Temporal and Spatial Pattern of Indole-3-acetic Acid Occurrence during Walnut Pistillate Flower Bud Differentiation as Revealed by Immunohistochemistry

We used anti-indole-3-acetic acid (IAA) monoclonal antibodies to monitor the temporal and spatial pattern of IAA during pistillate flower bud differentiation in the walnut ( Juglans regia ) cultivar Liaoning 1. Based on morphological changes, the process of pistillate flower bud differentiation was divided into five stages. The flower induction stage, which includes the early phase, midphase, and late phase, persisted from 25 Apr. to the end of May. The pedicel differentiation stage began on 5 June. The bract primordium stage began on 25 June and persisted through mid-March of the next year. Both the perianth and pistil differentiation stages persisted for nearly 2 weeks. During the floral induction period, little IAA was present in the shoot apical meristem (SAM); hence, the SAM may not always be a site of IAA production. IAA was obviously concentrated in cells of the first several layers of the SAM during pedicel primordium formation. High levels of IAA were also noted in the phyllome, young leaf tips, and vascular bundle of leaves and gemmae. This direct evidence indicates that no close relationship exists between IAA and physiological differentiation; instead, IAA may strongly affect morphogenesis. These findings comprise a first step toward elucidating the walnut flowering mechanism.