Phytolith spectra in respiratory aerial roots of some mangrove plants of the Indian Sunderbans and its efficacy in ancient deltaic environment reconstruction

This study reports phytolith diversity in respiratory aerial roots of some true mangrove plants for the first time. To identify the signal of phytoliths retrieved from these respiratory roots in the modern deltaic environments, we have analyzed the phytolith contents of pneumatophores and/or pneumatothodes of 13 mangrove species from 7 families inhabiting true mangrove forests, and compared their representation in assemblages of stems and leaves of 108 taxa and 26 surface soil samples collected from different deltaic sub-environments of the Indian Sunderbans along a salinity gradient. The result shows that most of the phytolith morphotypes occurring in these respiratory roots are redundant, having low taxonomic significance, and are produced in the stems of the same or different plants. Our study revealed that despite the morphological symmetry of the phytolith morphotypes, size differences of two morphotypes i.e. blocky polyhedral and blocky elongated bodies can be attributed to discriminate their source of origin reliably. Discriminant function analysis shows that 97.7% of stem and 66.5% aerating root data on blocky polyhedral morphotypes can be correctly classified. Blocky elongated morphotypes can accurately classify 75.7% of stem data and 81.1% of aerating root data. This means that size parameters of certain phytolith morphotypes can successfully discriminate between stem and respiratory aerial root. Study of surface and sub-surface sediments from a late Quaternary profile (4215 ± 35 BP at a depth of 390 cm and 2810 ± 25 BP at a depth of 30 cm) also confirms the conclusions. The implications of these findings lie in distinguishing true mangrove environments from other deltaic sub-environments, as pneumatophores/pneumatothodes are only produced in plants inhabiting tidal or intertidal sub-environments. The creation of a comprehensive analogue of aerating root phytoliths from the Indian Sunderbans has provided the necessary ground work for interpretation of late Quaternary environmental studies in the region.