Anatomical Basis for Seed Shattering in Kleingrass and Guineagrass1

Abstract

Anatomical studies of abscission layer development in kleingrass (Panicum coloratum L.) and guineagrass (P. maximum Jacq.) were conducted to determine the sequential histogenesis of the abscission layers which lead to seed shattering and to relate layer development to identifiable morphological stages of developing inflorescences. Two different abscission layers were present in each species. A primary layer was located in the pedicel near the base of the glumes, and a secondary layer extended across the rachilla which supports the fertile floret. The primary layer became histologically identifiable at the early boot stage of floral development in both species but was initiated earlier in guineagrass. By anthesis, cells in this layer were fully developed. Primary layer cell disintegration started 8 to 9 days after anthesis in kleingrass and continued until the spikelet was eventually attached to the pedicel only by the epidermal and vascular cells. Spikelet disarticulation normally occurred by the 14th day after anthesis in kleingrass. In guineagrass, the same progression of events accounted for seed abscission but they occurred over a shorter time span with shattering occurring 10 days after anthesis. The secondary abscission layer was not identifiable until the inflorescences had emerged from the leaf sheaths in both species. The layer consisted of a thin layer of oblong parenchyma cells extending across the rachilla. By anthesis the cells near the outer edge of the layer became sclerified and the sclerification process continued inwardly until abscission occurred 12 to 15 days following anthesis. The abscission mechanism at this layer was assumed to be mechanical, because there was no evidence of cell disintegration. The major contributor to seed shattering in both species was the primary abscission layer. In kleingrass, 81% of the seed shattered at the primary layer and 19% at the secondary layer.