Lamina Abortion in Terminal Bud-Scale Leaves of Populus deltoides during Dormancy Induction

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Abstract
Bud scales of terminal buds of P. deltoides Bartr. ex Marsh. are formed by enlarged stipules. The laminae of the 1st bud-scale leaves mature and abscise, but the lamina of the last bud-scale leaf (or leaves) senesces prematurely and aborts. The immature lamina of the aborting leaf turns black, shrivels and protrudes from the bud scales until physically dislodged during bud break the following spring. Senescence of the aborting bud-scale lamina begins with loss of starch and enlargement and vacuolation of cells abaxial to the dorsal bundle of the midrib at the lamina tip. These processes basipetally and laterally in the lamina, with degeneration becoming particularly evident in the spongy mesophyll. While lamina degeneration is under way, an abscission meristem begins to form distal to the stipules at the petiole base. Cell divisions initiating the abscission meristem are first evident in the reactivated adaxial meristem and they spread laterally and obliquely across the petiole base. The vascular bundles are isolated by a funnel-shaped, protective layer of corklike cells that extends downward in the leaf base. The deteriorated central trace vasculature extends deeper in the leaf base than that of the lateral traces. Evidently, lamina senescence and abortion are in some way related to degeneration of the central leaf trace that forms the dorsal bundle of the midrib and petiole.