The Spatial Distribution of Growth in the Extension Zone of Seedling Wheat Leaves

Two models of the distribution of relative elemental rates of elongation (RELEL) were tested for the extension zone (EZ) of the first foliage leaf of seedling wheat plants, by comparison to patterns of separation of rings and gyres in the walls of protoxylem vessels. One model, containing a defined growth maximum in the basal half of the EZ, is favoured in the literature and was derived from data published for perennial ryegrass. The other, containing a flat, broad maximum throughout the region of the EZ with stomates, was constructed from regressions of interstomatal distance against distance along the EZ in the first foliage leaf of wheat seedlings. The test strongly favoured the model with the flat maximum. Although the gibberellic acid (GA) insensitivity alleles Rht1 and Rht2 reduce length of extension zone (LEZ), leaf extension rate (LER) and final cell and leaf lengths, they had no effect on maximum RELEL. Results with an inhibitor of GA synthesis indicated that control of leaf elongation by the control of LEZ may be generalizable as a mechanism by which GA controls LER in the grass leaf.