LANDRUM, JAMES VICTOR. Biology Department, Washburn University, Topeka, KS 66621. - Light intensity as the primary factor for wide-band tracheid initiation and develoment in stems of Anacampseros (Portulacaceae).
Wide-band tracheids (WBTs) are novel variations of more common
tracheids, and are characterized by a wide, intrusive secondary wall.
Limited in systematic occurrence, WBTS are found only in three
angiosperm families: Aizoaceae, Cactaceae, and Portulacaceae. These
cells were hypothesized as evolutionary adaptations to water-stress
events; during water-stress events, common tracheids would collapse
whereas WBTs would retain functionality. Experiments using WBTs found
in stems of species in the genus Anacampseros (Portulacaceae)
show that light, not water-stress, is the primary controlling factor
for WBT initiation and development. Plants exposed to low, moderate,
and high light levels show correlative differences in WBT traits and
numbers. Light intensity is thus proposed as a pre-adaptive factor
that initiates WBT formation in preparation, not in response, to
Key words: Anacampseros, light, Portulacaceae, tracheid, wide-band