Echinodorus nenufar P. A. Calvino, Lehtonen, M. Waldbillig, T. Acuna & F. Alonso 2026, sp. nov.

Echinodorus nenufar P. A. Calviño, Lehtonen, M. Waldbillig, T. Acuña & F. Alonso sp. nov. Fig. 1 Type. Argentina • Salta: Departamento Gral. José de San Martín, temporary aquatic habitats along Route 53 near Padre Lozano (23°13'00.06"S, 63°47'55.95"W), 22 November 2024, fl., fr., P. A. Calviño and M. Waldbillig s. n. (holotype: mounted on A-B sheets SI - 226624, sheet A, barcode 147436; sheet B, barcode 147435; Fig. 2 A, B. Isotype: SI - 226624, sheet C, barcode 147423). Diagnosis. Echinodorus nenufar is readily distinguishable from all other members of the genus by its unique floral coloration — petals ranging from pale yellow to creamy white (versus consistently intense white in other species) — and the shape of its mature floating leaf blades, which are broadly cordate to ovate to suborbicular (versus typically ovoid, lanceolate, elliptic, or oval in congeners). Description. Herbs perennial or annual, rooted hydrophytes, either emergent or submerged. Rhizomes short, fusiform, erect, firm, 10–30 mm long, 15–25 mm in diameter, with fasciculate, fibrous roots, some of which are finely septate and spongy. Leaves submerged, with floating blades or emergent; venation campylodromous. In mature plants, emergent aerial blades ovate to oval, 9–16 (– 19) cm long × 8–12 (– 16) cm wide, with 10–13 veins, green to light green, lacking pellucid lines, apex obtuse to rounded, base cordate, often with basal lobes contiguous or overlapping to form an oval; petioles sheathing at base; epidermis with faint longitudinal striations. Juvenile emergent leaves. Blades ovate, 3–5 × 2–3 cm, with 5–7 veins and a cordate base. Floating leaves. Blades ovate to broadly ovate or suborbicular, with a cordate base whose basal lobes often overlap; (2.5 –) 7–15 (– 26) cm long × (1.5 –) 4–12 (– 18.5) cm wide; lacking translucent markings. Petioles from 10 up to 50 cm long and 2–8 mm in diameter, with 7–11 veins and indumentum composed of glandular, papillate trichomes bearing 1–3 dendritic or stellate hairs. Submerged leaves. Blades initially linear-lanceolate, later lanceolate, with attenuate bases, obtuse apices, and 3–5 veins. Petioles triangular in cross-section, winged at the base; early petioles subpetiolate, later ones up to 20 cm long and 5–10 mm in diameter. Juvenile emergent leaves ovate, 3–5 cm × 2–3 cm, with 5–7 veins and cordate base. Floating blades ovate to broadly ovate or suborbicular, base cordate with often overlapping basal lobes, (2.5 –) 7–15 (– 26) cm long × (1.5 –) 4–12 (– 18.5) cm wide, lacking translucent markings, petioles up to 50 cm long and 2–8 mm in diameter, with 7–11 veins and indumentum composed of glandular, papillate trichomes, bearing 1–3 dendritic or stellate hairs. Submerged blades initially linear-lanceolate, then lanceolate, with attenuate bases and obtuse apices, 3–5 veins, petioles triangular in cross-section, winged at base, early leaves subpteriolate (with a very short or partially differentiated petiole, intermediate between sessile and fully petiolate); later ones up to 20 cm long, 5–10 mm in diameter. Inflorescences racemose or paniculate, bearing 4–7 whorls of flowers, each whorl with 3–14 flowers, upright or decumbent, often proliferous, up to 45 cm tall and 10 cm wide. Rachis triangular in cross-section, longitudinally striated, puberulent, with some glandular and dendritic trichomes. Peduncles terete, up to 50 cm long and 7.5 mm in diameter, sparsely pilous. Bracts lanceolate, robust, 1.5–4.5 cm long, sometimes shorter, equal to or longer than subtended pedicels, with 10–17 veins, apex acute. Pedicels cylindrical, terete, ascending, accrescent to 0.5–3.5 cm long, 1.0– 1.5 mm in diameter. Flowers bisexual, 1.5–2.5 cm in diameter, ephemeral, each lasting a single day. Sepals three, green, erect, ovate, obtuse, concave, 10–14 - nerved, ca. 5 × 3 mm, abaxially bearing whitish to hyaline trichomes, filiform, branched, or dendritic. Petals three, obovate to orbicular, delicate, non-overlapping to slightly imbricate, pale yellow to creamy white, often with a deeper yellow basal blotch extending up to one-third of the petal length, ca. 12 × 13 mm. Stamens 19–26; filaments slender, ca. 3.5 mm long; anthers oblong, subbasifixed, versatile, ca. 3 mm long. Gynoecium apocarpous; style simple; carpels numerous. Fruits achenes, oblanceolate, ca. 1.5 × 0.5 mm, with 4–6 lateral ribs and 2–4 dorsal keels extending toward the persistent style beak, usually bearing 1–3 small irregular glands; style beak 0.2–0.4 mm long, erect, or lateral. Floral morphology and pigmentation are markedly variable. Most individuals display the typical cup-shaped corolla with erect petals, pale yellow to creamy white, each marked by a conspicuous yellow basal spot that fades gradually toward the center. A smaller proportion shows distinct morphotypes. In one variant, petals are shorter and uniformly deep yellow yet retain the cup-shaped configuration. Another, less frequent form, has fully expanded, non-overlapping, creamy-white petals with a pale yellow to lime-green basal blotch; rare individuals present slightly overlapping petals of similar hue and pattern. Morphological variation. Echinodorus nenufar exhibits pronounced morphological plasticity throughout its life cycle, thriving under both terrestrial and aquatic conditions. Germination and early development may occur either submerged or on exposed, periodically inundated soil. The highest degree of heterophylly is expressed in seedlings developing underwater. In submerged seedlings, the first two leaves are linear-lanceolate, followed by a pair of lanceolate blades; the fifth and sixth leaves become ovate (Fig. 4 A), and later growth gives rise to cordate-ovate floating leaves (Fig. 4 C). Mature individuals in aquatic habitats produce broadly ovate to suborbicular, cordate floating blades. At this stage, the root system remains fibrous, and petioles are initially short. In fully submerged plants, the leaf blades float at the surface, borne on elongated petioles that connect them to the rhizome. These petioles, reaching up to 55 cm in length, are slender and triangular to subcircular in cross-section, with a shield-shaped base. The blades are thin, ovate-cordate to oval-cordate. Developmentally earlier, narrower (lanceolate or elliptical) leaves may remain submerged or reach the surface. Larger floating leaves may become nearly circular, closely resembling those of Nymphaea (Nymphaeaceae), particularly before the emergence of aerial inflorescences. During maturation, individuals transition to an emergent phase characterized by robust ovate to oval, cordate leaves that typically surround the central inflorescence (Fig. 4 B). The basal lobes of these leaves may overlap and curve upward, forming a shallow spoon- or bowl-like shape, while older floating leaves persist toward the plant periphery. This phase generally coincides with the late stages of habitat desiccation. Emergent leaves in terrestrial individuals retain a cordate base, and their lobes may overlap to form a nearly perfect oval outline. When lying flat against the substrate, these leaves closely resemble those of Nymphaea. Seedlings germinating in terrestrial microhabitats often persist as dwarf rosettes with ovate, cordate-based leaves, forming dense clusters. These forms may later revert to aquatic morphologies upon flooding. Conversely, the sudden inundation of juvenile terrestrial plants induces the formation of elongated petioles, which raise newly produced leaves to the water surface, initiating the floating phase. Submerged leaves from earlier stages usually retain their shape, while newly developed ovate leaves may remain permanently submerged in some individuals. Trichomes are restricted to aerial parts. Leaf venation is occasionally tinted purple, and entirely purple leaves occur rarely. Variation in petal coloration and corolla form appears to be environmentally influenced, particularly by light intensity. Flowers with expanded, cream-colored petals are consistently found in plants growing in partial shade beneath trees, whereas cup-shaped, yellow to cream flowers predominate in full-sun habitats. Under cultivation (pers. obs.), an inflorescence collected from a sun-exposed wild plant produced pale cream flowers when grown under shaded aquarium conditions. Yellow pigmentation intensifies during senescence, coinciding with petal curling or retraction at the distal margins preceding wilting. Individual plants usually bear flowers of uniform color and form, suggesting that environmental effects operate at the level of the whole plant rather than on individual flowers. Floral opening follows a consistent diurnal sequence (Fig. 5). Around 05: 30 h, sepals begin to separate; by 06: 00 h, the floral bud opens in a rosaceous pattern at dawn. At approximately 07: 30 h, the petals unfold, and the corolla assumes an erect, cup-like shape. By midday (12: 00–13: 00 h), the corolla is fully expanded — flowers in full sun maintain a cup shape, while those in shaded conditions may become more expanded or reflexed. Around 14: 30 h, petals begin to retract subtly; by approximately 16: 00 h, the corolla may form a shallow bowl or close entirely, particularly in flowers that did not fully expand during anthesis. Senescence begins around 17: 00 h; petal closure accelerates under direct sunlight but is delayed in shade. Root structure in Echinodorus nenufar is notably variable and includes adaptations to temporary aquatic environments (Fig. 6). In several adult specimens (e. g., SI 226628) cultivated in partially submerged pots, the roots were spongy, white, and septate, surrounded by a velamen-like sheath that envelops and protects the fibrous axis. This velamen is semi-translucent and parchment-like when dry, imparting a pearly white appearance; when hydrated, it becomes gelatinous and flexible, presumably enhancing oxygen diffusion and providing mechanical protection. Specialized root hairs arise laterally through the velame