Comparative analysis of hypocotyl development in epiphytic, lignotuber-forming, and terrestrial Vaccinieae (Ericaceae)

A comparative analysis of hypocotyl development was undertaken with seedlings of three Vaccinieae (Ericaceae) species (Macleania pentaptera Hoerold. Macleania rupestris (Kunth) A.C. Sm., and Vaccinium angustifolium Ait.) to determine the developmental basis for enlarged hypocotyls leading to the development of woody tubers (lignotubers) in M. pentaptera and M. rupestris. Differences in hypocotyl development are apparent after the first true leaves are visible in each species. Vascular tissue in M. rupestris and V. angustifolium is composed primarily of axial columns of secondary xylem. Secondary xylem tissues in M. pentaptera remain mostly parenchymatous and form radial columns of cells through numerous periclinal divisions. Furthermore, the secondary xylem of M. pentaptera comprises random networks of interconnected, small xylem elements in comparison with the secondary xylem of M. rupestris and V. angustifolium. These differences in hypocotyl development persist through the first 200 d of hypocotyl development, and ultimately lead to the development of a large lignotuber in M. pentaptera. Given the large amounts of parenchymatous tissue, a secondary xylem of relatively short secondary xylem elements, and the absence of adventitious buds, we propose that the lignotubers of M. pentaptera are used for short term water storage, rather than regeneration.