Morphological Characteristics and In Situ Auxin Production during the Histogenesis of Staminate Flowers in Precocious Walnut

Monoclonal anti-indole acetic acid antibodies were used to monitor the temporal and spatial pattern of auxin during staminate flower differentiation in walnut (Juglans regia) cultivars Liaoning 1 and Liaoning 3. The relationship between morphological characteristics and histological structure was established. Seven stages of differentiation were recognized based on the visibility and color of the squama, bract, perianth, and rachis as follows: Stage 1, several bract primordia were present in the squama with catkins protruding from the squama as the only externally visible portion of the floret (Stage 1); the bract became externally visible, and the floret, perianth, and stamen primordia formed basipetally (Stage 2); the length of catkins were elongated, only bracts visible and getting brown (Stage 3); the bracts were brown and wrapped tightly, cellular specialization occurred to form a central core containing reproductive cells and tapetal cells that differentiated (Stage 4); the perianth became visible externally, reproductive cells and tapetal cells separated from the exterior layers of the anther wall (Stage 5); the anther walls were reduced to two cell layers (epidermis and endothecium) as the anthers became visible and matured (Stage 6); and the anther turned black, dehisced, and released its pollen grains (Stage 7). The histological differentiation of the flowers was related to auxin. The auxin signal was strongest in the shoot apical meristem (SAM) during bract primordia differentiation; thus, the SAM may be a site of auxin production. When the floral organs began centralized differentiation, auxin was distributed mainly in the differentiating tissues. Our findings indicate that a high level of auxin may strongly affect morphogenesis. Additionally, the tapetal and reproductive cells that arise during cellular specialization may be important for auxin production. The distribution of auxin was centralized in germ pores at the pollen grain surface, indicating that a high level of auxin induces pollen germination.