Pisum sativum cv. Telephone were grown in a 8:16/27:22 L:D controlled environment to prolong vegetative shoot development. Shoot apices were sampled at various times during the plastochron using a stipule length plastochron index. Stems were prefixed with 1-ethyl-3-(dimethyl-aminopropyl)-carbodiimide hydrochloride to stabilize the location of free endogenous indole-3-acetic acid (IAA) in the shoot apical meristem and leaf primordia. Serial transverse sections were immunolabeled with a commercially available primary monoclonal antibody against IAA. IAA location was visualized using Western Blue stabilized substrate for alkaline phosphatase secondary antibody. Quantitative comparisons between the amount of label on the halves of the shoot apical meristem adjacent to and opposite the youngest leaf primordium were performed to test the hypothesis that just prior to leaf primordium initiation a population of shoot apical meristem cells is transformed from auxin sink cells to auxin source cells.

Key words: Auxin, Immunolabelling, Leaf Primordia, Pisum sativum, Shoot Apical Meristem