Anatomical characteristics of vascular bundles in wheat spike in relation to positional differences of grain development.

BACKGROUND AND AIMS: The development of grains at different spikelet positions and floret positions in wheat is determined by the flow efficiency of the vascular bundle system in the spike. However, the developmental characteristic of vascular bundles in the wheat spike lacks systematic research, and the distribution and branching pattern remain unclear. METHODS: The developmental patterns of vascular bundles in the wheat rachis and rachilla were systematically studied using resin slicing and microscopic observation techniques. KEY RESULTS: The differentiation of vascular bundles in the rachis was earlier than that in the spikelet, and the vascular bundle network in the spike were completely formed before flowering. At the approach of the spikelet, the area of rachis vascular bundles on the spikelet side increased and new multiple vascular bundles were generated to flow into the spikelet. The vascular bundles entering the spikelet continued to branch, with a small portion flowing into the glume and the majority flowing into the first and second florets, respectively. Additionally, the rest vascular bundles flowed into the third floret, and the vascular bundles in the fourth and distal florets were derived from the branch of vascular bundles in the lower rachilla. The weight of grains in the middle and lower spikelet were significantly higher than those in the upper spikelet. The accumulation of main storage substances in the endosperm, including starch and protein, and grain weight at proximal G1 position in the middle spikelet was optimal compared to distal G3 position. CONCLUSIONS: Our observations suggest that the developmental pattern and spatial distribution of vascular bundles may contribute to positional differences in grain development within the wheat spike. The results provide the anatomical basis for understanding the vascular bundles development and grain synchronous development in the spike.