Procambial Initiation for the Vascular System in the Spikelet of Wheat

Abstract

Development of the procambial system of the spikelet of wheat is described from serial transections of successively older primordia selected before anthesis. The procambial median traces of the two glumes merge into the procambial system already in the rachis. More distally in the spikelet primordium, the procambial median traces of the first two lemmas, and then the major lateral traces of the two glumes differentiate in isolation and merge successively with the glume median traces linked to the rachis. Such mergers establish the procambial paired sympodia, which will become the junction of all procambial strands developing distally in the spikelet with the vascular system of the rachis. The paired sympodia form an arc opposite the rachis traces. The first procambial traces from the distal regions of the spikelet to augment the paired sympodia by basipetal mergers are the major laterals of the lemmas of the first two florets. The major lateral traces of all lemmas of the spikelet become interconnected by a series of n, n - 1 mergers between adjacent distal lemmas and between lemmas two and one directly with the paired sympodia. The procambial system for the sexual components of the florets is initiated in the proximal pair of florets, then successively in the distal florets. In each floret, the three isolated stamen traces merge into a unit whose flanks merge into the descending traces of the palea and of the lemma intermediate laterals. This composite of traces becomes an intrafloret link with the lemma's median and major lateral traces and, through them, an interfloret connection to the paired sympodia. The progress of procambial trace mergers in the spikelet primordium is helpful in explaining patterns of grain filling, confirming systematic relationships of wheat to its close relatives, and in revealing morphogenetic similarities between spikelets and axillary buds.