PROTOXYLEM MATURATION IN THE SEEDLING LEAF OF WHEAT

Protoxylem (PX) maturation was followed in vascular bundles of the first foliage leaf (L#1) of wheat seedlings, using clearings and sectioned materials to assess development changes. L#1 contained seven to 11 longitudinal bundles. The seven bundles with largest diameters contained PX vessels, but the number of bundles with PX varied with leaf length. Repeated vessel maturation maintained PX continuity as older vessels collapsed due to leaf extension. Thus, the number of PX vessels in each bundle was also a function of leaf length, and the PX content of a bundle was a function of the stage of leaf development and how soon during leaf development the bundle attained mature PX. Distance from the leaf tip to the start point for PX was greater for bundles later to mature. Thus, the pattern of distal start points in mature leaves reflected the progression of PX maturation from the midrib to bundles of lower rank. It is suggested that the involvement of more and more bundles in PX maturation as the leaf ages is a general occurrence in grass leaves, and that assessing how many bundles will have PX requires observations late in leaf development.