Developmental Anatomy of the Inflorescence of Bread Wheat (Triticum aestivum L.) during Normal Initiation and when Affected by 2, 4-D

Abstract

In wheat, the tip of the shoot apex normally consists of a core of irregularly arranged cells covered by two uniseriate, self perpetuating, layers (the dermatogen and the hypodermal layer): no third, inner layer (sub-hypodermal layer) is present. Leaf initiation involves periclinals in the cells of the dermatogen and hypodermal layers, but not the core. Buds involve many periclinals in the outer cells of the core, a few occasionally in the hypodermal layer but never any in the dermatogen. The appearance of 'double-ridges' signals inflorescence initiation. Each double-ridge is the equivalent of an axillary bud (the future spikelet bud) and its subtending leaf primordium. The initiation of the subtending leaf is normal : the initiation of the spikelet bud is characterized by periclinal divisions in the outer cells of the core, though some may also occur in cells of the hypodermal layer immediately outside : no periclinals are observed in the neighbouring dermatogen cells. All the above events concerned with leaf and bud initiation occur in an easily recognizable, strictly distichous, pattern. In plants affected by 2,4-dichlorophenoxyacetic acid the cellular pattern where double-ridges would have been arising, is badly disrupted, due mainly to increased cell divisions in the hypodermal layer and outer part of the core, though possibly including some in the dermatogen. The apex tip itself is unaffected, probably explaining why, when growth is resumed, it produces a succession of normal spikelets in the normal phyllotaxis.