Stable Carbon Isotope Ratio and Activities of PEP Carboxylase and PEP Carboxykinase in Pineapple Leaves CHANGES OF CONFORMATION STATES OF PHOSPHOENOLPYVATE CARBOXYKINASE FROM PINEAPPLE LEAF DURING BINDING WITH ITSSUBSTRATES The Identification of Photosynthetic Subpathways of Some C4 and CAM Plants