PATTERNS OF PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITY AND CYTOSOLIC PHDURING LIGHT ACTIVATION AND DARK DEACTIVATION IN C-3 AND C-4 PLANTS

The rate and extent of light activation of PEPC may be used as another criterion to distinguish C3 and C4 plants. Light stimulated phosphoen olypyruvate carboxylase (PEPC) in leaf discs Of C4 plants, the activit y being three times greater than that in the dark but stimulation of P EPC was limited about 30% over the dark-control in C3 species. The lig ht activation of PEPC in leaves of C3 plants was complete within 10 mi n, while maximum activation in C4 plants required illumination for mor e than 20 min, indicating that the relative pace of PEPC activation wa s slower in C4 plants than in C3 plants. Similarly, the dark-deactivat ion of the enzyme was also slower in leaves of C4 than in C3 species. The extent of PEPC stimulation in the alkaline pH range indicated that the dark-adapted form of the C4 enzyme is very sensitive to changes i n pH. The pH of cytosol-enriched cell sap extracted from illuminated l eaves of C4 plants was more alkaline than that of dark-adapted leaves. The extent of such light-dependent alkalization of cell sap was three times higher in C4 leaves than in C3 plants. The course of light-indu ced alkalization and dark-acidification of cytosol-enriched cell sap w as markedly similar to the pattern of light activation and dark-deacti vation of PEPC in Alternanthera pungens, a C4 plant. Our report provid es preliminary evidence that the photoactivation of PEPC in C4 plants may be mediated at least partially by the modulation of cytosolic pH.