Hydropotes of young and mature leaves in Nuphar lutea and Nymphaea alba (Nymphaeaceae): Formation, functions and phylogeny

Abstract Hydropotes are specialized trichomes in aquatic plants, especially in species of the Nympaeaceae and Cabombaceae; yet their function remains controversial. In this paper, we present data on the hydropotes formed on the abaxial surface of young etiolated leaves tightly packed in the rosette at the rhizome flattened apex in Nuphar lutea (L.) Sm. and Nymphaea alba L. (Nymphaeaceae). The edges of the leaf blade are twisted in 3–5 scrolls. In N. lutea, long glandular hydropotes secreting mucilage are formed on the leaf abaxial surface and inside scrolls. In N. alba, non-glandular three-celled hydropotes are present but each leaf inside the rosette is enclosed by a thin film without mucilage. Only three-celled hydropotes are formed on the abaxial epidermis of green floating leaves of both species. Based on the similarity in initiation and first development of glandular hydropotes and hydropotes, we propose to distinguish two types of hydropotes: type I consisting of an apical cell that continues to divide and type II – an apical cell terminating division. This is well demonstrated by the presence of both types of hydropotes in N. lutea. We assume the function of type I is the secretion of mucilage to protect and facilitate growth of young etiolated leaves inside the rosette and that of type II the accumulation and removal of substances of endogenic and exogenic origin, perhaps residuals of metabolism and/or toxins, in green submerged growing and floating mature leaves. Phylogenetic implications of these morphological structures in Nymphaeales are discussed.