolism during leaf ontogeny in Zea mays L.: Enzyme studies.

The activities of several enzymes, including ribulose-1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured as a function of leaf age in Z. mays. Mature leaf tissue had a RuDP-carboxylase activity of 296.7 μmol CO2 g(-1) fresh weight h(-1) and a PEP-carboxylase activity of 660.6 μmol CO2 g(-1) fresh weight h(-1). In young corn leaves the activity of the two enzymes was 11 and 29%, respectively, of the mature leaves. In senescent leaf tissue, RuDP carboxylase activity declined more rapidly than that of any of the other enzymes assayed. On a relative basis the activities of NADP malic enzyme (EC 1.1.1.40), aspartate (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2), and NAD malate dehydrogenase (EC 1.1.1.37) exceeded those of both PEP and RuDP carboxylase in young and senescent leaf tissue. Pulse-chase labeling experiments with mature and senescent leaf tissue show that the predominant C4 acid differs between the two leaf ages. Labeling of alanine in senescent tissue never exceeded 4% of the total (14)C remaining during the chase period, while in mature leaf tissue alanine accounted for 20% of the total after 60 s in (12)CO2. The activity of RuDP carboxylase during leaf ontogeny in Z. mays parallels the development of the activity of this enzyme in C3 plants.