Eects of Flooding and Drought on the Anatomy of Paspalum dilatatum

0305-7364/0 * For corre { Present Durham, NC Paspalum dilatatum occupies di€erent topographic positions in the Flooding Pampa, Argentina. Populations from di€erent positions are subjected to various regimes of ̄ooding and drought, both of which may occur in the same growing season. We investigated the constitutive and plastic anatomical traits of P. dilatatum populations from habitats with contrasting regimes of ̄ooding and drought. Both events a€ected root and sheath anatomy, and these e€ects were similar for clones from di€erent topographic positions. Flooding increased the aerenchymatous tissue in the root cortex and the leaf sheaths and decreased the number of root hairs per unit of root length. Drought decreased the diameter of root metaxylem vessels, thus lowering the risk of embolisms and increasing water̄ow resistance, and increased the number of root hairs, thereby increasing water uptake ability. In addition to these plastic responses, all clones showed constitutive characteristics that may confer an ability to withstand sudden events of ̄ooding or drought: a high proportion of aerenchyma, which may maintain aeration before plastic responses take place; sclerenchyma, which may prevent root and leaf sheath collapse by soil compaction; and a conspicuous endodermis, which may protect stelar tissues from desiccation. Both constitutive and plastic anatomical characteristics are likely to contribute to the ability of this species to occupy widely di€erent topographic positions and to resist temporal variations in water and oxygen availability. # 2001 Annals of Botany Company