Enzymes of starch and sucrose metabolism in Zea mays leaves

Mesophyll and bundle sheath cells of maize leaves were separated and enzymes of starch and sucrose metabolism assayed. The starch content and activities of ADPglucose (ADPG) starch synthetase and phosphorylase expressed both on a chlorophyll and a protein basis were much lower in mesophyll cells compared to bundle sheath preparations. Exposure of the leaves to continuous illumination for 2·5 days caused the starch content of mesophyll cells to rise greatly and led to considerable increases in ADPG starch synthetase and phosphorylase activity. In glasshouse grown leaves the bulk of invertase, sucrose phosphate synthetase, sucrose phosphatase, UDPglucose pyrophosphorylase and amylase was situated in the mesophyll layer. Sucrose synthetase, ADPG starch synthetase and phosphorylase were largely confined to the bundle sheath. No enzyme could be completely assigned to one particular cell layer. Upon continuous illumination both ADPG starch synthetase and phosphorylase increased in the mesophyll bythe same relative amount. The mesophyll is likely to be a major site for sucrose synthesis in maize leaves.