Anatomy of the Root-Shoot Junction in Wheat Seedlings with Respect to Internal Oxygen Transport and Root Growth Retardation by External Oxygen Shortage*

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Abstract
Anatomical alterations in the root-shoot junction following hypoxic conditions were studied in young wheat plants (Triticum aestivum L. cv. Hatri) grown in nutrient solution flushed with air or nitrogen gas. The root-shoot junction was characterized by densely packed tissues with only small intercellular spaces. Seven days of hypoxia did not alter the anatomy of this region, suggesting that it does not constitute an important pathway for oxygen diffusion from aerial shoot to the aerenchymatous roots. A likely alternative path for oxygen movement is the gas-filled interspace between coleoptile and shoot base. Roots emerging from more apical parts of the stem elongated more quickly in hypoxic conditions than those from more basal parts. This was related to the path length from the main point of entry of atmospheric oxygen into the plant. Additionally, oxygen shortage in the ambient root medium decreased the number of mitoses per root tip, as determined by the Feulgen method. This effect was most severe in the basally inserted roots, that are presumed to be the most oxygen deficient.