The utility of Nymphaeaceae sclereids in paleoenvironmental research

Abstract

As entomophilous plants, water lilies ( Nymphaea) and spatterdocks ( Nuphar) have low pollen production, thus can be under represented in the sediment record. These macrophytes produce distinctly shaped sclerenchyma tissue referred to as stone-cells, trichosclereids, astrosclereids or simply sclereids. This study examines the utility of using sclereids from two common species from the Nymphaeaceae Family as an alternative to their pollen. Histological studies of fresh tissues of Nymphaea odorata and Nuphar lutea show that each has distinct sclereids, but there has been confusion in the terminology used to designate their morphology. Some reports have referred to Nymphaeaceae sclereids as trichosclereids, but our histological studies show that the cells are more appropriately classified as polyramous, astrosclereids, librosclereids or rhizosclereids. Tissues from both species were treated using HCL, KOH, acetolysis and HF. We found that only the sclereids from N. lutea survived chemical treatments in a detectable form, indicating that sclereids from N. lutea can be a useful indicator of presence while the chance of observing sclereids from N. odorata in pollen preparations is very low, severely limiting the utility of the latter as a paleoecological indicator. Another limitation to using sclereids as a proxy is that they originate from tissues that require extended acetolysis treatments for release from the matrix. Finally, we examined sclereid abundance in sediments from a slough in the Florida Everglades, USA to determine if abundance of Nymphaeaceae sclereids correlates with Nymphaeaceae pollen and we find no significant correlation. Additional analyses are required to determine if empirical relationships exist amongst plant populations, pollen, sclereids and environmental conditions. A clear report of chemical treatments used and processing times are critical to verify results of studies utilizing sclereids.